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

Abstract

The embodiments concern a method comprising encoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer; encoding into the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer; encoding into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and deriving a scaling factor for the local illumination compensation that is applied to the sample of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is not clipped to avoid negative values. Also, technical equipment for implementing the method is disclosed.

Description

AN APPARATUS, A METHOD AND A COMPUTER PROGRAM FOR VIDEO CODING AND DECODINGTECHNICAL FIELD

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

[0002] A video codec may comprise an encoder which transforms input video into a compressed representation suitable for storage and / or transmission and a decoder that can uncompress the compressed video representation back into a viewable form, or either one of them. Typically, the encoder discards some information in the original video sequence in order to represent the video in a more compact form, for example at a lower bit rate.

[0003] In High Efficiency Video Coding (HEVC) standard, a picture may refer to a frame comprising a matrix of luma samples and possibly the corresponding chroma samples. Chroma samples may be either monochrome format or non-monochrome format. Monochrome samples contain single color component, whereas non-monochrome format contains three color components.SUMMARY

[0004] As an improvement to video coding, the present embodiments provide a solution to enable inter-layer reference prediction between layers having different chroma samples.

[0005] 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.

[0006] An apparatus according to a first aspect comprises means for encoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer; means for encoding into the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in thefirst layer; and means for encoding into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and means for deriving a scaling factor for the local illumination compensation that is applied to the sample of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to a reference block is not clipped to avoid negative values.

[0007] According to an embodiment, the apparatus further comprises means for encoding into the bitstream a second coding unit for the picture in the second layer a syntax element indicating reference prediction from another picture in the second layer; and means for encoding into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the second coding unit; and means for deriving a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the second layer, when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is clipped to avoid negative values.

[0008] According to an embodiment, the indication has been encoded into a parameter set or header that use of local illumination is enabled.

[0009] According to an embodiment, a syntax flag indicates whether or not clipping is applied.

[0010] According to an embodiment, the apparatus further comprises means for encoding into the bitstream a first chroma format syntax element for the first layer; and means for encoding into the bitstream a second chroma format syntax element for the second layer, wherein one of the first or second chroma format syntax elements indicates a monochrome format, and the other of the first and second chroma format syntax elements indicates the non-monochrome format.

[0011] According to an embodiment, the first layer further contains a first value of auxiliary picture identifier and the second layer further contains a second value of auxiliary picture identifier, wherein the second value of auxiliary picture identifier differs from the first value of auxiliary picture identifier.

[0012] According to an embodiment, the second layer is directly dependent from the first layer.

[0013] According to an embodiment, further comprising means for including the first layer as an inter-layer reference picture in a reference picture list of the current picture.

[0014] An apparatus according to a second aspect comprises 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 encode a bitstream comprising a first layer with afirst value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer; to encode into the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer; and to encode into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and to derive a scaling factor for the local illumination compensation that is applied to the sample of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to a reference block is not clipped to avoid negative values.

[0015] According to an embodiment, the apparatus is further caused to encode into the bitstream a second coding unit for the picture in the second layer a syntax element indicating reference prediction from another picture in the second layer; and encode into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the second coding unit; and derive a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the second layer, when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is clipped to avoid negative values.

[0016] According to an embodiment, the indication has been encoded into a parameter set or header that use of local illumination is enabled.

[0017] According to an embodiment, a syntax flag indicates whether or not clipping is applied.

[0018] According to an embodiment, the apparatus is further caused to encode into the bitstream a first chroma format syntax element for the first layer; and means for encoding into the bitstream a second chroma format syntax element for the second layer, wherein one of the first or second chroma format syntax elements indicates a monochrome format, and the other of the first and second chroma format syntax elements indicates the non-monochrome format.

[0019] According to an embodiment, the first layer further contains a first value of auxiliary picture identifier and the second layer further contains a second value of auxiliary picture identifier, wherein the second value of auxiliary picture identifier differs from the first value of auxiliary picture identifier.

[0020] According to an embodiment, the second layer is directly dependent from the first layer.

[0021] According to an embodiment, further comprising means for including the first layer as an inter-layer reference picture in a reference picture list of the current picture.

[0022] A method according to a third aspect comprises encoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layeridentifier, wherein the first layer is used for inter-layer reference prediction for the second layer; encoding into the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer; encoding into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and deriving a scaling factor for the local illumination compensation that is applied to the sample of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.

[0023] According to a fourth aspect, there is provided computer program product comprising computer program code configured to, when executed on at least one processor, cause an apparatus or a system to encode a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer; encode into the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer; encode into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and derive a scaling factor for the local illumination compensation that is applied to the sample of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.

[0024] According to a fifth aspect, there is provided an apparatus comprising means for decoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer; means for decoding from the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer; means for decoding from the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and means for deriving a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.

[0025] According to a sixth 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, whichwhen executed by said at least one processor, causes the apparatus to decode a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer; decode from the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer; decode from the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and derive a scaling factor for the local illumination compensation that is applied to the sample of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.

[0026] According to a seventh aspect, there is provided a method comprising decoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer; decoding from the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer; decoding from the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and deriving a scaling factor for the local illumination compensation that is applied to the sample of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.

[0027] According an eighth aspect, there is provided computer program product comprising computer program code configured to, when executed on at least one processor, cause an apparatus or a system to decode a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer; decode from the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer; decode from the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and derive a scaling factor for the local illumination compensation that is applied to the sample of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.

[0028] According to an embodiment, the computer program product is embodied on a non- transitory computer readable medium.

[0029] The method as described above also comprises steps for previous embodiments. The apparatuses and the computer readable storage mediums stored with code thereon, as described above, are thus arranged to carry out the above method and one or more of the embodiments related thereto.BRIEF DESCRIPTION OF THE DRAWINGS

[0030] For better understanding of the present invention, reference will now be made by way of example to the accompanying drawings in which:

[0031] Figures 1 shows schematically an encoder suitable for implementing embodiments of the invention

[0032] Figure 2 shows schematically a decoder suitable for implementing embodiments of the invention;

[0033] Figure 3 shows example of a primary picture and an auxiliary picture;

[0034] Figure 4 illustrates a flow chart of a method according to an embodiment of the invention;

[0035] Figure 5 illustrates a flow chart of a method according to another embodiment of the invention;

[0036] Figure 6 illustrates a flow chart of a method according to yet another embodiment of the invention;

[0037] Figure 7 shows schematically an electronic device employing embodiments of the invention;

[0038] Figure 8 shows schematically a user equipment suitable for employing embodiments of the invention;

[0039] Figure 9 further shows schematically electronic devices suitable for employing embodiments of the invention connected using wireless and wired network connections; and

[0040] Figure 10 shows a schematic diagram of an example multimedia communication system within which various embodiments may be implemented.DETAILED DESCRIPTON OF SOME EXAMPLE EMBODIMENTS

[0041] Before describing the embodiments further, a brief reference to evolution of video coding standardization is given. The present embodiments are suited within the context of next generation video coding standardization, e.g., H.267 video coding standard.

[0042] Video codec consists of an encoder that transforms the input video into a compressed representation suited for storage / transmission and a decoder that can uncompress the compressed video representation back into a viewable form. A video encoder and / or a video decoder may also be separate from each other, i.e. need not form a codec. Typically, an encoder discards some information in the original video sequence in order to represent the video in a more compact form (that is, at lower bitrate).

[0043] 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 may be 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).

[0044] 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.

[0045] 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 non-integer 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.

[0046] 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.

[0047] One outcome of the coding procedure is a set of coding parameters, such as motion vectors and quantized transform coefficients. Many parameters can be entropy-coded more efficiently if they are predicted first from spatially or temporally neighboring parameters. For example, a motion vector may be predicted from spatially adjacent motion vectors and only the difference relative to the motion vector predictor may be coded. Prediction of coding parameters and intra prediction may be collectively referred to as in-picture prediction.

[0048] 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. As discussed above, the encoder may discard and / or lose some information in the original video sequence in order to represent the video in a more compact form (that is, at lower bitrate). Figure 1 shows an encoder comprising elements to perform an encoding process. Figure 2 shows a decoder comprising elements to perform a decoding process. The encoder and the decoder are suitable for employing embodiments of the invention. In Figure 1, there are an image to be encoded (In); apredicted 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 (Pint™); mode selection (MS) and filtering (F).

[0049] In Figure 2, there are 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 (T'1); 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).

[0050] 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 may be 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.

[0051] 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. Contextbased coding may for example be context adaptive binary arithmetic coding (CAB AC) 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.

[0052] 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 isassociated 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.

[0053] 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 Organization 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).

[0054] Version 1 of the High Efficiency Video Coding (H.265 / HEVC a.k.a. HEVC) standard was developed by the Joint Collaborative Team - Video Coding (JCT-VC) of VCEG and MPEG. The standard was published by both parent standardization organizations, and it is referred to as ITU-T Recommendation H.265 and ISO / IEC International Standard 23008-2, also known as MPEG-H Part 2 High Efficiency Video Coding (HEVC). Later versions of H.265 / HEVC included scalable, multiview, fidelity range, three-dimensional, and screen content coding extensions which may be abbreviated SHVC, MV-HEVC, REXT, 3D-HEVC, and SCC, respectively.

[0055] 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.

[0056] Exploration towards a next video coding standard, which may become ITU-T H.267, is ongoing in JVET. An exploratory video codec, known as ECM or Enhanced Compression Model, is maintained by JVET.

[0057] 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.

[0058] Some key definitions, bitstream and coding structures, and concepts of some video coding 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 reference video coding specifications.

[0059] Some video coding specifications specify the bitstream syntax and semantics as well as the decoding process for error-free bitstreams, whereas the encoding process is not specified, but encoders must generate conforming bitstreams. Bitstream and decoder conformance can be verified with the Hypothetical Reference Decoder (HRD). Some video coding specifications 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 decoding process might not have been specified for erroneous bitstreams.

[0060] In many cases, the elementary unit for the input to an encoder and the output of a decoder is a picture. A picture given as an input to an encoder may also be referred to as a source picture, and a picture decoded by a decoded may be referred to as a decoded picture.

[0061] 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).

[0062] The video may be 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.

[0063] Samples of a sample array have a certain bit depth, such as 8 bits per sample or 10 bits per sample. A bit depth implicitly specifies a value range, which may be referred to as the full range. For example, the full range is from 0 to 255, inclusive, for 8 bits per sample, or from 0 to 1023, inclusive, for 10 bits per sample. The source video may use allocate a narrower sample value range than the full range. A specific value range, sometimes referred to as the studio range, has been specified in the ITU-T H.273 standard specifying coding-independent code points for video. A source value range may interchangeably be referred to as a source samplevalue range, and may be defined as the sample value range of the video that is given as input to a video encoder to be encoded.

[0064] In some video coding 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 comprise monochrome format and non-monochrome formats, and these 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.

[0065] In some video coding specifications, such as 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.

[0066] 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.

[0067] 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 threesample 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.

[0068] 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).

[0069] 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 signaled 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 signaled in the bitstream allowing the decoder to reproduce the intended structure of these units.

[0070] To be able to utilize parallel processing, images can be split into independently codable and decodable image segments (slices or tiles). Slices typically refer to image segments constructed of certain number of basic coding units that are processed in default coding or decoding order, while tiles typically refer to image segments that have been defined as rectangular image regions that are processed at least to some extend as individual frames.

[0071] 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 syntaxelements 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.

[0072] 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.

[0073] 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.

[0074] 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-compensatedprediction 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 colocated blocks in temporal reference pictures and the used motion field information is signaled among a list of motion field candidate list filled with motion field information of available adjacent / co-located blocks.

[0075] In some coding specifications, 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 short-term reference".

