Interaction between transform partitioning and primary / secondary transform type selection

Abstract

A method, apparatus, and computer-readable storage medium for encoding / decoding video data in a decoder, comprising: receiving a coded video bitstream of data blocks; extracting from the coded video bitstream transform partition types associated with the data blocks; extracting transform types of transforms associated with divided transform blocks from the data blocks as signaled in the coded video bitstream in response to the transform partition types belonging to a subset of a predefined set of transform partition types that each specify a partitioning pattern for dividing the data blocks into transform blocks, the transform types belonging to a first predefined set of transform types; and performing an inverse transform on the transform blocks according to the transform types.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 175,897, filed April 16, 2021, and U.S. Nonprovisional Patent Application No. 17 / 568,275, filed January 4, 2022, both of which are incorporated by reference in their entireties herein.

[0002] This disclosure describes a set of advanced video coding techniques. More specifically, the disclosed techniques involve the interplay between transform partitioning schemes and primary / secondary transform type selection in video encoding and decoding. [Background technology]

[0003] The discussion of the background art provided herein is intended to generally present the context of the present disclosure. The inventors' work is not admitted, expressly or impliedly, as prior art to the present disclosure to the extent that that work is described in this background section, together with aspects of the description that may not otherwise be admitted as prior art at the time of filing of this application.

[0004] Video coding and video decoding can be performed using inter-picture prediction with motion compensation. Uncompressed digital video can include a sequence of pictures, each having spatial dimensions of, for example, 1920x1080 luma samples and associated full-sampled or sub-sampled chroma samples. The sequence of pictures can have a fixed or variable picture rate (also called frame rate), for example, 60 pictures per second or 60 frames per second. Uncompressed video has specific bit rate requirements for streaming or data processing. For example, a video with a pixel resolution of 1920x1080, a frame rate of 60 frames / second, and 4:2:0 chroma subsampling with 8 bits per pixel per color channel requires a bandwidth close to 1.5 Gbit / s. One hour of such video requires more than 600 GByte of storage space.

[0005] One goal of video coding and video decoding may be the reduction of redundancy in an uncompressed input video signal through compression. Compression may help reduce the aforementioned bandwidth and / or storage space requirements, in some cases by more than one order of magnitude. Both lossless and lossy compression, and combinations thereof, may be used. Lossless compression refers to techniques where an exact copy of the original signal can be reconstructed from the compressed original signal by the decoding process. Lossy compression refers to a coding / decoding process where the original video information is not fully preserved during coding and cannot be fully recovered during decoding. When using lossy compression, the reconstructed signal may not be identical to the original signal, but the distortion between the original signal and the reconstructed signal is small enough to make the reconstructed signal useful for its intended application, even with some information loss. For video, lossy compression is widely adopted in many applications. The amount of distortion that can be tolerated depends on the application. For example, a user of a particular consumer video streaming application may tolerate higher distortion than a user of a movie or television broadcast application. The compression ratio achievable by a particular coding algorithm may be selected or adjusted to reflect different distortion tolerances. That is, generally speaking, higher distortion tolerance allows for coding algorithms that result in higher losses and higher compression ratios.

[0006] Video encoders and decoders can utilize techniques from a number of broad categories and steps, including, for example, motion compensation, Fourier transform, quantization, and entropy coding.

[0007] Video codec technology can include a technique known as intra-coding. In intra-coding, sample values ​​are represented without reference to samples or other data from previously reconstructed reference pictures. In some video codecs, a picture is spatially subdivided into blocks of samples. If all blocks of samples are coded in intra mode, the picture can be called an intra-picture. Intra-pictures and their derived pictures, such as independent decoder refresh pictures, can be used to reset the decoder state and can therefore be used as the first picture in a coded video bitstream and video session or as a still image. Samples of the block after intra prediction can then be transformed to the frequency domain, and the transform coefficients so generated can be quantized before entropy coding. Intra prediction refers to a technique that minimizes sample values ​​in the pre-transform domain. In some cases, the smaller the DC value after transformation and the smaller the AC coefficients, the fewer bits are needed for a given quantization step size to represent the block after entropy coding.

[0008] Conventional intra-coding, for example as known from MPEG-2 generation coding techniques, does not use intra-prediction. However, some newer video compression techniques include techniques that attempt to code / decode a block based on surrounding sample data and / or metadata that precedes the block of data being intra-coded or intra-decoded in decoding order, e.g., obtained during encoding and / or decoding of spatial neighbors. Such techniques are hereafter referred to as "intra-prediction" techniques. It should be noted that in at least some cases, intra-prediction uses reference data only from the current picture being reconstructed, and not from other reference pictures.

[0009] Intra prediction may take many different forms. When more than one of such techniques is available in a given video coding technique, the technique used may be referred to as an intra prediction mode. One or more intra prediction modes may be provided in a particular codec. In certain cases, a mode may have sub-modes and / or may be associated with various parameters, and the mode / sub-mode information and intra coding parameters of a block of video may be coded separately or jointly included in the codeword of the mode. Which codeword is used for a given mode, sub-mode, and / or parameter combination may affect the coding efficiency gains via intra prediction, and thus may also affect the entropy coding technique used to convert the codeword into a bitstream.

[0010] Certain modes of intra prediction were introduced in H.264, improved in H.265, and further improved in newer coding techniques such as Joint Search Model (JEM), Versatile Video Coding (VVC), and Benchmark Set (BMS). In general, in intra prediction, the predictor block can be formed using neighboring sample values ​​that become available. For example, the available values ​​of a particular set of neighboring samples along a particular direction and / or line can be copied to the predictor block. The reference to the direction used can be coded in the bitstream or can itself be predicted.

[0011] Referring to FIG. 1A, shown at the bottom right is a subset of nine predictor directions specified in the 33 possible predictor directions of H.265 (corresponding to the 33 angle modes of the 35 intra modes specified in H.265). The point where the arrows converge (101) represents the sample being predicted. The arrows represent the directions from which neighboring samples are used to predict sample 101. For example, arrow (102) indicates that sample (101) is predicted from one or more neighboring samples to the upper right, at an angle of 45 degrees from the horizontal. Similarly, arrow (103) indicates that sample (101) is predicted from one or more neighboring samples to the lower left of sample (101), at an angle of 22.5 degrees from the horizontal.

[0012] 1A, at the top left is depicted a square block (104) of 4x4 samples (indicated by a thick dashed line). The square block (104) contains 16 samples, each labeled with "S", its position in the Y dimension (e.g., row index), and its position in the X dimension (e.g., column index). For example, sample S21 is the second sample (from the top) in the Y dimension and the first sample (from the left) in the X dimension. Similarly, sample S44 is the fourth sample in both the Y and X dimensions in the block (104). Since the size of the block is 4x4 samples, S44 is at the bottom right. Also shown are examples of reference samples that follow a similar numbering scheme. The reference samples are labeled with R, their Y position (e.g., row number) and X position (column number) relative to the block (104). In both H.264 and H.265, predicted samples that neighbor the block being reconstructed are used.

[0013] Intra-picture prediction of block 104 may start by copying reference sample values ​​from neighboring samples according to a signaled prediction direction. For example, assume that the coded video bitstream includes signaling for this block 104 indicating the prediction direction of the arrow (102), i.e., the sample is predicted from one or more prediction samples to the upper right and at an angle of 45 degrees from the horizontal. In such a case, samples S41, S32, S23, S14 are predicted from the same reference sample R05. Then, sample S44 is predicted from reference sample R08.

[0014] In certain cases, especially when the orientation is not evenly divisible by 45 degrees, the values ​​of multiple reference samples may be combined, for example by interpolation, to calculate the reference sample.

[0015] The number of possible directions has increased as video coding technology continues to develop. In H.264 (2003), for example, nine different directions are available for intra prediction. This increases to 33 in H.265 (2013), and JEM / VVC / BMS can support up to 65 directions at the time of this disclosure. Experimental studies have been conducted to help identify the most suitable intra prediction directions, and those most suitable directions can be coded with a small number of bits using certain techniques of entropy coding, accepting certain bit penalties for the directions. Furthermore, the direction itself may be predicted from the neighboring directions used in the intra prediction of the decoded neighboring blocks.

[0016] FIG. 1B shows a schematic diagram (180) showing 65 intra prediction directions according to JEM to illustrate the increasing number of prediction directions in various coding techniques that have evolved over time.

[0017] The mapping of bits representing intra-prediction directions in a coded video bitstream to prediction directions can vary across video coding techniques, ranging from simple direct mappings of prediction directions to intra-prediction modes, for example, to complex adaptation schemes involving codewords, most likely modes, and similar techniques. In all cases, however, there may be certain directions of intra-prediction that are statistically less likely to occur in the video content than certain other directions. Because the goal of video compression is to reduce redundancy, in a well-designed video coding technique, these less likely directions are represented by more bits than the more likely directions.

[0018] Inter-picture prediction, or inter-prediction, may be based on motion compensation, in which sample data from a previously reconstructed picture or part thereof (reference picture) may be used to predict a newly reconstructed picture or picture part (e.g., block) after being spatially shifted in a direction indicated by a motion vector (hereafter MV). In some cases, the reference picture may be the same as the picture currently being reconstructed. The MV may have two dimensions X and Y, or three dimensions, with the third dimension being an indication of the reference picture to be used (similar to the temporal dimension).

[0019] In some video compression techniques, a current MV applicable to a particular area of ​​sample data can be predicted from other MVs, e.g., from other MVs related to other areas of sample data that are spatially adjacent to the area being reconstructed and that precede the current MV in decoding order. Doing so can significantly reduce the overall amount of data required to code the MV by relying on the removal of redundancy in correlated MVs, thereby increasing compression efficiency. MV prediction can work effectively because, for example, when coding an input video signal derived from a camera (known as natural video), areas larger than the area to which a single MV is applicable have a statistical likelihood to move in a similar direction in the video sequence, and therefore, in some cases, can be predicted using similar motion vectors derived from MVs of neighboring areas. As a result, the actual MV of a given area is similar or identical to the MV predicted from the surrounding MVs. Such MVs can further be represented with fewer bits after entropy coding than would be used if the MV was directly coded instead of predicted from the neighboring MV(s). In some cases, MV prediction can be an example of lossless compression of a signal (i.e., MV) derived from an original signal (i.e., sample stream). In other cases, the MV prediction itself may be lossy, for example due to rounding errors when computing a predictor from several surrounding MVs.

[0020] Various MV prediction mechanisms are described in H.265 / HEVC (ITU-T Rec. H.265, "High Efficiency Video Coding", December 2016). Among the many MV prediction mechanisms specified by H.265, the one described below is a technique that we will refer to as "spatial merging" from here on.

[0021] Specifically, referring to Figure 2, a current block (201) contains samples that are detected by the encoder during the motion search process as predictable from a previous block of the same size but spatially shifted. Instead of coding its MV directly, the MV can be derived from metadata associated with one or more reference pictures, e.g., from the last reference picture (in decoding order), using the MV associated with any one of the five surrounding samples represented as A0, A1, and B0, B1, B2 (202 to 206, respectively). In H.265, MV prediction can use predictors from the same reference picture that neighboring blocks use. Summary of the Invention [Means for solving the problem]

[0022] This disclosure describes various embodiments of methods, apparatus, and computer-readable storage media for video encoding and / or video decoding.

[0023] According to one aspect, an embodiment of the present disclosure provides a method for encoding / decoding video data in a decoder, the method including: receiving a coded video bitstream of a data block, extracting from the coded video bitstream a transform partition type associated with the data block, extracting a transform type of a transform associated with a divided transform block from the data block as signaled in the coded video bitstream in response to the transform partition type belonging to a subset of a predefined set of transform partition types each specifying a partitioning pattern for dividing the data block into transform blocks, the transform type belonging to a first predefined set of transform types, and performing an inverse transform on the transform block according to the transform type.

