WO2025214699A1 - An apparatus, a method and a computer program for video coding and decoding - Google Patents

Abstract

A method comprising: inferring or indicating a processing order comprising multiple processing stages; inferring or indicating a noninitial processing stage among the multiple processing stages; encoding a first indication that the noninitial processing stage is invoked to one or more indicated pictures; and associating the first indication with at least one reconstructed picture, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

Description

AN APPARATUS, A METHOD AND A COMPUTER PROGRAM FOR VIDEO CODING AND DECODING

TECHNICAL FIELD

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

BACKGROUND

[0002] Video coding specifications may enable the use of supplemental enhancement information (SEI) messages or alike. Several SEI messages are specified in H.264/AVC, H.265/HEVC, H.266/VVC, and H.274/VSEI standards, and the user data SEI messages enable organizations and companies to specify SEI messages for their own use. The standards may contain the syntax and semantics for the specified SEI messages, which the encoders are required to follow, but the decoders might not be required to process SEI messages for output order conformance. An SEI processing order (SPO) SEI message carries information indicating the preferred processing order, as determined by the encoder (i.e., the content producer), for different types of SEI messages that may be present in the bitstream.

[0003] In video coding, multiple picture processing stages are typically performed in cascade. The processing stages may include, for example, filtering for different purposes, film grain synthesis, and/or color conversion. One example of such processing stage is frame rate upsampling filtering, where a frame rate upsampling filter generates or interpolates one or more pictures between a pair of pictures given as input to the filter. [0004] Thus, the multiple picture processing stages may include a first processing stage, which processes, e.g. interpolates or extrapolates, pictures temporally, and a second later processing stage, which is intended to be applied to some but not necessarily all the interpolated or extrapolated pictures. Using the SPO SEI messages, the second processing stage may generally be invoked or activated by an SPO SEI message that is associated with a coded picture or a respective decoded picture. However, it is not possible to invoke or activate the second processing stage specifically for interpolated or extrapolated pictures. SUMMARY

[0005] Now, an improved method and technical equipment implementing the method has been invented, by which the above problems are alleviated. Various aspects include a method, an apparatus and a computer readable medium comprising a computer program, or a signal stored therein, which are characterized by what is stated in the independent claims. Various details of the embodiments are disclosed in the dependent claims and in the corresponding images and description.

[0006] The scope of protection sought for various embodiments of the invention is set out by the independent claims. The embodiments and features, if any, described in this specification that do not fall under the scope of the independent claims are to be interpreted as examples useful for understanding various embodiments of the invention.

[0007] According to a first aspect, there is provided an apparatus comprising means for inferring or indicating a processing order comprising multiple processing stages; means for inferring or indicating a noninitial processing stage among the multiple processing stages; means for encoding a first indication that the noninitial processing stage is invoked to one or more indicated pictures; and means for associating the first indication with at least one reconstructed picture, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

[0008] According to an embodiment, the noninitial processing stage is preceded by a preceding processing stage in the processing order, and the preceding processing stage involves interpolating or extrapolating one or more generated pictures based on at least one reconstructed picture.

[0009] According to an embodiment, the first indication further indicates that the noninitial processing stage is invoked for the one or more generated pictures.

[0010] According to an embodiment, the apparatus comprises means for deriving a candidate picture list comprising the one or more generated pictures; and means for encoding, in or along the first indication, a second indication that indicates a first picture among the candidate picture list to which the noninitial processing stage is invoked.

[0011] According to an embodiment, the candidate picture list comprises the at least one reconstructed picture and/or at least one processed reconstructed picture resulting from processing stages preceding the noninitial processing stage. [0012] According to an embodiment, the apparatus comprises means for indicating the processing order through indicating enabled and/or disabled in-loop filters.

[0013] According to an embodiment, the means for deriving the candidate picture list further comprises means for initiating the candidate picture list with pictures present in the decoded picture buffer, and for each processing stage in the processing order, means for replacing each of those pictures in the candidate picture list with a coinciding processed reconstructed picture resulting from the processing stage, if any, and means for inserting interpolated pictures and extrapolated pictures, if any, into the candidate picture list and placing pictures in the derived candidate picture list in an output order.

[0014] According to an embodiment, the candidate picture list comprises the pictures present in the decoded picture buffer in a pre-defined order, such as output order.

[0015] According to an embodiment, the apparatus comprises means for encoding the first indication in a syntax structure with normative decoding, such as a picture parameter set, a picture header, a slice header, a picture trailer data unit, or a reference picture marking update data unit.

[0016] According to an embodiment, the apparatus comprises means for associating the first indication with a reconstructed picture by referring to the syntax structure from the coded picture or including the syntax structure in the coded picture, wherein decoding of the coded picture results into the reconstructed picture.

[0017] As a second aspect, there is provided 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: infer or indicate a processing order comprising multiple processing stages; infer or indicate a noninitial processing stage among the multiple processing stages; encode a first indication that the noninitial processing stage is invoked to one or more indicated pictures; and associate the first indication with at least one reconstructed picture, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

[0018] A method according to a third aspect comprises inferring or indicating a processing order comprising multiple processing stages; inferring or indicating a noninitial processing stage among the multiple processing stages; encoding a first indication that the noninitial processing stage is invoked to one or more indicated pictures; and associating the first indication with at least one reconstructed picture, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

[0019] An apparatus according to a fourth aspect comprises means for identifying a processing order comprising multiple processing stages; means for identifying a noninitial processing stage among the multiple processing stages; means for decoding a first indication that at least one noninitial processing stage is invoked to one or more indicated pictures; and means for determining at least one reconstructed picture associated with the first indication, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

[0020] According to an embodiment, the noninitial processing stage is preceded by a preceding processing stage in the processing order, and the preceding processing stage involves interpolating or extrapolating one or more generated pictures based on at least one reconstructed picture.

[0021] According to an embodiment, the first indication further indicates that the noninitial processing stage is invoked for the one or more generated pictures.

[0022] According to an embodiment, the apparatus comprises means for deriving a candidate picture list comprising the one or more generated pictures; and means for decoding, from or along the first indication, a second indication that indicates a first picture among the candidate picture list to which the noninitial processing stage is invoked.

[0023] According to an embodiment, the candidate picture list comprises the at least one reconstructed picture and/or at least one processed reconstructed picture resulting from processing stages preceding the noninitial processing stage.

[0024] According to an embodiment, the apparatus comprises means for identifying the processing order through identifying enabled and/or disabled in-loop filters.

[0025] According to an embodiment, the means for deriving the candidate picture list further comprises means for initiating the candidate picture list with pictures present in the decoded picture buffer, and for each processing stage in the processing order, means for replacing each of those pictures in the candidate picture list with a coinciding processed reconstructed picture resulting from the processing stage, if any, and means for inserting interpolated pictures and extrapolated pictures, if any, into the candidate picture list and placing pictures in the derived candidate picture list in an output order. [0026] According to an embodiment, the candidate picture list comprises the pictures present in the decoded picture buffer in a pre-defined order, such as output order.

[0027] According to an embodiment, the apparatus comprises means for decoding the first indication in a syntax structure with normative decoding, such as a picture parameter set, a picture header, a slice header, a picture trailer data unit, or a reference picture marking update data unit.

[0028] According to an embodiment, the apparatus comprises means for determining the first indication being associated with a reconstructed picture by referring to the syntax structure from the coded picture or including the syntax structure in the coded picture, wherein decoding of the coded picture results into the reconstructed picture.

[0029] An apparatus according to a fifth aspect 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: identify a processing order comprising multiple processing stages; identify a noninitial processing stage among the multiple processing stages; decode a first indication that at least one noninitial processing stage is invoked to one or more indicated pictures; and determine at least one reconstructed picture associated with the first indication, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

[0030] A method according to a sixth aspect comprises identifying a processing order comprising multiple processing stages; identifying a noninitial processing stage among the multiple processing stages; decoding a first indication that at least one noninitial processing stage is invoked to one or more indicated pictures; and determining at least one reconstructed picture associated with the first indication, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

[0031] Computer readable storage media according to further aspects comprise code for use by an apparatus, which when executed by a processor, causes the apparatus to perform the above methods. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] For a more complete understanding of the example embodiments, reference is now made to the following descriptions taken in connection with the accompanying drawings in which:

[0033] Figures la and lb show schematically an encoder and a decoder, respectively, suitable for implementing embodiments of the invention;

[0034] Figure 2 shows a flow chart of a method according to an embodiment;

[0035] Figure 3 shows an example of indicating an invocation of interpolation or extrapolation of a picture to be used as an out picture or a reference picture according to an embodiment;

[0036] Figure 4 shows schematically an electronic device suitable for employing embodiments of the invention;

[0037] Figure 5 shows schematically a user equipment suitable for employing embodiments of the invention;

[0038] Figure 6 further shows schematically electronic devices suitable for employing embodiments of the invention connected using wireless and wired network connections; [0039] Figure 7 shows a schematic diagram of an example multimedia communication system within which various embodiments may be implemented.

DETAILED DESCRIPTON OF SOME EXAMPLE EMBODIMENTS

[0040] Video codec comprises an encoder that transforms the input video into a compressed representation suited for storage/transmission and a decoder that can decompress the compressed video representation back into a viewable form. An encoder may discard some information in the original video sequence in order to represent the video in a more compact form (that is, at lower bitrate).

[0041] 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).

[0042] In temporal prediction, the sources of prediction are previously decoded pictures (a.k.a. reference pictures). In intra block copy (IBC; a.k.a. intra-block-copy prediction), prediction is applied similarly to temporal prediction, but the reference picture is the current picture and only previously decoded samples can be referred in the prediction process. Inter-layer or inter-view prediction may be applied similarly to temporal prediction, but the reference picture is a decoded picture from another scalable layer or from another view, respectively. In some cases, inter prediction may refer to temporal prediction only, while in other cases inter prediction may refer collectively to temporal prediction and any of intra block copy, inter-layer prediction, and inter-view prediction provided that they are performed with the same or similar process than temporal prediction. Inter prediction or temporal prediction may sometimes be referred to as motion compensation or motion-compensated prediction.

[0043] Motion compensation can be performed either with full sample or sub-sample accuracy. In the case of full sample accurate motion compensation, motion can be represented as a motion vector with integer values for horizontal and vertical displacement and the motion compensation process effectively copies samples from the reference picture using those displacements. In the case of sub-sample accurate motion compensation, motion vectors are represented by fractional or decimal values for the horizontal and vertical components of the motion vector. In the case a motion vector is referring to a noninteger position in the reference picture, a sub-sample interpolation process is typically invoked to calculate predicted sample values based on the reference samples and the selected sub-sample position. The sub-sample interpolation process typically consists of horizontal filtering compensating for horizontal offsets with respect to full sample positions followed by vertical filtering compensating for vertical offsets with respect to full sample positions. However, the vertical processing can also be done before horizontal processing in some environments.

[0044] 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.

[0045] 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 inpicture prediction.

[0046] Figs, la and lb show an encoder and a decoder suitable for employing embodiments of the invention. A video codec consists of an encoder that transforms an input video into a compressed representation suited for storage/transmission and a decoder that can decompress the compressed video representation back into a viewable form. Typically, the encoder discards and/or loses some information in the original video sequence in order to represent the video in a more compact form (that is, at lower bitrate). An example of an encoding process is illustrated in Figure la. Figure 4a illustrates an image to be encoded (Iⁿ); a predicted representation of an image block (P'ⁿ); a prediction error signal (Dⁿ); a reconstructed prediction error signal (D'ⁿ); a preliminary reconstructed image (I'ⁿ); a final reconstructed image (R'ⁿ); a transform (T) and inverse transform (T’¹); a quantization (Q) and inverse quantization (Q ¹); entropy encoding (E); a reference frame memory (RFM); inter prediction (Pinter); intra prediction (Pintra); mode selection (MS) and filtering (F).

[0047] An example of a decoding process is illustrated in Figure lb. Figure lb illustrates a predicted representation of an image block (P'ⁿ); a reconstructed prediction error signal (D'ⁿ); a preliminary reconstructed image (I'ⁿ); a final reconstructed image (R'ⁿ); an inverse transform an inverse quantization (Q ¹ ); an entropy decoding (E'¹); a reference frame memory (RFM); a prediction (either inter or intra) (P); and filtering (F). [0048] Many hybrid video encoders 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). Video codecs may also provide a transform skip mode, which the encoders may choose to use. In the transform skip mode, the prediction error is coded in a sample domain, for example by deriving a sample-wise difference value relative to certain adjacent samples and coding the sample-wise difference value with an entropy coder.

[0049] Entropy coding/decoding may be performed in many ways. For example, context-based coding/decoding may be applied, where in both the encoder and the decoder modify the context state of a coding parameter based on previously coded/decoded coding parameters. Context-based coding may for example be context adaptive binary arithmetic coding (CABAC) or context-based variable length coding (CAVLC) or any similar entropy coding. Entropy coding/decoding may alternatively or additionally be performed using a variable length coding scheme, such as Huffman coding/decoding or Exp-Golomb coding/decoding. Decoding of coding parameters from an entropy-coded bitstream or codewords may be referred to as parsing.

[0050] The phrase along the bitstream (e.g. indicating along the bitstream) may be defined to refer to out-of-band transmission, signalling, 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, signalling, or storage) that is associated with the bitstream. For example, an indication along the bitstream may refer to metadata in a container file that encapsulates the bitstream.

[0051] 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).

[0052] Version 1 of the High Efficiency Video Coding (H.265/HEVC a.k.a. HEVC) standard was developed by the Joint Collaborative Team - Video Coding (JCT-VC) of VCEG and MPEG. The standard was published by both parent standardization organizations, and it is referred to as ITU-T Recommendation H.265 and ISO/IEC International Standard 23008-2, also known as MPEG-H Part 2 High Efficiency Video Coding (HEVC). Later versions ofH.265/HEVC included scalable, multiview, fidelity range, three-dimensional, and screen content coding extensions which may be abbreviated SHVC, MV-HEVC, REXT, 3D-HEVC, and SCC, respectively.

[0053] Versatile Video Coding (VVC) (MPEG-I Part 3), a.k.a. ITU-T H.266, is a video compression standard developed by the Joint Video Experts Team (JVET) of the Moving Picture Experts Group (MPEG), (formally ISO/IEC JTC1 SC29 WG11) and Video Coding Experts Group (VCEG) of the International Telecommunication Union (ITU) to be the successor to HEVC/H.265.

[0054] A specification of the AV 1 bitstream format and decoding process were developed by the Alliance for Open Media (AOM). The AVI specification was published in 2018. AOM is reportedly working on the AV2 specification. [0055] Some key definitions, bitstream and coding structures, and concepts of some video coding standards and specifications are described in this section for providing background for a video encoder, decoder, encoding method, decoding method, and a bitstream structure, wherein the embodiments may be implemented. It is to be understood that embodiments are not limited to the referenced video coding standards or specifications.

[0056] Many video coding standards or specifications specicy the bitstream syntax and semantics as well as the decoding process for error-free bitstreams. The encoding process may not be specified, but encoders must generate conforming bitstreams. It may be possible to verify bitstream and decoder conformance with the Hypothetical Reference Decoder (HRD) or alike. The standards may contain coding tools that help in coping with transmission errors and losses, but the use of the tools in encoding may be optional and in many standards or specifications no decoding process has been specified for erroneous bitstreams.

[0057] An elementary unit for the input to an encoder and an output of a decoder, respectively, may be a picture. A picture given as an input to an encoder may also be referred to as a source picture, and a picture decoded by a decoder may be referred to as a decoded picture or a reconstructed picture.

[0058] 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).

[0059] Typically, video is encoded in YUV or YCbCr color space as that is found to reflect some characteristics of human visual system and allows using lower quality representation for Cb and Cr channels as human perception is less sensitive to the chrominance fidelity those channels represent.

