An apparatus, a method for video coding and decoding using in-loop filter model candidate list

Abstract

A method comprising: receiving an image block unit of a frame, the image block unit comprising samples in color channels comprising at least one chrominance channel and one luminance channel; reconstructing samples of said color channels to produce reconstructed chrominance samples and reconstructed luminance samples; determining that a picture-level model list re-ordering is used; means for determining a reference picture; determining a list of cross-component models for the reference picture; generating a re-ordered list where the cross-component models of the determined list have been re-ordered; determining an index for a cross-component model; means for deriving the cross-component model from the re-ordered list with the given index; applying a first adaptive in-loop filter for the reconstructed luminance samples; and the derived cross-component model for the reconstructed luminance samples to obtain an additional input for reconstructing the chrominance samples.

Description

[0001] AN APPARATUS, A METHOD AND A COMPUTER PROGRAM FOR VIDEO CODING AND DECODING TECHNICAL FIELD The present invention relates to an apparatus, a method and a computer program for video coding and decoding. BACKGROUND In video coding, video and image samples may be encoded using color representations such as YUV or YCbCr consisting of one luminance (luma) and two chrominance (chroma) channels. In these cases the luminance channel, representing mostly the illumination of the scene, is coded at certain resolution, while the chrominance channels, typically representingdifferences between certain color components, may be coded at a second resolution lowerthan that of the luminance signal. The intention of this kind of a differential representation is to decorrelate the color components and be able to compress the data more efficiently. Adaptive in-loop filters (ALF) perform luma and chroma filtering in parallel fashion to lower the mean square error (MSE) between the reconstruction and the original samples. For example, in Versatile Video Coding (VVC) (MPEG-I Part 3), a.k.a. ITU-T H.266, the ALFfirst filters both luma and chroma in parallel, and subsequently also applies cross-component filtering (CC-ALF) to further improve the chroma. Only the luma samples are used as input to the CC-ALF in VVC. The chroma ALF uses chroma reconstruction buffer. SUMMARY As an improvement to the ALF, an enhanced method and technical equipment areintroduced herein. 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. An apparatus according to a first aspect comprises means for receiving an image block unit of a frame, the image block unit comprising samples in color channels comprising at least one chrominance channel and one luminance channel; means for reconstructing samples of said color channels to produce reconstructed chrominance samples and reconstructed luminance samples; means for determining that a picture-level model list re- ordering is used; means for determining a reference picture; means for determining a list ofcross-component models for the reference picture; means for generating a re-ordered listwhere the cross-component models of the determined list have been re-ordered; means fordetermining an index for a cross-component model; means for deriving the cross-componentmodel from the re-ordered list with the given index; means for applying a first adaptive in-loop filter for the reconstructed luminance samples; and means for applying the derivedcross-component model for the reconstructed luminance samples to obtain an additional input for reconstructing the chrominance samples. According to an embodiment, the apparatus further comprises means for computing ahistogram of the cross-component models used by all the image block units in the reference picture; and means for generating the re-ordered list by placing the most frequently occurring cross-component models at top of the list. According to an embodiment, the apparatus further comprises means for appending non-occurring models at end of the list using their numerical descending order. According to an embodiment, the apparatus further comprises means for decoding atleast one syntax element that determines whether the indices for cross-component models are determined at picture-level or at image block unit-level, whereupon if the indices aredetermined at picture-level, the apparatus comprises means for decoding at least one syntaxelement indicating the index of the cross-component models for all image block units in thepicture; or if the indices are not determined at picture-level, for each image block unit thathas adaptive in-loop filter chroma update enabled, means for decoding at least one syntaxelement indicating the index of the cross-component model for the given image block unit.According to an embodiment, the apparatus further comprises means for applying across-component adaptive in-loop filter for outputs of the first and second adaptive in-loop filters; and means for providing an output of the cross-component adaptive in-loop filter to the output of the second adaptive in-loop filter as an additional correction for the chrominance samples. According to an embodiment, the apparatus further comprises means for decoding thelist of cross-component models from a bitstream. According to an embodiment, the apparatus further comprises means for applying there-ordered model list to certain image block units in the current picture. According to an embodiment, the apparatus further comprises means for applying there-ordered model list to first image block unit of the current picture.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 receive an image block unit of a frame,the image block unit comprising samples in color channels comprising at least onechrominance channel and one luminance channel; reconstruct samples of said color channelsto produce reconstructed chrominance samples and reconstructed luminance samples; determine that a picture-level model list re-ordering is used; determine a reference picture;determine a list of cross-component models for the reference picture; generate a re-orderedlist where the cross-component models of the determined list have been re-ordered;determine an index for a cross-component model; derive the cross-component model fromthe re-ordered list with the given index; apply a first adaptive in-loop filter for thereconstructed luminance samples; apply the derived cross-component model for thereconstructed luminance samples to obtain an additional input for reconstructing the chrominance samples. According to an embodiment, the apparatus is further caused to compute a histogramof the cross-component models used by all the image block units in the reference picture; and to the re-ordered list by placing the most frequently occurring cross-component models at top of the list. According to an embodiment, the apparatus is further caused to append non-occurringmodels at end of the list using their numerical descending order. According to an embodiment, the apparatus is further caused to decode at least onesyntax element that determines whether the indices for cross-component models are determined at picture-level or at image block unit-level, whereupon if the indices aredetermined at picture-level, the apparatus is caused to decode at least one syntax elementindicating the index of the cross-component models for all image block units in the picture;or if the indices are not determined at picture-level, for each image block unit that hasadaptive in-loop filter chroma update enabled, the apparatus is caused to decode at least onesyntax element indicating the index of the cross-component model for the given image blockunit. According to an embodiment, the apparatus is further caused to apply a cross- component adaptive in-loop filter for outputs of the first and second adaptive in-loop filters; and to provide an output of the cross-component adaptive in-loop filter to the output of the second adaptive in-loop filter as an additional correction for the chrominance samples. According to an embodiment, the apparatus is further caused to decode the list of cross-component models from a bitstream. According to an embodiment, the apparatus is further caused to apply the re-ordered model list to certain image block units in the current picture. According to an embodiment, the apparatus is further caused to apply the re-orderedmodel list to first image block unit of the current picture.A method according to a third aspect comprises receiving an image block unit of aframe, the image block unit comprising samples in color channels comprising at least onechrominance channel and one luminance channel; reconstructing samples of said colorchannels to produce reconstructed chrominance samples and reconstructed luminance samples; determining that a picture-level model list re-ordering is used; means fordetermining a reference picture; determining a list of cross-component models for thereference picture; generating a re-ordered list where the cross-component models of thedetermined list have been re-ordered; determining an index for a cross-component model;means for deriving the cross-component model from the re-ordered list with the given index;applying a first adaptive in-loop filter for the reconstructed luminance samples; and thederived cross-component model for the reconstructed luminance samples to obtain an additional input for reconstructing the chrominance samples. 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 anapparatus or a system to: receive an image block unit of a frame, the image block unitcomprising samples in color channels comprising at least one chrominance channel and oneluminance channel; reconstruct samples of said color channels to produce reconstructedchrominance samples and reconstructed luminance samples; determine that a picture-level model list re-ordering is used; determine a reference picture; determine a list of cross-component models for the reference picture; generate a re-ordered list where the cross-component models of the determined list have been re-ordered; determine an index for across-component model; derive the cross-component model from the re-ordered list with thegiven index; apply a first adaptive in-loop filter for the reconstructed luminance samples;apply the derived cross-component model for the reconstructed luminance samples to obtainan additional input for reconstructing the chrominance samples. According to an embodiment, the computer program product is embodied on a non- transitory computer readable medium. The method as described above also comprises steps for previous embodiments. The apparatuses and the computer readable storage mediums stored with code thereon, asdescribed above, are thus arranged to carry out the above method and one or more of the embodiments related thereto. BRIEF DESCRIPTION OF THE DRAWINGS For better understanding of the present invention, reference will now be made by way of example to the accompanying drawings in which: Figures 1 shows schematically an encoder suitable for implementing embodimentsof the invention Figure 2 shows schematically a decoder suitable for implementing embodiments ofthe invention; Figure 3 shows locations of the samples used for derivation of parameters for aCross-Component Linear Model (CCLM); Figures 4a and 4b show examples of classification of luma samples into two classes in the sample domain, and in the spatial domain, respectively; Figure 5 shows an example of co-located reference sample areas consisting ofreconstructed luma and chroma samples defined for both luma and chroma for aConvolutional Cross-Component Model (CCCM);Figure 6 shows various examples of the dimensions of the filter kernel in CCCM;Figure 7 shows an example of four reference lines neighboring to a predictionblock; Figure 8 shows a matrix weighted intra prediction process;Figure 9 shows an example of combined inter / intra prediction (CIIP) mode appliedto the current coding unit; Figure 10 shows an example of a system level diagram of the CC-ALF process withrespect to the SAO, luma ALF and chroma ALF processes; Figure 11 shows an example of an ALF process; Figure 12 illustrates a flow chart of a method according to an embodiment of theinvention; Figure 13 shows an example of spatial locations of the north, south, west, and eastadjacent samples; Figure 14 shows an example of a plurality of CTUs having different ALF chroma settings; Figure 15 illustrates a flow chart of a method according to a further embodiment ofthe invention; Figure 16 illustrates a flow chart of a method according to a yet further embodimentof the invention; Figure 17 shows an example of a picture-level model list re-ordering;Figure 18 shows schematically an electronic device employing embodiments of theinvention; Figure 19 shows schematically a user equipment suitable for employingembodiments of the invention; Figure 20 further shows schematically electronic devices suitable for employingembodiments of the invention connected using wireless and wired network connections; andFigure 21 shows a schematic diagram of an example multimedia communicationsystem within which various embodiments may be implemented. DETAILED DESCRIPTON OF SOME EXAMPLE EMBODIMENTS 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, anencoder discards some information in the original video sequence in order to represent the video in a more compact form (that is, at lower bitrate). 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 acertain 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). 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 currentpicture 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 performedwith 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. 