WO2016185090A1

WO2016185090A1 - An apparatus, a method and a computer program for video coding and decoding

Info

Publication number: WO2016185090A1
Application number: PCT/FI2016/050327
Authority: WO
Inventors: Miska Hannuksela
Original assignee: Nokia Technologies Oy
Priority date: 2015-05-20
Filing date: 2016-05-18
Publication date: 2016-11-24
Also published as: GB2538531A; GB201508620D0

Abstract

A method comprising: obtaining a scalable video bitstream comprising base-layer pictures and enhancement-layer pictures, wherein a first set of base-layer pictures lacks dependencies on a second set of base-layer pictures and the second set of base-layer pictures is not used as a reference for inter-layer prediction of the enhancement-layer pictures;generating a media presentation description on the scalable video bitstream; generating one or more media segments from the scalable video bitstream; and providing, in at least one of the media presentation description and the media segments, one or more first indications for enabling to request the reception of the first set of base-layer pictures separately from the second set of base-layer pictures.

Description

AN APPARATUS, A METHOD AND A COMPUTER PROGRAM FOR VIDEO

CODING AND DECODING

TECHNICAL FIELD

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

BACKGROUND

Recently, Hypertext Transfer Protocol (HTTP) has been widely used for the delivery of realtime 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.

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. The Dynamic Adaptive Streaming over HTTP (DASH) standard has turned out to be a promising suite of protocols and formats for multimedia streaming applications.

The two most recent international video coding standards are known as the Advanced Video Coding (AVC) (also called H.264) and the High Efficiency Video Coding (HEVC) (also called H.265). Both AVC and HEVC also have multiple extensions, including the scalable extensions, known as SVC (scalable video coding) and SHVC (scalable high efficiency video coding).

The use of SVC in DASH has been studied widely, and most of the findings also apply to the use of SHVC in DASH. While SVC reduces the storage space, the downstream bitrate of SVC-coded DASH Representations is increased compared to that of an AVC bitstream with the same resolution and quality. A part of the increased downstream bit rate is caused by a greater number of HTTP GET requests, as HTTP messages have some header overhead. The greater number of HTTP GET requests may also leads to a worse bandwidth utilization. Moreover, the inferior rate-distortion performance of SVC when compared to AVC causes less efficient storage space use in HTTP caches, when a media presentation is requested only once or very few times.

SUMMARY

Now in order to at least alleviate the above problems, an improved method for generating a scalable bitstream is introduced herein. A method according to a first aspect comprises

obtaining a scalable video bitstream comprising base-layer pictures and enhancement-layer pictures, wherein a first set of base-layer pictures lacks dependencies on a second set of base-layer pictures and the second set of base-layer pictures is not used as a reference for inter-layer prediction of the enhancement-layer pictures;

generating a media presentation description on the scalable video bitstream;

generating one or more media segments from the scalable video bitstream; and providing, in at least one of the media presentation description and the media segments, one or more first indications for enabling to request the reception of the first set of base-layer pictures separately from the second set of base-layer pictures.

According to an embodiment, the method further comprises:

providing, in at least one of the media presentation description and the media segments, one or more second indications for concluding that the first set of the base-layer pictures is sufficient for decoding of the enhancement- layer pictures and that the first set of the base-layer pictures is decodable without the second set of the base-layer pictures.

According to an embodiment, said obtaining comprises:

encoding the base-layer pictures such that the first set of base-layer pictures has no dependencies on the second set of base-layer pictures; and

encoding the enhancement- layer pictures such that the first set of base-layer pictures are usable as a reference for inter-layer prediction of the enhancement-layer pictures and that the second set of base-layer pictures is not used as a reference for inter- layer prediction of the enhancement-layer pictures.

According to an embodiment, the first set of base-layer pictures consists of the base-layer pictures of a first set of temporal sub-layers; and the second set of base-layer pictures consists of the base-layer pictures of a second set of temporal sub-layers, wherein the first set of temporal sub-layers differs from the second set of temporal sub-layers.

According to an embodiment, the method further comprises:

associating a first set of Uniform Resource Locators (URLs) with the first set of base-layer pictures; and

including information indicative of the first set of URLs in the media presentation description.

According to an embodiment, the media presentation description complies with MPEG- DASH and the method further comprises:

associating a first Representation described in the media presentation description with the first set of base-layer pictures; and

associating a second Representation described in the media presentation description with the second set of base-layer pictures. According to an embodiment, the method further comprises:

including a first range of the first set of base-layer pictures and a second range of the second set of base-layer pictures into a media segment such that all coded video data of the second range of the first set of base-layer pictures succeeds within the media segment any coded video data of the first range of the first set of base-layer pictures; and

including, in the media segment; information of a byte range covering all coded video data of the first range of the first set of base-layer pictures.

A method according to a second aspect comprises

parsing a media presentation description on a scalable video bitstream;

parsing, from at least one of the media presentation description and a media segment, one or more first indications for enabling to request the reception of a first set of base-layer pictures separately from a second set of base-layer pictures, wherein a first set of base-layer pictures lacks dependencies on a second set of base-layer pictures and the second set of base-layer pictures is not used as a reference for inter-layer prediction of the

enhancement-layer pictures;

requesting, receiving, and decoding coded data from the first set of base-layer pictures on the basis of the one or more first indications;

requesting, receiving, and decoding coded data from the enhancement-layer pictures.

According to an embodiment, the method further comprises:

decoding, from at least one of the media presentation description and the media segments, one or more second indications for concluding that the first set of the base-layer pictures is sufficient for decoding of the enhancement- layer pictures and that the first set of the base-layer pictures is decodable without the second set of the base-layer pictures.

According to an embodiment, the method further comprises:

decoding information indicative of a first set of URLs from the media presentation description, wherein the first set of URLs is associated with the first set of base-layer pictures; and

requesting coded data from the first of base-layer pictures using at least a subset of the first set of URLs.

According to an embodiment, the media presentation description complies with MPEG- DASH, wherein:

a first Representation described in the media presentation description is associated with the first set of base-layer pictures; and

a second Representation described in the media presentation description is associated with the second set of base-layer pictures. According to an embodiment, the method further comprises:

decoding, from the media segment, information of a first byte range covering all coded video data of a first range of the first set of base-layer pictures; and

requesting coded data from the first of base-layer pictures through a byte range request of the first byte range.

Further aspects include at least apparatuses and computer program products/code stored on a non-transitory memory medium arranged to carry out the above methods. BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 shows schematically an electronic device employing embodiments of the invention; Figure 2 shows schematically a user equipment suitable for employing embodiments of the invention;

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

Figure 4 shows schematically an encoder suitable for implementing embodiments of the invention;

Figure 5 shows an example of a hierarchical data model used in DASH;

Figure 6a shows a conceptual illustration of a typical end-to-end system for DASH;

Figures 6b and 6c show examples of delivering DASH bitstreams and SVC-coded DASH bitstreams, respectively, to a client;

Figure 7 shows a flow chart of generating a bitstream according to an embodiment of the invention;

Figure 8 shows an example of arranging temporal sub-layers on a base layer and an enhancement layer according to an embodiment of the invention;

Figure 9 shows a flow chart of decoding a bitstream according to an embodiment of the invention;

Figure 10 shows an example of delivering SHVC-coded DASH bitstreams to a client according to an embodiment of the invention;

Figure 11 shows an example of a multi-view prediction structure according to an embodiment of the invention;

Figure 12 shows an example of a client requesting and receiving only a subset of views of Figure 11 according to an embodiment of the invention;

Figure 13 shows an example of a closed GOP structure encoded in a base layer and an open GOP structure encoded in the enhancement layer according to an embodiment of the invention;

Figure 14 shows the operation of a DASH client when representation up-switching takes place at the segment boundary of Figure 13; Figure 15 shows a schematic diagram of a decoder suitable for implementing embodiments of the invention; and

Figure 16 shows a schematic diagram of an example multimedia communication system within which various embodiments may be implemented.

DETAILED DESCRIPTION OF SOME EXAMPLE EMBODIMENTS

The following describes in further detail suitable apparatus and possible mechanisms for decreasing the downstream bitrate of streaming a scalable video bitstream. 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.

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.

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.

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 40 (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 42 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.

The apparatus 50 may comprise a controller 56 or processor 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.

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. 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).

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.

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.

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

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. 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.

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.

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.

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.

Real-time Transport Protocol (RTP) is widely used for real-time transport of timed media such as audio and video. RTP may operate on top of the User Datagram Protocol (UDP), which in turn may operate on top of the Internet Protocol (IP). RTP is specified in Internet Engineering Task Force (IETF) Request for Comments (RFC) 3550, available from www.ietf.org/rfc/rfc3550.txt. In RTP transport, media data is encapsulated into RTP packets. Typically, each media type or media coding format has a dedicated RTP payload format.

An RTP session is an association among a group of participants communicating with RTP. It is a group communications channel which can potentially carry a number of RTP streams. An RTP stream is a stream of RTP packets comprising media data. An RTP stream is identified by an SSRC belonging to a particular RTP session. SSRC refers to either a synchronization source or a synchronization source identifier that is the 32-bit SSRC field in the RTP packet header. A synchronization source is characterized in that all packets from the synchronization source form part of the same timing and sequence number space, so a receiver may group packets by synchronization source for playback. Examples of synchronization sources include the sender of a stream of packets derived from a signal source such as a microphone or a camera, or an RTP mixer. Each RTP stream is identified by a SSRC that is unique within the RTP session.

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.

Available media file format standards include ISO base media file format (ISO/IEC 14496-12, which may be abbreviated ISOBMFF), MPEG-4 file format (ISO/IEC 14496-14, also known as the MP4 format), file format for NAL unit structured video (ISO/IEC 14496-15) and 3 GPP file format (3 GPP TS 26.244, also known as the 3GP format). ISOBMFF is the base for derivation of all the above mentioned file formats (excluding the ISOBMFF itself). 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. One building block in the ISOBMFF is called a box. Each box may have 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 ISOBMFF may be considered to specify a hierarchical structure of boxes. Each box of the ISO base media file may be identified by a four-character code (4CC, fourCC). A four- character code may interchangeably be represented by a 32-bit unsigned integer (by assuming a certain conversion of characters to 8-bit values, a certain bit endianness, and a certain byte endianness). The header may provide information about the type and size of the box.

According to the ISOBMFF, a file may include media data and metadata that may be enclosed in separate boxes. In an example embodiment, 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 must be present. The movie (moov) box may include one or more tracks, and each track may reside in one corresponding track (trak) box. Each track is associated with a handler, identified by a four-character code, specifying the track type. Video, audio, and image sequence tracks can be collectively called media tracks, and they contain an elementary media stream. Other track types comprise hint tracks and timed metadata tracks. Tracks comprise samples, such as audio or video frames. A media track refers to samples (which may also be referred to as media samples) formatted according to a media compression format (and its encapsulation to the ISOBMFF). A hint track refers to hint samples, containing cookbook instructions for constructing packets for transmission over an indicated communication protocol. The cookbook instructions may include guidance for packet header construction and may include packet payload construction. In the packet payload construction, data residing in other tracks or items may be referenced. As such, for example, data residing in other tracks or items may be indicated by a reference as to which piece of data in a particular track or item is instructed to be copied into a packet during the packet construction process. A timed metadata track may refer to samples describing referred media and/or hint samples. For the presentation of one media type, one media track may be selected.

The 'trak' box contains a Sample Table box. The Sample Table box comprises e.g. all the time and data indexing of the media samples in a track. The Sample Table box is required to contain a Sample Description box. The Sample Description box includes an entry count field, specifying the number of sample entries included in the box. The Sample Description box is required to contain at least one sample entry. The sample entry format depends on the handler type for the track. Sample entries give detailed information about the coding type used and any initialization information needed for that coding.

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. In some examples, the media samples for the movie fragments may reside in an mdat 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.

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 (and hence are similar to chunks). 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 ISOBMFF 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).

A sample grouping in the ISOBMFF and its derivatives, such as the file format for NAL unit structured video (ISO/IEC 14496-15), 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 SampleToGroup box (sbgp box) represents the assignment of samples to sample groups; and (2) a SampleGroupDescription box (sgpd box) contains a sample group entry for each sample group describing the properties of the group. There may be multiple instances of the SampleToGroup and

SampleGroupDescription boxes based on different grouping criteria. These may be distinguished by a type field used to indicate the type of grouping. The 'sbgp' and the 'sgpd' boxes may be linked using the value of grouping type and, in some versions of the boxes, also the value of grouping_type_parameter. The 'sbgp' box indicates the index of the sample group description entry that a particular sample belongs to.

The Matroska file format is capable of (but not limited to) storing any of video, audio, picture, or subtitle tracks in one file. Matroska file extensions include .mkv for video (with subtitles and audio), .mk3d for stereoscopic video, .mka for audio-only files, and .mks for subtitles only. 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. 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). A video encoder may be used to encode an image sequence, as defined subsequently, and a video decoder may be used to decode a coded image sequence. A video encoder or an intra coding part of a video encoder or an image encoder may be used to encode an image, and a video decoder or an inter decoding part of a video decoder or an image decoder may be used to decode a coded image.

