WO2020008106A1

WO2020008106A1 - An apparatus, a method and a computer program for video coding and decoding

Info

Publication number: WO2020008106A1
Application number: PCT/FI2019/050485
Authority: WO
Inventors: Miska Hannuksela
Original assignee: Nokia Technologies Oy
Priority date: 2018-07-02
Filing date: 2019-06-20
Publication date: 2020-01-09
Also published as: EP3818716A4; EP3818716A1

Abstract

A method comprising: encoding an input picture sequence into at least a first bitstream and a second bitstream, said encoding comprising; encoding, into the first and the second bitstream, a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance; encoding, into the first and the second bitstream, other pictures as intermediate pictures, the intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content, wherein the intermediate pictures of first bitstream represent a first aspect of the content and the intermediate pictures of second bitstream represent a second different aspect of the content.

Description

AN APPARATUS, A METHOD AND A COMPUTER PROGRAM FOR VIDEO

CODING AND DECODING

TECHNICAL FIELD

[0001 ] The present invention relates to an apparatus, a method and a computer program for video coding and decoding.

BACKGROUND

[0002] Recently, the development of various multimedia streaming applications, especially 360-degree video or virtual reality (VR) applications, has advanced with big steps. In viewport-adaptive streaming, the bitrate is aimed to be reduced e.g. such that the primary viewport (i.e., the current viewing orientation) is transmitted at the best quality/resolution, while the remaining of 360-degree video is transmitted at a lower quality/resolution. When the viewing orientation changes, e.g. when the user turns his/her head when viewing the content with a head-mounted display (HMD), another version of the content needs to be streamed, matching the new viewing orientation.

[0003] There are several alternatives to deliver the viewport-dependent omnidirectional video. It can be delivered, for example, as equal-resolution High Efficiency Video Coding (HEVC) bitstreams with motion-constrained tile sets (MCTSs). Thus, several HEVC bitstreams of the same omnidirectional source content are encoded at the same resolution but different qualities and bitrates using motion-constrained tile sets.

[0004] A further method is called constrained inter-layer prediction (CILP). In CILP, certain input pictures are chosen to be encoded into two coded pictures in the same bitstream, the first referred to as a shared coded picture. A shared coded picture in a first bitstream is identical to the respective shared coded picture in a second bitstream. The encoding method facilitates decoding a first bitstream up to a selected shared coded picture, exclusive, and decoding a second bitstream starting from the respective shared coded picture. No intra-coded picture is required to start the decoding of the second bitstream, and consequently compression efficiency is improved compared to a conventional approach.

[0005] CILP enables the use of HEVC Main profile encoder and decoder. Moreover, CILP takes advantage of relatively low intra picture frequency. However, CILP suffers from being limited to a single resolution. For example, CILP cannot offer higher resolution than 4K on the viewport with 4K decoding capacity. It is nevertheless foreseeable that the development of the HMDs will require higher resolutions. Hence, multi-resolution viewport adaptation continues to be needed.

SUMMARY

[0006] Now in order to at least alleviate the above problems, an enhanced encoding method is introduced herein.

[0007] A method according to a first aspect comprises encoding an input picture sequence into at least a first bitstream and a second bitstream, said encoding comprising; encoding, into the first and the second bitstream, a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance; and encoding, into the first and the second bitstream, other pictures as intermediate pictures, the intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content, wherein the intermediate pictures of first bitstream represent a first aspect of the content and the intermediate pictures of second bitstream represent a second different aspect of the content.

[0008] An apparatus according to a second aspect comprises means for encoding an input picture sequence into at least a first bitstream and a second bitstream, said encoding comprising; means for encoding, into the first and the second bitstream, a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance; means for encoding, into the first and the second bitstream, other pictures as intermediate pictures, the intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content, wherein the intermediate pictures of first bitstream represent a first aspect of the content and the intermediate pictures of second bitstream represent a second different aspect of the content.

[0009] According to an embodiment, the apparatus further comprises means for encoding the intermediate pictures as motion-constrained tile sets (MCTSs).

[0010] According to an embodiment, the apparatus further comprises means for encoding the intermediate pictures in MCTSs to include a conditional anchor position for MCTSs, where the anchor position of an MCTS is applied when a shared coded picture is referenced in inter prediction. [001 1] According to an embodiment, the apparatus further comprises means for selecting collocated MCTSs of time-aligned intermediate pictures in different bitstreams prior to encoding such that the collocated MCTSs are mutually exclusive for rendering.

[0012] According to an embodiment, the apparatus further comprises means for encoding the intermediate pictures in MCTSs by initializing a motion vector candidate to a value that indicates the spatial location difference of a tile in an intermediate picture relative to the respective tile in the shared coded picture used as a reference picture.

[0013] According to an embodiment, the apparatus further comprises means for selecting the first aspect to be a first region of a projected omnidirectional picture format and the second different aspect to be a second region of the projected omnidirectional picture format, the first region differing from the second region.

[0014] According to an embodiment, the input picture sequence represents volumetric video, and the apparatus further comprises means for selecting the first aspect to be first visibility information and the second different aspect to be second visibility information, the first visibility information differing from the second visibility information.

[0015] A method according to a third aspect comprises receiving and decoding a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance; selecting at least one spatiotemporal unit among at least a first and second spatiotemporal unit, wherein the first spatiotemporal unit represents a first aspect of the content and the second spatiotemporal unit represents a second different aspect of the content; receiving the at least one spatiotemporal unit; merging the at least one spatiotemporal unit into an intermediate picture; and decoding intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content.

[0016] An apparatus according to a fourth aspect comprises means for receiving and decoding a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance; means for selecting at least one spatiotemporal unit among at least a first and second spatiotemporal unit, wherein the first spatiotemporal unit represents a first aspect of the content and the second spatiotemporal unit represents a second different aspect of the content; means for receiving the at least one spatiotemporal unit; means for merging the at least one spatiotemporal unit into an intermediate picture; and means for decoding intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content.

[0017] According to an embodiment, the apparatus further comprises means for identifying intermediate pictures encoded as motion-constrained tile sets (MCTSs) that are alternatives to each other.

[0018] According to an embodiment, the apparatus further comprises means for obtaining properties of the MCTSs that are alternatives to each other.

[0019] According to an embodiment, the apparatus further comprises means for selecting an alternative that suits its needs among the MCTSs that are alternatives to each other.

[0020] According to an embodiment, the apparatus further comprises means for determining the number, position, and size of MCTSs in the merged coded pictures.

[0021] An apparatus according to a fifth aspect comprises means for receiving a first bitstream and a second bitstream, wherein both the first bitstream and the second bitstream individually comprise a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance, and the first bitstream and the second bitstream comprise other pictures encoded as intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content, wherein the intermediate pictures of first bitstream represent a first aspect of the content and the intermediate pictures of second bitstream represent a second different aspect of the content; means for selecting a first spatiotemporal unit of the intermediate picture of the first bitstream and encapsulating the first spatiotemporal unit of the intermediate pictures of the first bitstream into a first tile or sub-picture track; means for selecting a second spatiotemporal unit of the intermediate picture of the second bitstream and encapsulating the second spatiotemporal unit of the intermediate pictures of the second bitstream into a second tile or sub-picture track; means for providing an indication and an identifier of a first group of tile or sub-picture tracks that are alternatives for extraction, the first group of tile or sub-picture tracks comprising the first and second tile or sub-picture tracks; means for creating a first set of samples into a collector track, the first set of samples natively comprising the set of shared coded pictures for the time instance; and means for creating a second set of samples into the collector track and associating the identifier of the first group of tile or sub-picture tracks to the second set of samples, the association intended to be resolved by selecting one of the tile or sub-picture tracks in the first group to be a source of extraction for including the first or the second spatiotemporal unit by reference into the second set of samples.

[0022] The further aspects relate to apparatuses and computer readable storage media stored with code thereon, which are arranged to carry out the above methods and one or more of the embodiments related thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] For better understanding of the present invention, reference will now be made by way of example to the accompanying drawings in which:

[0024] Figure 1 shows schematically an electronic device employing embodiments of the invention;

[0025] Figure 2 shows schematically a user equipment suitable for employing embodiments of the invention;

[0026] Figure 3 further shows schematically electronic devices employing embodiments of the invention connected using wireless and wired network connections;

[0027] Figure 4 shows schematically an encoder suitable for implementing

embodiments of the invention;

[0028] Figure 5 shows an example of MPEG Omnidirectional Media Format (OMAF) concept;

[0029] Figures 6a and 6b show two alternative methods for packing 360-degree video content into 2D packed pictures for encoding;

[0030] Figure 7 shows a process of forming a monoscopic equirectangular panorama picture;

[0031] Figure 8 shows an example of OMAF’s coordinate system;

[0032] Figure 9 shows an example of converting a spherical picture into a packed 2D picture;

[0033] Figure 10 shows an example of delivery of equal-resolution HEVC bitstreams with motion-constrained tile sets;

[0034] Figure 11 shows an example of constrained inter-layer prediction (CILP) encoding;

[0035] Figure 12 shows a flow chart of an encoding method according to an

embodiment of the invention; [0036] Figure 13 shows an example of the encoding method according to an

embodiment of the invention;

[0037] Figures l4a and l4b show an example of encapsulating MCTS-based bitstreams into a container file according to various embodiments of the invention;

[0038] Figure 15 shows a flow chart of an encapsulation method according to an embodiment of the invention;

[0039] Figure 16 shows an example of using different size of tiles according to an embodiment of the invention;

[0040] Figures l7a and l7b show an example of using extractors for encoding MCTS- based bitstreams according to various embodiments of the invention;

[0041 ] Figures 18a and 18b show an example of using more than two resolutions for encoding MCTS-based bitstreams according to various embodiments of the invention;

[0042] Figure 19 shows a flow chart of a decoding method according to an embodiment of the invention;

[0043] Figure 20 shows a schematic diagram of a decoder suitable for implementing embodiments of the invention; and

[0044] Figure 21 shows a schematic diagram of an example multimedia communication system within which various embodiments may be implemented.

DETAILED DESCRIPTON OF SOME EXAMPLE EMBODIMENTS

[0045] The following describes in further detail suitable apparatus and possible mechanisms for viewport adaptive streaming. In this regard reference is first made to Figures 1 and 2, where Figure 1 shows a block diagram of a video coding system according to an example embodiment as a schematic block diagram of an exemplary apparatus or electronic device 50, which may incorporate a codec according to an embodiment of the invention. Figure 2 shows a layout of an apparatus according to an example embodiment. The elements of Figs. 1 and 2 will be explained next.

[0046] The electronic device 50 may for example be a mobile terminal or user equipment of a wireless communication system. However, it would be appreciated that embodiments of the invention may be implemented within any electronic device or apparatus which may require encoding and decoding or encoding or decoding video images. [0047] The apparatus 50 may comprise a housing 30 for incorporating and protecting the device. The apparatus 50 further may comprise a display 32 in the form of a liquid crystal display. In other embodiments of the invention the display may be any suitable display technology suitable to display an image or video. The apparatus 50 may further comprise a keypad 34. In other embodiments of the invention any suitable data or user interface mechanism may be employed. For example the user interface may be

implemented as a virtual keyboard or data entry system as part of a touch-sensitive display.

[0048] The apparatus may comprise a microphone 36 or any suitable audio input which may be a digital or analogue signal input. The apparatus 50 may further comprise an audio output device which in embodiments of the invention may be any one of: an earpiece 38, speaker, or an analogue audio or digital audio output connection. The apparatus 50 may also comprise a battery (or in other embodiments of the invention the device may be powered by any suitable mobile energy device such as solar cell, fuel cell or clockwork generator). The apparatus may further comprise a camera capable of recording or capturing images and/or video. The apparatus 50 may further comprise an infrared port for short range line of sight communication to other devices. In other embodiments the apparatus 50 may further comprise any suitable short range communication solution such as for example a Bluetooth wireless connection or a USB/firewire wired connection.

[0049] The apparatus 50 may comprise a controller 56, processor or processor circuitry for controlling the apparatus 50. The controller 56 may be connected to memory 58 which in embodiments of the invention may store both data in the form of image and audio data and/or may also store instructions for implementation on the controller 56. The controller 56 may further be connected to codec circuitry 54 suitable for carrying out coding and decoding of audio and/or video data or assisting in coding and decoding carried out by the controller.

[0050] The apparatus 50 may further comprise a card reader 48 and a smart card 46, for example a UICC and UICC reader for providing user information and being suitable for providing authentication information for authentication and authorization of the user at a network.

[0051] The apparatus 50 may comprise radio interface circuitry 52 connected to the controller and suitable for generating wireless communication signals for example for communication with a cellular communications network, a wireless communications system or a wireless local area network. The apparatus 50 may further comprise an antenna 44 connected to the radio interface circuitry 52 for transmitting radio frequency signals generated at the radio interface circuitry 52 to other apparatus(es) and for receiving radio frequency signals from other apparatus(es).

[0052] The apparatus 50 may comprise a camera capable of recording or detecting individual frames which are then passed to the codec 54 or the controller for processing. The apparatus may receive the video image data for processing from another device prior to transmission and/or storage. The apparatus 50 may also receive either wirelessly or by a wired connection the image for coding/decoding. The structural elements of apparatus 50 described above represent examples of means for performing a corresponding function.

[0053] With respect to Figure 3, an example of a system within which embodiments of the present invention can be utilized is shown. The system 10 comprises multiple communication devices which can communicate through one or more networks. The system 10 may comprise any combination of wired or wireless networks including, but not limited to a wireless cellular telephone network (such as a GSM, UMTS, CDMA network etc.), a wireless local area network (WLAN) such as defined by any of the IEEE 802.x standards, a Bluetooth personal area network, an Ethernet local area network, a token ring local area network, a wide area network, and the Internet.

[0054] The system 10 may include both wired and wireless communication devices and/or apparatus 50 suitable for implementing embodiments of the invention.

[0055] For example, the system shown in Figure 3 shows a mobile telephone network 11 and a representation of the internet 28. Connectivity to the internet 28 may include, but is not limited to, long range wireless connections, short range wireless connections, and various wired connections including, but not limited to, telephone lines, cable lines, power lines, and similar communication pathways.

[0056] The example communication devices shown in the system 10 may include, but are not limited to, an electronic device or apparatus 50, a combination of a personal digital assistant (PDA) and a mobile telephone 14, a PDA 16, an integrated messaging device (IMD) 18, a desktop computer 20, a notebook computer 22. The apparatus 50 may be stationary or mobile when carried by an individual who is moving. The apparatus 50 may also be located in a mode of transport including, but not limited to, a car, a truck, a taxi, a bus, a train, a boat, an airplane, a bicycle, a motorcycle or any similar suitable mode of transport. [0057] The embodiments may also be implemented in a set-top box; i.e. a digital TV receiver, which may/may not have a display or wireless capabilities, in tablets or (laptop) personal computers (PC), which have hardware or software or combination of the encoder/decoder implementations, in various operating systems, and in chipsets, processors, DSPs and/or embedded systems offering hardware/software based coding.

[0058] Some or further apparatus may send and receive calls and messages and communicate with service providers through a wireless connection 25 to a base station 24. The base station 24 may be connected to a network server 26 that allows communication between the mobile telephone network 11 and the internet 28. The system may include additional communication devices and communication devices of various types.

[0059] The communication devices may communicate using various transmission technologies including, but not limited to, code division multiple access (CDMA), global systems for mobile communications (GSM), universal mobile telecommunications system (UMTS), time divisional multiple access (TDMA), frequency division multiple access (FDMA), transmission control protocol-internet protocol (TCP-IP), short messaging service (SMS), multimedia messaging service (MMS), email, instant messaging service (IMS), Bluetooth, IEEE 802.11 and any similar wireless communication technology. A communications device involved in implementing various embodiments of the present invention may communicate using various media including, but not limited to, radio, infrared, laser, cable connections, and any suitable connection.

[0060] In telecommunications and data networks, a channel may refer either to a physical channel or to a logical channel. A physical channel may refer to a physical transmission medium such as a wire, whereas a logical channel may refer to a logical connection over a multiplexed medium, capable of conveying several logical channels. A channel may be used for conveying an information signal, for example a bitstream, from one or several senders (or transmitters) to one or several receivers.

[0061] An MPEG-2 transport stream (TS), specified in ISO/IEC 13818-1 or equivalently in ITU-T Recommendation H.222.0, is a format for carrying audio, video, and other media as well as program metadata or other metadata, in a multiplexed stream. A packet identifier (PID) is used to identify an elementary stream (a.k.a. packetized elementary stream) within the TS. Hence, a logical channel within an MPEG-2 TS may be considered to correspond to a specific PID value. [0062] Available media file format standards include ISO base media file format (ISO/IEC 14496-12, which may be abbreviated ISOBMFF) and file format for NAF unit structured video (ISO/IEC 14496-15), which derives from the ISOBMFF.

[0063] Some concepts, structures, and specifications of ISOBMFF are described below as an example of a container file format, based on which the embodiments may be implemented. The aspects of the invention are not limited to ISOBMFF, but rather the description is given for one possible basis on top of which the invention may be partly or fully realized.

[0064] A basic building block in the ISO base media file format is called a box. Each box has a header and a payload. The box header indicates the type of the box and the size of the box in terms of bytes. A box may enclose other boxes, and the ISO file format specifies which box types are allowed within a box of a certain type. Furthermore, the presence of some boxes may be mandatory in each file, while the presence of other boxes may be optional. Additionally, for some box types, it may be allowable to have more than one box present in a file. Thus, the ISO base media file format may be considered to specify a hierarchical structure of boxes.

[0065] According to the ISO family of file formats, a file includes media data and metadata that are encapsulated into boxes. Each box is identified by a four character code (4CC) and starts with a header which informs about the type and size of the box.

[0066] In files conforming to the ISO base media file format, the media data may be provided in a media data‘mdat‘ box and the movie‘moov’ box may be used to enclose the metadata. In some cases, for a file to be operable, both of the‘mdat’ and‘moov’ boxes may be required to be present. The movie‘moov’ box may include one or more tracks, and each track may reside in one corresponding track‘trak’ box. A track may be one of the many types, including a media track that refers to samples formatted according to a media compression format (and its encapsulation to the ISO base media file format).

[0067] Movie fragments may be used e.g. when recording content to ISO files e.g. in order to avoid losing data if a recording application crashes, runs out of memory space, or some other incident occurs. Without movie fragments, data loss may occur because the file format may require that all metadata, e.g., the movie box, be written in one contiguous area of the file. Furthermore, when recording a file, there may not be sufficient amount of memory space (e.g., random access memory RAM) to buffer a movie box for the size of the storage available, and re-computing the contents of a movie box when the movie is closed may be too slow. Moreover, movie fragments may enable simultaneous recording and playback of a file using a regular ISO file parser. Furthermore, a smaller duration of initial buffering may be required for progressive downloading, e.g., simultaneous reception and playback of a file when movie fragments are used and the initial movie box is smaller compared to a file with the same media content but structured without movie fragments.

[0068] The movie fragment feature may enable splitting the metadata that otherwise might reside in the movie box into multiple pieces. Each piece may correspond to a certain period of time of a track. In other words, the movie fragment feature may enable interleaving file metadata and media data. Consequently, the size of the movie box may be limited and the use cases mentioned above be realized.

[0069] In some examples, the media samples for the movie fragments may reside in an mdat box, if they are in the same file as the moov box. For the metadata of the movie fragments, however, a moof box may be provided. The moof box may include the information for a certain duration of playback time that would previously have been in the moov box. The moov box may still represent a valid movie on its own, but in addition, it may include an mvex box indicating that movie fragments will follow in the same file. The movie fragments may extend the presentation that is associated to the moov box in time.

[0070] Within the movie fragment there may be a set of track fragments, including anywhere from zero to a plurality per track. The track fragments may in turn include anywhere from zero to a plurality of track runs, each of which document is a contiguous run of samples for that track. Within these structures, many fields are optional and can be defaulted. The metadata that may be included in the moof box may be limited to a subset of the metadata that may be included in a moov box and may be coded differently in some cases. Details regarding the boxes that can be included in a moof box may be found from the ISO base media file format specification. A self-contained movie fragment may be defined to consist of a moof box and an mdat box that are consecutive in the file order and where the mdat box contains the samples of the movie fragment (for which the moof box provides the metadata) and does not contain samples of any other movie fragment (i.e. any other moof box).

[0071] The track reference mechanism can be used to associate tracks with each other. The TrackReferenceBox includes box(es), each of which provides a reference from the containing track to a set of other tracks. These references are labeled through the box type (i.e. the four-character code of the box) of the contained box(es). [0072] The track grouping mechanism enables indication of groups of tracks, where each group shares a particular characteristic or the tracks within a group have a particular relationship. TrackGroupBox may be contained in a TrackBox. TrackGroupBox contains zero or more boxes derived from TrackGroupTypeBox. The particular characteristic or the relationship is indicated by the box type of the contained boxes. The contained boxes include an identifier, which can be used to conclude the tracks belonging to the same track group. The tracks that contain the same type of a contained box within the TrackGroupBox and have the same identifier value within these contained boxes belong to the same track group.

[0073] The ISO Base Media File Format contains three mechanisms for timed metadata that can be associated with particular samples: sample groups, timed metadata tracks, and sample auxiliary information. Derived specification may provide similar functionality with one or more of these three mechanisms.