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. Inter prediction, which may also be referred to as temporal prediction, motion compensation, or motion-compensated prediction, reduces temporal redundancy. In inter prediction the sources of prediction are previously decoded pictures. Intra prediction utilizes the fact that adjacent pixels within the same picture are likely to be correlated. Intra prediction can be performed in spatial or transform domain, i.e., either sample values or transform coefficients can be predicted. Intra prediction is typically exploited in intra coding, where no inter prediction is applied. One outcome of the coding procedure is a set of coding parameters, such as motion vectors and quantized transform coefficients. Many parameters can be entropy-coded more efficiently if they are predicted first from spatially or temporally neighboring parameters. For example, a motion vector may be predicted from spatially adjacent motion vectors and only the difference relative to the motion vector predictor may be coded. Prediction of coding parameters and intra prediction may be collectively referred to as in-picture prediction. 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 representthe video in a more compact form (that is, at lower bitrate). Figure 1 shows an encoder and 2shows a decoder suitable for employing embodiments of the invention. Figure 1 illustrates animage to be encoded (In); a predicted representation of an image block (P'n); a prediction error signal (Dn); a reconstructed prediction error signal (D'n); a preliminary reconstructed image (I'n); a final reconstructed image (R'n); a transform (T) and inverse transform (T-1); a quantization (Q) and inverse quantization (Q-1); entropy encoding (E); a reference frame memory (RFM); inter prediction (Pinter); intra prediction (Pintra); mode selection (MS) and filtering (F). Figure 2 illustrates a predicted representation of an image block (P'n); a reconstructedprediction 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). Many hybrid video encoders encode the video information in two phases. Firstly, pixelvalues 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 ofpixels, is coded. This may be done by transforming the difference in pixel values using aspecified transform (e.g., Discrete Cosine Transform (DCT) or a variant of it), quantizing thecoefficients 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. Entropy coding / decoding may be performed in many ways. For example, context- based coding / decoding may be applied, where in both the encoder and the decoder modify the context state of a coding parameter based on previously coded / decoded coding parameters. Context-based coding may for example be context adaptive binary arithmetic coding (CABAC) or context-based variable length coding (CAVLC) or any similar entropy coding. Entropy coding / decoding may alternatively or additionally be performed using a variable length coding scheme, such as Huffman coding / decoding or Exp-Golomb coding / decoding. Decoding of coding parameters from an entropy-coded bitstream or codewords may be referred to as parsing. 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 datais associated with the bitstream. The phrase decoding along the bitstream or alike may referto 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. The H.264 / AVC standard was developed by the Joint Video Team (JVT) of the Video Coding Experts Group (VCEG) of the Telecommunications Standardization Sector of International Telecommunication Union (ITU-T) and the Moving Picture Experts Group (MPEG) of International Organisation for Standardization (ISO) / International Electrotechnical Commission (IEC). The H.264 / AVC standard is published by both parent standardization organizations, and it is referred to as ITU-T Recommendation H.264 and ISO / IEC International Standard 14496-10, also known as MPEG-4 Part 10 Advanced Video Coding (AVC). There have been multiple versions of the H.264 / AVC standard, integrating new extensions or features to the specification. These extensions include Scalable Video Coding (SVC) and Multiview Video Coding (MVC). Version 1 of the High Efficiency Video Coding (H.265 / HEVC a.k.a. HEVC) standardwas developed by the Joint Collaborative Team – Video Coding (JCT-VC) of VCEG andMPEG. The standard was published by both parent standardization organizations, and it isreferred to as ITU-T Recommendation H.265 and ISO / IEC International Standard 23008-2,also known as MPEG-H Part 2 High Efficiency Video Coding (HEVC). Later versions ofH.265 / HEVC included scalable, multiview, fidelity range, three-dimensional, and screencontent coding extensions which may be abbreviated SHVC, MV-HEVC, REXT, 3D-HEVC,and SCC, respectively.Versatile Video Coding (VVC) (MPEG-I Part 3), a.k.a. ITU-T H.266, is a videocompression standard developed by the Joint Video Experts Team (JVET) of the MovingPicture Experts Group (MPEG), (formally ISO / IEC JTC1 SC29 WG11) and Video CodingExperts Group (VCEG) of the International Telecommunication Union (ITU) to be thesuccessor to HEVC / H.265. Some key definitions, bitstream and coding structures, and concepts of H.264 / AVC and HEVC 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. Some of the key definitions, bitstream and codingstructures, and concepts of H.264 / AVC are the same as in HEVC – hence, they are describedbelow jointly. Similarly to many earlier video coding standards, the bitstream syntax and semantics as well as the decoding process for error-free bitstreams are specified in H.264 / AVC and HEVC. The encoding process is not specified, but encoders must generate conforming bitstreams. Bitstream and decoder conformance can be verified with the Hypothetical Reference Decoder (HRD). The standards contain coding tools that help in coping with transmission errors and losses, but the use of the tools in encoding is optional and no decoding process has been specified for erroneous bitstreams. The elementary unit for the input to an H.264 / AVC or HEVC encoder and the output of an H.264 / AVC or HEVC decoder, respectively, is a picture. A picture given as an input to an encoder may also be referred to as a source picture, and a picture decoded by a decoded may be referred to as a decoded picture. 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 (forexample, YZX, also known as XYZ). The video may be encoded in YUV or YCbCr color space as that is found to reflectsome 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. In H.264 / AVC and HEVC, a picture may either be a frame or a field. A frame comprises a matrix of luma samples and possibly the corresponding chroma samples. A field is a set of alternate sample rows of a frame and may be used as encoder input, when the source signal is interlaced. Chroma sample arrays may be absent (and hence monochrome sampling may be in use) or chroma sample arrays may be subsampled when compared to luma sample arrays. Chroma formats may be summarized as follows:- In monochrome sampling there is only one sample array, which may be nominallyconsidered the luma array.- In 4:2:0 sampling, each of the two chroma arrays has half the height and half the width ofthe luma array.- In 4:2:2 sampling, each of the two chroma arrays has the same height and half the widthof the luma array.- In 4:4:4 sampling when no separate color planes are in use, each of the two chromaarrays has the same height and width as the luma array. In H.264 / AVC and HEVC, it is possible to code sample arrays as separate color planes into the bitstream and respectively decode separately coded color planes from the bitstream. When separate color planes are in use, each one of them is separately processed (by the encoder and / or the decoder) as a picture with monochrome sampling. A partitioning may be defined as a division of a set into subsets such that each element of the set is in exactly one of the subsets. When describing the operation of HEVC encoding and / or decoding, the following terms may be used. A coding block may be defined as an NxN block of samples for some value of N such that the division of a coding tree block into coding blocks is a partitioning. A coding tree block (CTB) may be defined as an NxN block of samples for some value of N such that the division of a component into coding tree blocks is a partitioning. A coding tree unit (CTU) may be defined as a coding tree block of luma samples, two corresponding coding tree blocks of chroma samples of a picture that has three sample arrays, or a coding tree block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax structures used to code the samples. A coding unit (CU) may be defined as a coding block of luma samples, two corresponding coding blocks of chroma samples of a picture that has three sample arrays, or a coding block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntaxstructures used to code the samples. A CU with the maximum allowed size may be named asLCU (largest coding unit) or coding tree unit (CTU) and the video picture is divided into non-overlapping LCUs. 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 predictionerror coding process for the samples in the said CU. Typically, a CU consists of a squareblock of samples with a size selectable from a predefined set of possible CU sizes. Each PUand TU can be further split into smaller PUs and TUs in order to increase granularity of theprediction 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 thepixels within that PU (e.g. motion vector information for inter predicted PUs and intraprediction directionality information for intra predicted PUs).Each TU can be associated with information describing the prediction error decoding process for the samples within the said TU (including e.g. DCT coefficient information). It is typically signalled at CU level whether prediction error coding is applied or not for each CU. In the case there is no prediction error residual associated with the CU, it can be consideredthere are no TUs for the said CU. The division of the image into CUs, and division of CUsinto PUs and TUs is typically signalled in the bitstream allowing the decoder to reproducethe intended structure of these units. 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. In HEVC, a picture can be partitioned in tiles, which are rectangular and contain an integer number of LCUs. In HEVC, the partitioning to tiles forms a regular grid, where heights and widths of tiles differ from each other by one LCU at the maximum. In HEVC, a slice is defined to be an integer number of coding tree units contained in one independent slice segment and all subsequent dependent slice segments (if any) that precede the next independent slice segment (if any) within the same access unit. In HEVC, a slice segment is defined to be an integer number of coding tree units ordered consecutively in the tile scan and contained in a single NAL unit. The division of each picture into slice segments is a partitioning. In HEVC, an independent slice segment is defined to be a slice segment for which the values of the syntax elements of the slice segment header are not inferred from the values for a preceding slice segment, and a dependent slice segment is defined to be a slice segment for which the values of some syntax elements of the slice segment header are inferred from the values for the preceding independent slice segment in decoding order. In HEVC, a slice header is defined to be the slice segment header of the independent slice segment that is a current slice segment or is the independent slice segment that precedes acurrent dependent slice segment, and a slice segment header is defined to be a part of a codedslice segment containing the data elements pertaining to the first or all coding tree unitsrepresented in the slice segment. The CUs are scanned in the raster scan order of LCUswithin tiles or within a picture, if tiles are not in use. Within an LCU, the CUs have a specificscan order. The decoder reconstructs the output video by applying prediction means similar to the encoder to form a predicted representation of the pixel blocks (using the motion or spatial information created by the encoder and stored in the compressed representation) and prediction error decoding (inverse operation of the prediction error coding recovering the quantized prediction error signal in spatial pixel domain). After applying prediction and prediction error decoding means the decoder sums up the prediction and prediction error signals (pixel values) to form the output video frame. The decoder (and encoder) can also apply additional filtering means to improve the quality of the output video before passing it for display and / or storing it as prediction reference for the forthcoming frames in the video sequence. Instead, or in addition to approaches utilizing sample value prediction and transform coding for indicating the coded sample values, a color palette based coding can be used.Palette based coding refers to a family of approaches for which a palette, i.e., a set of colorsand associated indexes, is defined and the value for each sample within a coding unit is expressed by indicating its index in the palette. Palette based coding can typically achieve good coding efficiency in coding units with a relatively small number of colors (such as image areas which are representing computer screen content, like text or simple graphics). In order to improve the coding efficiency of palette coding different kinds of palette index prediction approaches can be utilized, or the palette indexes can be run-length coded to be able to represent larger homogenous image areas efficiently. Also, in the case the CU contains sample values that are not recurring within the CU, escape coding can be utilized. Escape coded samples are transmitted without referring to any of the palette indexes. Instead, their values are indicated individually for each escape coded sample. The filtering may for example include one more of the following: deblocking, sampleadaptive offset (SAO), and / or adaptive loop filtering (ALF). H.264 / AVC includes adeblocking, whereas HEVC includes both deblocking and SAO. In typical video codecs the motion information is indicated with motion vectors associated with each motion compensated image block, such as a prediction unit. Each of these motion vectors represents the displacement of the image block in the picture to be coded (in the encoder side) or decoded (in the decoder side) and the prediction source block in one of the previously coded or decoded pictures. In order to represent motion vectorsefficiently those are typically coded