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). 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.

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. 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 simplified to encode only one layer or 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.

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. 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. 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. 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.

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.

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.

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).

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). Version 2 of H.265/HEVC included scalable, multiview, and fidelity range extensions, which may be abbreviated SHVC, MV-HEVC, and REXT, respectively. Version 2 of H.265/HEVC was published as ITU-T Recommendation H.265 (10/2014) and as Edition 2 of ISO/IEC 23008-2. There are currently ongoing standardization projects to develop further extensions to H.265/HEVC, including three-dimensional and screen content coding extensions, which may be abbreviated 3D- HEVC and SCC, respectively. 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.

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. 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.

In the description of existing standards as well as in the description of example embodiments, a syntax element may be defined as an element of data represented in the bitstream. A syntax structure may be defined as zero or more syntax elements present together in the bitstream in a specified order. In the description of existing standards as well as in the description of example embodiments, a phrase "by external means" or "through external means" may be used. For example, an entity, such as a syntax structure or a value of a variable used in the decoding process, may be provided "by external means" to the decoding process. The phrase "by external means" may indicate that the entity is not included in the bitstream created by the encoder, but rather conveyed externally from the bitstream for example using a control protocol. It may alternatively or additionally mean that the entity is not created by the encoder, but may be created for example in the player or decoding control logic or alike that is using the decoder. The decoder may have an interface for inputting the external means, such as variable values. 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 referred to as a source picture, and a picture decoded by a decoded may be referred to as a decoded picture.

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).

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 arrays (luma and two chroma) or the array or a single sample of the array that compose a picture in monochrome format.

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.

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.

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.

In H.264/AVC, a macroblock is a 16x16 block of luma samples and the corresponding blocks of chroma samples. For example, in the 4:2:0 sampling pattern, a macroblock contains one 8x8 block of chroma samples per each chroma component. In H.264/AVC, a picture is partitioned to one or more slice groups, and a slice group contains one or more slices. In H.264/AVC, a slice consists of an integer number of macroblocks ordered consecutively in the raster scan within a particular slice group.

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. In some video codecs, such as High Efficiency Video Coding (HEVC) codec, video pictures are divided into coding units (CU) covering the area of the picture. 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. 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. An LCU can be further split into a combination of smaller CUs, e.g. by recursively splitting the LCU and resultant CUs. Each resulting CU typically has at least one PU and at least one TU associated with it. 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).

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. 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.

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. 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 HEVC includes both deblocking and SAO.

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. Typical video codecs enable the use of uni-prediction, where a single prediction block is used for a block being (de)coded, and bi-prediction, where two prediction blocks are combined to form the prediction for a block being (de)coded. Some video codecs enable weighted prediction, where the sample values of the prediction blocks are weighted prior to adding residual information. For example, multiplicative weighting factor and an additive offset which can be applied. In explicit weighted prediction, enabled by some video codecs, a weighting factor and offset may be coded for example in the slice header for each allowable reference picture index. In implicit weighted prediction, enabled by some video codecs, the weighting factors and/or offsets are not coded but are derived e.g. based on the relative picture order count (POC) distances of the reference pictures.

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.

Typical video encoders utilize Lagrangian cost functions to find optimal coding modes, e.g. the desired Macroblock mode and associated motion vectors. This kind of cost function uses a weighting factor λ 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:

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). 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 macroblock or CU may be regarded as unavailable for intra prediction, if the neighboring macroblock or CU resides in a different slice. 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 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 startcode 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. NAL units consist of a header and payload.

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. 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 does not use any picture having a Temporalld greater than TID as inter prediction reference. A sub-layer or a temporal sub- layer may be defined to be a temporal scalable layer 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. NAL units can be categorized into Video Coding Layer (VCL) NAL units and non-VCL NAL units. In H.264/AVC, coded slice NAL units contain syntax elements representing one or more coded macroblocks, each of which corresponds to a block of samples in the

uncompressed picture. In HEVC, VCLNAL units contain syntax elements representing one or more CU.

In HEVC, a coded slice NAL unit can be indicated to be one of the following types: nal unit type Name of Content of NAL unit and RBSP

nal unit type syntax structure

0, TRAIL N, Coded slice segment of a non- 1 TRAIL R TSA, non-STSA trailing picture

slice_segment_layer_rbsp( ) 2, TSA_N, Coded slice segment of a TSA

3 TSA R picture

slice_segment_layer_rbsp( )

4, STSA_N, Coded slice segment of an STSA

5 STSA R picture

slice_layer_rbsp( )

6, RADL N, Coded slice segment of a RADL

7 RADL R picture

slice_layer_rbsp( )

8, RASL N, Coded slice segment of a RASL

9 RASL R, picture

slice_layer_rbsp( )

10, RSV VCL N10 Reserved // reserved non-RAP

12, RSV VCL N12 non-reference VCL NAL unit

14 RSV VCL N14 types

11, RSV VCL Rl 1 Reserved // reserved non-RAP

13, RSV VCL R13 reference VCL NAL unit types

15 RSV VCL R15

16, BLA W LP Coded slice segment of a BLA

17, IDR W RADL picture

18 BLA N LP slice_segment_layer_rbsp( )

19, IDR W RADL Coded slice segment of an IDR

20 IDR N LP picture

slice_segment_layer_rbsp( )

21 CRA NUT Coded slice segment of a CRA

picture

slice_segment_layer_rbsp( )

22, RS V IRAP VCL22.. Reserved // reserved RAP VCL

23 RSV IRAP VCL23 NAL unit types

24..31 RSV VCL24.. Reserved // reserved non-RAP

RSV VCL31 VCL NAL unit types

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.

A Random Access Point (RAP) picture, which may also be referred to as an intra random access point (IRAP) picture, is a picture where each slice or slice segment has nal unit type in the range of 16 to 23, inclusive. A IRAP picture in an independent layer contains only intra-coded slices. An IRAP picture belonging to a predicted layer with nuh layer id value currLayerld may contain P, B, and I slices, cannot use inter prediction from other pictures with nuh layer id equal to currLayerld, 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 with nuh layer id value currLayerld and all subsequent non-RASL pictures with nuh layer id equal to currLayerld in decoding order can be correctly decoded without performing the decoding process of any pictures with nuh layer id equal to currLayerld 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 layer with nuh layer id equal to currLayerld has been initialized (i.e. when LayerInitializedFlag[ refLayerld ] is equal to 1 for refLayerld equal to all nuh layer id values of the direct reference layers of the layer with nuh layer id equal to currLayerld). There may be pictures in a bitstream that contain only intra-coded slices that are not IRAP pictures. In HEVC a CRA picture may be the first picture in the bitstream in decoding order, or may appear later in the bitstream. CRA pictures in HEVC allow so-called leading pictures that follow the CRA picture in decoding order but precede it in output order. Some of the leading pictures, so-called RASL pictures, may use pictures decoded before the CRA picture as a reference. Pictures that follow a CRA picture in both decoding and output order are decodable if random access is performed at the CRA picture, and hence clean random access is achieved similarly to the clean random access functionality of an IDR picture.

A CRA picture may have associated RADL or RASL pictures. When a CRA picture is the first picture in the bitstream in decoding order, the CRA picture is the first picture of a coded video sequence in decoding order, and any associated RASL pictures are not output by the decoder and may not be decodable, as they may contain references to pictures that are not present in the bitstream.

A leading picture is a picture that precedes the associated RAP picture in output order. The associated RAP picture is the previous RAP picture in decoding order (if present). A leading picture is either a RADL picture or a RASL picture.

All RASL pictures are leading pictures of an associated BLA or CRA picture. When the associated RAP picture is a BLA picture or is the first coded picture in the bitstream, the RASL picture is not output and may not be correctly decodable, as the RASL picture may contain references to pictures that are not present in the bitstream. However, a RASL picture can be correctly decoded if the decoding had started from a RAP picture before the associated RAP picture of the RASL picture. RASL pictures are not used as reference pictures for the decoding process of non-RASL pictures. When present, all RASL pictures precede, in decoding order, all trailing pictures of the same associated RAP picture. In some drafts of the HEVC standard, a RASL picture was referred to a Tagged for Discard (TFD) picture.

All RADL pictures are leading pictures. RADL pictures are not used as reference pictures for the decoding process of trailing pictures of the same associated RAP picture. When present, all RADL pictures precede, in decoding order, all trailing pictures of the same associated RAP picture. RADL pictures do not refer to any picture preceding the associated RAP picture in decoding order and can therefore be correctly decoded when the decoding starts from the associated RAP picture.

When a part of a bitstream starting from a CRA picture is included in another bitstream, the RASL pictures associated with the CRA picture might not be correctly decodable, because some of their reference pictures might not be present in the combined bitstream. To make such a splicing operation straightforward, the NAL unit type of the CRA picture can be changed to indicate that it is a BLA picture. The RASL pictures associated with a BLA picture may not be correctly decodable hence are not be output/displayed. Furthermore, the RASL pictures associated with a BLA picture may be omitted from decoding.

A BLA picture may be the first picture in the bitstream in decoding order, or may appear later in the bitstream. Each BLA picture begins a new coded video sequence, and has similar effect on the decoding process as an IDR picture. However, a BLA picture contains syntax elements that specify a non-empty reference picture set. When a BLA picture has nal unit type equal to BLA W LP, it may have associated RASL pictures, which are not output by the decoder and may not be decodable, as they may contain references to pictures that are not present in the bitstream. When a BLA picture has nal unit type equal to BLA W LP, it may also have associated RADL pictures, which are specified to be decoded. When a BLA picture has nal unit type equal to BLA W RADL, it does not have associated RASL pictures but may have associated RADL pictures, which are specified to be decoded. When a BLA picture has nal unit type equal to BLA N LP, it does not have any associated leading pictures.

An IDR picture having nal unit type equal to IDR N LP does not have associated leading pictures present in the bitstream. An IDR picture having nal unit type equal to IDR W LP does not have associated RASL pictures present in the bitstream, but may have associated RADL pictures in the bitstream.

When the value of nal unit type is equal to TRAIL N, TSA_N, STSA_N, RADL N, RASL N, RSV_VCL_N10, RSV_VCL_N12, or RSV_VCL_N14, the decoded picture is not used as a reference for any other picture of the same temporal sub-layer. That is, in HEVC, when the value of nal unit type is equal to TRAIL N, TSA_N, STSA_N, RADL N, RASL N, RSV_VCL_N10, RSV_VCL_N12, or RSV_VCL_N14, the decoded picture is not included in any of RefPicSetStCurrBefore, RefPicSetStCurrAfter and RefPicSetLtCurr of any picture with the same value of Temporalld. A coded picture with nal unit type equal to TRAIL N, TSA N, STSA N, RADL N, RASL N, RSV VCL N10, RSV VCL N12, or RSV VCL N14 may be discarded without affecting the decodability of other pictures with the same value of Temporalld.

A trailing picture may be defined as a picture that follows the associated RAP picture in output order. Any picture that is a trailing picture does not have nal unit type equal to RADL N, RADL R, RASL N or RASL R. Any picture that is a leading picture may be constrained to precede, in decoding order, all trailing pictures that are associated with the same RAP picture. No RASL pictures are present in the bitstream that are associated with a BLA picture having nal unit type equal to BLA W RADL or BLA N LP. No RADL pictures are present in the bitstream that are associated with a BLA picture having

nal unit type equal to BLA N LP or that are associated with an IDR picture having nal unit type equal to IDR N LP. Any RASL picture associated with a CRA or BLA picture may be constrained to precede any RADL picture associated with the CRA or BLA picture in output order. Any RASL picture associated with a CRA picture may be constrained to follow, in output order, any other RAP picture that precedes the CRA picture in decoding order. In HEVC there are two picture types, the TSA and STSA picture types that can be used to indicate temporal sub-layer switching points. If temporal sub-layers with Temporalld up to N had been decoded until the TSA or STSA picture (exclusive) and the TSA or STSA picture has Temporalld equal to N+l, the TSA or STSA picture enables decoding of all subsequent pictures (in decoding order) having Temporalld equal to N+l . The TSA picture type may impose restrictions on the TSA picture itself and all pictures in the same sub-layer that follow the TSA picture in decoding order. None of these pictures is allowed to use inter prediction from any picture in the same sub-layer that precedes the TSA picture in decoding order. The TSA definition may further impose restrictions on the pictures in higher sub-layers that follow the TSA picture in decoding order. None of these pictures is allowed to refer a picture that precedes the TSA picture in decoding order if that picture belongs to the same or higher sublayer as the TSA picture. TSA pictures have Temporalld greater than 0. The STSA is similar to the TSA picture but does not impose restrictions on the pictures in higher sub-layers that follow the STSA picture in decoding order and hence enable up-switching only onto the sublayer where the STSA picture resides.