[0060] In many video coding standards or specifications, 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.

[0061] 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.

[0062] 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.

[0063] When describing the operation of HEVC encoding and/or decoding, the following terms may be used. A coding block may be defined as an NxN block of samples for some value of N such that the division of a coding tree block into coding blocks is a partitioning. A coding tree block (CTB) may be defined as an NxN block of samples for some value of N such that the division of a component into coding tree blocks is a partitioning. A coding tree unit (CTU) may be defined as a coding tree block of luma samples, two corresponding coding tree blocks of chroma samples of a picture that has three sample arrays, or a coding tree block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax structures used to code the samples. A coding unit (CU) may be defined as a coding block of luma samples, two corresponding coding blocks of chroma samples of a picture that has three sample arrays, or a coding block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax structures used to code the samples. A CU with the maximum allowed size may be named as LCU (largest coding unit) or coding tree unit (CTU) and the video picture is divided into non-overlapping LCUs.

[0064] A CU consists of one or more prediction units (PU) defining the prediction process for the samples within the CU and one or more transform units (TU) defining the prediction error coding process for the samples in the said CU. Typically, a CU consists of a square block of samples with a size selectable from a predefined set of possible CU sizes. Each PU and TU can be further split into smaller PUs and TUs in order to increase granularity of the prediction and prediction error coding processes, respectively. Each PU has prediction information associated with it defining what kind of a prediction is to be applied for the pixels within that PU (e.g. motion vector information for inter predicted PUs and intra prediction directionality information for intra predicted PUs).

[0065] 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.

[0066] To be able to utilize parallel processing, images can be split into independently codable and decodable image segments (slices or tiles). 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. [0067] 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.

[0068] Instead, or in addition to approaches utilizing sample value prediction and transform coding for indicating the coded sample values, a color palette based coding can be used. Palette based coding refers to a family of approaches for which a palette, i.e. a set of colors and associated indexes, is defined and the value for each sample within a coding unit is expressed by indicating its index in the palette. Palette based coding can typically achieve good coding efficiency in coding units with a relatively small number of colors (such as image areas which are representing computer screen content, like text or simple graphics). In order to improve the coding efficiency of palette coding different kinds of palette index prediction approaches can be utilized, or the palette indexes can be run-length coded to be able to represent larger homogenous image areas efficiently. Also, in the case the CU contains sample values that are not recurring within the CU, escape coding can be utilized. Escape coded samples are transmitted without referring to any of the palette indexes. Instead, their values are indicated individually for each escape coded sample. [0069] 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.

[0070] 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.

[0071] 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.

[0072] 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 inpicture prediction are turned off across slice boundaries, and the decoder operation takes this information into account for example when concluding which prediction sources are available. For example, samples from a neighboring CU may be regarded as unavailable for intra prediction, if the neighboring CU resides in a different slice.

[0073] A bitstream may be defined as a sequence of bits or a sequence of syntax structures. A bitstream format may constrain the order of syntax structures in the bitstream. [0074] A syntax element may be defined as an element of data represented in a bitstream. A syntax structure may be defined as zero or more syntax elements present together in a bitstream in a specified order.

[0075] An identifier may be defined as a syntax element that identifies a syntax structure. A value of the identifier may for example differ in different instances of the same syntax structure, such as a parameter set. A particular instance of the syntax structure may be referenced through its identifier value. For example, a parameter set that is referenced by the (de)coding of a coded video slice may be identified by providing the identifier value of the parameter set in a header of the coded video slice.

[0076] An indicator (ide) may be defined as a syntax element whose value indicates a selection among more than two values (for which semantics have been specified). An indicator syntax element may have _idc postfix in its name.

[0077] Syntax structures may be specified, for example, using arithmetic, logical, relational, bit-wise, and assignment operators similar to those available in many programming languages. For example, & may indicate a bit-wise ‘AND’ operation. Furthermore, syntax structures may be specified with reference to mathematical functions. [0078] Syntax structures and semantics may use the values of variables derived from the values of syntax elements. Naming conventions may be defined for variables. For example, variables may be named by a mixture of lower case and upper case letter and without any underscore characters. Variables starting with an upper case letter may be derived for the decoding of the current syntax structure and all depending syntax structures. Variables starting with an upper case letter may, in some cases, be used in the decoding process for later syntax structures without mentioning the originating syntax structure of the variable. Variables starting with a lower case letter may only be used in relation to the syntax structure or function they have been defined for.

[0079] An elementary unit for the output of an encoder and the input of a decoder, respectively, may be a Network Abstraction Layer (NAL) unit. For transport over packet- oriented networks or storage into structured files, NAL units may be encapsulated into packets or similar structures. A bytestream format has been specified in some video coding standardsfor transmission or storage environments that do not provide framing structures. The bytestream format separates NAL units from each other by attaching a start code in front of each NAL unit. To avoid false detection of NAL unit boundaries, encoders run a byte-oriented start code emulation prevention algorithm, which adds an emulation prevention byte to the NAL unit payload if a start code would have occurred otherwise. In order to enable straightforward gateway operation between packet- and stream-oriented systems, start code emulation prevention may always be performed regardless of whether the bytestream format is in use or not. A NAL unit may be defined as a syntax structure containing an indication of the type of data to follow and bytes containing that data in the form of an RBSP interspersed as necessary with emulation prevention bytes. A raw byte sequence pay load (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.

[0080] A bitstream may be defined to logically include a syntax structure, such as a NAL unit, when the syntax structure is transmitted along the bitstream but may be included in the bitstream according to the bitstream format. A bitstream may be defined to natively comprise a syntax structure, when the bitstream includes the syntax structure. [0081] In some coding formats or standards, a bitstream may be in the form of a network abstraction layer (NAL) unit stream or a byte stream, that forms the representation of coded pictures and associated data forming one or more coded video sequences.

[0082] In some coding formats, such as AVI, a bitstream may comprise a sequence of open bitstream units (OBUs). An OBU comprises a header and a payload, wherein the header identifies a type of the OBU. Furthermore, the header may comprise a size of the payload in bytes.

[0083] In some coding standards, NAL units include a header and payload. In some coding standards, the NAL unit header indicates the type of the NAL unit. In some coding standards, the NAL unit header indicates a scalability layer identifier (e.g., called nuh_layer_id in H.265/HEVC and H.266/VVC), which may be used, e.g., for indicating spatial or quality layers, views of a multiview video, or auxiliary layers (such as depth maps or alpha planes). In some coding standards, the NAL unit header includes a temporal sublayer identifier, which may be used for indicating temporal subsets of the bitstream, such as a 30-frames-per-second subset of a 60-frames-per-second bitstream.

[0084] Bitstreams or coded video sequences may be encoded to be temporally scalable as follows. Each picture may be assigned to a particular temporal sub-layer. A temporal sub-layer may be equivalently called a sub-layer, temporal sublayer, sublayer, or temporal level. Temporal sub-layers may be enumerated, e.g., from 0 upwards. The lowest temporal sub-layer, sub-layer 0, may be decoded independently. Pictures at temporal sub-layer 1 may be predicted from reconstructed pictures at temporal sub-layers 0 and 1. Pictures at temporal sub-layer 2 may be predicted from reconstructed pictures at temporal sub-layers 0, 1, and 2, and so on. In other words, a picture at temporal sub-layer N does not use any picture at temporal sub-layer greater than N as a reference for inter prediction. The bitstream created by excluding all pictures greater than or equal to a selected sub-layer value and including pictures remains conforming.

[0085] Each picture of a temporally scalable bitstream may be assigned with a temporal identifier (also known as TID, temporal layer identifier, sub-layer identifier, sublayer identifier, temporal sub-layer identifier, temporal sublayer identifier, or temporal layer ID), which may be, for example, assigned to a variable Temporalld. The temporal identifier may, for example, be indicated in a NAL unit header or in an OBU extension header. Temporalld equal to 0 corresponds to the lowest temporal level. The bitstream created by excluding all coded pictures having a Temporalld greater than or equal to a selected value and including all other coded pictures remains conforming. Consequently, a picture having Temporalld equal to tid value does not use any picture having a Temporalld greater than tid value as a prediction reference. In some video coding standards, a sub-layer or a temporal sub-layer may be defined to be a temporal scalable layer (or a temporal layer, TL) of a temporal scalable bitstream, consisting of VCL NAL units with a particular value of the Temporalld variable and the associated non-VCL NAL units.

[0086] 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 zerobased Temporalld variable may be derived as follows: Temporalld = temporal ! d_p his 1 - 1. The abbreviation TID may be used to interchangeably with the Temporalld variable. Temporalld equal to 0 corresponds to the lowest temporal level. The value of temporal ! d_plusl is required to be non-zero in order to avoid start code emulation involving the two NAL unit header bytes. nuh_layer_id can be understood as a scalability layer identifier.

[0087] NAL units can be categorized into Video Coding Layer (VCL) NAL units and non-VCL NAL units. VCL NAL units are typically coded slice NAL units. In HEVC, VCL NAL units contain syntax elements representing one or more CU.

[0088] A non-VCL NAL unit may be for example one of the following types: a video parameter set (VPS), a sequence parameter set (SPS), a picture parameter set (PPS), a supplemental enhancement information (SEI) NAL unit, an access unit delimiter, an end of sequence (EOS) NAL unit, an end of bitstream (EOB) 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.

[0089] 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.

[0090] 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.

[0091] A video parameter set RBSP may include parameters that can be referred to by one or more sequence parameter set RBSPs.

[0092] 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.

[0093] 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. [0094] Out-of-band transmission, signaling or storage can additionally or alternatively be used for other purposes than tolerance against transmission errors, such as ease of access or session negotiation. For example, a sample entry of a track in a file conforming to the ISO Base Media File Format may comprise parameter sets, while the coded data in the bitstream is stored elsewhere in the file or in another file. The phrase along the bitstream (e.g. indicating along the bitstream) or along a coded unit of a bitstream (e.g. indicating along a coded tile) 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 or the coded unit, respectively. The phrase decoding along the bitstream or along a coded unit of a 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 or the coded unit, respectively. [0095] A SEI NAL unit may contain one or more SEI messages, which are not required for the decoding of output pictures but may assist in related processes, such as picture output timing, rendering, error detection, error concealment, and resource reservation. [0096] A coded picture may be defined as a coded representation of a picture.

[0097] In some coding formats, picture unit (PU) may be defined as a set of data units, such as NAL units, that are associated with each other, are consecutive in decoding order, and contain exactly one coded picture. For example, certain non- video-coding data units, such as non-VCL NAL units, may be next to coded video data units in decoding order and the respective picture unit may comprise both these non-video-coding data units and the video coding data units of a coded picture.

[0098] In HEVC, a coded picture may be defined as a coded representation of a picture containing all coding tree units of the picture. In HEVC, an access unit (AU) may be defined as a set of NAL units that are associated with each other according to a specified classification rule, are consecutive in decoding order, and contain at most one picture with any specific value of nuh layer id. In addition to containing the VCL NAL units of the coded picture, an access unit may also contain non-VCL NAL units. Said specified classification rule may for example associate pictures with the same output time or picture output count value into the same access unit. [0099] Video coding standards, specifications or systems may allow that 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. In some video coding standards, 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.

[0100] In some coding formats, a coded video sequence (CVS) may be defined as a sequence of coded pictures in decoding order that is independently decodable and is followed by another coded video sequence or the end of the bitstream.

[0101] In some coding formats, such as AVI, a coded video sequence comprises one or more temporal units. A temporal unit consists of a series of OBUs starting from a temporal delimiter, optional sequence headers, optional metadata OBUs, a sequence of one or more frame headers, each followed by zero or more tile group OBUs as well as optional padding OBUs. A temporal unit may be defined to comprise all the OBUs that are associated with a specific, distinct time instant. A temporal unit may comprise a temporal delimiter OBU, and all the OBUs that follow, up to but not including the next temporal delimiter. A temporal delimiter OBU may be defined as an indication that the following OBUs will have a different presentation/decoding time stamp from the one of the last frame prior to the temporal delimiter.

[0102] In H.264/AVC, a coded video sequence is defined to be a sequence of consecutive access units in decoding order from an IDR access unit, inclusive, to the next IDR access unit, exclusive, or to the end of the bitstream, whichever appears earlier.

[0103] 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. 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.

[0104] 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.

[0105] A coded layer video sequence (CLVS) may be defined as a sequence of pictures and associated other data within the same scalable layer (e.g., with the same value of nuh layer id in WC) that is decodable independently of other pictures in the same layer. [0106] 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. [0107] Some codecs use a concept of picture order count (POC). A value of POC is derived for each picture and is non-decreasing with increasing picture position in output order. POC therefore indicates the output order of pictures. POC may be used in the decoding process for example for implicit scaling of motion vectors and for reference picture list initialization. Furthermore, POC may be used in the verification of output order conformance. The variable including a POC value of a picture may be referred to as PicOrderCntVal.

[0108] 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.

[0109] Output order may be defined as the order in which the decoded pictures are output from the decoded picture buffer (for the decoded pictures that are to be output from the decoded picture buffer).

[0110] Output time may be defined as a time when a decoded picture is to be output from a decoder or from the DPB of a decoder (for the decoded pictures that are to be output from the DPB), for example as specified by a hypothetical reference decoder specification according to the output timing DPB operation.

[0111] Pictures having the same output order may be defined to mean the same as pictures having the same output time.

[0112] Decoding order may be defined as the order in which syntax elements are processed by the decoding process. It may be required that syntax elements are ordered in a bitstream in their decoding order.

[0113] A decoder and/or an HRD may comprise a picture output process. The output process may be considered to be a process in which the decoder provides decoded and cropped pictures (also known as cropped decoded pictures) as the output of the decoding process. The output process may be a part of video coding standards, e.g., as a part of the hypothetical reference decoder specification. In output cropping, lines and/or columns of samples may be removed from decoded pictures according to a cropping rectangle to form output pictures. A cropped decoded picture may be defined as the result of cropping a decoded picture based on the conformance cropping window specified e.g., in the sequence parameter set that is referred to by the corresponding coded picture. Hence, it may be considered that the conformance cropping window specifies the cropping rectangle to form output pictures from decoded pictures.

[0114] One or more syntax structures for (decoded) reference picture marking may exist in a video coding system. An encoder generates an instance of a syntax structure e.g. in each coded picture, and a decoder decodes an instance of the syntax structure e.g. from each coded picture. For example, the decoding of the syntax structure may cause pictures to be adaptively marked as "used for reference" or "unused for reference".

[0115] A reference picture set (RPS) syntax structure of HEVC is an example of a syntax structure for reference picture marking. A reference picture set valid or active for a picture includes all the reference pictures that may be 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. The reference pictures that are kept marked as "used for reference" for any subsequent pictures in decoding order but that are not used as reference picture for the current picture or image segment may be considered inactive. For example, they might not be included in the initial reference picture list(s).

[0116] In some coding formats and codecs, a distinction is made between so-called short-term and long-term reference pictures. This distinction may affect some decoding processes such as motion vector scaling. Syntax structure(s) for marking reference pictures may be indicative of marking a picture as "used for long-term reference" or "used for shortterm reference" .

[0117] In some coding formats, reference picture for inter prediction may be 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 some codecs, 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. [0118] 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.

