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

Abstract

A method comprising: determining a context class list; determining a first superblock of a picture; determining a first context class from the context class list: determining a first bin value belonging to the first context class in the first superblock; calculating a value for a first probability estimate using the first bin value; encoding a syntax element using the first probability estimate; determining a second superblock of the picture; determining a second context class from said context class list; determining a second bin value belonging to the second context class in the second superblock; calculating a value for a second probability estimate using the second bin value; and encoding a syntax element using the second probability estimate

Description

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

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

[0002] Arithmetic coders, such as context adaptive binary arithmetic coder (CAB AC), are used in video coding for the purpose of compressing and decompressing different syntax elements efficiently. In the case of context adaptive arithmetic coding, each syntax element has an estimate of probabilities for its possible values. These probability estimates are referred to as contexts as they provide the context or model that describes the behavior of the syntax element or elements associated with the context. A context class list identifies the context classes that have been selected for spatial tuning of the arithmetic coder’s states. In CAB AC, separate lists of coding context classes are used for each superblock, such as a Coding Tree Unit (CTU).

[0003] Encoding and decoding of image and video content is typically done in a nested tree order within a Coding Tree Unit (CTU) starting from the top-left corner of the CTU and ending to the bottom-right corner of the CTU. When progressing to a new CTU, to be able to adjust the probabilities of the arithmetic coding engine towards their optimal values in the local neighborhood of syntax elements to be coded or decoded, some memory is needed to store previously calculated syntax element values in different neighborhoods in the picture together with the list of coding context classes of the corresponding superblock.

[0004] It can be easily concluded that the amount of such data can be relatively large, and it needs to be buffered for the time required for encoding / decoding of a plurality of CTUs. Storing such amount of data efficiently so as to enable to minimize the amount of memory required for the storage poses various challenges.SUMMARY

[0005] Now in order to at least alleviate the above problems, enhanced methods are introduced herein.

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

[0007] A method according to a first aspect comprises determining a context class list; determining a first superblock of a picture; determining a first context class from the context class list: determining a first bin value belonging to the first context class in the first superblock; calculating a value for a first probability estimate using the first bin value; encoding a syntax element using the first probability estimate; determining a second superblock of the picture; determining a second context class from said context class list; determining a second bin value belonging to the second context class in the second superblock; calculating a value for a second probability estimate using the second bin value; and encoding a syntax element using the second probability estimate.

[0008] A method according to a second aspect comprises determining a context class list; determining a first superblock of a picture; determining a first context class from the context class list: determining a first bin value belonging to the first context class in the first superblock; calculating a value for a first probability estimate using the first bin value; decoding a syntax element using the first probability estimate; determining a second superblock of the picture; determining a second context class from said context class list; determining a second bin value belonging to the second context class in the second superblock; calculating a value for a second probability estimate using the second bin value; and decoding a syntax element using the second probability estimate.

[0009] An apparatus according to a third aspect comprises means for determining a context class list; means for determining a first superblock of a picture; means for determining a first context class from the context class list: means for determining a first bin value belonging to the first context class in the first superblock; means for calculating a value for a first probability estimate using the first bin value; means for encoding a syntax element using the first probability estimate; means for determining a second superblock of the picture; means for determining a second context class from said context class list; means for determining a second bin value belonging to the second context class in the second superblock; means for calculating a value fora second probability estimate using the second bin value; and means for encoding a syntax element using the second probability estimate.

[0010] An apparatus according to a fourth aspect comprises means for determining a context class list; means for determining a first superblock of a picture; means for determining a first context class from the context class list: means for determining a first bin value belonging to the first context class in the first superblock; means for calculating a value for a first probability estimate using the first bin value; means for decoding a syntax element using the first probability estimate; means for determining a second superblock of the picture; means for determining a second context class from said context class list; means for determining a second bin value belonging to the second context class in the second superblock; means for calculating a value for a second probability estimate using the second bin value; and means for decoding a syntax element using the second probability estimate.

[0011] According to an embodiment, the apparatus comprises means for updating the context class list in the beginning of the picture, in the beginning of a slice or in the beginning of a superblock row.

[0012] According to an embodiment, the context class list has a determined maximum size.

[0013] According to an embodiment, the apparatus comprises means for constructing the context class list by including context classes from at least two superblocks.

[0014] According to an embodiment, the apparatus comprises means for constructing the context class list by including at most a determined maximum number of context classes from a single superblock.

[0015] According to an embodiment, the apparatus comprises means for constructing the context class list having a determined number of context classes calculated using an index of a coding unit.

[0016] According to an embodiment, the apparatus comprises means for storing the bin values associated with the context classes included in the context class list in superblock-specific buffers.

[0017] According to an embodiment, the apparatus comprises means for updating at least the first probability estimate by determining the first context class using the context class list and the stored bin value for the first superblock; and means for updating at least the second probabilityestimate by determining the second context class using the said context class list and the stored bin value for the second superblock.

[0018] According to an embodiment, the apparatus comprises means for updating at least one probability estimate relating to the first context class in the first superblock based on a stored bin value of the second superblock; and means for updating at least one probability estimate relating to the second context class in a third superblock based on a stored bin value of a fourth superblock, wherein the first and second context classes are determined using the same context class list.

[0019] According to an embodiment, the apparatus comprises means for sharing two context class lists between multiple superblocks, wherein a first context class list is used to store context classes for which bin values are stored for updating probability estimates for a next superblock row and a second context class list is used to identify what context classes are stored on a previous superblock row and used in updating probability estimates for the current superblock row.

[0020] According to an embodiment, the apparatus comprises means for resetting the first context class list before processing the first superblock in a superblock row; and means for skipping the resetting for the second context class list.

[0021] According to an embodiment, the superblock is a coding tree unit (CTU), a largest coding unit (LCU) or a macroblock.

[0022] As a fifth aspect, there is provided an apparatus comprising: at least one processor and at least one memory, said at least one memory stored with code thereon, which when executed by said at least one processor, causes the apparatus to perform at least: determine a context class list; determine a first superblock of a picture; determine a first context class from the context class list: determine a first bin value belonging to the first context class in the first superblock; calculate a value for a first probability estimate using the first bin value; encode a syntax element using the first probability estimate; determine a second superblock of the picture; determine a second context class from said context class list; determine a second bin value belonging to the second context class in the second superblock; calculate a value for a second probability estimate using the second bin value; and encode a syntax element using the second probability estimate.

[0023] As a sixth aspect, there is provided an apparatus comprising: at least one processor and at least one memory, said at least one memory stored with code thereon, which when executedby said at least one processor, causes the apparatus to perform at least: determine a context class list; determine a first superblock of a picture; determine a first context class from the context class list: determine a first bin value belonging to the first context class in the first superblock; calculate a value for a first probability estimate using the first bin value; decode a syntax element using the first probability estimate; determine a second superblock of the picture; determine a second context class from said context class list; determine a second bin value belonging to the second context class in the second superblock; calculate a value for a second probability estimate using the second bin value; and decode a syntax element using the second probability estimate.

[0024] The apparatuses and the computer readable storage mediums stored with code thereon, as described above, are thus arranged to carry out the above methods and one or more of the embodiments related thereto.BRIEF DESCRIPTION OF THE DRAWINGS

[0025] For better understanding of the present invention, reference will now be made by way of example to the accompanying drawings in which:

[0026] Figures la and lb show schematically an encoder and a decoder, respectively, suitable for implementing embodiments of the invention;

[0027] Figure 2 shows a typical segmentation of a picture to coding tree units (CTUs) or superblocks;

[0028] Figure 3 shows an example of a nested tree encoding / decoding order of coding units (CU) within a Coding Tree Unit (CTU);

[0029] Figure 4 shows a flow chart of an encoding method according to an embodiment;

[0030] Figure 5 shows a flow chart of a decoding method according to an embodiment;

[0031] Figure 6 shows an example memory array layout according to an embodiment;

[0032] Figure 7 shows schematically an electronic device suitable for employing embodiments of the invention;

[0033] Figure 8 shows schematically a user equipment suitable for employing embodiments of the invention;

[0034] Figure 9 further shows schematically electronic devices suitable for employing embodiments of the invention connected using wireless and wired network connections;

[0035] Figure 10 shows a schematic diagram of an example multimedia communication system within which various embodiments may be implemented.DETAILED DESCRIPTON OF SOME EXAMPLE EMBODIMENTS

[0036] Video codec consists of an encoder that transforms the input video into a compressed representation suited for storage / transmission and a decoder that can uncompress the compressed video representation back into a viewable form. A video encoder and / or a video decoder may also be separate from each other, i.e. need not form a codec. Typically, an encoder discards some information in the original video sequence in order to represent the video in a more compact form (that is, at lower bitrate).