[0074] A sample grouping in the ISO base media file format and its derivatives, such as the AVC file format and the SVC file format, may be defined as an assignment of each sample in a track to be a member of one sample group, based on a grouping criterion. A sample group in a sample grouping is not limited to being contiguous samples and may contain non-adjacent samples. As there may be more than one sample grouping for the samples in a track, each sample grouping may have a type field to indicate the type of grouping. Sample groupings may be represented by two linked data structures: (1) a SampleToGroupBox (sbgp box) represents the assignment of samples to sample groups; and (2) a SampleGroupDescriptionBox (sgpd box) contains a sample group entry for each sample group describing the properties of the group. There may be multiple instances of the SampleToGroupBox and SampleGroupDescriptionBox based on different grouping criteria. These may be distinguished by a type field used to indicate the type of grouping. SampleToGroupBox may comprise a grouping_type_parameter field that can be used e.g. to indicate a sub-type of the grouping.

[0075] The Matroska file format is capable of (but not limited to) storing any of video, audio, picture, or subtitle tracks in one file. Matroska may be used as a basis format for derived file formats, such as WebM. Matroska uses Extensible Binary Meta Language (EBML) as basis. EBML specifies a binary and octet (byte) aligned format inspired by the principle of XML. EBML itself is a generalized description of the technique of binary markup. A Matroska file consists of Elements that make up an EBML "document." Elements incorporate an Element ID, a descriptor for the size of the element, and the binary data itself. Elements can be nested. A Segment Element of Matroska is a container for other top-level (level 1) elements. A Matroska file may comprise (but is not limited to be composed of) one Segment. Multimedia data in Matroska files is organized in Clusters (or Cluster Elements), each containing typically a few seconds of multimedia data. A Cluster comprises BlockGroup elements, which in turn comprise Block Elements. A Cues Element comprises metadata which may assist in random access or seeking and may include file pointers or respective timestamps for seek points.

[0076] Video codec consists of an encoder that transforms the input video into a compressed representation suited for storage/transmission and a decoder that can uncompress the compressed video representation back into a viewable form. A video encoder and/or a video decoder may also be separate from each other, i.e. need not form a codec. Typically encoder discards some information in the original video sequence in order to represent the video in a more compact form (that is, at lower bitrate).

[0077] Typical hybrid video encoders, for example many encoder implementations of ITU-T H.263 and H.264, encode the video information in two phases. Firstly pixel values in a certain picture area (or“block”) are predicted for example by motion compensation means (finding and indicating an area in one of the previously coded video frames that corresponds closely to the block being coded) or by spatial means (using the pixel values around the block to be coded in a specified manner). Secondly the prediction error, i.e. the difference between the predicted block of pixels and the original block of pixels, is coded. This is typically done by transforming the difference in pixel values using a specified transform (e.g. Discrete Cosine Transform (DCT) or a variant of it), quantizing the coefficients and entropy coding the quantized coefficients. By varying the fidelity of the quantization process, encoder can control the balance between the accuracy of the pixel representation (picture quality) and size of the resulting coded video representation (file size or transmission bitrate).

[0078] In temporal prediction, the sources of prediction are previously decoded pictures (a.k.a. reference pictures). In intra block copy (IBC; a.k.a. intra-block-copy prediction), prediction is applied similarly to temporal prediction but the reference picture is the current picture and only previously decoded samples can be referred in the prediction process. Inter-layer or inter- view prediction may be applied similarly to temporal prediction, but the reference picture is a decoded picture from another scalable layer or from another view, respectively. In some cases, inter prediction may refer to temporal prediction only, while in other cases inter prediction may refer collectively to temporal prediction and any of intra block copy, inter-layer prediction, and inter- view prediction provided that they are performed with the same or similar process than temporal prediction. Inter prediction or temporal prediction may sometimes be referred to as motion compensation or motion- compensated prediction.

[0079] Inter prediction, which may also be referred to as temporal prediction, motion compensation, or motion-compensated prediction, reduces temporal redundancy. In inter prediction the sources of prediction are previously decoded pictures. Intra prediction utilizes the fact that adjacent pixels within the same picture are likely to be correlated. Intra prediction can be performed in spatial or transform domain, i.e., either sample values or transform coefficients can be predicted. Intra prediction is typically exploited in intra coding, where no inter prediction is applied.

[0080] One outcome of the coding procedure is a set of coding parameters, such as motion vectors and quantized transform coefficients. Many parameters can be entropy- coded more efficiently if they are predicted first from spatially or temporally neighboring parameters. For example, a motion vector may be predicted from spatially adjacent motion vectors and only the difference relative to the motion vector predictor may be coded.

Prediction of coding parameters and intra prediction may be collectively referred to as in picture prediction.

[0081] Figure 4 shows a block diagram of a video encoder suitable for employing embodiments of the invention. Figure 4 presents an encoder for two layers, but it would be appreciated that presented encoder could be similarly extended to encode more than two layers. Figure 4 illustrates an embodiment of a video encoder comprising a first encoder section 500 for a base layer and a second encoder section 502 for an enhancement layer. Each of the first encoder section 500 and the second encoder section 502 may comprise similar elements for encoding incoming pictures. The encoder sections 500, 502 may comprise a pixel predictor 302, 402, prediction error encoder 303, 403 and prediction error decoder 304, 404. Figure 4 also shows an embodiment of the pixel predictor 302, 402 as comprising an inter-predictor 306, 406, an intra-predictor 308, 408, a mode selector 310, 410, a filter 316, 416, and a reference frame memory 318, 418. The pixel predictor 302 of the first encoder section 500 receives 300 base layer images of a video stream to be encoded at both the inter-predictor 306 (which determines the difference between the image and a motion compensated reference frame 318) and the intra-predictor 308 (which determines a prediction for an image block based only on the already processed parts of current frame or picture). The output of both the inter-predictor and the intra-predictor are passed to the mode selector 310. The intra-predictor 308 may have more than one intra prediction modes. Hence, each mode may perform the intra-prediction and provide the predicted signal to the mode selector 310. The mode selector 310 also receives a copy of the base layer picture 300. Correspondingly, the pixel predictor 402 of the second encoder section 502 receives 400 enhancement layer images of a video stream to be encoded at both the inter-predictor 406 (which determines the difference between the image and a motion compensated reference frame 418) and the intra-predictor 408 (which determines a prediction for an image block based only on the already processed parts of current frame or picture). The output of both the inter-predictor and the intra-predictor are passed to the mode selector 410. The intra-predictor 408 may have more than one intra-prediction modes. Hence, each mode may perform the intra-prediction and provide the predicted signal to the mode selector 410. The mode selector 410 also receives a copy of the enhancement layer picture 400.

[0082] Depending on which encoding mode is selected to encode the current block, the output of the inter-predictor 306, 406 or the output of one of the optional intra-predictor modes or the output of a surface encoder within the mode selector is passed to the output of the mode selector 310, 410. The output of the mode selector is passed to a first summing device 321, 421. The first summing device may subtract the output of the pixel predictor 302, 402 from the base layer picture 300/enhancement layer picture 400 to produce a first prediction error signal 320, 420 which is input to the prediction error encoder 303, 403.

[0083] The pixel predictor 302, 402 further receives from a preliminary reconstructor 339, 439 the combination of the prediction representation of the image block 312, 412 and the output 338, 438 of the prediction error decoder 304, 404. The preliminary reconstructed image 314, 414 may be passed to the intra-predictor 308, 408 and to a filter 316, 416. The filter 316, 416 receiving the preliminary representation may filter the preliminary representation and output a final reconstructed image 340, 440 which may be saved in a reference frame memory 318, 418. The reference frame memory 318 may be connected to the inter-predictor 306 to be used as the reference image against which a future base layer picture 300 is compared in inter-prediction operations. Subject to the base layer being selected and indicated to be source for inter-layer sample prediction and/or inter-layer motion information prediction of the enhancement layer according to some embodiments, the reference frame memory 318 may also be connected to the inter-predictor 406 to be used as the reference image against which a future enhancement layer pictures 400 is compared in inter-prediction operations. Moreover, the reference frame memory 418 may be connected to the inter-predictor 406 to be used as the reference image against which a future enhancement layer picture 400 is compared in inter-prediction operations.

[0084] Filtering parameters from the filter 316 of the first encoder section 500 may be provided to the second encoder section 502 subject to the base layer being selected and indicated to be source for predicting the filtering parameters of the enhancement layer according to some embodiments.

[0085] The prediction error encoder 303, 403 comprises a transform unit 342, 442 and a quantizer 344, 444. The transform unit 342, 442 transforms the first prediction error signal 320, 420 to a transform domain. The transform is, for example, the DCT transform. The quantizer 344, 444 quantizes the transform domain signal, e.g. the DCT coefficients, to form quantized coefficients.

[0086] The prediction error decoder 304, 404 receives the output from the prediction error encoder 303, 403 and performs the opposite processes of the prediction error encoder 303, 403 to produce a decoded prediction error signal 338, 438 which, when combined with the prediction representation of the image block 312, 412 at the second summing device 339, 439, produces the preliminary reconstructed image 314, 414. The prediction error decoder may be considered to comprise a dequantizer 361, 461, which dequantizes the quantized coefficient values, e.g. DCT coefficients, to reconstruct the transform signal and an inverse transformation unit 363, 463, which performs the inverse transformation to the reconstructed transform signal wherein the output of the inverse transformation unit 363, 463 contains reconstructed block(s). The prediction error decoder may also comprise a block filter which may filter the reconstructed block(s) according to further decoded information and filter parameters.

[0087] The entropy encoder 330, 430 receives the output of the prediction error encoder 303, 403 and may perform a suitable entropy encoding/variable length encoding on the signal to provide error detection and correction capability. The outputs of the entropy encoders 330, 430 may be inserted into a bitstream e.g. by a multiplexer 508.

[0088] The H.264/AVC standard was developed by the Joint Video Team (JVT) of the Video Coding Experts Group (VCEG) of the Telecommunications Standardization Sector of International Telecommunication Union (ITU-T) and the Moving Picture Experts Group (MPEG) of International Organisation for Standardization (ISO) / International

Electrotechnical Commission (IEC). The H.264/AVC standard is published by both parent standardization organizations, and it is referred to as ITU-T Recommendation H.264 and ISO/IEC International Standard 14496-10, also known as MPEG-4 Part 10 Advanced Video Coding (AVC). There have been multiple versions of the H.264/AVC standard, integrating new extensions or features to the specification. These extensions include Scalable Video Coding (SVC) and Multiview Video Coding (MVC).

[0089] Version 1 of the High Efficiency Video Coding (H.265/HEVC a.k.a. HEVC) standard was developed by the Joint Collaborative Team - Video Coding (JCT-VC) of VCEG and MPEG. The standard was published by both parent standardization

organizations, and it is referred to as ITU-T Recommendation H.265 and ISO/IEC

International Standard 23008-2, also known as MPEG-H Part 2 High Efficiency Video Coding (HEVC). Later versions of H.265/HEVC included scalable, multiview, fidelity range extensions, , three-dimensional, and screen content coding extensions which may be abbreviated SHVC, MV-HEVC, REXT, 3D-HEVC, and SCC, respectively.

[0090] SHVC, MV-HEVC, and 3D-HEVC use a common basis specification, specified in Annex F of the version 2 of the HEVC standard. This common basis comprises for example high-level syntax and semantics e.g. specifying some of the characteristics of the layers of the bitstream, such as inter-layer dependencies, as well as decoding processes, such as reference picture list construction including inter- layer reference pictures and picture order count derivation for multi-layer bitstream. Annex F may also be used in potential subsequent multi-layer extensions of HEVC. It is to be understood that even though a video encoder, a video decoder, encoding methods, decoding methods, bitstream structures, and/or embodiments may be described in the following with reference to specific extensions, such as SHVC and/or MV-HEVC, they are generally applicable to any multi-layer extensions of HEVC, and even more generally to any multi-layer video coding scheme.

[0091 ] The standardization of the Versatile Video Coding (WC, H.266, or

H.266/WC) standard has been started in the Joint Video Experts Team (JVET) of ITU-T and MPEG.

[0092] Some key definitions, bitstream and coding structures, and concepts of

H.264/ AVC and HEVC are described in this section as an example of a video encoder, decoder, encoding method, decoding method, and a bitstream structure, wherein the embodiments may be implemented. Some of the key definitions, bitstream and coding structures, and concepts of H.264/AVC are the same as in HEVC - hence, they are described below jointly. The aspects of the invention are not limited to H.264/AVC or HEVC, but rather the description is given for one possible basis on top of which the invention may be partly or fully realized. Many aspects described below in the context of H.264/AVC or HEVC may apply to WC, and the aspects of the invention may hence be applied to WC.

[0093] Similarly to many earlier video coding standards, the bitstream syntax and semantics as well as the decoding process for error-free bitstreams are specified in

H.264/AVC and HEVC. The encoding process is not specified, but encoders must generate conforming bitstreams. Bitstream and decoder conformance can be verified with the Hypothetical Reference Decoder (HRD). The standards contain coding tools that help in coping with transmission errors and losses, but the use of the tools in encoding is optional and no decoding process has been specified for erroneous bitstreams.

[0094] The elementary unit for the input to an H.264/AVC or HEVC encoder and the output of an H.264/AVC or HEVC decoder, respectively, is a picture. A picture given as an input to an encoder may also be referred to as a source picture, and a picture decoded by a decoded may be referred to as a decoded picture.

[0095] The source and decoded pictures are each comprised of one or more sample arrays, such as one of the following sets of sample arrays:

Luma (Y) only (monochrome).

Luma and two chroma (YCbCr or YCgCo).

Green, Blue and Red (GBR, also known as RGB).

Arrays representing other unspecified monochrome or tri- stimulus color samplings (for example, YZX, also known as XYZ).

[0096] In the following, these arrays may be referred to as luma (or L or Y) and chroma, where the two chroma arrays may be referred to as Cb and Cr; regardless of the actual color representation method in use. The actual color representation method in use can be indicated e.g. in a coded bitstream e.g. using the Video Usability Information (VUI) syntax of H.264/AVC and/or HEVC. A component may be defined as an array or single sample from one of the three sample arrays (luma and two chroma) or the array or a single sample of the array that compose a picture in monochrome format. [0097] In H.264/AVC and HEVC, a picture may either be a frame or a field. A frame comprises a matrix of luma samples and possibly the corresponding chroma samples. A field is a set of alternate sample rows of a frame and may be used as encoder input, when the source signal is interlaced. Chroma sample arrays may be absent (and hence monochrome sampling may be in use) or chroma sample arrays may be subsampled when compared to luma sample arrays. Chroma formats may be summarized as follows:

In monochrome sampling there is only one sample array, which may be nominally considered the luma array.

In 4:2:0 sampling, each of the two chroma arrays has half the height and half the width of the luma array.

In 4:2:2 sampling, each of the two chroma arrays has the same height and half the width of the luma array.

In 4:4:4 sampling when no separate color planes are in use, each of the two chroma arrays has the same height and width as the luma array.

[0098] In H.264/AVC and HEVC, it is possible to code sample arrays as separate color planes into the bitstream and respectively decode separately coded color planes from the bitstream. When separate color planes are in use, each one of them is separately processed (by the encoder and/or the decoder) as a picture with monochrome sampling.

[0099] A partitioning may be defined as a division of a set into subsets such that each element of the set is in exactly one of the subsets.

[0100] When describing the operation of HEVC encoding and/or decoding, the following terms may be used. A coding block may be defined as an NxN block of samples for some value of N such that the division of a coding tree block into coding blocks is a partitioning. A coding tree block (CTB) may be defined as an NxN block of samples for some value of N such that the division of a component into coding tree blocks is a partitioning. A coding tree unit (CTU) may be defined as a coding tree block of luma samples, two corresponding coding tree blocks of chroma samples of a picture that has three sample arrays, or a coding tree block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax structures used to code the samples. A coding unit (CU) may be defined as a coding block of luma samples, two corresponding coding blocks of chroma samples of a picture that has three sample arrays, or a coding block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax structures used to code the samples. A CU with the maximum allowed size may be named as LCU (largest coding unit) or coding tree unit (CTU) and the video picture is divided into non-overlapping LCUs.

[0101] A CU consists of one or more prediction units (PU) defining the prediction process for the samples within the CU and one or more transform units (TU) defining the prediction error coding process for the samples in the said CU. Typically, a CU consists of a square block of samples with a size selectable from a predefined set of possible CU sizes. Each PU and TU can be further split into smaller PUs and TUs in order to increase granularity of the prediction and prediction error coding processes, respectively. Each PU has prediction information associated with it defining what kind of a prediction is to be applied for the pixels within that PU (e.g. motion vector information for inter predicted PUs and intra prediction directionality information for intra predicted PUs).

[0102] Each TU can be associated with information describing the prediction error decoding process for the samples within the said TU (including e.g. DCT coefficient information). It is typically signalled at CU level whether prediction error coding is applied or not for each CU. In the case there is no prediction error residual associated with the CU, it can be considered there are no TUs for the said CU. The division of the image into CUs, and division of CUs into PUs and TUs is typically signalled in the bitstream allowing the decoder to reproduce the intended structure of these units.

[0103] In HEVC, a picture can be partitioned in tiles, which are rectangular and contain an integer number of LCUs. In HEVC, the partitioning to tiles forms a regular grid, where heights and widths of tiles differ from each other by one LCU at the maximum. In HEVC, a slice is defined to be an integer number of coding tree units contained in one independent slice segment and all subsequent dependent slice segments (if any) that precede the next independent slice segment (if any) within the same access unit. In HEVC, a slice segment is defined to be an integer number of coding tree units ordered consecutively in the tile scan and contained in a single NAL unit. The division of each picture into slice segments is a partitioning. In HEVC, an independent slice segment is defined to be a slice segment for which the values of the syntax elements of the slice segment header are not inferred from the values for a preceding slice segment, and a dependent slice segment is defined to be a slice segment for which the values of some syntax elements of the slice segment header are inferred from the values for the preceding independent slice segment in decoding order. In HEVC, a slice header is defined to be the slice segment header of the independent slice segment that is a current slice segment or is the independent slice segment that precedes a current dependent slice segment, and a slice segment header is defined to be a part of a coded slice segment containing the data elements pertaining to the first or all coding tree units represented in the slice segment. The CUs are scanned in the raster scan order of LCUs within tiles or within a picture, if tiles are not in use. Within an LCU, the CUs have a specific scan order.

[0104] A motion-constrained tile set (MCTS) is such that the inter prediction process is constrained in encoding such that no sample value outside the motion-constrained tile set, and no sample value at a fractional sample position that is derived using one or more sample values outside the motion-constrained tile set, is used for inter prediction of any sample within the motion-constrained tile set. Additionally, the encoding of an MCTS is constrained in a manner that motion vector candidates are not derived from blocks outside the MCTS. This may be enforced by turning off temporal motion vector prediction of HEVC, or by disallowing the encoder to use the TMVP candidate or any motion vector prediction candidate following the TMVP candidate in the merge or AMVP candidate list for PUs located directly left of the right tile boundary of the MCTS except the last one at the bottom right of the MCTS. In general, an MCTS may be defined to be a tile set that is independent of any sample values and coded data, such as motion vectors, that are outside the MCTS. In some cases, an MCTS may be required to form a rectangular area. It should be understood that depending on the context, an MCTS may refer to the tile set within a picture or to the respective tile set in a sequence of pictures. The respective tile set may be, but in general need not be, collocated in the sequence of pictures.

[0105] It is noted that sample locations used in inter prediction may be saturated by the encoding and/or decoding process so that a location that would be outside the picture otherwise is saturated to point to the corresponding boundary sample of the picture. Hence, if a tile boundary is also a picture boundary, in some use cases, encoders may allow motion vectors to effectively cross that boundary or a motion vector to effectively cause fractional sample interpolation that would refer to a location outside that boundary, since the sample locations are saturated onto the boundary. In other use cases, specifically if a coded tile may be extracted from a bitstream where it is located on a position adjacent to a picture boundary to another bitstream where the tile is located on a position that is not adjacent to a picture boundary, encoders may constrain the motion vectors on picture boundaries similarly to any MCTS boundaries. [0106] The temporal motion-constrained tile sets SEI message of HEVC can be used to indicate the presence of motion-constrained tile sets in the bitstream.

[0107] It needs to be understood that even though some examples and embodiments are described with respect to MCTSs, they could be similarly realized with other similar concepts of independently decodab le spatio temporal units. Moreover, motion constraints for such spatiotemporal units could be specified similarly to how MCTSs above. Example of such spatiotemporal units include but are not limited to motion-constrained slices and motion-constrained pictures. A motion-constrained slice is such that the inter prediction process is constrained in encoding such that no syntax or derived variables outside the motion-constrained slice, no sample value outside the motion-constrained slice, and no sample value at a fractional sample position that is derived using one or more sample values outside the motion-constrained slice, is used for inter prediction of any sample within the motion-constrained slice. A motion-constrained picture is such that the inter prediction process is constrained in encoding such that no syntax or derived variables outside the motion-constrained picture without special consideration of picture boundaries, no sample value outside the motion-constrained picture without special consideration of picture boundaries, and no sample value at a fractional sample position that is derived using one or more sample values outside the motion-constrained picture without special consideration of picture boundaries, is used for inter prediction of any sample within the motion-constrained picture. Such special consideration of picture boundaries could for example be saturation of coordinates to be within picture boundaries and inferring blocks or motion vectors outside picture boundaries to be unavailable in a prediction process. When the phrase spatiotemporal unit is used in the context of a single time instance or single picture, it can be considered as a spatial unit, corresponding to a certain subset of a coded picture and, when decoded, a certain subset of a decoded picture area.