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. 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.

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

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. 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. In HEVC, the base VPS may be considered to comprise the video_parameter_set_rbsp( ) syntax structure without the vps_extension( ) syntax structure. The

video_parameter_set_rbsp( ) syntax structure was primarily specified already for HEVC version 1 and includes syntax elements which may be of use for base layer decoding. In HEVC, the VPS extension may be considered to comprise the vps_extension( ) syntax structure. The vps_extension( ) syntax structure was specified in HEVC version 2 primarily for multi-layer extensions and comprises syntax elements which may be of use for decoding of one or more non-base layers, such as syntax elements indicating layer dependency relations. The syntax element max_tid_il_ref_pics_plusl in the VPS extension can be used to indicate that non-IRAP pictures are not used a reference for inter-layer prediction and, if not so, which temporal sub-layers are not used as a reference for inter-layer prediction:

max_tid_il_ref_pics_plusl [ i ][ j ] equal to 0 specifies that non-IRAP pictures with

nuh layer id equal to layer_id_in_nuh[ i ] are not used as source pictures for inter-layer prediction for pictures with nuh layer id equal to layer_id_in_nuh[ j ].

max_tid_il_ref_pics_plusl [ i ][ j ] greater than 0 specifies that pictures with nuh layer id equal to layer_id_in_nuh[ i ] and Temporalld greater than max_tid_il_ref_pics_plusl [ i ][ j ] - 1 are not used as source pictures for inter-layer prediction for pictures with nuh layer id equal to layer_id_in_nuh[ j ]. When not present, the value of max_tid_il_ref_pics_plusl [ i ][ j ] is inferred to be equal to 7.

H.264/AVC and HEVC syntax allows many instances of parameter sets, and each instance is identified with a unique identifier. In order to limit the memory usage needed for parameter sets, the value range for parameter set identifiers has been limited. In H.264/AVC and HEVC, each slice header includes the identifier of the picture parameter set that is active for the decoding of the picture that contains the slice, and each picture parameter set contains the identifier of the active sequence parameter set. Consequently, the transmission of picture and sequence parameter sets does not have to be accurately synchronized with the transmission of slices. Instead, it is sufficient that the active sequence and picture parameter sets are received at any moment before they are referenced, which allows transmission of parameter sets "out- of-band" using a more reliable transmission mechanism compared to the protocols used for the slice data. For example, parameter sets can be included as a parameter in the session description for Real-time Transport Protocol (RTP) sessions. If parameter sets are transmitted in-band, they can be repeated to improve error robustness.

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 ISOBMFF 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. A coded picture is a coded representation of a picture.

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.

It may be required that coded pictures appear in certain order within an access unit. For example a coded picture with nuh layer id equal to nuhLayerldA may be required to precede, in decoding order, all coded pictures with nuh layer id greater than nuhLayerldA in the same access unit. An AU typically contains all the coded pictures that represent the same output time and/or capturing time. 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 HEVC and its current draft extensions, the EOB NAL unit is required to have nuh layer id equal to 0. In HEVC, a coded video sequence (CVS) may be defined, for example, as a sequence of access units that consists, in decoding order, of an IRAP access unit with NoRaslOutputFlag equal to 1 , followed by zero or more access units that are not IRAP access units with

NoRaslOutputFlag equal to 1 , including all subsequent access units up to but not including any subsequent access unit that is an IRAP access unit with NoRaslOutputFlag equal to 1. An IRAP access unit may be defined as an access unit in which the base layer picture is an IRAP picture. The value of NoRaslOutputFlag is equal to 1 for each IDR picture, each BLA picture, and each IRAP picture that is the first picture in that particular layer in the bitstream in decoding order, is the first IRAP picture that follows an end of sequence NAL unit having the same value of nuh layer id in decoding order. In multi-layer HEVC, the value of

NoRaslOutputFlag is equal to 1 for each IRAP picture when its nuh layer id is such that LayerInitializedFlag[ nuh layer id ] is equal to 0 and LayerInitializedFlag[ refLayerld ] is equal to 1 for all values of refLayerld equal to IdDirectRefLayer[ nuh layer id ][ j ], where j is in the range of 0 to NumDirectRefLayers[ nuh layer id ] - 1, inclusive. Otherwise, the value of NoRaslOutputFlag is equal to HandleCraAsBlaFlag. NoRaslOutputFlag equal to 1 has an impact that the RASL pictures associated with the IRAP picture for which the

NoRaslOutputFlag is set are not output by the decoder. There may be means to provide the value of HandleCraAsBlaFlag to the decoder from an external entity, such as a player or a receiver, which may control the decoder. HandleCraAsBlaFlag may be set to 1 for example by a player that seeks to a new position in a bitstream or tunes into a broadcast and starts decoding and then starts decoding from a CRA picture. When HandleCraAsBlaFlag is equal to 1 for a CRA picture, the CRA picture is handled and decoded as if it were a BLA picture. 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. 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. A Structure of Pictures (SOP) may be defined as one or more coded pictures consecutive in decoding order, in which the first coded picture in decoding order is a reference picture at the lowest temporal sub-layer and no coded picture except potentially the first coded picture in decoding order is a RAP picture. All pictures in the previous SOP precede in decoding order all pictures in the current SOP and all pictures in the next SOP succeed in decoding order all pictures in the current SOP. A SOP may represent a hierarchical and repetitive inter prediction structure. The term group of pictures (GOP) may sometimes be used interchangeably with the term SOP and having the same semantics as the semantics of SOP.

The bitstream syntax of H.264/AVC and HEVC indicates whether a particular picture is a reference picture for inter prediction of any other picture. Pictures of any coding type (I, P, B) can be reference pictures or non-reference pictures in H.264/AVC and HEVC.

In HEVC, a reference picture set (RPS) syntax structure and decoding process are used. A reference picture set valid or active for a picture includes all the reference pictures used as reference for the picture and all the reference pictures that are kept marked as "used for reference" for any subsequent pictures in decoding order. There are six subsets of the reference picture set, which are referred to as namely RefPicSetStCurrO (a.k.a.

RefPicSetStCurrBefore), RefPicSetStCurrl (a.k.a. RefPicSetStCurrAfter), RefPicSetStFollO, RefPicSetStFolll, RefPicSetLtCurr, and RefPicSetLtFoll. RefPicSetStFollO and

RefPicSetStFolll may also be considered to form jointly one subset RefPicSetStFoll. The notation of the six subsets is as follows. "Curr" refers to reference pictures that are included in the reference picture lists of the current picture and hence may be used as inter prediction reference for the current picture. "Foil" refers to reference pictures that are not included in the reference picture lists of the current picture but may be used in subsequent pictures in decoding order as reference pictures. "St" refers to short-term reference pictures, which may generally be identified through a certain number of least significant bits of their POC value. "Lt" refers to long-term reference pictures, which are specifically identified and generally have a greater difference of POC values relative to the current picture than what can be represented by the mentioned certain number of least significant bits. "0" refers to those reference pictures that have a smaller POC value than that of the current picture. "1" refers to those reference pictures that have a greater POC value than that of the current picture.

RefPicSetStCurrO, RefPicSetStCurrl , RefPicSetStFoUO and RefPicSetStFolU are collectively referred to as the short-term subset of the reference picture set. RefPicSetLtCurr and

RefPicSetLtFoll are collectively referred to as the long-term subset of the reference picture set. In HEVC, a reference picture set may be specified in a sequence parameter set and taken into use in the slice header through an index to the reference picture set. A reference picture set may also be specified in a slice header. A reference picture set may be coded independently or may be predicted from another reference picture set (known as inter-RPS prediction). In both types of reference picture set coding, a flag (used_by_curr_pic_X_flag) is additionally sent for each reference picture indicating whether the reference picture is used for reference by the current picture (included in a *Curr list) or not (included in a *Foll list). Pictures that are included in the reference picture set used by the current slice are marked as "used for reference", and pictures that are not in the reference picture set used by the current slice are marked as "unused for reference". If the current picture is an IDR picture, RefPicSetStCurrO, RefPicSetStCurrl , RefPicSetStFoUO, RefPicSetStFolU , RefPicSetLtCurr, and

RefPicSetLtFoll are all set to empty.

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 HEVC 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.

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.

A reference picture list, such as reference picture list 0 and reference picture list 1 , is typically 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 Temporalld or alike), or information on the prediction hierarchy such as GOP structure, or any combination thereof. Second, the initial reference picture list may be reordered by reference picture list reordering (RPLR) commands, also known as reference picture list modification syntax structure, which may be contained in slice headers. If reference picture sets are used, the reference picture list 0 may be initialized to contain RefPicSetStCurrO first, followed by RefPicSetStCurrl, followed by RefPicSetLtCurr.

Reference picture list 1 may be initialized to contain RefPicSetStCurrl first, followed by RefPicSetStCurrO. In HEVC, 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. In other words, in HEVC, reference picture list modification is encoded into a syntax structure comprising a loop over each entry in the final reference picture list, where each loop entry is a fixed-length coded index to the initial reference picture list and indicates the picture in ascending position order in the final reference picture list.

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.

In order to represent motion vectors efficiently in bitstreams, motion vectors may be coded differentially with respect to a block-specific predicted motion vector. In many video codecs, the predicted motion vectors are created in a predefined way, for example by calculating the median of the encoded or decoded motion vectors of the adjacent blocks. Another way to create motion vector predictions, sometimes referred to as advanced motion vector prediction (AMVP), 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, the reference index of previously coded/decoded picture can be predicted. The reference index is typically predicted from adjacent blocks and/or co-located blocks in temporal reference picture. Differential coding of motion vectors is typically disabled across slice boundaries.

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.

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.

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.

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.

Bit-depth scalability: Base layer pictures are coded at lower bit-depth (e.g. 8 bits) than enhancement layer pictures (e.g. 10 or 12 bits).

- Dynamic range scalability: Scalable layers represent a different dynamic range and/or images obtained using a different tone mapping function and/or a different optical transfer function.

Chroma format scalability: Base layer pictures provide lower spatial resolution in chroma sample arrays (e.g. coded in 4:2:0 chroma format) than enhancement layer pictures (e.g. 4:4:4 format).

Color gamut scalability: enhancement layer pictures have a richer/broader color representation range than that of the base layer pictures - for example the enhancement layer may have UHDTV (ITU-R BT.2020) color gamut and the base layer may have the ITU-R BT.709 color gamut.

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.

- 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).

Region-of- interest scalability (as described below).

Interlaced-to-progressive scalability (also known as field-to-frame scalability): coded interlaced source content material of the base layer is enhanced with an enhancement layer to represent progressive source content.

Hybrid codec scalability (also known as coding standard scalability): In hybrid codec scalability, the bitstream syntax, semantics and decoding process of the base layer and the enhancement layer are specified in different video coding standards. Thus, base layer pictures are coded according to a different coding standard or format than enhancement layer pictures. For example, the base layer may be coded with

H.264/AVC and an enhancement layer may be coded with an HEVC multi-layer extension. It should be understood that many of the scalability types may be combined and applied together. For example color gamut scalability and bit-depth scalability may be combined.

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, depth, bit-depth, chroma format, and/or color gamut 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. Various technologies for providing three-dimensional (3D) video content are currently investigated and developed. 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. More than two parallel views may be needed for applications which enable viewpoint switching or for autostereoscopic displays which may present a large number of views simultaneously and let the viewers to observe the content from different viewpoints.

A view may be defined as a sequence of pictures representing one camera or viewpoint. The pictures representing a view may also be called view components. In other words, a view component may be defined as a coded representation of a view in a single access unit. In multiview video coding, more than one view is coded in a bitstream. Since views are typically intended to be displayed on stereoscopic or multiview autostrereoscopic display or to be used for other 3D arrangements, they typically represent the same scene and are content-wise partly overlapping although representing different viewpoints to the content. Hence, interview prediction may be utilized in multiview video coding to take advantage of inter- view correlation and improve compression efficiency. One way to realize inter- view prediction is to include one or more decoded pictures of one or more other views in the reference picture list(s) of a picture being coded or decoded residing within a first view. View scalability may refer to such multiview video coding or multiview video bitstreams, which enable removal or omission of one or more coded views, while the resulting bitstream remains conforming and represents video with a smaller number of views than originally. Region of Interest (ROI) coding may be defined to refer to coding a particular region within a video at a higher fidelity.

ROI scalability may be defined as a type of scalability wherein an enhancement layer enhances only part of a reference- layer picture e.g. spatially, quality-wise, in bit-depth, and/or along other scalability dimensions. As ROI scalability may be used together with other types of scalabilities, it may be considered to form a different categorization of scalability types. There exists several different applications for ROI coding with different requirements, which may be realized by using ROI scalability. For example, an enhancement layer can be transmitted to enhance the quality and/or a resolution of a region in the base layer. A decoder receiving both enhancement and base layer bitstream might decode both layers and overlay the decoded pictures on top of each other and display the final picture.