differentially with respect to block specific predictedmotion vectors. In typical video codecs the predicted motion vectors are created in a predefined way, for example calculating the median of the encoded or decoded motion vectors of the adjacent blocks. Another way to create motion vector predictions is to generate a list of candidate predictions from adjacent blocks and / or co-located blocks in temporal reference pictures and signalling the chosen candidate as the motion vector predictor. In addition to predicting the motion vector values, it can be predicted which reference picture(s) are used for motion-compensated prediction and this prediction information may be represented for example by a reference index of previously coded / decoded picture. The reference index is typically predicted from adjacent blocks and / or co-located blocks in temporal reference picture. Moreover, typical high efficiency video codecs employ an additional motion information coding / decoding mechanism, often called merging / merge mode, where all the motion field information, which includes motion vector and corresponding reference picture index for each available reference picture list, is predicted and used without any modification / correction. Similarly, predicting the motion field information is carried out using the motion field information of adjacent blocks and / or co- located blocks in temporal reference pictures and the used motion field information is signalled among a list of motion field candidate list filled with motion field information of available adjacent / co-located blocks. In typical video codecs the prediction residual after motion compensation is first transformed with a transform kernel (like DCT) and then coded. The reason for this is that often there still exists some correlation among the residual and transform can in many cases help reduce this correlation and provide more efficient coding. Typical Video encoders may utilize Lagrangian cost functions to find optimal codingmodes, e.g., the desired Macroblock mode and associated motion vectors. This kind of costfunction uses a weighting factor λ to tie together the (exact or estimated) image distortion dueto lossy coding methods and the (exact or estimated) amount of information that is required to represent the pixel values in an image area: ^= ^ + ^^ (Eq. 1)where C is the Lagrangian cost to be minimized, D is the image distortion (e.g., MeanSquared Error) with the mode and motion vectors considered, and R the number of bitsneeded to represent the required data to reconstruct the image block in the decoder (including the amount of data to represent the candidate motion vectors). Scalable video coding refers to coding structure where one bitstream can contain multiple representations of the content at different bitrates, resolutions or frame rates. In these cases, the receiver can extract the desired representation depending on itscharacteristics (e.g., resolution that matches best the display device). Alternatively, a serveror a network element can extract the portions of the bitstream to be transmitted to thereceiver depending on e.g., the network characteristics or processing capabilities of thereceiver. A scalable bitstream may consist of a “base layer” providing the lowest qualityvideo 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 lowerlayers. For example, the motion and mode information of the enhancement layer can bepredicted from lower layers. Similarly, the pixel data of the lower layers can be used to create prediction for the enhancement layer. 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. In addition to quality scalability, there are also other scalability modes, such as^ Spatial scalability, where enhancement layer pictures are coded at a higherresolution than the base layer pictures; ^Bit-depth scalability, where enhancement layer pictures are coded at higherbit-depth (e.g.10 or 12 bits) than base layer pictures (e.g. 8 bits); ^Chroma format scalability, where enhancement layer pictures provide higherfidelity in chroma (e.g. coded in 4:4:4 chroma format) than base layer pictures (e.g.4:2:0 format). In all of the above scalability cases, base layer information could be used to code enhancement layer to minimize the additional bitrate overhead. 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 efficiencygains available. A reference frame based scalability codec can be implemented by utilizingthe same hardware or software implementation for all the layers, just taking care of the DPB management by external means. 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 thisinformation 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. An elementary unit for the output of an H.264 / AVC or HEVC encoder and the input of an H.264 / AVC or HEVC decoder, respectively, is a Network Abstraction Layer (NAL) unit. For transport over packet-oriented networks or storage into structured files, NAL units may be encapsulated into packets or similar structures. A bytestream format has been specified in H.264 / AVC and HEVC for transmission or storage environments that do not provide framing structures. The bytestream format separates NAL units from each other by attaching a start code in front of each NAL unit. To avoid false detection of NAL unit boundaries, encoders run a byte-oriented start code emulation prevention algorithm, which adds an emulation prevention byte to the NAL unit payload if a start code would have occurred otherwise. Inorder to enable straightforward gateway operation between packet- and stream-orientedsystems, 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. NAL units consist of a header and payload. In H.264 / AVC and HEVC, the NAL unit header indicates the type of the NAL unit. 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_plus1 indication for temporal level (may be required to be greater than or equal to 1) and a six-bit nuh_layer_id syntax element. The temporal_id_plus1 syntax element may be regarded as a temporal identifier for the NAL unit, and a zero-based TemporalIdvariable may be derived as follows: TemporalId = temporal_id_plus1 – 1. The abbreviationTID may be used to interchangeably with the TemporalId variable. TemporalId equal to 0corresponds to the lowest temporal level. The value of temporal_id_plus1 is required to be non-zero in order to avoid start code emulation involving the two NAL unit header bytes. The bitstream created by excluding all VCL NAL units having a TemporalId greater than or equal to a selected value and including all other VCL NAL units remains conforming. Consequently, a picture having TemporalId equal to tid_value does not use any picture having a TemporalId greater than tid_value as inter prediction reference. A sub-layer or a temporal sub-layer may be defined to be a temporal scalable layer (or a temporal layer, TL) of a temporal scalable bitstream, consisting of VCL NAL units with a particular value of the TemporalId variable and the associated non-VCL NAL units. nuh_layer_id can be understood as a scalability layer identifier. NAL units can be categorized into Video Coding Layer (VCL) NAL units and non-VCL NAL units. VCL NAL units are typically coded slice NAL units. In HEVC, VCL NALunits contain syntax elements representing one or more CU. A non-VCL NAL unit may be for example one of the following types: a sequence parameter set, a picture parameter set, a supplemental enhancement information (SEI) NALunit, an access unit delimiter, an end of sequence NAL unit, an end of bitstream NAL unit, ora filler data NAL unit. Parameter sets may be needed for the reconstruction of decoded pictures, whereas many of the other non-VCL NAL units are not necessary for the reconstruction of decoded sample values. Parameters that remain unchanged through a coded video sequence may be included in a sequence parameter set. In addition to the parameters that may be needed by the decoding process, the sequence parameter set may optionally contain video usability information (VUI), which includes parameters that may be important for buffering, picture output timing,rendering, and resource reservation. In HEVC a sequence parameter set RBSP includesparameters that can be referred to by one or more picture parameter set RBSPs or one or more SEI NAL units containing a buffering period SEI message. A picture parameter set contains such parameters that are likely to be unchanged in several coded pictures. A picture parameter set RBSP may include parameters that can be referred to by the coded slice NAL units of one or more coded pictures. In HEVC, a video parameter set (VPS) may be defined as a syntax structure containing syntax elements that apply to zero or more entire coded video sequences as determined by the content of a syntax element found in the SPS referred to by a syntax element found in the PPS referred to by a syntax element found in each slice segment header. A video parameter set RBSP may include parameters that can be referred to by one or more sequence parameter set RBSPs. The relationship and hierarchy between video parameter set (VPS), sequence parameter set (SPS), and picture parameter set (PPS) may be described as follows. VPS resides one level above SPS in the parameter set hierarchy and in the context of scalability and / or 3D video. VPS may include parameters that are common for all slices across all (scalability or view) layers in the entire coded video sequence. SPS includes the parameters that are common for all slices in a particular (scalability or view) layer in the entire coded video sequence, and may be shared by multiple (scalability or view) layers. PPS includes the parameters that are common for all slices in a particular layer representation (the representation of one scalability or view layer in one access unit) and are likely to be shared by all slices in multiple layer representations. VPS may provide information about the dependency relationships of the layers in a bitstream, as well as many other information that are applicable to all slices across all (scalability or view) layers in the entire coded video sequence. VPS may be considered to comprise two parts, the base VPS and a VPS extension, where the VPS extension may be optionally present. Out-of-band transmission, signaling or storage can additionally or alternatively be used for other purposes than tolerance against transmission errors, such as ease of access or session negotiation. For example, a sample entry of a track in a file conforming to the ISO Base Media File Format may comprise parameter sets, while the coded data in the bitstream is stored elsewhere in the file or in another file. The phrase along the bitstream (e.g. indicating along the bitstream) or along a coded unit of a bitstream (e.g. indicating along a coded tile) may be used in claims and described embodiments to refer to out-of-band transmission, signaling, or storage in a manner that the out-of-band data is associated withthe bitstream or the coded unit, respectively. The phrase decoding along the bitstream oralong a coded unit of a bitstream or alike may refer to decoding the referred out-of-band data(which may be obtained from out-of-band transmission, signaling, or storage) that isassociated with the bitstream or the coded unit, respectively.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. A coded picture is a coded representation of a picture. In HEVC, a coded picture may be defined as a coded representation of a picture containing all coding tree units of the picture. In HEVC, an access unit (AU) may be defined as a set of NAL units that are associated with each other according to a specified classification rule, are consecutive in decoding order, and contain at most one picture with any specific value of nuh_layer_id. In addition to containing the VCL NAL units of the coded picture, an access unit may also contain non-VCL NAL units. Said specified classification rule may for example associate pictures with the same output time or picture output count value into the same access unit. A bitstream may be defined as a sequence of bits, in the form of a NAL unit stream or a byte stream, that forms the representation of coded pictures and associated data forming one or more coded video sequences. A first bitstream may be followed by a second bitstream in the same logical channel, such as in the same file or in the same connection of a communication protocol. An elementary stream (in the context of video coding) may be defined as a sequence of one or more bitstreams. The end of the first bitstream may be indicated by a specific NAL unit, which may be referred to as the end of bitstream (EOB) NAL unit and which is the last NAL unit of the bitstream. In HEVC and its current draft extensions, the EOB NAL unit is required to have nuh_layer_id equal to 0. In 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. 