[0108] The decoder reconstructs the output video by applying prediction means similar to the encoder to form a predicted representation of the pixel blocks (using the motion or spatial information created by the encoder and stored in the compressed representation) and prediction error decoding (inverse operation of the prediction error coding recovering the quantized prediction error signal in spatial pixel domain). After applying prediction and prediction error decoding means the decoder sums up the prediction and prediction error signals (pixel values) to form the output video frame. The decoder (and encoder) can also apply additional filtering means to improve the quality of the output video before passing it for display and/or storing it as prediction reference for the forthcoming frames in the video sequence.

[0109] The filtering may for example include one more of the following: deblocking, sample adaptive offset (SAO), and/or adaptive loop filtering (ALF). H.264/AVC includes a deblocking, whereas HE VC includes both deblocking and SAO.

[01 10] In typical video codecs the motion information is indicated with motion vectors associated with each motion compensated image block, such as a prediction unit. Each of these motion vectors represents the displacement of the image block in the picture to be coded (in the encoder side) or decoded (in the decoder side) and the prediction source block in one of the previously coded or decoded pictures. In order to represent motion vectors efficiently those are typically coded differentially with respect to block specific predicted motion vectors. In typical video codecs the predicted motion vectors are created in a predefined way, for example calculating the median of the encoded or decoded motion vectors of the adjacent blocks. Another way to create motion vector predictions is to generate a list of candidate predictions from adjacent blocks and/or co-located blocks in temporal reference pictures and signalling the chosen candidate as the motion vector predictor. In addition to predicting the motion vector values, it can be predicted which reference picture(s) are used for motion-compensated prediction and this prediction information may be represented for example by a reference index of previously

coded/decoded picture. The reference index is typically predicted from adjacent blocks and/or co-located blocks in temporal reference picture. Moreover, typical high efficiency video codecs employ an additional motion information coding/decoding mechanism, often called merging/merge mode, where all the motion field information, which includes motion vector and corresponding reference picture index for each available reference picture list, is predicted and used without any modification/correction. Similarly, predicting the motion field information is carried out using the motion field information of adjacent blocks and/or co-located blocks in temporal reference pictures and the used motion field information is signalled among a list of motion field candidate list filled with motion field information of available adjacent/co-located blocks.

[0111] In typical video codecs the prediction residual after motion compensation is first transformed with a transform kernel (like DCT) and then coded. The reason for this is that often there still exists some correlation among the residual and transform can in many cases help reduce this correlation and provide more efficient coding. [01 12] Typical video encoders utilize Lagrangian cost functions to find optimal coding modes, e.g. the desired coding mode for a block and associated motion vectors. This kind of cost function uses a weighting factor l to tie together the (exact or estimated) image distortion due to lossy coding methods and the (exact or estimated) amount of information that is required to represent the pixel values in an image area:

C = D + R, (1) where C is the Lagrangian cost to be minimized, D is the image distortion (e.g. Mean Squared Error) with the mode and motion vectors considered, and R the number of bits needed to represent the required data to reconstruct the image block in the decoder (including the amount of data to represent the candidate motion vectors).

[0113] Video coding standards and specifications may allow encoders to divide a coded picture to coded slices or alike. In-picture prediction is typically disabled across slice boundaries. Thus, slices can be regarded as a way to split a coded picture to independently decodable pieces. In H.264/AVC and HEVC, in-picture prediction may be disabled across slice boundaries. Thus, slices can be regarded as a way to split a coded picture into independently decodable pieces, and slices are therefore often regarded as elementary units for transmission. In many cases, encoders may indicate in the bitstream which types of in picture prediction are turned off across slice boundaries, and the decoder operation takes this information into account for example when concluding which prediction sources are available. For example, samples from a neighboring CU may be regarded as unavailable for intra prediction, if the neighboring CU resides in a different slice.

[01 14] An elementary unit for the output of an H.264/AVC or HEVC encoder and the input of an H.264/AVC or HEVC decoder, respectively, is a Network Abstraction Layer (NAL) unit. For transport over packet-oriented networks or storage into structured files, NAL units may be encapsulated into packets or similar structures. A bytestream format has been specified in H.264/AVC and HEVC for transmission or storage environments that do not provide framing structures. The bytestream format separates NAL units from each other by attaching a start code in front of each NAL unit. To avoid false detection of NAL unit boundaries, encoders run a byte-oriented start code emulation prevention algorithm, which adds an emulation prevention byte to the NAL unit payload if a start code would have occurred otherwise. In order to enable straightforward gateway operation between packet- and stream-oriented systems, start code emulation prevention may always be performed regardless of whether the bytestream format is in use or not. A NAL unit may be defined as a syntax structure containing an indication of the type of data to follow and bytes containing that data in the form of an RBSP interspersed as necessary with emulation prevention bytes. A raw byte sequence payload (RBSP) may be defined as a syntax structure containing an integer number of bytes that is encapsulated in a NAL unit. An RBSP is either empty or has the form of a string of data bits containing syntax elements followed by an RBSP stop bit and followed by zero or more subsequent bits equal to 0.

[0115] NAL units consist of a header and payload. In H.264/AVC and HEVC, the NAL unit header indicates the type of the NAL unit

[01 16] In HEVC, a two-byte NAL unit header is used for all specified NAL unit types. The NAL unit header contains one reserved bit, a six-bit NAL unit type indication, a three- bit nuh_temporal_id_plusl indication for temporal level (may be required to be greater than or equal to 1) and a six-bit nuh layer id syntax element. The temporal_id_plusl syntax element may be regarded as a temporal identifier for the NAL unit, and a zero- based Temporalld variable may be derived as follows: Temporalld = temporal_id_plusl - 1. The abbreviation TID may be used to interchangeably with the Temporalld variable. Temporalld equal to 0 corresponds to the lowest temporal level. The value of

temporal_id_plusl is required to be non-zero in order to avoid start code emulation involving the two NAL unit header bytes. The bitstream created by excluding all VCL NAL units having a Temporalld greater than or equal to a selected value and including all other VCL NAL units remains conforming. Consequently, a picture having Temporalld equal to tid value does not use any picture having a Temporalld greater than tid value as inter prediction reference. A sub-layer or a temporal sub-layer may be defined to be a temporal scalable layer (or a temporal layer, TL) of a temporal scalable bitstream, consisting of VCL NAL units with a particular value of the Temporalld variable and the associated non-VCL NAL units nuh layer id can be understood as a scalability layer identifier.

[01 17] NAL units can be categorized into Video Coding Layer (VCL) NAL units and non-VCL NAL units. VCL NAL units are typically coded slice NAL units. In HEVC, VCL NAL units contain syntax elements representing one or more CU.

[0118] In HEVC, abbreviations for picture types may be defined as follows: trailing (TRAIL) picture, Temporal Sub-layer Access (TSA), Step-wise Temporal Sub-layer Access (STSA), Random Access Decodable Leading (RADL) picture, Random Access Skipped Leading (RASL) picture, Broken Link Access (BLA) picture, Instantaneous Decoding Refresh (IDR) picture, Clean Random Access (CRA) picture.

[0119] A Random Access Point (RAP) picture, which may also be referred to as an intra random access point (IRAP) picture in an independent layer contains only intra-coded slices. An IRAP picture belonging to a predicted layer may contain P, B, and I slices, cannot use inter prediction from other pictures in the same predicted layer, and may use inter- layer prediction from its direct reference layers. In the present version of HEVC, an IRAP picture may be a BLA picture, a CRA picture or an IDR picture. The first picture in a bitstream containing a base layer is an IRAP picture at the base layer. Provided the necessary parameter sets are available when they need to be activated, an IRAP picture at an independent layer and all subsequent non-RASL pictures at the independent layer in decoding order can be correctly decoded without performing the decoding process of any pictures that precede the IRAP picture in decoding order. The IRAP picture belonging to a predicted layer and all subsequent non-RASL pictures in decoding order within the same predicted layer can be correctly decoded without performing the decoding process of any pictures of the same predicted layer that precede the IRAP picture in decoding order, when the necessary parameter sets are available when they need to be activated and when the decoding of each direct reference layer of the predicted layer has been initialized . There may be pictures in a bitstream that contain only intra-coded slices that are not IRAP pictures.

[0120] A non-VCL NAL unit may be for example one of the following types: a sequence parameter set, a picture parameter set, a supplemental enhancement information (SEI) NAL unit, an access unit delimiter, an end of sequence NAL unit, an end of bitstream NAL unit, or a filler data NAL unit. Parameter sets may be needed for the reconstruction of decoded pictures, whereas many of the other non-VCL NAL units are not necessary for the reconstruction of decoded sample values.

[0121] Parameters that remain unchanged through a coded video sequence may be included in a sequence parameter set. In addition to the parameters that may be needed by the decoding process, the sequence parameter set may optionally contain video usability information (VUI), which includes parameters that may be important for buffering, picture output timing, rendering, and resource reservation. In HEVC a sequence parameter set RBSP includes parameters that can be referred to by one or more picture parameter set RBSPs or one or more SEI NAL units containing a buffering period SEI message. A picture parameter set contains such parameters that are likely to be unchanged in several coded pictures. A picture parameter set RBSP may include parameters that can be referred to by the coded slice NAL units of one or more coded pictures.

[0122] In HEVC, a video parameter set (VPS) may be defined as a syntax structure containing syntax elements that apply to zero or more entire coded video sequences as determined by the content of a syntax element found in the SPS referred to by a syntax element found in the PPS referred to by a syntax element found in each slice segment header.

[0123] A video parameter set RBSP may include parameters that can be referred to by one or more sequence parameter set RBSPs.

[0124] The relationship and hierarchy between video parameter set (VPS), sequence parameter set (SPS), and picture parameter set (PPS) may be described as follows. VPS resides one level above SPS in the parameter set hierarchy and in the context of scalability and/or 3D video. VPS may include parameters that are common for all slices across all (scalability or view) layers in the entire coded video sequence. SPS includes the parameters that are common for all slices in a particular (scalability or view) layer in the entire coded video sequence, and may be shared by multiple (scalability or view) layers. PPS includes the parameters that are common for all slices in a particular layer representation (the representation of one scalability or view layer in one access unit) and are likely to be shared by all slices in multiple layer representations.

[0125] VPS may provide information about the dependency relationships of the layers in a bitstream, as well as many other information that are applicable to all slices across all (scalability or view) layers in the entire coded video sequence. VPS may be considered to comprise two parts, the base VPS and a VPS extension, where the VPS extension may be optionally present.

[0126] Out-of-band transmission, signaling or storage can additionally or alternatively be used for other purposes than tolerance against transmission errors, such as ease of access or session negotiation. For example, a sample entry of a track in a file conforming to the ISO Base Media File Format may comprise parameter sets, while the coded data in the bitstream is stored elsewhere in the file or in another file. The phrase along the bitstream (e.g. indicating along the bitstream) may be used in claims and described embodiments to refer to out-of-band transmission, signaling, or storage in a manner that the out-of-band data is associated with the bitstream. The phrase decoding along the bitstream or alike may refer to decoding the referred out-of-band data (which may be obtained from out-of-band transmission, signaling, or storage) that is associated with the bitstream.

[0127] A SEI NAL unit may contain one or more SEI messages, which are not required for the decoding of output pictures but may assist in related processes, such as picture output timing, rendering, error detection, error concealment, and resource reservation. Several SEI messages are specified in H.264/AVC and HEVC, and the user data SEI messages enable organizations and companies to specify SEI messages for their own use. H.264/AVC and HEVC contain the syntax and semantics for the specified SEI messages but no process for handling the messages in the recipient is defined. Consequently, encoders are required to follow the H.264/AVC standard or the HEVC standard when they create SEI messages, and decoders conforming to the H.264/AVC standard or the HEVC standard, respectively, are not required to process SEI messages for output order conformance. One of the reasons to include the syntax and semantics of SEI messages in H.264/AVC and HEVC is to allow different system specifications to interpret the supplemental information identically and hence interoperate. It is intended that system specifications can require the use of particular SEI messages both in the encoding end and in the decoding end, and additionally the process for handling particular SEI messages in the recipient can be specified.

[0128] In HEVC, there are two types of SEI NAL units, namely the suffix SEI NAL unit and the prefix SEI NAL unit, having a different nal unit type value from each other. The SEI message(s) contained in a suffix SEI NAL unit are associated with the VCL NAL unit preceding, in decoding order, the suffix SEI NAL unit. The SEI message(s) contained in a prefix SEI NAL unit are associated with the VCL NAL unit following, in decoding order, the prefix SEI NAL unit.

[0129] A coded picture is a coded representation of a picture.

[0130] In HEVC, a coded picture may be defined as a coded representation of a picture containing all coding tree units of the picture. In HEVC, an access unit (AU) may be defined as a set of NAL units that are associated with each other according to a specified classification rule, are consecutive in decoding order, and contain at most one picture with any specific value of nuh layer id. In addition to containing the VCL NAL units of the coded picture, an access unit may also contain non-VCL NAL units. Said specified classification rule may for example associate pictures with the same output time or picture output count value into the same access unit.

[0131] A bitstream may be defined as a sequence of bits, in the form of a NAL unit stream or a byte stream, that forms the representation of coded pictures and associated data forming one or more coded video sequences. A first bitstream may be followed by a second bitstream in the same logical channel, such as in the same file or in the same connection of a communication protocol. An elementary stream (in the context of video coding) may be defined as a sequence of one or more bitstreams. The end of the first bitstream may be indicated by a specific NAL unit, which may be referred to as the end of bitstream (EOB) NAL unit and which is the last NAL unit of the bitstream. In HE VC and its current draft extensions, the EOB NAL unit is required to have nuh layer id equal to 0.

[0132] A coded video sequence may be defined as such a sequence of coded pictures in decoding order that is independently decodable and is followed by another coded video sequence or the end of the bitstream or an end of sequence NAL unit.

[0133] In HEVC, a coded video sequence may additionally or alternatively (to the specification above) be specified to end, when a specific NAL unit, which may be referred to as an end of sequence (EOS) NAL unit, appears in the bitstream and has nuh layer id equal to 0.

[0134] A group of pictures (GOP) and its characteristics may be defined as follows. A GOP can be decoded regardless of whether any previous pictures were decoded. An open GOP is such a group of pictures in which pictures preceding the initial intra picture in output order might not be correctly decodable when the decoding starts from the initial intra picture of the open GOP. In other words, pictures of an open GOP may refer (in inter prediction) to pictures belonging to a previous GOP. An HEVC decoder can recognize an intra picture starting an open GOP, because a specific NAL unit type, CRA NAL unit type, may be used for its coded slices. A closed GOP is such a group of pictures in which all pictures can be correctly decoded when the decoding starts from the initial intra picture of the closed GOP. In other words, no picture in a closed GOP refers to any pictures in previous GOPs. In H.264/AVC and HEVC, a closed GOP may start from an IDR picture. In HEVC a closed GOP may also start from a BLA W RADL or a BLA N LP picture. An open GOP coding structure is potentially more efficient in the compression compared to a closed GOP coding structure, due to a larger flexibility in selection of reference pictures.

[0135] A Decoded Picture Buffer (DPB) may be used in the encoder and/or in the decoder. There are two reasons to buffer decoded pictures, for references in inter prediction and for reordering decoded pictures into output order. As H.264/AVC and HE VC provide a great deal of flexibility for both reference picture marking and output reordering, separate buffers for reference picture buffering and output picture buffering may waste memory resources. Hence, the DPB may include a unified decoded picture buffering process for reference pictures and output reordering. A decoded picture may be removed from the DPB when it is no longer used as a reference and is not needed for output.

[0136] In many coding modes of H.264/AVC and HEVC, the reference picture for inter prediction is indicated with an index to a reference picture list. The index may be coded with variable length coding, which usually causes a smaller index to have a shorter value for the corresponding syntax element. In H.264/AVC and HEVC, two reference picture lists (reference picture list 0 and reference picture list 1) are generated for each bi- predictive (B) slice, and one reference picture list (reference picture list 0) is formed for each inter-coded (P) slice.

[0137] A reference picture list, such as the reference picture list 0 and the reference picture list 1, may be constructed in two steps: First, an initial reference picture list is generated. The initial reference picture list may be generated for example on the basis of frame num, POC, temporal id, or information on the prediction hierarchy such as a GOP structure, or any combination thereof. Second, the initial reference picture list may be reordered by reference picture list reordering (RPLR) syntax, also known as reference picture list modification syntax structure, which may be contained in slice headers. The initial reference picture lists may be modified through the reference picture list modification syntax structure, where pictures in the initial reference picture lists may be identified through an entry index to the list.

[0138] Many coding standards, including H.264/AVC and HEVC, may have decoding process to derive a reference picture index to a reference picture list, which may be used to indicate which one of the multiple reference pictures is used for inter prediction for a particular block. A reference picture index may be coded by an encoder into the bitstream is some inter coding modes or it may be derived (by an encoder and a decoder) for example using neighboring blocks in some other inter coding modes.

[0139] Several candidate motion vectors may be derived for a single prediction unit. For example, motion vector prediction HEVC includes two motion vector prediction schemes, namely the advanced motion vector prediction (AMVP) and the merge mode. In the AMVP or the merge mode, a list of motion vector candidates is derived for a PU. There are two kinds of candidates: spatial candidates and temporal candidates, where temporal candidates may also be referred to as TMVP candidates.

[0140] A candidate list derivation may be performed for example as follows, while it should be understood that other possibilities may exist for candidate list derivation. If the occupancy of the candidate list is not at maximum, the spatial candidates are included in the candidate list first if they are available and not already exist in the candidate list. After that, if occupancy of the candidate list is not yet at maximum, a temporal candidate is included in the candidate list. If the number of candidates still does not reach the maximum allowed number, the combined bi-predictive candidates (for B slices) and a zero motion vector are added in. After the candidate list has been constructed, the encoder decides the final motion information from candidates for example based on a rate-distortion

optimization (RDO) decision and encodes the index of the selected candidate into the bitstream. Likewise, the decoder decodes the index of the selected candidate from the bitstream, constructs the candidate list, and uses the decoded index to select a motion vector predictor from the candidate list.

[0141] A motion vector anchor position may be defined as a position (e.g., horizontal and vertical coordinates) within a picture area relative to which the motion vector is applied. A motion vector anchor position may be regarded as the initial point for the motion vector. While conventionally the motion vector anchor position has been inferred to be the same as the position of the block being predicted, it has been proposed that a horizontal offset and a vertical offset for adapting the motion vector anchor position could be encoded into the bitstream and/or decoded from the bitstream. A horizontal offset and a vertical offset for the anchor position may be given in the slice header, slice parameter set, tile header, tile parameter set, or the like. A motion vector anchor position other than the current block being prediction may be used conditionally based on pre-defined and/or indicated conditions, in which case the term conditional anchor position may be used. [0142] An example encoding method taking advantage of a motion vector anchor position comprises: encoding an input picture into a coded constituent picture;

reconstructing, as a part of said encoding, a decoded constituent picture corresponding to the coded constituent picture; encoding a spatial region into a coded tile, the encoding comprising: determining a horizontal offset and a vertical offset indicative of a region- wise anchor position of the spatial region within the decoded constituent picture; encoding the horizontal offset and the vertical offset; determining that a prediction unit at position of a first horizontal coordinate and a first vertical coordinate of the coded tile is predicted relative to the region- wise anchor position, wherein the first horizontal coordinate and the first vertical coordinate are horizontal and vertical coordinates, respectively, within the spatial region; indicating that the prediction unit is predicted relative to a prediction-unit anchor position that is relative to the region- wise anchor position; deriving a prediction- unit anchor position equal to sum of the first horizontal coordinate and the horizontal offset, and the first vertical coordinate and the vertical offset, respectively; determining a motion vector for the prediction unit; and applying the motion vector relative to the prediction-unit anchor position to obtain a prediction block.

[0143] An example decoding method wherein a motion vector anchor position is used comprises: decoding a coded tile into a decoded tile, the decoding comprising: decoding a horizontal offset and a vertical offset; decoding an indication that a prediction unit at position of a first horizontal coordinate and a first vertical coordinate of the coded tile is predicted relative to a prediction-unit anchor position that is relative to the horizontal and vertical offset; deriving a prediction-unit anchor position equal to sum of the first horizontal coordinate and the horizontal offset, and the first vertical coordinate and the vertical offset, respectively; determining a motion vector for the prediction unit; and applying the motion vector relative to the prediction-unit anchor position to obtain a prediction block.

[0144] Scalable video coding may refer to coding structure where one bitstream can contain multiple representations of the content, for example, at different bitrates, resolutions or frame rates. In these cases the receiver can extract the desired representation depending on its characteristics (e.g. resolution that matches best the display device). Alternatively, a server or a network element can extract the portions of the bitstream to be transmitted to the receiver depending on e.g. the network characteristics or processing capabilities of the receiver. A meaningful decoded representation can be produced by decoding only certain parts of a scalable bit stream. A scalable bitstream typically consists of a“base layer” providing the lowest quality video available and one or more

enhancement layers that enhance the video quality when received and decoded together with the lower layers. In order to improve coding efficiency for the enhancement layers, the coded representation of that layer typically depends on the lower layers. E.g. the motion and mode information of the enhancement layer can be predicted from lower layers. Similarly the pixel data of the lower layers can be used to create prediction for the enhancement layer.