[0076] In some coding specifications, a list of motion vector prediction candidates may comprise, but may not be limited to, two kinds of candidates: spatial candidates and temporal candidates, where temporal candidates may also be referred to as TMVP candidates or temporal motion vector predictor candidates. A reference index for a collocated picture that comprises the temporal motion vector predictor may be indicated by the encoder, e.g., in the slice header (e.g. as a collocated ref idx syntax element). TMVP may be obtained from the collocated picture from a pre-defined position in relation to the current prediction unit being coded or decoded. TMVP may be scaled according to the proportions of the picture order count differences of the reference picture of the temporal motion vector predictor, the collocated picture, and the current picture.

[0077] 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.

[0078] Typical Video encoders may utilize Lagrangian cost functions to find optimal coding modes, e.g., the desired Macroblock mode and associated motion vectors. This kind of cost function uses a weighting factor A to tie together the (exact or estimated) image distortion due to lossy coding methods and the (exact or estimated) amount of information that is required to represent the pixel values in an image area:C = D + R (Eq. 1) where C is the Lagrangian cost to be minimized, D is the image distortion (e.g., Mean Squared Error) with the mode and motion vectors considered, and R the number of bits needed to represent the required data to reconstruct the image block in the decoder (including the amount of data to represent the candidate motion vectors).

[0079] Coding standards may specify “profiles” and “levels.” A profile may be defined as a subset of algorithmic features of the standard (of the encoding algorithm or the equivalent decoding algorithm). In another definition, a profile is a specified subset of the syntax of the standard (and hence implies that the encoder may only use features that result into a bitstream conforming to that specified subset and the decoder may only support features that are enabled by that specified subset).

[0080] A level may be defined as a set of limits to the coding parameters that impose a set of constraints in decoder resource consumption. In another definition, a level is a defined set of constraints on the values that may be taken by the syntax elements and variables of the standard. These constraints may be simple limits on values. Alternatively or in addition, they may take the form of constraints on arithmetic combinations of values (e.g., picture width multiplied by picture height multiplied by number of pictures decoded per second). Other means for specifying constraints for levels may also be used. Some of the constraints specified in a level may for example relate to the maximum picture size, maximum bitrate and maximum data rate in terms of coding units, such as macroblocks, per a time period, such as a second. The same set of levels may be defined for all profiles. It may be preferable for example to increase interoperability of terminals implementing different profiles that most or all aspects of the definition of each level may be common across different profiles.

[0081] An indicated profile and level can be used to signal properties of a media stream and / or to signal the capability of a media decoder.

[0082] In HEVC, a reference picture set (RPS) syntax structure and decoding process are used. A reference picture set valid or active for a picture includes all the reference pictures used as reference for the picture and all the reference pictures that are kept marked as “used for reference” for any subsequent pictures in decoding order. There are six subsets of the reference picture set, which are referred to as namely RefPicSetStCurrO (a.k.a. RefPicSetStCurrBefore), RefPicSetStCurrl (a.k.a. RefPicSetStCurrAfter), RefPicSetStFollO, RefPicSetStFolll,RefPicSetLtCurr, and RefPicSetLtFoll. RefPicSetStFollO and RefPicSetStFolll may also be considered to form jointly one subset RefPicSetStFoll. The notation of the six subsets is as follows. “Curr” refers to reference pictures that are included in the reference picture lists of the current picture and hence may be used as inter prediction reference for the current picture. “Foil” refers to reference pictures that are not included in the reference picture lists of the current picture but may be used in subsequent pictures in decoding order as reference pictures. “St” refers to short-term reference pictures, which may generally be identified through a certain number of least significant bits of their POC value. “Lt” refers to long-term reference pictures, which are specifically identified and generally have a greater difference of POC values relative to the current picture than what can be represented by the mentioned certain number of least significant bits. “0” refers to those reference pictures that have a smaller POC value than that of the current picture. “1” refers to those reference pictures that have a greater POC value than that of the current picture. RefPicSetStCurrO, RefPicSetStCurrl, RefPicSetStFollO and RefPicSetStFolll are collectively referred to as the short-term subset of the reference picture set. RefPicSetLtCurr and RefPicSetLtFoll are collectively referred to as the long-term subset of the reference picture set.

[0083] In HEVC, a reference picture set may be specified in a sequence parameter set and taken into use in the slice header through an index to the reference picture set. A reference picture set may also be specified in a slice header. A reference picture set may be coded independently or may be predicted from another reference picture set (known as inter-RPS prediction). In both types of reference picture set coding, a flag (used_by_curr_pic_X_flag) is additionally sent for each reference picture indicating whether the reference picture is used for reference by the current picture (included in a *Curr list) or not (included in a *Foll list). Pictures that are included in the reference picture set used by the current slice are marked as “used for reference”, and pictures that are not in the reference picture set used by the current slice are marked as “unused for reference”. If the current picture is an IDR picture, RefPicSetStCurrO, RefPicSetStCurrl, RefPicSetStFollO, RefPicSetStFolll, RefPicSetLtCurr, and RefPicSetLtFoll are all set to empty.

[0084] 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. Since many video coding specifications, such as HEVC and VVC, 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 bufferingprocess 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.

[0085] A reference picture list may be defined as list of reference pictures that is available for use in inter prediction.

[0086] In some coding modes, the 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.

[0087] In some video coding specifications, such as HEVC and VVC, two reference picture lists (reference picture list 0 and reference picture list 1) are generated for a bi-predictive (B) slice, and one reference picture list (reference picture list 0) is formed for an inter-coded (P) slice.

[0088] In some video coding specifications, such as HEVC, a reference picture list, such as reference picture list 0 and reference picture list 1, is constructed in two steps: First, an initial reference picture list is generated. The initial reference picture list may be generated for example on the basis of picture order count (POC). Second, the initial reference picture list may be reordered by reference picture list reordering (RPLR) commands, also known as reference picture list modification syntax structure, which may be contained in slice headers. If reference picture sets are used, the reference picture list 0 may be initialized to contain RefPicSetStCurrO first, followed by RefPicSetStCurrl, followed by RefPicSetLtCurr. Reference picture list 1 may be initialized to contain RefPicSetStCurrl first, followed by RefPicSetStCurrO. In HEVC, the initial reference picture lists may be modified through the reference picture list modification syntax structure, where pictures in the initial reference picture lists may be identified through an entry index to the list. In other words, in HEVC, reference picture list modification is encoded into a syntax structure comprising a loop over each entry in the final reference picture list, where each loop entry is a fixed-length coded index to the initial reference picture list and indicates the picture in ascending position order in the final reference picture list.

[0089] In some video coding specifications, such as VVC, reference picture lists are indicated directly in a reference picture list syntax structure rather than indicating reference picture sets and using an initialization and optional reordering process as described above. When a picture is present in any reference picture list of the current picture (within active or inactive entries of any reference picture list), it marked as "used for long-term reference" or "used for short-term reference". When a picture is present in no reference picture list of the current picture, it is marked as "unused for reference". The abbreviation RPL may be used to refer to the reference picture list syntax structure and / or to one or more reference picture lists.

[0090] In some video coding specifications, such as HEVC or VVC, the decoding process may derive a reference index refldxLO for a current prediction unit or a current coding unit being (de)coded and, when the current prediction unit or the current coding unit is bi-predictive, another reference refldxLl. The variable refldxLO may be a reference index in relation to reference picture list 0, and the variable refldxLl may be a reference index in relation to reference picture list 1. The variables refldxLO and refldxLl may indicate the reference pictures for inter prediction of the current prediction unit or the current coding unit.

[0091] In some video coding specifications, such as HEVC, the decoding process may derive reference index arrays RefIdxL0[ x ][ y ] and RefldxLl [ x ][ y ], which include the reference index for reference picture list 0 and 1, respectively, for sample coordinates x (horizontal) and y (vertical), where sample coordinates may, for example, be luma sample coordinates, but could likewise be implemented for another granularity, such as in units of 4 luma samples or in units of the smallest width or height for a prediction unit.

[0092] Scalable video coding refers to coding structure where one bitstream can contain multiple representations of the content, for example at different bitrates, resolutions or frame rates. In these cases, the receiver can extract the desired representation depending on its characteristics (e.g., resolution that matches best the display device). Alternatively, a server or a network element can extract the portions of the bitstream to be transmitted to the receiver depending on e.g., the network characteristics or processing capabilities of the receiver. A scalable bitstream may consist of a “base layer” providing the lowest quality video available and one or more enhancement layers that enhance the video quality when received and decoded together with the lower layers. To improve coding efficiency for the enhancement layers, the coded representation of that layer typically depends on the lower layers. For example, the motion and mode information of the enhancement layer can be predicted from lower layers. Similarly, the pixel data of the lower layers can be used to create prediction for the enhancement layer.

[0093] A scalable video codec for quality scalability (also known as Signal-to-Noise or SNR) and / or spatial scalability may be implemented as follows. For a base layer, a conventional non- scalable video encoder and decoder is used. The reconstructed / decoded pictures of the base layer are included in the reference picture buffer for an enhancement layer. In H.264 / AVC, H.265 / HEVC, and similar codecs using reference picture list(s) for inter prediction, the base layer decoded pictures may be inserted into a reference picture list(s) for coding / decoding of an enhancement layer picture similarly to the decoded reference pictures of the enhancement layer. Consequently, the encoder may choose a base-layer reference picture as inter prediction reference and indicate its use typically with a reference picture index in the coded bitstream.The decoder decodes from the bitstream, for example from a reference picture index, that a base-layer picture is used as inter prediction reference for the enhancement layer. When a decoded base-layer picture is used as prediction reference for an enhancement layer, it is referred to as an inter-layer reference picture.

[0094] While the previous paragraph described a scalable video codec with two scalability layers with an enhancement layer and a base layer, it needs to be understood that the description can be generalized to any two layers in a scalability hierarchy with more than two layers. In this case, a second enhancement layer may depend on a first enhancement layer in encoding and / or decoding processes, and the first enhancement layer may therefore be regarded as the base layer for the encoding and / or decoding of the second enhancement layer. Furthermore, it needs to be understood that there may be inter-layer reference pictures from more than one layer in a reference picture buffer or reference picture lists of an enhancement layer, and each of these inter-layer reference pictures may be considered to reside in a base layer or a reference layer for the enhancement layer being encoded and / or decoded. Furthermore, it needs to be understood that other types of inter-layer processing than reference-layer picture upsampling may take place instead or additionally. For example, the bit-depth of the samples of the reference-layer picture may be converted to the bit-depth of the enhancement layer and / or the sample values may undergo a mapping from the color space of the reference layer to the color space of the enhancement layer.

[0095] In addition to quality scalability, there are also other scalability modes, such as• Spatial scalability, where enhancement layer pictures are coded at a higher resolution than the base layer pictures;• Bit-depth scalability, where enhancement layer pictures are coded at higher bitdepth (e.g. 10 or 12 bits) than base layer pictures (e.g. 8 bits).

[0096] In all of the above scalability cases, base layer information could be used to code enhancement layer to minimize the additional bitrate overhead.

[0097] Scalability can be enabled in two basic ways. For example, new coding modes may be introduced for performing prediction of pixel values or syntax from lower layers of the scalable representation. Alternatively, the lower layer pictures may be placed to the reference picture buffer (decoded picture buffer, DPB) of the higher layer. The first approach is more flexible and thus can provide better coding efficiency in most cases. However, the second, i.e., reference frame-based scalability, approach can be implemented very efficiently with minimal changes to single layer codecs while still achieving majority of the coding efficiency gains available. A reference frame-based scalability codec can be implemented by utilizing the same hardware orsoftware implementation for all the layers, just taking care of the DPB management by external means.

[0098] In scalable video coding, a scalability dimension may be defined as an indication of the type of scalability present in a bitstream. For example, scalability dimensions may be specified for texture vs. depth scalability, view scalability, spatial and / or quality scalability, and auxiliary video (e.g. alpha plane).