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 accessunits with NoRaslOutputFlag equal to 1, including all subsequent access units up to but notincluding 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 IDRpicture, each BLA picture, and each IRAP picture that is the first picture in that particularlayer 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. In HEVC, a coded video sequence may additionally or alternatively (to the specification above) be specified to end, when a specific NAL unit, which may be referred to as an end of sequence (EOS) NAL unit, appears in the bitstream and has nuh_layer_id equal to 0. A group of pictures (GOP) and its characteristics may be defined as follows. A GOP can be decoded regardless of whether any previous pictures were decoded. An open GOP is such a group of pictures in which pictures preceding the initial intra picture in output order might not be correctly decodable when the decoding starts from the initial intra picture of the open GOP. In other words, pictures of an open GOP may refer (in inter prediction) to pictures belonging to a previous GOP. An HEVC decoder can recognize an intra picture starting an open GOP, because a specific NAL unit type, CRA NAL unit type, may be used for its coded slices. A closed GOP is such a group of pictures in which all pictures can be correctly decoded when the decoding starts from the initial intra picture of the closed GOP. In other words, no picture in a closed GOP refers to any pictures in previous GOPs. In H.264 / AVC and HEVC, a closed GOP may start from an IDR picture. In HEVC a closed GOP may also start from a BLA_W_RADL or a BLA_N_LP picture. An open GOP coding structure is potentially more efficient in the compression compared to a closed GOP coding structure, due to a larger flexibility in selection of reference pictures. A Decoded Picture Buffer (DPB) may be used in the encoder and / or in the decoder. There are two reasons to buffer decoded pictures, for references in inter prediction and for reordering decoded pictures into output order. As H.264 / AVC and HEVC provide a great deal of flexibility for both reference picture marking and output reordering, separate buffers for reference picture buffering and output picture buffering may waste memory resources. Hence, the DPB may include a unified decoded picture buffering process for reference pictures and output reordering. A decoded picture may be removed from the DPB when it is no longer used as a reference and is not needed for output. In many coding modes of H.264 / AVC and HEVC, the reference picture for inter prediction is indicated with an index to a reference picture list. The index may be coded with variable length coding, which usually causes a smaller index to have a shorter value for the corresponding syntax element. In H.264 / AVC and HEVC, two reference picture lists (reference picture list 0 and reference picture list 1) are generated for each bi-predictive (B) slice, and one reference picture list (reference picture list 0) is formed for each inter-coded (P) slice. 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 issome inter coding modes or it may be derived (by an encoder and a decoder) for exampleusing neighboring blocks in some other inter coding modes. 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. 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 along-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 referencepicture (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 beaccompanied by other 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 blockor per reference index or alike);- a vertical motion vector component (which may be indicated e.g. per prediction block orper reference index or alike);- one or more parameters, such as picture order count difference and / or a relative cameraseparation 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 / ormotion information applies. 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 averagemerging candidates –Affine motion inter prediction– sub-block based temporal motion vector prediction– Adaptive motion vector resolution– 8x8 block-based motion compression for temporal motion prediction– High precision (1 / 16 pel) motion vector storage and motion compensation with 8-tap interpolation filter for luma component and 4-tap interpolation filter for chroma component –Triangular partitions– Combined intra and inter prediction– Merge with MVD (MMVD)– Symmetrical MVD coding– Bi-directional optical flow– Decoder side motion vector refinement– Bi-prediction with CU-level weight^ Transform, quantization and coefficients coding– Multiple primary transform selection with DCT2, DST7 and DCT8– Secondary transform for low frequency zone– Sub-block transform for inter predicted residual– Dependent quantization with max QP increased from 51 to 63– Transform coefficient coding with sign data hiding– Transform skip residual coding^ Entropy Coding– Arithmetic coding engine with adaptive double windows probability update^ In loop filter– In-loop reshaping– Deblocking filter with strong longer filter– Sample adaptive offset– Adaptive Loop Filter^ Screen content coding:– Current picture referencing with reference region restriction^ 360-degree video coding– Horizontal wrap-around motion compensation^ High-level syntax and parallel processing– Reference picture management with direct reference picture list signalling– Tile groups with rectangular shape tile groupsIn the coding tools listed above, the cross-component linear model (CCLM) predictionmode is used in the VVC to reduce the cross-component redundancy. Therein, the chromasamples are predicted based on the reconstructed luma samples of the same CU by using a linear model as follows: pred^(i, j) = α · rec^′(i, j) + β (Eq. 2a)where pred^(i, j) represents the predicted chroma samples in a CU and rec^ '(i, j) representsthe downsampled reconstructed luma samples of the same CU. Alternatively, the below equation may be used for CCLM: where >> operation denotes a bit shifting to right by value k.The CCLM parameters (α and β) are derived with at most four neighbouring chromasamples and their corresponding down-sampled luma samples. Suppose the current chroma block dimensions are W×H, then W’ and H’ are set as -W’ = W, H’ = H when LM mode is applied;- W’ =W + H when LM-A mode is applied;- H’ = H + W when LM-L mode is applied;Herein, LM-A mode refers to linear model_above, where only the above template (i.e. sample values from neighbouring positions above the CU) is used to calculate the linear model coefficients. To get more samples, the above template is extended to (W+H). LM-L mode, in turn, refers to linear model_left, where only left template (i.e. sample values fromneighbouring positions left to the CU) is used to calculate the linear model coefficients. To get more samples, the left template is extended to (H+W). For a non-square block, the above template is extended to W+W, the left template is extended to H+H. The above neighbouring positions are denoted as S[ 0, −1 ]…S[ W’ − 1, −1 ] and theleft neighbouring positions are denoted as S[ −1, 0 ]…S[ −1, H’ − 1 ]. Then the four samplesare selected as -S[W’ / 4, −1 ], S[ 3 * W’ / 4, −1 ], S[ −1, H’ / 4 ], S[ −1, 3 * H’ / 4 ] when LM mode isapplied and both above and left neighbouring samples are available; -S[ W’ / 8, −1 ], S[ 3 * W’ / 8, −1 ], S[ 5 * W’ / 8, −1 ], S[ 7 * W’ / 8, −1 ] when LM-Amode is applied or only the above neighbouring samples are available; -S[ −1, H’ / 8 ], S[ −1, 3 * H’ / 8 ], S[ −1, 5 * H’ / 8 ], S[ −1, 7 * H’ / 8 ] when LM-Lmode is applied or only the left neighbouring samples are available; The four neighbouring luma samples at the selected positions are down-sampled and compared four times to find two smaller values: x0A and x1A, and two larger values: x0B and x1B. Their corresponding chroma sample values are denoted as y0A, y1A, y0B and y1B. Then xA, xB, yA and yB are derived as: Finally, the linear model parameters ^ and ^ are obtained according to the followingequations:Figure 3 shows an example of the location of the left and above samples and thesample of the current block involved in the CCLM mode. The division operation to calculate parameter α is implemented with a look-up table. To reduce the memory required for storing the table, the diff value (difference between maximum and minimum values) and the parameter α are expressed by an exponential notation. For example, diff is approximated with a 4-bit significant part and an exponent. Consequently, the table for 1 / diff is reduced into 16 elements for 16 values of the significant part as follows:DivTable [ ] = { 0, 7, 6, 5, 5, 4, 4, 3, 3, 2, 2, 1, 1, 1, 1, 0 } (Eq. 5)This provides the benefit of both reducing the complexity of the calculation as well as the memory size required for storing the needed tables. To match the chroma sample locations for 4:2:0 video sequences, two types of downsampling filter are applied to luma samples to achieve 2 to 1 downsampling ratio in both horizontal and vertical directions. The selection of downsampling filter is specified by a SPS level flag. The two downsampling filters are as follows, which are corresponding to “type-0” and “type-2” content, respectively: It is noted that only one luma line (general line buffer in intra prediction) is used to make the down-sampled luma samples when the upper reference line is at the CTU boundary. This parameter computation is performed as part of the decoding process and is not just as an encoder search operation. As a result, no syntax is used to convey the α and β values to the decoder. For chroma intra mode coding, a total of 8 intra modes are allowed for chroma intra mode coding. Those modes include five traditional intra modes and three cross-component linear model modes (CCLM, LM_A, and LM_L). Chroma mode signalling and derivation process are shown in Table 1. Chroma mode coding directly depends on the intra prediction mode of the corresponding luma block. Since separate block partitioning structure for luma and chroma components is enabled in I slices, one chroma block may correspond to multiple luma blocks. Therefore, for Chroma DM mode, the intra prediction mode of the corresponding luma block covering the center position of the current chroma block is directly inherited. Corresponding luma intra prediction mode Chroma prediction mode 050 18 1 X ( 0 <= X <= 66 )0 66 0 0 0 01 50 66 50 50 502 18 18 66 18 183 1 1 1 66 14 0 50 18 1 X5 81 81 81 81 816 82 82 82 82 827 83 83 83 83 83Table 1. A single binarization table is used regardless of the value of sps_cclm_enabled_flag as shown in Table 2. Value of intra_chroma_pred_mode Bin string 4000 01001 01012 01103 01115 106 1107 111Table 2. In Table 2, the first bin indicates whether it is regular (0) or LM modes (1). If it is LM mode, then the next bin indicates whether it is LM_CHROMA (0) or not. If it is not LM_CHROMA, next 1 bin indicates whether it is LM_L (0) or LM_A (1). For this case, when sps_cclm_enabled_flag is 0, the first bin of the binarization table for the corresponding intra_chroma_pred_mode can be discarded prior to the entropy coding. Or, in other words, the first bin is inferred to be 0 and hence not coded. This single binarization table is used forboth sps_cclm_enabled_flag equal to 0 and 1 cases. The first two bins in Table 2 are contextcoded with its own context model, and the rest bins are bypass coded. In addition, in order to reduce luma-chroma latency in dual tree, when the 64x64 luma coding tree node is partitioned with Not Split (and intra sub-partitions (ISP) is not used forthe 64x64 CU) or QT, the chroma CUs in 32x32 / 32x16 chroma coding tree node are allowed to use CCLM in the following way: -If the 32x32 chroma node is not split or partitioned QT split, all chroma CUs in the 32x32node can use CCLM -If the 32x32 chroma node is partitioned with Horizontal BT, and the 32x16 child nodedoes not split or uses Vertical BT split, all chroma CUs in the 32x16 chroma node can use CCLM. In all the other luma and chroma coding tree split conditions, CCLM is not allowed for chroma CU. Multi-model LM (MMLM) The CCLM included in VVC is extended by adding three Multi-model LM (MMLM) modes. In each MMLM mode, the reconstructed neighbouring samples are classified into two classes using a threshold which is the average of the luma reconstructed neighbouring samples. The linear model of each class is derived using the Least-Mean-Square (LMS) method. For the CCLM mode, the LMS method is also used to derive the linear model. Figures 4a and 4billustrate two luma-to-chroma models obtained for luma (Y) threshold of 17 in sample domainand spatial domain, respectively. Each luma-to-chroma model has its own linear modelparameters α and β. As can be seen in Figure 4b, each luma-to-chroma model corresponds to aspatial segmentation of the content (i.e., they correspond to different objects or textures in the scene). Convolutional cross-component model (CCCM) An improved version of cross-component prediction, known as CCCM, uses 2D filter kernel to derive the luma-to-chroma model. The filter coefficients are derived decoder-side using reconstructed set of input data and chroma samples. For the filter coefficient derivation, co-located reference sample areas (consisting of reconstructed luma and chroma samples) are defined for both luma and chroma, as shown in Figure 5, where the typicallyused 4:2:0 chroma down-sampling has been applied. The reference sample area for a given block can be, for example, six lines above and left as shown in Figure 5, yet any number of reference lines (that can be realized by both the encoder and decoder) can be used. Generally, reference samples can contain any chroma and luma samples that have been reconstructed by both the encoder and decoder. Once the reference samples are determined the filter coefficients can be derived, for example, using different types of linear regression tools such as ordinary least-squares estimation, orthogonal matching pursuit, optimized orthogonal matching pursuit, ridge regression, or least absolute shrinkage and selection operator. The dimensions of the filter kernel can be for example 1x3 (1D vertical) 601, 3x1 (1Dhorizontal) 602, 3x3604, 7x7604 or any dimensions, and can be shaped (by selecting only asubset of all possible kernel locations) as a cross or a diamond (as shown in Figure 6) or as any given shape. When referring to the samples within the filter kernel the following notation is used: north (above), east (right), south (below), west (left) and center, as illustrated inFigure 6 using the letters N, E, S, W, C, respectively.The overall method of reconstructing chroma samples using convolution between a decoder-side obtained filter kernel and a set of input data is referred to as convolutional cross-component model (CCCM) here. The following steps can be applied to perform a CCCM operation:1) Define co-located reference areas over the luma and chroma components.2) Down-sample the luma samples to match the chroma grid (optional).3) Scan the luma and chroma samples of the reference area and collect available statistics(such as auto-correlation matrix and cross-correlation vector) based on the filter shape.4) Solve the filter coefficients by minimizing squared-error (or any other metric) based onthe available statistics (such as the auto-correlation matrix and cross-correlation vector).5) Calculate a predicted chroma block by convolving the down-sampled luma samples withthe filter kernel. Let us define the (possibly down-sampled) luma samples as a 2D array Y(x,y) indexed using horizontal x-coordinate and