[0145] In some scalable video coding schemes, a video signal can be encoded into a base layer and one or more enhancement layers. An enhancement layer may enhance, for example, the temporal resolution (i.e., the frame rate), the spatial resolution, or simply the quality of the video content represented by another layer or part thereof. Each layer together with all its dependent layers is one representation of the video signal, for example, at a certain spatial resolution, temporal resolution and quality level. In this document, we refer to a scalable layer together with all of its dependent layers as a“scalable layer representation”. The portion of a scalable bitstream corresponding to a scalable layer representation can be extracted and decoded to produce a representation of the original signal at certain fidelity.

[0146] Scalability modes or scalability dimensions may include but are not limited to the following:

Quality scalability: Base layer pictures are coded at a lower quality than

enhancement layer pictures, which may be achieved for example using a greater quantization parameter value (i.e., a greater quantization step size for transform coefficient quantization) in the base layer than in the enhancement layer. Quality scalability may be further categorized into fine-grain or fine-granularity scalability (FGS), medium-grain or medium-granularity scalability (MGS), and/or coarse- grain or coarse-granularity scalability (CGS), as described below.

Spatial scalability: Base layer pictures are coded at a lower resolution (i.e. have fewer samples) than enhancement layer pictures. Spatial scalability and quality scalability, particularly its coarse-grain scalability type, may sometimes be considered the same type of scalability.

View scalability, which may also be referred to as multiview coding. The base layer represents a first view, whereas an enhancement layer represents a second view. A view may be defined as a sequence of pictures representing one camera or viewpoint. It may be considered that in stereoscopic or two-view video, one video sequence or view is presented for the left eye while a parallel view is presented for the right eye.

Depth scalability, which may also be referred to as depth-enhanced coding. A layer or some layers of a bitstream may represent texture view(s), while other layer or layers may represent depth view(s).

[0147] It should be understood that many of the scalability types may be combined and applied together.

[0148] The term layer may be used in context of any type of scalability, including view scalability and depth enhancements. An enhancement layer may refer to any type of an enhancement, such as SNR, spatial, multiview, and/or depth enhancement. A base layer may refer to any type of a base video sequence, such as a base view, a base layer for SNR/spatial scalability, or a texture base view for depth-enhanced video coding.

[0149] A sender, a gateway, a client, or another entity may select the transmitted layers and/or sub-layers of a scalable video bitstream. Terms layer extraction, extraction of layers, or layer down-switching may refer to transmitting fewer layers than what is available in the bitstream received by the sender, the gateway, the client, or another entity. Layer up-switching may refer to transmitting additional layer(s) compared to those transmitted prior to the layer up-switching by the sender, the gateway, the client, or another entity, i.e. restarting the transmission of one or more layers whose transmission was ceased earlier in layer down-switching. Similarly to layer down-switching and/or up-switching, the sender, the gateway, the client, or another entity may perform down- and/or up- switching of temporal sub-layers. The sender, the gateway, the client, or another entity may also perform both layer and sub-layer down-switching and/or up-switching. Layer and sub-layer down-switching and/or up-switching may be carried out in the same access unit or alike (i.e. virtually simultaneously) or may be carried out in different access units or alike (i.e. virtually at distinct times).

[0150] A scalable video encoder for quality scalability (also known as Signal-to-Noise or SNR) and/or spatial scalability may be implemented as follows. For a base layer, a conventional non-scalable video encoder and decoder may be used. The

reconstructed/decoded pictures of the base layer are included in the reference picture buffer and/or reference picture lists for an enhancement layer. In case of spatial scalability, the reconstructed/decoded base-layer picture may be upsampled prior to its insertion into the reference picture lists for an enhancement-layer picture. The base layer decoded pictures may be inserted into a reference picture list(s) for coding/decoding of an enhancement layer picture similarly to the decoded reference pictures of the enhancement layer.

Consequently, the encoder may choose a base-layer reference picture as an inter prediction reference and indicate its use with a reference picture index in the coded bitstream. The decoder decodes from the bitstream, for example from a reference picture index, that a base-layer picture is used as an inter prediction reference for the enhancement layer. When a decoded base-layer picture is used as the prediction reference for an enhancement layer, it is referred to as an inter-layer reference picture.

[0151 ] While the previous paragraph described a scalable video codec with two scalability layers with an enhancement layer and a base layer, it needs to be understood that the description can be generalized to any two layers in a scalability hierarchy with more than two layers. In this case, a second enhancement layer may depend on a first enhancement layer in encoding and/or decoding processes, and the first enhancement layer may therefore be regarded as the base layer for the encoding and/or decoding of the second enhancement layer. Furthermore, it needs to be understood that there may be inter-layer reference pictures from more than one layer in a reference picture buffer or reference picture lists of an enhancement layer, and each of these inter-layer reference pictures may be considered to reside in a base layer or a reference layer for the enhancement layer being encoded and/or decoded. Furthermore, it needs to be understood that other types of inter layer processing than reference- layer picture upsampling may take place instead or additionally. For example, the bit-depth of the samples of the reference- layer picture may be converted to the bit-depth of the enhancement layer and/or the sample values may undergo a mapping from the color space of the reference layer to the color space of the enhancement layer.

[0152] A scalable video coding and/or decoding scheme may use multi- loop coding and/or decoding, which may be characterized as follows. In the encoding/decoding, a base layer picture may be reconstructed/decoded to be used as a motion-compensation reference picture for subsequent pictures, in coding/decoding order, within the same layer or as a reference for inter-layer (or inter-view or inter-component) prediction. The

reconstructed/decoded base layer picture may be stored in the DPB. An enhancement layer picture may likewise be reconstructed/decoded to be used as a motion-compensation reference picture for subsequent pictures, in coding/decoding order, within the same layer or as reference for inter-layer (or inter-view or inter-component) prediction for higher enhancement layers, if any. In addition to reconstructed/decoded sample values, syntax element values of the base/reference layer or variables derived from the syntax element values of the base/reference layer may be used in the inter-layer/inter-component/inter- view prediction.

[0153] Inter-layer prediction may be defined as prediction in a manner that is dependent on data elements (e.g., sample values or motion vectors) of reference pictures from a different layer than the layer of the current picture (being encoded or decoded). Many types of inter-layer prediction exist and may be applied in a scalable video

encoder/decoder. The available types of inter-layer prediction may for example depend on the coding profile according to which the bitstream or a particular layer within the bitstream is being encoded or, when decoding, the coding profile that the bitstream or a particular layer within the bitstream is indicated to conform to. Alternatively or

additionally, the available types of inter-layer prediction may depend on the types of scalability or the type of an scalable codec or video coding standard amendment (e.g. SHVC, MV-HEVC, or 3D-HEVC) being used.

[0154] A direct reference layer may be defined as a layer that may be used for inter layer prediction of another layer for which the layer is the direct reference layer. A direct predicted layer may be defined as a layer for which another layer is a direct reference layer. An indirect reference layer may be defined as a layer that is not a direct reference layer of a second layer but is a direct reference layer of a third layer that is a direct reference layer or indirect reference layer of a direct reference layer of the second layer for which the layer is the indirect reference layer. An indirect predicted layer may be defined as a layer for which another layer is an indirect reference layer. An independent layer may be defined as a layer that does not have direct reference layers. In other words, an independent layer is not predicted using inter-layer prediction. A non-base layer may be defined as any other layer than the base layer, and the base layer may be defined as the lowest layer in the bitstream. An independent non-base layer may be defined as a layer that is both an independent layer and a non-base layer.

[0155] Similarly to MVC, in MV-HEVC, inter-view reference pictures can be included in the reference picture list(s) of the current picture being coded or decoded. SHVC uses multi- loop decoding operation (unlike the SVC extension of H.264/AVC). SHVC may be considered to use a reference index based approach, i.e. an inter-layer reference picture can be included in a one or more reference picture lists of the current picture being coded or decoded (as described above).

[0156] For the enhancement layer coding, the concepts and coding tools of HEVC base layer may be used in SHVC, MV-HEVC, and/or alike. However, the additional inter-layer prediction tools, which employ already coded data (including reconstructed picture samples and motion parameters a.k.a motion information) in reference layer for efficiently coding an enhancement layer, may be integrated to SHVC, MV-HEVC, and/or alike codec.

[0157] A constituent picture may be defined as such part of an enclosing (de)coded picture that corresponds to a representation of an entire input picture. In addition to the constituent picture, the enclosing (de)coded picture may comprise other data, such as another constituent picture.

[0158] Frame packing may be defined to comprise arranging more than one input picture, which may be referred to as (input) constituent frames or constituent pictures, into an output picture. In general, frame packing is not limited to any particular type of constituent frames or the constituent frames need not have a particular relation with each other. In many cases, frame packing is used for arranging constituent frames of a stereoscopic video clip into a single picture sequence. The arranging may include placing the input pictures in spatially non-overlapping areas within the output picture. For example, in a side-by-side arrangement, two input pictures are placed within an output picture horizontally adjacently to each other. The arranging may also include partitioning of one or more input pictures into two or more constituent frame partitions and placing the constituent frame partitions in spatially non-overlapping areas within the output picture. The output picture or a sequence of frame-packed output pictures may be encoded into a bitstream e.g. by a video encoder. The bitstream may be decoded e.g. by a video decoder. The decoder or a post-processing operation after decoding may extract the decoded constituent frames from the decoded picture(s) e.g. for displaying.

[0159] Terms 360-degree video or virtual reality (VR) video may be used

interchangeably. They may generally refer to video content that provides such a large field of view that only a part of the video is displayed at a single point of time in typical displaying arrangements. For example, VR video may be viewed on a head-mounted display (HMD) that may be capable of displaying e.g. about lOO-degree field of view. The spatial subset of the VR video content to be displayed may be selected based on the orientation of the HMD. In another example, a typical flat-panel viewing environment is assumed, wherein e.g. up to 40-degree field-of-view may be displayed. When displaying wide-FOV content (e.g. fisheye) on such a display, it may be preferred to display a spatial subset rather than the entire picture.

[0160] MPEG Omnidirectional Media Format (OMAF) is described in the following by referring to Figure 5. A real-world audio-visual scene (A) is captured by audio sensors as well as a set of cameras or a camera device with multiple lenses and sensors. The acquisition results in a set of digital image/video (Bi) and audio (Ba) signals. The cameras/lenses typically cover all directions around the center point of the camera set or camera device, thus the name of 360-degree video.

[0161 ] Audio can be captured using many different microphone configurations and stored as several different content formats, including channel-based signals, static or dynamic (i.e. moving through the 3D scene) object signals, and scene-based signals (e.g., Higher Order Ambisonics). The channel-based signals typically conform to one of the loudspeaker layouts defined in CICP. In an omnidirectional media application, the loudspeaker layout signals of the rendered immersive audio program are binaraulized for presentation via headphones.

[0162] The images (Bi) of the same time instance are stitched, projected, and mapped onto a packed picture (D).

[0163] For monoscopic 360-degree video, the input images of one time instance are stitched to generate a projected picture representing one view. The breakdown of image stitching, projection, and region-wise packing process for monoscopic content is illustrated with Figure 6a and described as follows. Input images (Bi) are stitched and projected onto a three-dimensional projection structure that may for example be a unit sphere. The projection structure may be considered to comprise one or more surfaces, such as plane(s) or part(s) thereof. A projection structure may be defined as three-dimensional structure consisting of one or more surface(s) on which the captured VR image/video content is projected, and from which a respective projected picture can be formed. The image data on the projection structure is further arranged onto a two-dimensional projected picture (C). The term projection may be defined as a process by which a set of input images are projected onto a projected frame. There may be a pre-defined set of representation formats of the projected picture, including for example an equirectangular projection (ERP) format and a cube map projection (CMP) format. It may be considered that the projected picture covers the entire sphere.

[0164] Optionally, region- wise packing is then applied to map the projected picture onto a packed picture. If the region-wise packing is not applied, the packed picture is identical to the projected picture, and this picture is given as input to image/video encoding. Otherwise, regions of the projected picture are mapped onto a packed picture (D) by indicating the location, shape, and size of each region in the packed picture, and the packed picture (D) is given as input to image/video encoding. The term region-wise packing may be defined as a process by which a projected picture is mapped to a packed picture. The term packed picture may be defined as a picture that results from region- wise packing of a projected picture.

[0165] In the case of stereoscopic 360-degree video, the input images of one time instance are stitched to generate a projected picture representing two views, one for each eye. Both views can be mapped onto the same packed picture, as described below in relation to the Figure 6b, and encoded by a traditional 2D video encoder. Alternatively, each view of the projected picture can be mapped to its own packed picture, in which case the image stitching, projection, and region- wise packing is like described above with the Figure 6a. A sequence of packed pictures of either the left view or the right view can be independently coded or, when using a multiview video encoder, predicted from the other view.

[0166] The breakdown of image stitching, projection, and region- wise packing process for stereoscopic content where both views are mapped onto the same packed picture is illustrated with the Figure 6b and described as follows. Input images (Bi) are stitched and projected onto two three-dimensional projection structures, one for each eye. The image data on each projection structure is further arranged onto a two-dimensional projected picture (C_L for left eye, C_R for right eye), which covers the entire sphere. Frame packing is applied to pack the left view picture and right view picture onto the same projected picture. Optionally, region- wise packing is then applied to the pack projected picture onto a packed picture, and the packed picture (D) is given as input to image/video encoding. If the region- wise packing is not applied, the packed picture is identical to the projected picture, and this picture is given as input to image/video encoding.

[0167] The image stitching, projection, and region- wise packing process can be carried out multiple times for the same source images to create different versions of the same content, e.g. for different orientations of the projection structure. Similarly, the region-wise packing process can be performed multiple times from the same projected picture to create more than one sequence of packed pictures to be encoded.

[0168] 360-degree panoramic content (i.e., images and video) cover horizontally the full 360-degree field-of-view around the capturing position of an imaging device. The vertical field-of-view may vary and can be e.g. 180 degrees. Panoramic image covering 360-degree field-of-view horizontally and 180-degree field-of-view vertically can be represented by a sphere that can be mapped to a bounding cylinder that can be cut vertically to form a 2D picture (this type of projection is known as equirectangular projection). The process of forming a monoscopic equirectangular panorama picture is illustrated in Figure 7. A set of input images, such as fisheye images of a camera array or a camera device with multiple lenses and sensors, is stitched onto a spherical image. The spherical image is further projected onto a cylinder (without the top and bottom faces). The cylinder is unfolded to form a two-dimensional projected frame. In practice one or more of the presented steps may be merged; for example, the input images may be directly projected onto a cylinder without an intermediate projection onto a sphere. The projection structure for

equirectangular panorama may be considered to be a cylinder that comprises a single surface.

[0169] In general, 360-degree content can be mapped onto different types of solid geometrical structures, such as polyhedron (i.e. a three-dimensional solid object containing flat polygonal faces, straight edges and sharp comers or vertices, e.g., a cube or a pyramid), cylinder (by projecting a spherical image onto the cylinder, as described above with the equirectangular projection), cylinder (directly without projecting onto a sphere first), cone, etc. and then unwrapped to a two-dimensional image plane.

[0170] In some cases panoramic content with 360-degree horizontal field-of-view but with less than 180-degree vertical field-of-view may be considered special cases of panoramic projection, where the polar areas of the sphere have not been mapped onto the two-dimensional image plane. In some cases a panoramic image may have less than 360- degree horizontal field-of-view and up to 180-degree vertical field-of-view, while otherwise has the characteristics of panoramic projection format.

[0171] Region- wise packing information may be encoded as metadata in or along the bitstream. For example, the packing information may comprise a region-wise mapping from a pre-defined or indicated source format to the packed frame format, e.g. from a projected picture to a packed picture, as described earlier.

[0172] Rectangular region- wise packing metadata is described next: For each region, the metadata defines a rectangle in a projected picture, the respective rectangle in the packed picture, and an optional transformation of rotation by 90, 180, or 270 degrees and/or horizontal and/or vertical mirroring. Rectangles may for example be indicated by the locations of the top-left comer and the bottom-right comer. The mapping may comprise resampling. As the sizes of the respective rectangles can differ in the projected and packed pictures, the mechanism infers region-wise resampling.

[0173] Among others, region-wise packing provides signalling for the following usage scenarios:

- Additional compression for viewport-independent projections is achieved by

densifying sampling of different regions to achieve more uniformity across the sphere. For example, the top and bottom parts of ERP are oversampled, and region- wise packing can be applied to down-sample them horizontally.

Arranging the faces of plane-based projection formats, such as cube map projeciton, in an adaptive manner.

Generating viewport-dependent bitstreams that use viewport-independent projection formats. For example, regions of ERP or faces of CMP can have different sampling densities and the underlying projection structure can have different orientations.

Indicating regions of the packed pictures represented by an extractor track. This is needed when an extractor track collects tiles from bitstreams of different resolutions.

[0174] OMAF allows the omission of image stitching, projection, and region- wise packing and encode the image/video data in their captured format. In this case, images D are considered the same as images Bi and a limited number of fisheye images per time instance are encoded.

[0175] For audio, the stitching process is not needed, since the captured signals are inherently immersive and omnidirectional.

[0176] The stitched images (D) are encoded as coded images (Ei) or a coded video bitstream (Ev). The captured audio (Ba) is encoded as an audio bitstream (Ea). The coded images, video, and/or audio are then composed into a media file for file playback (F) or a sequence of an initialization segment and media segments for streaming (Fs), according to a particular media container file format. In this specification, the media container file format is the ISO base media file format. The file encapsulator also includes metadata into the file or the segments, such as projection and region- wise packing information assisting in rendering the decoded packed pictures.

[0177] The metadata in the file may include:

- the projection format of the projected picture,

- fisheye video parameters,

- the area of the spherical surface covered by the packed picture,

- the orientation of the projection structure corresponding to the projected picture relative to the global coordinate axes,

- region-wise packing information, and

- region- wise quality ranking (optional).

[0178] The segments Fs are delivered using a delivery mechanism to a player.

[0179] The file that the file encapsulator outputs (F) is identical to the file that the file decapsulator inputs (F'). A file decapsulator processes the file (F') or the received segments (F’s) and extracts the coded bitstreams (E’a, EV, and/or E’i) and parses the metadata. The audio, video, and/or images are then decoded into decoded signals (B'a for audio, and D' for images/video). The decoded packed pictures (D') are projected onto the screen of a head-mounted display or any other display device based on the current viewing orientation or viewport and the projection, spherical coverage, projection structure orientation, and region-wise packing metadata parsed from the file. Likewise, decoded audio (B'a) is rendered, e.g. through headphones, according to the current viewing orientation. The current viewing orientation is determined by the head tracking and possibly also eye tracking functionality. Besides being used by the renderer to render the appropriate part of decoded video and audio signals, the current viewing orientation may also be used the video and audio decoders for decoding optimization.

[0180] The process described above is applicable to both live and on-demand use cases.

[0181 ] The human eyes are not capable of viewing the whole 360 degrees space, but are limited to a maximum horizontal and vertical FoVs (HHFoV, HVFoV). Also, a HMD device has technical limitations that allow only viewing a subset of the whole 360 degrees space in horizontal and vertical directions (DHFoY, DYFoY)). [0182] At any point of time, a video rendered by an application on a HMD renders a portion of the 360 degrees video. This portion is defined here as viewport. A viewport is a window on the 360 world represented in the omnidirectional video displayed via a rendering display. A viewport may alternatively be defined as a region of omnidirectional image or video suitable for display and viewing by the user.

[0183] A viewport size may correspond to the field of view of the HMD or may have a smaller size, depending on the application. For the sake of clarity, we define as primary viewport the part of the 360 degrees space viewed by a user at any given point of time.

[0184] The coordinate system of OMAF consists of a unit sphere and three coordinate axes, namely the X (back-to-front) axis, the Y (lateral, side-to-side) axis, and the Z (vertical, up) axis, where the three axes cross at the centre of the sphere. The location of a point on the sphere is identified by a pair of sphere coordinates azimuth (f) and elevation (Q). Figure 8 specifies the relation of the sphere coordinates azimuth (f) and elevation (Q) to the X, Y, and Z coordinate axes.

[0185] A viewing orientation may be defined as triplet of azimuth, elevation, and tilt angle characterizing the orientation that a user is consuming the audio-visual content; in case of image or video, characterizing the orientation of the viewport.

[0186] Figure 9 illustrates the conversions from a spherical picture to a packed picture that could be used in content authoring and the corresponding conversions from a packed picture to a spherical picture to be rendered that could be used in an OMAF player. The example in this clause is described for a packed picture that appears in a projected omnidirectional video track. Similar description could be derived for an image item.

[0187] The content authoring could include the following ordered steps:

The source images provided as input are stitched to generate a sphere picture on the unit sphere per the global coordinate axes as indicated in a).

The unit sphere is then rotated relative to the global coordinate axes, as indicated in b). The amount of rotation to convert from the local coordinate axes to the global coordinate axes is specified by the rotation angles indicated in the RotationBox.

The local coordinate axes of the unit sphere are the axes of the coordinate system that has been rotated. The absence of RotationBox indicates that the local coordinate axes are the same as the global coordinate axes.

As illustrated in c), the spherical picture on the rotated unit sphere is then converted to a two-dimensional projected picture, for example using the equirectangular projection. When spatial packing of stereoscopic content is applied, two spherical pictures for the two views are converted to two constituent pictures, after which frame packing is applied to pack the two constituent pictures to one projected picture.

Rectangular region- wise packing could be applied to obtain a packed picture from the projected picture. One example of packing is depicted in c) and d). The dashed rectangles in c) indicate the projected regions on a projected picture, and the respective areas in d) indicate the corresponding packed regions. In this example, projected regions 1 and 3 are horizontally downsampled, while projected region 2 is kept at its original resolution.