[0099] Inter-layer prediction may be defined as prediction in a manner that is dependent on data elements (e.g., sample values or motion vectors) of reference pictures from a different layer than the layer of the current picture (being encoded or decoded). Many types of inter-layer prediction exist and may be applied in a scalable video encoder / decoder.

[0100] The types of inter-layer prediction may comprise, but may not be limited to, one or more of the following: inter-layer sample prediction, inter-layer motion prediction (a.k.a. inter-layer motion vector prediction), inter-layer residual prediction. In inter-layer sample prediction, at least a subset of the reconstructed sample values of a source picture for inter-layer prediction are used as a reference for predicting sample values of the current picture. In inter-layer motion prediction, at least a subset of the motion vectors of a source picture for inter-layer prediction are used as a reference for predicting motion vectors of the current picture. Typically, predicting information on which reference pictures are associated with the motion vectors is also included in inter-layer motion prediction. For example, the reference indices of reference pictures for the motion vectors may be inter-layer predicted and / or the picture order count or any other identification of a reference picture may be inter-layer predicted. In some cases, inter-layer motion prediction may also comprise prediction of block coding mode, header information, block partitioning, and / or other similar parameters. In some cases, coding parameter prediction, such as inter-layer prediction of block partitioning, may be regarded as another type of interlayer prediction. In inter-layer residual prediction, the prediction error or residual of selected blocks of a source picture for inter-layer prediction is used for predicting the current picture.

[0101] A direct reference layer may be defined as a layer that may be used for inter-layer prediction of another layer for which the layer is the direct reference layer. A direct predicted layer may be defined as a layer for which another layer is a direct reference layer. An indirect reference layer may be defined as a layer that is not a direct reference layer of a second layer but is a direct reference layer of a third layer that is a direct reference layer or indirect reference layer of a direct reference layer of the second layer for which the layer is the indirect reference layer. An indirect predicted layer may be defined as a layer for which another layer is an indirect reference layer. An independent layer may be defined as a layer that does not havedirect reference layers. In other words, an independent layer is not predicted using inter-layer prediction. A non-base layer may be defined as any other layer than the base layer, and the base layer may be defined as the lowest layer in the bitstream. An independent non-base layer may be defined as a layer that is both an independent layer and a non-base layer.

[0102] A source picture for inter-layer prediction may be defined as a decoded picture that either is, or is used in deriving, an inter-layer reference picture that may be used as a reference picture for prediction of the current picture. In multi-layer HEVC extensions, an inter-layer reference picture is included in an inter-layer reference picture set of the current picture. An inter-layer reference picture (ILRP) may be defined as a reference picture that may be used for inter-layer prediction of the current picture. In the coding and / or decoding process, the inter-layer reference pictures may be treated as long term reference pictures. A reference-layer picture may be defined as a picture in a direct reference layer of a particular layer or a particular picture, such as the current layer or the current picture (being encoded or decoded). A reference-layer picture may but need not be used as a source picture for inter-layer prediction. Sometimes, the terms reference-layer picture and source picture for inter-layer prediction may be used interchangeably.

[0103] A source picture for inter-layer prediction may be required to be in the same access unit as the current picture. In some cases, e.g. when no resampling, motion field mapping or other inter-layer processing is needed, the source picture for inter-layer prediction and the respective inter-layer reference picture may be identical. In some coding specifications and in some cases, e.g. when HEVC is in use and resampling is needed to match the sampling grid of the reference layer to the sampling grid of the layer of the current picture (being encoded or decoded), interlayer processing is applied to derive an inter-layer reference picture from the source picture for inter-layer prediction.

[0104] In some video coding specifications, such as HEVC, inter-layer reference picture set(s) or list(s) are derived for a current picture. These inter-layer reference picture lists may be assigned to variables RefPicSetlnterLayerO and RefPicSetlnterLayerl. The assignment of an inter-layer reference picture to RefPicSetlnterLayerO or RefPicSetlnterLayerl may be based on view identifier values. Reference picture list 0 and 1 may be initialized to comprise pictures in RefPicSetlnterLayerO and RefPicSetlnterLayerl, respectively, at a pre-defined order in relation temporal reference pictures.

[0105] In some video coding specifications, such as VVC, inclusion of inter-layer reference picture(s) in reference picture lists are indicated directly in a reference picture list syntax structure.

[0106] In some video coding specifications, such as HEVC, an inter-layer reference picture is temporarily assigned to be treated like a long-term reference picture for the decoding of a current picture. At the end of the decoding of the current picture, the reference picture marking of the inter-layer reference picture is restored (e.g., to "used for short-term reference").

[0107] In some video coding specifications, a reference picture that is not an ILRP may be referred to as a temporal reference picture. In other words, a temporal reference picture may be defined to be such a reference picture that resides in the same scalability layer as the current picture.

[0108] In some coding specifications, the spatial correspondence of a reference-layer picture and an enhancement-layer picture may be inferred or may be indicated with one or more types of so- called reference layer location offsets. In HEVC, reference layer location offsets may be included in the PPS by the encoder and decoded from the PPS by the decoder. Reference layer location offsets may be used for but are not limited to achieving region-of-interest (RO I) scalability. Reference layer location offsets may comprise one or more of scaled reference layer offsets, reference region offsets, and resampling phase sets. Scaled reference layer offsets may be considered to specify the horizontal and vertical offsets between the sample in the current picture that is collocated with the top-left luma sample of the reference region in a decoded picture in a reference layer and the horizontal and vertical offsets between the sample in the current picture that is collocated with the bottom-right luma sample of the reference region in a decoded picture in a reference layer. Another way is to consider scaled reference layer offsets to specify the positions of the comer samples of the upsampled reference region relative to the respective corner samples of the enhancement layer picture. The scaled reference layer offset values may be signed. Reference region offsets may be considered to specify the horizontal and vertical offsets between the top-left luma sample of the reference region in the decoded picture in a reference layer and the top-left luma sample of the same decoded picture as well as the horizontal and vertical offsets between the bottom-right luma sample of the reference region in the decoded picture in a reference layer and the bottom-right luma sample of the same decoded picture. The reference region offset values may be signed. A resampling phase set may be considered to specify the phase offsets used in resampling process of a source picture for interlayer prediction. Different phase offsets may be provided for luma and chroma components.

[0109] In reference picture resampling (RPR), which may also be called adaptive resolution change (ARC), the decoding process of a picture may refer to one or more previous reference pictures that have a different spatial resolution for inter prediction. Consequently, a resampling of the reference pictures for operation of the inter-picture prediction process may be applied. Ingeneral, resampling may be either picture-wise or block-wise. In picture-wise resampling, an entire reference picture is resampled, whereas in block-wise resampling, an area within a reference picture, such as a reference block for motion compensation, is resampled. The blockwise resampling may be performed as a part of the motion compensation process. Resampling may generally comprise downsampling and upsampling.

[0110] A scaling window may be signaled for and associated with a picture. Scaling windows of a picture and its reference picture may indicate the spatial correspondence between the pictures. Scaling windows may be used to achieve one or both of the following: i) a horizontal scaling ratio and a vertical scaling ratio may be derived based on the width ratio and the height ratio, respectively, of the scaling windows; ii) a left offset and a top offset for inter prediction may be derived. The horizontal and vertical scaling ratios may be used as resampling ratios for RPR. The left and top offsets may be used in deriving a spatial correspondence between a picture and its reference picture. The left and top offsets may be added in the reference picture location derivation through a motion vector scaled by a scaling ratio. For example, the top-left corner of the current block is mapped to the respective “anchor location” in the reference picture through the left and top offsets, and a motion vector scaled by a scaling ratio is then applied to obtain a reference block in relation to anchor location. In general, the scaling ratio may be 1 : 1, in which case the left and top offsets may be applied without resampling. It needs to be understood that there may be other means to signal scaling ratio(s), top offset, and / or left offset than scaling windows.

[0111] In VVC, a scaling window may be indicated in a PPS explicitly or implicitly. When indicated explicitly, pps scaling win left offset, pps scaling win right offset, pps scaling win top offset, and pps scaling win bottom offset specify the offsets of the scaling window boundaries relative to the left, right, top, and bottom boundaries of the conformance cropping window, respectively. When indicated implicitly, the scaling window is inferred to be identical to the conformance cropping window.

[0112] In some coding specifications, such as VVC, the spatial correspondence of an ILRP and a current picture may be indicated through their scaling windows.

[0113] A multi-layer bitstream is a bitstream comprising multiple layers, which may be, but are not limited to, base and enhancement layers as discussed above for scalable video coding. A multi-layer bitstream may additionally or alternatively comprise independent layers that do not have inter-layer prediction relationship between each other and may even represent different types of content.

[0114] Layers of a multi-layer bitstream may be identified by a layer identifier or layer ID. In some video coding specifications, such as HEVC and VVC, the layer ID is represented by the nuh layer id syntax element. In some video coding specifications, such as AVI, the spatial id syntax element of an extension of an OBU header (obu extension header) may be regarded as a layer ID.

[0115] Video coding standards and specifications may allow encoders to divide a coded picture to coded slices or alike. In-picture prediction is typically disabled across slice boundaries. Thus, slices can be regarded as a way to split a coded picture to independently decodable pieces. In H.264 / AVC and HEVC, in-picture prediction may be disabled across slice boundaries. Thus, slices can be regarded as a way to split a coded picture into independently decodable pieces, and slices are therefore often regarded as elementary units for transmission. In many cases, encoders may indicate in the bitstream which types of in-picture prediction are turned off across slice boundaries, and the decoder operation takes this information into account for example when concluding which prediction sources are available. For example, samples from a neighboring CU may be regarded as unavailable for intra prediction, if the neighboring CU resides in a different slice.

[0116] 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.

[0117] 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.

[0118] Syntax structures may be specified, for example, using arithmetic, logical, relational, bitwise, 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.

[0119] Bit-wise operations may be defined as follows:

[0120] & bit-wise "and". When operating on integer arguments, operates on a two's complement representation of the integer value. When operating on a binary argument that contains fewer bits than another argument, the shorter argument is extended by adding more significant bits equal to 0.

[0121] | bit- wise "or". When operating on integer arguments, operates on a two's complement representation of the integer value. When operating on a binary argument that contains fewer bits than another argument, the shorter argument is extended by adding more significant bits equal to 0.

[0122] Abit- wise "exclusive or". When operating on integer arguments, operates on a two's complement representation of the integer value. When operating on a binary argument that contains fewer bits than another argument, the shorter argument is extended by adding more significant bits equal to 0.

[0123] x » y arithmetic right shift of a two's complement integer representation of x by y binary digits. This function is defined only for non-negative integer values of y. Bits shifted into the most significant bits (MSBs) as a result of the right shift have a value equal to the MSB of x prior to the shift operation.

[0124] x « y arithmetic left shift of a two's complement integer representation of x by y binary digits. This function is defined only for non-negative integer values of y. Bits shifted into the least significant bits (LSBs) as a result of the left shift have a value equal to 0.

[0125] 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 uppercase 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.

[0126] Syntax structures may be specified with reference to syntax functions. Semantics and / or decoding process may be specified with reference to mathematical and other functions. The Clip3 function may be specified as follows: ; z < x ; z > y; otherwise

[0127] Video coding specifications may define an elementary unit that for the output an of an encoder and / or for the input to a decoder. For example, such an elementary unit may be an open bitstream unit (OBU), as specified e.g. in AVI, or a Network Abstraction Layer (NAL) unit, as specified e.g. in HEVC or VVC.