[0119] A reference picture list, such as the reference picture list 0 and the reference picture list 1, may be constructed in two steps: First, an initial reference picture list is generated. The initial reference picture list may be generated using an algorithm predefined in a standard. Such an algorithm may use e.g. POC and/or temporal sub-layer, as the basis. The algorithm may process reference pictures with particular marking(s), such as "used for reference", and omit other reference pictures, i.e. avoid inserting other reference pictures into the initial reference picture list. An example of such other reference picture is a reference picture marked as "unused for reference" but still residing in the decoded picture buffer waiting to be output from the decoder. Second, the initial reference picture list may be reordered through a specific syntax structure, such as reference picture list reordering (RPLR) commands of H.264/AVC or reference picture list modification syntax structure of HEVC or anything alike. Furthermore, the number of active reference pictures may be indicated for each list, and the use of the pictures beyond the active ones in the list as reference for inter prediction is disabled. One or both the reference picture list initialization and reference picture list modification may process only active reference pictures among those reference pictures that are marked as "used for reference" or alike.

[0120] In some coding standards, such as WC, a reference picture list may be indicated in a reference picture list syntax structure where active entries of the list may be used as a reference for predicting a current picture and inactive entries are not used as a reference for predicting the current picture but are maintained to be marked as "used for reference". Furthermore, a picture that is not included in any reference picture list for a current picture may be marked as "unused for reference".

[0121] HEVC comprises 35 intra prediction modes, including a DC, a planar, and 33 angular (directional) prediction modes. The DC and the planar mode are targeted at flat areas (i.e., the DC mode representing a block whose pixel values are constant across the block) or areas with few structure (i.e., the planar mode representing a block with pixel values gradually changing with a small planar gradient). The angular modes, in turn, provide directional prediction in a very granular way.

[0122] In comparison to the previous video coding standards, Versatile Video Codec (H.266/VVC) introduces a plurality of new coding tools, such as the following:

Intra prediction

67 intra mode with wide angles mode extension

Block size and mode dependent 4 tap interpolation filter

Position dependent intra prediction combination (PDPC) Cross component linear model intra prediction (CCLM) Multi-reference line intra prediction

Intra sub-partitions

Weighted intra prediction with matrix multiplication

Inter-picture prediction

Block motion copy with spatial, temporal, history-based, and pairwise average merging candidates

Affine motion inter prediction sub-block based temporal motion vector prediction

Adaptive motion vector resolution

8x8 block-based motion compression for temporal motion prediction

High precision (1/16 pel) motion vector storage and motion compensation with 8-tap interpolation filter for luma component and 4-tap interpolation filter for chroma component

Triangular partitions

Combined intra and inter prediction

Merge with MVD (MMVD)

Symmetrical MVD coding

Bi-directional optical flow

Decoder side motion vector refinement

Bi-prediction with CU-level weight

Transform, quantization and coefficients coding Multiple primary transform selection with DCT2, DST7 and DCT8 Secondary transform for low frequency zone Sub-block transform for inter predicted residual

Dependent quantization with max QP increased from 51 to 63 Transform coefficient coding with sign data hiding Transform skip residual coding

Entropy Coding

Arithmetic coding engine with adaptive double windows probability update In loop filter

In-loop reshaping

Deblocking filter with strong longer filter Sample adaptive offset Adaptive Loop Filter

Screen content coding:

Current picture referencing with reference region restriction 360-degree video coding

Horizontal wrap-around motion compensation

High-level syntax and parallel processing

Reference picture management with direct reference picture list signalling Tile groups with rectangular shape tile groups

Supplemental enhancement information (SEI) messages

[0123] Video coding specifications may enable the use of supplemental enhancement information (SEI) messages or alike. Some video coding specifications include SEI network abstraction layer (NAL) units, and some video coding specifications contain both prefix SEI NAL units and suffix SEI NAL units, where the former type can start a picture unit or alike and the latter type can end a picture unit or alike. An SEI NAL unit contains one or more SEI messages, which are not required for the decoding of output pictures but may assist in related processes, such as picture output timing, post-processing of decoded pictures, rendering, error detection, error concealment, and resource reservation. Several SEI messages are specified in H.264/AVC, H.265/HEVC, H.266/WC, and H.274/VSEI standards, and the user data SEI messages enable organizations and companies to specify SEI messages for their own use. The standards may contain the syntax and semantics for the specified SEI messages but a process for handling the messages in the recipient might not be defined. Consequently, encoders may be required to follow the standard specifying a SEI message when they create SEI message(s), and decoders might not be required to process SEI messages for output order conformance. One of the reasons to include the syntax and semantics of SEI messages in standards is to allow different system specifications to interpret the supplemental information identically and hence interoperate. It is intended that system specifications can require the use of particular SEI messages both in the encoding end and in the decoding end, and additionally the process for handling particular SEI messages in the recipient can be specified. SEI messages are generally not extended in future amendments or versions of the standard.

[0124] Some video coding specifications enable metadata OBUs. A metadata OBU comprises a type field, which specifies the type of metadata. A metadata OBU may be understood to be similar to an SEI NAL unit or an SEI message.

[0125] ITU-T Recommendation H.274, which is equivalent to ISO/LEC 23002-7, may be called "versatile supplemental enhancement information messages for coded video bitstreams" and be referred to as "versatile supplemental enhancement information" or VSEI. The VSEI standard specifies the syntax and semantics of video usability information (VUI) parameters and supplemental enhancement information (SEI) messages. The VUI parameters and SEI messages defined in the VSEI standard are designed to be conveyed within coded video bitstreams in a manner specified in a video coding specification or to be conveyed by other means determined by the specifications for systems that make use of such coded video bitstreams. The VSEI standard is intended for use with WC coded video bitstreams, although it is drafted in a manner intended to be sufficiently generic that it may also be used with other types of coded video bitstreams. VUI parameters and SEI messages may, for example, assist in processes related to decoding, display or other purposes. In the VSEI standard, the vui_parameters( payloadSize ) syntax structure is specified for the VUI, where payloadSize is an input argument indicating the number of bits in the VUI. [0126] The latest specification text for SEI processing order SEI message has been described in document JVET-AG2027-vl. The SEI processing order SEI message carries information indicating the preferred processing order, as determined by the encoder (i.e., the content producer), for different types of SEI messages that may be present in the bitstream. When an SEI processing order SEI message is present, it is present in the first access unit of the coded video sequence (CVS) or coded layer video sequence (CLVS). The SEI processing order SEI message persists in decoding order from the current access unit until the end of the CVS.

[0127] The syntax of SPO SEI message may be specified as follows:

[0128] The semantics of the SPO SEI message may be specified as follows.

[0129] The semantics of the SPO SEI message uses the concept of types of SEI messages. SEI messages that have different payloadType values are considered different types of SEI messages. Additionally, different SEI messages that have the same payloadType value but are differentiated by values of syntax elements in the SEI payload are considered different types of SEI messages. Such differentiation by values of syntax elements in the SEI payload is to be performed by comparing values sent using po_sei_prefix_data_bit[ i ][ j ] syntax elements (when present) or values sent as SEI messages within a processing order nesting SEI message (when present). For example, neural-network post-filter characteristics (NNPFC) SEI messages can be differentiated by having different nnpfc id values.

[0130] When the i-th SEI message sei A in any SPO SEI message has po_sei_wrapping_flag[ i ] and po_sei_prefix_flag[ i ] both equal to 0, there shall be no other SEI message seiB included in the same SPO SEI message or in a different SPO SEI message in the current CVS for which all of the following are true:

- The value of po_sei_payload_type[ i ] of seiB is the same as that for seiA.

- The value of po_sei_wrapping_flag[ i ] of seiB is equal to 0.

- The value of po_sei_prefix_flag[ i ] of seiB is equal to 1.

[0131] When an SPO SEI message with a particular value of po id is present in any access unit of a CVS, an SPO SEI message with that particular value of po id shall be present in the first access unit of the CVS in decoding order. The number of SEI messages and the payloadType codes of the SEI messages indicated within each SPO SEI message with the same value of po id persist in decoding order from the current access unit until the end of the CVS in output order.

[0132] The SPO SEI message can carry one or more SEI prefix indications of a particular payloadType. When present, each SEI prefix indication is a bit string that follows the SEI payload syntax of that value of payloadType and contains a number of complete syntax elements starting from the first syntax element in the SEI payload. These SEI prefix indications should provide sufficient information to determine the specific processing order for types of SEI messages having the same value of payloadType but a different preferred processing order.

[0133] po id contains an identifying number to identify the SPO SEI message.

[0134] A processing chain may be defined to consist of a list of types of SEI messages identified by an SPO SEI message in the preferred processing order indicated in the SPO SEI message. A processing chain may be additionally or alternatively defined to include one or more processing steps. A processing step may be interchangeably called a process or a processing stage. In some cases, a processing chain may comprise alternative or parallel processing steps. The list of types of SEI message of a processing chain may also comprise types of SEI messages that define properties, rather than processing steps, wherein the properties may, for example, describe the video content at the respective processing step.

[0135] Each type of SEI message in the processing chain indicated by an SPO SEI message is identified by the syntax elements po_sei_payload_type[ i ], po_sei_wrapping_flag[ i ], po_sei_processing_order[ i ] and, when present, po_num_bits_in_prefix_indication_minusl[ i ] and po_prefix_data_bit[ i ][ j ].

[0136] An SEI message type is not required to belong to any processing chain and may belong to any number of processing chains identified by SPO SEI messages with different po id values.

[0137] Each SEI message of an SEI message type identified within the SPO SEI message has the same persistence scope as if the SEI message was carried outside of the SPO SEI message and not identified within an SPO SEI message.

[0138] NOTE 1 - Processing chains can be alternatives to each other, i.e., such that at most processing chain is chosen to be applied, or they can be complementary, i.e., such that more than one processing chain is chosen and applied separately, with each processing chain generating one output.

[0139] po_num_sei_messages_minus2 plus 2 indicates the number of types of SEI messages for which the preferred order of processing is indicated in the SPO SEI message. [0140] po_sei_wrapping_flag[ i ] equal to 1 specifies that one or more processing order nesting SEI messages with both of the following constraints should be present: pon_target_po_id[ j ] with any value of j is equal to po id.

There is a k-th loop entry in the processing order nesting SEI message such that the payloadType of the k-th nested SEI message is equal to po_sei_payload_type[ i ] and pon_processing_order[ k ] is equal to po_sei_processing_order[ i ].

[0141] When po_sei_wrapping_flag[ i ] is equal to 0, an SEI message with payloadType equal to po_sei_payload_type[ i ] (and, when po_sei_prefix_flag[ i ] equal to 1, prefix data that matches the values of po_sei_prefix_data_bit[ i ][ j ]) should be present outside of the processing order nesting SEI message. [0142] NOTE 2 - po sei wrapping _flag[ i ] equal to 1 enables SEI messages to be carried within the processing order nesting SEI message to prevent such SEI messages from being incorrectly interpreted by decoders that do not process the SPO SEI message. Thus, po_sei_wrapping_flag[ i ] equal to 1 is intended to be used when po_sei_wrapping_flag[ i ] equal to 0 can lead to unintended results being produced by such decoders.

[0143] po_sei_importance_flag[ i ] indicates the degree of importance determined by the encoder for the type of SEI message with index i.

[0144] If the decoding system cannot interpret or does not support the functionality indicated by any indicated SEI message that has po_sei_importance_flag[ i ] equal to 1, it should ignore the entire SPO SEI message.

[0145] po_sei_payload_type[ i ] specifies the payloadType value of the i-th type of SEI message.

[0146] po_sei_prefix_flag[ i ] equal to 1 specifies that po_num_bits_in_prefix_indication_minusl[ i ] and some po_sei_prefix_data_bit[ i ][ j ] syntax elements are present. po_sei_prefix_flag[ i ] equal to 0 specifies that these syntax elements are not present.

[0147] SeiProcessingOrderSeiList is set to consist of the payloadType values 3, 4, 5, 19, 137, 142, 144, 147, 148, 149, 165, 177, 210, and 211. The value of po_sei_payload_type[ i ] for each i in the range of 0 to po_num_sei_messages_minus2 + 1 , inclusive, shall be equal to a value in SeiProcessingOrderSeiList.

[0148] po_sei_processing_order[ i ] indicates the preferred order of processing of the i-th type of SEI message for which preferred processing order information is provided in the SPO SEI message. For any two different integer values of m and n, po_sei_processing_order[ m ] less than po_sei_processing_order[ n ] indicates that the type of SEI message associated with index m should be processed before the type of SEI message associated with index n, and po_sei_processing_order[ m ] equal to po_sei_processing_order[ n ] indicates that there is no preferred order of processing between the types of SEI messages associated with indexes m and n (e.g., they can indicate different properties that are both applicable at that stage, or alternative processes that can be applied, or one can indicate a property and the other can indicate a process). [0149] For i greater than 0, po_sei_processing_order[ i ] shall be greater than or equal to po_sei_processing_order[ i - 1 ].

[0150] A processing chain has an initial processing stage (which may also be referred to as a root processing stage) from which the processing of the processing chain starts. In other words, the initial processing stage is the first processing stage that is performed when a decoding system performs the processing chain. When an SPO SEI message indicates a processing chain, the initial processing stage may be indicated with the lowest value of po_sei_processing_order[ i ] within the SPO SEI message.

[0151] A noninitial processing stage may be defined as any processing stage of a processing chain that is not the initial processing stage of the processing chain.

[0152] It is to be understood that SEI messages may or may not persist or be activated for all pictures. If an SPO SEI message defines a specific type of an SEI message with the lowest value of po_sei_processing_order[ i ] but no SEI message of that type persists for this processing chain and a particular picture, the initial processing stage for this particular picture may be defined by the first SEI message that is present as a type of an SEI message in the SEI processing order, defines a processing stage in this processing chain, and persists or is activated for this processing chain and the particular picture. Consequently, the noninitial processing stage(s) for this particular picture follow the initial processing stage for this particular picture.

[0153] po_num_bits_in_prefix_indication_minusl[ i ] and po_sei_prefix_data_bit[ i ][ j ], when present, have the same semantics as the num_bits_in_prefix_indication_minusl[ i ] and sei_prefix_data_bit[ i ][ j ] syntax elements of the SEI prefix indication SEI message, with prefix_sei_payload_type replaced by po_sei_payload_type[ i ].

[0154] When more than one SPO SEI message with a particular value of po id is present in a CVS, the values of po_num_sei_messages_minus2 and, for each value of i, the values of po_sei_wrapping_flag[ i ], po_sei_prefix_flag[ i ], po_sei_importance_flag[ i ], po_sei_payload_type[ i ], po_sei_processing_order[ i ] shall be the same as in the other SPO SEI messages in the CVS with the same value of po id.

[0155] po_byte_alignment_bit_equal_to_one shall be equal to 1. [0156] The processing order nesting (PON) SEI message includes one or more SEI messages that should be applied only as parts of the processing chain identified by an associated SEI processing order SEI message and should not be applied in a manner that would contradict with the processing chain identified by the associated SEI processing order SEI message. The latest specification text for the PON SEI message is also available in document JVET-AG2027-vl .

[0157] The syntax of the PON SEI message may be specified as follows:

[0158] The semantics of the PON SEI message may be specified as follows.

[0159] The SEI messages contained in a PON SEI message are referred to as PON- nested SEI messages.

[0160] NOTE - An encoder can include multiple PON SEI messages in the same access unit. For example, a first PON SEI message in an access unit can contain a PON-nested SEI message that applies to multiple processing chains and one or more other PON SEI messages in the same access unit that apply to a single processing chain only.

[0161] pon_num_po_ids_minusl plus 1 specifies the number of the SEI processing order SEI messages SEI associated with this PON SEI message.

[0162] pon_target_po_id[ i ] indicates the po id of the i-th associated SEI processing order SEI message.

[0163] pon_num_seis_minusl plus 1 specifies the number of the PON-nested SEI messages that are included in this PON SEI message.

[0164] pon_processing_order[ i ] specifies the position of the i-th processing-order- nested SEI message within the processing order defined by the associated SEI processing order SEI message. When i is greater than 0, pon_processing_order[ i ] shall be greater than or equal to pon_processing_order[ i - 1 ].