[0188] CoveragelnformationBox could be used to indicate content coverage, i.e., which part of the sphere is covered by the packed picture.

[0189] In order to map sample locations of a packed picture, such as that in d), to a unit sphere used in rendering illustrated in a), the OMAF player could perform the following ordered steps:

- A packed picture, such as that in d), is obtained as a result of decoding a picture from a video track or an image item.

If needed, chroma sample arrays of the packed picture are upsampled to the resolution of the luma sample array of the packed picture, and colour space conversion could also be performed.

If region- wise packing is indicated, the sample locations of the packed picture are converted to sample locations of the respective projected picture, such as that in c). Otherwise, the projected picture is identical to the packed picture.

If spatial frame packing of the projected picture is indicated, the sample locations of the projected picture are converted to sample locations of the respective constituent picture of the projected picture. Otherwise, the constituent picture of the projected picture is identical to the projected picture.

The sample locations of a constituent picture the projected picture are converted to sphere coordinates that are relative to local coordinate axes, as specified for the omnidirectional projection format being used. The resulting sample locations correspond to a sphere picture depicted in b). If rotation is indicated, the sphere coordinates relative to the local coordinate axes are converted to sphere coordinates relative to the global coordinate axes.

Otherwise, the global coordinate axes are identical to the local coordinate axes.

[0190] Extractors specified in ISO/IEC 14496-15 for H.264/AVC and HEVC enable compact formation of tracks that extract NAL unit data by reference. An extractor is a NAL-unit-like structure. A NAL-unit-like structure may be specified to comprise a NAL unit header and NAL unit payload like any NAL units, but start code emulation prevention (that is required for a NAL unit) might not be followed in a NAL-unit-like structure. Lor HEVC, an extractor contains one or more constructors. A sample constructor extracts, by reference, NAL unit data from a sample of another track. An in-line constructor includes NAL unit data. When an extractor is processed by a file reader that requires it, the extractor is logically replaced by the bytes resulting when resolving the contained constructors in their appearance order. Nested extraction may be disallowed, e.g. the bytes referred to by a sample constructor shall not contain extractors; an extractor shall not reference, directly or indirectly, another extractor. An extractor may contain one or more constructors for extracting data from the current track or from another track that is linked to the track in which the extractor resides by means of a track reference of type 'seal'.

[0191] The bytes of a resolved extractor are one of the following:

a) One entire NAL unit; note that when an Aggregator is referenced, both the included and referenced bytes are copied

b) More than one entire NAL unit

[0192] In both cases the bytes of the resolved extractor start with a valid length field and a NAL unit header.

[0193] The bytes of a sample constructor are copied only from the single identified sample in the track referenced through the indicated 'seal' track reference. The alignment is on decoding time, i.e. using the time-to-sample table only, followed by a counted offset in sample number. Extractors are a media-level concept and hence apply to the destination track before any edit list is considered. (However, one would normally expect that the edit lists in the two tracks would be identical).

[0194] The following syntax may be used:

class aligned(8) Extractor () {

NALUnitHeader ( ) ;

do { unsigned int(8) constructor_type;

if ( constructor_type == 0 )

SampleConstructor ( ) ;

else if ( constructor_type == 2 )

InlineConstructor ( ) ;

} while ( ! EndOfNALUnit ( ) )

}

[0195] The semantics may be defined as follows:

NALUnitHeader(): The first two bytes of HEVC NAL units. A particular nal unit type value indicates an extractor, e.g. nal unit type equal to 49.

constructor type specifies the constructor being used.

EndOfNALUnit() is a function that returns 0 (false) when more data follows in this extractor; otherwise it returns 1 (true).

[0196] The sample constructor (SampleConstructor) may have the following syntax: class aligned(8) SampleConstructor () {

unsigned int(8) track ref index;

signed int(8) sample offset;

unsigned int ( (lengthSizeMinusOne+1) *8)

data_offset;

unsigned int ( (lengthSizeMinusOne+1) *8)

data_length;

}

[0197] track ref index identifies the source track from which data is extracted track ref index is the index of the track reference of type 'seal'. The first track reference has the index value 1; the value 0 is reserved.

[0198] The sample in that track from which data is extracted is temporally aligned or nearest preceding in the media decoding timeline, i.e. using the time-to-sample table only, adjusted by an offset specified by sample offset with the sample containing the extractor sample offset gives the relative index of the sample in the linked track that shall be used as the source of information. Sample 0 (zero) is the sample with the same, or the closest preceding, decoding time compared to the decoding time of the sample containing the extractor; sample 1 (one) is the next sample, sample -1 (minus 1) is the previous sample, and so on.

[0199] data offset: The offset of the first byte within the reference sample to copy. If the extraction starts with the first byte of data in that sample, the offset takes the value 0. [0200] data length: The number of bytes to copy.

[0201 ] The syntax of the in-line constructor may be specified as follows:

class aligned(8) InlineConstructor () {

unsigned int(8) length;

unsigned int(8) inline data [length] ;

}

[0202] length: the number of bytes that belong to the InlineConstructor following this field.

[0203] inline data: the data bytes to be returned when resolving the in-line constructor.

[0204] A tile track specified in ISO/IEC 14496-15 enables storage of one or more temporal motion-constrained tile set as a track. When a tile track contains tiles of an HE VC base layer, the sample entry type 'hvtl' is used. When a tile track contains tiles of a non base layer, the sample entry type 'lhtl ' is used. A sample of a tile track consists of one or more complete tiles in one or more complete slice segments. A tile track is independent from any other tile track that includes VCL NAL units of the same layer as this tile track.

A tile track has a 'tbas' track reference to a tile base track. The tile base track does not include VCL NAL units. A tile base track indicates the tile ordering using a 'sabf track reference to the tile tracks. An HEVC coded picture corresponding to a sample in the tile base track can be reconstructed by collecting the coded data from the time-aligned samples of the tracks indicated by the 'sabf track reference in the order of the track references. It can therefore be understood that a tile base track includes coded video data of the referenced tile tracks by reference.

[0205] A sub-picture may be defined as a picture that represents a spatial subset of the original video content, which has been split into spatial subsets before video encoding at the content production side. A sub-picture bitstream may be defined as a bitstream that represents a spatial subset of the original video content, which has been split into spatial subsets before video encoding at the content production side. A sub-picture track may be defined as a track that is with spatial relationships to other track(s) originating from the same original video content and that represents a sub-picture bitstream. A sub-picture track conforms to the a conventional track format, such as 'hvcl' or 'hevl' defined for HEVC in ISO/IEC 14496-15. In an approach to generate sub-picture tracks, a source picture sequence is split into sub-picture sequences before encoding. A sub-picture sequence is then encoded independently from other sub-picture sequences as a single-layer bitstream, such as HEVC Main profile bitstream. The coded single-layer bitstream is encapsulated into a sub-picture track. The bitstream for a sub-picture track may be encoded with motion- constrained pictures, as defined later. In another approach to generate sub-picture tracks, a source picture sequence is encoded with motion-constrained tile sets into a bitstream, an MCTS sequence is extracted from the bitstream,, and a sub-picture track is generated by converting the MCTS sequence into a conforming bitstream e.g. through slice header modifications and encapsulating the generated bitstream into a track. Sub-picture tracks generated this way comprise motion-constrained pictures.

[0206] A collector track may be defined as a track that extracts implicitly or explicitly MCTSs or sub-pictures from other tracks. When resolved by a file reader, a collector track may represent a bitstream conforming to a video codec specification, such a HEVC or H.266/VVC. A collector track may for example extract MCTSs or sub-pictures to form a coded picture sequence where MCTSs or sub-pictures are arranged to a grid. For example, when a collector track extracts two MCTSs or sub-pictures, they may be arranged into a 2x1 grid of MCTSs or sub-pictures. A tile base track may be regarded as a collector track, and an extractor track that extracts MCTSs or sub-pictures from other tracks may be regarded as a collector track. A collector track may also be referred to as a collection track. A track that is a source for extracting to a collector track may be referred to as a collection item track.

[0207] To avoid creating an excessive number of extractor tracks (e.g., to avoid creating an extractor track for each combination of high-resolution and low-resolution tiles), tracks that are alternatives for extraction may be grouped with a mechanism described in the following. Likewise, to enable the use of the same tile base track for collocated tile tracks representing different bitrate versions of the same content, the following mechanism may be used.

[0208] A file writer indicates in a file that a track group, e.g. referred to as 'alte' track group, contains tracks that are alternatives to be used as a source for extraction.

[0209] The identifier for the 'alte' group may be taken from the same numbering space as the identifier for tracks. In other words, the identifier for the 'alte' group may be required to differ from all the track identifier values. Consequently, the 'alte' track group identifier may be used in places where track identifier is conventionally used. Specifically, the 'alte' track group identifier may be used as a track reference indicating the source for extraction. [0210] Members of the track group formed by this box are alternatives to be used as a source for extraction. Members of the track group with track group type equal to 'alte' are alternatives to be used as a source for 'seal' or 'sabf track reference. A

TrackReferenceTypeBox of reference type equal to track_ref_4cc may list the

track group id value(s) of an 'alte' track group(s) of containing the same

alte_track_ref_4cc value in addition to or instead of track ID values. For example, an extractor track may, through a 'seal' track reference, point to an 'alte' track group in addition to or instead of individual tracks. Any single track of the 'alte' track group is a suitable source for extraction. The source track for extraction may be changed at a position where the track switched to has a sync sample or a SAP sample of type 1 or 2.

[021 1 ] A uniform resource identifier (URI) may be defined as a string of characters used to identify a name of a resource. Such identification enables interaction with representations of the resource over a network, using specific protocols. A URI is defined through a scheme specifying a concrete syntax and associated protocol for the URI. The uniform resource locator (URL) and the uniform resource name (URN) are forms of URI. A URL may be defined as a URI that identifies a web resource and specifies the means of acting upon or obtaining the representation of the resource, specifying both its primary access mechanism and network location. A URN may be defined as a URI that identifies a resource by name in a particular namespace. A URN may be used for identifying a resource without implying its location or how to access it.

[0212] In many video communication or transmission systems, transport mechanisms, and multimedia container file formats, there are mechanisms to transmit or store a scalability layer separately from another scalability layer of the same bitstream, e.g. to transmit or store the base layer separately from the enhancement layer(s). It may be considered that layers are stored in or transmitted through separate logical channels. For example in ISOBMFF, the base layer can be stored as a track and each enhancement layer can be stored in another track, which may be linked to the base-layer track using so-called track references.

[0213] Many video communication or transmission systems, transport mechanisms, and multimedia container file formats provide means to associate coded data of separate logical channels, such as of different tracks or sessions, with each other. For example, there are mechanisms to associate coded data of the same access unit together. For example, decoding or output times may be provided in the container file format or transport mechanism, and coded data with the same decoding or output time may be considered to form an access unit.

[0214] Recently, Hypertext Transfer Protocol (HTTP) has been widely used for the delivery of real-time multimedia content over the Internet, such as in video streaming applications. Unlike the use of the Real-time Transport Protocol (RTP) over the User Datagram Protocol (UDP), HTTP is easy to configure and is typically granted traversal of firewalls and network address translators (NAT), which makes it attractive for multimedia streaming applications.

[0215] Several commercial solutions for adaptive streaming over HTTP, such as

Microsoft® Smooth Streaming, Apple® Adaptive HTTP Live Streaming and Adobe® Dynamic Streaming, have been launched as well as standardization projects have been carried out. Adaptive HTTP streaming (AHS) was first standardized in Release 9 of 3rd Generation Partnership Project (3GPP) packet-switched streaming (PSS) service (3GPP TS 26.234 Release 9:“Transparent end-to-end packet-switched streaming service (PSS); protocols and codecs”). MPEG took 3GPP AHS Release 9 as a starting point for the MPEG DASH standard (ISO/IEC 23009-1 :“Dynamic adaptive streaming over HTTP (DASH)-Part 1 : Media presentation description and segment formats,” International Standard, 2^nd Edition, , 2014). 3GPP continued to work on adaptive HTTP streaming in communication with MPEG and published 3GP-DASH (Dynamic Adaptive Streaming over HTTP; 3GPP TS 26.247:“Transparent end-to-end packet-switched streaming Service (PSS); Progressive download and dynamic adaptive Streaming over HTTP (3GP-DASH)”. MPEG DASH and 3GP-DASH are technically close to each other and may therefore be collectively referred to as DASH. Streaming systems similar to MPEG-DASH include for example HTTP Live Streaming (a.k.a. HLS), specified in the IETF RFC 8216. For a detailed description of said adaptive streaming system, all providing examples of a video streaming system, wherein the embodiments may be implemented, a reference is made to the above standard documents. The aspects of the invention are not limited to the above standard documents but rather the description is given for one possible basis on top of which the invention may be partly or fully realized.

[0216] In DASH, the multimedia content may be stored on an HTTP server and may be delivered using HTTP. The content may be stored on the server in two parts: Media Presentation Description (MPD), which describes a manifest of the available content, its various alternatives, their URL addresses, and other characteristics; and segments, which contain the actual multimedia bitstreams in the form of chunks, in a single or multiple files. The MDP provides the necessary information for clients to establish a dynamic adaptive streaming over HTTP. The MPD contains information describing media presentation, such as an HTTP-uniform resource locator (URL) of each Segment to make GET Segment request. To play the content, the DASH client may obtain the MPD e.g. by using HTTP, email, thumb drive, broadcast, or other transport methods. By parsing the MPD, the DASH client may become aware of the program timing, media-content availability, media types, resolutions, minimum and maximum bandwidths, and the existence of various encoded alternatives of multimedia components, accessibility features and required digital rights management (DRM), media-component locations on the network, and other content characteristics. Using this information, the DASH client may select the appropriate encoded alternative and start streaming the content by fetching the segments using e.g. HTTP GET requests. After appropriate buffering to allow for network throughput variations, the client may continue fetching the subsequent segments and also monitor the network bandwidth fluctuations. The client may decide how to adapt to the available bandwidth by fetching segments of different alternatives (with lower or higher bitrates) to maintain an adequate buffer.

[0217] In the context of DASH, the following definitions may be used: A media content component or a media component may be defined as one continuous component of the media content with an assigned media component type that can be encoded individually into a media stream. Media content may be defined as one media content period or a contiguous sequence of media content periods. Media content component type may be defined as a single type of media content such as audio, video, or text. A media stream may be defined as an encoded version of a media content component.

[0218] In DASH, a hierarchical data model is used to structure media presentation as follows. A media presentation consists of a sequence of one or more Periods, each Period contains one or more Groups, each Group contains one or more Adaptation Sets, each Adaptation Sets contains one or more Representations, each Representation consists of one or more Segments. A Group may be defined as a collection of Adaptation Sets that are not expected to be presented simultaneously. An Adaptation Set may be defined as a set of interchangeable encoded versions of one or several media content components. A

Representation is one of the alternative choices of the media content or a subset thereof typically differing by the encoding choice, e.g. by bitrate, resolution, language, codec, etc. The Segment contains certain duration of media data, and metadata to decode and present the included media content. A Segment is identified by a URI and can typically be requested by a HTTP GET request. A Segment may be defined as a unit of data associated with an HTTP-URL and optionally a byte range that are specified by an MPD.

[0219] An Initialization Segment may be defined as a Segment containing metadata that is necessary to present the media streams encapsulated in Media Segments. In ISOBMFF based segment formats, an Initialization Segment may comprise the Movie Box ('moov') which might not include metadata for any samples, i.e. any metadata for samples is provided in 'moof boxes.

[0220] A Media Segment contains certain duration of media data for playback at a normal speed, such duration is referred as Media Segment duration or Segment duration. The content producer or service provider may select the Segment duration according to the desired characteristics of the service. For example, a relatively short Segment duration may be used in a live service to achieve a short end-to-end latency. The reason is that Segment duration is typically a lower bound on the end-to-end latency perceived by a DASH client since a Segment is a discrete unit of generating media data for DASH. Content generation is typically done such a manner that a whole Segment of media data is made available for a server. Furthermore, many client implementations use a Segment as the unit for GET requests. Thus, in typical arrangements for live services a Segment can be requested by a DASH client only when the whole duration of Media Segment is available as well as encoded and encapsulated into a Segment. For on-demand service, different strategies of selecting Segment duration may be used.

[0221 ] A Segment may be further partitioned into Subsegments e.g. to enable downloading segments in multiple parts. Subsegments may be required to contain complete access units. Subsegments may be indexed by Segment Index box, which contains information to map presentation time range and byte range for each Subsegment. The Segment Index box may also describe subsegments and stream access points in the segment by signaling their durations and byte offsets. A DASH client may use the information obtained from Segment Index box(es) to make a HTTP GET request for a specific Subsegment using byte range HTTP request. If relatively long Segment duration is used, then Subsegments may be used to keep the size of HTTP responses reasonable and flexible for bitrate adaptation. The indexing information of a segment may be put in the single box at the beginning of that segment, or spread among many indexing boxes in the segment. Different methods of spreading are possible, such as hierarchical, daisy chain, and hybrid. This technique may avoid adding a large box at the beginning of the segment and therefore may prevent a possible initial download delay.

[0222] As explained above, DASH and other similar streaming systems provide a protocol and/or formats for multimedia streaming applications. A recent trend in streaming in order to reduce the streaming bitrate of VR video is the following: a subset of 360- degree video content covering the primary viewport (i.e., the current view orientation) is transmitted at the best quality/resolution, while the remaining of 360-degree video is transmitted at a lower quality/resolution.

[0223] Viewport-adaptive streaming may be realized through a tile-based encoding and streaming approaches. In these approaches, 360-degree content is encoded and made available in a manner that enables selective streaming of viewports from different encodings. Some of the tile-based approaches are described next.

[0224] An approach of tile-based encoding and streaming, which may be referred to as tile rectangle based encoding and streaming or sub-picture based encoding and streaming, may be used with any video codec, even if tiles similar to HE VC were not available in the codec or even if motion-constrained tile sets or alike were not implemented in an encoder. In tile rectangle based encoding, the source content is split into tile rectangle sequences (a.k.a. sub-picture sequences) before encoding. Each tile rectangle sequence covers a subset of the spatial area of the source content, such as full panorama content, which may e.g. be of equirectangular projection format. Each tile rectangle sequence is then encoded independently from each other as a single-layer bitstream. Several bitstreams may be encoded from the same tile rectangle sequence, e.g. for different bitrates. Each tile rectangle bitstream may be encapsulated in a file as its own track (or alike) and made available for streaming. At the receiver side the tracks to be streamed may be selected based on the viewing orientation. The client may receive tracks covering the entire omnidirectional content. Better quality or higher resolution tracks may be received for the current viewport compared to the quality or resolution covering the remaining, currently non- visible viewports. In an example, each track may be decoded with a separate decoder instance.

[0225] In an example of tile rectangle based encoding and streaming, each cube face may be separately encoded and encapsulated in its own track (and Representation). More than one encoded bitstream for each cube face may be provided, e.g. each with different spatial resolution. Players can choose tracks (or Representations) to be decoded and played based on the current viewing orientation. High-resolution tracks (or Representations) may be selected for the cube faces used for rendering for the present viewing orientation, while the remaining cube faces may be obtained from their low-resolution tracks (or

Representations) .

[0226] In an approach of tile-based encoding and streaming, encoding is performed in a manner that the resulting bitstream comprises motion-constrained tile sets. Several bitstreams of the same source content are encoded using motion-constrained tile sets.

[0227] In an approach, one or more motion-constrained tile set (MCTS) sequences are extracted from a bitstream, and each extracted motion-constrained tile set sequence is stored as a tile or sub-picture track (e.g. an HEVC tile track or an HEVC sub-picture track) in a file. A tile base track (e.g. an HEVC tile base track or a HEVC track comprising extractors to extract data from the sub-picture tracks) may be generated and stored in a file. The tile base track represents the bitstream by implicitly collecting motion-constrained tile sets from the tile set tracks or by explicitly extracting (e.g. by HEVC extractors) motion- constrained tile sets from the tile set tracks. Tile set tracks and the tile base track of each bitstream may be encapsulated in an own file, and the same track identifiers may be used in all files. At the receiver side the tile set tracks to be streamed may be selected based on the viewing orientation. The client may receive tile set tracks covering the entire omnidirectional content. Beter quality or higher resolution tile set tracks may be received for the current viewport compared to the quality or resolution covering the remaining, currently non- visible viewports.

[0228] In an example, equirectangular panorama content is encoded using motion- constrained tile sets. More than one encoded bitstream may be provided, e.g. with different spatial resolution and/or picture quality. Each motion-constrained tile set is made available in its own track (and Representation). Players can choose tracks (or Representations) to be decoded and played based on the current viewing orientation. High-resolution or high- quality tracks (or Representations) may be selected for tile sets covering the present primary viewport, while the remaining area of the 360-degree content may be obtained from low-resolution or low-quality tracks (or Representations).

[0229] In an approach, each received tile set track is decoded with a separate decoder or decoder instance. [0230] In another approach, a tile base track is utilized in decoding as follows. If all the received tile tracks originate from bitstreams of the same resolution (or more generally if the tile base tracks of the bitstreams are identical or equivalent, or if the initialization segments or other initialization data, such as parameter sets, of all the bitstreams is the same), a tile base track may be received and used to construct a bitstream. The constructed bitstream may be decoded with a single decoder.

[0231] It needs to be understood that tile-based encoding and streaming may be realized by splitting a source picture in tile rectangle sequences that are partly overlapping.