[0128] In some video codecs, 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 H.264 / AVC and HEVCfor transmission or storage environments that do not provide framing structures. The bytestream format separates NAL units from each other by attaching a start code in front of each NAL unit. To avoid false detection of NAL unit boundaries, encoders run a byte-oriented start code emulation prevention algorithm, which adds an emulation prevention byte to the NAL unit payload if a start code would have occurred otherwise. In order to enable straightforward gateway operation between packet- and stream-oriented systems, start code emulation prevention may always be performed regardless of whether the bytestream format is in use or not. A NAL unit may be defined as a syntax structure containing an indication of the type of data to follow and bytes containing that data in the form of an RBSP interspersed as necessary with emulation prevention bytes. A raw byte sequence payload (RBSP) may be defined as a syntax structure containing an integer number of bytes that is encapsulated in a NAL unit. An RBSP is either empty or has the form of a string of data bits containing syntax elements followed by an RBSP stop bit and followed by zero or more subsequent bits equal to 0.

[0129] In some video coding specifications, a bitstream may be defined as a sequence of bits, in the form of a NAL unit stream or a byte stream, that forms the representation of coded pictures and associated data forming one or more coded video sequences. A first bitstream may be followed by a second bitstream in the same logical channel, such as in the same file or in the same connection of a communication protocol. An elementary stream (in the context of video coding) may be defined as a sequence of one or more bitstreams. The end of the first bitstream may be indicated by a specific NAL unit, which may be referred to as the end of bitstream (EOB) NAL unit and which is the last NAL unit of the bitstream.

[0130] 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.

[0131] NAL units consist of 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), 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.

[0132] 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-layermay be equivalently called a sub-layer, temporal sublayer, sublayer, or temporal level. Temporal sub-layers may be enumerated, e.g., from 0 (zero) upwards. The lowest temporal sublayer, 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 sublayer 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 sublayer 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.

[0133] 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.

[0134] 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 ! d_plusl syntax element may be regarded as a temporal identifier for the NAL unit, and a zero-based Temporalld variable may be derived as follows: Temporalld = temporal_id_plusl - 1.

[0135] 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.

[0136] 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 NALunit, an end of bitstream NAL unit, or a filler data NAL unit. Parameter sets may be needed for the reconstruction of decoded pictures, whereas many of the other non-VCL NAL units are not necessary for the reconstruction of decoded sample values.

[0137] Some coding formats specify parameter sets that may carry parameter values needed for the decoding or reconstruction of decoded pictures. A parameter may be defined as a syntax element of a parameter set. A parameter set may be defined as a syntax structure that contains parameters and that can be referred to from or activated by another syntax structure, for example, using an identifier.

[0138] In some video coding specifications, parameters that remain unchanged through a coded video sequence may be included in an SPS. In some video coding specifications, parameters that remain unchanged through a coded layer video sequence may be included in an SPS. In addition to the parameters that may be needed by the decoding process, the SPS 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 PPS RBSPs or one or more SEI NAL units containing a buffering period SEI message. PPS contains such parameters that are likely to be unchanged in several coded pictures. PPS RBSP may include parameters that can be referred to by the coded slice NAL units of one or more coded pictures.

[0139] In some video coding specifications, such as 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 PSS referred to by a syntax element found in each slice segment header.

[0140] A VPS RBSP may include parameters that can be referred to by one or more SPS RBSPs.

[0141] The relationship and hierarchy between VPS, SPS, and 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.

[0142] 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.

[0143] Instead of or in addition to parameter sets at different hierarchy levels (e.g., sequence and picture), video coding formats may include header syntax structures, such as a sequence header or a picture header.

[0144] A sequence header may precede any other data of the coded video sequence in the bitstream order. It may be allowed to repeat a sequence header in the bitstream, e.g., to provide a sequence header at a random-access point.

[0145] A picture header may precede any coded video data for the picture in the bitstream order. A picture header may be interchangeably referred to as a frame header. Some video coding specifications may enable carriage of a picture header in a dedicated picture header NAL unit or a frame header OBU or alike. Some video coding specifications may enable carriage of a picture header in a NAL unit, OBU, or alike syntax structure that also contains coded picture data.

[0146] 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.

[0147] A coded picture is a coded representation of a picture.

[0148] 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.

[0149] 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.

[0150] In some coding formats, 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 include at most one coded picture at any scalability layer (e.g., with any specific value of nuh layer id in some coding formats, such as HEVC or VVC). In some coding formats, an access unit comprises one or more complete picture units. In some coding formats, in addition to including the VCL NAL units of a coded picture, an access unit may also include non-VCL NAL units associated with the coded picture. Said specified classification rule may, for example, associate pictures with the same output time or picture order count value into the same access unit.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] 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.

[0155] 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.

[0156] 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) that is decodable independently of other pictures in the same layer.

[0157] 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.

[0158] 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.

[0159] In many coding modes of H.264 / AVC and HEVC, the reference picture for inter prediction is indicated with an index to a reference picture list. The index may be coded with variable length coding, which usually causes a smaller index to have a shorter value for the corresponding syntax element. In H.264 / AVC and HEVC, two reference picture lists (reference picture list 0 and reference picture list 1) are generated for each bi-predictive (B) slice, and one reference picture list (reference picture list 0) is formed for each inter-coded (P) slice.

[0160] 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.

[0161] 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.

[0162] Motion parameter types or motion information may include but are not limited to one or more of the following types: an indication of a prediction type (e.g. intra prediction, uni -prediction, bi-prediction) and / or a number of reference pictures; an indication of a prediction direction, such as inter (a.k.a. temporal) prediction, inter-layer prediction, inter-view prediction, view synthesis prediction (VSP), and inter-component prediction (which may be indicated per reference picture and / or per prediction type and where in some embodiments inter-view and view-synthesis prediction may be jointly considered as one prediction direction) and / or an indication of a reference picture type, such as a short-term reference picture and / or a long-term reference picture and / or an inter-layer reference picture (which may be indicated e.g. per reference picture) a reference index to a reference picture list and / or any other identifier of a reference picture (which may be indicated e.g. per reference picture and the type of which may depend on the prediction direction and / or the reference picture type and which may be accompanied byother relevant pieces of information, such as the reference picture list or alike to which reference index applies); a horizontal motion vector component (which may be indicated e.g. per prediction block or per reference index or alike); a vertical motion vector component (which may be indicated e.g. per prediction block or per reference index or alike); one or more parameters, such as picture order count difference and / or a relative camera separation between the picture containing or associated with the motion parameters and its reference picture, which may be used for scaling of the horizontal motion vector component and / or the vertical motion vector component in one or more motion vector prediction processes (where said one or more parameters may be indicated e.g. per each reference picture or each reference index or alike); coordinates of a block to which the motion parameters and / or motion information applies, e.g. coordinates of the top-left sample of the block in luma sample units; extents (e.g. a width and a height) of a block to which the motion parameters and / or motion information applies.

[0163] 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 (MM VD)- 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

[0164] Multi-layer bitstreams

[0165] Both HEVC and VVC standards support multi-layer bitstreams comprising layers identified by a layer ID value in the NAL unit header. Multi-layer bitstreams can be used to support scalable or multi-view coding, simulcast layers, or auxiliary picture layers.

[0166] In VVC, the VPS provides syntax to indicate whether each layer is dependently coded, or coded as an independent layer. For layers being dependently coded, inter-layer reference prediction may be used. For each dependent layer, the layer(s) upon which it is dependent are signaled within a bitstream. The signaling can be done within the VPS

[0167] VVC defines an inter-layer reference picture (ILRP) as follows:A picture in the same access unit (AU) with the current picture, with nuh layer id less than the nuh layer id of the current picture, and is marked as "used for longterm reference".

[0168] The HEVC and VVC standards include a sps chroma format idc syntax element in the SPS to indicate if the chroma format of the sequence is monochrome, or non-monochrome, such as 4:2:0, 4:2:2, or 4:4:4. Monochrome sequences contain a single color component (i.e., the luma component), while the non-monochrome formats contain three color components (i.e., one luma component and two chroma components). The values for sps chroma format idc for different chroma formats are illustrated in the following table:

[0169] The Versatile Supplemental Enhancement Information (VSEI) standard includes a Scalability Dimension Information (SDI) SEI message which enables signaling of an auxiliary picture type for individual layers, including alpha channels and depth maps, and to signal information about associated primary picture layers. In a Committee Draft document for a future version of VSEI, an additional auxiliary picture type is defined for object masks.

[0170] In pictures depth maps, alpha channel, and object masks sources typically include only a single color component and can be coded using a monochrome color format.

[0171] The HEVC Video Usability Information (VUI) has a neutral chroma indication flag, which can be used to indicate that all chroma component sample values are equal to 1 « ( BitDepthC - 1 ). This flag can be set when monochrome content is coded in a non-monochromeformat having 3 color components (4:2:0, 4:2:2, or 4:4:4). The HEVC Main profile does not support the chroma format being monochrome.

[0172] The scalable HEVC extension (SHVC), multiview HEVC extension (MV-HEVC), and multilayer profiles of VVC allow inter-layer prediction of motion vectors using temporal motion vector prediction (TMVP), for improved bitrate efficiency when coding motion vectors. In HEVC temporal motion vector prediction, the co-located motion vector of a reference picture is used as a predictor for coding the motion vector of a coding unit of the current picture. Inter layer reference pictures (ILRPs) may be added to a reference picture list for the current picture and treated similarly to temporal reference pictures from the same layer by coding tools.

[0173] In the VVC VPS, the vps_direct_ref_layer_flag[ i ][ j ] syntax element is used to indicate if the layer with index] is not a direct reference layer for the layer with index i. There is a semantic constraint on the vps_direct_ref_layer_flag[ i ][ j ] syntax element that the sps chroma format idc values for the i-th and j -th layers are equal, as shown in the semantics that follow: vps_direct_ref_layer_flag[ i ][ j ] equal to 0 specifies that the layer with index] is not a direct reference layer for the layer with index i. vps direct ref layer flag [ i ][ j ] equal to 1 specifies that the layer with index] is a direct reference layer for the layer with index i.For any two different values of i and j, both in the range of 0 to vps max layers minusl, inclusive, when dependencyFlag[ i ][ j ] equal to 1, it is a requirement of bitstream conformance that the values of sps chroma format idc and sps_bitdepth_minus8 that apply to the i-th layer shall be equal to the values of sps chroma format idc and sps_bitdepth_minus8, respectively, that apply to the j -th layer.

[0174] The semantics for the vps_direct_ref_layer_flag[ i ][ j ] syntax element also include a requirement that the bit depth be identical for the i-th and -jth layers.

[0175] In the HEVC semantics for direct_dependency_flag[ i ][ j ], there is a semantic constraint that inter-layer prediction may not take place for different values of Auxld. direct_dependency_flag[ i ][ j ] equal to 0 specifies that the layer with index] is not a direct reference layer for the layer with index i. direct_dependency_flag[ i ][ j ] equal to 1 specifies that the layer with index j is a direct reference layer for the layer with index i. When direct_dependency_flag[ i ][ j ] is not present for i and j in the range of 0 to MaxLayersMinusl, it is inferred to be equal to 0.It is a requirement of bitstream conformance thatAuxldf IdDirectRefLayerf nuhLayerldA ][ j ] ] for any values of nuhLayerldA and j shall be equal to Auxldf nuhLayerldA ], when Auxldf nuhLayerldA ] is in the range of 0 to 2, inclusive.

[0176] This means that no prediction takes place between layers with a different value of Auxld, when Auxld is in the range of 0 to 2, inclusive.

[0177] The HEVC semantics include derivation of the variable DependencyFlag[ i ][ j ], based upon for direct_dependency_flag[ i ][ j ].

[0178] The VPS extension semantics of HEVC include direct dependency typef i ][ j ] syntax element, which indicates the type(s) of dependency between the layers identified by indices i and j, wherein dependency types include inter-layer sample prediction and inter-layer motion prediction.

[0179] The MV-HEVC extension to HEVC has a constraint in the Multiview Main profile that the chroma format is 4:2:0 for all layers.