[0024] According to another aspect, an embodiment of the present disclosure provides a method for encoding / decoding video data, the method including receiving a coded video bitstream of data blocks, extracting from the coded video bitstream transform partition types associated with data blocks of the video data, extracting from the coded video bitstream transform types associated with transform blocks of the data blocks in response to transform partition types belonging to a subset of a predefined set of transform partition types, and identifying by default a transform type of the data blocks in response to transform partition types not belonging to the predefined set of transform partition types.

[0025] According to another aspect, an embodiment of the present disclosure provides a method for encoding / decoding video data, the method including receiving a coded video bitstream of a data block, extracting from the coded video bitstream a transform type of a transform associated with a transform block of the data block, and extracting from the coded video bitstream a transform partition type associated with the data block in response to the transform type belonging to a predefined set of transform types.

[0026] According to another aspect, an embodiment of the present disclosure provides an apparatus for video encoding and / or video decoding, the apparatus including a memory storing instructions and a processor in communication with the memory, the processor configured to, when executing the instructions, cause the apparatus to perform the above-mentioned method for video decoding and / or video encoding.

[0027] According to yet another aspect, an embodiment of the present disclosure provides a non-transitory computer-readable medium storing instructions that, when executed by a computer for video decoding and / or video encoding, cause the computer to perform the above-mentioned method for video decoding and / or video encoding.

[0028] These and other aspects and their implementations are described in further detail in the drawings, specification, and claims.

[0029] Further features, nature and various advantages of the disclosed subject matter will become more apparent from the following detailed description and the accompanying drawings. [Brief description of the drawings]

[0030] [Figure 1A] FIG. 13 is a schematic diagram of an example subset of intra-prediction directional modes. [Figure 1B] FIG. 2 illustrates an exemplary intra-prediction direction. [Diagram 2] FIG. 2 is a schematic diagram illustrating a current block and its surrounding spatial merge candidates for motion vector prediction in one example. [Diagram 3] FIG. 3 is a schematic diagram illustrating a simplified block diagram of a communication system (300) according to an example embodiment. [Figure 4] FIG. 4 is a schematic diagram illustrating a simplified block diagram of a communication system (400) according to an example embodiment. [Diagram 5] FIG. 2 is a schematic diagram illustrating a simplified block diagram of a video decoder according to an example embodiment. [Figure 6] FIG. 1 is a schematic diagram illustrating a simplified block diagram of a video encoder according to an example embodiment. [Figure 7] FIG. 2 is a block diagram illustrating a video encoder according to another example embodiment. [Figure 8] FIG. 2 is a block diagram illustrating a video decoder according to another example embodiment. [Figure 9] FIG. 2 illustrates directional intra-prediction modes according to an exemplary embodiment of the present disclosure. [Figure 10] FIG. 2 illustrates a non-directional intra-prediction mode according to an exemplary embodiment of the present disclosure. [Figure 11] 1 illustrates a recursive intra-prediction mode according to an exemplary embodiment of the present disclosure. [Figure 12]4 illustrates transform block partitioning and scanning of intra-predicted blocks according to an exemplary embodiment of the present disclosure. [Figure 13] 1 illustrates transform block partitioning and scanning of inter-prediction blocks according to an exemplary embodiment of the present disclosure. [Figure 14] 4 illustrates a low frequency non-separable transformation process according to an exemplary embodiment of the present disclosure. [Figure 15] FIG. 2 illustrates various baseline-based intra-prediction schemes, according to an exemplary embodiment of the present disclosure. [Figure 16] FIG. 2 illustrates a non-recursive block division scheme according to an exemplary embodiment of the present disclosure. [Figure 17] 1 shows a flow chart according to an embodiment of the present disclosure. [Figure 18] FIG. 1 is a schematic diagram illustrating a computer system according to an exemplary embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] FIG. 3 illustrates a simplified block diagram of a communication system (300) according to one embodiment of the present disclosure. The communication system (300) includes, for example, a plurality of terminal devices that can communicate with each other via a network (350). For example, the communication system (300) includes a first pair of terminal devices (310) and (320) interconnected via the network (350). In the example of FIG. 3, the first pair of terminal devices (310) and (320) may perform unidirectional transmission of data. For example, the terminal device (310) may code video data (e.g., a stream of video pictures captured by the terminal device (310)) for transmission to the other terminal device (320) via the network (350). The encoded video data may be transmitted in the form of one or more coded video bitstreams. The terminal device (320) may receive the coded video data from the network (350), decode the coded video data to reconstruct the video pictures, and display the video pictures according to the reconstructed video data. One-way data transmission may be implemented, for example, for media serving applications.

[0032] In another example, the communication system (300) includes a second pair of terminal devices (330) and (340) performing bidirectional transmission of coded video data, which may be implemented, for example, during video conferencing applications. For the bidirectional transmission of data, in one example, each of the terminal devices (330) and (340) may code video data (e.g., of a stream of video pictures captured by that terminal device) for transmission to the other of the terminal devices (330) and (340) over the network (350). Each of the terminal devices (330) and (340) may also receive coded video data transmitted by the other of the terminal devices (330) and (340), decode the coded video data to recover the video pictures, and display the video pictures on an accessible display device according to the recovered video data.

[0033] In the example of FIG. 3, the terminal devices (310), (320), (330), and (340) may be implemented as a server, a personal computer, and a smartphone, although the applicability of the principles underlying the present disclosure is not so limited. The embodiments of the present disclosure may be implemented in desktop computers, laptop computers, tablet computers, media players, wearable computers, dedicated video conferencing equipment, and the like. The network (350) represents any number or type of network that conveys coded video data between the terminal devices (310), (320), (330), and (340), including, for example, wired (wired connection) and / or wireless communication networks. The communication network (350) 9 may exchange data over circuit-switched channels, packet-switched channels, and / or other types of channels. Exemplary networks include telecommunications networks, local area networks, wide area networks, and / or the Internet. For purposes of this discussion, the architecture and topology of the network (350) may not be important to the operation of the present disclosure unless explicitly described herein.

[0034] 4 illustrates an arrangement of video encoders and video decoders in a video streaming environment as an example of an application of the disclosed subject matter. The disclosed subject matter may be equally applied to other video-enabled applications including, for example, video conferencing, digital television broadcasting, gaming, virtual reality, storage of compressed video on digital media including CDs, DVDs, memory sticks, etc.

[0035] A video streaming system may include a video source (401), such as a video capture subsystem (413), which may include a digital camera, for creating a stream of uncompressed video pictures or images (402). In one example, the stream of video pictures (402) includes samples recorded by the digital camera of the video source 401. The stream of video pictures (402), shown in bold to emphasize its high amount of data compared to the encoded video data (404) (or coded video bitstream), may be processed by an electronic device (420) including a video encoder (403) coupled to the video source (401). The video encoder (403) may include hardware, software, or a combination thereof to enable or implement aspects of the disclosed subject matter as described in more detail below. The encoded video data (404) (or encoded video bitstream (404)), shown with thin lines to emphasize its low amount of data compared to the stream of uncompressed video pictures (402), can be stored directly on the streaming server (405) or on a downstream video device (not shown) for future use. One or more streaming client subsystems, such as the client subsystems (406) and (408) of FIG. 4, can access the streaming server (405) to obtain copies (407) and (409) of the encoded video data (404). The client subsystem (406) can include a video decoder (410), for example, in an electronic device (430). The video decoder (410) decodes an input copy (407) of the encoded video data and creates an output stream (411) of video pictures that is uncompressed and can be rendered on a display (412) (e.g., a display screen) or other rendering device (not shown). The video decoder 410 can be configured to perform some or all of the various functions described in this disclosure.In some streaming systems, the encoded video data (404), (407), and (409) (e.g., video bitstreams) may be encoded according to a particular video coding / compression standard. Examples of such standards include ITU-T Recommendation H.265. In one example, a video coding standard under development is informally known as Versatile Video Coding (VVC). The disclosed subject matter may be used in the context of VVC, as well as other video coding standards.

[0036] It should be noted that electronic devices 420 and 430 may include other components (not shown). For example, electronic device 420 may include a video decoder (not shown) and electronic device 430 may also include a video encoder (not shown).

[0037] 5 shows a block diagram of a video decoder (510) according to any of the following embodiments of the present disclosure. The video decoder (510) can be included in an electronic device (530). The electronic device (530) can include a receiver (531) (e.g., a receiving circuit). The video decoder (510) can be used in place of the video decoder (410) in the example of FIG. 4.

[0038] The receiver (531) may receive one or more coded video sequences to be decoded by the video decoder (510). In the same or another embodiment, one coded video sequence may be decoded at a time, with the decoding of each coded video sequence being independent of the other coded video sequences. Each video sequence may be associated with multiple video frames or video images. The coded video sequences are received from a channel (501), which may be a hardware / software link to a storage device that stores the coded video data, or a streaming source that transmits the coded video data. The receiver (531) may receive the coded video data along with other data, such as coded audio data and / or auxiliary data streams, which may be forwarded to respective processing circuits (not shown). The receiver (531) may separate the coded video sequences from other data. To combat network jitter, a buffer memory (515) may be located between the receiver (531) and the entropy decoder / parser (520) (hereafter "parser (520)"). In certain applications, the buffer memory (515) may be implemented as part of the video decoder (510). In other applications, the buffer memory (515) may be separate and external to the video decoder (510) (not shown). In still other applications, there may be a buffer memory (not shown) external to the video decoder (510), for example to combat network jitter, and there may be another buffer memory (515) internal to the video decoder (510), for example to handle playback timing. When the receiver (531) is receiving data from a storage / forwarding device with sufficient bandwidth and controllability, or from an isosynchronous network, the buffer memory (515) may be unnecessary or may be small. For use with best-effort packet networks such as the Internet, a buffer memory (515) of sufficient size may be required, and its size may be relatively large.Such a buffer memory may be implemented with an adaptive size and may be implemented, at least in part, in an operating system or similar element (not shown) external to the video decoder (510).

[0039] The video decoder (510) may include a parser (520) to recover symbols (521) from the coded video sequence. These categories of symbols include information used to manage the operation of the video decoder (510) and potentially information for controlling a rendering device such as a display (512) (e.g., a display screen) that may or may not be an integral part of the electronic device (530) but may be coupled to the electronic device (530) as shown in FIG. 5. The control information for the rendering device(s) may be in the form of a supplemental enhancement information (SEI message) or a video usability information (VUI) parameter set fragment (not shown). The parser (520) may parse / entropy decode the coded video sequence received by the parser (520). The coding of the entropy coded video sequence may be according to a video coding technique or standard and may be according to various principles including variable length coding, Huffman coding, arithmetic coding with or without context sensitivity, etc. The parser (520) may extract from the coded video sequence a set of subgroup parameters for at least one of the subgroups of pixels in the video decoder based on at least one parameter corresponding to the subgroup. The subgroups may include Groups of Pictures (GOPs), pictures, tiles, slices, macroblocks, coding units (CUs), blocks, transform units (TUs), prediction units (PUs), etc. The parser (520) may also extract information from the coded video sequence, such as transform coefficients (e.g., Fourier transform coefficients), quantization parameter values, motion vectors, etc.

[0040] The parser (520) can perform entropy decoding / parsing operations on the video sequence received from the buffer memory (515) to produce symbols (521).

[0041] The reconstruction of the symbols (521) may involve a number of different processing or functional units, depending on the type of video picture or portion thereof being coded (inter-picture and intra-picture, inter-block and intra-block, etc.), as well as other factors. The units that are included and how they are included may be controlled by subgroup control information parsed from the coded video sequence by the parser (520). The flow of such subgroup control information between the parser (520) and the following processing or functional units is not shown for the sake of simplicity.

[0042] Besides the functional blocks already mentioned, the video decoder (510) may be conceptually subdivided into several functional units, as described below. In an actual implementation operating under commercial constraints, many of these functional units may closely interact with each other and may be, at least partially, integrated with each other. However, in order to clearly explain the various functions of the disclosed subject matter, the following disclosure adopts a conceptual subdivision into functional units.