[0165] For each associated SEI processing order SEI message there shall be at least one value of i in the range of 0 to pon num seis minusl, inclusive, in the PON SEI message for which the associated SEI processing order SEI message has some entry k for which all of the following are true: po_sei_processing_order[ k ] is equal to pon_processing_order[ i ] po_sei_payload_type[ k ] is equal to the payloadType value of the i-th PON-nested SEI message.

When po_sei_prefix_flag[ k ] is equal to 1, po_sei_prefix_data_bit[ k ][ j ] for j in the range of 0 to po_num_bits_in_prefix_indication_minusl[ k ], inclusive, contain the same content as the po_num_bits_in_prefix_indication_minusl[ k ] plus 1 initial bits of the SEI message payload of the i-th PON-nested SEI message.

[0166] The i-th PON-nested SEI message should be applied as the k-th loop entry of the associated SEI processing order SEI message.

Fundamentals of neural networks

[0167] A neural network (NN) may be described as a computation graph including several layers of computation. Each layer includes one or more units, where each unit performs a computation. A unit is connected to one or more other units, and a connection may be associated with a weight. The weight may be used for scaling the signal passing through an associated connection. Weights are learnable parameters, for example, values which may be learned from training data. There may be other learnable parameters, such as those of batch-normalization layers.

[0168] In some neural networks, such as convolutional neural networks for image classification, initial layers (those close to the input data) extract semantically low-level features such as edges and textures in images, whereas intermediate layers extract more high-level features. After the feature extraction layers there may be one or more layers performing a certain task, such as classification, semantic segmentation, object detection, denoising, style transfer, super-resolution, and the like. [0169] Neural networks are being utilized in an ever-increasing number of applications for many different types of devices, for example, mobile phones, chat bots, loT devices, smart cars, voice assistants, and the like. Some of these applications include, but are not limited to, image and video analysis and processing, social media data analysis, device usage data analysis, and the like.

[0170] One of the properties of neural networks (and other machine learning tools), is that they are able to learn properties from input data, e.g. either in a supervised way or in an unsupervised way. Such learning is a result of a training algorithm, or of a meta-level neural network providing the training signal.

[0171] In general, the training algorithm includes changing some properties of the neural network so that its output is as close as possible to a desired output. For example, in the case of classification of objects in images, the output of the neural network may be used to derive a class or category index which indicates the class or category that the object in the input image belongs to. Training usually happens by minimizing or decreasing the output error, also referred to as the loss or loss function. Examples of losses are mean squared error, cross-entropy, and the like. In recent deep learning techniques, training is an iterative process, where at each iteration the algorithm modifies the weights of the neural network to make a gradual improvement in the network’s output, for example, gradually decrease the loss , by means of gradient descent technique. In one example, at each training iteration, gradients of the loss function with respect to one or more weights or parameters of the NN are computed, for example by backpropagation technique; the computed gradients are then used by an optimization routine, such as Adam or Stochastic Gradient Descent (SGD) to obtain an update to the one or more weights or parameters.

[0172] In various embodiment, the terms “model”, “neural network”, “neural net” and “network” may be used interchangeably, and also the weights of neural networks are sometimes referred to as learnable parameters or simply as parameters.

[0173] Training a neural network is an optimization process, but the final goal is different from the typical goal of optimization. In optimization, the only goal is to minimize a function. In machine learning, the goal of the optimization or training process is to make the model learn the properties of the data distribution from a limited training dataset. In other words, the goal is to learn to use a limited training dataset in order to learn to generalize to previously unseen data, for example, data which was not used for training the model. This is usually referred to as generalization. In practice, data is usually split into at least two sets, the training set and the validation set. The training set is used for training the network, for example, to modify its learnable parameters in order to minimize the loss. The validation set is used for checking the performance of the network on data, which was not used to minimize the loss, as an indication of the final performance of the model. In particular, the errors on the training set and on the validation set are monitored during the training process to understand the following: when the network is learning at all - in this case, the training set error should decrease, otherwise the model is in the regime of underfitting. when the network is learning to generalize - in this case, also the validation set error needs to decrease and be not too much higher than the training set error. For example, the validation set error should be less than 20% higher than the training set error. When the training set error is low, for example 10% of its value at the beginning of training, or with respect to a threshold that may have been determined based on an evaluation metric, but the validation set error is much higher than the training set error, or it does not decrease, or it even increases, the model is in the regime of overfitting. This means that the model has just memorized properties of the training set and performs well only on that set, but performs poorly on a set not used for training or tuning of its parameters.

[0174] The term generative artificial intelligence (Al), or generative modeling, or generative machine learning (and other similar terms), are commonly used to indicate a class of models learned from data, and/or training algorithms that are used to train those models, where those models are capable of generating new data and/or sampling from a learned probability distribution of data and/or extrapolating data. State-of-the-art generative models are based on neural networks.

[0175] Examples of generative models include (but are not limited to) diffusion models, Variational Auto-Encoder (VAE) models, Generative Adversarial Network (GAN) models, Transformers trained and/or used in a generative way such as with an auto-regressive loss functions, and the like. [0176] One example architecture of generative neural network for generating text is a Transformer-based “decoder” that is trained by using an auto-regressive loss function. Here, “decoder” may not refer to a decoder that is part of a codec performing compression of input data into a small bitstream, but it refers to a neural network that gets a set of input words or parts of words or tokens extracted from input words, and outputs a set of output words or parts of words or tokens. At inference time, such a NN is run in auto-regressive mode, where the generated word(s) or token(s) is provided as part of the input word(s) or token(s). In order for such a NN to generate data, it is trained to predict the next word(s) (or an estimate of a probability distribution over the next words) given a set of input words. The NN may be based on the Transformer architecture, which comprises the use of the self-attention mechanism, where an attention score is assigned to each input token or word based on all other input tokens or words, including the previously generated words or tokens. During training of a decoder-style Transformer architecture, the future data items (words or tokens) are masked so not to leak information from the future. In some cases, decoder-style Transformer architectures are referred to as “uni-directional”, because they use or process information from left-to-right, as opposed to some encoder-style Transformer architectures that are referred to as “bi-directional” (because they may use or process information from left-to-right and from right-to-left).

[0177] Another example of generative modeling is visual temporal extrapolation, where a picture is generated by a NN based at least on one or more previously decoded or generated pictures and on one or more other data items. The one or more previously decoded or generated pictures may be pictures decoded by a process that does not involve generative modeling, such as a traditional codec, e.g., a WC-compliant codec. The one or more data items may include parameters or features that describe the differences between the one or more previously decoded pictures and the current picture to be temporally extrapolated. Examples of such parameters are facial parameters (such as facial keypoints or facial landmarks and their positions or differential positions with respect to the facial landmarks of a previous picture), or parameters of other objects. The one or more data items may be signaled from encoder to decoder. However, it is to be understood that, in some examples or use cases, a neural network may perform visual temporal extrapolation based only on one or more previously decoded or generated pictures. [0178] View synthesis may be defined as a process that generates video for another viewpoint or camera position than that or those represented by the video or images given as input to the view synthesis process. A depth map may be defined as a picture that represents the distance or disparity of samples from the viewpoint or camera. An alpha mask (a.k.a. alpha map) may be used to provide transparency information for an associated image. A first value of an alpha mask may represent a fully opaque pixel, and a second value may represent a fully transparent pixel. Values between the first and second values may represent different levels of transparency between fully opaque and fully transparent. View synthesis, depth map generation or alpha mask generation may be performed using a neural network inference, but also methods not based on neural networks exist.

Neural Network Representation (NNR)

[0179] ISO/IEC 15938-17 (Compression of Neural Networks for Multimedia Content Description and Analysis) is also known as neural network representation (NNR) or neural network compression (NNC). NNR specifies a compressed representation of the parameters and/or weights of a trained neural network and a decoding process for the compressed representation. NNR complements the description of the network topology in existing neural network exchange formats. NNR is independent of a particular neural network exchange format and is interoperable with common neural network exchange formats.

[0180] NNR establishes a toolbox of compression methods, specifying (where applicable) the resulting elements of the compressed bitstream. All of these tools may be applied to the compression of entire neural networks, and some of them may also be applied to the compression of differential updates of neural networks with respect to a base network. Such differential updates are, for example, useful when models are redistributed after fine-tuning or transfer learning, or when providing versions of a neural network with different compression ratios. The support for incremental compression of updates of neural networks respective to a base model will be included in the 2^nd edition of NNR, which is currently being standardized. [0181] NNR comprises the syntax format, semantics, associated decoding process requirements, parameter sparsification, parameter transformation methods, parameter quantization, entropy coding method and integration/signaling within existing exchange formats.

[0182] An NNR bitstream may conform to ISO/IEC 15938-17. NNR bitstream or NNR data in a channel may comprise a sequence of NNR Units. An NNR Unit may be regarded as a basic high-level syntax structure in an NNR bitstream, and may include three syntax elements or structures: NNR Unit Size, NNR unit header, and NNR unit payload.

Neural-network post-filters

[0183] Version 3 of the VSEI standard includes the specification of the neural-network post-filter characteristics (NNPFC) and neural -network post-filter activation (NNPFA) supplemental enhancement information (SEI) messages. Extensions to NNPFC SEI message are being specified for version 4 of the VSEI standard, and at the time of writing this disclosure the latest draft is available in document JVET-AG2034-vl.

[0184] The syntax structure specifying the NNPFC SEI message may be called nn_post_filter_characteristics. The syntax structure specifying the NNPFA SEI message may be called nn_post_filter_activation.

[0185] The NNPFC SEI message comprises the nnpfc_id syntax element, which includes an identifying number that may be used to identify a post-processing filter.

[0186] The NNPFC SEI message syntax comprises nnpfc_base_flag. nnpfc_base_flag equal to 1 specifies that the SEI message specifies the base NNPF. nnpfc base flag equal to 0 specifies that the SEI message specifies an update relative to the base NNPF.

[0187] The following constraints apply to the value of nnpfc base flag:

When an NNPFC SEI message is the first NNPFC SEI message, in decoding order, that has a particular nnpfc id value within the current CL VS, the value of nnpfc base flag is required to be equal to 1.

All NNPFC SEI messages in a CLVS that have a particular nnpfc_id value and nnpfc base flag equal to 1 are required to have identical SEI payload content.

[0188] When nnpfc base flag is equal to 0, the following applies: This SEI message defines an update relative to the preceding base NNPF in decoding order with the same nnpfc id value. Updates are not cumulative but rather each update is applied on the base NNPF, which is the NNPF specified by the first NNPFC SEI message, in decoding order, that has a particular nnpfc id value within the current CLVS. The NNPF defined by this SEI message is obtained by applying the update defined by this SEI message relative to the base NNPF with the same nnpfc_id value.

This SEI message pertains to the current decoded picture and all subsequent decoded pictures of the current layer, in output order, until the end of the current CLVS or up to but excluding the decoded picture that follows the current decoded picture in output order within the current CLVS and is associated with a subsequent NNPFC SEI message, in decoding order, having nnpfc_base_flag equal to 0 and that particular nnpfc id value within the current CLVS, whichever is earlier.

[0189] The NNPFC SEI message comprises the nnpfc mode idc syntax element, the semantics of which may be defined as follows:

[0190] nnpfc mode idc equal to 1 specifies that the base post-processing filter or the update relative to the base post-processing filter associated with the nnpfc id value is a neural network identified by the Uniform Resource Identifier (URI) nnpfc uri with the format identified by the tag URI nnpfc tag uri.

[0191] nnpfc mode idc equal to 0 indicates that this SEI message contains an ISO/IEC 15938-17 bitstream that specifies the base post-processing filter or updates relative to the base post-processing filter with the same nnpfc id value.

[0192] When nnpfc mode idc is equal to 0 and nnpfc base flag is equal to 0, the update may be obtained by decoding the coded neural network bitstream included in the NNPFC SEI message.

[0193] The NNPFC SEI message may also comprise:

Purpose of the post-processing filter, which may comprise, but may not be limited to, one or more of the following:

• Visual quality improvement; • Chroma upsampling from the 4:2:0 chroma format to the 4:2:2 or 4:4:4 chroma format, or from the 4:2:2 chroma format to the 4:4:4 chroma format;

• Increasing the width or height of the input picture;

• Frame rate upsampling;

• Bit depth upsampling; or

• Colorization.

• Temporal extrapolation (i.e., generating one or more future pictures) Formatting of the input tensors that are given as input to the neural network inference

Formatting of the output tensors that are resulting from the neural network inference; and Characterization of the complexity of the neural network.

[0194] The NNPFC SEI message syntax includes the nnpfc_num_input_pics_minusl syntax element. nnpfc_num_input_pics_minusl plus 1 specifies the number of pictures used as input for the NNPF. The variable numlnputPics may be set equal to nnpfc_num_input_pics_minusl + 1.

[0195] A frame rate upsampling filter may interchangeably be called a picture rate upsampling filter. Such a filter generates or interpolates one or more pictures between a pair of pictures given as input to the filter. It is also possible to have a frame rate upsampling filter where the number of input pictures may be greater than 2. Such a frame rate upsampling filter may generate pictures between more than one pair of input pictures. A frame rate upsampling filter may comprise a neural network, in which case the generation of the interpolated pictures between a pair of input pictures is performed by the inference of the neural network. It is possible to have a frame rate upsampling filter that extrapolates a picture before input picture(s) or after input picture(s), instead of or in addition to between input pictures.

[0196] When the filtering purpose comprises frame rate upsampling, the NNPFC SEI message includes nnpfc_interpolated_pics[ i ] syntax elements for the values of i in the range of 0, inclusive, to nnpfc_num_input_pics_minusl, exclusive. nnpfc_interpolated_pics[ i ] specifies the number of interpolated pictures generated by the NNPF between the i-th and the ( i + 1 )-th picture used as input for the NNPF. [0197] It is to be understood that the terms visual temporal extrapolation, temporal extrapolation, and video prediction may be used interchangeably. Visual temporal extrapolation may be defined as a method, algorithm, or process that generates one or more pictures in the future given one or more past pictures as input or generates one or more pictures in the past given one or more subsequent pictures as input. Visual temporal extrapolation may be realized by, but is not necessarily based on or limited to, neural network inference.

[0198] Use cases for visual temporal extrapolation include, but are not limited to:

• Very low delay computer vision for domains like robotics and autonomous driving, where extrapolated future pictures facilitate anticipatory decision making;

• Increase of the rendered picture rate in very low-latency applications, such as cloud gaming, relative to the decoded picture rate;

• Generating multiple potential future pictures representing the same future timestamp based on potential future user actions, such as the upcoming player actions in cloud gaming. From the generated multiple potential future pictures, one or more are selected for further processing, such as for serving as reference picture(s) for inter prediction and/or displaying;

• Reduction of the end-to-end delay in low-latency applications through extrapolating and displaying future pictures before they are received; and

• Video generation, such as generative face video, for very low bitrate video coding. [0199] When the filtering purpose comprises temporal extrapolation, the NNPFC SEI message includes the nnpfc_extrapolated_pics_minusl syntax element. nnpfc_extrapolated_pics_minusl plus 1 specifies the number of extrapolated pictures generated by the NNPF subsequent to all input pictures for the NNPF in output order. [0200] The NNPFC SEI message syntax may comprise an indication, which may be called nnpfc_absent_input_pic_zero_flag, that indicates how pictures that would not originate from the current bitstream are expected to be replaced in the input tensor. nnpfc_absent_input_pic_zero_flag equal to 1 indicates that the NNPF expects an input picture that is not present in the current bitstream to be represented sample arrays with sample values equal to 0. nnpfc_absent_input_pic_flag equal to 0 indicates that the NNPF expects an input picture that is not present in the current bitstream to be represented by the closest input picture in output order within the current bitstream.

