Method and apparatus for video encoding

Abstract

To provide methods, apparatuses, and non-transitory computer-readable storage mediums for video encoding / decoding.SOLUTION: An apparatus includes processing circuitry that decodes prediction information of a current block in a current picture that is a part of a coded video sequence. The prediction information indicates a cross component filtering (CCF) process for the current block. The processing circuitry generates filtered reconstruction samples of the current block by applying the CCF process to at least one of predicted samples, residual values, or reconstruction samples of the current block. The filtered reconstruction samples of the current block are used for reconstruction of a subsequent block. The processing circuitry reconstructs the current block and the subsequent block based on the filtered reconstruction samples of the current block.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] Incorporation by Reference This application claims the benefit of priority to U.S. Patent Application No. 17 / 315,989, entitled "METHOD AND APPARATUS FOR VIDEO CODING," filed May 10, 2021, which in turn claims the benefit of priority to U.S. Provisional Application No. 63 / 079,322, entitled "CROSS-COMPONENT FILTERING ON BLOCK-LEVEL RECONSTRUCTION," filed September 16, 2020. The entire disclosures of the prior applications are incorporated herein by reference in their entireties.

[0002] This disclosure describes embodiments generally related to video encoding. [Background technology]

[0003] The background discussion provided herein is intended to provide a general context for the present disclosure. The presently described work of the inventors, as well as aspects of the specification that do not otherwise qualify as prior art at the time of filing, are not admitted, expressly or impliedly, as prior art to the present disclosure to the extent described in this background section.

[0004] Video encoding and decoding can be performed using inter-image prediction with motion compensation. Uncompressed digital video can include a series of images, each having spatial dimensions of, for example, 1920 x 1080 luminance samples and associated chrominance samples. The series of images can have a fixed or variable image rate (also informally known as the frame rate), for example, 60 images per second or 60 Hz. Uncompressed video has significant bitrate requirements. For example, 1080p60 4:2:0 video (1920 x 1080 luminance sample resolution at a 60 Hz frame rate) with 8 bits per sample requires a bandwidth approaching 1.5 Gbit / s. One hour of such video requires more than 600 gigabytes of storage space.

[0005] One goal of video encoding and decoding can be to reduce redundancy in an input video signal through compression. Compression can help reduce the aforementioned bandwidth and / or storage space requirements, sometimes by more than two orders of magnitude. Both lossless and lossy compression, as well as combinations thereof, can be used. Lossless compression refers to techniques that can reconstruct an exact copy of the original signal from a compressed version. When lossy compression is used, the reconstructed signal may not be identical to the original signal, but the distortion between the original and reconstructed signal is small enough to make the reconstructed signal useful for its intended application. For video, lossy compression is widely used. The amount of acceptable distortion varies depending on the application; for example, users of certain consumer streaming applications may tolerate higher distortion than users of television distribution applications. The achievable compression ratio can reflect higher acceptable distortion / allowable distortion can result in higher compression ratios.

[0006] Video encoders and decoders can utilize techniques from several broad categories, including, for example, motion compensation, 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 a previously reconstructed reference image. In some video codecs, an image is spatially subdivided into blocks of samples. If all blocks of samples are coded in intra mode, the image may be an intra-image. Intra-images and their derivatives, such as independent decoder refresh images, can be used to reset the decoder state and thus can be used as the first image in a coded video bitstream and video session, or as still images. Samples of intra-blocks can be transformed, and the transform coefficients can be quantized before entropy coding. Intra-prediction can be a technique that minimizes sample values in the pre-transform domain. In some cases, the smaller the DC value and AC coefficients after the transform, the fewer bits required for a given quantization step size to represent the block after entropy coding.

[0008] Conventional intra-coding, such as that known from MPEG-2 generation encoding techniques, does not use intra-prediction. However, some newer video compression techniques include techniques that attempt to do so from surrounding sample data and / or metadata obtained during the encoding / decoding of spatially adjacent and preceding blocks of data in decoding order. Such techniques are hereinafter referred to as "intra-prediction" techniques. It should be noted that, at least in some cases, intra-prediction uses only reference data from the current image being reconstructed, and does not use reference data from a reference image.