The spatial correspondence of a reference- layer picture and an enhancement-layer picture may be inferred or may be indicated with one or more types of so-called reference layer location offsets. In HEVC, reference layer location offsets may be included in the PPS by the encoder and decoded from the PPS by the decoder. Reference layer location offsets may be used for but are not limited to achieving ROI scalability. Reference layer location offsets may comprise one or more of scaled reference layer offsets, reference region offsets, and resampling phase sets. Scaled reference layer offsets may be considered to specify the horizontal and vertical offsets between the sample in the current picture that is collocated with the top-left luma sample of the reference region in a decoded picture in a reference layer and the horizontal and vertical offsets between the sample in the current picture that is collocated with the bottom-right luma sample of the reference region in a decoded picture in a reference layer. Another way is to consider scaled reference layer offsets to specify the positions of the corner samples of the upsampled reference region relative to the respective corner samples of the enhancement layer picture. The scaled reference layer offset values may be signed.

Reference region offsets may be considered to specify the horizontal and vertical offsets between the top-left luma sample of the reference region in the decoded picture in a reference layer and the top-left luma sample of the same decoded picture as well as the horizontal and vertical offsets between the bottom-right luma sample of the reference region in the decoded picture in a reference layer and the bottom-right luma sample of the same decoded picture. The reference region offset values may be signed. A resampling phase set may be considered to specify the phase offsets used in resampling process of a source picture for inter-layer prediction. Different phase offsets may be provided for luma and chroma components.

Some scalable video coding schemes may require IRAP pictures to be aligned across layers in a manner that either all pictures in an access unit are IRAP pictures or no picture in an access unit is an IRAP picture. Other scalable video coding schemes, such as the multi-layer extensions of HEVC, may allow IRAP pictures that are not aligned, i.e. that one or more pictures in an access unit are IRAP pictures, while one or more other pictures in an access unit are not IRAP pictures. Scalable bitstreams with IRAP pictures or similar that are not aligned across layers may be used for example for providing more frequent IRAP pictures in the base layer, where they may have a smaller coded size due to e.g. a smaller spatial resolution. A process or mechanism for layer- wise start-up of the decoding may be included in a video decoding scheme. Decoders may hence start decoding of a bitstream when a base layer contains an IRAP picture and step-wise start decoding other layers when they contain IRAP pictures. In other words, in a layer- wise start-up of the decoding mechanism or process, decoders progressively increase the number of decoded layers (where layers may represent an enhancement in spatial resolution, quality level, views, additional components such as depth, or a combination) as subsequent pictures from additional enhancement layers are decoded in the decoding process. The progressive increase of the number of decoded layers may be perceived for example as a progressive improvement of picture quality (in case of quality and spatial scalability).

A layer- wise start-up mechanism may generate unavailable pictures for the reference pictures of the first picture in decoding order in a particular enhancement layer. Alternatively, a decoder may omit the decoding of pictures preceding, in decoding order, the IRAP picture from which the decoding of a layer can be started. These pictures that may be omitted may be specifically labeled by the encoder or another entity within the bitstream. For example, one or more specific NAL unit types may be used for them. These pictures, regardless of whether they are specifically marked with a NAL unit type or inferred e.g. by the decoder, may be referred to as cross-layer random access skip (CL-RAS) pictures. The decoder may omit the output of the generated unavailable pictures and the decoded CL-RAS pictures.

Scalability may be enabled in two basic ways. Either by introducing new coding modes for performing prediction of pixel values or syntax from lower layers of the scalable

representation or by placing the lower layer pictures to a reference picture buffer (e.g. a decoded picture buffer, DPB) of the higher layer. The first approach may be more flexible and thus may provide better coding efficiency in most cases. However, the second, reference frame based scalability, approach may be implemented efficiently with minimal changes to single layer codecs while still achieving majority of the coding efficiency gains available. Essentially a reference frame based scalability codec may be implemented by utilizing the same hardware or software implementation for all the layers, just taking care of the DPB management by external means. 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.

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.

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.

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. The types of inter- layer prediction may comprise, but are not limited to, one or more of the following: inter-layer sample prediction, inter-layer motion prediction, inter-layer residual prediction. In inter-layer sample prediction, at least a subset of the reconstructed sample values of a source picture for inter-layer prediction are used as a reference for predicting sample values of the current picture. In inter-layer motion prediction, at least a subset of the motion vectors of a source picture for inter-layer prediction are used as a reference for predicting motion vectors of the current picture. Typically, predicting information on which reference pictures are associated with the motion vectors is also included in inter-layer motion prediction. For example, the reference indices of reference pictures for the motion vectors may be inter-layer predicted and/or the picture order count or any other identification of a reference picture may be inter-layer predicted. In some cases, inter-layer motion prediction may also comprise prediction of block coding mode, header information, block partitioning, and/or other similar parameters. In some cases, coding parameter prediction, such as inter- layer prediction of block partitioning, may be regarded as another type of inter- layer prediction. In inter- layer residual prediction, the prediction error or residual of selected blocks of a source picture for inter-layer prediction is used for predicting the current picture. In multiview-plus-depth coding, such as 3D-HEVC, cross-component inter-layer prediction may be applied, in which a picture of a first type, such as a depth picture, may affect the inter-layer prediction of a picture of a second type, such as a conventional texture picture. For example, disparity-compensated inter-layer sample value and/or motion prediction may be applied, where the disparity may be at least partially derived from a depth picture.

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.

A source picture for inter-layer prediction may be defined as a decoded picture that either is, or is used in deriving, an inter-layer reference picture that may be used as a reference picture for prediction of the current picture. In multi-layer HEVC extensions, an inter-layer reference picture is included in an inter-layer reference picture set of the current picture. An inter-layer reference picture may be defined as a reference picture that may be used for inter-layer prediction of the current picture. In the coding and/or decoding process, the inter-layer reference pictures may be treated as long term reference pictures.

A source picture for inter-layer prediction may be required to be in the same access unit as the current picture. In some cases, e.g. when no resampling, motion field mapping or other inter- layer processing is needed, the source picture for inter-layer prediction and the respective inter-layer reference picture may be identical. In some cases, e.g. when resampling is needed to match the sampling grid of the reference layer to the sampling grid of the layer of the current picture (being encoded or decoded), inter-layer processing is applied to derive an inter-layer reference picture from the source picture for inter-layer prediction. Examples of such inter-layer processing are described in the next paragraphs. Inter-layer sample prediction may be comprise resampling of the sample array(s) of the source picture for inter-layer prediction. The encoder and/or the decoder may derive a horizontal scale factor (e.g. stored in variable ScaleFactorX) and a vertical scale factor (e.g. stored in variable ScaleFactorY) for a pair of an enhancement layer and its reference layer for example based on the reference layer location offsets for the pair. If either or both scale factors are not equal to 1 , the source picture for inter- layer prediction may be resampled to generate an inter- layer reference picture for predicting the enhancement layer picture. The process and/or the filter used for resampling may be pre-defined for example in a coding standard and/or indicated by the encoder in the bitstream (e.g. as an index among pre-defined resampling processes or filters) and/or decoded by the decoder from the bitstream. A different resampling process may be indicated by the encoder and/or decoded by the decoder and/or inferred by the encoder and/or the decoder depending on the values of the scale factor. For example, when both scale factors are less than 1 , a pre-defined downsampling process may be inferred; and when both scale factors are greater than 1 , a pre-defined upsampling process may be inferred. Additionally or alternatively, a different resampling process may be indicated by the encoder and/or decoded by the decoder and/or inferred by the encoder and/or the decoder depending on which sample array is processed. For example, a first resampling process may be inferred to be used for luma sample arrays and a second resampling process may be inferred to be used for chroma sample arrays.

SHVC enables the use of weighted prediction or a color-mapping process based on a 3D lookup table (LUT) for (but not limited to) color gamut scalability. The 3D LUT approach may be described as follows. The sample value range of each color components may be first split into two ranges, forming up to 2x2x2 octants, and then the luma ranges can be further split up to four parts, resulting into up to 8x2x2 octants. Within each octant, a cross color component linear model is applied to perform color mapping. For each octant, four vertices are encoded into and/or decoded from the bitstream to represent a linear model within the octant. The color-mapping table is encoded into and/or decoded from the bitstream separately for each color component. Color mapping may be considered to involve three steps: First, the octant to which a given reference- layer sample triplet (Y, Cb, Cr) belongs is determined. Second, the sample locations of luma and chroma may be aligned through applying a color component adjustment process. Third, the linear mapping specified for the determined octant is applied. The mapping may have cross-component nature, i.e. an input value of one color component may affect the mapped value of another color component. Additionally, if inter- layer resampling is also required, the input to the resampling process is the picture that has been color-mapped. The color-mapping may (but needs not to) map samples of a first bit- depth to samples of another bit-depth.

Inter-layer motion prediction may be realized as follows. A temporal motion vector prediction process, such as TMVP of H.265/HEVC, may be used to exploit the redundancy of motion data between different layers. This may be done as follows: when the decoded base-layer picture is upsampled, the motion data of the base-layer picture is also mapped to the resolution of an enhancement layer. If the enhancement layer picture utilizes motion vector prediction from the base layer picture e.g. with a temporal motion vector prediction mechanism such as TMVP of H.265/HEVC, the corresponding motion vector predictor is originated from the mapped base-layer motion field. This way the correlation between the motion data of different layers may be exploited to improve the coding efficiency of a scalable video coder. In SHVC and/or alike, inter-layer motion prediction may be performed by setting the inter-layer reference picture as the collocated reference picture for TMVP derivation. 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). 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.

A coding standard or system may refer to a term operation point or alike, which may indicate the scalable layers and/or sub-layers under which the decoding operates and/or may be associated with a sub-bitstream that includes the scalable layers and/or sub-layers being decoded. In HEVC, an operation point is defined as bitstream created from another bitstream by operation of the sub-bitstream extraction process with the another bitstream, a target highest Temporalld, and a target layer identifier list as inputs.

The VPS of HEVC specifies layer sets and HRD parameters for these layer sets. A layer set may be used as the target layer identifier list in the sub-bitstream extraction process. In

HEVC, a layer set may be defined as set of layers represented within a bitstream created from another bitstream by operation of the sub-bitstream extraction process with the another bitstream, the target highest Temporalld equal to 6, and the target layer identifier list equal to the layer identifier list associated with the layer set as inputs.

An output layer may be defined as a layer whose decoded pictures are output by the decoding process. The output layers may depend on which subset of the multi-layer bitstream is decoded. The pictures output by the decoding process may be further processed, e.g. a color space conversion from the YUV color space to RGB may be performed, and they may be displayed. However, further processing and/or displaying may be considered to be processes external of the decoder and/or the decoding process and might not take place.

In multi-layer video bitstreams, an operation point definition may include a consideration a target output layer set. For example, an operation point may be defined as a bitstream that is created from another bitstream by operation of the sub-bitstream extraction process with the another bitstream, a target highest temporal sub-layer (e.g. a target highest Temporalld), and a target layer identifier list as inputs, and that is associated with a set of output layers.

Alternatively, another term, such as an output operation point, may be used when referring to an operation point and the associated set of output layers. For example, in MV-HEVC/SHVC, an output operation point may be defined as a bitstream that is created from an input bitstream by operation of the sub-bitstream extraction process with the input bitstream, a target highest Temporalld, and a target layer identifier list as inputs, and that is associated with a set of output layers.

An output layer set (OLS) may be defined as a set of layers consisting of the layers of one of the specified layer sets, where one or more layers in the set of layers are indicated to be output layers. An output layer may be defined as a layer of an output layer set that is output when the decoder and/or the HRD operates using the output layer set as the target output layer set. In MV-HEVC/SHVC, the variable TargetOlsIdx may specify which output layer set is the target output layer set by setting TargetOlsIdx equal to the index of the output layer set that is the target output layer set. A target output layer set may be defined as the output layer set for which the index is equal to TargetOlsIdx. TargetOlsIdxmay be set for example by the HRD and/or may be set by external means, for example by a player or alike through an interface provided by the decoder. In MV-HEVC/SHVC, an output layer may be defined as a layer of an output layer set that is output when TargetOlsIdx is equal to the index of the output layer set.