[0201] The NNPFC SEI message syntax may comprise an indication, which may be called nnpfc auxiliary inp idc, that indicates if auxiliary input data in addition to sample array(s) of input picture(s) is present in the input tensor of the NNPF. nnpfc auxiliary inp idc greater than 0 indicates that auxiliary input data is present in the input tensor of the NNPF. Specific semantics may be specified for specific non-zero values of nnpfc auxiliary inp idc. nnpfc auxiliary inp idc equal to 0 indicates that auxiliary input data is not present in the input tensor.

[0202] The NNPFA SEI message specifies the neural-network post-processing filter (NNPF) that may be used for post-processing filtering for the current picture, or for postprocessing filtering for the current picture and one or more other pictures. The NNPFA SEI message comprises the nnpfa target id syntax element, which indicates that the neural-network post-processing filter with nnpfc id equal to nnpfa target id may be used for post-processing filtering for the indicated persistence. The indicated persistence may be the current picture only (indicated by nnpfa_persistence_flag equal to 0). Alternatively, the NNPF activation may be indicated to be persistent by nnpfa_persistence_flag equal to 1, in which case the persistence of the NNPF activation may last until the end of the current CLVS or the next picture, in output order, in the current layer associated with a NNPFA SEI message with the same nnpfa target id as the current SEI message.

[0203] When nnpfa_persistence_flag is equal to 0 in an NNPFA SEI message that is not included in a PON SEI message and is present in a picture unit, the NNPFA SEI message activates the NNPF for the cropped decoded picture decoded from the picture unit. When nnpfa_persistence_flag is equal tol in an NNPFA SEI message not included in a PON SEI message, the NNPFA SEI message activates the NNPF for each cropped decoded picture to which the NNPFA SEI message persists as described above.

[0204] The NNPFA SEI message syntax may comprise a syntax element indicative if the base post-processing filter or the latest post-processing filter is activated, where the latest post-processing filter is defined by the base post-processing filter relative to which the latest filter update, if any, has been applied. The syntax element may be called nnpfa target base flag. nnpfa target base flag equal to 1 specifies that the target NNPF is the base NNPF with nnpfc id equal to nnpfa target id. nnpfa target base flag equal to 0 specifies that the target NNPF is the NNPF specified by the last NNPFC SEI message with nnpfc id equal to nnpfa target id that precedes the first VCL NAL unit of the current picture in decoding order and is not a repetition of the NNPFC SEI message that contains the base NNPF.

[0205] The NNPFA SEI message syntax may comprise indications which ones of the filtered pictures corresponding to the input pictures are output by the NNPF process. For the i-th input picture that is filtered by the NNPF, the NNPFA SEI message syntax may comprise nnpfa_output_flag[ i ] syntax element, which when equal to 0, specifies that the filtered picture is not output by the NNPF process, and when equal to 1, specifies that the filtered picture is output by the NNPF process.

[0206] In relation to an NNPFA SEI message, two sets of pictures may be defined, namely nnpfcTargetPictures and nnpfaTargetPictures. nnpfcTargetPictures may be defined to be the set of pictures to which the last NNPFC SEI message with nnpfc id equal to nnpfa target id that precedes the current NNPFA SEI message in decoding order pertains. nnpfaTargetPictures may be defined to be the set of pictures for which the target NNPF is activated by the current NNPFA SEI message. It may be required for a conforming bitstream that any picture included in nnpfaTargetPictures shall also be included in nnpfcT argetPictur es .

[0207] An NNPF process comprises performing the NNPF inference for given input pictures. The NNPF inference may be performed in a patch-wise manner so that the entire picture area gets filtered. The NNPF inference may be followed by outputting NNPF- generated pictures in their increasing index order, where all NNPF -generated pictures that were interpolated by the NNPF are output and those NNPF -generated pictures that correspond to any input pictures to the NNPF are output as specified in the semantics of the NNPFA SEI message.

[0208] A general post-processing filtering process using NNPFs may be described as follows. Input to this process is a bitstream Bitstr eamToFilter. Output of this process is a list of NNPF output pictures ListNnpfOutputPics. First, BitstreamToFilter is decoded, and the list CroppedDecodedPictures is set to be the list of the cropped decoded pictures in output order resulted from decoding BitstreamToFilter. Second, the filtering process for one picture, as described below, is repeatedly invoked, in output order, for each cropped decoded picture that is in CroppedDecodedPictures and for which one or more NNPFs are activated. The order of the pictures in ListNnpfOutputPics is in output order. It may be required that within ListNnpfOutputPics there shall be no more than one picture pertaining to any particular output time instance. When for any particular picture in CroppedDecodedPictures there are multiple NNPFs activated and only one the NNPFs is allowed to be chosen to be applied although any of the NNPFs may be chosen, the above constraint shall apply regardless of which NNPF is chosen to be applied to the particular picture.

[0209] A filtering process for one picture using an NNPF may be described as follows. The filtering process for one picture using an NNPF may be applied to each cropped decoded picture, referred to as the current picture, that is in CroppedDecodedPictures and for which one or more NNPFs are activated. When applying an NNPF to the current picture, the filtered and/or interpolated pictures are generated by the NNPF by applying the NNPF process to the current picture. When applying an NNPF to the current picture, the order of the pictures generated by the NNPF by applying the NNPF process being stored into the output tensor of the NNPF is in output order. When the applied NNPF is the last NNPF that is applied to the current picture, the pictures generated by the NNPF and output by the NNPF process are included into ListNnpfOutputPics, in the same order as when the pictures are stored into the output tensor of the NNPF.

[0210] The use of NNPFC and NNPF A SEI messages for WC has been described in version 3 of the versatile video coding (VVC) standard. It is to be understood that NNPFC and NNPFA SEI message may be similarly used for any other video coding specification. [0211] When NNPFC and NNPFA SEI messages are used for WC, a decoder selects input pictures for the NNPF. The input pictures may be selected in reverse output order starting from a picture for which the NNPF is activated through an NNPFA SEI message. The input pictures may be indexed, starting from index 0 that is assigned for the picture for which the NNPF is activated through an NNPFA SEI message. In an example, the decoder selects the input picture with index i, where i is greater than 0, to be the latest cropped decoded output picture, in output order, that precedes the input picture with index i-1 in output order. When there is no cropped decoded output picture, in output order, that precedes the input picture with index i-1 in output order as a result of decoding the bitstream, it may be considered that the input picture with index i is not present in the current bitstream (e.g., missing) and the subsequent input pictures, when any, with index i+1 to numlnputPics-l, inclusive, are likewise missing. A missing input picture may be treated like described above in relation to nnpfc_absent_input_pic_zero_flag syntax element.

[0212] When NNPFC and NNPFA SEI messages are used for WC and a picture rate upsampling NNPF that interpolates pictures between a single pair of input pictures is activated persistently until the end of the bitstream, the NNPF is applied repeatedly at the end of the bitstream for different sets of input pictures up to but excluding a set of input pictures that would cause creation of any interpolated picture after the last picture of the bitstream in output order. In these sets of input pictures, some of the pictures may be missing and may be, for example, replaced by the last picture within the bitstream in output order.

[0213] In video coding, multiple picture processing stages are typically performed in cascade. The processing stages may include, but may not be limited to, filtering for different purposes, film grain synthesis, and/or color conversion. One example of such processing stage is the frame rate upsampling filtering (a.k.a. picture rate upsampling filtering) mentioned above. The frame rate upsampling filter generates or interpolates one or more pictures between a pair of pictures given as input to the filter.

[0214] Thus, the multiple picture processing stages may include a first processing stage, which interpolates or extrapolates pictures temporally, and a second (later, but not necessarily subsequent) processing stage, which is intended to be applied to some but not necessarily all the interpolated or extrapolated pictures.

[0215] Using the SEI processing order (SPO) SEI messages, the second processing stage may generally be invoked or activated by an SPO SEI message that is associated with a coded picture or a respective decoded picture. However, it is not possible to invoke or activate the second processing stage specifically for interpolated or extrapolated pictures. Particularly, when the second processing stage is a neural-network post-filtering (NNPF), it cannot be indicated by an NNPFA SEI message, whether the pictures that have been created by temporal interpolation or extrapolation are filtered with the base NNPF or an updated NNPF.

[0216] In the following, an enhanced method for defining the processing order as well as indicating specific picture(s) for a processing stage will be described in more detail, in accordance with various embodiments.

[0217] The method, which is disclosed in Figure 2, comprises inferring or indicating (200) a processing order comprising multiple processing stages; inferring or indicating (202) a noninitial processing stage among the multiple processing stages; encoding (204) a first indication that the noninitial processing stage is invoked to one or more indicated pictures; and associating (206) the first indication with at least one reconstructed picture, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

[0218] Thus, a processing order for multiple picture processing stages is indicated. A noninitial processing stage is invoked to one or more indicated pictures by using an encoded first indication. The first indication is associated with at least one reconstructed picture, which is a different picture than the one or more indicated pictures.

[0219] According to an embodiment, the noninitial processing stage is preceded by a preceding processing stage in the processing order, and the preceding processing stage involves interpolating or extrapolating one or more generated pictures based on at least one reconstructed picture. Embodiments may interchangeably refer to a preceding processing stage or to a first processing stage. It is to be understood that the term "a first processing stage" does not necessarily refer to the first processing stage of a processing chain, but to any processing stage that precedes the noninitial processing stage in the processing chain. [0220] It is noted that the method is not only intended to address the limitations of SPO SEI messages or neural-network post-filtering (NNPF) and the related SEI message(s), but to be applied to any generic situation, where the multiple picture processing stages include a first processing stage, which generates one or more pictures that do not correspond to any of the input pictures of the first processing stage, and a later processing stage, which is intended to be applied to some but not necessarily all one or more generated pictures.

[0221] In some embodiments, the multiple picture processing stages include a first processing stage that comprises visual spatial extrapolation that generates one or more regions that do not correspond spatially to any of the regions in the input pictures of the first processing stage, and a subsequent processing stage which is intended to be applied to at least some of the one or more generated regions.

[0222] The first processing stage may comprise, but may not be limited to, one or more of the following: picture rate upsampling (a.k.a. temporal interpolation), temporal extrapolation, view synthesis, depth map generation, alpha map generation. The first processing may comprise a neural network inference but may alternatively or additionally comprise other means.

[0223] It may be defined that the one or more generated pictures do not correspond to any of the input pictures of the first processing stage, since the one or more generated pictures represent different pictures than the any of the input pictures, for example in terms of output time, output order, view, or video signal type (e.g., input pictures may be texture pictures, whereas the generated pictures may represent depth or alpha maps), or spatial coordinates (e.g., when performing visual spatial extrapolation, one or more areas in the output frame is not present in the input frame because those areas were extrapolated).

[0224] When the first processing stage comprises spatial extrapolation, a generated picture may be defined as the picture that comprises one or more generated regions resulting from the spatial extrapolation, and it may be defined that the one or more generated pictures do not correspond to any of the input pictures of the first processing stage, since the one or more generated pictures comprise one or more generated regions not present in the input pictures of the first processing stage. A generated region may be defined to be a region that was generated by spatial extrapolation.

[0225] Accordingly, examples of cases where an encoder may apply the method and/or one or more of the following embodiments may include, but are not limited to, one or more of the following:

An interpolated picture may need to be enhanced if the interpolator NN (e.g., the neural network performing frame-rate upsampling) is low-complexity (or anyway low- capacity), thus a visual enhancement filter needs to be applied only on the interpolated pictures.

Two or more consecutive frames (in output or display order) may belong to different temporal sublayers and may have different qualities. A visual enhancement filter may be applied only to pictures that are output or interpolated by a frame-rate upsampling process based on one or two low-quality pictures (e.g., high temporal sublayer). Example: pictures with index (in output/display order) 0 and 2 are high quality (e.g., with temporal sublayer identifier 0 and 2), pictures 4 and 6 are low quality (e.g., TID 4 and 6). Picture 1 is interpolated based on pictures 0 and 2 (both high quality) and will likely be high quality, thus pictures 0, 1 , 2 may not need to be filtered. Picture 3 is interpolated based on pictures 2 (high quality) and 4 (low quality) and will likely be lower quality than picture 2, thus it may need to be visually enhanced. Picture 5 is interpolated based on pictures 4 and 6 (both low quality) and will likely be low quality, likely even lower quality than pictures 4 and 6, thus the visual enhancement filter can be applied either to pictures 4, 5, and 6 or only to picture 5.

An NNPF has temporal extrapolation purpose and outputs multiple speculative coinciding output pictures, out of which one output picture is subsequently selected by an encoding system for further processing. The extrapolated picture for further processing is indicated through an input picture selection mechanism for the next processing stage of an SEI processing order.

An encoder finetunes a quality-enhancement NNPF based on interpolated pictures of a random access segment and sends an NNPF update through an NNPFC SEI message. An encoder indicates the use of the base quality-enhancement NNPF for the current picture and the updated NNPF for the interpolated pictures that have been generated by activating the picture rate upsampling filter for the current picture. It is noted that the current picture may be any picture in the random access segment.

[0226] According to an embodiment, the first indication indicates also that the noninitial processing stage is invoked for the one or more generated pictures.

[0227] Herein, the preceding processing stage can be any processing stage that precedes the noninitial processing stage in the processing order. The preceding processing stage may or may not be the previous processing stage relative to the noninitial processing stage.

[0228] According to an embodiment, the method further comprises deriving a candidate picture list comprising the one or more generated pictures; and encoding, in or along the first indication, a second indication that indicates a first picture among the candidate picture list to which the noninitial processing stage is invoked. [0229] According to an embodiment, the candidate picture list comprises the at least one reconstructed picture and/or at least one processed reconstructed picture resulting from processing stages preceding the noninitial processing stage.

[0230] Any of the embodiments relating to post-processing stages may be respectively realized for in-loop processing. For example, the preceding processing stage may comprise picture rate upsampling and/or temporal extrapolation, and the noninitial processing stage may comprise film grain synthesis and/or a quality enhancement filter. An encoder may encode indication(s) in a syntax structure that is normatively decoded by a decoder, as opposed to encoding indication(s) as supplemental enhancement information or alike. For example, the indication(s) may be included in a picture trailer data unit and/or a reference picture marking update data unit, as discussed more in detail further below.

[0231] A video coding scheme may provide encoders the possibility to indicate invocation of interpolation or extrapolation of a picture, which may be used as an output of the decoder and/or as a reference picture for inter prediction. Since the (de)coding order of pictures may differ from the output order, the invocation indication may concern one or more selected pictures that the encoder indicates. The invocation indication is included in a picture unit that is not necessarily used as input in the invoked processing.

[0232] In the following examples, the noninitial processing stage involves interpolating or extrapolating one or more generated pictures based on at least one reconstructed picture, and the preceding processing stage may comprise decoding or reconstructing the at least one reconstructed picture.

[0233] Figure 3 illustrates an example, where the output order is from left to right and the decoding order is indicated by increasing frame identifiers (included in the boxes indicating pictures). The encoder may determine that rather than coding picture 8 conventionally, it is interpolated (e.g., using a picture rate upsampling NN) from pictures 6 and 5.

[0234] It may be beneficial that an indication to interpolate picture 8 is associated with picture 7, since while picture 8 can be interpolated right after picture 6 has been reconstructed, one or more picture memory buffer(s) may be saved if picture 8 is generated only after picture 7 has been reconstructed. [0235] In this example, after reconstructing picture 7, an encoder and/or a decoder generates a candidate picture list that comprises pictures 1, 7, 6, and 5. The encoder encodes an indication and/or the decoder decodes an indication that pictures 6 and 5 from the candidate picture list are used for invoking the preceding processing stage (picture rate upsampling).