[0009] Intra-prediction can take many different forms. If two or more such techniques can be used in a given video coding technique, the technique in use can be coded in intra-prediction mode. In certain cases, a mode can have sub-modes and / or parameters, which may be coded separately or included in a mode codeword. Which codeword to use for a given mode, sub-mode, and / or parameter combination can affect the coding efficiency gain through intra-prediction, as can 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 refined in new coding techniques such as the Joint Search Model (JEM), Versatile Video Coding (VVC), and Benchmark Set (BMS). Predictor blocks can be formed using neighboring sample values belonging to already available samples. The sample values of the neighboring samples are copied into the predictor block according to their direction. The reference to the direction in use may be coded in the bitstream or may be predicted itself.

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

[0012] 1A , a square block (104) of 4×4 samples is shown in the upper left (indicated by a bold 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 in the Y dimension (from the top) and the first sample in the X dimension (from the left). Similarly, sample S44 is the fourth sample of block (104) in both the Y and X dimensions. Because the size of the block is 4×4 samples, S44 is located in the lower right. Also shown are reference samples, which follow a similar numbering scheme. The reference samples are labeled R, their Y position (e.g., row index) and X position (column index) relative to block (104). In both H.264 and H.265, predicted samples are adjacent to the block being reconstructed, and therefore, negative values need not be used.

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

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

[0015] As video coding technology has evolved, the number of possible directions has increased. In H.264 (2003), nine different directions could be represented. This increased to 33 in H.265 (2013), and JEM / VVC / BMS can support up to 65 directions at the time of disclosure. Experiments have been performed to identify the most likely directions, and specific techniques in entropy coding are used to represent these more likely directions with fewer bits, at the expense of certain penalties for less likely directions. Furthermore, the direction itself may be predictable from neighboring directions used in neighboring blocks that have already been decoded.

[0016] FIG. 1B shows a schematic diagram (105) showing 65 intra-prediction directions by JEM to illustrate the increasing number of prediction directions over time.

[0017] The mapping of intra-prediction direction bits within a coded video bitstream to represent directions may vary across video coding techniques and may range, for example, from simple direct mapping of prediction directions to complex adaptive schemes including intra-prediction modes, codewords, recursive modes, and similar techniques. However, in all cases, there may be certain directions that are statistically less likely to occur in video content than certain other directions. Because the goal of video compression is to reduce redundancy, these less likely directions are represented by more bits than more likely directions in well-performing video coding techniques.

[0018] Motion compensation can be a lossy compression technique and can refer to a technique in which blocks of sample data from a previously reconstructed image or portion thereof (reference image) are spatially shifted in a direction indicated by a motion vector (hereinafter MV) and then used to predict a newly reconstructed image or image portion. In some cases, the reference image may be the same as the image currently being reconstructed. The MV can have two dimensions, X and Y, or three dimensions, with the third dimension indicating the reference image in use (the latter may indirectly be a temporal dimension).

[0019] In some video compression techniques, the MV applicable to a particular region of sample data can be predicted from other MVs, e.g., from those associated with another region of sample data that is spatially adjacent to the region being reconstructed and precedes that MV in decoding order. Doing so can significantly reduce the amount of data required to encode the MV, thereby eliminating redundancy and improving compression. For example, when encoding an input video signal obtained from a camera (known as natural video), MV prediction can work effectively because there is a statistical possibility that regions larger than the region to which a single MV is applicable move in a similar direction. Therefore, in some cases, MV prediction can be performed using similar MVs derived from MVs in neighboring regions. As a result, the MV detected for a given region is similar or identical to the MV predicted from surrounding MVs, which, after entropy coding, can be represented with fewer bits than would be used to directly encode the MV. In some cases, MV prediction can be an example of lossless compression of a signal (i.e., MV) derived from the original signal (i.e., sample stream). In other cases, MV prediction itself may be lossy, for example, due to rounding errors when calculating 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 provided by H.265, the one described here is a technique hereafter referred to as "spatial merging".