[0043] The first unit is a scalar / inverse transform unit (551), which may receive quantized transform coefficients as well as control information from the parser (520) including information indicating which type of inverse transform to use, block size, quantization coefficients / parameters, quantization scaling matrices, etc. The scalar / inverse transform unit (551) may output a block comprising sample values ​​that may be input to an aggregator (555).

[0044] In some cases, the output samples of the scaler / inverse transform (551) may relate to intra-coded blocks, i.e., blocks that do not use prediction information from a previously reconstructed picture, but may use prediction information from a previously reconstructed portion of the current picture. Such prediction information may be provided by an intra-picture prediction unit (552). In some cases, the intra-picture prediction unit (552) may generate blocks of the same size and shape as the block being reconstructed using information of surrounding blocks that have already been reconstructed and stored in a current picture buffer (558). The current picture buffer (558) may, for example, buffer a partially reconstructed and / or a fully reconstructed current picture. The aggregator (555) may, in some implementations, add prediction information generated by the intra-prediction unit (552) to the output sample information provided by the scaler / inverse transform unit (551) on a sample-by-sample basis.

[0045] In other cases, the output samples of the scaler / inverse transform unit (551) may relate to an inter-coded, potentially motion-compensated block. In such cases, the motion compensated prediction unit (553) may access the reference picture memory (557) to fetch samples used for inter-picture prediction. After motion compensating the fetched samples according to the symbols (521) related to the block, these samples may be added to the output of the scaler / inverse transform unit (551) by the aggregator (555) to generate output sample information (the output of unit 551 may be referred to as a residual sample or residual signal). The address in the reference picture memory (557) from which the motion compensated prediction unit (553) fetches the prediction sample may be controlled by a motion vector available to the motion compensated prediction unit (553) in the form of a symbol (521) that may have, for example, an X component, a Y component (shift), and a reference picture component (time). Motion compensation may also include interpolation of sample values ​​fetched from a reference picture memory (557) when sub-sample accurate motion vectors are used, and may be associated with a motion vector prediction mechanism, etc.

[0046] The output samples of the aggregator (555) may be subjected to various loop filtering techniques in the loop filter unit (556). Video compression techniques may include in-loop filter techniques that are controlled by parameters included in the coded video sequence (also called the coded video bitstream) and made available to the loop filter unit (556) as symbols (521) from the parser (520), but may also be responsive to previously reconstructed and loop filtered sample values ​​as well as meta-information obtained during decoding of previous portions (in decoding order) of the coded picture or coded video sequence. As described in more detail below, several types of loop filters may be included as part of the loop filter unit 556 in various orders.

[0047] The output of the loop filter unit (556) may be a sample stream that can be output to a rendering device (512) and also stored in a reference picture memory (557) for use in future inter-picture prediction.

[0048] Once a particular coded picture is fully reconstructed, it can be used as a reference picture for future inter-picture prediction. For example, once a coded picture corresponding to a current picture is fully reconstructed and the coded picture is identified (e.g., by the parser (520)) as a reference picture, the current picture buffer (558) can become part of the reference picture memory (557), and any unused current picture buffer can be reallocated before beginning reconstruction of the next coded picture.

[0049] The video decoder (510) may perform decoding operations according to a given video compression technique adopted in a standard such as ITU-T Rec. H.265. The coded video sequence may conform to the syntax specified by the video compression technique or standard being used, in the sense that the coded video sequence adheres to both the syntax of the video compression technique or standard and to a profile documented in the video compression technique. In particular, a profile may select certain tools from among all tools available in the video compression technique or standard as tools that are dedicated to use only under that profile. To conform to a standard, the complexity of the coded video sequence may be within a range defined by a level of the video compression technique or standard. In some cases, the level limits the maximum picture size, maximum frame rate, maximum reconstruction sample rate (e.g., measured in megasamples per second), maximum reference picture size, etc. The limits set by the level may be further limited in some cases by a hypothetical reference decoder (HRD) specification and metadata for HRD buffer management signaled in the coded video sequence.

[0050] In some example embodiments, the receiver (531) may receive additional (redundant) data along with the encoded video. The additional data may be included as part of the coded video sequence(s). The additional data may be used by the video decoder (510) to properly decode the data and / or to more accurately recover the original video data. The additional data may be in the form of, for example, temporal, spatial, or signal-to-noise ratio (SNR) enhancement layers, redundant slices, redundant pictures, forward error correction codes, etc.

[0051] 6 shows a block diagram of a video encoder (603) according to an exemplary embodiment of the present disclosure. The video encoder (603) may be included in an electronic device (620). The electronic device (620) may further include a transmitter (640) (e.g., a transmission circuit). The video encoder (603) may be used in place of the video encoder (403) of the example of FIG.

[0052] The video encoder (603) may receive video samples from a video source (601) (which in the example of FIG. 6 is not part of the electronic device (620)), which may capture the video image(s) to be coded by the video encoder (603). In another example, the video source (601) may be implemented as part of the electronic device (620).

[0053] The video source (601) may provide a source video sequence to be coded by the video encoder (603) in the form of a digital video sample stream that may be of any suitable bit depth (e.g., 8-bit, 10-bit, 12-bit, ...), any color space (e.g., BT.601 Y CrCb, RGB, XYZ...), and any suitable sampling structure (e.g., Y CrCb 4:2:0, Y CrCb 4:4:4). In a media serving system, the video source (601) may be a storage device that may store previously prepared video. In a video conferencing system, the video source (601) may be a camera that captures local image information as a video sequence. The video data may be provided as a number of separate pictures or images that give motion when viewed in sequence. The picture itself may be organized as a spatial array of pixels, each pixel may contain one or more samples depending on the sampling structure, color space, etc. that is being used. Those skilled in the art can easily understand the relationship between pixels and samples. The following description focuses on samples.

[0054] According to some example embodiments, the video encoder (603) may code and compress pictures of a source video sequence into a coded video sequence (643) in real time or under any other time constraint required by the application. Enforcing an appropriate coding rate constitutes one function of the controller (650). In some embodiments, the controller (650) may be operatively coupled to other functional units and control them as described below. For the sake of simplicity, coupling is not shown. Parameters set by the controller (650) may include rate control related parameters (picture skip, quantizer, lambda value of rate distortion optimization technique, etc.), picture size, Group of Pictures (GOP) layout, maximum motion vector search range, etc. The controller (650) may be configured to have other appropriate functions associated with the video encoder (603) optimized for a particular system design.

[0055] In some example embodiments, the video encoder (603) may be configured to operate in a coding loop. As an oversimplified explanation, in one example, the coding loop may include a source coder (630) (e.g., responsible for creating symbols, such as a symbol stream, based on an input picture to be coded and reference picture(s)) and a (local) decoder (633) embedded in the video encoder (603). The decoder (633) reconstructs the symbols to create sample data in a similar manner as a (remote) decoder would create them, even if the embedded decoder 633 processes the video stream coded by the source coder 630 without entropy coding (since in the video compression techniques contemplated in the disclosed subject matter, any compression between the symbols and the coded video bitstream may be lossless). The reconstructed sample stream (sample data) is input to a reference picture memory (634). Since decoding of the symbol stream leads to bit-exact results regardless of the location of the decoder (local or remote), the contents in the reference picture memory (634) are also bit-exact between the local and remote encoders. In other words, the prediction part of the encoder "sees" exactly the same sample values ​​as the reference picture samples that the decoder will "see" when using the prediction during decoding. This basic principle of reference picture synchrony (and the resulting drift if synchrony cannot be maintained, e.g., due to channel errors) is used to improve coding quality.

[0056] The operation of the "local" decoder (633) may be the same as the operation of a "remote" decoder, such as the video decoder (510), already described in detail above in conjunction with Figure 5. Referring also briefly to Figure 5, however, because symbols are available and the encoding / decoding of symbols into a coded video sequence by the entropy coder (645) and parser (520) may be lossless, the entropy decoding portion of the video decoder (510), including the buffer memory (515) and parser (520), may not be fully implemented in the local decoder (633) within the encoder.

[0057] At this point, it can be said that any decoder technology, except for parsing / entropy decoding, which may only exist in the decoder, may also necessarily need to exist in the corresponding encoder in substantially the same functional form. For this reason, the subject matter of the disclosure may focus on the decoder operation, which is similar to the decoding part of the encoder. Thus, the description of the encoder technology may be omitted, since it is the reverse of the decoder technology described in general. Only in certain areas or aspects, a more detailed description of the encoder is given below.

[0058] In operation, in some example implementations, the source coder (630) may perform motion-compensated predictive coding, which predictively codes an input picture with reference to one or more previously coded pictures from a video sequence designated as "reference pictures." In this manner, the coding engine (632) codes color channel differences (or residuals) between pixel blocks of the input picture and pixel blocks of the reference picture(s) that may be selected as prediction reference(s) to the input picture. The term "residual" and its adjective form "residual difference" may be used interchangeably.

[0059] The local video decoder (633) may decode the coded video data of pictures that may be designated as reference pictures based on the symbols created by the source coder (630). The operation of the coding engine (632) may advantageously be a lossy process. When the coded video data may be decoded in a video decoder (not shown in FIG. 6), the restored video sequence may be a replica of the source video sequence, usually with some errors. The local video decoder (633) may replicate the decoding process that may be performed by the video decoder on the reference pictures and store the reconstructed reference pictures in a reference picture cache (634). In this way, the video encoder (603) may locally store (without transmission errors) copies of reconstructed reference pictures that have common content with reconstructed reference pictures obtained by a far-end (remote) video decoder.

[0060] The predictor (635) may perform a prediction search for the coding engine (632). That is, for a new picture to be coded, the predictor (635) may search the reference picture memory (634) for sample data (as candidate reference pixel blocks) or specific metadata such as reference picture motion vectors, block shapes, etc., that may serve as suitable prediction references for the new picture. The predictor (635) may operate on sample blocks on a pixel block by pixel block basis to find suitable prediction references. In some cases, as determined by the search results obtained by the predictor (635), the input picture may have prediction references drawn from multiple reference pictures stored in the reference picture memory (634).

[0061] The controller (650) can manage the coding operations of the source coder (630), including, for example, setting the parameters and subgroup parameters used to encode the video data.

[0062] The output of all the aforementioned functional units may undergo entropy coding in an entropy coder (645), which converts the symbols produced by the various functional units into a coded video sequence by lossless compression of the symbols according to techniques such as Huffman coding, variable length coding, arithmetic coding, etc.

[0063] The transmitter (640) can buffer the coded video sequence created by the entropy coder (645) and prepare it for transmission over a communication channel (660), which may be a hardware / software link to a storage device that stores the encoded video data. The transmitter (640) can merge the coded video data from the video coder (603) with other data to be transmitted, such as coded audio data and / or auxiliary data streams (sources not shown).

[0064] The controller (650) can manage the operation of the video encoder (603). During coding, the controller (650) can assign a particular coded picture type to each coded picture, which may affect the coding techniques that may be applied to the respective picture. For example, pictures may often be assigned as one of the following picture types:

[0065] An intra picture (I-picture) may be one that can be coded and decoded without using any other picture in a sequence as a predictor. Some video codecs allow different types of intra pictures, including, for example, independent decoder refresh ("IDR") pictures. Those skilled in the art are aware of these variations of I-pictures and their respective uses and characteristics.

[0066] A predicted image (P picture) may be encoded and decoded using intra- or inter-prediction, which uses at most one motion vector and reference index to predict the sample values ​​of each block.

[0067] Bidirectionally predicted images (B-pictures) may be encoded and decoded using intra- or inter-prediction, which uses up to two motion vectors and reference indices to predict the sample values ​​of each block. Similarly, multi-predicted images may use more than two reference images and associated metadata for the reconstruction of a single block.