[0236] The indication may be included in a syntax structure that follows the coded picture data in a picture unit. The syntax structure may, for example, be, but not limited to, any of the following:

A picture trailer data unit (which may alternatively have other names, such as a coded picture completion data unit). This data unit may indicate the end of coded data of a picture or indicate the end of a picture unit.

A reference picture marking update data unit, which may indicate changes to be made to the reference picture marking after the decoding of a coded picture, which may include invocation of a picture rate upsampling or temporal extrapolation to generate pictures, and/or marking of pictures as short-term reference, long-term reference, or unused for reference.

[0237] A data unit referred above may, for example, be a NAL unit or an OBU or a part thereof.

[0238] The above method and embodiments may be applied to indicating in-loop processing to be performed in a decoding process. Therein, one or more of the following embodiments may be applied.

[0239] According to an embodiment, the method further comprises indicating the processing order through indicating enabled and/or disabled in-loop filters.

[0240] Herein, the in-loop filters may be in an order that may be pre-defined, e.g., in a coding standard, but some in-loop filters may be disabled.

[0241] According to an embodiment, deriving the candidate picture list further comprises initiating the candidate picture list with pictures present in a decoded picture buffer; and for each processing stage in the processing order, replacing each of said pictures in the candidate picture list with a coinciding processed reconstructed picture resulting from the processing stage, if any, and inserting interpolated pictures and extrapolated pictures, if any, into the candidate picture list and placing them such that all pictures in the updated candidate picture list are in output order.

[0242] According to an embodiment, the candidate picture list comprises the pictures present in the decoded picture buffer in a pre-defined order.

[0243] According to an embodiment, the pre-defined order is selected based on the type of the first processing stage.

[0244] According to an embodiment, when the type of the first processing stage is picture rate upsampling and/or temporal extrapolation, the pre-defined order is the output order.

[0245] According to an embodiment, when the type of the first processing stage is view synthesis, the pre-defined order is a view order.

[0246] According to an embodiment, when the first processing stage results into one or more generated pictures that have different video signal type(s) than that or those of the input pictures to the first processing stage, the pre-defined order is specified based on video signal type.

[0247] According to an embodiment, when the type of the first processing stage is spatial extrapolation, the pre-defined order may, for example, be the output order.

[0248] According to an embodiment, when the first processing stage has many types, the combination of the pre-defined orders for the single types of the first processing stage may be performed in a pre-defined order. For example, when the first processing stage performs both picture rate upsampling and depth map generation, the resulting generated texture picture may be included in the candidate picture list in output order, followed by the depth map pictures in output order.

[0249] According to an embodiment, the method further comprises encoding the first indication in a syntax structure with normative decoding.

[0250] Such syntax structure with normative decoding may include, for example, a picture parameter set, a picture header, a slice header, a picture trailer data unit, or a reference picture marking update data unit.

[0251] According to an embodiment, the method further comprises associating the first indication with a reconstructed picture by referring to the syntax structure from the coded picture or including the syntax structure in the coded picture, wherein decoding of the coded picture results into the reconstructed picture.

[0252] Herein, the reference to the syntax structure from the coded picture may be, for example, a parameter set identifier value. Including the syntax structure in the coded picture may be carried out, for example, by including the picture header or the slice header as a part of the coded picture.

[0253] The above method and embodiments may be applied to indicating postprocessing to be performed in a decoding process. Therein, one or more of the following embodiments may be applied. Additionally, embodiments for decoding and/or postprocessing are described.

[0254] According to an embodiment, the method further comprises indicating the processing order through encoding an SEI processing order SEI message. According to an embodiment, the method further comprises determining the processing order through decoding an SEI processing order SEI message.

[0255] Thus, for indicating post-processing stages, such as picture rate upsampling or temporal extrapolation, an appropriately defined SPO SEI message may be used.

[0256] According to an embodiment, the method further comprises indicating the noninitial processing stage through encoding an SEI processing order SEI message. According to an embodiment, the method further comprises determining the noninitial processing stage through decoding an SEI processing order SEI message.

[0257] According to an embodiment, the method further comprises indicating the noninitial processing stage by encoding one or more indications in an SEI processing order SEI message. According to an embodiment, the method further comprises determining the noninitial processing stage by decoding one or more indications from an SEI processing order SEI message. For example, the one or more indications may comprise a flag for each type of an SEI message, hereafter po_gen_pic_inp_allowed_flag[ i ], indicated in an SPO SEI message. po_gen_pic_inp_allowed_flag[ i ] equal to 1 specifies that the i-th type of an SEI message specifies a noninitial processing stage that may take one or more generated pictures as input. po_gen_pic_inp_allowed_flag[ i ] equal to 0 specifies that the i-th type of an SEI message does not specify a noninitial processing stage and takes no generated pictures as input. [0258] According to an embodiment, the method further comprises indicating the noninitial processing stage through encoding a processing order nesting SEI message. According to an embodiment, the method further comprises determining the noninitial processing stage through decoding a processing order nesting SEI message.

[0259] According to an embodiment, the method further comprises indicating the noninitial processing stage by encoding one or more indications in an processing order nesting SEI message. According to an embodiment, the method further comprises determining the noninitial processing stage by decoding one or more indications from an processing order nesting message. For example, the one or more indications may comprise a flag for each PON-nested SEI message, hereafter pon_gen_pic_inp_allowed_flag[ i ], indicated in a PON SEI message. pon_gen_pic_inp_allowed_flag[ i ] equal to 1 specifies that the i-th PON-nested SEI message specifies a noninitial processing stage that takes one or more generated pictures as input. pon_gen_pic_inp_allowed_flag[ i ] equal to 0 specifies that the i-th PON-nested SEI message does not specify a noninitial processing stage and takes no generated pictures as input.

[0260] According to an embodiment, the method further comprises identifying candidate noninitial processing stages following a processing stage that generates one or more generated pictures in the processing chain specified by the SEI processing order SEI message, identifying if the processing stage that generates one or more generated pictures persists or is active for a particular picture, and, if so, determining the noninitial processing stage to be such a processing stage among the candidate noninitial processing stages that persists or is active for the particular picture.

[0261] Hence, indications may be added in a PON SEI message to indicate which ones of the candidate pictures, consisting of the current picture and the interpolated and extrapolated pictures generated in any previous processing stages, the semantics of the PON-nested SEI messages apply. Thus, for example, a PON-nested NNPFA SEI message can be indicated to apply to only the interpolated pictures but not the current picture.

[0262] According to an embodiment, the method further comprises encoding the first indication into the processing order nesting SEI message; and associating the first indication with the at least one reconstructed picture through including the processing order nesting SEI message in a picture unit comprising a coded picture whose decoding results into the at least one reconstructed picture.

[0263] The PON SEI message may thus be used for indicating, by the first indication, that the noninitial processing stage is invoked to one or more indicated pictures and for associating the first indication with at least one reconstructed picture.

[0264] According to an embodiment, deriving the candidate picture list further comprises, for each processing stage in the processing order, replacing each of those pictures in the candidate picture list with a coinciding output picture resulting from the processing stage, if any, and inserting interpolated pictures and extrapolated pictures, if any, into the candidate picture list and placing them such that all pictures in the updated candidate picture list are in output order.

[0265] According to an embodiment, the first indication comprises or is accompanied by a syntax element (hereafter, pon_curr_pic_association_flag) that specifies whether the semantics of the PON-nested SEI messages in this PON SEI message apply to the processed current picture. According to an embodiment, an encoder includes pon_curr_pic_association_flag in the PON SEI message to specify whether the semantics of the PON-nested SEI messages in this PON SEI message apply to the processed current picture. According to an embodiment, a decoder decodes pon_curr_pic_association_flag from the PON SEI message to conclude whether the semantics of the PON-nested SEI messages in this PON SEI message apply to the processed current picture.

[0266] According to an embodiment, the first indication comprises or is accompanied by a syntax element (hereafter, pon gen pic association flag) that specifies whether the semantics of the PON-nested SEI messages in this PON SEI message apply to all the pictures in the candidate picture list that comprises only generated pictures (and not reconstructed pictures or processed reconstructed pictures). According to an embodiment, an encoder includes pon gen pic association flag in the PON SEI message to specify whether the semantics of the PON-nested SEI messages in this PON SEI message apply to all the pictures in the candidate picture list that comprises only generated pictures.

According to an embodiment, a decoder decodes pon_gen_pic_association_flag from the PON SEI message to conclude whether the semantics of the PON-nested SEI messages in this PON SEI message apply all the pictures in the candidate picture list that comprises only generated pictures.

[0267] The following discloses an example of the syntax of a PON SEI message for carrying out one or more of the above embodiments, where the PON SEI message provides nesting association to the current picture and/or all generated candidate pictures:

[0268] For each value of pon_processing_order[ i ] present in this SEI message, the following applies:

Let the list of pictures, denoted candGenPicList, consist of the following pictures in output order: o The pictures that have been generated by applying the post-processing stages of the associated SPO SEI message that have po_sei_processing_order[ i ] less than pon_processing_order[ i ] in this SEI message for the current picture and are associated with an output time that is not among the output times of the decoded pictures. When several pictures in candGenPicList have the same output time, they are ordered in the same order in candGenPicList as they appear in the list of NNPF output pictures.

Let the processed current picture be the picture that is associated with the same output time as that of the current picture and has been output by the post-processing stage of the associated SPO SEI message that has the greatest value of po_sei_processing_order[ i ] less than pon_processing_order[ i ] in this SEI message and outputs a picture that has an output time equal to that of the current picture.

[0269] pon_curr_pic_association_flag equal to 1 specifies that the semantics of the PON-nested SEI messages in this PON SEI message apply to the processed current picture. pon_curr_pic_association_flag equal to 0 specifies that the semantics of the PON-nested SEI messages in this PON SEI message do not apply to the processed current picture. [0270] pon_gen_pic_association_all_flag equal to 1 specifies that the semantics of the PON-nested SEI messages in this PON SEI message apply to all the pictures in candGenPicList. pon_gen_pic_association_flag equal to 0 specifies that the semantics of the PON-nested SEI messages in this PON SEI message do not apply to all the pictures in candGenPicList.

[0271 ] It is a requirement of bitstream conformance that pon_curr_pic_association_flag

+ pon gen pic association all flag shall be greater than or equal to 1.

[0272] According to an embodiment, the first indication comprises or is accompanied by one or more syntax elements that identify the generated pictures to which the semantics of the PON-nested SEI messages in this PON SEI message apply among the candidate picture list that comprises only generated pictures (and not reconstructed pictures or processed reconstructed pictures). According to an embodiment, an encoder includes one or more syntax elements in the PON SEI message to identify the generated pictures to which the semantics of the PON-nested SEI messages in this PON SEI message apply. According to an embodiment, a decoder decodes one or more syntax elements from the PON SEI message identifying the generated pictures to which the semantics of the PON- nested SEI messages in this PON SEI message apply.

[0273] Another example of the syntax of a PON SEI message (also referred to as option 2) for carrying out one or more of the above embodiments is disclosed below, where the PON SEI message provides nesting association to any candidate picture:

[0274] For each value of pon_processing_order[ i ] present in this SEI message, the list of pictures candGenPicList and the set of pictures candPicSet are derived as follows:

Let the list of pictures, denoted candGenPicList, consist of the following pictures in output order:

- The pictures that have been generated by applying the post-processing stages of the associated SPO SEI message that have po_sei_processing_order[ i ] less than pon_processing_order[ i ] in this SEI message for the current picture and are associated with an output time that is not among the output times of the decoded pictures. When several pictures in candGenPicList have the same output time, they are ordered in the same order in candGenPicList as they appear in the list of NNPF output pictures.

Let the set of pictures, denoted candPicSet, consist of the following pictures:

- All pictures in candGenPicList. - The picture that is associated with the same output time as that of the current picture and has been output the post-processing stage of the associated SPO SEI message that has the greatest value of po_sei_processing_order[ i ] less than pon_processing_order[ i ] in this SEI message and outputs a picture that has an output time equal to that of the current picture. This picture is called the processed current picture below.

[0275] pon_association_all_flag equal to 1 specifies that the semantics of each PON- nested SEI message in this PON SEI message apply individually to each picture in candPicSet. pon association all flag equal to 0 specifies that the semantics of the PON- nested SEI messages in this PON SEI message may not apply to each picture in candPicSet.

[0276] pon_curr_pic_association_flag equal to 1 specifies that the semantics of the PON-nested SEI messages in this PON SEI message apply to the processed current picture. pon_curr_pic_association_flag equal to 0 specifies that the semantics of the PON-nested SEI messages in this PON SEI message do not apply to the processed current picture.

[0277] pon_gen_pic_association_all_flag equal to 1 specifies that the semantics of the PON-nested SEI messages in this PON SEI message apply to all the pictures in candGenPicList. pon_gen_pic_association_flag equal to 0 specifies that the semantics of the PON-nested SEI messages in this PON SEI message do not apply to all the pictures in candGenPicList. When pon_curr_pic_association_flag is present and equal to 1 , pon gen pic association all flag is inferred to be equal to 0.

[0278] pon_association_cnt_minusl plus 1 specifies the count of pictures among candGenPicList to which the semantics of the PON-nested SEI messages in this PON SEI message apply.

[0279] pon_association_diff_minusl[ i ] is used to specify the i-th picture among candGenPicList to which the semantics of the PON-nested SEI messages in this PON SEI message apply. The variable assocGenPic[ j ], which specifies the j-th index of the picture among candGenPicList to which the semantics of the PON-nested SEI messages in this PON SEI message apply, is derived as follows: assocGenPic[ 0 ] = pon_association_diff_minusl [ 0 ] for( j = 1 ; j <= pon association cnt minusl ; j++ ) assocGenPic[ j ] = assocGenPic[ j - 1 ] + pon_association_diff_minusl[ j ] + 1 [0280] pon_alignment_zero_bit shall be equal to 0.

[0281] According to an embodiment, the method further comprises initiating the candidate picture list by cropped decoded pictures. [0282] Thus, at least in some embodiments, the candidate picture list may be initiated by cropped decoded pictures, as explained in the examples below.

[0283] Referring to the examples cases where an encoder may apply the method and/or one or more of the embodiments mentioned above, said examples can be realized with the proposed syntax as follows:

[0284] Example 1 : An interpolated picture is enhanced since the interpolator NNPF has low complexity and produces relatively poor picture quality. Thus, a visual enhancement filter is to be applied only on the interpolated pictures, but not to the cropped decoded pictures.

[0285] In an embodiment, an encoder or any other entity includes NNPFA and PON SEI messages in a bitstream as follows: Each picture unit for which the interpolator NNPF is activated includes a PON SEI message with a PON-nested NNPFA SEI message to activate the quality enhancement NNPF and the following syntax element values of the PON SEI message: pon association all flag = 0 (option 2 only) pon_curr_pic_association_flag = 0 pon gen pic association all flag = 1

[0286] Example 2: Consecutive pictures in output order belong to different temporal sublayers and have different qualities in a hierarchical fashion. An interpolator NNPF generates a picture 1, 3, 5, ... between each pair of cropped decoded pictures 0, 2, 4, 6, etc. Interpolated picture 1 will likely be of high quality and needs not be filtered by a quality enhancement NNPF. Interpolated picture 3 will likely be of lower quality than interpolated picture 1, and thus is to be visually enhanced with the quality enhancement NNPF. In an embodiment, an encoder or any other entity includes NNPFA and PON SEI messages in a bitstream as follows:

Picture unit 2 does not contain NNPFA SEI message to active the quality enhancement NNPF.

Picture unit 4 contains a PON SEI message with a PON-nested NNPFA SEI message and the following syntax element values of the PON SEI message: o pon association all flag = 0 (option 2 only) o pon_curr_pic_association_flag = 0 o pon_gen_pic_association_all flag = 1

[0287] Example 3: An NNPF has temporal extrapolation purpose and outputs multiple speculative coinciding output pictures, out of which one output picture is subsequently selected by an encoding system for further processing. The extrapolated picture for further processing can be indicated with option 2 as follows.