vertical y-coordinate. Let us also define the co-located chroma samples as a 2D array C(x,y) and the filter kernel (i.e., coefficients) as 3x3 array F(i,j). On a sample level we define the convolution between Y and F as, When using other data terms, such as the non-linear square-root term, the appended convolution becomes,where ^^ are filter coefficients that reside outside of the 2D filter kernel yet have been obtained asa part of the system of linear equations that were used to solve the 2D filter coefficients in Step 4 above. Similarly, we can add the bias term to the convolution with, Multiple reference line (MRL) intra prediction Multiple reference line (MRL) intra prediction uses more reference lines for intra prediction. In Figure 7, an example of 4 reference lines is depicted, where the samples of segments A and F are not fetched from reconstructed neighbouring samples but padded with the closest samples from Segment B and E, respectively. HEVC intra-picture prediction uses the nearest reference line (i.e., reference line 0). In MRL, 2 additional lines (reference line 1 and reference line 3) are used. The index of selected reference line (mrl_idx) is signalled and used to generate intra predictor. For reference line idx, which is greater than 0, only include additional reference line modes in MPM list and only signal mpm index without remaining mode. The reference line index is signalled before intra prediction modes, and Planar mode is excluded from intra prediction modes in case a nonzero reference line index is signalled. MRL is disabled for the first line of blocks inside a CTU to prevent using extended reference samples outside the current CTU line. Also, PDPC is disabled when additional line is used. For MRL mode, the derivation of DC value in DC intra prediction mode for non-zero reference line indices are aligned with that of reference line index 0. MRL requires the storage of 3 neighboring luma reference lines with a CTU to generate predictions. The CCLM tool also requires 3 neighboring luma reference lines for its down-sampling filters. The definition of MLR to use the same 3 lines is aligned as CCLM to reduce the storage requirements for decoders. Intra sub-partitions (ISP) The intra sub-partitions (ISP) divides luma intra-predicted blocks vertically or horizontally into 2 or 4 sub-partitions depending on the block size. For example, minimum block size for ISP is 4x8 (or 8x4). If block size is greater than 4x8 (or 8x4) then the corresponding block is divided by 4 sub-partitions. It has been noted that the M×128 (with M≤64) and 128×N (with N≤64) ISP blocks could generate a potential issue with the 64×64 VDPU. For example, an M×128 CU in the single tree case has an M×128 luma TB and two corresponding M / 2×64 chroma TBs. If the CU uses ISP, then the luma TB will be divided into four M×32 TBs (only the horizontal split is possible), each of them smaller than a 64×64 block. However, in the current design of ISP chroma blocks are not divided. Therefore, both chroma components will have a size greater than a 32×32 block. Analogously, a similar situation could be created with a 128×N CU using ISP. Hence, these two cases are an issue for the 64×64 decoder pipeline. For this reason, the CU sizes that can use ISP is restricted to a maximum of 64×64. All sub-partitions fulfil the condition of having at least 16 samples. Matrix weighted Intra Prediction (MIP) Matrix weighted intra prediction (MIP) method is a newly added intra prediction technique into VVC. For predicting the samples of a rectangular block of width W and height H, matrix weighted intra prediction (MIP) takes one line of H reconstructed neighbouring boundary samples left of the block and one line of W reconstructed neighbouring boundary samples above the block as input. If the reconstructed samples are unavailable, they are generated as it is done in the conventional intra prediction. The generation of the prediction signal is based on the following three steps, which are averaging, matrix vector multiplication and linear interpolation as shown in Figure 8. Inter prediction in VVC Merge list may include the following candidate(s) 1) Spatial MVP from spatial neighbour CUs2) Temporal MVP from collocated CUs3) History-based MVP from a FIFO table4) Pairwise average MVP (using the candidates already in the list)5) Zero MVs.Merged mode width motion vector difference (MMVD) is to signal MVDs and a resolution index after signaling merge candidate. In Symmetric MVD, motion information of list-1 is derived from motion information of list-0 in bi-prediction case. In Affine prediction, several motion vectors are indicated / signaled for different corners of a block, which are used to derive the motion vectors of sub-block. In affine merge, affine motion information of a block is generated based on the normal or affine motion information of the neighboring blocks. In Sub-block-based temporal motion vector prediction, motion vectors of sub-blocks of the current block are predicted from a proper subblocks in the reference frame which are indicated by the motion vector of a spatial neighboring block (if available). In Adaptive motion vector resolution (AMVR), precision of MVD is signaled for each CU. In Bi-prediction with CU-level weight, an index indicated the weight values for weighted average of two prediction block. Bi-directional optical flow (BDOF) refines the motion vectors in bi-prediction case. BDOF generates two prediction blocks using the signaled motion vectors. Then a motion refinement is calculated two minimize the error between two prediction blocks using their gradient values. The final prediction blocks are refined using the motion refinement and gradient values. Bi-prediction with CU-level weight (BCW) and weighted prediction (WP) In HEVC, the bi-prediction signal is generated by averaging two prediction signals obtained from two different reference pictures and / or using two different motion vectors. In VVC, the bi-prediction mode is extended beyond simple averaging to allow weighted averaging of the two prediction signals. ^bi-pred = ^(8 − ^) ∗ ^^ + ^ ∗ ^^ + 4^ ≫ 3Five weights are allowed in the weighted averaging bi-prediction, w∈{-2,3,4,5,10}. For each bi-predicted CU, the weight w is determined in one of two ways: 1) for a non-merge CU, the weight index is signalled after the motion vector difference; 2) for a merge CU, the weight index is inferred from neighbouring blocks based on the merge candidate index. BCW is only applied to CUs with 256 or more luma samples (i.e., CU width times CU height is greater than or equal to 256). For low-delay pictures, all 5 weights are used. For non-low-delay pictures, only 3 weights (w∈{3,4,5}) are used. -At the encoder, fast search algorithms are applied to find the weight index withoutsignificantly increasing the encoder complexity. These algorithms are summarized as follows. For further details readers are referred to the VTM software and document JVET-L0646. When combined with AMVR, unequal weights are only conditionally checked for 1-pel and 4-pel motion vector precisions if the current picture is a low-delay picture. -When combined with affine, affine ME will be performed for unequal weights if and onlyif the affine mode is selected as the current best mode. -When the two reference pictures in bi-prediction are the same, unequal weights are onlyconditionally checked. -Unequal weights are not searched when certain conditions are met, depending on thePOC distance between current picture and its reference pictures, the coding QP, and the temporal level. The BCW weight index is coded using one context coded bin followed by bypass coded bins. The first context coded bin indicates if equal weight is used; and if unequal weight is used, additional bins are signalled using bypass coding to indicate which unequal weight is used. Weighted prediction (WP) is a coding tool supported by the H.264 / AVC and HEVC standards to efficiently code video content with fading. Support for WP was also added into the VVC standard. WP allows weighting parameters (weight and offset) to be signalled for each reference picture in each of the reference picture lists L0 and L1. Then, during motion compensation, the weight(s) and offset(s) of the corresponding reference picture(s) are applied. WP and BCW are designed for different types of video content. In order to avoid interactions between WP and BCW, which will complicate VVC decoder design, if a CU uses WP, then the BCW weight index is not signalled, and w is inferred to be 4 (i.e. equal weight is applied). For a merge CU, the weight index is inferred from neighbouring blocks based on the merge candidate index. This can be applied to both normal merge mode and inherited affine merge mode. For constructed affine merge mode, the affine motion information is constructed based on the motion information of up to 3 blocks. The BCW index for a CU using the constructed affine merge mode is simply set equal to the BCW index of the first control point MV. In VVC, CIIP and BCW cannot be jointly applied for a CU. When a CU is coded with CIIP mode, the BCW index of the current CU is set to 2, e.g. equal weight. Combined inter and intra prediction (CIIP) In VVC, when a CU is coded in merge mode, if the CU contains at least 64 luma samples (that is, CU width times CU height is equal to or larger than 64), and if both CU width and CU height are less than 128 luma samples, an additional flag is signaled to indicate if the combined inter / intra prediction (CIIP) mode is applied to the current CU. As its name indicates, the CIIP prediction combines an inter prediction signal with an intra prediction signal. The inter prediction signal in the CIIP mode ^^^^^^is derived using the same inter prediction process applied to regular merge mode; and the intra prediction signal ^^^^^^is derived following the regular intra prediction process with the planar mode. Then, the intra and inter prediction signals are combined using weighted averaging, where the weight value is calculated depending on thecoding modes of the top and left neighbouring blocks (depicted in Figure 9) as follows:- If the top neighbor is available and intra coded, then set isIntraTop to 1, otherwise setisIntraTop to 0; -If the left neighbor is available and intra coded, then set isIntraLeft to 1, otherwise setisIntraLeft to 0; -If (isIntraLeft + isIntraTop) is equal to 2, then wt is set to 3;- Otherwise, if (isIntraLeft + isIntraTop) is equal to 1, then wt is set to 2;- Otherwise, set wt to 1.The CIIP prediction is formed as follows: ^CIIP = ^(4 − ^^) ∗ ^^^^^^ + ^^ ∗ ^^^^^^ + 2^ ≫ 2Local illumination compensation (LIC) 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 α and an offset β, which forms a linear equation, that is, α*p[x]+β to compensate illumination changes, where p[x] is a reference sample pointed to by MV at a location x on reference picture. Since α and β 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. The local illumination compensation proposed in JVET-O0066 is used in ECM for uni-prediction inter CUs with the following modifications. • Intra neighbor samples can be used in LIC parameter derivation; • LIC is disabled for blocks with less than 32 luma samples; • For both non-subblock and affine modes, LIC parameter derivation is performed based on the template block samples corresponding to the current CU, instead of partial template block samples corresponding to first top-left 16x16 unit; • Samples of the reference block template are generated by using MC with the block MV without rounding it to integer-pel precision. Handling of out-of-boundary samples (OOB) In bi-directional motion compensation the out of boundary (OOB) prediction samples are discarded and only the non-OOB predictors, when available, are used to generate the final predictor. Specifically, let ^^^_^ and ^^^_^ denote the position of one prediction sample in^,^ ^,^one current block, ^^_^^^,^^and ^^_^^^,^^(x = 0,1) denote the MV of the current block; and ^^^^^^^^^^^^^ are the positions of four boundariesof the picture. One prediction sample is regarded as OOB when at least one of the following conditions is satisfied: (^^^_^^,^ + ^^_^^^,^^) > (^^^^^^^^^^^^+half_pixel), (^^^_^ + ^^_^^^^,^ ^,^) < (^^^^^^^^^^^- half_pixel),(^^^_^^,^ + ^^_^^^,^^) > (^^^^^^^^^^^^^+ half_pixel), (^^^_^^ + ^^_^^^,^ ^,^ ) < (^^^^^^^^^^- half_pixel)where half_pixel is equal to 8 that represents the half-pel sample distance in the 1 / 16-pel sampleprecision. After examining the OOB condition for each sample, the final prediction samples of one bi-directional block are generated as follows: ^^ ^^If ^ is OOB and ^ is non-OOB^,^ ^,^^^^^^ ^^= ^^^,^^,^^^ ^^else if ^ is non-OOB and ^ is OOB^,^ ^,^^^^^^^^= ^^^,^^,^else OOB checking process is also applicable when BCW is enabled. It is noted that this sample-adaptive bi-prediction process only applies to prediction units for which at least a reference bock is first detected as partially or entirely out-of-bounds.Thus, a block-level OOB criteria is first checked. If both prediction blocks are non-OOB, thenthe usual bi-prediction takes place. In-loop filters There are totally three in-loop filters in VVC. Besides a deblocking filter and a SAO (the two loop filters in HEVC), adaptive loop filters (ALF) are applied. The ALF comprisesluma ALF, chroma ALF and cross-component ALF (CC-ALF). The ALF filtering process is designed so that luma ALF, chroma ALF and CC-ALF can be executed in parallel. The order of the filtering process in the VVC is the deblocking filter, the SAO and the ALF. The SAO in VVC is the same as that in HEVC. In VVC, an Adaptive Loop Filter (ALF) with block-based filter adaption is applied. For the luma component, one among 25 filters is selected for each 4×4 block, based on the direction and activity of local gradients. In VVC, two diamond filter shapes are used. The 7×7 diamond shape is applied forluma component and the 5×5 diamond shape is applied for chroma components. A classification index C for each 4x4 luma component block is derived based on its directionality ^ and a quantized value of activity ^^, as follows:To calculate ^ and ^^, gradients of the horizontal, vertical and two diagonal directionare first calculated using 1-D Laplacian:

[0002] where indices ^ and ^ refer to the coordinates of the upper left sample within the 4 × 4 block and^(^, ^) indicates a reconstructed sample at coordinate To reduce the complexity of block classification, the subsampled 1-D Laplacian calculation is applied. The same subsampled positions are used for gradient calculation of all directions. Then ^ maximum and minimum values of the gradients of horizontal and vertical directions are set as: The maximum and minimum values of the gradient of two diagonal directions are set as: ^^^^ ^^^^,^^ = ^^^(^^^, ^^^), ^ ^^^,^^ = ^^^(^^^ , ^^^)To derive the value of the directionality ^, these values are compared against each other and with two thresholds ^^and ^^: Step 1. If both are true, ^ is set to 0.Step 2. If ^^^^⁄ ^^^^^^⁄ ^^^^,^^^,^ > ^^^,^^^^^,^^ , continue from Step 3; otherwise continue fromStep 4. Step 3. set to 2; otherwise ^ is set toStep 4. otherwise ^ is set toThe activity value ^ is calculated as: ^is further quantized to the range of 0 to 4, inclusively, and the quantized value isdenoted as ^^. For chroma components in a picture, no classification method is applied. Before filtering each 4×4 luma block, geometric transformations such as rotation or diagonal and vertical flipping are applied to the filter coefficients f(k,l) and to the corresponding filter clipping values c(k,l) depending on gradient values calculated for that block. This is equivalent to applying these transformations to the samples in the filter support region. The idea is to make different blocks to which ALF is applied more similar by aligning their directionality. Three geometric transformations, including diagonal, vertical flip and rotation are introduced: Diagonal: ^^(^, ^) = ^(^, ^), ^^(^, ^) = ^(^, ^),Vertical flip: ^^(^, ^) = ^(^, ^ − ^ − 1), ^^(^, ^) = ^(^, ^ − ^ − 1)Rotation: ^^(^, ^) = ^(^ − ^ − 1, ^), ^^(^, ^) = ^(^ − ^ − 1, ^)where ^ is the size of the filter and 0 ≤ ^, ^ ≤ ^ − 1 are coefficients coordinates, such thatlocation (0,0) is at the upper left corner and location (^ − 1, ^ − 1) is at the lower right corner.The transformations are applied to the filter coefficients f (k, l) and to the clipping values ^(^, ^) depending on gradient values calculated for that block. The relationshipbetween the transformation and the four gradients of the four directions are summarized in the following table. Gradient values Transformationgd2 < gd1 and gh < gv No transformationgd2 < gd1 and gv < gh Diagonalgd1 < gd2 and gh < gv Vertical flipgd1 < gd2 and gv < gh RotationTable: Mapping of the gradient calculated for one block and the transformations At decoder side, when ALF is enabled for a CTB, each sample R(i,j) within the CU is filtered, resulting in sample value R'(i,j) as shown below, where f(k,l) denotes the decoded filter coefficients, K(x,y) is the clipping function and c(k,l) denotes the decoded clipping parameters. The variable k and l vary between -L / 2 andL / 2 where L denotes the filter length. The clipping function ^(^, ^) = min(^, max(−^, ^)), which corresponds to thefunction Clip3 (-y,y,x). The clipping operation introduces non-linearity to make ALF more efficient by reducing the impact of neighbor sample values that are too different with the current sample value. CC-ALF uses luma sample values to refine each chroma component by applying an adaptive, linear filter to the luma channel and then using the output of this filtering operation for chroma refinement. Figure 10 provides a system level diagram of the CC-ALF processwith respect to the SAO, luma ALF and chroma ALF processes. The ALF pipeline thereincomprises applying luma ALF to the reconstructed luma samples and applying chroma ALF to the reconstructed chroma samples. The reconstructed luma samples are further used as an input to CC-ALF to obtain a chroma correction, which is then added to the output of the chroma ALF. Filtering in CC-ALF is accomplished by applying a linear, diamond shaped filter tothe luma channel. One filter is used for each chroma channel, and the operation is expressed as where (^, ^) is chroma component i location being refined (^^, ^^) is the luma locationbased on (^, ^), ^^ is filter support area in luma component, ^^(^^, ^^) represents the filtercoefficients. Thus, the luma filter support is the region collocated with the current chromasample after accounting for the spatial scaling factor between the luma and chroma planes. In the VVC reference software, CC-ALF filter coefficients are computed by minimizing the mean square error (MSE) of each chroma channels with respect to the original chroma content. To achieve this, the VTM algorithm uses a coefficient derivation process similar to the one used for chroma ALF. Specifically, a correlation matrix is derived,and the coefficients are computed using a Cholesky decomposition solver in an attempt tominimize a mean square error metric. In designing the filters, a maximum of 8 CC-ALF filters can be designed and transmitted per picture. The resulting filters are then indicated for each of the two chroma channels on a CTU basis. Thus, the ALFs perform luma and chroma filtering in parallel fashion to lower the mean square error (MSE) between the reconstruction and the original samples. I.e., in VVC the ALF first filters both luma and chroma in parallel, and subsequently also applies cross- component filtering (CC-ALF) to further improve the chroma. Only the luma samples are used as input to the CC-ALF in VVC. The chroma ALF uses chroma reconstruction buffer. However, using luma samples only as an input to CC-ALF and / or chroma reconstruction buffers for refining the chroma components may not be optimal all cases. This is especially true if the correlation between the luma component and the chroma components of the sample is smaller than in a typical situation.Figure 11 illustrates a filtering pipeline, where luma input to CC-ALF 1118, 1119 isthe output of the ALF luma stage 1110. The CCCM model derivation 1120 can be placedafter the SAO Luma stage 1105, before the ALF luma stage 1110. The optional input paths toCCCM filtering stage 1115 are used when some of the output samples of CCCM filteringderivation 1120 stage are to be blended with the SAO output Cb, Cr samples 1106, 1107.In Figure 11 an input obtained from the CCCM filter 1115 is provided to the ALFchroma 1112, and the output of the chroma ALF 1115 is provided directly to the cross-component adaptive in-loop filter, such as the CC-ALF Cb 1118 and CC-ALF Cr 1119.Therein, the output of the ALF chroma 1112, together with the output of the first adaptive in-loop filter, i.e. ALF luma 1110 all used as inputs for the CC-ALF Cb 1118 and CC-ALF Cr1119. The outputs of the cross-component adaptive in-loop chroma filters 1118, 1119 (CC-ALF Cb, CC-ALF Cr) are then added to the outputs of the ALF chroma 1112 as anadditional correction for the chroma samples.In the example of Figure 11, one of the aims is to obtain maximum benefit from thefiltering stage of CCCM. A single cross-component model, such as a baseline model is often not adequate and multiple models need to be utilized in different parts of the picture and sequence to improve the quality of the reconstructed chroma samples. Now an improved method for chroma filtering is introduced, which will be describedin below. In particular, the use of multiple cross-component models in ALF chroma update isdescribed. In the following, concept of multiple models is presented and a description of how the models are selected by the decoder is provided together with model list re-orderingtechniques for improving the coding efficiency. In practice, the present embodiments have aneffect to the filtering pipeline of Figure 11, and in particular to the CCCM model derivation stage 1120. The objective quality (e.g., PSNR) of reconstructed chroma samples can be improvedby applying cross-component filtering during the ALF process. In the example of Figure 11,representing the conventional ALF chroma update, it is signaled at CTU-level, which CTUs make use of the cross-component filter. However, in such example, only the default cross- shaped 2D filter is considered. Due to the present embodiments, improved performance can be obtained by utilizing different filter kernels (also referred to as “models”) for cross- component filtering. Figure 12 illustrates a flowchart of a method according to an embodiment of the invention. The method comprises steps for ^receiving 1210 an image block unit of a frame, the image block unit comprisingsamples in color channels comprising at least one chrominance channel and oneluminance channel; ^reconstructing 1220 samples of said color channels to produce reconstructedchrominance samples and reconstructed luminance samples; ^determining 1230 a list of cross-component models;^ determining 1240 an index in the list of cross-component models;^ deriving 1250 a cross-component model from the list with the given index;^ applying 1260 a first adaptive in-loop filter for the reconstructed luminance samples;^ applying 1270 the derived cross-component model for the reconstructed luminancesamples to obtain an additional input for reconstructing the chrominance samples. The aforementioned list of cross-component models can be according to the followingexample: model index 0. center, north, south, east, west, nonlinear, bias model index 1. center, biasmodel index 2. center, nonlinear, biasmodel index 3. center, north, south, east, west, biasmodel index 4. center, north, south, nonlinear, bias model index 5. center, east, west, nonlinear, biasmodel index 6. center, north-east, north, north-west, east, west, south-east, south,south-west, nonlinear, bias model index 7. center, north-east, north, north-west, east, west, south-east, south,south-west, bias where north, south, west and east are spatial locations adjacent to the center chroma sample. Figure 13 shows an example of the spatial locations, where center chroma sample isrepresented by a location (x, y), whereupon north has a location (x, y-1), south has a location(x, y+1), west has a location (x-1, y) and east has a location (x+1, y). It is to be appreciatedthat the list of cross-component models as presented above is only an example and any otherconfiguration of adjacent samples and nonlinear terms can be used. The list can have an arbitrary number of model elements as long as the number is fixed or otherwise known by the decoder. Figure 14 illustrates an example of a decoded picture 1400 utilizing multi-model ALFapproach . In the picture of Figure 14, each CTU has its own ALF chroma parameter including tool on / off selection and model index. The on / off selection may be simplyindicated with 1 / 0, where “1” may stand for appliance of the model (it is to be appreciated that any one or more indices may be used to indicate the enablement). The model for thecorresponding CTU is indicated with a corresponding index. In the example of Figure 14, forexample, the top-left CTU 1410 has ALF chroma update enabled “1”, and uses cross- component model 0. The CTU 1411 on the right side of the top-left CTU 1410 has notenabled the ALF chroma update “0”, whereupon there is no need to decode the rest of the index. According to present embodiments, the encoder performs a rate-distortionoptimization for different cross-component models, and based on the result, the encoder decides at CTU-level which (if any) cross-component model performs best. Thus, theencoder is configured to determine for each CTU (or for entire picture), whether or not theALF chroma update is enabled, and if so, which cross-component model is to be applied. The encoder may signal the determined information to the decoder in various ways. For example, the encoder may use explicit signaling by transmitting a syntax element indicating the model index at CTU-level. Alternatively, the encoder may send a picture-level syntax element defining which model is used for all CTUs. Similarly, the decoder can determine the cross-component model per CTU or per picture depending on the signaling used by the encoder. For example, in respect of explicit signaling, the decoder receives a syntax element for determining the model index at CTU- level. As another example, the decoder receives a picture-level syntax element that defines which model is used for all CTUs. According to the embodiment further improving the previous embodiment, the modellist is available both at the encoder and at the decoder. According to an embodiment further improving the previous embodiment(s), themodel list can be initialized at the decoder at picture level to a fixed list or to an inferred list. The inferred list may be based on signaling received from the encoder.According to an embodiment further improving the previous embodiment(s), thesignaling of any of the syntax elements, such as an element for determining the model index,can be performed using variable length coding, fixed length coding, Huffman coding, arithmetic coding or any other suitable coding method. According to an embodiment further improving the previous embodiment(s), any ofthe syntax elements for determining the model indices can be signaled, for example, via Sequence Parameter Set (SPS), Picture Parameter Set (PPS), Adaptation Parameter Set(APS) or, for example, together with any other sequence, picture, slice, sub-picture, CTU-level or other partition-level parameter set. According to an embodiment further improving the previous embodiment(s), themulti-model concept is not limited to model indices only but can include other settings such as block size, block overlap, and other filtering related parameters. According to an embodiment further improving the previous embodiment(s), thedecoder can obtain the model list from the decoded bitstream. Figure 15 shows a flowchart of a method according to a further embodiment. Such amethod comprises ^receiving 1510 an image block unit of a frame, the image block unit comprisingsamples in color channels comprising at least one chrominance channel and oneluminance channel; ^reconstructing 1520 samples of said color channels to produce reconstructedchrominance samples and reconstructed luminance samples; ^determining 1530 a list of cross-component models and initializing the list to havemodels in descending numerical order; ^determining 1540 an index in the list of cross-component models;^ deriving 1550 a cross-component model from the list with the given index;^ re-ordering 1560 the list of cross-component models;^ applying 1570 a first adaptive in-loop filter for the reconstructed luminancesamples; ^applying 1580 the derived cross-component model for the reconstructed luminancesamples to obtain an additional input for reconstructing the chrominance samples.As shown in Figure 15, the model list is re-ordered at each CTU afterdecoding / determining the model index. According to an embodiment, the list is re-orderedusing the most recently accessed model index. The re-ordering affects to the list in such a manner, that the most recently used model is moved to the beginning of the model list. Thisis beneficial, for example, when variable-length coding (such as Golomb, Rice, Huffmancoding, arithmetic coding) is used to signal the model index, as shorter (or more frequent) indices will have shorter codes. Practically this means that after each CTU, the currently used model is moved to the top of the list and the relative order of other models is maintained. The re-ordering is discussed in more detailed, with the following examples. In below,there is the initial cross-component model list before decoding any CTUs:center north south east west nonlinear biascenter biascenter nonlinear biascenter north south east west biascenter north south nonlinear biascenter east west nonlinear biascenter north- north north- east west south- south south- nonlinear biaseast west east west center north- north north- east west south- south south-bias east west east west After decoding a CTU that uses model index 1, the above list is permuted by moving the model at index 1 to the top of the list while maintaining the relative order of the other entries in the list. The re-ordered list is thus as follows: 0center bias1 center north south east west nonlinear bias2 center nonlinear bias3 center north south east west bias4 center north south nonlinear bias5 center east west nonlinear bias6 center north- north north- east west south- south south- nonlinear biaseast west east west 7center north- north north- east west south- south south-bias east west east west According to the embodiment further improving the previous embodiment, the modellist (both the initial list and the re-ordered list) is available both at the encoder and at the decoder. According to an embodiment further improving the previous embodiment(s), themodel list can be initialized at the decoder at picture level to a fixed list or to an inferred list. According to an embodiment further improving the previous embodiment(s), thesignaling of any of the syntax elements, such as an element for determining the model index,can be performed using variable length coding, fixed length coding, Huffman coding, arithmetic coding or any other suitable coding method. According to an embodiment further improving the previous embodiment(s), thesyntax elements for determining the model indices can be signaled, for example, via SPS, PPS, APS or, for example, together with any other sequence, picture, slice, sub-picture, CTU-level or other partition-level parameter set. According to an embodiment further improving the previous embodiment(s), thecross-component model list may be re-ordered by the decoder at picture-level or at CTU- level. According to an embodiment further improving the previous embodiment(s), the usageof model list re-ordering can be determined by the decoder from the decoded bitstream. Figure 16 shows a flowchart of a method according to a further embodiment. Such amethod comprises ^receiving 1610 an image block unit of a frame, the image block unit comprisingsamples in color channels comprising at least one chrominance channel and oneluminance channel; ^reconstructing 1620 samples of said color channels to produce reconstructedchrominance samples and reconstructed luminance samples; ^determining 1625 that a picture-level model list re-ordering is used;^ determining 1630 a reference picture;^ determining 1640 a list of cross-component models for the reference picture;^ generating 1650 a re-ordered list where the cross-component models of thedetermined list have been re-ordered; ^determining 1660 an index for a cross-component model;^ deriving 1670 the cross-component model from the re-ordered list with the givenindex; ^applying 1680 a first adaptive in-loop filter for the reconstructed luminancesamples; ^applying 1690 the derived cross-component model for the reconstructed luminancesamples to obtain an additional input for reconstructing the chrominance samples. According to an alternative for the model list re-ordering, as shown in Figure 16, thelist is re-ordered at picture-level after the model indices for all CTUs have been determined.Figure 17 illustrates an example for such re-ordering. The initial list 1700 comprises modelindices 0 – 5 having corresponding models. For example model index 0 is for a model center“C”, north “N”, south “S”, east “E”, west “W”, non-linear “NL”, bias “B”. This list is usedby the CTUs of the reference picture “POC 6” 1710. A histogram 1720 is formed of the ALFchroma parameters for the reference picture POC 61710. The histogram shows the indexnumber 0 – 5 for the models, and the number of occurrences of each model in the referencepicture 1710. For example, model having index “0” has been used by six CTUs in thereference picture 1710, while model having index “5” has been used by two CTUs in thereference picture 1710. The list is reordered based on the histogram 1720. This means thatthe model appearing in the top of the list will be the one that was mostly used in the referencepicture 1710, i.e., model having index “0”. The next model in the list will then be the modelthat had index “3” in the initial list, and so on. The reordered list 1730 is then used as theinitial list for current picture “POC 7” 1740. Thus, the re-ordering may be based on ahistogram of the indices used by CTUs, wherein more frequently occurring model indices are placed at the top of the list. Indices that have zero occurrences are appended to list using descending numerical order. In implementation, the decoder receives a syntax element that indicates the decoder touse the picture-level re-ordered list. The decoder then derives the reference picture based on which the decoder then performs the model list re-ordering. The re-ordered list is then used by the current picture. The reference picture may be derived from a reference picture list, for example reference picture lists L0 or L1. According to an embodiment further improving the previous embodiment, the modellist can be initialized at the decoder at picture level to a fixed list or to an inferred list. According to an embodiment further improving the previous embodiment(s), thesignaling of any of the syntax elements, such as an element for determining the model index,can be performed using variable length coding, fixed length coding, Huffman coding, arithmetic coding or any other suitable coding method. According to an embodiment further improving the previous embodiment(s), thesyntax elements for determining the model indices can be signaled, for example, via SPS, PPS, APS or, for example, together with any other sequence, picture, slice, sub-picture, CTU-level or other partition-level parameter set. According to an embodiment further improving the previous embodiment(s), thecross-component model list may be re-ordered by the decoder at picture-level or at CTU- level. According to an embodiment further improving the previous