[0068] A source picture may generally be spatially subdivided into multiple sample coding blocks (e.g., blocks of 4x4, 8x8, 4x8, or 16x16 samples each) and coded block by block. Blocks may be predictively coded with reference to other (already coded) blocks as determined by the coding assignment applied to the block's respective picture. For example, blocks of an I picture may be non-predictively coded or predictively coded with reference to already coded blocks of the same picture (spatial or intra prediction). Pixel blocks of a P picture may be predictively coded via spatial prediction or via temporal prediction with reference to one previously coded reference picture. Blocks of a B picture may be predictively coded by spatial prediction or via temporal prediction with reference to one or two previously coded reference pictures. Source pictures or intermediate processed pictures may be subdivided into other types of blocks for other purposes. The division of coding blocks and other types of blocks may or may not follow the same method, as described in more detail below.

[0069] The video encoder (603) may perform coding operations in accordance with a given video coding technique or standard, such as ITU-T Rec. H.265. In its operations, the video encoder (603) may perform various compression operations, including predictive coding operations that exploit temporal and spatial redundancy in the input video sequence. Thus, the coded video data may conform to a syntax specified by the video coding technique or standard being used.

[0070] In some example embodiments, the transmitter (640) may transmit additional data along with the encoded video. The source coder (630) may include such data as part of the coded video sequence. The additional data may include temporal / spatial / SNR enhancement layers, other forms of redundant data such as redundant pictures or slices, SEI messages, VUI parameter set fragments, etc.

[0071] Video may be captured as multiple source pictures (video pictures) in a time sequence. Intra-picture prediction (often abbreviated as intra-prediction) exploits spatial correlation in a given picture, while inter-picture prediction exploits temporal or other correlation between pictures. For example, a particular picture being coded / decoded, called the current picture, may be divided into blocks. If a block in the current picture resembles a reference block in a previously coded yet buffered reference picture in the video, it may be coded by a vector, called a motion vector. A motion vector points to a reference block in a reference picture and may have a third dimension that identifies the reference picture if multiple reference pictures are used.

[0072] In some exemplary embodiments, bi-prediction techniques can be used for inter-picture prediction. According to such bi-prediction techniques, two reference pictures are used, such as a first reference picture and a second reference picture, both of which follow the current picture in the video in decoding order (but may be in the past or future, respectively, in display order). A block in the current picture can be coded by a first motion vector that points to a first reference block in the first reference picture and a second motion vector that points to a second reference block in the second reference picture. A block can be jointly predicted by a combination of the first reference block and the second reference block.

[0073] Additionally, merge mode techniques may be used to improve coding efficiency in inter-picture prediction.

[0074] According to some exemplary embodiments of the present disclosure, predictions such as inter-picture prediction and intra-picture prediction are performed on a block-by-block basis. For example, a picture in a sequence of video pictures is divided into coding tree units (CTUs) for compression, and the CTUs in a picture may have the same size, such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTU may include three parallel coding tree blocks (CTBs), namely, one luma CTB and two chroma CTBs. Each CTU may be recursively quad-tree partitioned into one or more coding units (CUs). For example, a CTU of 64×64 pixels may be partitioned into one CU of 64×64 pixels, or four CUs of 32×32 pixels. Each of one or more of the 32×32 blocks may be further partitioned into four CUs of 16×16 pixels. In some example embodiments, each CU may be analyzed during encoding to determine the encoding of that CU among various prediction types, such as inter prediction type and intra prediction type. A CU may be divided into one or more prediction units (PUs) according to temporal predictability and / or spatial predictability. In general, each PU includes one luma prediction block (PB) and two chroma PBs. In one embodiment, the prediction operation in coding (encoding / decoding) is performed on a prediction block basis. The division of a CU into PUs (or PBs of different color channels) may be performed in various spatial patterns. A luma PB or a chroma PB may include a matrix of sample values ​​(e.g., luma values), such as, for example, 8×8 pixels, 16×16 pixels, 8×16 pixels, 16×8 pixels, etc.

[0075] 7 shows a diagram of a video encoder (703) according to another exemplary embodiment of this disclosure. The video encoder (703) is configured to receive a processed block (e.g., a predictive block) of sample values ​​in a current video picture in a sequence of video pictures and to encode the processed block into a coded picture that is part of a coded video sequence. The exemplary video encoder (703) may be used in place of the example video encoder (403) of FIG. 4.

[0076] For example, the video encoder (703) receives a matrix of sample values ​​for a processing block, such as a prediction block of 8x8 samples. The video encoder (703) then determines, for example using rate distortion optimization (RDO), whether the processing block is best coded using intra-mode, inter-mode, or bi-predictive mode. If it is determined that the processing block is coded in intra-mode, the video encoder (703) may code the processing block into a coded picture using intra-prediction techniques, and if it is determined that the processing block is coded in inter-mode or bi-predictive mode, the video encoder (703) may code the processing block into a coded picture using inter-prediction techniques or bi-prediction techniques, respectively. In some exemplary embodiments, a merge mode may be used as a sub-mode of inter-picture prediction, in which a motion vector is derived from one or more motion vector predictors without the benefit of coded motion vector components outside the predictors. In some other exemplary embodiments, there may be motion vector components applicable to the current block. Accordingly, the video encoder (703) may include components not explicitly shown in FIG. 7, such as a mode decision module, to determine the prediction mode of a processing block.

[0077] In the example of FIG. 7, the video encoder (703) includes an inter-encoder (730), an intra-encoder (722), a residual calculator (723), a switch (726), a residual encoder (724), a general controller (721), and an entropy encoder (725), coupled together as shown in the exemplary configuration of FIG.

[0078] The inter encoder (730) is configured to receive samples of a current block (e.g., a processing block), compare the block to one or more reference blocks in a reference picture (e.g., blocks in previous and subsequent pictures in display order), generate inter prediction information (e.g., a description of redundancy information, motion vectors, merge mode information according to an inter coding technique), and calculate an inter prediction result (e.g., a predicted block) based on the inter prediction information using any suitable technique. In some examples, the reference picture is a decoded reference picture that has been decoded based on the encoded video information using a decoding unit 633 incorporated in the example encoder 620 of FIG. 6 (shown as a residual decoder 728 of FIG. 7, as described in more detail below).

[0079] The intra encoder (722) is configured to receive samples of a current block (e.g., a processing block), compare the block to previously coded blocks in the same picture, generate transformed quantized coefficients, and possibly also generate intra prediction information (e.g., intra prediction direction information according to one or more intra encoding techniques). The intra encoder (722) may calculate an intra prediction result (e.g., a predicted block) based on the intra prediction information and a reference block in the same picture.

[0080] The generic controller (721) may be configured to determine generic control data and control other components of the video encoder (703) based on the generic control data. In one example, the generic controller (721) determines a prediction mode of the block and provides a control signal to the switch (726) based on the prediction mode. For example, if the prediction mode is an intra mode, the generic controller (721) controls the switch (726) to select an intra mode result for use by the residual calculator (723) and controls the entropy encoder (725) to select intra prediction information and include the intra prediction information in the bitstream, and if the predication mode of the block is an inter mode, the generic controller (721) controls the switch (726) to select an inter prediction result for use by the residual calculator (723) and controls the entropy encoder (725) to select inter prediction information and include the inter prediction information in the bitstream.

[0081] The residual calculator (723) may be configured to calculate a difference (residual data) between a received block and a prediction result for a block selected from the intra-encoder (722) or the inter-encoder (730). The residual encoder (724) may be configured to encode the residual data to generate transform coefficients. For example, the residual encoder (724) may be configured to transform the residual data from the spatial domain to the frequency domain to generate transform coefficients. The transform coefficients then undergo a quantization process to obtain quantized transform coefficients. In various exemplary embodiments, the video encoder (703) also includes a residual decoder (728). The residual decoder (728) is configured to perform an inverse transform and generate decoded residual data. The decoded residual data may be used by the intra-encoder (722) and the inter-encoder (730) as appropriate. For example, the inter-encoder (730) can generate decoded blocks based on the decoded residual data and the inter-prediction information, and the intra-encoder (722) can generate decoded blocks based on the decoded residual data and the intra-prediction information. The decoded blocks are appropriately processed to generate decoded pictures, which can be buffered in a memory circuit (not shown) and used as reference pictures.

[0082] The entropy encoder (725) is configured to format a bitstream to include the encoded blocks and to perform entropy coding. The entropy encoder (725) is configured to include various information in the bitstream. For example, the entropy encoder (725) may be configured to include general control data, selected prediction information (e.g., intra-prediction information or inter-prediction information), residual information, and other suitable information in the bitstream. Residual information may not be present when coding a block in a merged sub-mode of either an inter mode or a bi-prediction mode.

[0083] 8 shows a diagram of an example video decoder (810) according to another embodiment of the present disclosure. The video decoder (810) is configured to receive coded pictures that are part of a coded video sequence and decode the coded pictures to generate reconstructed pictures. In one example, the video decoder (810) may be used in place of the example video decoder (410) of FIG. 4.

[0084] In the example of FIG. 8, the video decoder (810) includes an entropy decoder (871), an inter-decoder (880), a residual decoder (873), a reconstruction module (874), and an intra-decoder (872), all coupled together as shown in the exemplary configuration of FIG. 8.

[0085] The entropy decoder (871) may be configured to recover from the coded picture certain symbols that represent syntax elements of which the coded picture is composed. Such symbols may include, for example, prediction information (e.g., intra-mode, inter-mode, bi-predictive mode, merged sub-mode, or another sub-mode) that may identify the mode in which the block is coded, certain samples or metadata used for prediction by the intra-decoder (872) or inter-decoder (880), residual information, for example in the form of quantized transform coefficients, etc. In one example, if the prediction mode is an inter-mode or bi-predictive mode, the inter-prediction information is provided to the inter-decoder (880), and if the prediction type is an intra-prediction type, the intra-prediction information is provided to the intra-decoder (872). The residual information may undergo inverse quantization and is provided to the residual decoder (873).

[0086] The inter decoder (880) may be configured to receive the inter prediction information and generate inter prediction results based on the inter prediction information.

[0087] The intra decoder (872) may be configured to receive intra prediction information and to generate a prediction result based on the intra prediction information.

[0088] The residual decoder (873) may be configured to perform inverse quantization to extract inverse quantized transform coefficients, and process the inverse quantized transform coefficients to transform the residual from the frequency domain to the spatial domain. The residual decoder (873) may also utilize certain control information (to include quantization parameters (QP)), which may be provided by the entropy decoder (871) (datapath not shown as this may only be a small amount of control information).

[0089] The reconstruction module (874) may be configured to combine, in the spatial domain, the residual as output by the residual decoder (873) and the prediction result (possibly as output by an inter-prediction module or an intra-prediction module) to form a reconstructed block that forms part of a reconstructed picture as part of the reconstructed video. It should be noted that other suitable operations, such as deblocking operations, may also be performed to improve visual quality.

[0090] It should be noted that the video encoders (403), (603), and (703) and the video decoders (410), (510), and (810) may be implemented using any suitable technique. In some exemplary embodiments, the video encoders (403), (603), and (703) and the video decoders (410), (510), and (810) may be implemented using one or more integrated circuits. In another embodiment, the video encoders (403), (603), and (603) and the video decoders (410), (510), and (810) may be implemented using one or more processors executing software instructions.

[0091] Returning to the intra prediction process, samples in a block (e.g., a luma or chroma prediction block, or a coding block if not further divided into prediction blocks) are predicted by samples of adjacent lines, next adjacent lines, or one or more other lines, or a combination thereof, to generate a prediction block. The residual between the actual block being coded and the prediction block may be processed by a transform followed by quantization. Various intra prediction modes may be made available, and parameters related to intra prediction mode selection and other parameters may be signaled in the bitstream. The various intra prediction modes may relate, for example, to one or more line positions for predicting samples, the direction in which the prediction samples are selected from predicting one or more lines, and other special intra prediction modes.