[0037] Typical hybrid video encoders, for example many encoder implementations of ITU-T H.263 and H.264, encode the video information in two phases. Firstly, pixel values in a certain picture area (or “block”) are predicted for example by motion compensation means (finding and indicating an area in one of the previously coded video frames that corresponds closely to the block being coded) or by spatial means (using the pixel values around the block to be coded in a specified manner). Secondly the prediction error, i.e. the difference between the predicted block of pixels and the original block of pixels, is coded. This is typically done by transforming the difference in pixel values using a specified transform (e.g. Discrete Cosine Transform (DCT) or a variant of it), quantizing the coefficients and entropy coding the quantized coefficients. By varying the fidelity of the quantization process, encoder can control the balance between the accuracy of the pixel representation (picture quality) and size of the resulting coded video representation (file size or transmission bitrate).

[0038] In temporal prediction, the sources of prediction are previously decoded pictures (a.k.a. reference pictures). In intra block copy (IBC; a.k.a. intra-block-copy prediction), prediction is applied similarly to temporal prediction but the reference picture is the current picture and only previously decoded samples can be referred in the prediction process. Interlayer or inter-view prediction may be applied similarly to temporal prediction, but the reference picture is a decoded picture from another scalable layer or from another view, respectively. In some cases, inter prediction may refer to temporal prediction only, while in other cases inter prediction may refer collectively to temporal prediction and any of intra block copy, inter-layer prediction, and inter-view prediction provided that they are performed with the same or similarprocess than temporal prediction. Inter prediction or temporal prediction may sometimes be referred to as motion compensation or motion-compensated prediction.

[0039] 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 be also be done before horizontal processing in some environments.

[0040] 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.

[0041] 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.

[0042] Figs, la and lb show an encoder and a decoder suitable for employing embodiments of the invention. A video codec consists of an encoder that transforms an input video into a compressed representation suited for storage / transmission and a decoder that can decompress thecompressed video representation back into a viewable form. Typically, the encoder discards and / or loses some information in the original video sequence in order to represent the video in a more compact form (that is, at lower bitrate). An example of an encoding process is illustrated in Figure la. Figure 4a illustrates an image to be encoded (In); a predicted representation of an image block (P'n); a prediction error signal (Dn); a reconstructed prediction error signal (D'n); a preliminary reconstructed image (I'n); a final reconstructed image (R'n); a transform (T) and inverse transform (T4); a quantization (Q) and inverse quantization (Q ); entropy encoding (E); a reference frame memory (RFM); inter prediction (Pinter); intra prediction (Pintra); mode selection (MS) and filtering (F).

[0043] An example of a decoding process is illustrated in Figure lb. Figure lb illustrates a predicted representation of an image block (P'n); a reconstructed prediction error signal (D'n); a preliminary reconstructed image (I'n); a final reconstructed image (R'n); an inverse transform (T‘1); an inverse quantization (Q ); an entropy decoding (E ); a reference frame memory (RFM); a prediction (either inter or intra) (P); and filtering (F).

[0044] Many hybrid video encoders encode the video information in two phases. Firstly pixel values in a certain picture area (or “block”) are predicted for example by motion compensation means (finding and indicating an area in one of the previously coded video frames that corresponds closely to the block being coded) or by spatial means (using the pixel values around the block to be coded in a specified manner). Secondly the prediction error, i.e. the difference between the predicted block of pixels and the original block of pixels, is coded. This is typically done by transforming the difference in pixel values using a specified transform (e.g. Discrete Cosine Transform (DCT) or a variant of it), quantizing the coefficients and entropy coding the quantized coefficients. By varying the fidelity of the quantization process, encoder can control the balance between the accuracy of the pixel representation (picture quality) and size of the resulting coded video representation (file size or transmission bitrate). Video codecs may also provide a transform skip mode, which the encoders may choose to use. In the transform skip mode, the prediction error is coded in a sample domain, for example by deriving a sample-wise difference value relative to certain adjacent samples and coding the sample- wise difference value with an entropy coder.

[0045] Entropy coding / decoding may be performed in many ways. For example, contextbased coding / decoding may be applied, where in both the encoder and the decoder modify thecontext 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.

[0046] The phrase along the bitstream (e.g. indicating along the bitstream) may be defined to refer to out-of-band transmission, signalling, or storage in a manner that the out-of-band data is associated with the bitstream. The phrase decoding along the bitstream or alike may refer to decoding the referred out-of-band data (which may be obtained from out-of-band transmission, signalling, or storage) that is associated with the bitstream. For example, an indication along the bitstream may refer to metadata in a container file that encapsulates the bitstream.

[0047] 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).

[0048] Version 1 of the High Efficiency Video Coding (H.265 / HEVC a.k.a. HEVC) standard was developed by the Joint Collaborative Team - Video Coding (JCT-VC) of VCEG and MPEG. The standard was published by both parent standardization organizations, and it is referred to as ITU-T Recommendation H.265 and ISO / IEC International Standard 23008-2, also known as MPEG-H Part 2 High Efficiency Video Coding (HEVC). Later versions of H.265 / HEVC included scalable, multiview, fidelity range, three-dimensional, and screen content coding extensions which may be abbreviated SHVC, MV-HEVC, REXT, 3D-HEVC, and SCC, respectively.

[0049] Versatile Video Coding (VVC) (MPEG-I Part 3), a.k.a. ITU-T H.266, is a video compression standard developed by the Joint Video Experts Team (JVET) of the Moving Picture Experts Group (MPEG), (formally ISO / LEC JTC1 SC29 WG11) and Video Coding Experts Group (VCEG) of the International Telecommunication Union (ITU) to be the successor to HEVC / H.265.

[0050] 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 coding structures, and concepts of H.264 / AVC are the same as in HEVC - hence, they are described below jointly.

[0051] 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.

[0052] 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.

[0053] The source and decoded pictures are each comprised of one or more sample arrays, such as one of the following sets of sample arrays:Luma (Y) only (monochrome).Luma and two chroma (YCbCr or YCgCo).Green, Blue and Red (GBR, also known as RGB).Arrays representing other unspecified monochrome or tri-stimulus color samplings (for example, YZX, also known as XYZ).

[0054] Typically, video is encoded in YUV or YCbCr color space as that is found to reflect some characteristics of human visual system and allows using lower quality representation forCb and Cr channels as human perception is less sensitive to the chrominance fidelity those channels represent.

[0055] In H.264 / AVC and HEVC, a picture may either be a frame or a field. A frame comprises a matrix of luma samples and possibly the corresponding chroma samples. A field is a set of alternate sample rows of a frame and may be used as encoder input, when the source signal is interlaced. Chroma sample arrays may be absent (and hence monochrome sampling may be in use) or chroma sample arrays may be subsampled when compared to luma sample arrays.Chroma formats may be summarized as follows:In monochrome sampling there is only one sample array, which may be nominally considered the luma array.In 4:2:0 sampling, each of the two chroma arrays has half the height and half the width of the luma array.In 4:2:2 sampling, each of the two chroma arrays has the same height and half the width of the luma array.In 4:4:4 sampling when no separate color planes are in use, each of the two chroma arrays has the same height and width as the luma array.

[0056] 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.

[0057] 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.

[0058] 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 ofluma samples, two corresponding coding blocks of chroma samples of a picture that has three sample arrays, or a coding block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax structures used to code the samples. A CU with the maximum allowed size may be named as LCU (largest coding unit) or coding tree unit (CTU) and the video picture is divided into non-overlapping LCUs.

[0059] A CU consists of one or more prediction units (PU) defining the prediction process for the samples within the CU and one or more transform units (TU) defining the prediction error coding process for the samples in the said CU. Typically, a CU consists of a square block of samples with a size selectable from a predefined set of possible CU sizes. Each PU and TU can be further split into smaller PUs and TUs in order to increase granularity of the prediction and prediction error coding processes, respectively. Each PU has prediction information associated with it defining what kind of a prediction is to be applied for the pixels within that PU (e.g. motion vector information for inter predicted PUs and intra prediction directionality information for intra predicted PUs).

[0060] Each TU can be associated with information describing the prediction error decoding process for the samples within the said TU (including e.g. DCT coefficient information). It is typically signalled at CU level whether prediction error coding is applied or not for each CU. In the case there is no prediction error residual associated with the CU, it can be considered there are no TUs for the said CU. The division of the image into CUs, and division of CUs into PUs and TUs is typically signalled in the bitstream allowing the decoder to reproduce the intended structure of these units.