[0180] The 3D-HEVC extension to HEVC enables coding of multiple views, for which each view contains a texture layer and a depth layer. The 3D-HEVC extension includes several additional coding tools for efficient coding of depth. Decoder implementations of 3D-HEVC require support for these additional coding tools.

[0181] The Joint Video Experts Team (JVET) is developing the Enhanced Compression Model (ECM) as an exploratory project, as documented in JVET-AH2025 “Algorithm description of Enhanced Compression Model 13 (ECM 13).” The ECM includes a Local Illuminance Compensation (LIC) tool, as described in JVET-AH2025:

[0182] LIC is an inter prediction technique to model local illumination variation between current block and its prediction block as a function of that between current block template and reference block template. The parameters of the function can be denoted by a scale a and an offset P, which forms a linear equation, that is, a*p[x]+P to compensate illumination changes, where p[x] is a reference sample pointed to by MV at a location x on reference picture. Since a and P can be derived based on current block template and reference block template, no signaling overhead is required for them, except that an LIC flag is signaled for AMVP mode to indicate the use of LIC.

[0183] In multi-layer bitstream, primary picture layers are typically coded using nonmonochrome format, i.e., color formats with three color components to represent YUV, RGB, or XYZ color spaces. Auxiliary pictures, such as alpha channels and depth maps, commonly typically have monochrome source formats.

[0184] Figure 3 shows an example of a texture picture 310 and a depth channel picture 320 wherein the texture picture 310 is so called “primary picture” and depth channel picture 320 is so called “auxiliary picture”. It is to be noted that the texture picture 310 in Figure 3 is coded using color formats, even though for reproduction reasons the texture picture 310 is shown as black-and-white picture.

[0185] Auxiliary pictures 320 and their associated primary pictures 310 represent different aspects of the same scene containing the same objects and often have visible similarities, as illustrated in the example in Figure 3. Object motion in auxiliary picture 320 and their associated primary pictures 310 is expected to be similar, because the same objects are represented in both, implying correlation between the motion vectors in a primary picture layer and an auxiliary picture layer.

[0186] According to the VVC standard, as it is currently written, layers that have different values of chroma format syntax element, i.e., sps chroma format idc are not allowed to have interlayer reference prediction. This disables inter-layer motion prediction and inter-layer sample prediction.

[0187] In the HEVC extensions for MV-HEVC and SHVC the profiles require that all layers have the same value of chroma format syntax element, i.e., chroma format idc.

[0188] Currently, new Multilayer profiles have been proposed for HEVC in JVET-AI0045, wherein the new Multilayer profiles include Multilayer 4:4:4 10 and Multiview 4:4:4 12. These profiles allow support for monochrome as well as non-monochrome, i.e., 4:0:0, 4:2:2, and 4:4:4 chroma formats. However, the profiles have a restriction that reference layers must have the same chroma format as the current layer. These profiles also have a restriction that reference layers must have the same bit depth as the current layer.

[0189] Now an improved method is provided that enables inter-layer reference prediction between layers with different chroma formats to improve coding efficiency. The present embodiments enable auxiliary picture to use inter-layer reference prediction from an associated texture primary picture. The auxiliary picture comprises monochrome layers containing, for example depth, alpha, or object masks, and an associated texture primary picture comprises nonmonochrome layers. Term “non-monochrome layer” in this description refers to layer with three color components.

[0190] The various embodiments are provided, where i) a first embodiment enables a monochrome layer to use inter-layer reference prediction from a non-monochrome layer for a video codec similar to VVC. Inter-layer sample prediction and motion vector prediction are supported.ii) a second embodiment enables a monochrome layer to use inter-layer reference prediction from a non-monochrome layer for new profiles for HEVC. Inter-layers sample prediction and motion vector prediction are supported. iii) a third embodiment complements the first embodiment to enable the non-monochrome layer to use inter-layer reference prediction from the monochrome layer. Inter-layer sample prediction and motion vector prediction are supported. iv) a fourth embodiment enables a layer to use inter-layer motion prediction from a layer with a different bit depth; v) a fifth embodiment modifies the LIC tool to improves the coding efficiency for interlayer sample prediction of auxiliary picture using associated primary pictures as interlayer reference pictures; vi) a sixth embodiment modifies default reference picture list construction when a reference layer has a different chroma forma or bit depths than the current layer.

[0191] Figure 4 illustrates a flowchart of a method for encoding according to embodiment i) - iv) of the invention. The method comprises encoding 410 a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier wherein either of the first layer or the second layer is directly dependent on the other so that one is used for inter-layer reference prediction for the other; encoding 420 into a bitstream a syntax element indicating the inter-layer reference prediction between the first layer and the second layer; encoding 430 into a bitstream a first chroma format syntax element for the first layer and encoding into a bitstream a second chroma format syntax element for the second layer, wherein one of the first or second chroma format syntax elements indicates a monochrome format, and the other of the first or second chroma format syntax elements indicates the non- monochrome format.

[0192] The syntax element indicating inter-layer reference prediction between the first layer and the second layer can be encoded to parameter set, such as a VPS, or a header. The first chroma format syntax element for the first layer can be encoded to a parameter set, such as a SPS, or a header. The second chroma format syntax element for the second layer can be encoded into a parameter set, such as a SPS, or header.

[0193] The embodiments i) - iv) are discussed in more detailed manner in the following:

[0194] Embodiment i) Inter-layer prediction of monochrome from non-monochrome reference for V VC-like codec

[0195] This embodiment refers to VVC. However, the teachings given in this section, are applicable in any codec that evolves from or has similar functionalities and requirements as VVC.

[0196] A monochrome layer may be indicated with chroma format syntax element, such as sps chroma format idc equal to 0. According to the embodiment i) the monochrome layer is dependently predicted from a reference layer, being a non-monochrome layer, with 4:2:0, 4:2:2, or 4:4:4 chroma format (with chroma format syntax element, such as sps chroma format idc equal to 1, 2, or 3, respectively.)

[0197] In this embodiment, the semantics vps_direct_ref_layer_flag[ i ][ j ] syntax element in the VVC VPS are modified, wherein vps_direct_ref_layer_flag[ i ][ j ] indicates that coded pictures in the layer with layer identifier (ID) associated with the index i may use inter-layer reference prediction from a coded picture in the same access unit in the layer with layer ID associated with the index j. The semantics from the VVC VPS for this syntax element related to the values of sps chroma format idc that apply to the i-th and j -th layers are modified. These changes can be used in a new Multilayer profile defined for VVC or for a new video coding standard that is similar to VVC.

[0198] The constraint appearing currently in the semantics of the vps_direct_ref_layer_flag[ i ][ j ] syntax element in the VPS is removed, thanks to the present embodiments. The constraint has specified that for any two different values of i and j, both in the range of 0 to vps max layers minusl, inclusive, when dependencyFlag[ i ][ j ] equal to 1, it is a requirement of bitstream conformance that the values of sps chroma format idc that applies to the i-th layer shall be equal to the values of sps chroma format idc that applies to the j -th layer.

[0199] Instead, as a result of the present embodiments, the following condition is added to the semantics of the vps_direct_ref_layer_flag[ i ][ j ] syntax element in the VPS:- It is a requirement of bitstream conformance that the value of sps chroma format idc that applies to the i-th layer shall be equal to 0 or equal to the value of sps chroma format idc that applies to the j -th layer.

[0200] With this modification, in a multi-layer bitstream, pictures in a monochrome layer may be coded using inter-layer sample prediction and / or inter-layer motion vector prediction from pictures in a non-monochrome layer in the same access unit. When coding the monochrome current picture, the direct dependent non-monochrome layer may be included as an inter-layer reference picture (ILRP) in a reference picture list of the current picture. Temporal motionvector prediction (TMVP) may be selected for a Coding Unit (CU) in the current monochrome picture that uses the reference index corresponding to that non-monochrome ILRP, such that the motion vector predictor is formed using a motion vector in the ILRP.

[0201] Inter-layer sample prediction may also be used when coding the current monochrome picture, using the luma samples of the non-monochrome layer ILRP.

[0202] To use this embodiment, a multi-layer bitstream is encoded to contain a first layer with a first value of layer ID, and a second layer with a second value of layer ID that is directly dependent on the first layer, wherein the layer ID of the first layer being less than the layer ID of the second layer. According to an embodiment, in the Video Parameter Set (VPS), vps_direct_ref_layer_flag[ i ][ j ] is set to 1, where i corresponds to second layer with the second value of layer ID and j corresponds to the first layer with the first value of layer ID. According to an embodiment, in the Sequence Parameter Set (SPS), the sps chroma format idc syntax element for the first layer is set equal to 1, 2, or 3, representing 4:2:0, 4:2:2, or 4:4:4 non- monochrome format, which contains three color components. The sps chroma format idc syntax element for the second layer may be set equal to 0, representing monochrome which contains 1 color component.

[0203] Because vps_direct_ref_layer_flag[ i ][ j ] is equal to 1, a reference list for a coded picture in the second layer may contain the ILRP. When a CU is coded, the motion vector of the co-located block in the ILRP may be used as a predictor for coding the current CU motion vector, either as merge candidate or a motion vector predictor candidate.

[0204] Embodiment ii) Inter-layer prediction of monochrome from non-monochrome reference for new HEVC profiles

[0205] This embodiment refers to HEVC. However, the teachings given in this section, are applicable in any codec that evolves from or has similar functionalities and requirements as HEVC.

[0206] Embodiment ii) provides the similar functionality of the embodiment i) for new profiles defined for HEVC, to enable a monochrome layer, indicated with chroma format syntax element, i.e., chroma format idc equal to 0, to have a dependent reference layer being a non- monochrome layer with 4:2:0, 4:2:2, or 4:4:4 chroma format (with chroma format syntax element, i.e., chroma format idc equal to 1, 2, or 3, respectively.)

[0207] Currently an auxiliary picture layer may be associated with its own scalability dimension value, Auxld (“auxiliary picture identifier”). Layers with Auxld greater than 0 contain auxiliary pictures. A layer carries only one type of auxiliary pictures, and the type of auxiliary pictures included in a layer is indicated by its Auxld value. For example, Auxld equal to 1 may indicatealpha planes and Auxld equal to 2 may indicate depth pictures. Primary pictures have Auxld equal to 0. The HEVC Multilayer 4:4:4 and Multilayer 4:4:4 10 profiles proposed in JVET- AI0045 currently contains the following restriction:- For the current layer, referred to as layerA and having nuh layer id equal to i, and another layer layerB with nuh layer id equal to j, when layerB is a reference layer of layerA, the value of chroma format idc for layerA and the value of chroma format idc for layerB shall be identical, and Auxldf i ] shall be equal to Auxld[ j ].

[0208] According to the present embodiments, this restriction is replaced in a profile definition with the following:- For the current layer, referred to as layerA and having nuh layer id equal to i, and another layer layerB with nuh layer id equal to j, when layerB is a reference layer of layerA, the value of chroma format idc for layerA shall either be equal to 0 or equal to the value of chroma format idc for layer B.

[0209] This replacement removes the restriction that Auxld[ i ] shall be equal to Auxld[ j ], and loosens the restrictions on the values of chroma format idc of layerA and layerB.

[0210] To use this embodiment, a multi-layer bitstream is encoded to contain a first layer with a first value of layer ID , and first value of AuxID, and a second layer with a second value of layer ID that is directly dependent on the first layer, with the layer ID of the first layer being less than the layer ID of the second layer and a second value of Auxld that differs from the first value of Auxld in the first layer. In the Video Parameter Set (VPS), vps_direct_ref_layer_flag[ i ][ j ] is set to 1, where i corresponds to second layer with the second value of layer ID and j corresponds to the first layer with the first value of layer ID. In the Sequence Parameter Set (SPS), the sps chroma format idc syntax element for the first layer is set equal to 1, 2, or 3, representing 4:2:0, 4:2:2, or 4:4:4 non-monochrome format, which contains 3 color components. The sps chroma format idc syntax element for the second layer is set equal to 0, representing monochrome which contains 1 color component. The second layer shall have a profile indicator value that indicates a profile with the restriction proposed above .