[0021] Referring to Figure 1C, the current block (111) may contain samples discovered by the encoder during the motion search process, which are predictable from a spatially shifted previous block of the same size. Instead of directly encoding its MV, the MV may be derived from metadata associated with one or more reference pictures, e.g., the most recent (in decoding order) reference picture, using MVs associated with any one of five surrounding samples, denoted A0, A1, and B0, B1, and B2 (112-116, respectively). In H.265, MV prediction may use predictors from the same reference picture as neighboring blocks. Summary of the Invention [Means for solving the problem]

[0022] An aspect of the present disclosure provides an apparatus for video encoding / decoding. The apparatus includes a processing circuit that decodes prediction information for a current block in a current image that is part of a coded video sequence. The prediction information indicates a cross-component filtering (CCF) process for the current block. The processing circuit generates filtered reconstructed samples for the current block by applying the CCF process to at least one of predicted samples, residual values, or reconstructed samples of the current block. The filtered reconstructed samples of the current block are used to reconstruct a subsequent block. The processing circuit reconstructs the current block and the subsequent block based on the filtered reconstructed samples of the current block.

[0023] In one embodiment, the processing circuit generates filtered reconstructed samples of the current block by applying a CCF process to the output of at least one of the inverse quantization process and the inverse transform process of the current block.

[0024] In one embodiment, the processing circuit reconstructs the current block by adding the output of the CCF process, which includes the offset value of the current block, to the chroma reconstructed samples of the current block.

[0025] In one embodiment, the processing circuit reconstructs the current block by adding the output of the CCF process, which includes the offset value of the current block, to the chroma residual value of the current block.

[0026] In one embodiment, the processing circuit reconstructs the current block by adding the output of the CCF process, which includes the offset values for the current block, to the chroma prediction samples of the current block.

[0027] In one embodiment, the filter coefficients of the CCF process are included in the prediction information.

[0028] In one embodiment, the filter coefficients of the CCF process are predefined constants.

[0029] In one embodiment, the filter coefficients of the CCF process used in the current image are determined based on the filter coefficients of the CCF process used in another image and an offset value of the filter coefficients of the CCF process used in the current image.

[0030] In one embodiment, the filter coefficients of the CCF process used for one color component are determined based on the filter coefficients of the CCF process used for another color component.

[0031] In one embodiment, the filter coefficients of the CCF process are determined based on the sample values of the current block determined before the CCF process is applied.

[0032] In one embodiment, whether the CCF process is enabled is determined based on one of the partitioning scheme, prediction mode, block width, block height, transform coefficients, and quantization parameters.

[0033] An aspect of the present disclosure provides a method for video encoding / decoding. In the method, prediction information of a current block in a current image that is part of a coded video sequence is decoded. The prediction information indicates a CCF process for the current block. Filtered reconstructed samples of the current block are generated by applying the CCF process to at least one of predicted samples, residual values, or reconstructed samples of the current block. The filtered reconstructed samples of the current block are used to reconstruct subsequent blocks. The current block and subsequent blocks are reconstructed based on the filtered reconstructed samples of the current block.

[0034] Aspects of the present disclosure also provide a non-transitory computer-readable medium storing instructions that, when executed by at least one processor, cause the at least one processor to perform any one or combination of methods for video decoding.

[0035] Further features, nature and various advantages of the disclosed subject matter will become more apparent from the following detailed description and accompanying drawings. [Brief explanation of the drawings]