[0061] To be able to utilize parallel processing, images can be split into independently codable and decodable image segments (slices or tiles). In HEVC, a picture can be partitioned in tiles, which are rectangular and contain an integer number of LCUs. In HEVC, the partitioning to tiles forms a regular grid, where heights and widths of tiles differ from each other by one LCU at the maximum. In HEVC, a slice is defined to be an integer number of coding tree units contained in one independent slice segment and all subsequent dependent slice segments (if any) that precede the next independent slice segment (if any) within the same access unit. In HEVC, a slice segment is defined to be an integer number of coding tree units ordered consecutively in the tile scan and contained in a single NAL unit. The division of each picture into slice segments is a partitioning. In HEVC, an independent slice segment is defined to be a slice segment for whichthe values of the syntax elements of the slice segment header are not inferred from the values for a preceding slice segment, and a dependent slice segment is defined to be a slice segment for which the values of some syntax elements of the slice segment header are inferred from the values for the preceding independent slice segment in decoding order. In HEVC, a slice header is defined to be the slice segment header of the independent slice segment that is a current slice segment or is the independent slice segment that precedes a current dependent slice segment, and a slice segment header is defined to be a part of a coded slice segment containing the data elements pertaining to the first or all coding tree units represented in the slice segment. The CUs are scanned in the raster scan order of LCUs within tiles or within a picture, if tiles are not in use. Within an LCU, the CUs have a specific scan order.

[0062] 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.

[0063] Instead, or in addition to approaches utilizing sample value prediction and transform coding for indicating the coded sample values, a color palette based coding can be used. Palette based coding refers to a family of approaches for which a palette, i.e. a set of colors and associated indexes, is defined and the value for each sample within a coding unit is expressed by indicating its index in the palette. Palette based coding can typically achieve good coding efficiency in coding units with a relatively small number of colors (such as image areas which are representing computer screen content, like text or simple graphics). In order to improve the coding efficiency of palette coding different kinds of palette index prediction approaches can be utilized, or the palette indexes can be run-length coded to be able to represent larger homogenous image areas efficiently. Also, in the case the CU contains sample values that are not recurring within the CU, escape coding can be utilized. Escape coded samples are transmitted withoutreferring to any of the palette indexes. Instead, their values are indicated individually for each escape coded sample.

[0064] The filtering may for example include one more of the following: deblocking, sample adaptive offset (SAO), and / or adaptive loop filtering (ALF). H.264 / AVC includes a deblocking, whereas HEVC includes both deblocking and SAO.

[0065] In typical video codecs the motion information is indicated with motion vectors associated with each motion compensated image block, such as a prediction unit. Each of these motion vectors represents the displacement of the image block in the picture to be coded (in the encoder side) or decoded (in the decoder side) and the prediction source block in one of the previously coded or decoded pictures. In order to represent motion vectors efficiently those are typically coded differentially with respect to block specific predicted motion vectors. In typical video codecs the predicted motion vectors are created in a predefined way, for example calculating the median of the encoded or decoded motion vectors of the adjacent blocks. Another way to create motion vector predictions is to generate a list of candidate predictions from adjacent blocks and / or co-located blocks in temporal reference pictures and signalling the chosen candidate as the motion vector predictor. In addition to predicting the motion vector values, it can be predicted which reference picture(s) are used for motion-compensated prediction and this prediction information may be represented for example by a reference index of previously coded / decoded picture. The reference index is typically predicted from adjacent blocks and / or co-located blocks in temporal reference picture. Moreover, typical high efficiency video codecs employ an additional motion information coding / decoding mechanism, often called merging / merge mode, where all the motion field information, which includes motion vector and corresponding reference picture index for each available reference picture list, is predicted and used without any modification / correction. Similarly, predicting the motion field information is carried out using the motion field information of adjacent blocks and / or co-located blocks in temporal reference pictures and the used motion field information is signalled among a list of motion field candidate list filled with motion field information of available adjacent / co-located blocks.

[0066] 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 oftenthere still exists some correlation among the residual and transform can in many cases help reduce this correlation and provide more efficient coding.

[0067] Video coding standards and specifications may allow encoders to divide a coded picture to coded slices or alike. In-picture prediction is typically disabled across slice boundaries. Thus, slices can be regarded as a way to split a coded picture to independently decodable pieces. In H.264 / AVC and HEVC, in-picture prediction may be disabled across slice boundaries. Thus, slices can be regarded as a way to split a coded picture into independently decodable pieces, and slices are therefore often regarded as elementary units for transmission. In many cases, encoders may indicate in the bitstream which types of in-picture prediction are turned off across slice boundaries, and the decoder operation takes this information into account for example when concluding which prediction sources are available. For example, samples from a neighboring CU may be regarded as unavailable for intra prediction, if the neighboring CU resides in a different slice.

[0068] An elementary unit for the output of an H.264 / AVC or HEVC encoder and the input of an H.264 / AVC or HEVC decoder, respectively, is a Network Abstraction Layer (NAL) unit. For transport over packet-oriented networks or storage into structured files, NAL units may be encapsulated into packets or similar structures. A bytestream format has been specified in H.264 / AVC and HEVC for transmission or storage environments that do not provide framing structures. The bytestream format separates NAL units from each other by attaching a start code in front of each NAL unit. To avoid false detection of NAL unit boundaries, encoders run a byte- oriented start code emulation prevention algorithm, which adds an emulation prevention byte to the NAL unit payload if a start code would have occurred otherwise. In order to enable straightforward gateway operation between packet- and stream-oriented systems, start code emulation prevention may always be performed regardless of whether the bytestream format is in use or not. A NAL unit may be defined as a syntax structure containing an indication of the type of data to follow and bytes containing that data in the form of an RBSP interspersed as necessary with emulation prevention bytes. A raw byte sequence payload (RBSP) may be defined as a syntax structure containing an integer number of bytes that is encapsulated in a NAL unit. An RBSP is either empty or has the form of a string of data bits containing syntax elements followed by an RBSP stop bit and followed by zero or more subsequent bits equal to 0.

[0069] NAL units consist of a header and pay load. In H.264 / AVC and HEVC, the NAL unit header indicates the type of the NAL unit

[0070] In HEVC, a two-byte NAL unit header is used for all specified NAL unit types. The NAL unit header contains one reserved bit, a six-bit NAL unit type indication, a three-bit nuh_temporal_id_plusl indication for temporal level (may be required to be greater than or equal to 1) and a six-bit nuh layer id syntax element. The temporal_id_plusl syntax element may be regarded as a temporal identifier for the NAL unit, and a zero-based Temporalld variable may be derived as follows: Temporalld = temporal ! d_plusl - 1. The abbreviation TID may be used to interchangeably with the Temporalld variable. Temporalld equal to 0 corresponds to the lowest temporal level. The value of temporal_id_plusl is required to be non-zero in order to avoid start code emulation involving the two NAL unit header bytes. The bitstream created by excluding all VCL NAL units having a Temporalld greater than or equal to a selected value and including all other VCL NAL units remains conforming. Consequently, a picture having Temporalld equal to tid value does not use any picture having a Temporalld greater than tid value as inter prediction reference. A sub-layer or a temporal sub-layer may be defined to be a temporal scalable layer (or a temporal layer, TL) of a temporal scalable bitstream, consisting of VCL NAL units with a particular value of the Temporalld variable and the associated non-VCL NAL units. nuh layer id can be understood as a scalability layer identifier.

[0071] NAL units can be categorized into Video Coding Layer (VCL) NAL units and non- VCL NAL units. VCL NAL units are typically coded slice NAL units. In HEVC, VCL NAL units contain syntax elements representing one or more CU.

[0072] A non-VCL NAL unit may be for example one of the following types: a sequence parameter set, a picture parameter set, a supplemental enhancement information (SEI) NAL unit, an access unit delimiter, an end of sequence NAL unit, an end of bitstream NAL unit, or a filler data NAL unit. Parameter sets may be needed for the reconstruction of decoded pictures, whereas many of the other non-VCL NAL units are not necessary for the reconstruction of decoded sample values.

[0073] Parameters that remain unchanged through a coded video sequence may be included in a sequence parameter set. In addition to the parameters that may be needed by the decoding process, the sequence parameter set may optionally contain video usability information (VUI), which includes parameters that may be important for buffering, picture output timing, rendering,and resource reservation. In HEVC a sequence parameter set RBSP includes parameters that can be referred to by one or more picture parameter set RBSPs or one or more SEI NAL units containing a buffering period SEI message. A picture parameter set contains such parameters that are likely to be unchanged in several coded pictures. A picture parameter set RBSP may include parameters that can be referred to by the coded slice NAL units of one or more coded pictures.

[0074] 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.

[0075] A video parameter set RBSP may include parameters that can be referred to by one or more sequence parameter set RBSPs.

[0076] 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.

[0077] 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.

[0078] 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 amanner that the out-of-band data is associated with the bitstream or the coded unit, respectively. The phrase decoding along the bitstream or along a coded unit of a bitstream or alike may refer to decoding the referred out-of-band data (which may be obtained from out-of-band transmission, signaling, or storage) that is associated with the bitstream or the coded unit, respectively.