[0211] A reference list for a coded picture in the second layer may contain the ILRP. When a CU is coded, the motion vector of the co-located block in the ILRP may be used as a predictor for coding the current CU motion vector, either as merge candidate or a motion vector predictor candidate.

[0212] Embodiment iii): Inter-layer prediction of non-monochrome from monochrome reference

[0213] This embodiment refers to VVC. However, the teachings given in this section, are applicable in any future codec that evolves from or has similar functionalities and requirements as VVC-codec.

[0214] Embodiment iii) provides the capabilities of the embodiment i) and additionally allows a non-monochrome layer (with 4:2:0, 4:2:2, or 4:4:4 chroma format) to have a dependent reference layer with monochrome chroma format. This can be used, for example, to scalably coded color content by encoding a base monochrome layer and a color enhancement layer with inter-layer prediction from the base layer.

[0215] Embodiment iii) modifies the VPS semantics for vps_direct_ref_layer_flag[ i ][ j ] and defines a new process to predict chroma samples when they are not present in a direct reference layer.

[0216] The following is added to the semantics of the vps_direct_ref_layer_flag[ i ][ j ] syntax element in the VPS:

[0217] The following are requirements of bitstream conformance:One or more of the following conditions shall be met with respect of chroma format syntax element:- The value of sps chroma format idc that applies to the i-th layer is equal 0- The value of sps chroma format idc that applies to the j -th layer is equal to 0- The value of sps chroma format idc that applies to the i-th layer is equal to the value of sps chroma format idc that applies to the j -th layer

[0218] Embodiment iii) enables inter-layer motion vector prediction when the base and enhancement layers either have the same chroma format or when either the base layer or the enhancement layer has a monochrome chroma format. Inter-layer motion vector prediction is not enabled when the base and enhancement layers both have non-monochrome format containing three color components but when the chroma format differs, as that would require a modification in the decoding process for adjusting the motion vector applied to chroma components from what is defined in VVC.

[0219] Differences in chroma format between a current layer and a direct dependent layer as proposed do not affect the decoding process for motion vector prediction, because in VVC the motion vectors signaled for a CU are used in the motion vector prediction process for both luma and chroma components and not separately signaled.

[0220] However, a modification is proposed to handle inter-layer sample prediction of a current picture in a current non-monochrome layer from a picture in a monochrome layer, becausedecoded samples in the decoded picture buffer for the inter-layer reference picture would otherwise not be available.

[0221] Two options for the modification of the decoding process are provided. A first option A) is to disallow inter-layer sample prediction of chroma components when the direct dependent layer has a different chroma format than the current layer. A second option B) is to define a process to derive missing chroma planes.

[0222] To use this embodiment, a multi-layer bitstream is encoded to contain a first layer with a first value of layer ID, and a second layer with a second value of layer ID that is directly dependent on the first layer, with the layer ID of the first layer being less than the layer ID of the second layer.

[0223] In the Video Parameter Set (VPS), vps_direct_ref_layer_flag[ i ][ j ] is set to 1, where i corresponds to second layer with the second value of layer ID and j corresponds to the first layer with the first value of layer ID. In the Sequence Parameter Set (SPS), the sps chroma format idc syntax element for the first layer is set equal to 0, representing monochrome which contains 1 color component. The sps chroma format idc syntax element for the second layer is set equal to 1, 2, or 3, representing 4:2:0, 4:2:2, or 4:4:4 nonmonochrome format, which contains 3 color components.

[0224] Option A for the inter-layer sample prediction of Embodiment iii)

[0225] In the first option A, a constraint is imposed on the current CU to not to allow inter-layer sample prediction when a direct reference layer is monochrome layer and when the current layer is a non-monochrome layer having three color components. This avoids decoding process references to missing chroma component samples in a monochrome inter-layer reference picture (ILRP).

[0226] According to the present embodiments, the following constraint is added to the Derivation process for motion vector components and reference indices:- It is a requirement of bitstream conformance that if refldxLX is an ILRP and sps chroma format idc of the current picture is greater than sps chroma format idc of the refldxLX, inter_pred_idc[ xCb ][ yCb ] shall not be equal to PRED LX or PRED BI.

[0227] Option B for inter-layer sample prediction of Embodiment iii)

[0228] In the second option, a decoding process is defined to generate unavailable chroma sample arrays by setting them to a mid-gray value, which is determined according to the bit depth of the picture.

[0229] This process may be invoked by the decoding process for a coded picture when the value of sps chroma format idc of the current picture is greater than 0 and an entry in the RPL is an ILRP with sps chroma format idc equal to 0,

[0230] According to the present embodiments, the following step is added to the decoding process for a coded picture:- When the value of sps chroma format idc of the current picture is greater than 0 and an entry in the RPL is an ILRP with sps chroma format idc equal to 0, the decoding process for generating unavailable chroma sample arrays is invoked.

[0231] The proposed decoding process for generating unavailable chroma sample arrays is provided below:

[0232] General decoding process for generating unavailable chroma sample arrays:This process may be invoked once per coded picture when the current picture sps chroma format idc is greater than 0 and an entry in the RPL is an ILRP with sps chroma format idc equal to 0.When this process is invoked, the following applies:- For each RefPicList[ i ][ j ], with i in the range of 0 to 1, inclusive, and j in the range of 0 to num_ref_entries[ i ][ Rplsldx[ i ] ] - 1, inclusive, that is an ILRP, chroma sample arrays of the reference picture are generated as specified in the following clause:Generation of unavailable chroma sample arrays for one picture When this process is invoked, unavailable chroma sample arrays are generated as follows:The value of each element in the sample arrays SCb and SCr for the picture is set equal to 1 « ( BitDepth - 1 )

[0233] Embodiment iv): Inter-layer motion prediction between pictures with different chroma formats or bit depths

[0234] Embodiment iv) allows inter-layer motion prediction between layers with different chroma formats or bit depths.

[0235] Embodiments presented in this section allow an encoder to select a collected picture for TMVP in a manner that the collocated picture has a different bit depth and / or chroma format compared to those of the current picture being encoded. However, the embodiments presented in this section constrain the encoder to select the reference indices in a manner that inter-layersample prediction does not refer to ILRPs that have a different bit depth or chroma format compared to those of the current picture.

[0236] The next embodiments refer to HEVC. However, the teachings given in this section, are applicable in any codec that evolves from or has similar functionalities and requirements as HEVC.

[0237] The HEVC Multilayer 4:4:4 and Multilayer 4:4:4 10 profiles proposed in JVET-AI0045 currently contains the following restriction:- For the current layer, referred to as layerA and having nuh layer id equal to i, and another layer layerB with nuh layer id equal to j, when layerB is a reference layer of layerA and the value of chroma format idc for layerB shall be identical, the value of bit_depth_luma_minus8 for layerA and the value of bit_depth_luma_minus8 for layerB shall be identical, the value of bit_depth_chroma_minus8 for layerA and the value of bit_depth_chroma_minus8 for layerB shall be identical, and Auxld[ i ] shall be equal to Auxld[ j ].

[0238] According to the present embodiments, this requirement is modified to allow differences in bit depth between the reference layer and the current layer for inter-layer motion prediction, while still disallowing the bit depths to differ for inter-layer sample prediction. Prediction of motion vectors is not impacted by the bit depth of the samples. However, sample prediction with different bit depths would benefit from an additional operation to adjust the bit depth of the samples of the reference.

[0239] In an embodiment, the proposed new requirement is as follows, with the modified text bolded in below:- For the current layer, referred to as layerA and having nuh layer id equal to i, and another layer layerB with nuh layer id equal to j, when layerB is a reference layer of layerA and RefPicList0[ RefIdxL0[ x ][ y ] ] or RefPicListl[ RefIdxLl[ x ][ y ] ] for any valid values of x and y is equal to an entry of RefPicSetlnterLayerO or RefPicSetlnterLayerl for layerB, the value of chroma format idc for layerA and the value of chroma format idc for layerB shall be identical, the value of bit_depth_luma_minus8 for layerA and the value of bit_depth_luma_minus8 for layerB shall be identical, the value of bit_depth_chroma_minus8 for layerA and the value of bit_depth_chroma_minus8 for layerB shall be identical, and Auxld[ i ] shall be equal to Auxld[ j ].

[0240] In an embodiment, which may be applied to VVC, a proposed new requirement is as follows (expressed with terms, variables, and syntax elements applicable to VVC): It is arequirement of bitstream conformance that when RefPicListOf refldxLO ] or RefPicListlf refldxLl ] is an ILRP, sps chroma format idc applicable to the ILRP shall be equal to sps chroma format idc applicable to the current picture and sps_bitdepth_minus8 applicable to the ILRP shall be equal to the sps_bitdepth_minus8 applicable to the current picture.

[0241] Embodiment v): Local Illumination Compensation

[0242] Figure 5 illustrates a flowchart of a method for encoding according to embodiment v) of the invention. The method comprises- encoding 510 a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer;- encoding 520 into a bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer;- encoding 530 into a bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and- deriving 540 a scaling factor for the local illumination compensation that is applied to the sample of a reference block of the first layer when forming the sample prediction for the at least one coding unit, , whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.

[0243] The embodiments i) - iv) enable inter-layer sample prediction of an auxiliary picture layer from a primary picture layer, such as predicting depth, alpha, or an object mask from texture.

[0244] Auxiliary pictures and their associated primary pictures represent the same scene and often have visual similarities. However, the sample values of an auxiliary picture might not be well predicted with inter-sample prediction from the primary picture.

[0245] The Local Illumination Compensation (LIC) tool in the ECM derives a scale factor, a, and an offset, P, that are applied to a reference block when forming the sample predictor for a coding unit (CU). The scale factor and offset parameters are derived using a template of neighboring samples of the current block and the reference block.

[0246] When used with the embodiments i) and ii), LIC can be used for inter-layer sample prediction. With a first layer that is a direct reference layer for a second layer, a scale factor, a,and an offset, P, are applied to the samples of a block in the first layer to form the sample predictor for the CU in the second layer.

[0247] According to current version of ECM 13, in the LIC implementation the scale factor a is clipped to avoid negative values.

[0248] After the scale factor, referred to below as scale, is derived from the template. ECM 13 applies a clipping operation as follows, using the Clip3 operator. scale = Clip3( 0, MaxVal, scale)

[0249] When LIC is used for inter prediction within a single layer when illumination changes are present in the content, the sample values are expected to be positively correlated, so clipping to avoid a negative scale factor may be beneficial.

[0250] However, the samples of an auxiliary picture and its associated primary picture may have positive or negative correlation. For example, objects that are closer to the camera have lighter depth values when depth is coded as disparity. The close objects might darker luma values in the primary texture picture, in which case there is a negative correlation.

[0251] Therefore, according to embodiment v) two options C) and D) are proposed to enable negative scale factors for LIC. Both options are related to signaling, an indication is encoded into a bitstream, where in the indication indicates a use of local illumination compensation with inter-layer reference prediction. When the LIC is enabled, a scaling factor applied to a reference block is not to be clipped.

[0252] Option C) for the embodiment v)

[0253] In this option, a condition is added to the clipping operation of the scale factor, so that it is not applied when the reference picture is an inter-layer reference picture (ILRP). Therefore, during coding process, it is determined whether or not the reference picture is an ILRP, and if so, the clipping operation is not applied.

[0254] The following operation scale = Clip3( 0, MaxVal, scale) is thus replaced with the following due to the present embodimetns: if the reference picture is not an ILRP scale = Clip3( 0, MaxVal, scale)

[0255] Option D) for the embodiment v)

[0256] In this option, a syntax element flag is added to a parameter set, such as the sequence parameter set (SPS) or picture parameter set (PPS), and / or picture header or slice header to indicate that clipping of the LIC scale factor take place.