[0036] [Figure 1A] FIG. 2 is a schematic diagram of an example subset of intra-prediction modes. [Figure 1B] FIG. 1 is a diagram of an exemplary intra-prediction direction. [Figure 1C] FIG. 1 is a schematic diagram of a current block and its surrounding spatial merge candidates in one example. [Figure 2] FIG. 1 is a schematic diagram of a simplified block diagram of a communication system according to one embodiment. [Figure 3] FIG. 1 is a schematic diagram of a simplified block diagram of a communication system according to one embodiment. [Figure 4] FIG. 2 is a schematic diagram of a simplified block diagram of a decoder according to one embodiment. [Figure 5] FIG. 2 is a schematic diagram of a simplified block diagram of an encoder according to one embodiment. [Figure 6] FIG. 4 is a block diagram of an encoder according to another embodiment. [Figure 7] FIG. 10 is a block diagram of a decoder according to another embodiment. [Figure 8A] FIG. 1 illustrates an exemplary diamond filter shape for an adaptive loop filter (ALF), according to an embodiment. [Figure 8B] FIG. 1 illustrates an exemplary diamond filter shape for an adaptive loop filter (ALF), according to an embodiment. [Figure 9A] 10A-10C illustrate exemplary subsampled positions used for gradient calculations in the vertical, horizontal, and two diagonal directions, respectively, according to an embodiment. [Figure 9B] 10A-10C illustrate exemplary subsampled positions used for gradient calculations in the vertical, horizontal, and two diagonal directions, respectively, according to an embodiment. [Figure 9C] 10A-10C illustrate exemplary subsampled positions used for gradient calculations in the vertical, horizontal, and two diagonal directions, respectively, according to an embodiment. [Figure 9D] 10A-10C illustrate exemplary subsampled positions used for gradient calculations in the vertical, horizontal, and two diagonal directions, respectively, according to an embodiment. [Figure 10] FIG. 10 illustrates an exemplary modified block classification applied to a luminance component, according to one embodiment. [Figure 11] FIG. 10 illustrates an exemplary modified ALF for luminance components at a virtual boundary, according to some embodiments. [Figure 12] FIG. 1 illustrates an exemplary maximum coding unit (LCU) aligned image quadtree partitioning, according to one embodiment. [Figure 13] FIG. 10 illustrates exemplary quadtree split flags encoded in z-order, according to one embodiment. [Figure 14A] FIG. 1 illustrates an exemplary arrangement of a cross-component adaptive loop filter (CC-ALF), according to one embodiment. [Figure 14B]FIG. 10 illustrates an exemplary linear diamond-shaped filter applied to the luma channel of each chroma component during CC-ALF operation, according to one embodiment. [Figure 15] FIG. 1 illustrates an exemplary direction search for an 8x8 block, according to one embodiment. [Figure 16] FIG. 1 illustrates an exemplary subspace projection according to one embodiment. [Figure 17] FIG. 1 illustrates an exemplary CCF applied after the inverse transform of a block. [Figure 18] 1 is an exemplary flowchart according to one embodiment of the present disclosure. [Figure 19] FIG. 1 is a schematic diagram of a computer system according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0037] I. Video Decoder and Encoder Systems

[0038] FIG. 2 illustrates a simplified block diagram of a communication system (200) according to one embodiment of the present disclosure. The system (200) includes multiple terminals capable of communicating, for example, via a network (250). For example, the communication system (200) includes a first pair of terminal devices (210) and (220) interconnected via the network (250). In the example of FIG. 2, the first pair of terminal devices (210) and (220) perform unidirectional data transmission. For example, the terminal device (210) can encode video data (e.g., a stream of video images captured by the terminal device (210)) for transmission to the other terminal device (220) via the network (250). The encoded video data can be transmitted in the form of one or more coded video bitstreams. The terminal device (220) can receive the encoded video data from the network (250), decode the encoded data to reconstruct the video image, and display the video image according to the reconstructed video data. One-way data transmission may be common in media serving applications, for example.

[0039] In another example, the communication system 200 includes a second pair of terminal devices 230 and 240 that perform bidirectional transmission of encoded video data, such as may occur during a video conference. In one example, for bidirectional transmission of data, each of the terminal devices 230 and 240 can encode video data (e.g., a stream of video images captured by the terminal device) for transmission to the other of the terminal devices 230 and 240 over the network 250. Each of the terminal devices 230 and 240 can also receive the encoded video data transmitted by the other of the terminal devices 230 and 240, decode the encoded video data to reconstruct the video images, and display the video images on an accessible display device in accordance with the reconstructed video data.

[0040] In the example of FIG. 2 , the terminal devices 210, 220, 230, and 240 may be depicted as a server, a personal computer, and a smartphone, although the principles of the present disclosure may not be so limited. Embodiments of the present disclosure apply to laptop computers, tablet computers, media players, and / or dedicated videoconferencing devices. The network 250 represents any number of networks that convey coded video data between the terminal devices 210, 220, 230, and 240, including, for example, wired (cabled) and / or wireless communication networks. The communication network 250 may exchange data over circuit-switched and / or packet-switched 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 250 may not be important to the operation of the present disclosure, unless otherwise described herein below.

[0041] 3 illustrates the placement of a video encoder and a video decoder in a streaming environment as an example of an application of the disclosed subject matter. The disclosed subject matter is equally applicable to other video-enabled applications, including, for example, video conferencing, digital TV, and storage of compressed video on digital media including CDs, DVDs, memory sticks, etc.