A sender, a gateway, a client, or alike 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, gateway, client, or alike. Layer up-switching may refer to transmitting additional layer(s) compared to those transmitted prior to the layer up-switching by the sender, gateway, client, or alike, 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, gateway, client, or alike may perform down- and/or up-switching of temporal sub-layers. The sender, gateway client, or alike 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). While a constant set of output layers suits well use cases and bitstreams where the highest layer stays unchanged in each access unit, they may not support use cases where the highest layer changes from one access unit to another. It has therefore been proposed that encoders can specify the use of alternative output layers within the bitstream and in response to the specified use of alternative output layers decoders output a decoded picture from an alternative output layer in the absence of a picture in an output layer within the same access unit. Several possibilities exist how to indicate alternative output layers. For example, as specified in HEVC, the alternative output layer set mechanism may be constrained to be used only for output layer sets containing only one output layer, and an output-layer-set-wise flag (alt_output_layer_flag[ olsldx ] in HEVC) may be used for specifying that any direct or indirect reference layer of the output layer may serve as an alternative output layer for the output layer of the output layer set. When more than one alternative output layer is enabled to be used, it may be specified that the first direct or indirect inter-layer reference picture present in the access unit in descending layer identifier order down to the indicated minimum alternative output layer is output.

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. 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.

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 3 GPP 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). MPEG DASH and 3GP-DASH are technically close to each other and may therefore be collectively referred to as DASH. Some concepts, formats, and operations of DASH are described below as an example of a video streaming system, wherein the embodiments may be implemented. The aspects of the invention are not limited to DASH, but rather the description is given for one possible basis on top of which the invention may be partly or fully realized.

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.

In DASH, hierarchical data model is used to structure media presentation as shown in Figure 5. 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 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.

The DASH MPD complies with Extensible Markup Language (XML) and is therefore specified through elements and attribute as defined in XML. The MPD may be specified using the following conventions: Elements in an XML document may be identified by an upper-case first letter and may appear in bold face as Element. To express that an element Elementl is contained in another element Element2, one may write Element2.Elementl. If an element's name consists of two or more combined words, camel-casing may be used, e.g. ImportantElement. Elements may be present either exactly once, or the minimum and maximum occurrence may be defined by <minOccurs> ... <maxOccurs>. Attributes in an

XML document may be identified by a lower-case first letter as well as they may be preceded by a '@'-sign, e.g. @attribute. To point to a specific attribute @attribute contained in an element Element, one may write Element @attribute. If an attribute's name consists of two or more combined words, camel-casing may be used after the first word, e.g.

@veryImportant Attribute. Attributes may have assigned a status in the XML as mandatory (M), optional (O), optional with default value (OD) and conditionally mandatory (CM).

In DASH, an independent representation may be defined as a representation that can be processed independently of any other representations. An independent representation may be understood to comprise an independent bitstream or an independent layer of a bitstream. A dependent representation may be defined as a representation for which Segments from its complementary representations are necessary for presentation and/or decoding of the contained media content components. A dependent representation may be understood to comprise e.g. a predicted layer of a scalable bitstream. A complementary representation may be defined as a representation which complements at least one dependent representation. A complementary representation may be an independent representation or a dependent representation. Dependent Representations may be described by a Representation element that contains a @dependencyld attribute. Dependent Representations can be regarded as regular Representations except that they depend on a set of complementary Representations for decoding and/or presentation. The @dependencyld contains the values of the @id attribute of all the complementary Representations, i.e. Representations that are necessary to present and/or decode the media content components contained in this dependent

Representation.

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.

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.

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. 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.

Sub-Representations are embedded in regular Representations and are described by the

SubRepresentation element. SubRepresentation elements are contained in a

Representation element. The SubRepresentation element describes properties of one or several media content components that are embedded in the Representation. It may for example describe the exact properties of an embedded audio component (e.g., codec, sampling rate, etc.), an embedded sub-title (e.g., codec) or it may describe some embedded lower quality video layer (e.g. some lower frame rate, etc.). Sub-Representations and

Representation share some common attributes and elements. In case the @level attribute is present in the SubRepresentation element, the following applies:

Sub-Representations provide the ability for accessing a lower quality version of the Representation in which they are contained. In this case, Sub-Representations for example allow extracting the audio track in a multiplexed Representation or may allow for efficient fast-forward or rewind operations if provided with lower frame rate; The Initialization Segment and/or the Media Segments and/or the Index Segments shall provide sufficient information such that the data can be easily accessed through HTTP partial GET requests. The details on providing such information are defined by the media format in use.

When ISOBMFF Segments are used, the following applies:

o The Initialization Segment contains the Level Assignment box.

o The Subsegment Index box ('ssix') is present for each Subsegment. o The attribute @level specifies the level to which the described Sub- Representation is associated to in the Subsegment Index. The information in Representation, Sub-Representation and in the Level Assignment ('leva') box contains information on the assignment of media data to levels, o Media data should have an order such that each level provides an enhancement compared to the lower levels. If the @level attribute is absent, then the SubRepresentation element is solely used to provide a more detailed description for media streams that are embedded in the Representation.

The ISOBMFF includes the so-called level mechanism to specify subsets of the file. Levels follow the dependency hierarchy so that samples mapped to level n may depend on any samples of levels m, where m <= n, and do not depend on any samples of levels p, where p > n. For example, levels can be specified according to temporal sub-layer (e.g., temporal ld of SVC or MVC or Temporalld of HEVC). Levels may be announced in the Level Assignment ('leva') box contained in the Movie Extends ('mvex') box. Levels cannot be specified for the initial movie. When the Level Assignment box is present, it applies to all movie fragments subsequent to the initial movie. For the context of the Level Assignment box, a fraction is defined to consist of one or more Movie Fragment boxes and the associated Media Data boxes, possibly including only an initial part of the last Media Data Box. Within a fraction, data for each level appears contiguously. Data for levels within a fraction appears in increasing order of level value. All data in a fraction shall be assigned to levels. The Level Assignment box provides a mapping from features, such as scalability layers or temporal sublayers, to levels. A feature can be specified through a track, a sub-track within a track, or a sample grouping of a track. For example, the Temporal Level sample grouping may be used to indicate a mapping of the pictures to temporal levels, which are equivalent to temporal sublayers in HEVC. That is, HEVC pictures of a certain Temporalld value may be mapped to the a particular temporal level using the Temporal Level sample grouping (and the same can be repeated for all Temporalld values). The Level Assignment box can then refer to the

Temporal Level sample grouping in the indicated mapping to levels. The Level Assignment box includes the syntax element padding flag. padding flag is equal to 1 indicates that a conforming fraction can be formed by concatenating any positive integer number of levels within a fraction and padding the last Media Data box by zero bytes up to the full size that is indicated in the header of the last Media Data box. For example, padding flag can be set equal to 1 when each fraction contains two or more AVC, SVC, or MVC tracks of the same video bitstream, the samples for each track of a fraction are contiguous and in decoding order in a Media Data box, and the samples of the first AVC, SVC, or MVC level contain extractor NAL units for including the video coding NAL units from the other levels of the same fraction.

The Subsegment Index box ('ssix') provides a mapping from levels (as specified by the Level Assignment box) to byte ranges of the indexed subsegment. In other words, this box provides a compact index for how the data in a subsegment is ordered according to levels into partial subsegments. It enables a client to easily access data for partial subsegments by downloading ranges of data in the subsegment. When the Subsegment Index box is present, each byte in the subsegment is assigned to a level. If the range is not associated with any information in the level assignment, then any level that is not included in the level assignment may be used. There is 0 or 1 Subsegment Index boxes present per each Segment Index box that indexes only leaf subsegments, i.e. that only indexes subsegments but no segment indexes. A

Subsegment Index box, if any, is the next box after the associated Segment Index box. A Subsegment Index box documents the subsegment that is indicated in the immediately preceding Segment Index box. Each level may be assigned to exactly one partial subsegment, i.e. byte ranges for one level are contiguous. Levels of partial subsegments are assigned by increasing numbers within a subsegment, i.e., samples of a partial subsegment may depend on any samples of preceding partial subsegments in the same subsegment, but not the other way around. For example, each partial subsegment contains samples having an identical temporal sub-layer and partial subsegments appear in increasing temporal sub-layer order within the subsegment. When a partial subsegment is accessed in this way, the final Media Data box may be incomplete, that is, less data is accessed than the length indication of the Media Data Box indicates is present. The length of the Media Data box may need adjusting, or padding may be used. The padding flag in the Level Assignment Box indicates whether this missing data can be replaced by zeros. If not, the sample data for samples assigned to levels that are not accessed is not present.

It may be required that for any dependent Representation X that depends on complementary Representation Y, the m-th Subsegment of X and the n-t Subsegment of Y shall be non- overlapping whenever m is not equal to n. It may be required that for dependent

Representations the concatenation of the Initialization Segment with the sequence of

Subsegments of the dependent Representations, each being preceded by the corresponding Subsegment of each of the complementary Representations in order as provided in the @dependencyld attribute shall represent a conforming Subsegment sequence conforming to the media format as specified in the @mimeType attribute for this dependent Representation.

MPEG-DASH defines segment-container formats for both ISOBMFF and MPEG-2 Transport Streams. Other specifications may specify segment formats based on other container formats. For example, a segment format based on Matroska container file format has been proposed and may be summarized as follows. When Matroska files are carried as DASH segments or alike, the association of DASH units and Matroska units may be specified as follows. A subsegment (of DASH) may be are defined as one or more consecutive Clusters of Matroska- encapsulated content. An Initialization Segment of DASH may be required to comprise the EBML header, Segment header (of Matroska), Segment Information (of Matroska) and Tracks, and may optionally comprise other level 1 elements and padding. A Segment Index of DASH may comprise a Cues Element of Matroska.

DASH supports rate adaptation by dynamically requesting Media Segments from different Representations within an Adaptation Set to match varying network bandwidth. When a DASH client switches up/down Representation, coding dependencies within Representation have to be taken into account. A Representation switch may only happen at a random access point (RAP), which is typically used in video coding techniques such as H.264/AVC. In DASH, a more general concept named Stream Access Point (SAP) is introduced to provide a codec-independent solution for accessing a Representation and switching between

Representations. In DASH, a SAP is specified as a position in a Representation that enables playback of a media stream to be started using only the information contained in

Representation data starting from that position onwards (preceded by initialising data in the Initialisation Segment, if any). Hence, Representation switching can be performed in SAP. Several types of SAP have been specified, including the following. SAP Type 1 corresponds to what is known in some coding schemes as a "Closed GOP random access point" (in which all pictures, in decoding order, can be correctly decoded, resulting in a continuous time sequence of correctly decoded pictures with no gaps) and in addition the first picture in decoding order is also the first picture in presentation order. SAP Type 2 corresponds to what is known in some coding schemes as a "Closed GOP random access point" (in which all pictures, in decoding order, can be correctly decoded, resulting in a continuous time sequence of correctly decoded pictures with no gaps), for which the first picture in decoding order may not be the first picture in presentation order. SAP Type 3 corresponds to what is known in some coding schemes as an "Open GOP random access point", in which there may be some pictures in decoding order that cannot be correctly decoded and have presentation times less than intra-coded picture associated with the SAP.

As described above, the client or player may request Segments or Subsegments to be transmitted from different representations similarly to how the transmitted layers and/or sublayers of a scalable video bitstream may be determined. Terms representation down-switching or bitstream down-switching may refer to requesting or transmitting a lower bitrate representation than what was requested or transmitted (respectively) previously. Terms representation up-switching or bitstream up-switching may refer to requesting or transmitting a higher bitrate representation than what was requested or transmitted (respectively) previously. Terms representation switching or bitstream switching may refer collectively to representation or bitstream up- and down-switching and may also or alternatively cover switching of representations or bitstreams of different viewpoints.

Streaming systems similar to MPEG-DASH include for example HTTP Live Streaming (a.k.a. HLS), specified in the IETF Internet Draft draft-pantos-http-live-streaming-13 (and other versions of the same Internet Draft). As a manifest format corresponding to the MPD, HLS uses an extended M3U format. M3U is a file format for multimedia playlists, originally developed for audio files. An M3U Playlist is a text file that consists of individual lines, and each line is a URI, blank, or starts with the character '#' indicating a tag or a comment. A URI line identifies a media segment or a Playlist file. Tags begin with #EXT. The HLS

specification specifies a number of tags, which may be regarded as key-value pairs. The value part of tags may comprise an attribute list, which is a comma-separated list of attribute-value pairs, where an attribute-value pair may be considered to have the syntax

AttributeName= Attribute Value. Hence, tags of HLS M3U8 files may be considered similar to Elements in MPD or XML, and attributes of HLS M3U8 files may be considered similar to Attributes in MPD or XML. Media segments in HLS are formatted according to the MPEG-2 Transport Stream and contain a single MPEG-2 Program. Each media segment is

recommended to start with a Program Association Table (PAT) and a Program Map Table (PMT).

Figure 6a provides a conceptual illustration of a typical end-to-end system for DASH. The media content is provided by an origin server, which is typically a conventional web (HTTP) server. The origin server may be connected with a Content Delivery Network (CDN) over which the streamed content is delivered to and stored in edge servers. The MPD allows signaling of multiple base URLs for the content, which can be used to announce the availability of the content in different edge servers. Alternatively, the content server may be directly connected to the Internet. Web proxies may reside on the path of routing the HTTP traffic between the DASH clients and the origin or edge server from which the content is requested. Web proxies cache HTTP messages and hence can serve clients' requests with the cached content. They are commonly used by network service providers, since they reduce the required network bandwidth from the proxy towards origin or edge servers. For end-users HTTP caching provides shorter latency. DASH clients are connected to the Internet through an access network, such as a mobile cellular network.