[0257] The flag can be coded with a single bit, in u(l) format, and signaled if the LIC feature is enabled.

[0258] An example section of a syntax table for a PPS is shown below, along with semantics for the proposed flag.

[0259] When present, pps lic scale clip flag equal to 1 specifies that clipping is applied to the scale factor derivation for local illumination compensation, pps lic scale clip flag equal to 1 specifies that clipping is not applied to the scale factor derivation for local illumination compensation.

[0260] This allows an encoder to select whether or not the LIC feature is used as general remapping that can have a negative scaling factor. An encoder may select to set the flag when inter-layer prediction from an auxiliary picture of a different type. An encoder may select to set the flag for other types of content where it improves coding efficiency.

[0261] Embodiment vi): Reference picture list construction depending on chroma format and bit depth

[0262] Figure 6 illustrates a flowchart of a method for encoding according to embodiment vi) of the invention. The method comprises- encoding 610 a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer;- encoding 620 into the bitstream for a coded picture of the second layer a syntax element indicating a motion vector prediction from a reference picture in the first layer;- determining 630 that the reference picture has different chroma format or different bit depth than the current picture;- modifying 640 a reference picture list by inactivating the reference picture having different chroma format or different bit depth in the reference picture list; and- encoding 650 the coded picture into the bitstream with reference to the modified reference picture list.

[0263] In some video coding formats, such as VVC, active entries of reference picture lists 0 and 1 for a current picture include the reference pictures that are used for sample prediction and motion vector prediction for the current picture. In VVC, the reference picture for the motion vector prediction is indicated with ph collocated from lO flag and ph collocated ref idx in the picture header (when reference picture lists are indicated in the picture header) or with sh collocated from lO flag and sh collocated ref idx in the slice header (when reference picture lists are indicated in the slice header).

[0264] In an embodiment, reference picture lists are constructed as follows:1) Reference picture lists are constructed conventionally, for example as specified inVVC.2) A collocated reference picture to be used for motion vector prediction is identified from the reference picture lists. For decoding, the collocated reference picture is decoded from indications in the bitstream, such as ph collocated from lO flag and ph collocated ref idx, or sh collocated from lO flag and sh coll ocated ref i dx .3) Reference picture(s) that have a chroma format or bit depth not equal to those for the current picture are removed from the reference picture lists or moved to be inactive entries. This may result into a reducing the number of active entries of the reference picture lists. These modified reference picture lists are used in subsequent decoding processes of the current picture, e.g., for sample prediction.

[0265] In an embodiment, the chroma format and the bit depth of an ILRP for step 3 above are obtained from the sequence parameter set applying to the layer that comprises the ILRP.

[0266] In an embodiment, the chroma format and the bit depth of a picture that is not an ILRP for step 3 are inferred to be the same as those for the current picture.

[0267] In an embodiment, a syntax element indicative whether a collocated picture for TMVP is an ILRP may be added in a parameter set (e.g., a PPS) or a header (e.g., a picture header). For example, pps collocated ilrp flag may be added to the PPS syntax structure. The presence of the syntax element may be conditional on TMVP being enabled. The syntax element causes the selection of the collocated picture as follows:If the syntax element indicates that ILRP is used for indicating a collocated picture for TMVP (e.g., if pps collocated ilrp flag is equal to 1), a list of ILRPs for the current picture may be derived and the collocated picture for TMVP may be indicated with another syntaxelement e.g. in the picture header or in the slice header, where that another syntax element indicates an index to the list of ILRPs. If the list of ILRPs only contains a single ILRP, the another syntax element may be absent and the single ILRP may be inferred to be used as the collocated picture for TMVP.Otherwise (e.g., when pps collocated ilrp flag is equal to 0), the collocated picture for TMVP may be indicated conventionally through an index to reference picture list 0 or 1, e.g. as specified in HEVC or VVC. Reference picture lists 0 and 1 are derived to include ILRPs that have same chroma format and bit depth as those of the current picture.

[0268] In another variation of the previous embodiment, the list of ILRPs is derived to include only those ILRPs that have a different chroma format or bit depth compared to those of the current picture.

[0269] Benefits of embodiments in this section may include shorter reference picture lists for sample prediction, which may cause shorter codewords to indicate reference indices for sample prediction.

[0270] Benefits of this embodiment may include shorter reference picture lists for sample prediction, which may cause shorter codewords to indicate reference indices for sample prediction.

[0271] In previous, method according to various aspects have been discussed. Next the technical implementation will be described from a point of view of various apparatuses.

[0272] An apparatus according to an aspect comprises means for encoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier wherein either of the first layer or the second layer is directly dependent on the other so that one is used for inter-layer reference prediction for the other; means for encoding into a bitstream a syntax element indicating the inter-layer reference prediction between the first layer and the second layer; means for encoding into a bitstream a first chroma format syntax element for the first layer; and means for encoding into a bitstream a second chroma format syntax element for the second layer, wherein one of the first or second chroma format syntax elements indicates a monochrome format, and the other of the first and second chroma format syntax elements indicates the non-monochrome format. These means comprise 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 the method as illustrated in Figure 4 and various embodiments of it.

[0273] According to an improvement, the apparatus according to the previous aspect further comprises means for encoding the syntax element indicating inter-layer reference prediction to a parameter set or a header.

[0274] According to an improvement to any of the previous paragraphs, the apparatus further comprises means for encoding the first chroma format syntax element for the first layer to a parameter set or a header, and for encoding the second chroma format syntax element for the second layer to a parameter set or a header.

[0275] According to an improvement to any of the previous paragraphs, the apparatus further comprises means for using a motion vector of a co-located block in the inter-layer reference picture as a predictor, either as merge candidate or a motion vector predictor candidate, for a motion vector of a current coding unit to be coded.

[0276] According to a further improvement to any of the previous paragraphs, the first layer further contains a first value of auxiliary picture identifier and the second layer further contains a second value of auxiliary picture identifier, wherein the second value of auxiliary picture identifier differs from the first value of auxiliary picture identifier.

[0277] According to a further improvement to the previous paragraph, the second layer has a profile indicator value indicating a profile with the restriction defining that when the first layer is directly dependent from the second layer, the first chroma format syntax element is equal to 0 or equal to the second chroma format syntax value.

[0278] According to a further improvement to any of the previous paragraphs, the second layer is directly dependent from the first layer.

[0279] According to an improvement, the apparatus according to any of the previous paragraphs further comprises means for coding the second layer using an inter-layer sample prediction and / or inter-layer motion vector prediction from pictures in the first layer.

[0280] According to an improvement, the apparatus according to any of the previous paragraphs further comprises means for including the first layer as an inter-layer reference picture in a reference picture list of the current picture.

[0281] According to a further improvement to any of the previous paragraphs, the first layer is directly dependent from the second layer.

[0282] According to an improvement, the apparatus according to any of the previous paragraphs further comprises means for encoding a second layer as a base layer, and the first layer as an enhancement layer with inter-layer prediction from the base layer.

[0283] An apparatus according to a second aspect comprises means for decoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a secondvalue of layer identifier wherein either of the first layer or the second layer is used for interlayer reference prediction for the other; means for decoding from the bitstream a syntax element indicating the inter-layer reference prediction between the first layer and the second layer; means for decoding from the bitstream a first chroma format syntax element for the first layer; and means for decoding from the bitstream a second chroma format syntax element for the second layer, wherein one of the first or second chroma format syntax elements indicates a monochrome format, and the other of the first or second chroma format syntax elements indicates the non-monochrome format.

[0284] According to an improvement, the first layer is a non-monochrome layer and is directly dependent from the second layer being monochrome layer.

[0285] According to an improvement, the apparatus according to any of the previous paragraphs further comprising means for decoding a second layer as a base layer, and the first layer as an enhancement layer with inter-layer prediction from the base layer.

[0286] According to an improvement, the apparatus according to any of the previous paragraphs further comprising means for generating missing chroma sample arrays of a non-monochrome layer based on bit depth of a current picture.

[0287] An apparatus according to a third aspect comprises means for encoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier wherein either of the first layer or the second layer is used for interlayer reference prediction for the other; means for encoding into the bitstream a syntax element indicating the inter-layer reference prediction between the first layer and the second layer; means for encoding into the bitstream a first syntax element for the first layer indicating a first bit depth; and means for encoding into the bitstream a second syntax element for the second layer indicating a first bit depth; wherein the second bit depth is different from the first bit depth.

[0288] A method according to a fourth aspect comprises encoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier wherein either of the first layer or the second layer is used for inter-layer reference prediction for the other; encoding into the bitstream a syntax element indicating the inter-layer reference prediction between the first layer and the second layer; encoding into the bitstream a first syntax element for the first layer indicating a first bit depth; and encoding into the bitstream a second syntax element for the second layer indicating a first bit depth; wherein the second bit depth is different from the first bit depth.

[0289] An apparatus according to a fifth aspect comprises means for decoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier wherein either of the first layer or the second layer is used for interlayer reference prediction for the other; means for decoding from the bitstream a syntax element indicating the inter-layer reference prediction between the first layer and the second layer; means for decoding from the bitstream a first syntax element for the first layer indicating a first bit depth; and means for decoding from the bitstream a second syntax element for the second layer indicating a first bit depth; wherein the second bit depth is different from the first bit depth.

[0290] A method according to a sixth aspect comprises decoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier wherein either of the first layer or the second layer is used for inter-layer reference prediction for the other; decoding from the bitstream a syntax element indicating the inter-layer reference prediction between the first layer and the second layer; decoding from the bitstream a first syntax element for the first layer indicating a first bit depth; and decoding from the bitstream a second syntax element for the second layer indicating a first bit depth; wherein the second bit depth is different from the first bit depth.

[0291] An apparatus according to a seventh aspect comprises 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 encode a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier wherein either of the first layer or the second layer is used for inter-layer reference prediction for the other; encode into the bitstream a syntax element indicating the inter-layer reference prediction between the first layer and the second layer; encode into the bitstream a first syntax element for the first layer indicating a first bit depth; and encode into the bitstream a second syntax element for the second layer indicating a first bit depth; wherein the second bit depth is different from the first bit depth.

[0292] An apparatus according to an eighth aspect comprises 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 decode a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier wherein either of the first layer or the second layer is used for inter-layer reference prediction for the other; decode from the bitstream a syntax element indicating the inter-layer reference prediction between the first layer and the second layer; decode from the bitstream a first syntax element forthe first layer indicating a first bit depth; and decode from the bitstream a second syntax element for the second layer indicating a first bit depth; wherein the second bit depth is different from the first bit depth.

[0293] An apparatus according to a further aspect comprises means for encoding a bitstream comprising a first layer with a first value of layer ID and a second layer with a second value of layer ID, wherein the first layer is used for inter-layer reference prediction for the second layer; means for encoding into the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer; and means for encoding into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and means for deriving a scaling factor for the local illumination compensation that is applied to the sample of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to a reference block is not clipped to avoid negative values. These means comprise 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 the method as illustrated in Figure 5 and various embodiments of it.

[0294] According to an improvement, the apparatus according to the previous aspect further comprises means for encoding into the bitstream a second coding unit for the picture in the second layer a syntax element indicating reference prediction from another picture in the second layer; and means for encoding into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the second coding unit; and means for deriving a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the second layer, when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is clipped to avoid negative values.

[0295] According to an improvement to the previous paragraphs, the indication has been encoded into a parameter set or header that use of local illumination is enabled.

[0296] According to an improvement to the previous paragraphs, a syntax flag indicates whether or not clipping is applied.