[0288] In an embodiment, an encoder or any other entity includes a PON SEI message in a bitstream as follows: The picture unit has a PON SEI message included in a suffix SEI NAL unit and with a PON-nested SEI message specifying the further processing step and the following syntax element values of the PON SEI message: pon association all flag = 0 pon_curr_pic_association_flag = 0 pon gen pic association all flag = 0 pon association cnt minusl = 0 pon association diff minusl [ 0 ] with such a value that the derived assocGenPic[ 0 ] is the index of the selected generated output picture for further processing.

[0289] Example 4: An encoder finetunes a quality-enhancement NNPF based on interpolated pictures of a random access segment and sends an NNPF update through an NNPFC SEI message. In an embodiment, an encoder or any other entity indicates the use of the base quality-enhancement NNPF for the current picture and the updated NNPF for the interpolated pictures that have been generated by activating the picture rate upsampling filter for the current picture as follows:

[0290] The current picture unit includes a PON SEI message ponA including an NNPFA SEI message that indicates the use of the base quality-enhancement NNPF for the current picture as follows: pon association all flag = 0 (option 2 only) pon_curr_pic_association_flag = 1 pon gen pic association all flag = 0 (inferred)

[0291] The current picture unit also includes a PON SEI message ponB including an NNPFA SEI message that indicates the use of an updated quality-enhancement NNPF for the interpolated pictures as follows: pon association all flag = 0 (option 2 only) pon_curr_pic_association_flag = 0 pon gen pic association all flag = 1

[0292] According to an embodiment, an encoder encodes into an NNPFC SEI message and/or a decoder decodes from an NNPFC SEI message one or more syntax elements indicative a proportional output times of extrapolated pictures, when temporal extrapolation is among the filtering purposes indicated by the NNPFC SEI message. The indicated output times may, for example, indicate a proportion between the output time of the current picture (for which the NNPF is activated) and the next picture in output order. For example, a number of relative clock ticks from the output time of the current picture to the output time of the next picture in output order may be pre-defined in the semantics of the NNPFC SEI message or indicated by the NNPFC SEI message, and the indicated output times of the extrapolated pictures may be indicated in units of these clock ticks. An example of syntax and semantics is provided below.

[0293] nnpfc_num_ticks_to_next_pic_minus2 plus 2 specifies the number of relative clock ticks from the output time of the current picture to the output time of the next picture in output order.

[0294] nnpfc_extrapolated_pic_output_diff[ i ] is used to derive the relative output time of the i-th extrapolated picture. The variable extrapolatedPicOutputTime[ i ], specifying the relative output time of the i-th extrapolated picture in units of the relative clock ticks specified above, is derived as follows:

[0295] If i is equal to 0, extrapolatedPicOutputTime[ 0 ] is set equal to 1 + nnpfc_extrapolated_pic_output_diff[ 0 ].

[0296] Otherwise (i is greater than 0), extrapolatedPicOutputTime[ i ] is set equal to extrapolatedPicOutputTime[ i - 1 ] + nnpfc_extrapolated_pic_output_diff[ i ]. [0297] It is a requirement of bitstream conformance that extrapolatedPicOutputTime[ i ] shall be less than nnpfc_num_ticks_to_next_pic_minus2 + 2.

[0298] When several extrapolated pictures have the same output time, any constraints that require output times to differ may be relaxed and further processing may be limited to only one picture of those that have the same output time.

[0299] According to embodiment, an encoder includes a flag, hereafter po_sei_process_flag[ i ], in an SPO SEI message to indicate if the i-th type of an SEI message indicates a process or a property. According to embodiment, a decoder decodes a flag, hereafter po_sei_process_flag[ i ], from an SPO SEI message to indicate if the i-th type of an SEI message indicates a process or a property.

[0300] According to an embodiment, when po_sei_process_flag[ i ] is equal to 0, it indicates a property of the previous process that is associated with the same processing order value.

[0301] According to an embodiment, an encoder includes one or more flags, hereafter po_sei_alternative_flag[ i ], to indicate between a cascade of processes that the same processing order value in any order and selection among the alternative processes with the same processing order value. According to an embodiment, a decoder decodes one or more flags, hereafter po_sei_alternative_flag[ i ], to determine between a cascade of processes that the same processing order value in any order and selection among the alternative processes with the same processing order value.

[0302] According to an embodiment, po_sei_alternative_flag[ i ] equal to 1 for i greater than 0 specifies that the processing of the i-th SEI message type is an alternative to the processing of the j -th SEI message type where the value of j is the greatest value less than i for which po_sei_processing_order[ j ] is equal to po_sei_processing_order[ i ] and po_sei_process_flag[ j ] is equal to 1.

[0303] According to an embodiment, po_sei_alternative_flag[ i ] equal to 0 for i greater than 0 specifies that the processing of the i-th SEI message type is not an alternative to the j-th SEI message type for any value of j in the range of 0 to i - 1, inclusive.

[0304] In some embodiments, poldx is set equal to po_processing_order[ i ], a list of values procList[ poldx ] consists of the values of j in increasing order such that po_sei_processing_order[ j ] is equal to poldx and po_sei_process _flag[ j ] is equal to 1, and maxProcIdx is set equal to the count of entries in procList[ poldx ] minus 1.

[0305] According to an embodiment, an encoder encodes an SPO SEI message as follows:

An encoder sets po_sei_alternative_flag[ procList[ poldx ][ 0 ] ] equal to 0. When po_sei_alternative_flag[ procList[ poldx ][ k ] ] is equal to 1 for any value of k in the range of 0 to maxProcIdx, inclusive, an encoder sets po_sei_alternative_flag[ procList[ poldx ][ k ] ] equal to 1 for all values of k in the range of 1 to maxProcIdx, inclusive.

[0306] According to an embodiment, a decoder decodes an SPO SEI message as follows: When po_sei_alternative_flag[ procList[ poldx ][ k ] ] is equal to 0 for any value of k in the range of 1 to maxProcIdx, inclusive, the decoder processes all the SEI messages types with index equal to procList[ poldx ][ k ] for all values of k in the range of 0 to maxProcIdx, inclusive, in a cascade (but in any order).

[0307] It is noted that in the embodiments above, the po_sei_alternative_flag[ i ] is equal to 0 for the first process having a particular processing order value. It is to be understood that if the processes of a particular processing order value are alternative processes to each other, embodiments may be similarly realized by requiring encoders to set the value of po_sei_alternative_flag[ i ] equal to 1 for all these processes. In this case, the semantics of po_sei_alternative_flag[ i ] equal to 1 indicates that all the processes having the same processing order value in the SPO SEI message are alternatives to each other.

[0308] In an embodiment, po_sei_alternative_flag[ i ] equal to 1 specifies that the processing of the i-th SEI message type is an alternative to the processing of the (i - 1 )-th SEI message type. po_sei_alternative_flag[ i ] equal to 0 specifies that the processing of the i-th SEI message type is not an alternative to the processing of the (i - 1 )-th SEI message type. It may be required that when i is equal to 0 or po_sei_processing_order[ i ] is not equal to po_sei_processing_order[ i - 1 ], po_sei_alternative_flag[ i ] shall be equal to 0. It may be required that when po_sei_alternative_flag[ i ] is equal to 1, the i-th SEI message type and the (i - l)-th SEI message type shall indicate processes. According to an embodiment, an encoder encodes an SPO SEI message including one or more po sei alternative _flag[ i ] syntax elements as specified in this paragraph. According to an embodiment, a decodes decodes an SPO SEI message including one or more po_sei_alternative_flag[ i ] syntax elements as specified in this paragraph. According to an embodiment, when i is greater than 0, po_sei_processing_order[ i ] is equal to po_sei_processing_order[ i - 1 ], and po_sei_alternative_flag[ i ] is equal to 0, a decoder processes both the i-th and ( i - 1 )-th SEI message types in a cascade (but in any order). [0309] An apparatus according to an aspect comprises means for inferring or indicating a processing order comprising multiple processing stages; means for inferring or indicating a noninitial processing stage among the multiple processing stages; means for encoding a first indication that the noninitial processing stage is invoked to one or more indicated pictures; and means for associating the first indication with at least one reconstructed picture, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

[0310] According to an embodiment, the noninitial processing stage is preceded by a preceding processing stage in the processing order, and the preceding processing stage involves interpolating or extrapolating one or more generated pictures based on at least one reconstructed picture.

[0311] According to an embodiment, the first indication further indicates that the noninitial processing stage is invoked for the one or more generated pictures.

[0312] According to an embodiment, the apparatus comprises means for deriving a candidate picture list comprising the one or more generated pictures; and means for encoding, in or along the first indication, a second indication that indicates a first picture among the candidate picture list to which the noninitial processing stage is invoked.

[0313] According to an embodiment, the candidate picture list comprises the at least one reconstructed picture and/or at least one processed reconstructed picture resulting from processing stages preceding the noninitial processing stage.

[0314] According to an embodiment, the apparatus comprises means for indicating the processing order through indicating enabled and/or disabled in-loop filters.

[0315] According to an embodiment, the means for deriving the candidate picture list further comprises means for initiating the candidate picture list with pictures present in the decoded picture buffer, and for each processing stage in the processing order, means for replacing each of those pictures in the candidate picture list with a coinciding processed reconstructed picture resulting from the processing stage, if any, and means for inserting interpolated pictures and extrapolated pictures, if any, into the candidate picture list and placing pictures in the derived candidate picture list in an output order.

[0316] According to an embodiment, the candidate picture list comprises the pictures present in the decoded picture buffer in a pre-defined order, such as output order.

[0317] According to an embodiment, the apparatus comprises means for encoding the first indication in a syntax structure with normative decoding, such as a picture parameter set, a picture header, a slice header, a picture trailer data unit, or a reference picture marking update data unit.

[0318] According to an embodiment, the apparatus comprises means for associating the first indication with a reconstructed picture by referring to the syntax structure from the coded picture or including the syntax structure in the coded picture, wherein decoding of the coded picture results into the reconstructed picture.

[0319] As a further aspect, there is provided 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: infer or indicate a processing order comprising multiple processing stages; infer or indicate a noninitial processing stage among the multiple processing stages; encode a first indication that the noninitial processing stage is invoked to one or more indicated pictures; and associate the first indication with at least one reconstructed picture, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

[0320] According to an embodiment, the noninitial processing stage is preceded by a preceding processing stage in the processing order, and the preceding processing stage involves interpolating or extrapolating one or more generated pictures based on at least one reconstructed picture.

[0321] According to an embodiment, the first indication further indicates that the noninitial processing stage is invoked for the one or more generated pictures.

[0322] According to an embodiment, the apparatus comprises code causing the apparatus to derive a candidate picture list comprising the one or more generated pictures; and encode, in or along the first indication, a second indication that indicates a first picture among the candidate picture list to which the noninitial processing stage is invoked.

[0323] According to an embodiment, the candidate picture list comprises the at least one reconstructed picture and/or at least one processed reconstructed picture resulting from processing stages preceding the noninitial processing stage.

[0324] According to an embodiment, the apparatus comprises code causing the apparatus to indicate the processing order through indicating enabled and/or disabled inloop filters.

[0325] According to an embodiment, the comprises code causing the apparatus to derive the candidate picture list further comprises code causing the apparatus to initiate the candidate picture list with pictures present in the decoded picture buffer, and for each processing stage in the processing order, replace each of those pictures in the candidate picture list with a coinciding processed reconstructed picture resulting from the processing stage, if any, and insert interpolated pictures and extrapolated pictures, if any, into the candidate picture list and placing pictures in the derived candidate picture list in an output order.

[0326] According to an embodiment, the candidate picture list comprises the pictures present in the decoded picture buffer in a pre-defined order, such as output order.

[0327] According to an embodiment, the apparatus comprises code causing the apparatus to encode the first indication in a syntax structure with normative decoding, such as a picture parameter set, a picture header, a slice header, a picture trailer data unit, or a reference picture marking update data unit.

[0328] According to an embodiment, the apparatus comprises code causing the apparatus to associate the first indication with a reconstructed picture by referring to the syntax structure from the coded picture or including the syntax structure in the coded picture, wherein decoding of the coded picture results into the reconstructed picture.

[0329] The decoding aspect may be implemented by an apparatus comprising means for identifying a processing order comprising multiple processing stages; means for identifying a noninitial processing stage among the multiple processing stages; means for decoding a first indication that at least one noninitial processing stage is invoked to one or more indicated pictures; and means for determining at least one reconstructed picture associated with the first indication, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

[0330] According to an embodiment, the noninitial processing stage is preceded by a preceding processing stage in the processing order, and the preceding processing stage involves interpolating or extrapolating one or more generated pictures based on at least one reconstructed picture.

[0331] According to an embodiment, the first indication further indicates that the noninitial processing stage is invoked for the one or more generated pictures.

[0332] According to an embodiment, the apparatus comprises means for deriving a candidate picture list comprising the one or more generated pictures; and means for decoding, from or along the first indication, a second indication that indicates a first picture among the candidate picture list to which the noninitial processing stage is invoked.

[0333] According to an embodiment, the candidate picture list comprises the at least one reconstructed picture and/or at least one processed reconstructed picture resulting from processing stages preceding the noninitial processing stage.

[0334] According to an embodiment, the apparatus comprises means for identifying the processing order through identifying enabled and/or disabled in-loop filters.

[0335] According to an embodiment, the means for deriving the candidate picture list further comprises means for initiating the candidate picture list with pictures present in the decoded picture buffer, and for each processing stage in the processing order, means for replacing each of those pictures in the candidate picture list with a coinciding processed reconstructed picture resulting from the processing stage, if any, and means for inserting interpolated pictures and extrapolated pictures, if any, into the candidate picture list and placing pictures in the derived candidate picture list in an output order.

[0336] According to an embodiment, the candidate picture list comprises the pictures present in the decoded picture buffer in a pre-defined order, such as output order.

[0337] According to an embodiment, the apparatus comprises means for decoding the first indication in a syntax structure with normative decoding, such as a picture parameter set, a picture header, a slice header, a picture trailer data unit, or a reference picture marking update data unit. [0338] According to an embodiment, the apparatus comprises means for determining the first indication being associated with a reconstructed picture by referring to the syntax structure from the coded picture or including the syntax structure in the coded picture, wherein decoding of the coded picture results into the reconstructed picture.

[0339] The decoding aspects may likewise be implemented in 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: identify a processing order comprising multiple processing stages; identify a noninitial processing stage among the multiple processing stages; decode a first indication that at least one noninitial processing stage is invoked to one or more indicated pictures; and determine at least one reconstructed picture associated with the first indication, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

[0340] According to an embodiment, the noninitial processing stage is preceded by a preceding processing stage in the processing order, and the preceding processing stage involves interpolating or extrapolating one or more generated pictures based on at least one reconstructed picture.

[0341] According to an embodiment, the first indication further indicates that the noninitial processing stage is invoked for the one or more generated pictures.

[0342] According to an embodiment, the apparatus comprises code causing the apparatus to derive a candidate picture list comprising the one or more generated pictures; and decode, from or along the first indication, a second indication that indicates a first picture among the candidate picture list to which the noninitial processing stage is invoked. [0343] According to an embodiment, the candidate picture list comprises the at least one reconstructed picture and/or at least one processed reconstructed picture resulting from processing stages preceding the noninitial processing stage.

[0344] According to an embodiment, the apparatus comprises code causing the apparatus to identify the processing order through identifying enabled and/or disabled inloop filters.