The term downstream may be defined as the direction from a server to the client. In the context of DASH, the server may be e.g. an origin server, an edge server of a CDN, or a web proxy server, and the client may be a DASH client. The term downstream bitrate in DASH may therefore refer to the bitrate needed or used to convey streamed media content to a DASH client.

As explained above, DASH and other similar streaming systems provide an attractive protocol and/or formats for multimedia streaming applications. Figure 6b illustrates a conventional manner of facilitating DASH by encoding multiple versions of the same original content, for example with different spatial resolutions. In Figure 6b, two versions (high- resolution and low-resolution) are offered and hence need to be stored in a storage system connected with an origin server. The client device dynamically chooses one of the versions. In Figure 6b, the client chooses the high-resolution (HR) version, and hence the downstream bitrate equals to the bitrate of the HR version. Note that the term access network in Figure 6b may be considered to include the Internet. In some cases, the system of Figure 6b may comprise one or more web proxies, which may serve the client at least a subset of the requested Segments instead of the edge server or the origin server. In some cases, the system of Figure 6a may exclude the CDN and the edge server. Figure 6c illustrates an example of using scalable video coding in DASH. The asserted benefits of scalable video coding include reduced storage space. In other words, the total byte count of the base layer (BL) and the enhancement layer (EL) is less than the total byte count of the single-layer streams of the respective resolutions. Another asserted benefit of scalable video coding is that since all clients will receive the BL part of the bitstream, it is more likely to be cached on an edge server (assuming automatic non-selective caching, e.g. regular HTTP proxy caches) or is more useful to be pushed to an edge server (assuming an active forwarding mechanism in a CDN) compared to the low-resolution stream of the same resolution in conventional non-scalable streaming. Similarly to Figure 6b, it is noted that the term access network in Figure 6c may be considered to include the Internet. In some cases, the system of Figure 6c may comprise one or more web proxies, which may serve the client at least a subset of the requested Segments instead of the edge server or the origin server. In some cases, the system of Figure 6c may exclude the CDN and the edge server. However, disadvantages of streaming of scalable video include an increased downstream bitrate (in the high-resolution streaming). In other words, the total bitrate of BL and EL together is greater than the bitrate of the single-layer high-resolution stream. Another disadvantage of scalable video coding is the increased decoding complexity in terms of both computational complexity and memory usage. It is noted that for clients accessing the BL only, there is no difference in the rate-distortion (RD) performance regardless whether scalable coding or multiple versions are used. Now in order to at least alleviate the above disadvantages, an enhanced method for encapsulating a scalable video bitstream for streaming is presented hereinafter. The method may comprise generating a scalable bitstream or a scalable bitstream may be provided for the method. In the bitstream encapsulating method, which is disclosed in Figure 7, a scalable video bitstream comprising base-layer pictures and enhancement- layer pictures is obtained (700), wherein a first set of base-layer pictures lacks dependencies on a second set of base-layer pictures and the second set of base-layer pictures is not used as a reference for inter- layer prediction of the enhancement-layer pictures. The scalable video bitstream may be obtained through encoding an uncompressed video sequence or the scalable video bitstream may be provided for or received by the encapsulating method. A media presentation description is generated (702) on the scalable video bitstream, and one or more media segments are generated (704) from the scalable video bitstream. In at least one of the media presentation description and the media segments, one or more first indications are provided (706) for enabling to request the reception of the first set of base-layer pictures separately from the second set of base-layer pictures.

Thus, a scalable bitstream is encoded in a manner that only a first subset of the base-layer pictures may be used as a reference for inter-layer prediction, while the remaining base-layer pictures are not used as a reference for inter- layer prediction. For example, SHVC or MV- HEVC bitstreams may have more than one temporal sub-layer and the base layer be created so that only certain, but not all temporal sub-layers may be used as a reference for inter-layer prediction (i.e., the value of max_tid_il_ref_pics_plusl for the base layer is less than the number of temporal sub-layers in the base layer).

The first subset of the base-layer pictures is arranged in segments in a manner that enables DASH clients to access only that subset of the base layer. The subset of the base layer may also be referred to as thinned base layer (thinned BL). For example, if only certain (but not all) temporal sub-layers may be used a reference for inter-layer prediction, clients need to obtain only those temporal sub-layers for correct decoding of the enhancement layer. Figure 8 presents an example where the temporal sub-layers 0 and 1 may be used for inter- layer prediction, while temporal sub-layers 2 and 3 are not used for inter-layer prediction. The first subset of the base-layer pictures therefore consists of the base-layer pictures at temporal sub- layers 0 and 1.

One or more indications are generated into the media presentation description and/or media segments to facilitate DASH clients

to conclude that for decoding of the enhancement layer only the first subset of the base-layer pictures is required; and

- to request only the first subset of the base-layer pictures.

Hence, a decoding operation of such bitstream for obtaining the enhancement layer but not the base layer may comprise, as disclosed in Figure 9, parsing (900) a media presentation description on a scalable video bitstream; parsing (902), from at least one of the media presentation description and a media segment, one or more indications for enabling to request the reception of a first set of base-layer pictures separately from a second set of base-layer pictures, wherein a first set of base-layer pictures lacks dependencies on a second set of base- layer pictures and the second set of base-layer pictures is not used as a reference for inter- layer prediction of the enhancement- layer pictures; requesting, receiving, and decoding (904) coded data from the first set of base-layer pictures on the basis of the one or more indications; and requesting, receiving, and decoding (906) coded data from the enhancement- layer pictures.

When receiving a multi-layer HEVC bitstream over DASH, a DASH client may need to choose which Representations and/or Sub-Representations are received and correspondingly which output layer set or operation point is consumed. In addition to the information in the MPD (e.g. the spatial resolution), the client may use information in the MIME type of the associated Representation and/or Sub-Representation. The output layer set and highest Temporalld corresponding to the Representations or Sub-Representations may be obtained by the client e.g. from the MIME type for example as optional MIME parameters. The output layer set being consumed may be for example such that only the enhancement layer is an output layer, while the base layer is not an output layer.

Consequently, a DASH client that decodes and displays the enhancement layer but does not display the base layer may operate as follows:

1) one or more indications are parsed indicating that for decoding of the enhancement layer only the first subset of the base-layer pictures is required;

2) one or more HTTP URLs to request the first subset of the base-layer pictures are

resolved on the basis of one or more indications;

3) the first subset of the base-layer pictures is obtained and decoded using the one or more HTTP URLs; and 4) the enhancement- layer pictures are decoded, wherein the first subset of the base-layer pictures may be used.

As a consequence, the downstream bitrate consists of the bitrate of the first subset of the base- layer pictures (i.e. thinned BL) and of the enhancement-layer pictures as illustrated in Figure 10. While this is still somewhat higher than the downstream bitrate for a single-layer non- scalable bitstream (with a spatial resolution and other characteristics corresponding to the enhancement layer), the achieved bitrate is lower than that for conventional streaming of scalable video bitstreams. Moreover, the achieved storage space is significantly than that for the respective multiple single-layer non-scalable bitstreams and only moderately higher than for conventional scalable bitstreams. Moreover, caching and cache hit ratio of the base layer is improved, as at least a subset of the base layer is requested by all clients unlike when multiple single-layer bitstreams is used. Similarly to Figures 6b and 6c, it is noted that the term access network in Figure 10 may be considered to include the Internet. In some cases, the system of Figure 10 may comprise one or more web proxies, which may serve the client at least a subset of the requested Segments instead of the edge server or the origin server. In some cases, the system of Figure 10 may exclude the CDN and the edge server.

In the following, various options for encoding a scalable video bitstream are presented.

According to an embodiment, the first set of base-layer pictures consists of pictures of certain picture type(s); and the second set of base-layer pictures consists of pictures that have a different picture type than said certain picture type(s). It may be indicated, e.g. by an encoder, in the bitstream which picture types belong to said certain picture type(s) or it may be pre- defined e.g. in a coding standard which picture types belong to said certain picture type(s). It may also be indicated, e.g. by an encoder, in the bitstream that pictures of said certain picture type(s) may be used as a reference for inter-layer prediction and/or that pictures not having said certain picture type(s) are not used as a reference for inter-layer prediction. Alternatively, it may be pre-defined e.g. in a coding standard that pictures of said certain picture type(s) may be used as a reference for inter- layer prediction and/or that pictures not having said certain picture type(s) are not used as a reference for inter-layer prediction.

According to an embodiment, the first set of base-layer pictures consists of IRAP pictures; and the second set of base-layer pictures consists of non-IRAP pictures. An encoder for a multi-layer HEVC extension may indicate that IRAP pictures may be used as a reference for inter-layer prediction while other pictures are not used as a reference for inter-layer prediction by including max_tid_il_ref_pics_plusl equal to 0 in the bitstream for the pair of the base layer and the enhancement layer.

The SHVC enables picture-wise indications for inter-layer prediction. Therein, a discardable picture is defined such that it is neither used as a reference picture for inter prediction (within the same layer) nor as a reference picture for inter-layer prediction.

A non-output-layer skip (NOLS) picture may be used as a reference for inter prediction of subsequent NOLS or discardable pictures in decoding order until the next unconstrained reference picture in decoding order with the same or lower Temporalld value than that of the current NOLS picture. Moreover, the NOLS picture is not used as a reference for inter-layer prediction and not used as a reference for inter prediction of any unconstrained reference picture or intra-layer non-reference picture. According to an embodiment, the second set of base-layer pictures comprises discardable and/or NOLS pictures and the first set of base-layer pictures consists of the remaining base- layer pictures. It is noted that this does not preclude some discardable and/or NOLS pictures being present in the first set of base-layer pictures. It needs to be understood that encoding may also be performed by combining some of the above embodiments. For example, the second set of base-layer pictures may comprise a second set of temporal sub-layers as well as discardable and/or NOLS pictures regardless of their temporal sub-layer. According to an embodiment, a bitstream is encoded so that the base layer contains more than one sub-layer and inter-layer prediction is applied adaptively as follows. The cost of turning off inter-layer prediction from a BL picture to the EL picture (e.g. in terms of additional bitrate) and all the pictures depending on the current picture may be approximated or derived. The benefit of turning off inter-layer prediction from a BL picture to the EL picture in downstream bitrate may be approximated or derived, assuming that the respective BL picture and the pictures depending on it reside in the second set of base-layer pictures. In some cases, multi-pass encoding may be performed, where said cost and benefit may be approximated or derived e.g. for each access unit sequence- or GOP-wise after which the final encoding or bitstream construction may be performed. In some cases, the cost and benefit may be approximated or derived for the current EL picture being encoded and a selection to turn inter-layer prediction on or off may be performed e.g. based on empirical threshold(s). As a result, the formation of the first set of BL pictures and the second set of BL pictures may be performed in smaller units than entire sub-layers of the base layer. According to an embodiment, the enhancement-layer pictures have the same temporal sublayer structure (i.e., Temporalld values) as the base-layer pictures of the respective access units. According to another embodiment, the enhancement- layer pictures may have different temporal sub-layer structure (i.e., Temporalld values) as the base-layer pictures of the respective access units, even though the (de)coding order of pictures may remain the same in the base layer and in the enhancement layer. In the following, various options for arranging the Media Segments, Representations, and the related signaling are presented.

Separate Representations

According to an embodiment, a first Representation is formed from the first subset of the base layer, and a second Representation the remaining subset of the base layer. A third

Representation is formed from the enhancement layer. In this case, the enhancement- layer Representation is indicated in the MPD to depend on the first Representation but not the second Representation. That may be done by setting the @dependencyld attribute of the second Representation and the third Representation to point to the value of the @id attribute of the first Representation.

When the Representations are arranged as described above, a client that consumes the base layer requests segments of both the first Representation and the second Representation. A client that consumes (displays) the enhancement layer requests segments of both the first Representation and the third Representation, but not of the second Representation.

According to an embodiment, Segments of the second Representation are arranged to include coded data from the first Representation by reference. For example, extractors as specified in ISO/IEC 14496-15 may be used this purpose. An extractor specified in ISO/IEC 14496-15 comprises an enumerated track reference (to indicate the track containing extracted data, i.e. here the track of the first Representation, containing the pictures of the first base layer representation) and a decoding time difference (to indicate a file format sample in the referenced track to the decoding time of the current file format sample, e.g. here a sample of a track containing the picturs of the second set of base layer pictures). An extractor specified in ISO/IEC 14496-15 also comprises an indicated byte range from the referred sample of the referred track (e.g. the track containing the base layer) by reference into the track containing the extractor.