[0042] The streaming system can include a capture subsystem (313), which can include, for example, a video source (301), such as a digital camera, that creates a stream of uncompressed video images (302). In one example, the stream of video images (302) includes samples captured by the digital camera. The stream of video images (302), shown in bold to emphasize its high data volume compared to the encoded video data (304) (or encoded video bitstream), can be processed by an electronic device (320) that includes a video encoder (303) coupled to the video source (301). The video encoder (303) can 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 (304) (or encoded video bitstream (304)), shown in thin to emphasize its low data volume compared to the stream of video images (302), can be stored on a streaming server (305) for future use. One or more streaming client subsystems, such as the client subsystems (306) and (308) of Figure 3, can access the streaming server (305) to retrieve copies (307) and (309) of the encoded video data (304). The client subsystem (306) can include, for example, a video decoder (310) within an electronic device (330). The video decoder (310) decodes the incoming copy (307) of the encoded video data and creates an outgoing stream of video images (311) that can be rendered on a display (312) (e.g., a display screen) or other rendering device (not shown). In some streaming systems, the encoded video data (304), (307), and (309) (e.g., a video bitstream) can be encoded according to a particular video encoding / compression standard. An example of such a standard is ITU-T Recommendation H.265. In one example, a video encoding standard under development is informally known as Versatile Video Coding (VVC).The disclosed subject matter can be used in the context of a VVC.

[0043] It should be noted that the electronic devices 320 and 330 may include other components (not shown). For example, the electronic device 320 may include a video decoder (not shown), and the electronic device 330 may also include a video encoder (not shown).

[0044] 4 shows a block diagram of a video decoder (410) according to one embodiment of the present disclosure. The video decoder (410) may be included in an electronic device (430). The electronic device (430) may include a receiver (431) (e.g., a receiving circuit). In the example of FIG. 3, the video decoder (410) may be used in place of the video decoder (310).

[0045] The receiver (431) can receive one or more coded video sequences to be decoded by the video decoder (410), one coded video sequence at a time, in the same or another embodiment, with the decoding of each coded video sequence being independent of the other coded video sequences. The coded video sequences may be received from a channel (401), which may be a hardware / software link to a storage device that stores the coded video data. The receiver (431) can 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 usage entities (not shown). The receiver (431) can separate the coded video sequences from other data. To address network jitter, a buffer memory (415) may be coupled between the receiver (431) and the entropy decoder / parser (420) (hereinafter, "parser (420)"). In certain applications, the buffer memory (415) is part of the video decoder (410). In other applications, it may be external to the video decoder (410) (not shown). In still other applications, there may be a buffer memory (not shown) external to the video decoder (410), for example, to deal with network jitter, and there may also be another buffer memory (415) internal to the video decoder (410), for example, to handle playout timing. When the receiver (431) is receiving data from a store-and-forward device with sufficient bandwidth and controllability, or from an equivalent synchronous network, the buffer memory (415) may not be required, or may be small. For use with best-effort packet networks such as the Internet, a buffer memory (415) may be required, which may be relatively large, advantageously of adaptable size, and may be implemented at least in part in an operating system or similar element (not shown) other than the video decoder (410).

[0046] The video decoder (410) may include a parser (420) for reconstructing symbols (421) from the coded video sequence. These symbol categories include information used to manage the operation of the video decoder (410) and potential information for controlling a rendering device, such as a rendering device (412) (e.g., a display screen) that is not an integral part of the electronic device (430) but may be coupled to the electronic device (430) as shown in FIG. 4. The control information for the rendering device 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 (420) can parse / entropy decode the received coded video sequence. The coding of the coded video sequence can follow a video coding technique or standard and can follow various principles, including variable-length coding, Huffman coding, arithmetic coding with or without context sensitivity, etc. The parser (420) can extract a set of subgroup parameters for at least one subgroup of pixels in the video decoder from the coded video sequence based on at least one parameter corresponding to the group. The subgroups may include groups of pictures (GOPs), images, tiles, slices, macroblocks, coding units (CUs), blocks, transform units (TUs), prediction units (PUs), etc. The parser (420) may also extract transform coefficients, quantizer parameter values, MVs, etc. from the coded video sequence information.

[0047] The parser (420) can perform entropy decoding / parsing operations on the video sequence received from the buffer memory (415) to produce symbols (421).