[0092] For example, the set of intra-prediction modes (interchangeably referred to as "intra modes") may include a predetermined number of directional intra-prediction modes. As described above in connection with the example implementation of FIG. 1, these intra-prediction modes may correspond to a predetermined number of directions in which an out-of-block sample is selected as a prediction of a sample predicted within a particular block. In another particular example implementation, eight main directional modes may be supported and predefined, corresponding to angles from 45 degrees to 207 degrees relative to the horizontal axis.

[0093] In some other implementations of intra prediction, the directional intra modes may be further extended to angle sets with finer granularity to further exploit more diverse spatial redundancy in the directional texture. For example, the above 8-angle implementation may be configured to provide eight nominal angles, designated V_PRED, H_PRED, D45_PRED, D135_PRED, D113_PRED, D157_PRED, D203_PRED, and D67_PRED, as shown in FIG. 9, and a predetermined number (e.g., 7) finer angles may be added for each nominal angle. Such extensions allow a larger total number (e.g., 56 in this example) of directional angles to be utilized for intra prediction, corresponding to the same number of predetermined directional intra modes. The prediction angle may be represented by the nominal intra angle plus an angle delta. In the above specific example with seven finer angle directions for each nominal angle, the angle delta may be -3 to 3 times the step size of 3 degrees.

[0094] In some implementations, instead of or in addition to the above directional intra modes, a certain number of non-directional intra prediction modes may also be predefined and made available. For example, five non-directional intra modes called smooth intra prediction modes may be specified. These non-directional intra mode prediction modes may be specifically called DC, PAETH, SMOOTH, SMOOTH_V, and SMOOTH_H intra modes. Prediction of samples of a particular block under these exemplary non-directional modes is shown in FIG. 10. As an example, FIG. 10 shows a 4×4 block 1002 predicted by samples from an upper neighboring line and / or a left neighboring line. A particular sample 1010 in the block 1002 may correspond to a directly above sample 1004 of the sample 1010 in the upper neighboring line of the block 1002, a left above sample 1006 of the sample 1010 as the intersection of the upper neighboring line and the left neighboring line, and a directly left sample 1008 of the sample 1010 in the left neighboring line of the block 1002. In the exemplary DC intra prediction mode, the average of the left and top neighboring samples 1008 and 1004 may be used as a predictor of the sample 1010. In the exemplary PAETH intra prediction mode, the top, left, and top-left reference samples 1004, 1008, and 1006 may be fetched, and then whichever value among these three reference samples is closest to (top+left-top-left) may be set as the predictor of the sample 1010. In the example of the SMOOTH_V intra prediction mode, the sample 1010 may be predicted by quadratic interpolation in the vertical direction of the top-left neighboring sample 1006 and the left neighboring sample 1008. For the exemplary SMOOTH_H intra prediction mode, the sample 1010 may be predicted by quadratic interpolation in the horizontal direction of the top-left neighboring sample 1006 and the top neighboring sample 1004. For the exemplary SMOOTH intra prediction mode, the sample 1010 may be predicted by an average of quadratic interpolation in the vertical and horizontal directions. The above implementation of the non-directional intra mode is merely given as a non-limiting example, other adjacent lines and other non-directional selections of samples and methods of combining prediction samples to predict a particular sample within a prediction block are also possible.

[0095] The selection of a particular intra-prediction mode by the encoder from the above directional or non-directional modes at various coding levels (picture, slice, block, unit, etc.) may be signaled in the bitstream. In some example implementations, the example eight nominal directional modes may be signaled first along with the five non-angle smooth modes (a total of 13 options). Then, if the signaled mode is one of the eight nominal angle intra modes, an index is further signaled to indicate the selected angle delta to the corresponding signaled nominal angle. In some other example implementations, all intra-prediction modes may be indexed together (e.g., 56 directional modes plus 5 non-directional modes to generate 61 intra-prediction modes) for signaling.

[0096] In some example implementations, the example 56 or other number of directional intra-prediction modes may be implemented using a unified directional predictor that projects each sample of a block to a reference subsample position and interpolates the reference sample by a 2-tap bilinear filter.

[0097] In some implementations, additional filter modes, called FILTER INTRA modes, can be designed to capture the decaying spatial correlation with the reference on the edge. In these modes, in addition to the out-of-block samples, the prediction samples within the block can be used as the intra prediction reference samples for some patches within the block. These modes can be, for example, predefined and made available for intra prediction of at least the luma block (or only the luma block). A predefined number (e.g., 5) of filter intra modes can be predesigned, each of which is represented by a set of n-tap filters (e.g., 7-tap filters) that reflect the correlation between a sample in a 4×2 patch and its n neighbors. In other words, the weight coefficients of the n-tap filters can be position-dependent. Taking 8×8 blocks, 4×2 patches, and 7-tap filtering as an example, the 8×8 block 1102 may be divided into eight 4×2 patches, as shown in FIG. 11. These patches are shown as B0, B1, B1, B3, B4, B5, B6, and B7 in FIG. 11. For each patch, its seven neighboring patches, denoted by R0-R7 in FIG. 11, can be used to predict the samples in the current patch. For patch B0, all neighboring elements may already be reconstructed. However, for other patches, some of the neighboring elements may be in the current block and therefore not reconstructed, in which case the predicted values ​​of the nearest neighboring elements are used as a reference. For example, all neighboring elements of patch B7 as shown in FIG. 11 have not been reconstructed, so the predicted samples of the neighboring elements of patch B7 are used instead.

[0098] In some implementations of intra prediction, one color component may be predicted using one or more other color components. The color components may be any components of YCrCb, RGB, XYZ color space, etc. For example, prediction of a chroma component (e.g., a chroma block) from a luma component (e.g., a luma reference sample), called chroma from luma, or CfL, may be performed. In some example implementations, most cross-color predictions are only allowed from luma to chroma. For example, a chroma sample in a chroma block may be modeled as a linear function of the corresponding reconstructed luma sample. CfL prediction may be performed as follows: CfL(α)=α×L AC +DC (1)

[0099] In the formula, L AC α represents the AC contribution of the luma component, α represents a parameter of the linear model, and DC represents the DC contribution of the chroma component. For example, an AC component is obtained for each sample of the block, and a DC component is obtained for the entire block. Specifically, the reconstructed luma samples may be subsampled to the chroma resolution, and then the average luma value (DC of luma) may be subtracted from each luma value to form the AC contribution in luma. The luma AC contribution is then used in the linear mode of equation (1) to predict the AC value of the chroma component. Instead of requiring the decoder to calculate scaling parameters to approximate or predict the chroma AC component from the luma AC contribution, an exemplary CfL implementation can determine the parameter α based on the original chroma samples and signal them in the bitstream. This reduces the complexity of the decoder and results in a more accurate prediction. As for the DC contribution of the chroma component, it may be calculated using an intra DC mode in the chroma content in some exemplary implementations.

[0100] In some example implementations of reference lines, multi-line intra prediction may be used. In these implementations, two or more reference lines may be available for selection in intra prediction, and the encoder determines and signals the reference line used to generate the intra predictor. The reference line index may be signaled before the intra prediction mode, and only the most probable mode is allowed if a non-zero reference line index is signaled. With reference to FIG. 15, an example of four reference lines (reference line 0 to reference line 3) is shown, and each reference line is composed of six segments, namely segments A to F (indicated as 1502 to 1512), along with a top-left reference sample. Furthermore, segments A and F may be padded with the nearest samples from segments B and E, respectively.

[0101] A transform of the residual of either the intra-predicted block or the inter-predicted block may then be performed, followed by quantization of the transform coefficients. For the purpose of performing the transform, both intra-coded and inter-coded blocks may be further divided into multiple transform blocks (the term "unit transform" is usually used to represent a collection of three color channels, but may also be used interchangeably as "transform unit", e.g., a "coding unit" includes a luma coding block and a chroma coding block) before the transform. In some implementations, a maximum division depth of the coding block (or prediction block) may be specified (the term "coding block" may be used interchangeably with "coding block"). For example, such division may not exceed two levels. The division of the prediction block into transform blocks may be handled differently for intra-predicted and inter-predicted blocks. However, in some implementations, such division may be similar between intra-predicted and inter-predicted blocks.

[0102] In some example implementations, for intra-coding blocks, transform partitioning may be done such that all transform blocks have the same size, and the transform blocks are coded in raster scan order. An example of such transform block partitioning of an intra-coding block is shown in FIG. 12. Specifically, FIG. 12 shows that a coding block 1202 is divided into 16 transform blocks of the same block size via a mid-level quadtree partition 1204, as indicated by 1206. An example raster scan order for coding is indicated by the ordered arrows in FIG. 12.

[0103] In some example implementations, and for inter-coding blocks, transform unit partitioning may be done recursively with partitioning depth up to a predetermined number of levels (e.g., two levels). As shown in FIG. 13, partitioning can stop or continue recursively at any level for any sub-partition. In particular, FIG. 13 illustrates an example where a block 1302 is partitioned into four quadtree sub-blocks 1304, one of the sub-blocks is further partitioned into four second-level transform blocks, while the partitioning of the other sub-block stops after the first level, resulting in a total of seven transform blocks of two different sizes. An example raster scan order for coding is further illustrated by the ordered arrows in FIG. 13. Although FIG. 13 illustrates an example implementation of quadtree partitioning of square transform blocks up to two levels, in some generating implementations, transform partitioning can support 1:1 (square), 1:2 / 2:1, and 1:4 / 4:1 transform block shapes and sizes ranging from 4×4 to 64×64. In some example implementations, if the coding block is 64x64 or smaller, transform block partitioning may be applied only to the luma component (in other words, the chroma transform block is the same as the coding block under that condition). Otherwise, if the width or height of the coding block is greater than 64, both the luma coding block and the chroma coding block may be implicitly partitioned into multiples of min(W,64)xmin(H,64) and min(W,32)xmin(H,32) transform blocks, respectively.

[0104] In some example implementations, as shown in FIG. 16, another alternative example scheme for partitioning a coding block or a predictive block into transform blocks is further illustrated. As shown in FIG. 16, instead of using recursive transform partitioning, a predefined set of partition types may be applied to a coding block according to the transform type of the coding block. In the particular example shown in FIG. 16, one of six example partition types may be applied to partition a coding block into various numbers of transform blocks. Such a scheme may be applied to either a coding block or a predictive block. In this disclosure, the term "partition type" may generally refer to the manner in which a block (e.g., a predictive block or a coding block) is partitioned, and may refer to a "transform partition type", a "predictive block partition type", or a "coding block partition type". Furthermore, for the description under "transform partition type", the same concept also applies to "coding block partition type", and vice versa.

[0105] More specifically, the partitioning scheme of FIG. 16 provides up to six partition types for any given transform type, as shown in FIG. 16. In this scheme, every coding block or predictive block may be assigned a transform type, for example, based on a rate-distortion cost. In one example, the partition type assigned to a coding block or predictive block may be determined based on the transform type of the coding block or predictive block. As shown by the four partition types illustrated in FIG. 16, a particular partition type may correspond to the partition size and pattern (or partition type) of the transform block. The correspondence between various transform types and various partition types may be predefined. An exemplary correspondence is shown below with capitalized labels indicating the transform types that may be assigned to a coding block or predictive block based on the rate-distortion cost.

[0106] ·PARTITION_NONE: Allocate the transformation size equal to the block size.

[0107] ·PARTITION_SPLIT: Allocates a transformation size that is 1 / 2 the width and 1 / 2 the height of the block size.

[0108] ·PARTITION_HORZ: Allocates a transformation size with the same width as the block size and 1 / 2 the height of the block size.

[0109] ·PARTITION_VERT: Allocates a transformation size that is half the width of the block size and the same height as the block size.

[0110] ·PARTITION_HORZ4: Allocates a transformation size with the same width as the block size and 1 / 4 of the block size in height.

[0111] ·PARTITION_VERT4: Allocates a transformation size that is 1 / 4 the width of the block size and the same height as the block size.