[0079] 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.

[0080] A coded picture is a coded representation of a picture.

[0081] 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.

[0082] 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.

[0083] 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.

[0084] 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 NoRaslOutputFlagequal to 1, followed by zero or more access units that are not IRAP access units with NoRaslOutputFlag equal to 1, including all subsequent access units up to but not including any subsequent access unit that is an IRAP access unit with NoRaslOutputFlag equal to 1. An IRAP access unit may be defined as an access unit in which the base layer picture is an IRAP picture. The value of NoRaslOutputFlag is equal to 1 for each IDR picture, each BLA picture, and each IRAP picture that is the first picture in that particular layer in the bitstream in decoding order, is the first IRAP picture that follows an end of sequence NAL unit having the same value of nuh layer id in decoding order. There may be means to provide the value of HandleCraAsBlaFlag to the decoder from an external entity, such as a player or a receiver, which may control the decoder. HandleCraAsBlaFlag may be set to 1 for example by a player that seeks to a new position in a bitstream or tunes into a broadcast and starts decoding and then starts decoding from a CRA picture. When HandleCraAsBlaFlag is equal to 1 for a CRA picture, the CRA picture is handled and decoded as if it were a BLA picture.

[0085] 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.

[0086] 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.

[0087] 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 forreordering 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.

[0088] 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.

[0089] Many coding standards, including H.264 / AVC and HEVC, may have decoding process to derive a reference picture index to a reference picture list, which may be used to indicate which one of the multiple reference pictures is used for inter prediction for a particular block. A reference picture index may be coded by an encoder into the bitstream is some inter coding modes or it may be derived (by an encoder and a decoder) for example using neighboring blocks in some other inter coding modes.

[0090] 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.

[0091] 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, interlayer prediction, inter- view prediction, view synthesis prediction (VSP), and intercomponent prediction (which may be indicated per reference picture and / or perprediction type and where in some embodiments inter-view and view-synthesis prediction may be jointly considered as one prediction direction) and / or an indication of a reference picture type, such as a short-term reference picture and / or a long-term reference picture and / or an inter-layer reference picture (which may be indicated e.g. per reference picture) a reference index to a reference picture list and / or any other identifier of a reference picture (which may be indicated e.g. per reference picture and the type of which may depend on the prediction direction and / or the reference picture type and which may be accompanied 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 block or per reference index or alike); a vertical motion vector component (which may be indicated e.g. per prediction block or per reference index or alike); one or more parameters, such as picture order count difference and / or a relative camera separation between the picture containing or associated with the motion parameters and its reference picture, which may be used for scaling of the horizontal motion vector component and / or the vertical motion vector component in one or more motion vector prediction processes (where said one or more parameters may be indicated e.g. per each reference picture or each reference index or alike); coordinates of a block to which the motion parameters and / or motion information applies, e.g. coordinates of the top-left sample of the block in luma sample units; extents (e.g. a width and a height) of a block to which the motion parameters and / or motion information applies.

[0092] In comparison to the previous video coding standards, Versatile Video Codec (H.266 / WC) introduces a plurality of new coding tools, such as the following:• Intra prediction- 67 intra mode with wide angles mode extension- Block size and mode dependent 4 tap interpolation filter- Position dependent intra prediction combination (PDPC)- Cross component linear model intra prediction (CCLM)- Multi-reference line intra prediction- Intra sub-partitions- Weighted intra prediction with matrix multiplication• Inter-picture prediction- Block motion copy with spatial, temporal, history-based, and pairwise average merging candidates- Affine motion inter prediction- sub-block based temporal motion vector prediction- Adaptive motion vector resolution- 8x8 block-based motion compression for temporal motion prediction- High precision (1 / 16 pel) motion vector storage and motion compensation with 8- tap interpolation filter for luma component and 4-tap interpolation filter for chroma component- Triangular partitions- Combined intra and inter prediction- Merge with MVD (MMVD)- Symmetrical MVD coding- Bi-directional optical flow- Decoder side motion vector refinement- Bi-prediction with CU-level weight• Transform, quantization and coefficients coding- Multiple primary transform selection with DCT2, DST7 and DCT8- Secondary transform for low frequency zone- Sub-block transform for inter predicted residual- Dependent quantization with max QP increased from 51 to 63- Transform coefficient coding with sign data hiding- Transform skip residual coding• Entropy Coding- Arithmetic coding engine with adaptive double windows probability update• In loop filter- In-loop reshaping- Deblocking filter with strong longer filter- Sample adaptive offset- Adaptive Loop Filter• Screen content coding:- Current picture referencing with reference region restriction• 360-degree video coding- Horizontal wrap-around motion compensation• High-level syntax and parallel processing- Reference picture management with direct reference picture list signalling- Tile groups with rectangular shape tile groups

[0093] Regarding entropy coding, both H.265 / HEVC and H.266 / VVC standards use context adaptive binary arithmetic coder (CAB AC) for the purpose of compressing and decompressing different syntax elements efficiently. As a binary arithmetic coder, CABAC encodes and decodes syntax elements of size of 1 bit. Other alternative is to use non-binary arithmetic coders where encoded and decoded symbols can represent syntax elements with more than one bit of information. The following is applicable to both binary and non-binary arithmetic coders, although examples are given for the case of binary arithmetic coding for simplicity.

[0094] In the case of context adaptive arithmetic coding, each syntax element has an estimate of probabilities for its possible values. These probability estimates are referred to as contexts as they provide the context or model that describes the behavior of the syntax element or elements associated with the context. A different set of contexts, referred to as a context class, may be used e.g. for luma and chroma components. A context class list includes the context identifiers (Cid) for those context classes that have been selected for spatial tuning of the arithmetic coder’s states. In CABAC, separate lists of coding context classes are used for each superblock, such as a Coding Tree Unit (CTU).

[0095] Efficiency of the arithmetic coder depends heavily on the accuracy of the probability estimates generated for the contexts. For example, the H.266 standard estimates the probabilities of syntax element being 0 or 1 using two estimators with different characteristics and calculates the active probability for the context using an average of the two estimators. In that case, the estimators have different adaptation rates, the first one is reacting faster to the changes in the encoded and decoded syntax element values and the second one reacts slower estimating morethe long-term trend of the values of the syntax element. The first estimate can then be considered to have a shorter adaptation window and the second estimate a longer one.

[0096] Figure 2 shows a typical segmentation of a picture to coding tree units (CTUs) or superblocks (So - Sn) which serve as the basis for further segmentation into finer coding units of different shapes and provide also a framework for some of the coding tools to initialize or update their states.

[0097] Encoding and decoding of image and video content is typically performed in a nested tree order within a Coding Tree Unit (CTU) starting from the coding unit (CU) or other coding block in the top-left corner of a CTU and ending to the coding unit (CU) or other coding block in the bottom-right corner of the CTU. This is illustrated in Figure 3, where arbitrary CTUs of a picture are encoded or decoded. The CTUs (CTUMI, CTUM2, CTUMS) of the CTU row M have been encoded / decoded. CTUM2 illustrates the nested tree order within the CTU, wherein the numbered (0 - 6) coding units (CUs) or other coding blocks have been encoded / decoded in the numbered order. The CTUs of the CTU row N are currently encoded / decoded, wherein the encoding / decoding of CTUNI is completed through the numbered (0 - 8) nested tree order. The encoding / decoding of CTUN2 starts from the CU (0) in the top-left corner of CTUN2.

[0098] As a result, when the encoding or decoding of a specific CTU, such as CTUNI, is completed, the probability estimates of the arithmetic coder are tuned for the data present in the bottom-right corner area of the CTU. When progressing to a new CTU, such as CTUN2, the encoding or decoding process continues again from the coding unit (CU) or other coding block in the top-left corner of the new CTU (CTUN2), where the probability estimates or context states of the arithmetic coder are not necessarily even close to ideal as the processing has jumped to a different spatial location in the picture.

[0099] To be able to adjust the probabilities of the arithmetic coding engine towards their optimal values in the local neighborhood of syntax elements to be coded or decoded, some memory is needed to store those bins or syntax element values in different neighborhoods in the picture together with the list of coding context classes of the corresponding superblock, such as the CTU. For example, bins or syntax element values from CUs or other coding blocks of a previously encoded CTU, such as CTUM2, locating spatially close to the top-left corner of the new CTU (CTUN2), such as coding units 1, 2 5 and 6 of CTUM2, may be more beneficial for adjusting the probabilities of the arithmetic coding engine. It can be easily concluded that theamount of such data can be relatively large, and it needs to be buffered for the time required for encoding / decoding of a plurality of CTUs. Such amount of data needs to be stored efficiently to be able minimize the amount of memory required for the storage.

[0100] Now an improved method for using coding contexts is introduced.