[0048] The reconstruction of the symbols (421) can involve several different units, depending on the type of coded video picture or portion thereof (e.g., inter- and intra-pictures, inter- and intra-blocks), and other factors. Which units are involved, and how, can be controlled by subgroup control information parsed from the coded video sequence by the parser (420). The flow of such subgroup control information between the parser (420) and the following units is not shown for clarity.

[0049] In addition to the functional blocks already mentioned, the decoder (410) may be conceptually subdivided into several functional units, as described below. In an actual implementation operating under commercial constraints, many of these units will interact closely with each other and may be at least partially integrated with each other. However, for purposes of describing the disclosed subject matter, the following conceptual subdivision into functional units is appropriate:

[0050] The first unit may be a scalar / inverse transform unit (451), which may receive quantized transform coefficients as well as control information from the parser (420) as symbols (421), including the transform to use, block size, quantization coefficients, quantization scaling matrix, etc. The scalar / inverse transform unit (451) may output blocks containing sample values that may be input to an aggregator (455).

[0051] In some cases, the output samples of the scalar / inverse transform unit (451) may relate to intra-coded blocks, i.e., blocks that do not use prediction information from a previously reconstructed image but can use prediction information from a previously reconstructed portion of the current image. Such prediction information may be provided by an intra-image prediction unit (452). In some cases, the intra-image prediction unit (452) generates a block of the same size and shape as the block being reconstructed using surrounding, already reconstructed information fetched from a current image buffer (458). The current image buffer (458), for example, buffers a partially reconstructed current image and / or a fully reconstructed current image. The aggregator (455) may add, on a sample-by-sample basis, the prediction information generated by the intra-prediction unit (452) to the output sample information provided by the scalar / inverse transform unit (451).

[0052] In other cases, the output samples of the scaler / inverse transform unit (451) may relate to an inter-coded, potentially motion-compensated block. In such cases, the motion-compensated prediction unit (453) may access a reference picture memory (457) to fetch samples used for prediction. After motion-compensating the fetched samples according to the symbols (421) associated with the block, the aggregator (455) may add these samples to the output of the scaler / inverse transform unit (451) (in this case, referred to as residual samples or residual signals) to generate output sample information. The addresses in the reference picture memory (457) from which the motion-compensated prediction unit (453) fetches prediction samples may be controlled by MVs, which may be available to the motion-compensated prediction unit (453) in the form of symbols (421), which may have, for example, X, Y, and reference picture components. Motion compensation may also include interpolation of sample values fetched from the reference picture memory (457) when sub-sample accurate MVs are used, MV prediction mechanisms, and the like.

[0053] The output samples of the aggregator (455) can be subjected to various loop filtering techniques in a loop filter unit (456). Video compression techniques can include in-loop filtering techniques, which are controlled by parameters contained in the coded video sequence (also called a coded video bitstream) and made available to the loop filter unit (456) as symbols (421) from the parser (420), but can also be responsive to previously reconstructed and loop filtered sample values as well as to meta-information obtained during decoding of previous (decoding order) portions of the coded image or coded video sequence.

[0054] The output of the loop filter unit (456) may be a sample stream that can be output to the rendering device (412) as well as stored in a reference image memory (457) for use in future inter-image prediction.

[0055] Once a particular coded image is fully reconstructed, it can be used as a reference image for future predictions. For example, once the coded image corresponding to the current image is fully reconstructed and the coded image is identified as a reference image (e.g., by the parser (420)), the current image buffer (458) can become part of the reference image memory (457), and a new current image buffer can be reallocated before starting the reconstruction of the next coded image.

[0056] The video decoder (410) can perform decoding operations according to a predetermined video compression technique in a standard, such as ITU-T Rec. H.265. An encoded video sequence can conform to the syntax specified by the video compression technique or standard being used, in the sense that the encoded video sequence conforms to both the syntax of the video compression technique or standard and a profile documented in the video compression technique or standard. Specifically, a profile can select specific tools from all tools available in the video compression technique or standard as the only tools usable by that profile. Compliance also requires that the complexity of the encoded video sequence be within a range defined by the level of the video compression technique or standard. In some cases, the level imposes limitations on the maximum picture size, maximum frame rate, maximum reconstruction sample rate (e.g., measured in megasamples per second), maximum reference picture size, etc. The limitations set by the level can, in some cases, be further constrained by a hypothetical reference decoder (HRD) specification and metadata for HRD buffer management signaled in the encoded video sequence.