[0112] In the above example, all the partition types shown in Figure 16 include uniform transform sizes for the partitioned transform blocks. This is not a limitation but merely an example. In some other implementations, mixed transform block sizes may be used for the partitioned transform blocks in a particular partition type (or pattern).

[0113] Then, each of the above obtained transform blocks may undergo a primary transform, which essentially moves the residuals in the transform block from the spatial domain to the frequency domain. In some implementations of the actual primary transform, multiple transform sizes (ranging from 4 points to 64 points for each of the two dimensions) and transform shapes (square; rectangular with width / height ratios of 2:1 / 1:2, and 4:1 / 1:4) may be allowed to support the above exemplary extended coding block partitions.

[0114] Turning to the actual primary transform, in some exemplary implementations, the 2D transform process may include the use of hybrid transform kernels, which may be composed of, for example, different 1-D transforms for each dimension of the coding residual transform block. Exemplary 1-D transform kernels may include, but are not limited to: a) 4-point, 8-point, 16-point, 32-point, 64-point DCT-2; b) 4-point, 8-point, 16-point asymmetric DST's (DST-4, DST-7) and their flipped versions; c) 4-point, 8-point, 16-point, 32-point equivalent transforms. The selection of the transform kernel used for each dimension may be based on a rate-distortion (RD) criterion. For example, the basis functions of DCT-2 and asymmetric DST's that may be implemented are listed in Table 1. [Table 1]

[0115] In some example implementations, the availability of hybrid transform kernels for a particular primary transform implementation can be based on transform block size and prediction mode. Examples of dependencies are listed in Table 2. For chroma components, transform type selection may be performed in an implicit manner. In an example, for intra prediction residuals, the transform type may be selected according to the intra prediction mode, as specified in FIG. 3. For inter prediction residuals, the transform type for a chroma block may be selected according to the transform type selection of the co-located luma block. Thus, for chroma components, there is no transform type signaling in the bitstream. [Table 2] [Table 3]

[0116] Some implementations may perform a secondary transform on the primary transform coefficients. For example, as shown in FIG. 14, a LFNST (low frequency separable transform), also known as a reduced secondary transform, may be applied between the forward primary transform and quantization (at the encoder) and between the inverse quantization and the inverse primary transform (at the decoder side) to further decorrelate the primary transform coefficients. In essence, the LFNST may take a portion of the primary transform coefficients, e.g., a low frequency portion (hence "reduced" from the complete set of primary transform coefficients of the transform block), to proceed to the secondary transform. In an example of LFNST, a 4×4 non-separable transform or an 8×8 non-separable transform may be applied according to the transform block size. For example, a 4×4 LFNST may be applied to a small transform block (e.g., min(width, height)<8), and an 8×8 LFNST may be applied to a large transform block (e.g., min(width, height)>8). For example, when an 8x8 transform block is subjected to a 4x4 LFNST, only the low frequency 4x4 portion of the 8x8 primary transform coefficients undergoes a further secondary transform.

[0117] As specifically shown in FIG. 14, a transform block may be 8×8 (or 16×16). Thus, a forward primary transform 1402 of the transform block generates an 8×8 (or 16×16) primary transform coefficient matrix 1404, with each square unit representing a 2×2 (or 4×4) portion. The input to the forward LFNST may not be, for example, the entire 8×8 (or 16×16) primary transform coefficients. For example, a 4×4 (or 8×8) LFNST may be used for the secondary transform. Thus, as shown in the shaded area (top left) 1406, only the 4×4 (or 8×8) low frequency primary transform coefficients of the primary transform coefficient matrix 1404 may be used as input to the LFNST. The remaining portion of the primary transform coefficient matrix may not undergo a secondary transform. In this way, after the secondary transform, the portions of the primary transform coeffects affected by the LFNST become secondary transform coefficients, while the remaining portions not affected by the LFNST (e.g., the unshaded portions of matrix 1404) retain their corresponding primary transform coefficients. In some example implementations, the remaining portions not subject to the secondary transform may be set to all zero coefficients.

[0118] An example application of the non-separable transform used in LFNST is described below. To apply the exemplary 4×4 LFNST, a 4×4 input block X (representing, for example, a 4×4 low frequency portion of a block of primary transform coefficients such as the shaded portion 1406 of the primary transform matrix 1404 in FIG. 14) can be expressed as follows:

number

[0119] This 2D input matrix is ​​first linearized or converted to a vector in the exemplary order:

number

number

[0120] The inseparable transformation of the 4×4 LFNST is then [Number] can be calculated as [Number] indicates the output conversion coefficient vector, and T is a 16×16 conversion matrix. The obtained 16×1 coefficient vector [Number] is then back-scanned as a 4×4 block using the scan order of that block (e.g., horizontal, vertical, or diagonal). Coefficients with smaller indices can be placed at coefficients with smaller scan indices within the 4×4 coefficient block. In this way, the redundancy of the primary conversion coefficient X can be further exploited via the second conversion T, thereby providing additional compression enhancement.

[0121] The above exemplary LFNST is based on a direct matrix multiplication approach for applying inseparable transformations and as a result is performed in a single pass without multiple iterations. In some further exemplary implementations, the dimension of the inseparable transformation matrix (T) of the example 4×4 LFNST can be further reduced in order to minimize the computational complexity and memory space requirements for storing the conversion coefficients. Such an implementation can be referred to as a reduced separable transform (RST). More specifically, the main concept of RST is to map an N (where N is 4×4 = 16 in the above example but could be equal to 64 for an 8×8 block) dimensional vector to an R dimensional vector in a different space, and N / R (R < N) represents the dimensionality reduction factor. Thus, instead of an N×N conversion matrix, the RST matrix becomes an R×N matrix as follows. [Number]

[0122] In Equation 7, the R rows of the transformation matrix are a reduced R basis of the N-dimensional space. Thus, the transformation transforms an input vector or N dimensions into a reduced R-dimensional output vector. Thus, as shown in Figure 14, the secondary transform coefficients 1408 transformed from the primary coefficients 1406 are reduced in dimension by a factor or N / R. The three squares around 1408 in Figure 14 may be padded with zeros.

[0123] The inverse transform matrix of the RTS may be the transpose of its forward transform. For an exemplary 8×8 LFNST (contrasted with the 4×4 LFNST above for more detailed explanation here), an exemplary reduction factor of 4 may be applied, and thus the 64×64 direct non-separable transform matrix is ​​correspondingly reduced to a 16×64 direct matrix. Furthermore, in some implementations, some, but not all, of the input primary coefficients may be linearized into the input vectors of the LFNST. For example, only some of the example 8×8 input primary transform coefficients may be linearized into the X vector above. In a particular example, of the four 4×4 quadrants of the 8×8 primary transform coefficient matrix, the bottom right (high frequency coefficients) may be excluded, and only the other three quadrants are linearized into a 64×1 vector using a predetermined scan order rather than a 48×1 vector. In such implementations, the non-separable transform matrix may be further reduced from 16×64 to 16×48.

[0124] Thus, the exemplary reduced 48×16 inverse RST matrix can be used at the decoder side to generate the top-left, top-right, and bottom-left 4×4 quadrants of the 8×8 core (primary) transform coefficients. Specifically, if a further reduced 16×48 RST matrix is ​​applied instead of a 16×64 RST with the same transform set configuration, the non-separable secondary transform takes as input 48 matrix elements vectorized from the three 4×4 quadrant blocks of the 8×8 primary coefficient block, excluding the bottom-right 4×4 block. In such an implementation, the omitted bottom-right 4×4 primary transform coefficient is ignored in the secondary transform. This further reduced transform converts the 48×1 vector into a 16×1 output vector, which is inverse scanned into a 4×4 matrix to fill 1408 in FIG. 14. The three squares of secondary transform coefficients surrounding 1408 may be zero-padded.

[0125] With the help of such a reduction in the dimensions of the RST, the memory usage for storing all the LFNST matrices is reduced. In the above example, for example, the memory usage can be reduced from 10 KB to 8 KB with a reasonably small performance degradation compared to an implementation without dimensionality reduction.

[0126] In some implementations, to reduce complexity, LFNST may be further restricted to be applicable only if all coefficients outside the portion of primary transform coefficients subject to LFNST (e.g., outside 1406 portion of 1404 in FIG. 14) are not significant. Thus, when LFNST is applied, all primary-only transform coefficients (e.g., the unshaded portion of primary coefficient matrix 1404 in FIG. 14) may be close to zero. Such a restriction allows for adjustment of LFNST index signaling at the least significant positions, thus avoiding additional coefficient scans that may be required to check for significant coefficients at certain positions when this restriction does not apply. In some implementations, the worst-case processing (in terms of multiplications per pixel) of LFNST may be restricted to non-separable transforms of 4×4 and 8×8 blocks to 8×16 and 8×48 transforms, respectively. In these cases, for other sizes less than 16, the final significant scan position must be less than 8 when LFNST is applied. For blocks with shapes of 4×N and N×4 and N>8, the above restriction means that LFNST is no longer applied once to the top-left 4×4 region. Since all primary-only coefficients are 0 when LFNST is applied, the number of operations required for the primary transform is reduced in such cases. From the encoder's point of view, the quantization of coefficients can be simplified when the LFNST transform is tested. Rate-distortion optimized quantization (RDO) can be done on the first 16 coefficients, at most (in scan order), and the remaining coefficients can be set to 0.

[0127] In some example implementations, the available RST kernels may be specified as several transform sets, with each transform set including several non-separable transform matrices. For example, there may be a total of four transform sets and two non-separable transform matrices (kernels) for each transform set used in LFNST. These kernels may be pre-trained offline and thus data driven. The offline trained transform kernels may be stored in memory or hard-coded into the encoding or decoding device for use during the encoding / decoding process. The selection of the transform set during the encoding or decoding process may be determined by the intra prediction mode. The mapping from intra prediction mode to transform sets may be predefined. An example of such a predefined mapping is shown in Table 4. For example, as shown in Table 4, if one of the three cross-component linear model (CCLM) modes (INTRA_LT_CCLM, INTRA_T_CCLM, or INTRA_L_CCLM) is used for the current block (i.e., 81<=predModeIntra<=83), transform set 0 may be selected for the current chroma block. For each transform set, the selected non-separable secondary transform candidates may be further specified by an explicitly signaled LFNST index, e.g., the index may be signaled in the bitstream once per intra CU after the transform coefficients. [Table 4]

[0128] Since LFNST is restricted in the above implementations to be applicable only when all coefficients outside the first coefficient subgroup or portion are not significant, the coding of the LFNST index may depend on the position of the lowest coefficient. Furthermore, the LFNST index may be context coded, but it is independent of the intra prediction mode, and only the first bin may be context coded. Furthermore, LFNST may be applied to intra CUs in both intra and inter slices, and both luma and chroma. If dual tree is enabled, the LFNST indexes for luma and chroma may be signaled separately. For inter slices (dual tree is disabled), a single LFNST index is signaled and used for both luma and chroma.

[0129] In some example implementations, when an intra sub-partitioning (ISP) mode is selected, LFNST may be disabled and RST index may not be signaled because performance improvement is likely to be limited even if RST is applied to all feasible partition blocks. Furthermore, disabling RST of ISP prediction residuals may reduce coding complexity. In some further implementations, when a multiple linear regression intra prediction (MIP) mode is selected, LFNST may also be disabled and RST index may not be signaled.

[0130] Considering that due to existing maximum transform size limitations (e.g., 64×64), large CUs larger than 64×64 (or any other predetermined size representing the maximum transform block size) are implicitly split (e.g., TU tiling), LFNST index lookup can increase data buffering by 4 times for a certain number of decoding pipeline stages. Thus, in some implementations, the maximum size allowed for LFNST may be limited to, for example, 64×64. In some implementations, LFNST may be enabled with DCT2 only as the primary transform.