Alternatively or additionally, bitstreams with motion-constrained tile sets may be generated from the same source content with different tile grids or tile set grids. We could then imagine the 360 degrees space divided into a discrete set of viewports, each separate by a given distance (e.g., expressed in degrees), so that the omnidirectional space can be imagined as a map of overlapping viewports, and the primary viewport is switched discretely as the user changes his/her orientation while watching content with a HMD. When the overlapping between viewports is reduced to zero, the viewports could be imagined as adjacent non-overlapping tiles within the 360 degrees space.

[0232] As explained above, in viewport-adaptive streaming the primary viewport (i.e., the current viewing orientation) is transmitted at the best quality/resolution, while the remaining of 360-degree video is transmitted at a lower quality/resolution. When the viewing orientation changes, e.g. when the user turns his/her head when viewing the content with a head-mounted display, another version of the content needs to be streamed, matching the new viewing orientation. In general, the new version can be requested starting from a stream access point (SAP), which are typically aligned with (Sub)segments. In single-layer video bitstreams, SAPs are intra-coded and hence costly in terms of rate- distortion performance. Conventionally, relatively long SAP intervals and consequently relatively long (Sub)segment durations in the order of seconds are hence used. Thus, the delay (here referred to as the viewport quality update delay) in upgrading the quality after a viewing orientation change (e.g. a head turn) is conventionally in the order of seconds and is therefore clearly noticeable and annoying.

[0233] There are several alternatives to deliver the viewport-dependent omnidirectional video. It can be delivered, for example, as equal-resolution HEVC bitstreams with motion- constrained tile sets (MCTSs). Thus, several HEVC bitstreams of the same omnidirectional source content are encoded at the same resolution but different qualities and bitrates using motion-constrained tile sets. The MCTS grid in all bitstreams is identical. In order to enable the client the use of the same tile base track for reconstructing a bitstream from MCTSs received from different original bitstreams, each bitstream is encapsulated in its own file, and the same track identifier is used for each tile track of the same tile grid position in all these files. HEVC tile tracks are formed from each motion-constrained tile set sequence, and a tile base track is additionally formed. The client parses tile base track to implicitly reconstruct a bitstream from the tile tracks. The reconstructed bitstream can be decoded with a conforming HEVC decoder.

[0234] Clients can choose which version of each MCTS is received. The same tile base track suffices for combining MCTSs from different bitstreams, since the same track identifiers are used in the respective tile tracks.

[0235] Figure 10 shows an example how tile tracks of the same resolution can be used for tile-based omnidirectional video streaming. A 4x2 tile grid has been used in forming of the motion-constrained tile sets. Two HEVC bitstreams originating from the same source content are encoded at different picture qualities and bitrates. Each bitstream is

encapsulated in its own file wherein each motion-constrained tile set sequence is included in one tile track and a tile base track is also included. The client chooses the quality at which each tile track is received based on the viewing orientation. In this example the client receives tile tracks 1, 2, 5, and 6 at a particular quality and tile tracks 3, 4, 7, and 8 at another quality. The tile base track is used to order the received tile track data into a bitstream that can be decoded with an HEVC decoder.

[0236] Another method for enabling viewport-dependent delivery of omnidirectional video is called constrained inter-layer prediction (CILP). CILP is illustrated by referring to Figure 11, which shows how the input picture sequence is encoded into two or more bitstreams, each representing the entire input picture sequence, i.e., the same input pictures are encoded in the bitstreams or a subset of the same input pictures, potentially with a reduced picture rate, are encoded in the bitstreams.

[0237] Certain input pictures are chosen to be encoded into two coded pictures in the same bitstream, the first referred to as a shared coded picture, and the two coded pictures may be referred to as a shared coded picture pair. A shared coded picture is either intra coded or uses only other shared coded pictures (or the respective reconstructed pictures) as prediction references. A shared coded picture in a first bitstream (of the encoded two or more bitstreams) is identical to the respective shared coded picture in a second bitstream (of the encoded two or more bitstreams), wherein "identical" may be defined to be identical coded representation, potentially excluding certain high-level syntax structures, such as SEI messages, and/or identical reconstructed picture. Any picture subsequent to a particular shared coded picture in decoding order is not predicted from any picture that precedes the particular shared coded picture and is not a shared coded picture.

[0238] A shared coded picture may be indicated to be a non-output picture. As a response to decoding a non-output picture indication, the decoder does not output the reconstructed shared coded picture. The encoding method facilitates decoding a first bitstream up to a selected shared coded picture, exclusive, and decoding a second bitstream starting from the respective shared coded picture. No intra-coded picture is required to start the decoding of the second bitstream, and consequently compression efficiency is improved compared to a conventional approach. CILP enables the use of HEVC Main profile encoder and decoder.

[0239] However, CILP suffers from being limited to a single resolution. For example, CILP cannot offer higher resolution than 4K on the viewport with 4K decoding capacity. Today’s display panels in HMD are often Quad HD or wide Quad HD, corresponding approximately to 6K panorama resolution. Effective 6K resolution is therefore desired for the content on the viewport.

[0240] By the end of 2019, a typical real-time video decoding capacity in devices might increase to 8K (e.g. 8192x4096). However, the display panel resolutions in HMD might also increase to 8K while field-of-view increase is proportionally smaller (e.g. 140 degrees), thus corresponding approximately to 10K panorama resolution. Effective resolution or effective panorama resolution may be defined as the resolution of an ERP picture from which the viewport originates, or if the content is coded using a projection format other than ERP, the resolution of an ERP picture in which the spherical sampling density corresponds to the region from which the viewport originates from. In multi resolution viewport-dependent streaming approaches, the effective resolution is typically greater than the resolution of the streamed or decoded content.

[0241 ] On the other hand, in the multi-resolution viewport-adaptive streaming as described by OMAF, viewport switching requires intra-coding, which is costly in bitrate and causes a spike in streaming bitrate. Bitrate variation typically causes a need for keeping the client buffer occupancy to a sufficient level to avoid interruptions in the playback and hence involve greater end-to-end and motion-to-high-quality delays.

Moreover, each distinct viewing orientation requires an extractor track being available.

[0242] Hence, an enhanced multi-resolution viewport adaptation continues to be needed.

[0243] OMAF version 1 facilitates three degrees of freedom (3DoF) content consumption, meaning that a viewport can be selected with any azimuth and elevation range and tilt angle that are covered by the omnidirectional content but the content is not adapted to any translational changes of the viewing position. The viewport-dependent streaming scenarios above have also been designed for 3DoF although could potentially be adapted to a different number of degrees of freedom.

[0244] Virtual reality is a rapidly developing area of technology in which image or video content, sometimes accompanied by audio, is provided to a user device such as a user headset (a.k.a. head-mounted display). As is known, the user device may be provided with a live or stored feed from a content source, the feed representing a virtual space for immersive output through the user device. Currently, many virtual reality user devices use so-called three degrees of freedom (3DoF), which means that the head movement in the yaw, pitch and roll axes are measured and determine what the user sees, i.e. to determine the viewport. It is known that rendering by taking the position of the user device and changes of the position into account can enhance the immersive experience. Thus, an enhancement to 3DoF is a six degrees-of- freedom (6DoF) virtual reality system, where the user may freely move in Euclidean space as well as rotate their head in the yaw, pitch and roll axes. Six degrees-of- freedom virtual reality systems enable the provision and consumption of volumetric content. Volumetric content comprises data representing spaces and/or objects in three-dimensions from all angles, enabling the user to move fully around the space and/or objects to view them from any angle. Such content may be defined by data describing the geometry (e.g. shape, size, position in a three-dimensional space) and attributes such as colour, opacity and reflectance. The data may also define temporal changes in the geometry and attributes at given time instances, similar to frames in two- dimensional video.

[0245] A viewpoint may be defined as the point or space from which the user views the scene; it usually corresponds to a camera position. Slight head motion does not imply a different viewpoint. A viewing position may be defined as the position within a viewing space from which the user views the scene. A viewing space may be defined as a 3D space of viewing positions within which rendering of image and video is enabled and VR experience is valid.

[0246] Typical representation formats for volumetric content include triangle meshes, point clouds and voxels. Temporal information about the content may comprise individual capture instances, i.e. frames or the position of objects as a function of time.

[0247] Advances in computational resources and in three-dimensional acquisition devices enable reconstruction of highly-detailed volumetric representations. Infrared, laser, time-of- flight and structured light technologies are examples of how such content may be constructed. The representation of volumetric content may depend on how the data is to be used. For example, dense voxel arrays may be used to represent volumetric medical images. In three-dimensional graphics, polygon meshes are extensively used.

Point clouds, on the other hand, are well suited to applications such as capturing real-world scenes where the topology of the scene is not necessarily a two-dimensional surface or manifold. Another method is to code three-dimensional data to a set of texture and depth maps. Closely related to this is the use of elevation and multi-level surface maps. For the avoidance of doubt, embodiments herein are applicable to any of the above technologies.

[0248] “Voxel” of a three-dimensional world corresponds to a pixel of a two- dimensional world. Voxels exist in a three-dimensional grid layout. An octree is a tree data structure used to partition a three-dimensional space. Octrees are the three-dimensional analog of quadtrees. A sparse voxel octree (SVO) describes a volume of a space containing a set of solid voxels of varying sizes. Empty areas within the volume are absent from the tree, which is why it is called“sparse”.

[0249] A three-dimensional volumetric representation of a scene may be determined as a plurality of voxels on the basis of input streams of at least one multicamera device. Thus, at least one but preferably a plurality (i.e. 2, 3, 4, 5 or more) of multicamera devices may be used to capture 3D video representation of a scene. The multicamera devices are distributed in different locations in respect to the scene, and therefore each multicamera device captures a different 3D video representation of the scene. The 3D video

representations captured by each multicamera device may be used as input streams for creating a 3D volumetric representation of the scene, said 3D volumetric representation comprising a plurality of voxels. Voxels may be formed from the captured 3D points e.g. by merging the 3D points into voxels comprising a plurality of 3D points such that for a selected 3D point, all neighbouring 3D points within a predefined threshold from the selected 3D point are merged into a voxel without exceeding a maximum number of 3D points in a voxel.

[0250] Voxels may also be formed through the construction of the sparse voxel octree. Each leaf of such a tree represents a solid voxel in world space; the root node of the tree represents the bounds of the world. The sparse voxel octree construction may have the following steps: 1) map each input depth map to a world space point cloud, where each pixel of the depth map is mapped to one or more 3D points; 2) determine voxel attributes such as colour and surface normal vector by examining the neighbourhood of the source pixel(s) in the camera images and the depth map; 3) determine the size of the voxel based on the depth value from the depth map and the resolution of the depth map; 4) determine the SVO level for the solid voxel as a function of its size relative to the world bounds; 5) determine the voxel coordinates on that level relative to the world bounds; 6) create new and/or traversing existing SVO nodes until arriving at the determined voxel coordinates; 7) insert the solid voxel as a leaf of the tree, possibly replacing or merging attributes from a previously existing voxel at those coordinates. Nevertheless, the size of voxel within the 3D volumetric representation of the scene may differ from each other. The voxels of the 3D volumetric representation thus represent the spatial locations within the scene.

[0251] A volumetric video frame may be regarded as a complete sparse voxel octree that models the world at a specific point in time in a video sequence. Voxel attributes contain information like colour, opacity, surface normal vectors, and surface material properties. These are referenced in the sparse voxel octrees (e.g. colour of a solid voxel), but can also be stored separately.

[0252] Point clouds are commonly used data structures for storing volumetric content. Compared to point clouds, sparse voxel octrees describe a recursive subdivision of a finite volume with solid voxels of varying sizes, while point clouds describe an unorganized set of separate points limited only by the precision of the used coordinate values.

[0253] In technologies such as dense point clouds and voxel arrays, there may be tens or even hundreds of millions of points. In order to store and transport such content between entities, such as between a server and a client over an IP network, compression is usually required.

[0254] User’s position can be detected relative to content provided within the volumetric virtual reality content, e.g. so that the user can move freely within a given virtual reality space, around individual objects or groups of objects, and can view the objects from different angles depending on the movement (e.g. rotation and location) of their head in the real world. In some examples, the user may also view and explore a plurality of different virtual reality spaces and move from one virtual reality space to another one.

[0255] The angular extent of the environment observable or hearable through a rendering arrangement, such as with a head-mounted display, may be called the visual field of view (FOV). The actual FOV observed or heard by a user depends on the inter pupillary distance and on the distance between the lenses of the virtual reality headset and the user’s eyes, but the FOV can be considered to be approximately the same for all users of a given display device when the virtual reality headset is being worn by the user.

[0256] When viewing volumetric content from a single viewing position, a portion (often half) of the content may not be seen because it is facing away from the user. This portion is sometimes called“back facing content”.

[0257] A volumetric image/video delivery system may comprise providing a plurality of patches representing part of a volumetric scene, and providing, for each patch, patch visibility information indicative of a set of directions from which a forward surface of the patch is visible. A volumetric image/video delivery system may further comprise providing one or more viewing positions associated with a client device, and processing one or more of the patches dependent on whether the patch visibility information indicates that the forward surface of the one or more patches is visible from the one or more viewing positions.

[0258] Patch visibility information is data indicative of where in the volumetric space the forward surface of the patch can be seen. For example, patch visibility information may comprise a visibility cone, which may comprise a visibility cone direction vector (X, Y, Z) and an opening angle (A). The opening angle (A) defines a set of spatial angles from which the forward surface of the patch can be seen. In another example, the patch visibility metadata may comprise a definition of a bounding sphere surface and sphere region metadata, identical or similar to that specified by the omnidirectional media format (OMAF) standard (ISO/IEC 23090-2). The bounding sphere surface may for example be defined by a three-dimensional location of the centre of the sphere, and the radius of the sphere. When the viewing position collocates with the bounding sphere surface, the patch may be considered visible within the indicated sphere region. In general, the geometry of the bounding surface may also be something other than a sphere, such as cylinder, cube, or cuboid. Multiple sets of patch visibility metadata may be defined for the same three- dimensional location of the centre of the bounding surface, but with different radii (or information indicative of the distance of the bounding surface from the three-dimensional location). Indicating several pieces of patch visibility metadata may be beneficial to handle occlusions.

[0259] A volumetric image/video delivery system may comprise one or more patch culling modules. One patch culling module may be configured to determine which patches are transmitted to a user device, for example the rendering module of the headset. Another patch culling module may be configured to determine which patches are decoded. A third patch culling module may be configured to determine which decoded patches are passed to rendering. Any combination of patch culling modules may be present or active in a volumetric image/video delivery or playback system. Patch culling may utilize the patch visibility information of patches, the current viewing position, the current viewing orientation, the expected future viewing positions, and/or the expected future viewing orientations.

[0260] In some cases, each volumetric patch may be projected to a two-dimensional colour (or other form of texture) image and to a corresponding depth image, also known as a depth map. This conversion enables each patch to be converted back to volumetric form at a client rendering module of the headset using both images.

[0261 ] In some cases, a source volume of a volumetric image, such as a point cloud frame, may be projected onto one or more projection surfaces. Patches on the projection surfaces may be determined, and those patches may be arranged onto one or more two- dimensional frames. As above, texture and depth patches may be formed similarly shows a projection of a source volume to a projection surface, and inpainting of a sparse projection. In other words, a three-dimensional (3D) scene model, comprising geometry primitives such as mesh elements, points, and/or voxel, is projected onto one or more projection surfaces. These projection surface geometries may be "unfolded" onto 2D planes (typically two planes per projected source volume: one for texture, one for depth). The "unfolding" may include determination of patches. 2D planes may then be encoded using standard 2D image or video compression technologies. Relevant projection geometry information may be transmitted alongside the encoded video files to the decoder. The decoder may then decode the coded image/video sequence and perform the inverse projection to regenerate the 3D scene model object in any desired representation format, which may be different from the starting format e.g. reconstructing a point cloud from original mesh model data.

[0262] It should be understood that volumetric image/video can comprise, additionally or alternatively to texture and depth, other types of patches, such as reflectance, opacity or transparency (e.g. alpha channel patches), surface normal, albedo, and/or other material or surface attribute patches.

[0263] Two-dimensional form of patches may be packed into one or more atlases. Texture atlases are known in the art, comprising an image consisting of sub-images, the image being treated as a single unit by graphics hardware and which can be compressed and transmitted as a single image for subsequent identification and decompression.

Geometry atlases may be constructed similarly to texture atlases. Texture and geometry atlases may be treated as separate pictures (and as separate picture sequences in case of volumetric video), or texture and geometry atlases may be packed onto the same frame, e.g. similarly to how frame packing is conventionally performed. Atlases may be encoded as frames with an image or video encoder.

[0264] The sub-image layout in an atlas may also be organized such that it is possible to encode a patch or a set of patches having similar visibility information into spatiotemporal units that can be decoded independently of other spatiotemporal units. For example, a tile grid, as understood in the context of High Efficiency Video Coding (HEVC), may be selected for encoding and an atlas may be organized in a manner such that a patch or a group of patches having similar visibility information can be encoded as a motion- constrained tile set (MCTS).

[0265] In some cases, one or more (but not the entire set of) spatiotemporal units may be provided and stored as a track, as is understood in the context of the ISO base media file format, or as any similar container file format structure. Such a track may be referred to as a patch track. Patch tracks may for example be sub-picture tracks, as understood in the context of OMAF, or tile tracks, as understood in the context of ISO/IEC 14496-15.

[0266] In some cases, several versions of the one or more atlases are encoded. Different versions may include, but are not limited to, one or more of the following: different bitrate versions of the one or more atlases at the same resolution; different spatial resolutions of the atlases; and different versions for different random access intervals; these may include one or more intra-coded atlases (where every picture can be randomly accessed). [0267] In some cases, combinations of patches from different versions of the texture atlas may be prescribed and described as metadata, such as extractor tracks, as will be understood in the context of OMAF and/or ISO/IEC 14496-15.

When the total sample count of a texture atlas and, in some cases, of the respective geometry pictures and/or other auxiliary pictures (if any) exceeds a limit, such as a level limit of a video codec, a prescription may be authored in a manner so that the limit is obeyed. For example, patches may be selected from a lower-resolution texture atlas according to subjective importance. The selection may be performed in a manner that is not related to the viewing position. The prescription may be accompanied by metadata characterizing the obeyed limit(s), e.g. the codec Level that is obeyed.

A prescription may be made specific to a visibility cone (or generally to a specific visibility) and hence excludes the patches not visible in the visibility cone. The selection of visibility cones for which the prescriptions are generated may be limited to a reasonable number, such that switching from one prescription to another is not expected to occur frequently. The visibility cones of prescriptions may overlap to avoid switching back and forth between two prescriptions. The prescription may be accompanied by metadata indicative of the visibility cone (or generally visibility information).

A prescription may use a specific grid or pattern of independent spatiotemporal units. For example, a prescription may use a certain tile grid, wherein tile boundaries are also MCTS boundaries. The prescription may be accompanied by metadata indicating potential sources (e.g. track groups, tracks, or representations) that are suitable as spatiotemporal units.

[0268] In some cases, a patch track forms a Representation in the context of DASH. Consequently, the Representation element in DASH MPD may provide metadata on the patch, such as patch visibility metadata, related to the patch track. Clients may select patch Representations and request (Sub)segments from the selected Representations on the basis of patch visibility metadata.

[0269] Now an improved method for enabling multi-resolution viewport-dependent delivery and/or decoding of omnidirectional video is introduced. The method may additionally or alternatively be used for point cloud video or volumetric video for enabling delivery and/or decoding that is adaptive to viewing orientation and/or viewing position and may involve the content in multiple resolutions.

[0270] The method according to an aspect, as shown in Figure 12, comprises encoding (1200) an input picture sequence into at least a first bitstream and a second bitstream, said encoding comprising; encoding (1202), into the first and the second bitstream, a set of shared coded pictures per a time instance comprising a complete representation of the content for the time instance; and encoding (1204), into the first and the second bitstream, other pictures as intermediate pictures, the intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content, wherein the intermediate pictures of first bitstream represent a first aspect of the content and the intermediate pictures of second bitstream represent a second different aspect of the content.

[0271] Herein, the complete representation may for example comprise the content at different resolutions, and/or the 360° or volumetric content for all viewing orientations, and/or the volumetric content for a comprehensive set of viewing positions. The different aspect of the content represented by the intermediate pictures of different bitstreams may be such that, for example, the intermediate pictures of the first bitstream may cover a first orientation range of 360° content at high resolution and the intermediate pictures of the second bitstream may cover a second orientation range of 360° content at high resolution, the first and second orientation ranges partly or fully differing from each other.

[0272] Thus, the method enables multi-resolution viewport-adaptive streaming. Hence, higher effective resolutions on the viewport can be achieved than would otherwise be enabled by the decoding capability. Moreover, thanks to the common set of of shared coded pictures for all bitstreams, intra pictures are not required for viewport switching. As a result, shared coded pictures can be encoded at relatively short intervals without significant penalty in compression performance, hence providing frequent viewport switch points and low motion-to-high-quality latency. Moreover, a viewport change does not cause a considerable spike in streaming bitrate.

[0273] According to an embodiment, the method further comprises encoding the set of shared coded pictures periodically. For example, the set of shared coded pictures may be encoded on the basis of a (Sub)segment duration.

[0274] According to an embodiment, the method further comprises encoding the intermediate pictures as motion-constrained tile sets (MCTSs). Encoding a picture as motion-constrained tile sets may be defined as encoding a picture that is partitioned to motion-constrained tile sets, i.e. contains no coded video data outside the MCTSs. The MCTSs of different bitstreams may be encoded in a manner that enables selection of MCTSs from different bitstreams to be merged to form a conforming bitstream. Thus, then a client may select the MCTSs according to its needs and the merge the selected MCTSs into a bitstream.