[0297] According to an improvement to the previous paragraphs the apparatus further comprises means for encoding into the bitstream a first chroma format syntax element for the first layer; and means for encoding into the bitstream a second chroma format syntax element for the second layer, wherein one of the first or second chroma format syntax elements indicates amonochrome format, and the other of the first and second chroma format syntax elements indicates the non-monochrome format.

[0298] According to an improvement to the previous paragraphs the first layer further contains a first value of auxiliary picture identifier and the second layer further contains a second value of auxiliary picture identifier, wherein the second value of auxiliary picture identifier differs from the first value of auxiliary picture identifier.

[0299] According to an improvement to the previous paragraphs, the second layer is directly dependent from the first layer.

[0300] According to an improvement to the previous paragraphs, the apparatus further comprises means for including the first layer as an inter-layer reference picture in a reference picture list of the current picture.

[0301] An apparatus according to yet further aspect comprises means for encoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer; means for encoding into the bitstream for a coded picture of the second layer a syntax element indicating a motion vector prediction from a reference picture in the first layer; means for determining that the reference picture has different chroma format or different bit depth than the current picture; means for modifying a reference picture list by inactivating the reference picture having different chroma format or different bit depth in the reference picture list; and means for encoding the coded picture into the bitstream with reference to the modified reference picture list. These means comprise 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 the method as illustrated in Figure 6 and various embodiments of it.

[0302] According to an improvement to the previous paragraph, the apparatus further comprises means for inactivating the reference picture by removing the reference picture from the reference picture list or by moving the reference picture to be an inactive entry.

[0303] According to an improvement to the previous paragraphs, the apparatus further comprises means for encoding into a bitstream a first chroma format syntax element for the first layer and encoding into a bitstream a second chroma format syntax element for the second layer, wherein one of the first or second chroma format syntax elements indicates a monochrome format, and the other of the first or second chroma format syntax elements indicates the non-monochrome format.

[0304] The apparatus according to various aspects may comprise e.g. all or a subset of the functional units disclosed in any of the Figures 1, 7 - 10 for implementing the embodiments.

[0305] 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.

[0306] Figure 7 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 8 shows a layout of an apparatus according to an example embodiment.

[0307] 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.

[0308] 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.

[0309] 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.

[0310] 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 mayfurther 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.

[0311] 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.

[0312] 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).

[0313] 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.

[0314] With respect to Figure 9, 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.

[0315] The system 10 may include both wired and wireless communication devices and / or apparatus 50 suitable for implementing embodiments of the invention.

[0316] For example, the system shown in Figure 20 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.

[0317] 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.

[0318] 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.

[0319] 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.

[0320] 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.

[0321] Figure 10 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 ofnetwork. 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.

[0322] 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.

[0323] 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.

[0324] 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.

[0325] 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.

[0326] The system includes one or more receivers 1560, typically capable of receiving, demodulating, and de-capsulating the transmitted signal into a coded media bitstream. The coded media bitstream may be transferred to a recording storage 1570. The recording storage 1570may 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 randomaccess memory. The format of the coded media bitstream in the recording storage 2170 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.

[0327] 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.

[0328] 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 1580, and Tenderer 1590 may reside in the same physical device or they may be included in separate devices.

[0329] 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 aSubsegment 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.

[0330] 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.

[0331] In the above, some embodiments have been described with reference to and / or using terminology of HEVC and / or VVC. It needs to be understood that embodiments may be similarly realized with any video encoder and / or video decoder.

[0332] In the above, some example embodiments have been described with the help of syntax of the bitstream. It needs to be understood, however, that the corresponding structure and / or computer program may reside at the encoder for generating the bitstream and / or at the decoder for decoding the bitstream.

[0333] In the above, some example embodiments have been described in relation to requirements of bitstream conformance. It is to be understood that the embodiments apply to an encoder that outputs a bitstream and has corresponding elements to realize the requirements of bitstream conformance. Furthermore, it is to be understood that embodiments apply to an entity thatverifies the correctness of a bitstream and has corresponding elements to check whether the requirements of bitstream conformance are fulfilled.

[0334] In the above, where the example embodiments have been described with reference to an encoder, it needs to be understood that the resulting bitstream and the decoder may have corresponding elements in them. Likewise, where the example embodiments have been described with reference to a decoder, it needs to be understood that the encoder may have structure and / or computer program for generating the bitstream to be decoded by the decoder.

[0335] The embodiments of the invention described above describe the codec in terms of separate encoder and decoder apparatus in order to assist the understanding of the processes involved. However, it would be appreciated that the apparatus, structures and operations may be implemented as a single encoder-decoder apparatus / structure / operation. Furthermore, it is possible that the coder and decoder may share some or all common elements.

[0336] Although the above examples describe embodiments of the invention operating within a codec within an electronic device, it would be appreciated that the invention as defined in the claims may be implemented as part of any video codec. Thus, for example, embodiments of the invention may be implemented in a video codec which may implement video coding over fixed or wired communication paths.

[0337] Thus, user equipment may comprise a video codec such as those described in embodiments of the invention above. It shall be appreciated that the term user equipment is intended to cover any suitable type of wireless user equipment, such as mobile telephones, portable data processing devices or portable web browsers.

[0338] Furthermore, elements of a public land mobile network (PLMN) may also comprise video codecs as described above.

[0339] In general, the various embodiments of the invention may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the invention may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

[0340] The embodiments of this invention may be implemented by computer software executable by a data processor of the mobile device, such as in the processor entity, or by hardware, or by a combination of software and hardware. Further in this regard it should be noted that any blocks of the logic flow as in the Figures may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD.

[0341] The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor-based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may include one or more of general-purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multi-core processor architecture, as non-limiting examples.

[0342] 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.

[0343] 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.

[0344] The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention.

Claims

CLAIMS:

1. An apparatus, comprising- means for encoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer;- means for encoding into the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer;- means for encoding into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and- means for deriving a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.

2. The apparatus according to claim 1, further comprising means for encoding into the bitstream a second coding unit for the picture in the second layer a syntax element indicating reference prediction from another picture in the second layer; and means for encoding into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the second coding unit; and means for deriving a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the second layer, when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is clipped to avoid negative values.

3. The apparatus according to claim 1, wherein the indication has been encoded into a parameter set or header that use of local illumination compensation is enabled.

4. The apparatus according to claim 1, wherein a syntax flag indicates whether or not clipping is applied.

675. The apparatus according to claim 1, further comprising means for encoding into the bitstream a first chroma format syntax element for the first layer; and means for encoding into the bitstream a second chroma format syntax element for the second layer, wherein one of the first or second chroma format syntax elements indicates a monochrome format, and the other of the first and second chroma format syntax elements indicates the non-monochrome format.

6. The apparatus according to 1, wherein first layer further contains a first value of auxiliary picture identifier and the second layer further contains a second value of auxiliary picture identifier, wherein the second value of auxiliary picture identifier differs from the first value of auxiliary picture identifier.

7. The apparatus according to claim 1, wherein the second layer is directly dependent from the first layer.

8. The apparatus according to claim 7, further comprising means for including the first layer as an inter-layer reference picture in a reference picture list of the current picture.

9. A method comprising:- encoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer;- encoding into the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer;- encoding into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and- deriving a scaling factor for the local illumination compensation that is applied to the sample of a reference block of the first layer when forming the sample prediction for the at least one coding unit, , whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.

10. The method according to claim 9, further comprising encoding into the bitstream a second coding unit for the picture in the second layer a syntax element indicating reference prediction from another picture in the second layer; and encoding into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the second coding unit; and deriving a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the second layer, when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is clipped to avoid negative values.

11. An apparatus, comprising- means for decoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer;- means for decoding from the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer;- means for decoding from the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and- means for deriving a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.

12. The apparatus according to claim 11, further comprising means for decoding from the bitstream a second coding unit for the picture in the second layer a syntax element indicating reference prediction from another picture in the second layer; and means for decoding from the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the second coding unit; and means for deriving a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the second layer, when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is clipped to avoid negative values.

13. The apparatus according to claim 11, wherein the indication has been decoded from a parameter set or header that use of local illumination compensation is enabled.

14. The apparatus according to claim 11, further comprising means for decoding a syntax flag indicating whether or not clipping is to be applied, and in response to decoding the syntax flag, in case clipping is not to be applied, the scaling factor applied to the reference block is not clipped to avoid negative values; in case clipping is to be applied the scaling factor applied to the reference block is clipped to avoid negative values.

15. The apparatus according to any of the claims 11, further comprising means for decoding from the bitstream a first chroma format syntax element for the first layer; and means for decoding from the bitstream a second chroma format syntax element for the second layer, wherein one of the first or second chroma format syntax elements indicates a monochrome format, and the other of the first and second chroma format syntax elements indicates the non-monochrome format.

16. A method comprising:- decoding a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer;- decoding from the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer;- decoding from the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and- deriving a scaling factor for the local illumination compensation that is applied to the sample of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.7017. 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- encode a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer;- encode into the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer;- encode into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and- derive a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.

18. The apparatus according to claim 17, further being caused to encode into the bitstream a second coding unit for the picture in the second layer a syntax element indicating reference prediction from another picture in the second layer; and to encode into the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the second coding unit; and to derive a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the second layer, when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is clipped to avoid negative values.

19. The apparatus according to claim 17, wherein the indication has been encoded into a parameter set or header that use of local illumination compensation is enabled.

20. The apparatus according to claim 17, wherein a syntax flag indicates whether or not clipping is applied.

21. The apparatus according to claim 17, further being caused to encode into the bitstream a first chroma format syntax element for the first layer; and encode into the bitstream a second chromaformat syntax element for the second layer, wherein one of the first or second chroma format syntax elements indicates a monochrome format, and the other of the first and second chroma format syntax elements indicates the non-monochrome format.

22. The apparatus according to claim 17, wherein first layer further contains a first value of auxiliary picture identifier and the second layer further contains a second value of auxiliary picture identifier, wherein the second value of auxiliary picture identifier differs from the first value of auxiliary picture identifier.

23. The apparatus according to claim 17, wherein the second layer is directly dependent from the first layer.

24. The apparatus according to claim 17, further being caused to include the first layer as an interlayer reference picture in a reference picture list of the current picture.

25. 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- decode a bitstream comprising a first layer with a first value of layer identifier and a second layer with a second value of layer identifier, wherein the first layer is used for inter-layer reference prediction for the second layer;- decode from the bitstream for at least one coding unit of a coded picture of the second layer a syntax element indicating reference prediction from a picture in the first layer;- decode from the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the at least one coding unit; and- derive a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the first layer when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is not clipped to avoid negative values.

26. The apparatus according to claim 25, further being caused to decode from the bitstream a second coding unit for the picture in the second layer a syntax element indicating referenceprediction from another picture in the second layer; and to decode from the bitstream for the at least one coding unit an indication of a use of local illumination compensation for forming a sample prediction for the second coding unit; and to derive a scaling factor for the local illumination compensation that is applied to the samples of a reference block of the second layer, when forming the sample prediction for the at least one coding unit, whereupon the scaling factor applied to the reference block is clipped to avoid negative values.

27. The apparatus according to claim 25, wherein the indication has been decoded from a parameter set or header that use of local illumination compensation is enabled.

28. The apparatus according to claim 25, further being caused to decode a syntax flag indicating whether or not clipping is to be applied, and in response to decoding the syntax flag, in case clipping is not to be applied, the scaling factor applied to the reference block is not clipped to avoid negative values; in case clipping is to be applied the scaling factor applied to the reference block is clipped to avoid negative values.

29. The apparatus according to any of the claims 25, further being caused to decode from the bitstream a first chroma format syntax element for the first layer; and decode from the bitstream a second chroma format syntax element for the second layer, wherein one of the first or second chroma format syntax elements indicates a monochrome format, and the other of the first and second chroma format syntax elements indicates the non-monochrome format.

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