[0131] In some other implementations, a secondary intra transform (IST) is provided for the luma component, for example, by defining 12 sets of secondary transforms, with, for example, three kernels in each set. An intra mode dependent index may be used for transform set selection. Kernel selection within a set may be based on a signaled syntax element. IST may be enabled when either DCT2 or ADST is used as both horizontal and vertical primary transforms. In some implementations, a 4×4 non-separable transform or an 8×8 non-separable transform may be selected according to the block size. If min(tx_width,tx_height)<8, a 4×4 IST may be selected. For larger blocks, an 8×8 IST may be used, where tx_width&tx_height correspond to the width and height of the transform block, respectively. The input to the IST may be low frequency primary transform coefficients in zigzag scan order.

[0132] Various transforms in a video coding or decoding process, e.g., either the primary transform of samples in a residual block or the secondary transform of blocks of a primary transform coefficient process, may not always be efficient in capturing directional texture patterns, such as edges that are oriented at 45 degrees (e.g., substantially away from the horizontal or vertical directions), when using only separable transform schemes. As mentioned above, in some example implementations, one or more non-separable transform designs may be used for the secondary transform of the primary transform coefficients.

[0133] The transform block partitioning and transform types applied to the partitioned transform blocks may be interrelated. For example, certain transform types may be more suitable for certain partition types. For example, the transform partitioning scheme illustrated in FIG. 16 and described above presents a non-recursive partition type compared to a recursive partition such as the partition described above in FIG. 13. If all available transform types were permitted for the partitioned transform blocks under all available partition patterns (e.g., the transform partition types of FIG. 16), the encoder would need to perform optimization in a large parameter space when determining which transform partition types to use to obtain the transform blocks and which transform types to use for each partitioned transform block. In practice, a certain set of transform types may generally be more suitable than other transform types for a certain type of transform partition type. In various implementations described below, the interaction between transform partition types and transform types may be considered and used to arrive at a scheme that restricts the transform types permitted for a certain partition type, as well as restricting the partition types permitted for a certain transform type. Such an implementation may enable a reduction in optimization space for the encoder when determining the transform partition pattern and transform type selection for each split transform block, especially in situations where non-recursive transform partitioning is applied.

[0134] These example implementations may be used separately or combined in any order or manner. In the above and following discussion, terms such as "coded block", "coding block" and the like may be used to refer to a picture unit on which prediction or transformation is performed. A coding block may be a luma coding block or a chroma coding block. In some circumstances, a coded / coding block may refer to a prediction block. The term "block size" is used to refer to the width or height of a coding block, or the maximum value of the width and height, or the minimum value of the width and height, or the size of a region (width*height), or the aspect ratio (width:height, or height:width).

[0135] Multiple candidate primary conversion types

[0136] In one embodiment, there may be multiple candidate primary transform types for a block. The block may include transform blocks resulting from partitions. The selection and / or signaling of the primary transform type may be limited to a predefined set of transform partition types. The predefined set of transform partition types may be a subset of a larger set of available transform partition types (e.g., the full set of partition types in FIG. 16). In other words, the selection of the primary transform is made and signaled only if the transform partition type of the block belongs to the predefined set of transform partition types. Otherwise, for example, if the transform partition type is of another type, a default transform type may be used instead of being selected and signaled.

[0137] In one implementation, the primary transform type may include at least one of: discrete cosine transform (DCT) type 1 to DCT type 8; asymmetric discrete sine transform (ADST); discrete sine transform (DST) type 1 to DST type 8; line graph transform (LGT); or Karhunen-Loeve transform (KLT).

[0138] In one implementation, the predefined set of transform partition types includes only PARTITION_NONE, i.e., the transform block size is equal to the prediction block (or coding block) size, for example, of the various transform partition types of Figure 16. Thus, a primary transform type may be selected and / or signaled only if the transform type belongs to this predefined set.

[0139] In one implementation, the number of partitions may also be taken into consideration to determine a predefined set of transform partition types. For example, only if the number of partitions of a particular transform partition type is equal to or less than a predefined threshold, the primary transform type is considered to be part of a predefined set of transform partition types from which it may be selected and / or signaled. In one implementation, the predefined threshold may be an integer between 1 and 16.

[0140] Multiple possible secondary transformation types

[0141] In one embodiment, there may be multiple candidate secondary transform types for a block. The block may include transform blocks resulting from partitions. The selection and / or signaling of the secondary transform type may apply only to a predefined set of transform partition types. The predefined set of transform partition types may be a subset of a larger set of available transform partition types (e.g., the full set of transform partition types in FIG. 16). In other words, the selection of the secondary transform is made and signaled only if the transform partition type of the block belongs to the predefined set of transform partition types. Otherwise, for example, if the transform partition type is of another type, a default secondary transform type may be used instead of being selected and signaled, or a secondary transform may not be performed.

[0142] In one implementation, the secondary transform type may include a KLT, which may be composed of different kernels.

[0143] In one implementation, the predefined set of transform partition types may include only PARTITION_NONE, i.e., the transform block size is equal to the prediction block (or coding block) size, for example, of the various transform partition types of Figure 16. Thus, a secondary transform type may be selected and / or signaled only if the transform type belongs to this predefined set.

[0144] In one implementation, the number of partitions may also be taken into consideration to determine a predefined set of transform partition types. For example, only if the number of partitions of a particular transform partition type is equal to or less than a predefined threshold, the secondary transform type is considered to be part of a predefined set of transform partition types that may be selected and / or signaled. In one implementation, the predefined threshold may be an integer between 1 and 16.

[0145] In one implementation, the selection and / or signaling of the secondary transform type may be based on a combination of a transform partition type and a primary transform type. The combination may include a transform partition type in a predefined set of transform partition types and a primary transform type in a predefined set of transform types. For example, the secondary transform type may need to be selected and / or signaled only if the transform partition type is PARTITION_NONE and the primary transform type used for the block is DCT or ADST. Otherwise, for example, if the transform partition type is of another type, a default secondary transform type may be used instead of being selected and signaled, or a secondary transform may not be performed.

[0146] Transformation-related signaling

[0147] In this disclosure, various signaling mechanisms are disclosed that aim to increase signaling efficiency by taking into account the order of transform-related syntax elements / parameters.

[0148] In one embodiment, the transform partition type information may be signaled before the primary / secondary transform type selection information. The primary / secondary transform type selection needs to be signaled only when the transform partition belongs to a predefined set of transform partition types, e.g., PARTITION_NONE. Otherwise, if the transform partition does not belong to a predefined set of transform partition types, the primary / secondary transform type selection may not need to be signaled. Instead, the primary / secondary transform type may be derived as a predefined default transform type.

[0149] In one embodiment, the primary / secondary transform type selection information may be signaled before the transform partition type information, in which case the selection and / or signaling of the transform partition type may depend on the primary / secondary transform type selection information.

[0150] In one implementation, the transform partition type information may only need to be signaled if the primary transform type belongs to a predefined set of transform types. Otherwise, the transform partition type information may not need to be signaled. As an example, the predefined set of transform types may include, but is not limited to, DCT type 1 to DCT type 8, ADST, DST type 1 to DST type 8, LGT, and KLT.

[0151] In one implementation, the transform partition type information may only need to be signaled if the secondary transform type belongs to a predefined set of transform types. By way of example, the predefined set of transform types may include, but is not limited to, a specific KLT having a kernel associated with a predefined KLT index. Otherwise, if the secondary transform type does not belong to a predefined set of transform types, the transform partition type information may not need to be signaled. Instead, the transform partition type information may be derived as a predefined default transform partition type, such as PARTITION_NONE.

[0152] 17 shows an example method 1700 for decoding video data. The method 1700 includes the following steps: receiving a coded video bitstream of a data block at step 1710, extracting from the coded video bitstream a transform partition type associated with the data block at step 1720, extracting a transform type of a transform associated with a partitioned transform block from the data block as signaled in the coded video bitstream in response to the transform partition type belonging to a subset of a predefined set of transform partition types each specifying a partitioning pattern for partitioning the data block into transform blocks, the transform type belonging to a first predefined set of transform types, and performing an inverse transform on the transform block according to the transform type.

[0153] In embodiments of the present disclosure, any of the steps and / or actions may be combined or arranged in any quantity or order as desired. Two or more steps and / or actions may be performed in parallel.

[0154] The embodiments of the present disclosure may be used separately or combined in any order. Furthermore, each of the methods (or embodiments), the encoder, and the decoder may be implemented by a processing circuit (e.g., one or more processors or one or more integrated circuits). In one example, the one or more processors execute a program stored on a non-transitory computer-readable medium. The embodiments of the present disclosure may be applied to luma blocks or chroma blocks.

[0155] The techniques described above may be implemented as computer software using computer-readable instructions physically stored on one or more computer-readable media. For example, Figure 18 illustrates a computer system (1800) suitable for implementing certain embodiments of the disclosed subject matter.

[0156] The computer software may be coded using any suitable machine code or computer language that may undergo assembly, compilation, linking, or similar mechanisms to produce code containing instructions that may be executed, such as by one or more computer central processing units (CPUs) and / or graphics processing units (GPUs), either directly, or through interpretation and execution of microcode.

[0157] The instructions may be executed in various types of computers or components thereof including, for example, personal computers, tablet computers, servers, smart phones, gaming consoles, Internet of Things devices, and the like.

[0158] 18 with respect to computer system (1800) are exemplary in nature and are not intended to suggest any limitation as to the scope of use or functionality of the computer software implementing the embodiments of the present disclosure, nor should the arrangement of components be interpreted as having any dependency or requirement relating to any one or combination of components illustrated in the exemplary embodiment of computer system (1800).

[0159] The computer system (1800) may include certain human interface input devices. Such human interface input devices may respond to input by one or more human users, for example, via tactile input (keystrokes, swipes, data glove movements, etc.), audio input (voice, clapping, etc.), visual input (gestures, etc.), and olfactory input (not shown). The human interface devices may also be used to capture certain media that are not necessarily directly related to conscious human input, such as audio (speech, music, ambient sounds, etc.), images (scanned images, photographic images obtained from a still image camera, etc.), and video (two-dimensional video, three-dimensional video including stereoscopic video, etc.).

[0160] The input human interface devices may include one or more (one of each is shown) of a keyboard (1801), a mouse (1802), a trackpad (1803), a touch screen (1810), a data glove (not shown), a joystick (1805), a microphone (1806), a scanner (1807), and a camera (1808).

[0161] The computer system (1800) may also include certain human interface output devices. Such human interface output devices may stimulate one or more of the senses of a human user, for example, through haptic output, sound, light, and smell / taste. Such human interface output devices may include haptic output devices (e.g., haptic feedback via a touch screen (1810), data gloves (not shown), or joystick (1805), although there may also be haptic feedback devices that do not function as input devices), audio output devices (such as speakers (1809), headphones (not shown)), visual output devices (such as screens (1810) including CRT screens, LCD screens, plasma screens, OLED screens (each with or without touch screen input capability, each with or without haptic feedback capability, some of which may be capable of outputting two-dimensional visual output, or output in more than three dimensions by means of stereographic output, etc.), virtual reality glasses (not shown), holographic displays, and smoke tanks (not shown)), and printers (not shown).

[0162] The computer system (1800) may also include human accessible storage devices and their associated media, such as optical media (1821) including CD / DVD ROM / RW (1820) having media such as CDs / DVDs, thumb drives (1822), removable hard drives or solid state drives (1823), legacy magnetic media such as tapes and floppy disks (not shown), dedicated ROM / ASIC / PLD based devices such as security dongles (not shown), etc.

[0163] Those skilled in the art should also understand that the term "computer-readable medium" as used in connection with the presently disclosed subject matter does not encompass transmission media, carrier waves, or other transitory signals.