[0275] According to an embodiment, the method further comprises encoding the intermediate pictures in MCTSs to include a conditional motion vector anchor position for MCTSs, where the anchor position of an MCTS is applied when a shared coded picture is referenced in inter prediction. Each of the intermediate pictures comprises, for a particular time instance, a subset of the content and/or parts of the content at a lower resolution.

[0276] According to an embodiment, the method further comprises selecting collocated MCTSs of time-aligned intermediate pictures in different bitstreams prior to encoding such that the collocated MCTSs are mutually exclusive. In other words, at most one of the collocated MCTSs needs to be selected for MCTS merging regardless of the circumstances (e.g. regardless of the viewing orientation or viewing position). This embodiment is hereafter referred to as the mutually exclusive collocated MCTS embodiment. For example, when high-resolution tiles of the intermediate pictures of a single bitstream cover a hemisphere, collocated MCTSs of time-aligned intermediate pictures in different bitstreams for 360° video may be selected to be of opposite orientations, assuming that the field of view of the viewport is less than 180° and thus at most one of the collocated MCTSs of the bitstreams is needed to cover the viewport regardless of the viewport orientation.

[0277] According to an embodiment, the method further comprises encoding the intermediate pictures in MCTSs by initializing a motion vector candidate to a value that indicates the spatial location difference of a tile in an intermediate picture relative to the respective tile in the shared coded picture used as a reference picture. This embodiment is explained in further details below in a section called "seed motion vector".

[0278] According to an embodiment, the method further comprises encoding several versions of multi-resolution 360° content such that all MCTSs are encoded in high resolution at least in one encoded version. For example, two encoded bitstreams are needed in the multi-resolution tiling layout presented below. The encoding of each bitstream comprises: Encoding a set of shared coded pictures per a time instance, such that it covers the content in all resolutions. Sets of shared coded pictures may be encoded

periodically, e.g. on the basis of (Sub)segment duration. Two shared coded pictures per a time instance are needed in the multi-resolution tiling layouts, which are presented further below.

Encoding intermediate pictures in MCTSs as described in any of the embodiments.

[0279] An example embodiment using cubemaps is illustrated in Figure 13. The content is downsampled to two spatial resolutions, which in the example illustration originally use 3x2 cube face arrangement. When 4K decoding capability (e.g. HEVC Level 5.1) is available, cube face sizes can be 1408x1408 and 704x704, respectively, yielding about 5.5K effective resolution, and sets of shared coded pictures may be coded for example every eighth time instance. Respectively, if 8K decoding capability is available, cube faces sizes can be 2816x2816 and 1408x1408, respectively, yielding about 10K effective resolution. The cube faces are partitioned in tiles of equal size. Each tile may be coded as a motion-constrained tile set, not depending on any other tiles than collocated tiles in its reference pictures. Prior to encoding, the partitioned cube faces are arranged into a tile grid of 5^X 12 tiles, each of which for the 4K decoding capability have tile column width and tile row height equal to 352 luma samples. Each set of shared coded pictures comprises all the cube faces of a time instance in both resolutions. Each of the intermediate pictures comprises half of the high-resolution cube face quadrants and the complementary half of the low-resolution quadrants, such that each of the intermediate pictures covers 360°.

[0280] According to an embodiment, a set of tiles of the high-resolution version that cover the same content as a single tile in the low-resolution version (i.e., a quadrant of a cube face) forms a motion-constrained tile set.

[0281 ] It is remarked that while the example in Figure 13 illustrates the use of the same MCTS grid in the shared coded pictures and in the intermediate pictures, the shared coded pictures may have a different MCTS grid than the intermediate pictures. For example, in the examples of Figure 13, an MCTS of the high-resolution content in the shared coded picture may comprise one cube face. Any MCTS grid may be selected for shared coded pictures, or shared coded pictures need not comprise MCTSs at all.

[0282] According to an embodiment, the method further comprises encoding point cloud or volumetric video content into several versions such that all patches are encoded in a set of shared coded pictures and a subset of patches is encoded in intermediate pictures. The encoding of each bitstream may comprise:

Encoding a set of shared coded pictures per a time instance, such that it includes all the patches of the content in all resolutions. Sets of shared coded pictures may be encoded periodically, e.g. on the basis of (Sub)segment duration.

Encoding intermediate pictures using any MCTS encoding arrangement embodiment. An MCTS comprises one or more entire patches. The patches for an MCTS may be selected for example on the basis that they are visible in a certain viewing cone or according to the visibility information for the patches. For example, two bitstreams may be coded, where a first bitstream comprises high- resolution patches for a viewing cone covering a hemisphere, and a second bitstream comprises high-resolution patches for a viewing cone covering the complementary hemisphere.

[0283] Encapsulation of the coded content

[0284] According to an embodiment, each MCTS or motion-constrained slice is encapsulated as a tile or sub-picture track or alike. Consequently, a client may request them individually from a server.

[0285] According to an embodiment, a collection track is generated as a tile base track specified in ISO/IEC 14496-15 or with a similar approach.

[0286] According to ISO/IEC 14496-15, a tile base track indicates the tile ordering using a 'sabf track reference to the tile tracks. According to an embodiment, a 'sabf track reference to an 'alte' track group (or a track group with some other four-character code) comprising tile tracks is allowed. A parser resolves a 'sabf track reference to an 'alte' track group by selecting one of the tracks in the track group for reconstructing a bitstream.

[0287] A picture unit may be defined as a set of NAL units that contain all VCL NAL units of a coded picture and their associated non-VCL NAL units. The bitstream may be reconstructed on the basis of a tile base track as follows:

1. For a tile base track and a number of tile tracks carrying VCL NAL units of one layer, a picture unit is firstly reconstructed to consist of the following NAL units in the order listed:

a. If the picture unit corresponds to a sample that is the first sample of a set of samples associated with a sample entry, the parameter sets and SEI NAL units contained in the sample entry b. NAL units in the sample of the tile base track

c. NAL units in the samples of the tile tracks in the order of the 'sabt' track

references. A 'sabf track reference may refer to an 'alte' track group. When a 'sabf track reference refers to an 'alte' track group, one of the tracks in the track group is selected and the NAL units of that track are included in the picture unit.

Then the following steps apply, in the order listed, for the above-reconstructed picture unit: a. If there is one or more than one EOS NAL unit present, an EOS NAL unit is placed at the end of the picture unit and any other EOS NAL unit is removed. b. If there is one or more than one EOB NAL unit present, one EOB NAL unit is placed at the end of the picture unit; any other EOB NAL unit is removed.

2. If only one layer is involved, the reconstructed picture unit is the access unit.

Otherwise, the access unit is reconstructed from all the picture units of the involved layers.

3. Finally, the bitstream is reconstructed from the reconstructed access units.

[0288] The above-described embodiment may be used when collocated MCTSs of time- aligned intermediate pictures in different bitstreams were selected prior to encoding to be such that at most one of them needs to be selected for MCTS merging regardless of the circumstances (e.g. regardless of the viewing orientation or viewing position). Figure l4a shows an example embodiment of MCTS numbering applied to the example of Figure 13.

[0289] The encapsulation of the bitstreams into a container file is illustrated in Figure l4b. The coded video content is encapsulated into tile tracks. Each pair of tile tracks that are collocated are indicated to form an 'alte' track group and is labelled with an identifier (1001, 1002, ..., 1060). The tile base track has a 'sabf track reference that refers to track group identifiers in the order the respective MCTSs appear in the decoding order. The samples of the tile base track may be empty, i.e. of size 0, or may contain non-VCL NAL units e.g. applying to entire picture(s), such as SEI NAL unit(s) comprising temporal motion-constrained tile set SEI message(s).

[0290] Another aspect of the invention relates to encapsulation of the coded content into one or more container files. The operation may include, as shown in Figure 15:

receiving (1500) a first bitstream and a second bitstream, wherein o both the first bitstream and the second bitstream individually comprise a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance, and

o the first bitstream and the second bitstream comprise other pictures encoded as intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content, wherein the intermediate pictures of first bitstream represent a first aspect of the content and the intermediate pictures of second bitstream represent a second different aspect of the content;

selecting (1502) a first spatiotemporal unit of the intermediate picture of the first bitstream and encapsulating the first spatiotemporal unit of the intermediate pictures of the first bitstream into a first tile or sub-picture track;

selecting (1504) a second spatiotemporal unit of the intermediate picture of the second bitstream and encapsulating the second spatiotemporal unit of the intermediate pictures of the second bitstream into a second tile or sub-picture track; providing (1506) an indication and an identifier of a first group of tile or sub- picture tracks that are alternatives for extraction, the first group of tile or sub- picture tracks comprising the first and second tile or sub-picture tracks;

creating (1508) a first set of samples into a collector track, the first set of samples natively comprising the set of shared coded pictures for the time instance; and creating (1509) a second set of samples into the collector track and associating the identifier of the first group of tile or sub-picture tracks to the second set of samples, the association intended to be resolved by selecting one of the tile or sub-picture tracks in the first group to be a source of extraction for including the first or the second spatiotemporal unit by reference into the second set of samples.

[0291] It is remarked that ISO/IEC 14496-15 disallows VCL NAL units included natively in a tile base track. The phrases "included natively" or "present natively" or alike may incur that inclusion does not take place by reference, e.g. using an extractor.

Consequently, in above-described embodiment, shared coded pictures need to have the same tile grid structure as that for the intermediate pictures. Moreover, since respective shared coded pictures in different bitstreams are identical, they are redundantly stored in multiple tracks. This also leads to delivery and caching inefficiency. These disadvantages can be overcome with the following embodiment: Samples of a tile base track are allowed to include VCL NAL units. When a sample of a tile base track includes VCL NAL units, the referenced tile tracks are disallowed to have a time-aligned sample or are required to have an empty sample (of size equal to 0).

[0292] According to an embodiment, the MCTSs or motion-constrained slices are encapsulated in tiles of different size. An example embodiment for using tiles of different size from cubemaps is illustrated in Figure 16. The content is downsampled to two spatial resolutions, which in the example illustration originally use 3x2 cube face arrangement. When 4K decoding capability (e.g. HEVC Level 5.1) is available, cube face sizes can be 1408x1408 and 704x704, respectively, yielding about 5.5K effective resolution, and sets of shared coded pictures may be coded for example every eighth time instance.

Respectively, if 8K decoding capability is available, cube faces sizes can be 2816x2816 and 1408x1408, respectively, yielding about 10K effective resolution. The cube faces are partitioned into four quadrants, each of which is coded as an MCTS or a motion- constrained slice. Prior to encoding, the cube face quadrants are arranged into a tile grid of 3x6 tiles, which for the 4K decoding capability have tile column widths equal to 704, 704, and 352 luma samples, and a constant tile row height of 704 luma samples. Each set of shared coded pictures comprises all the cube faces of a time instance in both resolutions. Each of the intermediate pictures comprises half of the high-resolution cube face quadrants and the complementary half of the low-resolution quadrants, such that each of the intermediate pictures covers 360°.

[0293] According to an embodiment, a collection track is generated. It natively includes the shared coded pictures and extracts (i.e., includes by reference) the MCTSs from tile or sub-picture tracks for the other coded pictures. Extractors, as or like defined in ISO/IEC 14496-15, may be used as the mechanism for inclusion-by reference.

[0294] According to an embodiment for tiles of the same size, one collection track is created for all viewing orientations, as illustrated in an example shown in Figure l7a. Extractors refer to groups of tile or sub-picture tracks that are alternatives for extraction and may be included in the same track group, such as the same 'alte' track group. For example, such a group may comprise all tile or sub-picture tracks that are of the same width and height and have the same slice header length. Since the same group contains tiles originating from both high- and low-resolution versions, clients may select the number of tiles from high- and low-resolution original content flexibly. [0295] According to an embodiment for 360-degree content conforming to OMAF vl, one collection track is created per a distinct viewing orientation, in which case extracted MCTSs have a pre-defined location in each collection track.

[0296] According to an embodiment for tiles of different size, one collection track is created for all viewing orientations, as illustrated in an example shown in Figure l7b. Extractors refer to groups of tile or sub-picture tracks that are alternatives for extraction, wherein the size of tiles or sub-pictures within the group is the same, but may be different between the alternative groups. For example, such a group may comprise all tile or sub- picture tracks that are of the same width and height and have the same slice header length.

[0297] Seed motion vector

[0298] It is noted that the embodiment for video encoding and/or video decoding involving a seed motion vector described next may be applied together with or

independently of other embodiments. Conventionally, when a spatial neighbour of a prediction unit or alike would be outside a tile or slice, it would be treated as unavailable in the motion vector prediction process of the prediction unit. In the embodiment, when a spatial neighbour of the prediction unit would be outside a tile or slice, the respective candidate motion vector and reference index (hereafter referred to as the seed motion vector and the seed reference index, respectively) are set to be equal to inferred values or values indicated in a higher level syntax structure, such as a slice header or a picture parameter set. In an alternative embodiment, when no spatial motion vector candidates are available for a prediction unit, the candidate list for motion vector prediction is appended with the seed motion vector and the seed reference index. These embodiments enable offsetting the indicated motion vector difference with a difference of the tile position in the current (intermediate picture) relative to the respective tile position in the shared coded picture.

[0299] According to an embodiment, the encoder sets the seed motion vector to reflect the spatial location difference of the current tile and the respective tile in the previous shared coded picture (in decoding order) containing the respective tile the seed reference index to point to that previous shared coded picture. Both the seed motion vector and the seed reference index are indicated in a higher level syntax structure, such as a slice header or a picture parameter set. The decoder decodes the seed motion vector and the seed reference index from the higher level syntax structure. The encoder and/or the decoder uses the seed motion vector and the seed reference index whenever a spatial neighbour of a prediction unit or alike would be outside a tile or slice.

[0300] According to an embodiment, the encoder and/or the decoder uses the seed motion vector and the seed reference index only for particular prediction units within a tile or a slice, such as only for the first prediction unit within the tile or slice or only for the top- and left-most prediction units within the tile or slice.

[0301] According to an embodiment, the encoder and/or the decoder uses the seed motion vector and the seed reference index as a spatial motion vector candidate when all conventional spatial motion vector candidates or the respective blocks are unavailable, i.e. outside the current tile or the current slice or coded with a mode not involving a motion vector, such as intra coding.

[0302] According to an embodiment, the seed reference index is inferred (e.g. to be the previous shared coded picture that is indicated to be an active reference picture for the current picture) or indicated in yet higher level syntax structure, such as a sequence parameter set.

[0303] According to an embodiment, the seed motion vector and, if needed, the seed reference index are rewritten on the basis of selecting an MCTS to be merged into an intermediate picture to be decoded. The rewritten seed motion vector corresponds to the horizontal and vertical difference of the selected MCTS and the corresponding area in a shared coded picture.

[0304] According to an embodiment, rather than a seed reference index, another means for indicating a reference picture is used. Such other means may include but are not limited to an index to a reference picture set; a picture order count or another syntax element or variable characteristic to a particular reference picture; and a picture type, from which a reference picture is resolved as the previous reference picture, in decoding order, of that picture type.

[0305] Generalization on number of pictures in a set of shared coded pictures and/or on the number of bitstreams

[0306] Examples were presented above with reference to two shared coded pictures per time instance. It needs to be understood that embodiments are not limited to any particular number of shared coded pictures per time instance.

[0307] Examples were presented above with two resolutions. It needs to be understood that embodiments are not limited to any particular number of resolutions. Figure l8a shows an example embodiment, wherein cubemap sequences are prepared in three resolutions. A set of shared coded pictures comprises three coded pictures, and three bitstreams are encoded.

[0308] When 4K decoding capability (e.g. HEVC Level 5.1) is available, MCTS size can be 384x384, yielding about 6K effective resolution in the highest resolution, and sets of shared coded pictures may be coded for example every eighth time instance. The cube faces are partitioned in tiles of equal size. Each tile may be coded as a motion-constrained tile set, not depending on any other tiles than collocated tiles in its reference pictures. Prior to encoding, the partitioned cube faces are arranged into a tile grid of 7^c6 tiles. Each set of shared coded pictures comprises all the cube faces of a time instance in all resolutions. Each of the intermediate pictures comprises selected of tiles of each resolution, such that the time-aligned intermedia pictures of the bitstreams collectively cover 360° in all resolutions.

[0309] As illustrated in Figure 18b, the client may use different strategies of selecting

MCTSs for intermediate pictures from different bitstreams, such as:

36 and 6 MCTSs originating from the highest and lowest resolution cubemaps, respectively. 36 MCTSs of the highest resolution cover a third of the cubemap, e.g. about l35°x 135° field of view, whereas 6 MCTSs of the lowest resolution fully cover the cubemap.

24, 16, and 2 MCTSs originating from the highest, middle, and lowest resolution cubemaps, respectively.

[0310] It is noted that while in the presented examples the shared coded pictures were encoded to enable viewport switching capability, they may also be used for bitrate adaptation capability. If the pixel count limitations (e.g. constraint of 4K decoding capacity) allow, additional shared coded pictures could be encoded for bitrate adaptation capability. Alternatively, bitrate adaptation could be enabled conventionally by stream switching starting from intra-coded pictures.

[031 1 ] Examples were presented above with particular tile and/or MCTS grids. It needs to be understood that embodiments are not limited to these tile and/or MCTS grids but similarly apply to other tile and/or MCTS grids too.

[0312] For signaling the metadata of tile or sub-picture tracks or alike, any known method may be used. For example, a region-wise packing box and/or a 2D or spherical region- wise quality ranking box may be present for each tile or sub-picture track of 360° video. In another example, metadata may be present for each tile or sub-picture track of volumetric video.

[0313] Region- wise quality ranking metadata may be present in or along a video or image bitstream. Quality ranking values of quality ranking regions may be relative to other quality ranking regions of the same bitstream or the same track or quality ranking regions of other tracks. Region-wise quality ranking metadata can be indicated for example by using the SphereRegionQualityRankingBox or the 2DRegionQualityRankingBox, which are specified as a part of MPEG Omnidirectional Media Format.

SphereRegionQualityRankingBox provides quality ranking values for sphere regions, i.e., regions defined on sphere domain, while 2DRegionQualityRankingBox provides quality ranking values for rectangular regions on decoded pictures (and potentially a leftover region covering all areas not covered by any of the rectangular regions). Quality ranking values indicate a relative quality order of quality ranking regions. When quality ranking region A has a non-zero quality ranking value less than that of quality ranking region B, quality ranking region A has a higher quality than quality ranking region B. When the quality ranking value is non-zero, the picture quality within the entire indicated quality ranking region may be defined to be approximately constant. In general, the boundaries of the quality ranking sphere or 2D regions may or may not match with the boundaries of the packed regions or the boundaries of the projected regions specified in region- wise packing metadata.

[0314] Another aspect of the invention relates to the client operation. The operation may include, as shown in Figure 19, receiving and decoding (1900) a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance; selecting (1902) at least one spatiotemporal unit among at least a first and second spatiotemporal unit, wherein the first spatiotemporal unit represents a first aspect of the content and the second spatiotemporal unit represents a second different aspect of the content; receiving (1904) the at least one spatiotemporal unit; (1906) merging the at least one spatiotemporal unit into an intermediate picture; and decoding (1908) the intermediate picture having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content.

[0315] As stated above, MCTSs of different bitstreams are encoded in a manner that enables selection of MCTSs from different bitstreams to be merged to form a conforming bitstream. [0316] Therefore the client may carry out one or more of the following embodiments in a client-driven video delivery system, such as DASH, before requesting and receiving suitable parts of different encoded bitstreams. Alternatively or additionally, the client may carry out one or more of the following embodiments when playing the different bitstreams available in the client, i.e. stored locally in the client. One or more of the following embodiments may be applied to a server-driven video delivery system by the server rather than the client provided that the client delivers prevailing and/or expected viewing information, such as viewport, viewing position, and/or viewing cone to the server.

[0317] According to an embodiment, the client identifies MCTSs that are alternatives to each other. For example, tile or sub-picture tracks that are alternatives to each other may be included in the content encapsulation phase in the same track group of a particular type (e.g. 'alte' track group) and indicated in the container file. Similarly, tile or sub-picture Representations that are alternatives to each other may be indicated in the MPD, e.g. by including them in the same Adaptation Set or by indicating them with a property descriptor of a particular type.

[0318] According to an embodiment, the client obtains properties of the MCTSs that are alternatives to each other. For example, the content coverage or region-wise quality may be indicated for MCTSs of 360° video or patch visibility information, such as visibility cone(s), may be indicated for MCTSs of point clouds or volumetric video. Moreover, the resolution or sampling density or alike may be obtained for MCTSs.

[0319] According to an embodiment, the client selects an alternative that suits its needs among the MCTSs that are alternatives to each other. For example, the client may select the MCTS having high sampling density when it is within or close to the viewport, if the alternatives have low sampling density and outside the viewport. In another example, the client may select the MCTS that contains patches within or close to the current viewing cone, if the alternatives are outside the viewing cone.

[0320] According to an embodiment, the client determines the number, position, and size of MCTSs (e.g. in terms of number of tile rows and columns, and/or in terms of tile column widths and heights) in the merged coded pictures. The determination may be performed by selecting a collector track or collector Representation, e.g. partly on the basis of decoding capacity requirements, such as codec profile and level, indicated for the collector track or the collector Representation, and the decoding capability available in the client. Alternatively, the determination may be performed by selecting the number, position, and size of MCTSs such that the available decoding capability in the client is sufficient for decoding the resulting merged bitstream. The above-described selection among alternative MCTSs may be performed separately for distinct MCTS positions in the merged coded picture.