embodiment(s), the usageof model list re-ordering can be determined by the decoder from the decoded bitstream. According to an embodiment further improving the previous embodiment(s), whenderiving the model list based on the histogram, for any matching number of counts order in numerical descending order based on order in the initial list. According to an embodiment further improving the previous embodiment(s), the re-ordered model list derived from a reference picture may be applied only to certain CTUs in the current picture, or for example only at the first CTU of the current picture. An apparatus according to an aspect comprises means for receiving an image blockunit of a frame, the image block unit comprising samples in color channels comprising atleast one chrominance channel and one luminance channel; means for reconstructing samples of said color channels to produce reconstructed chrominance samples and reconstructedluminance samples; means for determining a list of cross-component models; means fordetermining an index in the list of cross-component models; means for deriving a cross-component model from the list with the given index; means for applying a first adaptive in-loop filter for the reconstructed luminance samples; and means for applying the derivedcross-component model for the reconstructed luminance samples to obtain an additionalinput for reconstructing the chrominance samples. These means comprise at least oneprocessor 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 12 and various embodiments of it. An apparatus according to another aspect comprises means for receiving an imageblock unit of a frame, the image block unit comprising samples in color channels comprising at least one chrominance channel and one luminance channel; means for reconstructing samples of said color channels to produce reconstructed chrominance samples andreconstructed luminance samples; means for determining a list of cross-component modelsand initializing the list to have models in descending numerical order; means for determiningan index in the list of cross-component models; means for deriving a cross-component modelfrom the list with the given index; means for re-ordering the list of cross-component models;means for applying a first adaptive in-loop filter for the reconstructed luminance samples;and means for applying the derived cross-component model for the reconstructed luminancesamples to obtain an additional input for reconstructing the chrominance samples. Thesemeans 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 15 and various embodiments of it. An apparatus according to another aspect comprises means for receiving an image block unit of a frame, the image block unit comprising samples in color channels comprising at least one chrominance channel and one luminance channel; means for reconstructing samples of said color channels to produce reconstructed chrominance samples and reconstructed luminance samples; means for determining that a picture-level model list re- ordering is used; means for determining a reference picture; means for determining a list ofcross-component models for the reference picture; means for generating a re-ordered listwhere the cross-component models of the determined list have been re-ordered; means fordetermining an index for a cross-component model; means for deriving the cross-componentmodel from the re-ordered list with the given index; means for applying a first adaptive in-loop filter for the reconstructed luminance samples; means for applying the derived cross- component model for the reconstructed luminance samples to obtain an additional input forreconstructing the chrominance samples. These means comprise at least one processor and atleast 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 16 and various embodiments of it. Such apparatuses according to various aspects may comprise e.g. all or a subset of thefunctional units disclosed in any of the Figures 1, 2, 10, 11, 18 – 21 for implementing theembodiments. 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. Figure 18 shows a schematic block diagram of an exemplary apparatus or electronicdevice 50, which may incorporate a codec according to an embodiment of the invention. Figure 19 shows a layout of an apparatus according to an example embodiment. The electronic device 50 may for example be a mobile terminal or user equipment of a wireless communication system. However, it would be appreciated that embodiments of the invention may be implemented within any electronic device or apparatus which may require encoding and decoding or encoding or decoding video images. The apparatus 50 may comprise a housing 30 for incorporating and protecting the device. The apparatus 50 further may comprise a display 32 in the form of a liquid crystal display. In other embodiments of the invention the display may be any suitable display technology suitable to display an image or video. The apparatus 50 may further comprise a keypad 34. In other embodiments of the invention any suitable data or user interfacemechanism may be employed. For example, the user interface may be implemented as avirtual keyboard or data entry system as part of a touch-sensitive display. The apparatus may comprise a microphone 36 or any suitable audio input which may be a digital or analogue signal input. The apparatus 50 may further comprise an audio output device which in embodiments of the invention may be any one of: an earpiece 38, speaker, or an analogue audio or digital audio output connection. The apparatus 50 may also comprise a battery (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 furthercomprise any suitable short range communication solution such as for example a Bluetoothwireless connection or a USB / firewire wired connection. The apparatus 50 may comprise a controller 56, processor or processor circuitry forcontrolling the apparatus 50. The controller 56 may be connected to memory 58 which in embodiments of the invention may store both data in the form of image and audio data and / or may also store instructions for implementation on the controller 56. The controller 56 may further be connected to codec circuitry 54 suitable for carrying out coding and decoding of audio and / or video data or assisting in coding and decoding carried out by the controller. The apparatus 50 may further comprise a card reader 48 and a smart card 46, for example a UICC and UICC reader for providing user information and being suitable for providing authentication information for authentication and authorization of the user at a network. The apparatus 50 may comprise radio interface circuitry 52 connected to the controller and suitable for generating wireless communication signals for example for communication with a cellular communications network, a wireless communications system or a wireless local area network. The apparatus 50 may further comprise an antenna 44 connected to the radio interface circuitry 52 for transmitting radio frequency signals generated at the radio interface circuitry 52 to other apparatus(es) and for receiving radio frequency signals from other apparatus(es). The apparatus 50 may comprise a camera capable of recording or detecting individual frames which are then passed to the codec 54 or the controller for processing. The apparatus may receive the video image data for processing from another device prior to transmission and / or storage. The apparatus 50 may also receive either wirelessly or by a wired connectionthe image for coding / decoding. The structural elements of apparatus 50 described aboverepresent examples of means for performing a corresponding function. With respect to Figure 20, an example of a system within which embodiments of the present invention can be utilized is shown. The system 10 comprises multiple communication devices which can communicate through one or more networks. The system 10 may comprise any combination of wired or wireless networks including, but not limited to a wireless cellular telephone network (such as a GSM, UMTS, CDMA network etc.), a wireless local area network (WLAN) such as defined by any of the IEEE 802.x standards, a Bluetooth personal area network, an Ethernet local area network, a token ring local area network, a wide area network, and the Internet. The system 10 may include both wired and wireless communication devices and / or apparatus 50 suitable for implementing embodiments of the invention. For example, the system shown in Figure 20 shows a mobile telephone network 11 anda representation of the internet 28. Connectivity to the internet 28 may include, but is not limited to, long range wireless connections, short range wireless connections, and various wired connections including, but not limited to, telephone lines, cable lines, power lines, and similar communication pathways. The example communication devices shown in the system 10 may include, but are not limited to, an electronic device or apparatus 50, a combination of a personal digital assistant (PDA) and a mobile telephone 14, a PDA 16, an integrated messaging device (IMD) 18, a desktop computer 20, a notebook computer 22. The apparatus 50 may be stationary or mobile when carried by an individual who is moving. The apparatus 50 may also be located in amode of transport including, but not limited to, a car, a truck, a taxi, a bus, a train, a boat, anairplane, a bicycle, a motorcycle or any similar suitable mode of transport. The embodiments may also be implemented in a set-top box; i.e. a digital TV receiver, which may / may not have a display or wireless capabilities, in tablets or (laptop) personal computers (PC), which have hardware or software or combination of the encoder / decoder implementations, in various operating systems, and in chipsets, processors, DSPs and / or embedded systems offering hardware / software based coding. Some or further apparatus may send and receive calls and messages and communicate with service providers through a wireless connection 25 to a base station 24. The base station 24 may be connected to a network server 26 that allows communication between the mobile telephone network 11 and the internet 28. The system may include additional communication devices and communication devices of various types. The communication devices may communicate using various transmission technologies including, but not limited to, code division multiple access (CDMA), global systems for mobile communications (GSM), universal mobile telecommunications system (UMTS), time divisional multiple access (TDMA), frequency division multiple access (FDMA), transmission control protocol-internet protocol (TCP-IP), short messaging service (SMS), multimedia messaging service (MMS), email, instant messaging service (IMS), Bluetooth, IEEE 802.11 and any similar wireless communication technology. A communications device involved in implementing various embodiments of the present invention may communicate using various media including, but not limited to, radio, infrared, laser, cable connections, and any suitable connection. Figure 21 is a graphical representation of an example multimedia communicationsystem within which various embodiments may be implemented. A data source 2110 provides a source signal in an analog, uncompressed digital, or compressed digital format, or any combination of these formats. An encoder 2120 may include or be connected with a pre- processing, such as data format conversion and / or filtering of the source signal. The encoder 2120 encodes the source signal into a coded media bitstream. It should be noted that a bitstream to be decoded may be received directly or indirectly from a remote device located within virtually any type of network. Additionally, the bitstream may be received from local hardware or software. The encoder 2120 may be capable of encoding more than one media type, such as audio and video, or more than one encoder 2120 may be required to code different media types of the source signal. The encoder 2120 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 2120 is represented to simplify the description without a lack of generality. It should be further understood that, although text and examples contained herein may specifically describe an encoding process, one skilled in the art would understand that the same concepts and principles also apply to the corresponding decoding process and vice versa. The coded media bitstream may be transferred to a storage 2130. The storage 2130 may comprise any type of mass memory to store the coded media bitstream. The format of the coded media bitstream in the storage 2130 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 2120 or the storage 2130 may comprise the file generator, or the file generator is operationally attached to either the encoder 2120 or the storage 2130. Some systems operate “live”, i.e. omit storage and transfer coded media bitstream from the encoder 2120 directly to the sender 2140. The coded media bitstream may then be transferred to the sender 2140, 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 encoder21, the storage 2130, and the server 2140 may reside in the same physical device or they may be included in separate devices. The encoder 2120 and server 2140 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 2120 and / or in the server 2140 to smooth out variations in processing delay, transfer delay, and coded media bitrate. The server 2140 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 2140 encapsulates the coded media bitstream into packets. For example, when RTP is used, the server 2140 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 2140, but for the sake of simplicity, the following description only considers one server 2140. If the media content is encapsulated in a container file for the storage 2130 or for inputting the data to the sender 2140, the sender 2140 may comprise or be operationallyattached to a “sending file parser” (not shown in the figure). In particular, if the container fileis not transmitted as such but at least one of the contained coded media bitstream is encapsulated for transport over a communication protocol, a sending file parser locates appropriate parts of the coded media bitstream to be conveyed over the communication protocol. The sending file parser may also help in creating the correct format for the communication protocol, such as packet headers and payloads. The multimedia container file may contain encapsulation instructions, such as hint tracks in the ISOBMFF, for encapsulation of the at least one of the contained media bitstream on the communication protocol. The server 2140 may or may not be connected to a gateway 2150 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 notedthat the system may generally comprise any number gateways or alike, but for the sake of simplicity, the following description only considers one gateway 2150. The gateway 2150 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 andforking of data streams, and manipulation of data stream according to the downlink and / orreceiver capabilities, such as controlling the bit rate of the forwarded stream according to prevailing downlink network conditions. The gateway 2150 may be a server entity in various embodiments. The system includes one or more receivers 2160, typically capable of receiving, de- modulating, and de-capsulating the transmitted signal into a coded media bitstream. The coded media bitstream may be transferred to a recording storage 2170. The recording storage 2170 may comprise any type of mass memory to store the coded media bitstream. The recording storage 2170 may alternatively or additively comprise computation memory, such as random access 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 2160 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 2170 and transfer coded media bitstream from the receiver 2160 directly to the decoder 2180. 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 2170, while any earlier recorded data is discarded from the recording storage 2170. The coded media bitstream may be transferred from the recording storage 2170 to the decoder 2180. 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 inthe figure) is used to decapsulate each coded media bitstream from the container file. Therecording storage 2170 or a decoder 2180 may comprise the file parser, or the file parser is attached to either recording storage 2170 or the decoder 2180. It should also be noted that the system may include many decoders, but here only one decoder 2180 is discussed to simplify the description without a lack of generality. The coded media bitstream may be processed further by a decoder 2180, whose output is one or more uncompressed media streams. Finally, a renderer 2190 may reproduce the uncompressed media streams with a loudspeaker or a display, for example. The receiver 2160, recording storage 2170, decoder 2180, and renderer 2190 may reside in the same physical device or they may be included in separate devices. A sender 2140 and / or a gateway 150 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 2140 and / or a gateway 2150 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 2160 or prevailing conditions, such as throughput, of the network over which the bitstream is conveyed. In other words, the receiver 2160 may initiate switching between representations. A request from the receiver can be, e.g., a request for a Segment or a Subsegment from a different representation than earlier, a request for a change of transmitted scalability layers and / or sub-layers, or a change of a rendering device having different capabilities compared to the previous one. A request for a Segment may be an HTTP GET request. A request for a Subsegment may be an HTTP GET request with a byte range. Additionally or alternatively, bitrate adjustment or bitrate adaptation may be used for example for providing so-called fast start-up in streaming services, where the bitrate of the transmitted stream is lower than the channel bitrate after starting or random-accessing the streaming in order to start playback immediately and to achieve a buffer occupancy level that tolerates occasional packet delays and / or retransmissions. Bitrate adaptation may include multiple representation or layer up-switching and representation or layer down-switching operations taking place in various orders. A decoder 2180 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 2180 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. In the above, some embodiments have been described with reference to and / or usingterminology of HEVC and / or VVC. It needs to be understood that embodiments may besimilarly realized with any video encoder and / or video decoder. In the above, where the example embodiments have been described with reference to an encoder, it needs to be understood that the resulting bitstream and the decoder may have corresponding elements in them. Likewise, where the example embodiments have been described with reference to a decoder, it needs to be understood that the encoder may have structure and / or computer program for generating the bitstream to be decoded by the decoder. The embodiments of the invention described above describe the codec in terms of separate encoder and decoder apparatus in order to assist the understanding of the processes involved. However, it would be appreciated that the apparatus, structures and operations may be implemented as a single encoder-decoder apparatus / structure / operation. Furthermore, it is possible that the coder and decoder may share some or all common elements. Although the above examples describe embodiments of the invention operating within a codec within an electronic device, it would be appreciated that the invention as defined in the claims may be implemented as part of any video codec. Thus, for example, embodiments of the invention may be implemented in a video codec which may implement video coding over fixed or wired communication paths. Thus, user equipment may comprise a video codec such as those described in embodiments of the invention above. It shall be appreciated that the term user equipment is intended to cover any suitable type of wireless user equipment, such as mobile telephones, portable data processing devices or portable web browsers. Furthermore, elements of a public land mobile network (PLMN) may also comprisevideo codecs as described above. In general, the various embodiments of the invention may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the invention may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof. The embodiments of this invention may be implemented by computer software executable by a data processor of the mobile device, such as in the processor entity, or by hardware, or by a combination of software and hardware. Further in this regard it should be noted that any blocks of the logic flow as in the Figures may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD. The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor-based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multi-core processor architecture, as non-limiting examples. Embodiments of the inventions may be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate. Programs, such as those provided by Synopsys, Inc. of Mountain View, California and Cadence Design, of San Jose, California automatically route conductors and locate components on a semiconductor chip using well established rules of design as well as libraries of pre-stored design modules. Once the design for a semiconductor circuit has been completed, the resultant design, in a standardized electronic format (e.g., Opus, GDSII, or thelike) may be transmitted to a semiconductor fabrication facility or “fab” for fabrication.The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention.