[0057] In one embodiment, the receiver (431) can receive additional (redundant) data along with the coded video. The additional data may be included as part of the coded video sequence. The additional data may be used by the video decoder (410) to properly decode the data and / or to more accurately reconstruct 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 images, forward error correction codes, etc.

[0058] 5 shows a block diagram of a video encoder (503) according to one embodiment of the present disclosure. The video encoder (503) is included in an electronic device (520). The electronic device (520) includes a transmitter (540) (e.g., a transmitting circuit). In the example of FIG. 3, the video encoder (503) can be used in place of the video encoder (303).

[0059] The video encoder (503) can receive video samples from a video source (501) (not part of the electronic device (520) in the example of FIG. 5), which can capture video images to be encoded by the video encoder (503). In another example, the video source (501) is part of the electronic device (520).

[0060] The video source (501) can provide a source video sequence to be encoded by the video encoder (503) in the form of a digital video sample stream, which can be of any suitable bit depth (e.g., 8-bit, 10-bit, 12-bit, etc.), any color space (e.g., BT.601 Y CrCB, RGB, etc.), 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 (501) can be a storage device that stores previously prepared video. In a video conferencing system, the video source (501) can be a camera that captures local image information as a video sequence. The video data can be provided as multiple individual images that, when viewed sequentially, impart motion. The image itself can be organized as a spatial array of pixels, each of which can contain one or more samples, depending on the sampling structure, color space, etc., in use. Those skilled in the art can readily understand the relationship between pixels and samples. The following discussion focuses on samples.

[0061] According to one embodiment, the video encoder (503) can encode and compress images of a source video sequence into an encoded video sequence (543) in real time or under any other time constraint required by the application. Applying an appropriate encoding rate is one function of the controller (550). In some embodiments, the controller (550) controls and is functionally coupled to other functional units as described below. For clarity, coupling is not shown. Parameters set by the controller (550) can include rate control-related parameters (picture skip, quantizer, lambda value for rate-distortion optimization techniques, etc.), picture size, group of pictures (GOP) layout, maximum MV-allowed reference region, etc. The controller (550) can be configured with other appropriate functions associated with the video encoder (503) optimized for a particular system design.

[0062] In some embodiments, the video encoder (503) is configured to operate in an encoding loop. As an overly simplified explanation, in one example, the encoding loop can include a source coder (530) (e.g., responsible for creating symbols, such as a symbol stream, based on an input image to be encoded and a reference image) and a (local) decoder (533) embedded in the video encoder (503). The decoder (533) reconstructs the symbols to create sample data in a manner similar to how a (remote) decoder creates sample data (because the compression between the symbols and the encoded video bitstream is lossless in the video compression techniques considered in the disclosed subject matter). The reconstructed sample stream (sample data) is input to a reference image memory (534). Because decoding of the symbol stream produces bit-exact results regardless of the location of the decoder (local or remote), the contents in the reference image memory (534) are also bit-exact between the local and remote encoders. In other words, the predictive portion of the encoder "sees" the exact same sample values as the decoder "sees" when using prediction during decoding. This basic principle of reference image synchrony (and the resulting drift if synchrony cannot be maintained, for example due to channel errors) is also used in some prior art.

[0063] The operation of the "local" decoder (533) may be the same as the operation of a "remote" decoder, such as the video decoder (410), which has already been described in detail above in connection with Figure 4. However, again with brief reference to Figure 4, because symbols are available and the encoding / decoding of the symbols into an encoded video sequence by the entropy coder (545) and parser (420) may be lossless, the entropy decoding portion of the video decoder (410), including the buffer memory (415) and parser (420), may not be fully implemented in the local decoder (533).

[0064] An observation that can be made at this point is that any decoder technology, with the exception of analysis / entropy decoding, that exists in a decoder must exist in substantially identical functional form in the corresponding encoder. For this reason, the disclosed subject matter focuses on decoder operation. Descriptions of encoder technologies can be omitted, as they may be the inverse of the decoder technology that is described generically. Only in certain areas are more detailed descriptions necessary and are provided below.