It is noted that the first Representation may also be used for fast-forward playback, i.e.

playback at a pace that is faster than real-time, because it contains pictures at a relatively low picture rate but most decoders can decode these pictures at least at typical video picture rates (e.g. 25 or 30 Hz), hence resulting into a fast playback.

Single Representation, separate levels

According to an embodiment, a first level, as specified in ISOBMFF, is formed from the first subset of the base layer, and a second level, as specified in ISOBMFF, is formed from the second subset of the base layer. The enhancement layer is included as a third level in the same Representation as the base layer. In this case, the SubRepresentation element of the DASH MPD can be used to indicate the dependencies between levels. It is indicated e.g. in the MPD that the enhancement-layer depends on the first level but not the second level. This may be done using the @dependencyLevel attribute of the SubRepresentation element of the DASH MPD, which specifies the set of Sub-Representations within this Representation that this Sub- Representation depends on in the decoding and/or presentation process as a whitespace- separated list of @level values. Consequently, the @dependencyLevel attribute of the SubRepresentation element for the second level is set in the MPD to be equal to the @level attribute of the SubRepresentation element for the first level, and the same applies also to the @dependencyLevel attribute of the SubRepresentation element for the third level.

When the Representations and Segments are arranged as described above, a client may operate as follows to fetch coded data of a Segment:

1) The client issues an HTTP GET request with a byte range such that the initial Segment Index box, the first Subsegment Index box, and the first Movie Fragment box (as well as potentially some media data), and subsequently receives them.

2) It parses from the Subsegment Index box the byte ranges of associated with different levels and issues HTTP GET requests with byte ranges accordingly. A client that consumes the base layer issues requests for the byte ranges of the first and second level, and a client that consumes the enhancement layer issues requests for the byte ranges of the first and third level (but not the second level).

3) If there are subsequent Subsegments within the Segment, the client parses from the Segment Index box the byte offset of the next Subsegment and issues an HTTP GET request with a byte range such that the next Segment Index box, the respective Subsegment Index box, and the respective Movie Fragment box (as well as potentially some media data) and subsequently receives them. Then, it continues from step 2.

Representation with separate levels for the base layer, a dependent Representation for the enhancement layer

According to an embodiment, the base layer is included in a first Representation and the enhancement layer is included in a second Representation. A first level, as specified in ISOBMFF, is formed from the first subset of the base layer, and a second level, as specified in ISOBMFF, is formed from the second subset of the base layer.

The DASH MPD is appended to indicate which levels of the first Representation the second Representation depends on. For example, the @dependencyLevel attribute for the

Representation element may be specified and may be allowed to be present only of

@dependencyld is present for the same Representation element. When the

@dependencyLevel attribute is absent and @dependencyld is present, the dependent

Representation may depend on any levels of the complementary Representation.

Consequently, the amendment of @dependencyLevel for the Representation element is compatible with older DASH versions and implementations. When the @dependencyLevel attribute is present it may be defined to contain a comma-separated list of sub-representation dependencies. The number of list items may be required to be the same as the number of values in the @dependencyld attribute, and a list item with index i provides the sub- representation dependencies for the Representation with @id equal to the list item with index i of the @dependencyld attribute. For example, the @dependencyld and the

@dependencyLevel of the second Representation may be set as follows:

@dependencyld="repl" <!-- the first Representation— >

@dependencyLevel="0" <!-- the first level within the first Representation— >

A more complex example (not related to the presented coded data arrangement in this embodiment) may be:

@dependencyld="repl rep2"

@dependencyLevel="0, 0 1" <!~depends on level 0 of repl and levels 0 and 1 of rep2— >

When the Representations and Segments are arranged as described above, a client that consumes the enhancement layer may operate as follows to fetch coded data of a Segment:

1) The client issues an HTTP GET request with a byte range such that the initial Segment Index box, the first Subsegment Index box, and the first Movie Fragment box (as well as potentially some media data) of the first Representation and subsequently receives them.

2) It parses from the Subsegment Index box the byte ranges of associated with different levels and issues HTTP GET requests with byte ranges accordingly. A client issues requests for the byte ranges of the first level (but not the second level).

3) If there are subsequent Subsegments within the Segment, the client parses from the Segment Index box the byte offset of the next Subsegment and issues an HTTP GET request with a byte range such that the next Segment Index box, the respective

Subsegment Index box, and the respective Movie Fragment box (as well as potentially some media data) and subsequently receives them. Then, it continues from step 2.

In parallel (with a separate TCP connection) or sequentially (with the same TCP connection), the client also requests data of the enhancement-layer Representation.

In the embodiments described above, any type of scalabilility and scalable coding may be used with the invention, including e.g. SNR scalability (picture quality scalability), spatial scalability, bit-depth scalability, color gamut scalability, dynamic range scalability, region-of- interest (ROI) scalability, view scalability (e.g. view selection of inter-view predicted bitstream), and depth scalability (e.g. enhancing a color image with the respective depth map). In an example embodiment, multiview video may be encoded in a manner that inter- view prediction is enabled between selected views in a constrained manner as described in various embodiments, e.g. within access units of certain temporal sub-layers (e.g. up to and including sub-layer N) and disabled in access units of other temporal sub-layers (e.g. sub-layers above sub-layer N). The first set of base-layer pictures may thus include a subset of the base view that may be used as reference for inter- view prediction, and the second set of base-layer pictures may thus include a subset of the base view that is not used as reference for inter- view prediction. In an embodiment in which the multiview video is prepared is described above, the first set of base layer pictures as well as the non-base view may be received and decoded in order to display a non-base view that is inter- view-predicted from the base view. If the base view is displayed too or switching of the displayed view from the non-base view to the base view takes place, the second set of base layer pictures may be received (in addition to the first set). In case of said switching of the displayed view, the second set of base layer pictures may be requested and received for such a time range of the base view that has been already buffered in the client so as to display the base view with full picture rate as early as possible.

Above, embodiments were described with two layers (base and enhancement layer).

Embodiments can be similarly realized with any number of layers greater than two. The base layer may in general be an independent layer and needs not be the base layer (layer 0 in the bitstream).

According to an embodiment, an encoder receives as input pictures of a multitude of views (where the pictures of a view may be the pictures obtained with a particular camera in a multi- camera capturing equipment or arrangement). The encoder encodes the multitude of views into one or more bitstreams. The views may, but need not, be coded with inter-view prediction. The base quality layer for at least some of the views (which may or may not itself be inter- view predicted) may be arranged into a first set of pictures and a second set of pictures according to other embodiments. An enhanced quality layer may be available for each view or for some views, e.g. providing SNR, spatial, bit-depth, color gamut, dynamic range, and/or ROI scalability in relation to the respective base quality layer. This arrangement is illustrated in Figure 11. In the figure, all views and all layers have the same temporal inter prediction structure, even though the temporal prediction is illustrated only for two layers in the figure. In the figure, three temporal sub-layers (0 to 2) are present and inter-layer prediction takes place only from temporal sub-layer 0 and is not done in temporal sub-layers 1 and 2. In the figure, inter-view prediction is not applied between the enhanced quality layers, although the example could be applied similarly to that case too.

Similarly to other embodiments, a media presentation description is generated on said one or more bitstreams, and one or more media segments are generated from said one or more bitstreams. In at least one of the media presentation description and the media segments, one or more first indications are provided for enabling to request the reception of the first set of pictures of the base quality separately of a particular view from the second set of pictures of the base quality of the same view.

According to an embodiment, at least two views are received of one or more bitstreams (as described above), e.g. for displaying at a stereoscopic or multiview display arrangement, such as a head-mounted display (a.k.a. virtual reality headset). The at least two views may be received for example using DASH or a similar approach. The at least two views may be received e.g. in parallel, e.g. using a separate TCP connection for each of the at least two views, or segment-wise sequentially, e.g. receiving a Segment of a first Representation (representing a first view) first, followed by receiving a Segment of the respective time range and of a second Representation (representing a second view). The at least two views may be a subset of the multitude of view being available, and the at least two views selected for reception may dynamically change e.g. based on the users head and/or gaze direction when using a head-mounted display. The at least two views may also comprise views that are not presently displayed for a user but which may be displayed if the user e.g. changes the head direction. At least for some of the views an enhancement quality layer is requested and received by a client. For example, the enhancement quality layer may be requested and received for those views that are presently displayed. Only the first set of pictures may be requested and received for the base quality layer of those views for which an enhanced quality layer is also received. In the example of Figure 12 the enhanced quality layers of views 2 and 3 (out of views 0 to 3) are received. The received pictures are illustrated with black outline and font, whereas the non-received pictures are illustrated with gray outline and text.

Consequently, only the temporal sub-layer 0 of the base quality layers is received, whereas all temporal sub-layers are received for views 2 and 3 of the enhanced quality layers, while no pictures of the enhanced quality layers of views 0 and 1 are received.

In an embodiment, which may be applied together with or independently of other

embodiments, a DASH client according to the implementation handles intra pictures starting open GOPs, such as CRA pictures in HEVC, as follows: When a first segment of an independent representation is received while the previous segment of that independent representation is not received (e.g. due to representation switching, seeking, or tuning in to a live stream), the DASH client instructs the decoding of the first segment to omit the decoding or outputting of the leading pictures of an open GOP starting the segment, if any. For example, in case of HEVC decoding, the DASH client may set HandleCraAsBlaFlag equal to 1 for the decoding process. The DASH client can (but need not) exclude the leading pictures, such as RASL pictures in HEVC, of the initial open GOP starting from the segment that is provided to the decoder. When a first segment of a dependent representation is received while the previous segment of that dependent representation is not received (e.g. due to

representation up-switching), the DASH client instructs the decoding to omit the decoding or outputting of the leading pictures of an open GOP starting the segment and/or to apply layer- wise startup. For example, in case of multi-layer HEVC extensions, the DASH client may set HandleCraAsBlaFlag equal to 1 for the enhancement layer decoding process. The DASH client can (but need not) exclude the CL-RAS pictures and/or the leading pictures, such as RASL pictures in HEVC, of the initial open GOP starting from the segment that is provided to the decoder. In an embodiment, which may be applied together with or independently of other embodiments, a closed GOP structure is encoded in a reference layer, such as the base layer, while the respective pictures in the enhancement layer are encoded with an open GOP structure. This is illustrated in Figure 13 for multi-layer HEVC encoding, but can be similarly performed with other video coding schemes too. Figure 13 illustrates a two-layer bitstream, where each layer is assumed to form its own Representation. Such a bitstream facilitates seamless representation switching, even though an open GOP structure is used in the enhancement layer (i.e. in a dependent representation). As described above, the DASH client can instruct the decoding not to output the RASL pictures and CL-RAS pictures, if any.

Consequently the RASL pictures and the CL-RAS pictures, if any, of the initial CRA picture of the enhancement layer will not get output by the decoder. Instead, the decoder can output the respective pictures from the base layer, and hence the up-switching operation is seamless. If the alternative output layer mechanism or alike is in use, the decoder is required to output the respective pictures from the base layer. Representation down-switching also happens seamlessly due to closed GOP structure. It is noted that other embodiments can be realized by encoding an open GOP structure in the enhancement layer while omitting the encoding of an IRAP picture or alike from the base layer, i.e. the IRAP pictures or alike need not be aligned across layers. In this case, representation down-switching is seamless due to multi-loop decoding and representation up-switching happens similarly to what is described above but typically involves both CL-RAS and RASL pictures being omitted from output.

In an embodiment, the base layer is encoded to comprise a first set of base-layer pictures lacks dependencies on a second set of base-layer pictures and a second set of base-layer pictures is not used as a reference for inter-layer prediction of the enhancement-layer pictures according to other embodiments. However, base-layer pictures corresponding to CL-RAS pictures, if any, and those leading pictures, such as the RADL pictures in the case of HEVC, of the base layer of the first GOP of a segment are also included in the first set regardless whether they may be used for inter-layer prediction or are not used for inter-layer prediction. This requirement of the first set and second set of base-layer pictures may be pre-defined, for example in a standard, or it may be indicated, e.g. in an MPD, that Representations or Sub- Representations follow such partitioning to the first and second set of base-layer pictures. In Figure 13, base-layer pictures with Temporalld (TID) equal to 0 may be used as reference for inter-layer prediction, while base-layer pictures with Temporalld greater than 0 are not used as reference for inter-layer prediction. Hence, in the example of Figure 13, the first set of base-layer pictures comprises the base-layer pictures with Temporalld equal to 0 and the

RADL pictures that immediately follow a Segment boundary in output order, and the second set of base-layer pictures comprises the base-layer pictures that are on in the first set. Figure 14 illustrates the operation of a DASH client when representation up-switching takes place at the illustrated segment boundary. While the DASH client has received (and decoded) the first set and the second set of base-layer pictures for the segment immediately preceding the illustrated segment boundary, the DASH client receives the first set of base-layer pictures (but not the second set of base-layer pictures) and the enhancement-layer pictures of the segment starting from the illustrated segment boundary. As the first set of base-layer pictures of that segment contains the initial RADL pictures and as the respective RASL pictures are not output by the decoding process, the RADL pictures are decoded and output by the decoding process. Similarly, as the first set of base-layer pictures of that segment contains the BL pictures corresponding to CL-RAS pictures, if any, and as the respective CL-RAS pictures are not output by the decoding process, these BL pictures are decoded and output by the decoding process. It is noted that other embodiments can be realized by decoding an open GOP structure in the enhancement layer while the base layer need not contain respective IRAP pictures or alike, i.e. the IRAP pictures or alike need not be aligned across layers. It is also noted that other embodiments can be realized similarly when the base layer segment is received in its entirety. In this case, constrains on using the base-layer pictures as reference for inter-layer prediction need not be applied in encoding.