[0101] An encoding method according to an aspect is shown in Figure 4, where the method comprises determining (400) a context class list; determining (402) a first superblock of a picture; determining (404) a first context class from the context class list: determining (406) a first bin value belonging to the first context class in the first superblock; calculating (408) a value for a first probability estimate using the first bin value; encoding (410) a syntax element using the first probability estimate; determining (412) a second superblock of the picture; determining (414) a second context class from said context class list; determining (416) a second bin value belonging to the second context class in the second superblock; calculating (418) a value for a second probability estimate using the second bin value; and encoding (420) a syntax element using the second probability estimate.

[0102] A decoding method according to another aspect is shown in Figure 5, where the method comprises determining (500) a context class list; determining (502) a first superblock of a picture; determining (504) a first context class from the context class list: determining (506) a first bin value belonging to the first context class in the first superblock; calculating (508) a value for a first probability estimate using the first bin value; decoding (510) a syntax element using the first probability estimate; determining (512) a second superblock of the picture; determining (514) a second context class from said context class list; determining (516) a second bin value belonging to the second context class in the second superblock; calculating (518) a value for a second probability estimate using the second bin value; and decoding (520) a syntax element using the second probability estimate.

[0103] Thus, in the methods, a subset of arithmetic coding context classes is selected to be included in the set of context classes to be allocated for future use to adjust the probability states of the arithmetic coder regarding a plurality of superblocks, such as CTUs. The same set of context classes may be initially allocated for all CTUs of a picture. Using a common, but a limited size context class list for a plurality of CTUs enables more efficient memory usage.

[0104] It is noted that the term ‘superblock’ is used herein to refer to a coding unit with the maximum allowed size. In H.265 / HEVC and H.266 / VVC, this refers to LCU (largest codingunit) or CTU (coding tree unit), but in other video coding standards another term, such as macroblock, may be used instead. VP9 video coding standard refers to superblocks as the coding unit with the maximum allowed size. Herein, terms ‘superblock’ and ‘CTU’ may be used interchangeably.

[0105] In the following, various embodiments for the methods are described. It is noted that the same embodiments are applicable to both encoding and decoding processes.

[0106] Hence, a location-based update process for the state variables of an arithmetic codec may be triggered when starting to encode or decode the current CTU. For each, or for some determined set of CTUs, a selected number of bin values associated with different context classes can be stored to be used by the update process.

[0107] According to an embodiment, the number of context classes for which bin values are stored is limited to N. N can be a pre-determined number, such as 96, 128, 192 or 256, or a number that is derived using a set of given rules. Alternatively, N may be indicated in a video or image bitstream.

[0108] A context class list, i.e. a list of context classes that have been included in the set of context classes for which bin values are stored, is constructed and maintained by the arithmetic coder. Each context class in such context class list is further associated with stored bin values for the context class. Bin values can be stored in different ways. For example, there can be an array of bin values and a counter indicating how many bin values are currently included for a given context class. Each context class can be represented by its unique identifier or context ID in the context class list. Such identifier can be, for example, an integer number.

[0109] Context classes for which bin values are stored can be determined by selecting the first N context classes fulfilling given conditions in the order syntax elements with their associated context classes appear in the bitstream. According to an embodiment, the bottom coding units (CUs) of the CTU are included as candidate CUs for the process. Different selection can be made in alternative embodiments.

[0110] According to an embodiment, the method further comprises updating the context class list in the beginning of a picture, in the beginning of a slice or in the beginning of a superblock row.

[0111] Thus, the same context class list may be used for the whole picture, per slice or per a superblock row. The context class list may be updated partly, where a subset of context classes isremoved from the context class list, or entirely, where all the context classes included earlier in the context class list are removed. An alternative way to reset the list is to initialize it with a context class list of some other picture or a slice. A reset typically includes also clearing all the stored bin values and possible related counters that are associated with different context classes. New context classes may be included in the context class list as those appear in the bitstream, if given conditions for inclusion of new context classes are fulfilled.

[0112] According to an embodiment, the context class list has a determined maximum size.

[0113] By limiting the size of the context class list efficient memory usage is facilitated. Moreover, one condition for the inclusion of new context classes is that the determined maximum size of context class list may not be exceeded.

[0114] According to an embodiment, the method further comprises constructing the context class list by including context classes from at least two superblocks.

[0115] The selected set of context classes may then include context class from multiple coding tree units.

[0116] According to an embodiment, the method further comprises constructing the context class list by including at most a determined maximum number of context classes from a single superblock.

[0117] Thus, by setting a maximum number of context classes that can be included from a single coding tree unit enables to generalize the set of selected context classes as the set can then not be filled using syntax elements from a single coding tree unit alone.

[0118] Some of the embodiments can be illustrated by the following pseudo code example, where N is the maximum number of context classes whose bins can be stored, and M is the maximum number of bins that can be stored for each such context class: numberOfActiveContextClasses = 0 contextClassStorage = emptyQ for a CTU in picture: for a CU located on the bottom of the CTU: for a bin used to represent syntax elements of the CU: contextclass = contextClassOf(bin) if contextclass is already in contextClassStorage:if numberOfBinsStored for contextClass < M: store bin to contextClassStorage for contextClass else if numberOfActiveContextClasses < N: include contextClass in contextClassStorage increase numberOfActiveContextClasses by 1 store bin to contextClassStorage for contextClass

[0119] According to an embodiment, the method further comprises constructing the context class list having a determined number of context classes calculated using an index of a coding unit.

[0120] As an alternative example, the number of context classes that can be included in the context class list can be determined as a function of the index of the CTU which allows building the context class list more gradually and potentially allow more representative selection of context classes in the context class list. For example, the maximum number of context classes for a given CTU can be determined as minimum of X*CTUind and N, where X is the parameter controlling the speed of new context class inclusion in the list, CTUind is the index of the CTU in coding order and N is the total maximum size of the context class list. In this case the pseudo code example can be reformulated as: numberOfActiveContextClasses = 0 contextClassStorage = emptyQ for a CTU in picture: maxNumContexts = min(X * CTUind, N) for a CU located on the bottom of the CTU: for a bin used to represent syntax elements of the CU: contextClass = contextClassOf(bin) if contextClass is already in contextClassStorage: if numberOfBinsStored for contextClass < M: store bin to contextClassStorage for contextClass else if numberOfActiveContextClasses < maxNumContexts: include contextClass in contextClassStorageincrease numberOfActiveContextClasses by 1 store bin to contextClassStorage for contextclass

[0121] As a yet another example, the maximum number of context classes that can be included for a specific CTU can be limited to a certain value. In that case, the maximum number of context classes that cannot be exceeded when processing a given CTU can be determined as minimum of number of context classes in the beginning of a CTU processing plus Y and N, where Y is the parameter controlling how many new context classes can be included for a CTU and N is the total maximum size of the context class list. In this case the pseudo example can be reformulated as: numberOfActiveContextClasses = 0 contextClassStorage = emptyQ for a CTU in picture: maxNumContexts = min(numberOfActiveContextClasses + Y, N) for a CU located on the bottom of the CTU: for a bin used to represent syntax elements of the CU: contextclass = contextClassOf(bin) if contextclass is already in contextClassStorage: if numberOfBinsStored for contextClass < M: store bin to contextClassStorage for contextClass else if numberOfActiveContextClasses < maxNumContexts: include contextClass in contextClassStorage increase numberOfActiveContextClasses by 1 store bin to contextClassStorage for contextClass

[0122] According to an embodiment, the method comprises storing the bin values associated with the context classes included in the context class list in superblock-specific buffers.

[0123] In the above embodiments, a single context class list per picture or slice is enough, thus enabling to share the context class list between multiple CTUs, but each CTU may further use own bin storage buffer to store the bin values associated with the context classes included in the context class list. As an example, storage buffers for bin values may include two elements: one for the stored bins and one for a counter that indicates how many bins associated with acertain context class have been stored. Alternatively, there may be two arrays of elements. One array containing the stored bin values and other array containing the counters indicating numbers of stored bins for each context class. The context class list can be implemented, for example, as a list or vector or array or other kind of a data structure. The context class list can include, for example, integer numbers or context identifiers uniquely identifying a context class in the set of all context classes.

[0124] Applying a shared context class list to multiple CTUs enables to minimize the need for storing context identifiers or context IDs and the number of context class lists used in the encoding or decoding is smaller than the number of bin value storage lists. Such configuration allows multiple CTUs to use the same context class list to determine which bin values are stored or used for updating probability estimates of the arithmetic coder.

[0125] Update of the state variables of the arithmetic coder can be performed in different ways using the stored bin values. For example, it can be done based on discontinuities in scanning of coding units within a picture, wherein bins or syntax element values can be stored in memory to be used to update probability states of later coding units or coding tree units. As another example, the states of an arithmetic encoder may be updated based on buffered context index values and associated bin values, where the context indexes to be buffered are determined by selecting a certain number of context index values and associated bin values in the order they appear in a source coding tree unit.