[0321 ] According to an embodiment, the client merges, in coded domain, the selected MCTSs into coded picture(s) of a bitstream and decodes the bitstream.

[0322] The selection process by the client as described above may be performed e.g. on Segment or Subsegment basis. Alternatively or additionally, the selection process by the client may be performed whenever viewing orientation and/or viewing position changes, e.g. to an extent that could cause a different selection of MCTSs.

[0323] The embodiments as described above may facilitate to achieve significant advantages. For example, multi-resolution viewport-adaptive streaming is enabled by the embodiments. Hence, higher effective resolutions on the viewport can be achieved than would otherwise be enabled by the decoding capability. For example, 5.5K or 6K effective panorama resolution may be achieved with 4K decoding capability or respectively 10K or 12K effective panorama resolution may be achieved with 8K decoding capability.

[0324] It is expected that the method according to the embodiments provides streaming bitrate reduction for multi-resolution viewport-adaptive streaming (VAS).

[0325] Since intra pictures are not required for viewport switching, the embodiments enable frequent switch points (i.e., shared coded pictures) and hence low motion-to -high- quality latency. In the examples with two bitstreams, N can be set equal to 8, matching the typical prediction hierarchy (”GOP”) length of modem codecs. With N=8, a 5.5K effective resolution on the viewport is reached with 4K decoding capacity and viewport switching may take place every 8 pictures.

[0326] Since intra pictures are not required for viewport switching, a viewport change does not cause a spike in streaming bitrate. Instead, the bitrate is relatively stable regardless of the viewing orientation or its changes, unlike in prior art solution. Bitrate variation typically causes a need for keeping the client buffer occupancy to a sufficient level to avoid interruptions in the playback and hence involve greater end-to-end and motion-to-high-quality delays.

[0327] The embodiments utilizing either the conditional anchor position for MCTSs or the seed motion vector enable clients select MCTSs from different resolutions in a flexible manner e.g. based on the field of view that the highest resolution MCTSs need to cover and/or the expected changes in viewing orientation.

[0328] The mutually exclusive collocated MCTS embodiment can be fully implemented with existing codecs, such as HEVC and H.264/AVC.

[0329] Figure 20 shows a block diagram of a video decoder suitable for employing embodiments of the invention. Figure 20 depicts a structure of a two-layer decoder, but it would be appreciated that the decoding operations may similarly be employed in a single- layer decoder.

[0330] The video decoder 550 comprises a first decoder section 552 for a base layer and a second decoder section 554 a predicted layer. Block 556 illustrates a demultiplexer for delivering information regarding base layer pictures to the first decoder section 552 and for delivering information regarding predicted layer pictures to the second decoder section 554. Reference P’n stands for a predicted representation of an image block. Reference D’n stands for a reconstructed prediction error signal. Blocks 704, 804 illustrate preliminary reconstructed images (Tn). Reference R’n stands for a final reconstructed image. Blocks 703, 803 illustrate inverse transform

Blocks 702, 802 illustrate inverse quantization (Q ¹). Blocks 701, 801 illustrate entropy decoding (E ¹). Blocks 705, 805 illustrate a reference frame memory (RFM). Blocks 706, 806 illustrate prediction (P) (either inter prediction or intra prediction). Blocks 707, 807 illustrate filtering (F). Blocks 708, 808 may be used to combine decoded prediction error information with predicted base

layer/predicted layer images to obtain the preliminary reconstructed images (Tn).

Preliminary reconstructed and filtered base layer images may be output 709 from the first decoder section 552 and preliminary reconstructed and filtered base layer images may be output 809 from the first decoder section 554.

[0331] Herein, the decoder should be interpreted to cover any operational unit capable to carry out the decoding operations, such as a player, a receiver, a gateway, a

demultiplexer and/or a decoder.

[0332] Figure 21 is a graphical representation of an example multimedia

communication system within which various embodiments may be implemented. A data source 1510 provides a source signal in an analog, uncompressed digital, or compressed digital format, or any combination of these formats. An encoder 1520 may include or be connected with a pre-processing, such as data format conversion and/or filtering of the source signal. The encoder 1520 encodes the source signal into a coded media bitstream. It should be noted that a bitstream to be decoded may be received directly or indirectly from a remote device located within virtually any type of network. Additionally, the bitstream may be received from local hardware or software. The encoder 1520 may be capable of encoding more than one media type, such as audio and video, or more than one encoder 1520 may be required to code different media types of the source signal. The encoder 1520 may also get synthetically produced input, such as graphics and text, or it may be capable of producing coded bitstreams of synthetic media. In the following, only processing of one coded media bitstream of one media type is considered to simplify the description. It should be noted, however, that typically real-time broadcast services comprise several streams (typically at least one audio, video and text sub-titling stream). It should also be noted that the system may include many encoders, but in the figure only one encoder 1520 is represented to simplify the description without a lack of generality. It should be further understood that, although text and examples contained herein may specifically describe an encoding process, one skilled in the art would understand that the same concepts and principles also apply to the corresponding decoding process and vice versa.

[0333] The coded media bitstream may be transferred to a storage 1530. The storage 1530 may comprise any type of mass memory to store the coded media bitstream. The format of the coded media bitstream in the storage 1530 may be an elementary self- contained bitstream format, or one or more coded media bitstreams may be encapsulated into a container file, or the coded media bitstream may be encapsulated into a Segment format suitable for DASH (or a similar streaming system) and stored as a sequence of Segments. If one or more media bitstreams are encapsulated in a container file, a file generator (not shown in the figure) may be used to store the one more media bitstreams in the file and create file format metadata, which may also be stored in the file. The encoder 1520 or the storage 1530 may comprise the file generator, or the file generator is operationally attached to either the encoder 1520 or the storage 1530. Some systems operate“live”, i.e. omit storage and transfer coded media bitstream from the encoder 1520 directly to the sender 1540. The coded media bitstream may then be transferred to the sender 1540, also referred to as the server, on a need basis. The format used in the transmission may be an elementary self-contained bitstream format, a packet stream format, a Segment format suitable for DASH (or a similar streaming system), or one or more coded media bitstreams may be encapsulated into a container file. The encoder 1520, the storage 1530, and the server 1540 may reside in the same physical device or they may be included in separate devices. The encoder 1520 and server 1540 may operate with live real-time content, in which case the coded media bitstream is typically not stored permanently, but rather buffered for small periods of time in the content encoder 1520 and/or in the server 1540 to smooth out variations in processing delay, transfer delay, and coded media bitrate.

[0334] The server 1540 sends the coded media bitstream using a communication protocol stack. The stack may include but is not limited to one or more of Real-Time Transport Protocol (RTP), User Datagram Protocol (UDP), Hypertext Transfer Protocol (HTTP), Transmission Control Protocol (TCP), and Internet Protocol (IP). When the communication protocol stack is packet-oriented, the server 1540 encapsulates the coded media bitstream into packets. For example, when RTP is used, the server 1540

encapsulates the coded media bitstream into RTP packets according to an RTP payload format. Typically, each media type has a dedicated RTP payload format. It should be again noted that a system may contain more than one server 1540, but for the sake of simplicity, the following description only considers one server 1540.

[0335] If the media content is encapsulated in a container file for the storage 1530 or for inputting the data to the sender 1540, the sender 1540 may comprise or be operationally attached to a "sending file parser" (not shown in the figure). In particular, if the container file is not transmitted as such but at least one of the contained coded media bitstream is encapsulated for transport over a communication protocol, a sending file parser locates appropriate parts of the coded media bitstream to be conveyed over the communication protocol. The sending file parser may also help in creating the correct format for the communication protocol, such as packet headers and payloads. The multimedia container file may contain encapsulation instructions, such as hint tracks in the ISOBMFF, for encapsulation of the at least one of the contained media bitstream on the communication protocol.

[0336] The server 1540 may or may not be connected to a gateway 1550 through a communication network, which may e.g. be a combination of a CDN, the Internet and/or one or more access networks. The gateway may also or alternatively be referred to as a middle-box. For DASH, the gateway may be an edge server (of a CDN) or a web proxy. It is noted that the system may generally comprise any number gateways or alike, but for the sake of simplicity, the following description only considers one gateway 1550. The gateway 1550 may perform different types of functions, such as translation of a packet stream according to one communication protocol stack to another communication protocol stack, merging and forking of data streams, and manipulation of data stream according to the downlink and/or receiver capabilities, such as controlling the bit rate of the forwarded stream according to prevailing downlink network conditions. The gateway 1550 may be a server entity in various embodiments.

[0337] The system includes one or more receivers 1560, typically capable of receiving, de-mo dulating, and de-capsulating the transmitted signal into a coded media bitstream. The coded media bitstream may be transferred to a recording storage 1570. The recording storage 1570 may comprise any type of mass memory to store the coded media bitstream. The recording storage 1570 may alternatively or additively comprise computation memory, such as random access memory. The format of the coded media bitstream in the recording storage 1570 may be an elementary self-contained bitstream format, or one or more coded media bitstreams may be encapsulated into a container file. If there are multiple coded media bitstreams, such as an audio stream and a video stream, associated with each other, a container file is typically used and the receiver 1560 comprises or is attached to a container file generator producing a container file from input streams. Some systems operate“live,” i.e. omit the recording storage 1570 and transfer coded media bitstream from the receiver 1560 directly to the decoder 1580. In some systems, only the most recent part of the recorded stream, e.g., the most recent 10-minute excerption of the recorded stream, is maintained in the recording storage 1570, while any earlier recorded data is discarded from the recording storage 1570.

[0338] The coded media bitstream may be transferred from the recording storage 1570 to the decoder 1580. If there are many coded media bitstreams, such as an audio stream and a video stream, associated with each other and encapsulated into a container file or a single media bitstream is encapsulated in a container file e.g. for easier access, a file parser (not shown in the figure) is used to decapsulate each coded media bitstream from the container file. The recording storage 1570 or a decoder 1580 may comprise the file parser, or the file parser is attached to either recording storage 1570 or the decoder 1580. It should also be noted that the system may include many decoders, but here only one decoder 1570 is discussed to simplify the description without a lack of generality

[0339] The coded media bitstream may be processed further by a decoder 1570, whose output is one or more uncompressed media streams. Finally, a renderer 1590 may reproduce the uncompressed media streams with a loudspeaker or a display, for example. The receiver 1560, recording storage 1570, decoder 1570, and Tenderer 1590 may reside in the same physical device or they may be included in separate devices.

[0340] A sender 1540 and/or a gateway 1550 may be configured to perform switching between different representations e.g. for switching between different viewports of 360- degree video content, view switching, bitrate adaptation and/or fast start-up, and/or a sender 1540 and/or a gateway 1550 may be configured to select the transmitted

representation(s). Switching between different representations may take place for multiple reasons, such as to respond to requests of the receiver 1560 or prevailing conditions, such as throughput, of the network over which the bitstream is conveyed. In other words, the receiver 1560 may initiate switching between representations. A request from the receiver can be, e.g., a request for a Segment or a Subsegment from a different representation than earlier, a request for a change of transmitted scalability layers and/or sub-layers, or a change of a rendering device having different capabilities compared to the previous one. A request for a Segment may be an HTTP GET request. A request for a Subsegment may be an HTTP GET request with a byte range. Additionally or alternatively, bitrate adjustment or bitrate adaptation may be used for example for providing so-called fast start-up in streaming services, where the bitrate of the transmitted stream is lower than the channel bitrate after starting or random-accessing the streaming in order to start playback immediately and to achieve a buffer occupancy level that tolerates occasional packet delays and/or retransmissions. Bitrate adaptation may include multiple representation or layer up-switching and representation or layer down-switching operations taking place in various orders.

[0341] A decoder 1580 may be configured to perform switching between different representations e.g. for switching between different viewports of 360-degree video content, view switching, bitrate adaptation and/or fast start-up, and/or a decoder 1580 may be configured to select the transmitted representation(s). Switching between different representations may take place for multiple reasons, such as to achieve faster decoding operation or to adapt the transmitted bitstream, e.g. in terms of bitrate, to prevailing conditions, such as throughput, of the network over which the bitstream is conveyed.

Faster decoding operation might be needed for example if the device including the decoder 1580 is multi-tasking and uses computing resources for other purposes than decoding the video bitstream. In another example, faster decoding operation might be needed when content is played back at a faster pace than the normal playback speed, e.g. twice or three times faster than conventional real-time playback rate.

[0342] In the above, some embodiments have been described with reference to and/or using terminology of HEVC. It needs to be understood that embodiments may be similarly realized with any video encoder and/or video decoder with respective terms of other codecs. For example, rather than tiles or tile sets, embodiments could be realized with rectangular slice groups of H.264/AVC.

[0343] Certain example embodiments have been described above in relation to tiles. It needs to be understood that other embodiments apply equally to any other types of picture partitioning units or spatiotemporal units, such as slices or slice groups. Likewise, embodiments apply equally to a mixture of two or more types of picture partitioning units or spatiotemporal units, such as a mixture of tiles and slices.

[0344] Certain example embodiments have been described above in relation to MCTSs. It needs to be understood that other embodiments apply equally to any other types of motion-constrained spatiotemporal units, such as motion-constrained slices. Likewise, embodiments apply equally to a mixture of two or more types of motion-constrained spatiotemporal units, such as a mixture of MCTSs and motion-constrained slices.

[0345] In the above, some embodiments have been described with reference to segments, e.g. as defined in MPEG-DASH. It needs to be understood that embodiments may be similarly realized with subsegments, e.g. as defined in MPEG-DASH.

[0346] In the above, some embodiments have been described in relation to ISOBMFF, e.g. when it comes to segment format. It needs to be understood that embodiments could be similarly realized with any other file format, such as Matroska, with similar capability and/or structures as those in ISOBMFF.

[0347] In the above, some embodiments have been descried with reference to the terms tile track. It needs to be understood that embodiments can be realized similarly with sub picture tracks.

[0348] In the above, some embodiments have been described with reference to the term tile base track. It needs to be understood that embodiments can be realized with any type of collector tracks, rather than just tile base tracks. More specifically, the embodiments can be realized with extractor tracks instead of tile base tracks.

[0349] In the above, some embodiments have been described with reference to the term extractor track. It needs to be understood that embodiments can be realized with any type of collector tracks, rather than just extractor tracks. More specifically, the embodiments can be realized with tile base tracks instead of extractor tracks. Moreover, embodiments can be realized by using both extractor tracks and tile base tracks, e.g. in the same file or for different Representations included in the same MPD.

[0350] In the above, where the example embodiments have been described with reference to an encoder, it needs to be understood that the resulting bitstream and the decoder may have corresponding elements in them. Likewise, where the example embodiments have been described with reference to a decoder, it needs to be understood that the encoder may have structure and/or computer program for generating the bitstream to be decoded by the decoder.

[0351 ] The embodiments of the invention described above describe the codec in terms of separate encoder and decoder apparatus in order to assist the understanding of the processes involved. However, it would be appreciated that the apparatus, structures and operations may be implemented as a single encoder-decoder apparatus/structure/operation. Furthermore, it is possible that the coder and decoder may share some or all common elements.

[0352] Although the above examples describe embodiments of the invention operating within a codec within an electronic device, it would be appreciated that the invention as defined in the claims may be implemented as part of any video codec. Thus, for example, embodiments of the invention may be implemented in a video codec which may implement video coding over fixed or wired communication paths.

[0353] Thus, user equipment may comprise a video codec such as those described in embodiments of the invention above. It shall be appreciated that the term user equipment is intended to cover any suitable type of wireless user equipment, such as mobile telephones, portable data processing devices or portable web browsers.

[0354] Furthermore elements of a public land mobile network (PLMN) may also comprise video codecs as described above.

[0355] In general, the various embodiments of the invention may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller,

microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the invention may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

[0356] The embodiments of this invention may be implemented by computer software executable by a data processor of the mobile device, such as in the processor entity, or by hardware, or by a combination of software and hardware. Further in this regard it should be noted that any blocks of the logic flow as in the Figures may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD.

[0357] The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as

semiconductor-based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multi-core processor architecture, as non-limiting examples.

[0358] Embodiments of the inventions may be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

[0359] Programs, such as those provided by Synopsys, Inc. of Mountain View, California and Cadence Design, of San Jose, California automatically route conductors and locate components on a semiconductor chip using well established rules of design as well as libraries of pre-stored design modules. Once the design for a semiconductor circuit has been completed, the resultant design, in a standardized electronic format (e.g., Opus, GDSII, or the like) may be transmitted to a semiconductor fabrication facility or "fab" for fabrication.

[0360] The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention.

Claims

CLAIMS:

1. A method comprising:

encoding an input picture sequence into at least a first bitstream and a second bitstream, said encoding comprising;

encoding, into the first and the second bitstream, a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance;

encoding, into the first and the second bitstream, other pictures as intermediate pictures, the intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content, wherein the intermediate pictures of the first bitstream represent a first aspect of the content and the intermediate pictures of the second bitstream represent a second different aspect of the content.

2. An apparatus comprising

means for encoding an input picture sequence into at least a first bitstream and a second bitstream, said encoding comprising;

means for encoding, into the first and the second bitstream, a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance;

means for encoding, into the first and the second bitstream, other pictures as intermediate pictures, the intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content, wherein the intermediate pictures of the first bitstream represent a first aspect of the content and the intermediate pictures of the second bitstream represent a second different aspect of the content.

3. The apparatus according to claim 2, further comprising:

means for encoding the intermediate pictures as motion-constrained tile sets

(MCTSs).

4. The apparatus according to claim 3, further comprising: means for encoding the intermediate pictures in MCTSs to include a conditional anchor position for MCTSs, where the anchor position of an MCTS is applied when a shared coded picture is referenced in inter prediction.

5. The apparatus according to claim 3, further comprising:

means for selecting collocated MCTSs of time-aligned intermediate pictures in different bitstreams prior to encoding such that the collocated MCTSs are mutually exclusive for rendering.

6. The apparatus according to claim 3, further comprising:

means for encoding the intermediate pictures in MCTSs by initializing a motion vector candidate to a value that indicates spatial location difference of a tile in an intermediate picture relative to a respective tile in the shared coded picture used as a reference picture.

7. The apparatus according to any of claims 2 - 6, further comprising

means for selecting the first aspect to be a first region of a projected omnidirectional picture format and the second different aspect to be a second region of the projected omnidirectional picture format, the first region differing from the second region.

8. The apparatus according to any of claims 2 - 7, wherein the input picture sequence represents volumetric video, and the apparatus further comprises:

means for selecting the first aspect to be first visibility information and the second different aspect to be second visibility information, the first visibility information differing from the second visibility information.

9. A method comprising:

receiving and decoding a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance;

selecting at least one spatiotemporal unit among at least a first and second spatiotemporal unit, wherein the first spatiotemporal unit represents a first aspect of the content and the second spatiotemporal unit represents a second different aspect of the content; receiving the at least one spatiotemporal unit;

merging the at least one spatiotemporal unit into an intermediate picture; and decoding intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content.

10. An apparatus comprising

means for receiving and decoding a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance;

means for selecting at least one spatiotemporal unit among at least a first and second spatiotemporal unit, wherein the first spatiotemporal unit represents a first aspect of the content and the second spatiotemporal unit represents a second different aspect of the content;

means for receiving the at least one spatiotemporal unit;

means for merging the at least one spatiotemporal unit into an intermediate picture; and

means for decoding intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content.

11. The apparatus according to claim 10, further comprising:

means for identifying intermediate pictures encoded as motion-constrained tile sets (MCTSs) that are alternatives to each other.

12. The apparatus according to claim 11, further comprising:

means for obtaining properties of the MCTSs that are alternatives to each other.

13. The apparatus according to claim 11 or 12, further comprising:

means for selecting an alternative that suits its needs among the MCTSs that are alternatives to each other.

14. The apparatus according to any of claims 11 - 13, further comprising:

means for determining a number, a position, and a size of MCTSs in the merged coded pictures.

15. An apparatus comprising

means for receiving a first bitstream and a second bitstream, wherein both the first bitstream and the second bitstream individually comprise a set of shared coded pictures per a time instance comprising a complete representation of a content for the time instance, and wherein the first bitstream and the second bitstream comprise other pictures encoded as intermediate pictures having a width and height equal to the width and height of a shared coded picture and corresponding to a time instance of the content, wherein the intermediate pictures of first bitstream represent a first aspect of the content and the intermediate pictures of second bitstream represent a second different aspect of the content;

means for selecting a first spatiotemporal unit of the intermediate picture of the first bitstream and encapsulating the first spatiotemporal unit of the intermediate pictures of the first bitstream into a first tile or sub-picture track;

means for selecting a second spatiotemporal unit of the intermediate picture of the second bitstream and encapsulating the second spatiotemporal unit of the intermediate pictures of the second bitstream into a second tile or sub-picture track;

means for providing an indication and an identifier of a first group of tile or sub- picture tracks that are alternatives for extraction, the first group of tile or sub-picture tracks comprising the first and second tile or sub-picture tracks;

means for creating a first set of samples into a collector track, the first set of samples natively comprising the set of shared coded pictures for the time instance; and means for creating a second set of samples into the collector track and associating the identifier of the first group of tile or sub-picture tracks to the second set of samples, the association intended to be resolved by selecting one of the tile or sub-picture tracks in the first group to be a source of extraction for including the first or the second spatiotemporal unit by reference into the second set of samples.