Claims

CLAIMS:

1. An apparatus comprising^ means for receiving an image block unit of a frame, the image block unitcomprising samples in color channels comprising at least one chrominance channeland one luminance channel; ^means for reconstructing samples of said color channels to produce reconstructedchrominance samples and reconstructed luminance samples; ^means for determining that a picture-level model list re-ordering is used;^ means for determining a reference picture;^ means for determining a list of cross-component models for the reference picture;^ means for generating a re-ordered list where the cross-component models of thedetermined list have been re-ordered; ^means for determining an index for a cross-component model;^ means for deriving the cross-component model from the re-ordered list with thegiven index; ^means for applying a first adaptive in-loop filter for the reconstructed luminancesamples; and^ means for applying the derived cross-component model for the reconstructedluminance samples to obtain an additional input for reconstructing the chrominance samples.

2. The apparatus according to claim 1, further comprising means for computing a histogram of the cross-component models used by all theimage block units in the reference picture; means for generating the re-ordered list by placing the most frequently occurring cross-component models at top of the list.

3. The apparatus according to claim 2, further comprising means for appending non-occurringmodels at end of the list using their numerical descending order.

4. The apparatus according to any of the claims 1 to 3, further comprising means for decoding at least one syntax element that determines whether the indices forcross-component models are determined at picture-level or at image block unit-level, whereupon if the indices are determined at picture-level, the apparatus comprises means fordecoding at least one syntax element indicating the index of the cross-componentmodels for all image block units in the picture; orif the indices are not determined at picture-level, for each image block unit that hasadaptive in-loop filter chroma update enabled, means for decoding at least one syntaxelement indicating the index of the cross-component model for the given image blockunit.

5. The apparatus according to any of the claims 1 to 4, further comprising means for applying a cross-component adaptive in-loop filter for outputs of the first and second adaptive in-loop filters; andmeans for providing an output of the cross-component adaptive in-loop filter to the output of the second adaptive in-loop filter as an additional correction for the chrominance samples.

6. The apparatus according to any preceding claim, further comprising means for decoding thelist of cross-component models from a bitstream.

7. The apparatus according to any preceding claim, further comprising means for applying the re-ordered model list to certain image block units in the current picture.

8. The apparatus according to claims 1 to 6, further comprising means for applying the re-ordered model list to first image block unit of the current picture.

9. A method comprising: ^receiving an image block unit of a frame, the image block unit comprisingsamples in color channels comprising at least one chrominance channel and oneluminance channel; ^reconstructing samples of said color channels to produce reconstructedchrominance samples and reconstructed luminance samples; ^determining that a picture-level model list re-ordering is used;^ determining a reference picture;^ determining a list of cross-component models for the reference picture;^ generating a re-ordered list where the cross-component models of the determinedlist have been re-ordered; ^determining an index for a cross-component model;^ deriving the cross-component model from the re-ordered list with the given index;^ applying a first adaptive in-loop filter for the reconstructed luminance samples;^ applying the derived cross-component model for the reconstructed luminancesamples to obtain an additional input for reconstructing the chrominance samples.

10. The method according to claim 9, further comprising computing a histogram of the cross-component models used by all the image block units in the reference picture; and generating the re-ordered list by placing the most frequently occurring cross-component models at top of the list.

11. The method according to claim 10, further comprising appending non-occurring models atend of the list using their numerical descending order.

12. The method according to any preceding claims 9 to 11, further comprisingdecoding at least one syntax element that determines whether the indices for cross-component models are determined at picture-level or at image block unit-level, whereuponif the indices are determined at picture-level, decoding at least one syntax elementindicating the index of the cross-component models for all image block units in thepicture; or if the indices are not determined at picture-level, for each image block unit that hasadaptive in-loop filter chroma update enabled, decoding at least one syntax elementindicating the index of the cross-component model for the given image block unit.

13. The method according to claim 9 or 12, further comprisingapplying a cross-component adaptive in-loop filter for outputs of the first and second adaptive in-loop filters; and providing an output of the cross-component adaptive in-loop filter to the output of the second adaptive in-loop filter as an additional correction for the chrominance samples.

14. The apparatus according to any preceding claim 9 to13, further comprising applying the re-ordered model list to certain image block units in the current picture.

15. The apparatus according to claims 9 to 13, further comprising applying the re-ordered modellist to first image block unit of the current picture.

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