[0126] According to an embodiment, the method comprises updating at least the first probability estimate by determining the first context class using the context class list and the stored bin value for the first superblock; and updating at least the second probability estimate by determining the second context class using the said context class list and the stored bin value for the second superblock.

[0127] According to an embodiment, the method comprises updating at least one probability estimate relating to the first context class in the first superblock based on a stored bin value of the second superblock; and updating at least one probability estimate relating to the second context class in a third superblock based on a stored bin value of a fourth superblock, wherein the first and second context classes are determined using the same context class list.

[0128] The above embodiments are examples of the benefits of sharing the same context class list between multiple CTUs (superblocks), wherein a probability estimate relating to the certaincontext class in certain CTU can be updated based on a stored bin value of another (neighboring) CTU.

[0129] According to an embodiment, the method comprises sharing two context class lists between multiple superblocks, wherein a first context class list is used to store context classes for which bin values are stored for updating probability estimates for a next superblock row and a second context class list is used to identify what context classes are stored on a previous superblock row and used in updating probability estimates for the current superblock row.

[0130] Thus, an alternative embodiment operates on CTU (i.e. superblock) row basis. Herein, two context class lists are maintained, where a first context class list is used for collecting context classes for a current CTU row and a second context class list represents the final context class list for the previous CTU row, i.e. the CTU row above the current CTU row. When context update process in invoked for a CTU before the encoding or decoding of the CTU starts, the second context class list is used to determine which context are updated using the stored bin values for each of the CTU above the current CTU. When storing the selected bin values for the current CTU, the first context class list is used. This allows selecting different context classes for each CTU row and may provide better coding efficiency in some scenarios with the cost of maintaining two context class lists instead of one.

[0131] According to an embodiment, the method comprises resetting the first context class list before processing the first superblock in a superblock row; and skipping the resetting for the second context class list.

[0132] With such arrangements the first and second context class list may be swapped at the end of the CTU row as the context class list that was used to determined which contexts are updated for the current CTU row are no longer needed and it can then be cleared and used as the context class list for the new context classes selected for buffering bin values on the next CTU row.

[0133] Some of the above embodiments are illustrated by an example of Figure 6, where an example memory array layout is depicted. ContextList table demonstrates a possible way to implement a context class list, which is shared between multiple CTUs, preferably for the whole picture. The context class list includes the context identifiers (Cid) for those context classes that have been selected for spatial tuning of the arithmetic coder’s states. In the example, there are as many BinStorage tables as there are CTU columns. It is noted that one BinStorage table for eachCTU column is typically enough, since the same table can be first used to update the probability estimates of the arithmetic coder in the beginning of processing one CTU and then cleared and used to store the bin values related to that CTU as the processing proceeds.

[0134] For illustrative purposes, in this example the same BinStorage table is used to store the bin values (Bins) and the counters (C) indicating how many bin values have been stored for a CTU. It is nevertheless noted that in practical implementation, there is no need to store the context identifiers in the BinStorage tables as the slot number can be used to determine which context classes in ContextUist table relate to each set of stored bins in the BinStorage tables. Also, there is no need to store the slot numbers shown in the different tables as the slot numbers are typically available as the memory offsets from the beginning of each array.

[0135] An apparatus according to an aspect comprises means for determining a context class list; means for determining a first superblock of a picture; means for determining a first context class from the context class list: means for determining a first bin value belonging to the first context class in the first superblock; means for calculating a value for a first probability estimate using the first bin value; means for encoding a syntax element using the first probability estimate; means for determining a second superblock of the picture; means for determining a second context class from said context class list; means for determining a second bin value belonging to the second context class in the second superblock; means for calculating a value for a second probability estimate using the second bin value; and means for encoding a syntax element using the second probability estimate.

[0136] An apparatus according to an aspect comprises means for determining a context class list; means for determining a first superblock of a picture; means for determining a first context class from the context class list: means for determining a first bin value belonging to the first context class in the first superblock; means for calculating a value for a first probability estimate using the first bin value; means for decoding a syntax element using the first probability estimate; means for determining a second superblock of the picture; means for determining a second context class from said context class list; means for determining a second bin value belonging to the second context class in the second superblock; means for calculating a value for a second probability estimate using the second bin value; and means for decoding a syntax element using the second probability estimate.

[0137] According to an embodiment, the apparatus comprises means for updating the context class list in the beginning of the picture, in the beginning of a slice or in the beginning of a superblock row.

[0138] According to an embodiment, the context class list has a determined maximum size.

[0139] According to an embodiment, the apparatus comprises means for constructing the context class list by including context classes from at least two superblocks.

[0140] According to an embodiment, the apparatus comprises means for constructing the context class list by including at most a determined maximum number of context classes from a single superblock.

[0141] According to an embodiment, the apparatus comprises means for constructing the context class list having a determined number of context classes calculated using an index of a coding unit.

[0142] According to an embodiment, the apparatus comprises means for storing the bin values associated with the context classes included in the context class list in superblock-specific buffers.

[0143] According to an embodiment, the apparatus comprises means for updating at least the first probability estimate by determining the first context class using the context class list and the stored bin value for the first superblock; and means for updating at least the second probability estimate by determining the second context class using the said context class list and the stored bin value for the second superblock.

[0144] According to an embodiment, the apparatus comprises means for updating at least one probability estimate relating to the first context class in the first superblock based on a stored bin value of the second superblock; and means for updating at least one probability estimate relating to the second context class in a third superblock based on a stored bin value of a fourth superblock, wherein the first and second context classes are determined using the same context class list.

[0145] According to an embodiment, the apparatus comprises means for sharing two context class lists between multiple superblocks, wherein a first context class list is used to store context classes for which bin values are stored for updating probability estimates for a next superblock row and a second context class list is used to identify what context classes are stored on aprevious superblock row and used in updating probability estimates for the current superblock row.

[0146] According to an embodiment, the apparatus comprises means for resetting the first context class list before processing the first superblock in a superblock row; and means for skipping the resetting for the second context class list.

[0147] According to an embodiment, the superblock is a coding tree unit (CTU), a largest coding unit (LCU) or a macroblock.

[0148] As a further aspect, there is provided an apparatus comprising: at least one processor and at least one memory, said at least one memory stored with code thereon, which when executed by said at least one processor, causes the apparatus to perform at least: determine a context class list; determine a first superblock of a picture; determine a first context class from the context class list: determine a first bin value belonging to the first context class in the first superblock; calculate a value for a first probability estimate using the first bin value; encode a syntax element using the first probability estimate; determine a second superblock of the picture; determine a second context class from said context class list; determine a second bin value belonging to the second context class in the second superblock; calculate a value for a second probability estimate using the second bin value; and encode a syntax element using the second probability estimate.

[0149] As a yet further aspect, there is provided an apparatus comprising: at least one processor and at least one memory, said at least one memory stored with code thereon, which when executed by said at least one processor, causes the apparatus to perform at least: determine a context class list; determine a first superblock of a picture; determine a first context class from the context class list: determine a first bin value belonging to the first context class in the first superblock; calculate a value for a first probability estimate using the first bin value; decode a syntax element using the first probability estimate; determine a second superblock of the picture; determine a second context class from said context class list; determine a second bin value belonging to the second context class in the second superblock; calculate a value for a second probability estimate using the second bin value; and decode a syntax element using the second probability estimate.

[0150] According to an embodiment, the apparatus comprises code configured to cause the apparatus to update the context class list in the beginning of the picture, in the beginning of a slice or in the beginning of a superblock row.

[0151] According to an embodiment, the context class list has a determined maximum size.

[0152] According to an embodiment, the apparatus comprises code configured to cause the apparatus to construct the context class list by including context classes from at least two superblocks.

[0153] According to an embodiment, the apparatus comprises code configured to cause the apparatus to construct the context class list by including at most a determined maximum number of context classes from a single superblock.

[0154] According to an embodiment, the apparatus comprises code configured to cause the apparatus to construct the context class list having a determined number of context classes calculated using an index of a coding unit.

[0155] According to an embodiment, the apparatus comprises code configured to cause the apparatus to store the bin values associated with the context classes included in the context class list in superblock-specific buffers.

[0156] According to an embodiment, the apparatus comprises code configured to cause the apparatus to update at least the first probability estimate by determining the first context class using the context class list and the stored bin value for the first superblock; and code configured to cause the apparatus to update at least the second probability estimate by determining the second context class using the said context class list and the stored bin value for the second superblock.