[0345] According to an embodiment, the code causing the apparatus to derive the candidate picture list further comprises code causing the apparatus to initiate the candidate picture list with pictures present in the decoded picture buffer, and for each processing stage in the processing order, replace each of those pictures in the candidate picture list with a coinciding processed reconstructed picture resulting from the processing stage, if any, and insert interpolated pictures and extrapolated pictures, if any, into the candidate picture list and placing pictures in the derived candidate picture list in an output order. [0346] According to an embodiment, the candidate picture list comprises the pictures present in the decoded picture buffer in a pre-defined order, such as output order.

[0347] According to an embodiment, the apparatus comprises code causing the apparatus to decode the first indication in a syntax structure with normative decoding, such as a picture parameter set, a picture header, a slice header, a picture trailer data unit, or a reference picture marking update data unit.

[0348] According to an embodiment, the apparatus comprises code causing the apparatus to determine the first indication being associated with a reconstructed picture by referring to the syntax structure from the coded picture or including the syntax structure in the coded picture, wherein decoding of the coded picture results into the reconstructed picture.

[0349] Such apparatuses may comprise e.g. all or a subset of the functional units disclosed in any of the appended Figures la, lb, and 4 - 7 for implementing the embodiments.

[0350] Such an apparatus further comprises code, stored in said at least one memory, which when executed by said at least one processor, causes the apparatus to perform one or more of the embodiments disclosed herein.

[0351] Figure 4 shows 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 5 shows a layout of an apparatus according to an example embodiment. [0352] 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. [0353] 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. [0354] The apparatus may comprise a microphone 36 or any suitable audio input which may be a digital or analogue signal input. The apparatus 50 may further comprise an audio output device which in embodiments of the invention may be any one of: an earpiece 38, speaker, or an analogue audio or digital audio output connection. The apparatus 50 may also comprise a battery (or in other embodiments of the invention the device may be powered by any suitable mobile energy device such as solar cell, fuel cell or clockwork generator). The apparatus may further comprise a camera capable of recording or capturing images and/or video. The apparatus 50 may further comprise an infrared port for short range line of sight communication to other devices. In other embodiments the apparatus 50 may further comprise any suitable short range communication solution such as for example a Bluetooth wireless connection or a USB/firewire wired connection.

[0355] The apparatus 50 may comprise a controller 56, processor or processor circuitry for controlling the apparatus 50. The controller 56 may be connected to memory 58 which in embodiments of the invention may store both data in the form of image and audio data and/or may also store instructions for implementation on the controller 56. The controller 56 may further be connected to codec circuitry 54 suitable for carrying out coding and decoding of audio and/or video data or assisting in coding and decoding carried out by the controller.

[0356] 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.

[0357] 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).

[0358] The apparatus 50 may comprise a camera capable of recording or detecting individual frames which are then passed to the codec 54 or the controller for processing. The apparatus may receive the video image data for processing from another device prior to transmission and/or storage. The apparatus 50 may also receive either wirelessly or by a wired connection the image for coding/decoding. The structural elements of apparatus 50 described above represent examples of means for performing a corresponding function. [0359] With respect to Figure 6, 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.

[0360] The system 10 may include both wired and wireless communication devices and/or apparatus 50 suitable for implementing embodiments of the invention.

[0361] For example, the system shown in Figure 9 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.

[0362] 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.

[0363] 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. [0364] 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.

[0365] 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.

[0366] Figure 7 is a graphical representation of an example multimedia communication system within which various embodiments may be implemented. A data source 1510 provides a source signal in an analog, uncompressed digital, or compressed digital format, or any combination of these formats. An encoder 1520 may include or be connected with a pre-processing, such as data format conversion and/or filtering of the source signal. The encoder 1520 encodes the source signal into a coded media bitstream. It should be noted that a bitstream to be decoded may be received directly or indirectly from a remote device located within virtually any type of network. Additionally, the bitstream may be received from local hardware or software. The encoder 1520 may be capable of encoding more than one media type, such as audio and video, or more than one encoder 1520 may be required to code different media types of the source signal. The encoder 1520 may also get synthetically produced input, such as graphics and text, or it may be capable of producing coded bitstreams of synthetic media. In the following, only processing of one coded media bitstream of one media type is considered to simplify the description. It should be noted, however, that typically real-time broadcast services comprise several streams (typically at least one audio, video and text sub-titling stream). It should also be noted that the system may include many encoders, but in the figure only one encoder 1520 is represented to simplify the description without a lack of generality. It should be further understood that, although text and examples contained herein may specifically describe an encoding process, one skilled in the art would understand that the same concepts and principles also apply to the corresponding decoding process and vice versa.

[0367] 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.

[0368] 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.

[0369] 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.

[0370] The server 1540 may or may not be connected to a gateway 1550 through a communication network, which may e.g. be a combination of a CDN, the Internet and/or one or more access networks. The gateway may also or alternatively be referred to as a middle-box. For DASH, the gateway may be an edge server (of a CDN) or a web proxy. It is noted that the system may generally comprise any number gateways or alike, but for the sake of simplicity, the following description only considers one gateway 1550. The gateway 1550 may perform different types of functions, such as translation of a packet stream according to one communication protocol stack to another communication protocol stack, merging and forking of data streams, and manipulation of data stream according to the downlink and/or receiver capabilities, such as controlling the bit rate of the forwarded stream according to prevailing downlink network conditions. The gateway 1550 may be a server entity in various embodiments.

[0371] The system includes one or more receivers 1560, typically capable of receiving, de-modulating, 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.

[0372] 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 1580 is discussed to simplify the description without a lack of generality.

[0373] The coded media bitstream may be processed further by a decoder 1580, whose output is one or more uncompressed media streams. Finally, a Tenderer 1590 may reproduce the uncompressed media streams with a loudspeaker or a display, for example. The receiver 1560, recording storage 1570, decoder 1570, and Tenderer 1590 may reside in the same physical device or they may be included in separate devices.

[0374] Processing implied by SEI messages as described in various embodiments may be performed by the decoder 1580, or by the Tenderer 1590, or both by the decoder 1580 and the Tenderer 1590, for example depending on the type of the processing. In another alternative, processing implied by SEI messages as described in various embodiments may be performed by a post-processor, which may be get the output of the decoder 1580 and provide its output to the Tenderer 1590. In yet another alternative, processing implied by SEI messages as described in various embodiments may be performed by two or more of the decoder 1580, the post-processor, or the Tenderer 1590.

[0375] A sender 1540 and/or a gateway 1550 may be configured to perform switching between different representations e.g. for switching between different viewports of 360- degree video content, view switching, bitrate adaptation and/or fast start-up, and/or a sender 1540 and/or a gateway 1550 may be configured to select the transmitted representation(s). Switching between different representations may take place for multiple reasons, such as to respond to requests of the receiver 1560 or prevailing conditions, such as throughput, of the network over which the bitstream is conveyed. In other words, the receiver 1560 may initiate switching between representations. A request from the receiver can be, e.g., a request for a Segment or a Subsegment from a different representation than earlier, a request for a change of transmitted scalability layers and/or sub-layers, or a change of a rendering device having different capabilities compared to the previous one. A request for a Segment may be an HTTP GET request. A request for a Subsegment may be an HTTP GET request with a byte range. Additionally or alternatively, bitrate adjustment or bitrate adaptation may be used for example for providing so-called fast start-up in streaming services, where the bitrate of the transmitted stream is lower than the channel bitrate after starting or random-accessing the streaming in order to start playback immediately and to achieve a buffer occupancy level that tolerates occasional packet delays and/or retransmissions. Bitrate adaptation may include multiple representation or layer up-switching and representation or layer down-switching operations taking place in various orders.

[0376] A decoder 1580 may be configured to perform switching between different representations e.g. for switching between different viewports of 360-degree video content, view switching, bitrate adaptation and/or fast start-up, and/or a decoder 1580 may be configured to select the transmitted representation(s). Switching between different representations may take place for multiple reasons, such as to achieve faster decoding operation or to adapt the transmitted bitstream, e.g. in terms of bitrate, to prevailing conditions, such as throughput, of the network over which the bitstream is conveyed.

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

[0377] In the above, some embodiments have been described with reference to and/or using terminology of HEVC and/or WC. It needs to be understood that embodiments may be similarly realized with any video encoder and/or video decoder.

[0378] In the above, some embodiments have been described with reference to the term processing chain. It is to be understood that a processing chain may define, but may not be limited to, a cascade of processes. Embodiments may be realized with any post-processing graph, such as a simple directed graph of processes, wherein a simple directed graph may be defined as a directed graph without loops.

[0379] In the above, some embodiments have been described with reference to SEI messages. It needs to be understood that embodiments may be similarly realized with any other similar syntax structures, such as metadata OBUs or registered ITU-T T.35 metadata. [0380] In the above, some embodiments have been described in relation to particular syntax elements and/or syntax structures. It needs to be understood that corresponding embodiments for encoding may be realized by including encoding steps for creating the particular syntax elements and/or syntax structures. Similarly, it needs to be understood that corresponding embodiments for decoding may be realized by including decoding steps for reading the particular syntax elements and/or syntax structures. Furthermore, when the decoded syntax elements and/or syntax structures imply certain processing, such as certain processing order of SEI messages, corresponding embodiments for decoding may include such processing steps.

[0381] In the above, some embodiments have been described with reference to encoding. It needs to be understood that said encoding may comprise one or more of the following: encoding source image data into a bitstream, encapsulating the encoded bitstream in a container file and/or in packet(s) or stream(s) of a communication protocol, and announcing or describing the bitstream in a content description, such as the Media Presentation Description (MPD) of ISO/IEC 23009-1 (known as MPEG-DASH) or the IETF Session Description Protocol (SDP). Similarly, some embodiments have been described with reference to decoding. It needs to be understood that said decoding may comprise one or more of the following: decoding image data from a bitstream, decapsulating the bitstream from a container file and/or from packet(s) or stream(s) of a communication protocol, and parsing a content description of the bitstream, [0382] In the above, where the example embodiments have been described with reference to an encoder or an encoding method, it needs to be understood that the resulting bitstream and the decoder or the decoding method 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.

[0383] It is to be understood that when embodiments refer to encoding or an encoder, embodiments may be similarly realized with other processing or entities, such as transcoding or a transcoder. In another example, SEI messages as described in embodiments may be generated as a post-processing for encoding.

[0384] It is to be understood that when embodiments refer to decoding or a decoder, embodiments may be similarly realized with other processing or entities, such as postprocessing (for decoding), a post-processor, rendering, or a Tenderer.

[0385] In general, the various embodiments of the invention may be implemented in hardware or special purpose circuits or any combination thereof. While various aspects of the invention may be illustrated and described as block diagrams 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.

[0386] 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.

[0387] 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.

[0388] 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 examples. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention.

Claims

1. An apparatus comprising: means for inferring or indicating a processing order comprising multiple processing stages; means for inferring or indicating a noninitial processing stage among the multiple processing stages; means for encoding a first indication that the noninitial processing stage is invoked to one or more indicated pictures; and means for associating the first indication with at least one reconstructed picture, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

2. The apparatus according to claim 1, wherein the noninitial processing stage is preceded by a preceding processing stage in the processing order, and the preceding processing stage involves interpolating or extrapolating one or more generated pictures based on at least one reconstructed picture.

3. The apparatus according to claim 2, wherein the first indication further indicates that the noninitial processing stage is invoked for the one or more generated pictures.

4. The apparatus according to claim 2 or 3, comprising: means for deriving a candidate picture list comprising the one or more generated pictures; and means for encoding, in or along the first indication, a second indication that indicates a first picture among the candidate picture list to which the noninitial processing stage is invoked.

5. The apparatus according to claim 4, wherein the candidate picture list comprises the at least one reconstructed picture and/or at least one processed reconstructed picture resulting from processing stages preceding the noninitial processing stage.

6. The apparatus according to any preceding claim, comprising: means for indicating the processing order through indicating enabled and/or disabled in-loop filters.

7. The apparatus according to claim 4 or 5, wherein the means for deriving the candidate picture list further comprises means for initiating the candidate picture list with pictures present in the decoded picture buffer, and for each processing stage in the processing order, means for replacing each of those pictures in the candidate picture list with a coinciding processed reconstructed picture resulting from the processing stage, if any, and means for inserting interpolated pictures and extrapolated pictures, if any, into the candidate picture list and placing pictures in the derived candidate picture list in an output order.

8. The apparatus according to any preceding claim, wherein the candidate picture list comprises the pictures present in the decoded picture buffer in a pre-defined order, such as output order.

9. The apparatus according to any preceding claim, comprising means for encoding the first indication in a syntax structure with normative decoding, such as a picture parameter set, a picture header, a slice header, a picture trailer data unit, or a reference picture marking update data unit.

10. The apparatus according to any preceding claim, comprising means for associating the first indication with a reconstructed picture by referring to the syntax structure from the coded picture or including the syntax structure in the coded picture, wherein decoding of the coded picture results into the reconstructed picture.

11. A method comprising: inferring or indicating a processing order comprising multiple processing stages; inferring or indicating a noninitial processing stage among the multiple processing stages; encoding a first indication that the noninitial processing stage is invoked to one or more indicated pictures; and associating the first indication with at least one reconstructed picture, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

12. An apparatus comprising: means for identifying a processing order comprising multiple processing stages; means for identifying a noninitial processing stage among the multiple processing stages; means for decoding a first indication that at least one noninitial processing stage is invoked to one or more indicated pictures; and means for determining at least one reconstructed picture associated with the first indication, wherein the one or more indicated pictures differ from the at least one reconstructed picture.

13. The apparatus according to claim 12, wherein the noninitial processing stage is preceded by a preceding processing stage in the processing order, and the preceding processing stage involves interpolating or extrapolating one or more generated pictures based on at least one reconstructed picture.

14. The apparatus according to claim 13, wherein the first indication further indicates that the noninitial processing stage is invoked for the one or more generated pictures.

15. The apparatus according to claim 13 or 14, comprising: means for deriving a candidate picture list comprising the one or more generated pictures; and means for decoding, from or along the first indication, a second indication that indicates a first picture among the candidate picture list to which the noninitial processing stage is invoked.

16. The apparatus according to claim 15, wherein the candidate picture list comprises the at least one reconstructed picture and/or at least one processed reconstructed picture resulting from processing stages preceding the noninitial processing stage.

17. The apparatus according to any of claims 12 - 16, comprising: means for identifying the processing order through identifying enabled and/or disabled in-loop filters.

18. The apparatus according to claim 15 or 16, wherein the means for deriving the candidate picture list further comprises means for initiating the candidate picture list with pictures present in the decoded picture buffer, and for each processing stage in the processing order, means for replacing each of those pictures in the candidate picture list with a coinciding processed reconstructed picture resulting from the processing stage, if any, and means for inserting interpolated pictures and extrapolated pictures, if any, into the candidate picture list and placing pictures in the derived candidate picture list in an output order.

19. The apparatus according to any of claims 12 - 18, wherein the candidate picture list comprises the pictures present in the decoded picture buffer in a pre-defined order, such as output order.

20. The apparatus according to any of claims 12 - 19, comprising means for decoding the first indication in a syntax structure with normative decoding, such as a picture parameter set, a picture header, a slice header, a picture trailer data unit, or a reference picture marking update data unit.

21. The apparatus according to any of claims, comprising means for determining the first indication being associated with a reconstructed picture by referring to the syntax structure from the coded picture or including the syntax structure in the coded picture, wherein decoding of the coded picture results into the reconstructed picture.

22. A method comprising: identifying a processing order comprising multiple processing stages; identifying a noninitial processing stage among the multiple processing stages; decoding a first indication that at least one noninitial processing stage is invoked to one or more indicated pictures; and determining at least one reconstructed picture associated with the first indication, wherein the one or more indicated pictures differ from the at least one reconstructed picture.