[0164] The computer system (1800) may also include an interface (1854) to one or more communication networks (1855). The networks may be, for example, wireless, wired, optical. The networks may further be local, wide area, metropolitan, vehicular and industrial, real-time, delay tolerant, etc. Examples of networks include local area networks such as Ethernet, wireless LAN, cellular networks including GSM, 3G, 4G, 5G, LTE, etc., television wired or wireless wide area digital networks including cable television, satellite television, and terrestrial broadcast television, vehicular and industrial including CANbus, etc. Certain networks typically require an external network interface adapter attached to a particular general-purpose data port (e.g., a USB port of the computer system (1800)) or peripheral bus (1849), while other networks are typically integrated into the core of the computer system (1800) by attachment to a system bus described below (e.g., an Ethernet interface to a PC computer system or a cellular network interface to a smartphone computer system). Using any of these networks, computer system (1800) can communicate with other entities. Such communication may be one-way, receive only (e.g., television broadcast), one-way transmit only (e.g., a CANbus to a particular CANbus device), or two-way, e.g., to other computer systems using local or wide area digital networks. Specific protocols and protocol stacks may be used with each of these networks and network interfaces, as described above.

[0165] The aforementioned human interface devices, human accessible storage devices, and network interfaces may be connected to a core (1840) of the computer system (1800).

[0166] The cores (1840) may include one or more central processing units (CPUs) (1841), graphics processing units (GPUs) (1842), dedicated programmable processing units in the form of field programmable gate areas (FPGAs) (1843), hardware accelerators for specific tasks (1844), graphics adapters (1850), and the like. These devices may be connected via a system bus (1848) along with read only memory (ROM) (1845), random access memory (1846), internal mass storage devices such as internal hard drives that are not accessible to the user, SSDs, and the like (1847). In some computer systems, the system bus (1848) may be accessible in the form of one or more physical plugs to allow expansion with additional CPUs, GPUs, and the like. Peripheral devices may be connected directly to the system bus (1848) of the cores or may be connected via a peripheral bus (1849). In one example, a screen (1810) may be connected to the graphics adapter (1850). Peripheral bus architectures include PCI, USB, and the like.

[0167] The CPU (1841), GPU (1842), FPGA (1843), and accelerator (1844) can execute certain instructions that, in combination, can constitute the aforementioned computer code. The computer code can be stored in ROM (1845) or RAM (1846). Transient data can also be stored in RAM (1846), while permanent data can be stored, for example, in internal mass storage device (1847). Rapid storage and retrieval of any memory device can be made possible through the use of cache memory, which can be closely associated with one or more of the CPU (1841), GPU (1842), mass storage device (1847), ROM (1845), RAM (1846), etc.

[0168] The computer-readable medium can bear computer code for performing various computer-implemented operations. The medium and computer code may be those specially designed and constructed for the purposes of the present disclosure, or they may be of the type well known and available to those skilled in the computer software arts.

[0169] As a non-limiting example, a computer system (1800) having the architecture, and in particular a core (1840), may provide functionality as a result of a processor(s) (including CPUs, GPUs, FPGAs, accelerators, etc.) executing software embodied in one or more tangible computer-readable media. Such computer-readable media may be the user-accessible mass storage devices introduced above, as well as media associated with specific storage of the core (1840) that is non-transitory in nature, such as the core internal mass storage (1847) or ROM (1845). Software implementing various embodiments of the present disclosure may be stored in such devices and executed by the core (1840). The computer-readable media may include one or more memory devices or chips, depending on the particular needs. The software may cause the core (1840), and in particular the processors therein (including CPUs, GPUs, FPGAs, etc.) to perform certain processes or certain portions of certain processes described herein, including defining data structures stored in RAM (1846) and modifying such data structures according to the software-defined processes. Additionally, or alternatively, a computer system may provide functionality as a result of logic hardwired into circuitry or otherwise embodied therein (e.g., accelerator (1844)), which may operate in place of or in conjunction with software to perform particular processes or portions of particular processes described herein. References to software may encompass logic, and vice versa, where appropriate. Where appropriate, references to computer-readable media may encompass circuitry (such as an integrated circuit (IC)) that stores software for execution, circuitry that embodies logic for execution, or both. The present disclosure encompasses any appropriate combination of hardware and software.

[0170] While this disclosure has described several exemplary embodiments, there are modifications, substitutions, and various substitute equivalents which are within the scope of this disclosure. It will thus be appreciated that those skilled in the art will be able to devise numerous systems and methods which, although not explicitly shown or described herein, embody the principles of the present disclosure and are therefore within its spirit and scope. Appendix A: Acronyms JEM: Joint Exploration Model VVC: Versatile Video Coding BMS: Benchmark Set MV: Motion Vector HEVC: High Efficiency Video Coding SEI: Supplemental Enhancement Information VUI: Video Usability Information GOP: Group of Pictures TU: conversion unit PU: Prediction unit CTU: Coding Tree Unit CTB: coding tree block PB: Predicted block HRD: Hypothetical Reference Decoder SNR: Signal to Noise Ratio CPU: Central Processing Unit GPU: Graphics Processing Unit CRT: cathode ray tube LCD: Liquid crystal display OLED: Organic Light Emitting Diode CD:Compact Disc DVD: Digital Video Disc ROM: Read-Only Memory RAM: Random Access Memory ASIC: Application Specific Integrated Circuit PLD: Programmable Logic Device LAN: Local Area Network GSM: Global System for Mobile Communications LTE: Long-Term Evolution CANBus: Controller Area Network Bus USB: Universal Serial Bus PCI: Peripheral Component Interconnect FPGA: Field Programmable Gate Area SSD: Solid State Drive IC: Integrated Circuit HDR: High Dynamic Range SDR: Standard Dynamic Range JVET: Joint Video Exploration Team MPM: Most Probable Mode WAIP: Wide-angle Intra Prediction CU: coding unit PU: Prediction unit TU: conversion unit CTU: Coding Tree Unit PDPC: Position-dependent prediction combination ISP: Intra Subpartition SPS: Sequence Parameter Settings PPS: Picture Parameter Set APS: Adaptive Parameter Set VPS: Video Parameter Set DPS: Decoding Parameter Set ALF: Adaptive Loop Filter SAO: Sample Adaptive Offset CC-ALF: Cross-component adaptive loop filter CDEF: Constrained Directional Enhancement Filter CCSO: Cross component sample offset LSO: Local Sample Offset LR: Loop recovery filter AV1:AOMedia Video1 AV2:AOMedia Video2 [Explanation of symbols]

[0171] 101 Samples 102 Arrow 103 Arrow 104 Square Block 201 Block 300 Communication Systems 310 Terminal Equipment 320 Terminal Equipment 330 Terminal Equipment 350 Network 400 Communication Systems 401 Video Source 402 Stream 403 Video Encoder 404 Video Data, Video Bitstream 405 Streaming Server 406 Client Subsystem 407 Input copy, video data 410 Video Decoder 411 Output Stream 412 Display 413 Video Capture Subsystem 420 Electronic equipment 430 Electronic equipment 501 Channel 510 Video Decoder 512 Rendering devices, displays 515 Buffer Memory 520 Parser 521 Symbols 530 Electronic equipment 531 Receiver 551 Reverse conversion unit 552 Intra-picture prediction unit 553 Motion Compensation Prediction Unit 555 Aggregator 556 Loop Filter Unit 557 Reference Picture Memory 558 Picture Buffer 601 Video Sources 603 Video Coder, Video Encoder 620 Electronic equipment 630 Source Coder 632 Coding Engine 633 Local Video Decoder, Decoding Unit 634 Reference Picture Cache, Reference Picture Memory 635 Predictor 640 Transmitter 643 Video Sequences 645 Entropy Coder 650 Controller 660 Communication Channels 703 Video Encoder 721 General-purpose controller 722 Intra Encoder 723 Residual Calculator 724 Residual Encoder 725 Entropy Encoder 726 Switch 728 Residual Decoder 730 InterEncoder 810 Video Decoder 871 Entropy Decoder 872 Intra Decoder 873 Residual Decoder 874 Reconstruction Module 880 Interdecoder 1002 Block 1004 Top Sample 1006 Top left sample 1008 Left Sample 1010 Sample Block 1102 1202 coded blocks 1204 Quadtree splitting 1302 Block 1304 Quadtree Subblock 1402 Forward Primary Transformation 1404 Primary transformation coefficient matrix 1406 Primary coefficients, shaded area 1408 Secondary Conversion Factors 1800 Computer Systems 1801 Keyboard 1802 Mouse 1803 Trackpad 1805 Joystick 1806 Mike 1807 Scanner 1808 Camera 1809 Audio Output Device Speaker 1810 Touch Screen 1821 Optical media 1822 Thumb Drive 1823 Solid State Drive 1840 Core 1843 Field Programmable Gate Area (FPGA) 1844 Hardware Accelerator 1845 Read-Only Memory (ROM) 1846 Random Access Memory 1847 Core Internal Mass Storage 1848 System Bus 1849 Peripheral Bus 1850 Graphics Adapter 1854 Interface 1855 Communication Network

Claims

1. 1. A method for decoding video data in a decoder, comprising: receiving a coded video bitstream of data blocks; extracting from the coded video bitstream a transform partition type associated with the data block; in response to the transform partition type belonging to a subset of a predefined set of transform partition types, each transform partition type specifying a partitioning pattern for dividing the data block into transform blocks, and a number of transform partitions associated with each transform partition type in the subset of the predefined set of transform partition types is less than or equal to a predefined threshold; extracting, from a syntax element signaled in a coded video bitstream, a transform type of a transform associated with a transform block split from the data block, the transform type belonging to a first predefined set of transform types; performing an inverse transform on the transform block according to the transform type; in response to the transformation partition type not belonging to the subset of the predefined set of transformation partition types; performing an inverse transform on the transform block according to a default transform type; A method comprising:

2. in response to the transformation partition type not belonging to the subset of the predefined set of transformation partition types; The method of claim 1 , further comprising determining the transform type associated with the transform block to be a predefined default transform type.

3. The transform is a primary transform, and the first predefined set of transform types is Discrete Cosine Transform (DCT) type 1 to DCT type 8, Asymmetric Discrete Sine Transform (ADST), Discrete Sine Transform (DST) Type 1 to DST Type 8, Line graph transformation (LGT), and 3. The method of claim 1 or 2, comprising a Karhunen-Loeve transformation (KLT).

4. The method of claim 1 , wherein the subset of the predefined set of transform partition types consists of PARTITION_NONE for no transform block partitioning.

5. The method of claim 1 , wherein the predefined threshold comprises an integer greater than or equal to 1 and less than or equal to 16.

6. The method of claim 1 , wherein the transform is a secondary transform and the first predefined set of transform types includes a KLT.

7. The method of claim 1 , wherein the subset of the predefined set of transform partition types consists of PARTITION_NONE for no transform block partitioning.

8. The method of claim 1 , wherein a number of transform partitions associated with each transform partition type in the subset of the predefined set of transform partition types is less than or equal to a predefined threshold.

9. The method of claim 1 , wherein the transform type further indicates that a type of a primary transform associated with the transform block belongs to a second predefined set of transform types.

10. 10. The method of claim 9, wherein the subset of the predefined set of transform partition types consists of PARTITION_NONE for no transform block partitioning, and the second predefined set of transform types consists of DCT and ADST.

11. 1. A method for decoding video data, comprising the steps of: receiving a coded video bitstream of data blocks; extracting from the coded video bitstream a transform partition type associated with a data block of the video data; extracting, from a syntax element signaled in the coded video bitstream, a transform type associated with a transform block of the block of data in response to the transform partition type belonging to a subset of a predefined set of transform partition types and a number of transform partitions associated with each transform partition type in the subset of the predefined set of transform partition types being less than or equal to a predefined threshold; identifying the conversion type of the data block by default in response to the conversion partition type not belonging to the predefined set of conversion partition types; A method comprising:

12. The method of claim 11 , wherein the subset of the predefined set of transform partition types consists of PARTITION_NONE for no transform block partitioning.

13. The method according to claim 11 or 12, wherein the transformation type comprises a primary transformation type or a secondary transformation type.

14. 14. An apparatus comprising circuitry configured to carry out a method according to any one of claims 1 to 13.

15. A computer program product causing one or more processors to carry out the method according to any one of claims 1 to 13.

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