[0157] According to an embodiment, the apparatus comprises code configured to cause the apparatus to update at least one probability estimate relating to the first context class in the first superblock based on a stored bin value of the second superblock; and code configured to cause the apparatus to update at least one probability estimate relating to the second context class in a third superblock based on a stored bin value of a fourth superblock, wherein the first and second context classes are determined using the same context class list.

[0158] According to an embodiment, the apparatus comprises code configured to cause the apparatus to share two context class lists between multiple superblocks, wherein a first context class list is used to store context classes for which bin values are stored for updating probabilityestimates for a next superblock row and a second context class list is used to identify what context classes are stored on a previous superblock row and used in updating probability estimates for the current superblock row.

[0159] According to an embodiment, the apparatus comprises code configured to cause the apparatus to reset the first context class list before processing the first superblock in a superblock row; and code configured to cause the apparatus to skip the resetting for the second context class list.

[0160] According to an embodiment, the superblock is a coding tree unit (CTU), a largest coding unit (LCU) or a macroblock.

[0161] Such apparatuses may comprise e.g. all or a subset of the functional units disclosed in any of the appended Figures la, lb, and 7 - 10 for implementing the embodiments.

[0162] 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.

[0163] Figure 7 shows a schematic block diagram of an exemplary apparatus or electronic device 50, which may incorporate a codec according to an embodiment of the invention. Figure 8 shows a layout of an apparatus according to an example embodiment.

[0164] 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.

[0165] The apparatus 50 may comprise a housing 30 for incorporating and protecting the device. The apparatus 50 further may comprise a display 32 in the form of a liquid crystal display. In other embodiments of the invention the display may be any suitable display technology suitable to display an image or video. The apparatus 50 may further comprise a keypad 34. In other embodiments of the invention any suitable data or user interface mechanism may be employed. For example, the user interface may be implemented as a virtual keyboard or data entry system as part of a touch-sensitive display.

[0166] 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 ananalogue audio or digital audio output connection. The apparatus 50 may also comprise a battery (or in other embodiments of the invention the device may be powered by any suitable mobile energy device such as solar cell, fuel cell or clockwork generator). The apparatus may further comprise a camera capable of recording or capturing images and / or video. The apparatus 50 may further comprise an infrared port for short range line of sight communication to other devices. In other embodiments the apparatus 50 may further comprise any suitable short range communication solution such as for example a Bluetooth wireless connection or a USB / firewire wired connection.

[0167] The apparatus 50 may comprise a controller 56, processor or processor circuitry for controlling the apparatus 50. The controller 56 may be connected to memory 58 which in embodiments of the invention may store both data in the form of image and audio data and / or may also store instructions for implementation on the controller 56. The controller 56 may further be connected to codec circuitry 54 suitable for carrying out coding and decoding of audio and / or video data or assisting in coding and decoding carried out by the controller.

[0168] 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.

[0169] 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).

[0170] The apparatus 50 may comprise a camera capable of recording or detecting individual frames which are then passed to the codec 54 or the controller for processing. The apparatus may receive the video image data for processing from another device prior to transmission and / or storage. The apparatus 50 may also receive either wirelessly or by a wired connection the image for coding / decoding. The structural elements of apparatus 50 described above represent examples of means for performing a corresponding function.

[0171] With respect to Figure 9, an example of a system within which embodiments of the present invention can be utilized is shown. The system 10 comprises multiple communication devices which can communicate through one or more networks. The system 10 may comprise any combination of wired or wireless networks including, but not limited to a wireless cellular telephone network (such as a GSM, UMTS, CDMA network etc.), a wireless local area network (WLAN) such as defined by any of the IEEE 802.x standards, a Bluetooth personal area network, an Ethernet local area network, a token ring local area network, a wide area network, and the Internet.

[0172] The system 10 may include both wired and wireless communication devices and / or apparatus 50 suitable for implementing embodiments of the invention.

[0173] For example, the system shown in Figure 9 shows a mobile telephone network 11 and a representation of the internet 28. Connectivity to the internet 28 may include, but is not limited to, long range wireless connections, short range wireless connections, and various wired connections including, but not limited to, telephone lines, cable lines, power lines, and similar communication pathways.

[0174] The example communication devices shown in the system 10 may include, but are not limited to, an electronic device or apparatus 50, a combination of a personal digital assistant (PDA) and a mobile telephone 14, a PDA 16, an integrated messaging device (IMD) 18, a desktop computer 20, a notebook computer 22. The apparatus 50 may be stationary or mobile when carried by an individual who is moving. The apparatus 50 may also be located in a mode of transport including, but not limited to, a car, a truck, a taxi, a bus, a train, a boat, an airplane, a bicycle, a motorcycle or any similar suitable mode of transport.

[0175] 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.

[0176] 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 mobiletelephone network 11 and the internet 28. The system may include additional communication devices and communication devices of various types.

[0177] 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.

[0178] Figure 10 is a graphical representation of an example multimedia communication system within which various embodiments may be implemented. A data source 1510 provides a source signal in an analog, uncompressed digital, or compressed digital format, or any combination of these formats. An encoder 1520 may include or be connected with a preprocessing, such as data format conversion and / or filtering of the source signal. The encoder 1520 encodes the source signal into a coded media bitstream. It should be noted that a bitstream to be decoded may be received directly or indirectly from a remote device located within virtually any type of network. Additionally, the bitstream may be received from local hardware or software. The encoder 1520 may be capable of encoding more than one media type, such as audio and video, or more than one encoder 1520 may be required to code different media types of the source signal. The encoder 1520 may also get synthetically produced input, such as graphics and text, or it may be capable of producing coded bitstreams of synthetic media. In the following, only processing of one coded media bitstream of one media type is considered to simplify the description. It should be noted, however, that typically real-time broadcast services comprise several streams (typically at least one audio, video and text sub-titling stream). It should also be noted that the system may include many encoders, but in the figure only one encoder 1520 is represented to simplify the description without a lack of generality. It should be further understood that, although text and examples contained herein may specifically describean 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.

[0179] The coded media bitstream may be transferred to a storage 1530. The storage 1530 may comprise any type of mass memory to store the coded media bitstream. The format of the coded media bitstream in the storage 1530 may be an elementary self-contained bitstream format, or one or more coded media bitstreams may be encapsulated into a container file, or the coded media bitstream may be encapsulated into a Segment format suitable for DASH (or a similar streaming system) and stored as a sequence of Segments. If one or more media bitstreams are encapsulated in a container file, a file generator (not shown in the figure) may be used to store the one more media bitstreams in the file and create file format metadata, which may also be stored in the file. The encoder 1520 or the storage 1530 may comprise the file generator, or the file generator is operationally attached to either the encoder 1520 or the storage 1530. Some systems operate “live”, i.e. omit storage and transfer coded media bitstream from the encoder 1520 directly to the sender 1540. The coded media bitstream may then be transferred to the sender 1540, also referred to as the server, on a need basis. The format used in the transmission may be an elementary self-contained bitstream format, a packet stream format, a Segment format suitable for DASH (or a similar streaming system), or one or more coded media bitstreams may be encapsulated into a container file. The encoder 1520, the storage 1530, and the server 1540 may reside in the same physical device or they may be included in separate devices. The encoder 1520 and server 1540 may operate with live real-time content, in which case the coded media bitstream is typically not stored permanently, but rather buffered for small periods of time in the content encoder 1520 and / or in the server 1540 to smooth out variations in processing delay, transfer delay, and coded media bitrate.

[0180] The server 1540 sends the coded media bitstream using a communication protocol stack. The stack may include but is not limited to one or more of Real-Time Transport Protocol (RTP), User Datagram Protocol (UDP), Hypertext Transfer Protocol (HTTP), Transmission Control Protocol (TCP), and Internet Protocol (IP). When the communication protocol stack is packet-oriented, the server 1540 encapsulates the coded media bitstream into packets. For example, when RTP is used, the server 1540 encapsulates the coded media bitstream into RTP packets according to an RTP payload format. Typically, each media type has a dedicated RTPpayload format. It should be again noted that a system may contain more than one server 1540, but for the sake of simplicity, the following description only considers one server 1540.

[0181] If the media content is encapsulated in a container file for the storage 1530 or for inputting the data to the sender 1540, the sender 1540 may comprise or be operationally attached to a "sending file parser" (not shown in the figure). In particular, if the container file is not transmitted as such but at least one of the contained coded media bitstream is encapsulated for transport over a communication protocol, a sending file parser locates appropriate parts of the coded media bitstream to be conveyed over the communication protocol. The sending file parser may also help in creating the correct format for the communication protocol, such as packet headers and payloads. The multimedia container file may contain encapsulation instructions, such as hint tracks in the ISOBMFF, for encapsulation of the at least one of the contained media bitstream on the communication protocol.