In an embodiment, which may be applied together with or independently of other

embodiments, an encoder, an MPD creator, a segment encapsulator, and/or alike may determine that an EL CRA picture is a SAP of type 1 or 2, when the EL is an output layer of an OLS associated with the Representation comprising the EL and when the reference layers of the EL are alternative output layers. On the basis of said determination, related SAP signaling in the MPD and/or in the Media Segments (e.g. within a Segment Index box) may be created.

The use of scalable video coding in DASH, both in conventional manner and in the manner described above, is beneficial compared to multiple single-layer non-scalable bitstreams when it comes to flexibility in down-switching. In the MPEG-DASH group, there has been identified a need to achieve segment-independent SAP (stream access point) signaling. One of the key targets is to achieve lower end-to-end latency for streaming of live events. For that purpose, segments of shorter duration are planned to be used, but in order to avoid a penalty in compression efficiency, not all segments are planned to start with a SAP (which typically corresponds to an intra coded picture or, in the HEVC context, an IRAP picture).

Consequently, in the case of multiple single-layer non-scalable bitstreams, Representation down-switching is possible only at the beginning of those Segments that start with a SAP. However, in the case of scalable video coding, the base layer is consistently received. When multi-loop decoding of the scalable bitstream is performed, down-switching is enabled at the beginning of any Segment regardless of whether it starts with a SAP. Consequently, it is likely that the client can target at a lower buffer occupancy level in the case of scalable video bitstreams because a faster reaction to throughput reaction can be made than in the case of multiple single-layer bitstreams. Consequently, a lower end-to-end latency is achieved.

When compared to multiple single-layer non-scalable bitstreams, the proposed solution provides a good trade-off that

- reduces the storage space needs in origin and edge servers,

improves the cache hit ratio (for edge servers and proxy caches), requires only a moderate (e.g. less than 10%) downstream bitrate increase, and requires only a moderate additional decoding complexity (for decoding of BL at relative low picture rate). Regarding the options for transmission connections and protocols, the embodiments may be realized with various protocols enabling to request segments or sub-segments, including but not limited to different versions of HTTP, such as HTTP/1.1 and HTTP/2.0, WebSockets, and SPDY. It also needs to be understood that the embodiments may be applied in cases where no requests, such as HTTP GET, are made by a client, e.g. when a client receives data through a broadcast, such as a broadcast service according to 3 GPP Multimedia Broadcast/Multicast Service (MBMS). Furthermore, it needs to be understood that while the term HTTP URL is used in some embodiments, the protocol part of the URL may similarly indicate another protocol, such as HTTPS. The embodiments may be realized with various configurations to establish and use TCP connections, including but not limited to the following.

As discussed above a separate TCP connection may be used for downloading the

enhancement layer segments. In the case that bandwidth rapidly decreases and there appears a danger of a pause in playback, this enables easier and faster termination of the reception of the enhancement layer segments.

Alternatively or additionally, the same TCP connection may be used for base-layer and enhancement-layer segments. In that case, the HTTP GET requests may be pipelined. The client may select the order of HTTP GET requests in a manner that a greater duration of base- layer segments gets buffers in the client (compared to the buffered duration of enhancement- layer segments). It is also possible thatthe duration of enhancement layer segments or sub- segments is shorter than that of the base-layer segments or sub-segments in order to be able to react more quickly in throughput changes.

Embodiments may be realized with various types of access links and link layer protocols and their configurations for access links. For example, if QoS specified by 3GPP is in use, the TCP connection for enhancement layer segments could run on a best-effort QoS while the base layer segments could have a guaranteed QoS channel.

Figure 15 shows a block diagram of a video decoder suitable for employing embodiments of the invention. Figure 15 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. The video decoder 550 comprises a first decoder section 552 for base view components and a second decoder section 554 for non-base view components. Block 556 illustrates a demultiplexer for delivering information regarding base view components to the first decoder section 552 and for delivering information regarding non-base view components 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 (I'n). Reference R'n stands for a final reconstructed image. Blocks 703, 803 illustrate inverse transform ( ¹). 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 view/non- base view components to obtain the preliminary reconstructed images (I'n). Preliminary reconstructed and filtered base view images may be output 709 from the first decoder section 552 and preliminary reconstructed and filtered base view images may be output 809 from the first decoder section 554. 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.

Figure 16 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 preprocessing, 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.

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.

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.

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. 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 system includes one or more receivers 1560, typically capable of receiving, demodulating, 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. 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 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 renderer 1590 may reside in the same physical device or they may be included in separate devices.

A sender 1540 and/or a gateway 1550 may be configured to perform switching between different representations e.g. for 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. 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.

A decoder 1580 may be configured to perform switching between different representations e.g. for 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 580 is multi-tasking and uses computing resources for other purposes than decoding the scalable 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. The speed of decoder operation may be changed during the decoding or playback for example as response to changing from a fast-forward play from normal playback rate or vice versa, and consequently multiple layer up-switching and layer down- switching operations may take place in various orders.

In the above, example embodiments have been described in the context of multi-layer HEVC extensions, such as SHVC and MV-HEVC. It needs to be understood that embodiments could be similarly realized in any other multi-layer coding scenario. Some descriptions above specifically refer to SHVC or MV-HEVC or both, while it needs to be understood that the descriptions could similarly refer to any multi-layer HEVC extension or any other multi-layer coding scenario. Some descriptions above refer to HEVC as a collective term to include the base version of the HEVC standard and all extensions of the HEVC standard, i.e. the HEVC version 1, single-layer extensions (e.g. REXT, screen content coding), and multi-layer extensions (MV-HEVC, SHVC, 3D-HEVC). In the above, some terms are interchangeably referred sometimes with an upper-case initial and other times with a lower-case initial. For example, terms Representation and

representation are interchangeable in the DASH context.

In the above, some embodiments have been described in relation to DASH or MPEG-DASH. It needs to be understood that embodiments could be similarly realized with any other similar streaming system, and/or any similar protocols as those used in DASH, and/or any similar segment and/or manifest formats as those used in DASH, and/or any similar client operation as that of a DASH client. The embodiments specifically referring to the MPD may be similarly realized with any other manifest or presentation description format.

In the above, some embodiments have been described in relation to ISOBMFF-based segment format. It needs to be understood that embodiments could be similarly realized with any other segment format, such as a Matroska-based segment format or an MPEG2-TS-based segment format.

Some embodiments have been described with reference to segments. It is to be understood that those embodiments could be similarly described with reference to subsegments.

In the above, some embodiments have been described in relation to a low-resolution representation or bitstream and to a high-resolution representation or bitstream, indicating the spatial resolution of the representations differ and consequently that resampling in terms of spatial resolution is used in the embodiments. It needs to be understood that in addition to or instead of differing spatial resolution, the representations may have other types of relation or relations and may require another type of resampling or inter-layer process or processes. For example the bit depth and/or color gamut of the representations may differ, and resampling similar to that used in SHVC color gamut scalability may be used in the embodiments.

Embodiments are not limited to one type of resampling, but for example resampling in terms of spatial, bit-depth, and color gamut may be applied together. In the above, some embodiments have been described with a two-layer bitstream. It needs to be understood that embodiments can be similarly realized with a bitstream having more than two layers. Moreover, while some embodiments have been described in relation to the term base layer, embodiments can be realized similarly by referring to the term independent layer. It also needs to be understood that a predicted layer may have more than one direct reference layer. Embodiments can be similarly realized in such cases too. Furthermore, it needs to be understood that the partitioning to the first set of pictures and the second set of pictures of a reference layer may be carried out differently depending on which predicted layer uses the reference layer. For example, a first enhancement layer may enable inter-layer prediction from the base-layer pictures with Temporalld equal to 0 and 1 (and omit inter-layer prediction from pictures with other Temporalld values), while a second enhancment layer may enable inter-layer prediction from the base-layer pictures with Temporalld equal to 0 (and omit inter- layer prediction from pictures with other Temporalld values). The base layer may be divided into three sets of pictures, set A consisting of pictures with Temporalld equal to 0, set B consisting of pictures with Temporalld equal to 1, and set C consisting of pictures with Temporalld greater than 1. For the first enhancement layer, the first set of base-layer pictures in different embodiments may be considered to contain sets A and B, and the second set of base-layer pictures in different embodiments may be considered to contain set C. Likewise, for the second enhancement layer, the first set of base-layer pictures in different embodiments may be considered to contain set A, and the second set of base-layer pictures in different embodiments may be considered to contain sets B and C. In the above, where the example embodiments have been described with reference to adaptive streaming over HTTP, it needs to be understood that embodiments could be similarly realized with other transport mechanisms, such as:

Real-time Transport Protocol (RTP): a first RTP stream may carry the first set of base- layer pictures, a second RTP stream may carry the second set of base-layer pictures, and a third RTP may carry the enhancement-layer pictures. When displaying the enhancement-layer pictures, the receiver may choose to receive and/or the sender or gateway may choose to transmit only the first RTP stream and the third RTP stream. MPEG-2 transport stream (TS): Similarly to above, the first set of base-layer pictures, the second set of base-layer pictures and the enhancement layer pictures may each be associated with a different packet identifier (PID) value. The receiver may choose to process only the PIDs of the first set of base-layer pictures and the enhancement layer pictures.

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. 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.

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

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.

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.

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.

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.

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.

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 the claims.

Claims

CLAIMS:

1. A method comprising:

generating a media presentation description on the scalable video bitstream;

2. A method of claim 1, further comprising:

3. A method of claim 1, wherein said obtaining comprises:

4. A method of claim 1, wherein

the first set of base-layer pictures consists of the base-layer pictures of a first set of temporal sub-layers; and

the second set of base-layer pictures consists of the base-layer pictures of a second set of temporal sub-layers, wherein the first set of temporal sub-layers differs from the second set of temporal sub-layers.

5. A method of claim 1, further comprising:

6. A method of claim 5, wherein the media presentation description complies with MPEG- DASH and the method further comprises: associating a first Representation described in the media presentation description with the first set of base-layer pictures; and

associating a second Representation described in the media presentation description with the second set of base-layer pictures.

7. A method of claim 1, further comprising:

8. An apparatus comprising

at least one processor and at least one memory, said at least one memory stored with code thereon, which when executed by said at least one processor, causes the apparatus to perform at least

generating a media presentation description on the scalable video bitstream;

9. An apparatus according to claim 8, further comprising code causing the apparatus to perform:

10. An apparatus of claim 8, further comprising code causing the apparatus to perform said obtaining by:

11. An apparatus of claim 8, wherein

12. An apparatus of claim 8, further comprising code causing the apparatus to perform:

13. An apparatus of claim 12, wherein the media presentation description complies with MPEG-DASH and the apparatus further comprises code causing the apparatus to perform:

14. An apparatus of claim 8, further comprising code causing the apparatus to perform:

15. A computer readable storage medium stored with code thereon for use by an apparatus, which when executed by a processor, causes the apparatus to perform:

generating a media presentation description on the scalable video bitstream; generating one or more media segments from the scalable video bitstream; and providing, in at least one of the media presentation description and the media segments, one or more first indications for enabling to request the reception of the first set of base-layer pictures separately from the second set of base-layer pictures.

16. A method comprising:

parsing a media presentation description on a scalable video bitstream;

enhancement-layer pictures;

17. A method of claim 16, further comprising:

18. A method of claim 16, further comprising:

19. A method of claim 18, wherein the media presentation description complies with MPEG- DASH, wherein:

a second Representation described in the media presentation description is associated with the second set of base-layer pictures.

20. A method of claim 16, further comprising:

21. An apparatus comprising

parsing a media presentation description on a scalable video bitstream;

enhancement-layer pictures;

22. An apparatus of claim 21, further comprising code causing the apparatus to perform:

23. An apparatus of claim 21, further comprising code causing the apparatus to perform:

24. An apparatus of claim 23, wherein the media presentation description complies with MPEG-DASH, wherein:

25. An apparatus of claim 21, further comprising code causing the apparatus to perform:

26. A computer readable storage medium stored with code thereon for use by an apparatus, which when executed by a processor, causes the apparatus to perform:

parsing a media presentation description on a scalable video bitstream;

enhancement-layer pictures;