[0182] The server 1540 may or may not be connected to a gateway 1550 through a communication network, which may e.g. be a combination of a CDN, the Internet and / or one or more access networks. The gateway may also or alternatively be referred to as a middle-box. For DASH, the gateway may be an edge server (of a CDN) or a web proxy. It is noted that the system may generally comprise any number gateways or alike, but for the sake of simplicity, the following description only considers one gateway 1550. The gateway 1550 may perform different types of functions, such as translation of a packet stream according to one communication protocol stack to another communication protocol stack, merging and forking of data streams, and manipulation of data stream according to the downlink and / or receiver capabilities, such as controlling the bit rate of the forwarded stream according to prevailing downlink network conditions. The gateway 1550 may be a server entity in various embodiments.

[0183] The system includes one or more receivers 1560, typically capable of receiving, demodulating, and de-capsulating the transmitted signal into a coded media bitstream. The coded media bitstream may be transferred to a recording storage 1570. The recording storage 1570 may comprise any type of mass memory to store the coded media bitstream. The recording storage 1570 may alternatively or additively comprise computation memory, such as random access memory. The format of the coded media bitstream in the recording storage 1570 may be an elementary self-contained bitstream format, or one or more coded media bitstreams may be encapsulated into a container file. If there are multiple coded media bitstreams, such as an audiostream and a video stream, associated with each other, a container file is typically used and the receiver 1560 comprises or is attached to a container file generator producing a container file from input streams. Some systems operate “live,” i.e. omit the recording storage 1570 and transfer coded media bitstream from the receiver 1560 directly to the decoder 1580. In some systems, only the most recent part of the recorded stream, e.g., the most recent 10-minute excerption of the recorded stream, is maintained in the recording storage 1570, while any earlier recorded data is discarded from the recording storage 1570.

[0184] The coded media bitstream may be transferred from the recording storage 1570 to the decoder 1580. If there are many coded media bitstreams, such as an audio stream and a video stream, associated with each other and encapsulated into a container file or a single media bitstream is encapsulated in a container file e.g. for easier access, a file parser (not shown in the figure) is used to decapsulate each coded media bitstream from the container file. The recording storage 1570 or a decoder 1580 may comprise the file parser, or the file parser is attached to either recording storage 1570 or the decoder 1580. It should also be noted that the system may include many decoders, but here only one decoder 1580 is discussed to simplify the description without a lack of generality.

[0185] The coded media bitstream may be processed further by a decoder 1580, whose output is one or more uncompressed media streams. Finally, a Tenderer 1590 may reproduce the uncompressed media streams with a loudspeaker or a display, for example. The receiver 1560, recording storage 1570, decoder 1580, and Tenderer 1590 may reside in the same physical device or they may be included in separate devices.

[0186] A sender 1540 and / or a gateway 1550 may be configured to perform switching between different representations e.g. for switching between different viewports of 360-degree video content, view switching, bitrate adaptation and / or fast start-up, and / or a sender 1540 and / or a gateway 1550 may be configured to select the transmitted representation(s). Switching between different representations may take place for multiple reasons, such as to respond to requests of the receiver 1560 or prevailing conditions, such as throughput, of the network over which the bitstream is conveyed. In other words, the receiver 1560 may initiate switching between representations. A request from the receiver can be, e.g., a request for a Segment or a Subsegment from a different representation than earlier, a request for a change of transmitted scalability layers and / or sub-layers, or a change of a rendering device having differentcapabilities 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.

[0187] A decoder 1580 may be configured to perform switching between different representations e.g. for switching between different viewports of 360-degree video content, view switching, bitrate adaptation and / or fast start-up, and / or a decoder 1580 may be configured to select the transmitted representation(s). Switching between different representations may take place for multiple reasons, such as to achieve faster decoding operation or to adapt the transmitted bitstream, e.g. in terms of bitrate, to prevailing conditions, such as throughput, of the network over which the bitstream is conveyed. Faster decoding operation might be needed for example if the device including the decoder 1580 is multi-tasking and uses computing resources for other purposes than decoding the video bitstream. In another example, faster decoding operation might be needed when content is played back at a faster pace than the normal playback speed, e.g. twice or three times faster than conventional real-time playback rate.

[0188] In the above, some embodiments have been described with reference to and / or using terminology of HEVC and / or WC. It needs to be understood that embodiments may be similarly realized with any video encoder and / or video decoder.

[0189] 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.

[0190] 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 beimplemented as a single encoder-decoder apparatus / structure / operation. Furthermore, it is possible that the coder and decoder may share some or all common elements.

[0191] 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.

[0192] 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.

[0193] Furthermore, elements of a public land mobile network (PLMN) may also comprise video codecs as described above.

[0194] 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.

[0195] 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.

[0196] 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.

[0197] 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.

[0198] Programs, such as those provided by Synopsys, Inc. of Mountain View, California and Cadence Design, of San Jose, California automatically route conductors and locate components on a semiconductor chip using well established rules of design as well as libraries of pre-stored design modules. Once the design for a semiconductor circuit has been completed, the resultant design, in a standardized electronic format (e.g., Opus, GDSII, or the like) may be transmitted to a semiconductor fabrication facility or "fab" for fabrication.

[0199] 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 determining a context class list; means for determining a first superblock of a picture; means for determining a first context class from the context class list: means for determining a first bin value belonging to the first context class in the first superblock; means for calculating a value for a first probability estimate using the first bin value; means for encoding a syntax element using the first probability estimate; means for determining a second superblock of the picture; means for determining a second context class from said context class list; means for determining a second bin value belonging to the second context class in the second superblock; means for calculating a value for a second probability estimate using the second bin value; and means for encoding a syntax element using the second probability estimate.

2. An apparatus comprising means for determining a context class list; means for determining a first superblock of a picture; means for determining a first context class from the context class list: means for determining a first bin value belonging to the first context class in the first superblock; means for calculating a value for a first probability estimate using the first bin value; means for decoding a syntax element using the first probability estimate; means for determining a second superblock of the picture; means for determining a second context class from said context class list; means for determining a second bin value belonging to the second context class in the second superblock;means for calculating a value for a second probability estimate using the second bin value; and means for decoding a syntax element using the second probability estimate.

3. The apparatus according to claim 1 or 2, comprising means for updating the context class list in the beginning of the picture, in the beginning of a slice or in the beginning of a superblock row.

4. The apparatus according to any preceding claim, wherein the context class list has a determined maximum size.

5. The apparatus according to any preceding claim, comprising means for constructing the context class list by including context classes from at least two superblocks.

6. The apparatus according to any preceding claim, comprising means for constructing the context class list by including at most a determined maximum number of context classes from a single superblock.

7. The apparatus according to any preceding claim, comprising means for constructing the context class list having a determined number of context classes calculated using an index of a coding unit.

8. The apparatus according to any preceding claim, comprising means for storing the bin values associated with the context classes included in the context class list in superblock-specific buffers.

9. The apparatus according to any preceding claim, comprising means for updating at least the first probability estimate by determining the first context class using the context class list and the stored bin value for the first superblock; andmeans for updating at least the second probability estimate by determining the second context class using the said context class list and the stored bin value for the second superblock.

10. The apparatus according to any preceding claim, comprising means for updating at least one probability estimate relating to the first context class in the first superblock based on a stored bin value of the second superblock; and means for updating at least one probability estimate relating to the second context class in a third superblock based on a stored bin value of a fourth superblock, wherein the first and second context classes are determined using the same context class list.

11. The apparatus according to claim 1 or 2, comprising means for sharing two context class lists between multiple superblocks, wherein a first context class list is used to store context classes for which bin values are stored for updating probability estimates for a next superblock row and a second context class list is used to identify what context classes are stored on a previous superblock row and used in updating probability estimates for the current superblock row.

12. The apparatus according to claim 11, comprising means for resetting the first context class list before processing the first superblock in a superblock row; and means for skipping the resetting for the second context class list.

13. The apparatus according to any preceding claim, wherein the superblock is a coding tree unit (CTU), a largest coding unit (LCU) or a macroblock.

14. A method comprising determining a context class list; determining a first superblock of a picture; determining a first context class from the context class list:determining a first bin value belonging to the first context class in the first superblock; calculating a value for a first probability estimate using the first bin value; encoding a syntax element using the first probability estimate; determining a second superblock of the picture; determining a second context class from said context class list; determining a second bin value belonging to the second context class in the second superblock; calculating a value for a second probability estimate using the second bin value; and encoding a syntax element using the second probability estimate.

15. A method comprising: determining a context class list; determining a first superblock of a picture; determining a first context class from the context class list: determining a first bin value belonging to the first context class in the first superblock; calculating a value for a first probability estimate using the first bin value; decoding a syntax element using the first probability estimate; determining a second superblock of the picture; determining a second context class from said context class list; determining a second bin value belonging to the second context class in the second superblock; calculating a value for a second probability estimate using the second bin value; and decoding a syntax element using the second probability estimate.

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