[0065] In operation, in some examples, the source coder (530) may perform motion-compensated predictive coding, which predictively codes an input image with reference to one or more previously coded images from a video sequence designated as “reference frames.” In this manner, the coding engine (532) codes differences between pixel blocks of the input image and pixel blocks of reference images that may be selected as predictive references for the input image.

[0066] The local video decoder (533) can decode the encoded video data of an image that may be designated as a reference image based on the symbols created by the source coder (530). The operation of the encoding engine (532) can advantageously be a lossy process. When the encoded video data can be decoded by a video decoder (not shown in FIG. 5), the reconstructed video sequence can be a replica of the source video sequence, typically with some errors. The local video decoder (533) can replicate the decoding process that may be performed by the video decoder on the reference image and store the reconstructed reference image in a reference image cache (534). In this way, the video encoder (503) can locally store copies of reconstructed reference frames that have common content as the reconstructed reference image obtained by the far-end video decoder (without transmission errors).

[0067] The predictor (535) can perform a predictive search for the encoding engine (532). That is, for a new image to be encoded, the predictor (535) can search the reference image memory (534) for sample data (as candidate reference pixel blocks) or specific metadata, such as reference image MVs, block shapes, etc., that can serve as suitable predictive references for the new image. The predictor (535) can operate on a sample block by pixel block basis to find suitable predictive references. In some cases, as determined by the search results obtained by the predictor (535), the input image may have predictive references extracted from multiple reference images stored in the reference image memory (534).

[0068] The controller (550) can manage the encoding operations of the source coder (530), including, for example, setting the parameters and subgroup parameters used to encode the video data.

[0069] The output of all the aforementioned functional units may be entropy coded in an entropy coder (545), which converts the symbols produced by the various functional units into a coded video sequence by losslessly compressing the symbols according to techniques such as Huffman coding, variable length coding, or arithmetic coding.

[0070] The transmitter (540) can buffer the coded video sequence created by the entropy coder (545) and prepare it for transmission over a communication channel (560), which can be a software / hardware link to a storage device that stores the coded video data. The transmitter (540) can merge the coded video data from the video coder (503) with other data to be transmitted, such as coded audio data and / or auxiliary data streams (sources not shown).

[0071] The controller (550) can manage the operation of the video encoder (503). During encoding, the controller (550) can assign each encoded image a particular encoded image type, which can affect the encoding technique that can be applied to the respective image. For example, images are often assigned as one of the following image types:

[0072] An intra-picture (I-picture) may be one that can be coded and decoded without using other pictures in the sequence as a source of prediction. Some video codecs allow various 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.

[0073] A predicted image (P-image) may be one that can be coded and decoded using intra-prediction or inter-prediction, which uses at most one MV and reference index to predict the sample values of each block.

[0074] Bidirectionally predicted images (B-images) may be those that can be coded and decoded using intra- or inter-prediction, which uses up to two MVs and reference indices to predict the sample values of each block. Similarly, multiple predicted images may use more than two reference images and associated metadata for the reconstruction of a single block.

[0075] A source image is generally spatially subdivided into multiple sample blocks (e.g., blocks of 4x4, 8x8, 4x8, or 16x16 samples each) and may be coded block by block. Blocks may be predictively coded with reference to other (already coded) blocks as determined by the coding assignment applied to each image of the block. For example, blocks of an I image may be non-predictively coded, or they may be predictively coded with reference to already coded blocks of the same image (spatial prediction or intra prediction). Pixel blocks of a P image may be predictively coded via spatial prediction or via temporal prediction with reference to one previously coded reference image. Blocks of a B image may be predictively coded via spatial prediction or via temporal prediction with reference to one or two previously coded reference images.

[0076] The video encoder (503) may perform encoding operations according to a predetermined video encoding technique or standard, such as ITU-T Rec. H.265. In doing so, the video encoder (503) may perform various compression operations, including predictive encoding operations that exploit temporal and spatial redundancy in the input video sequence. Thus, the encoded video data may conform to a syntax specified by the video encoding technique or standard being used.

[0077] In one embodiment, the transmitter (540) can transmit additional data along with the coded video. The source coder (530) can include such data as part of the coded video sequence. The additional data can include temporal / spatial / SNR enhancement layers, other forms of redundant data such as redundant pictures and slices, SEI messages, VUI parameter set fragments, etc.

[0078] Video may be captured as multiple source images (video images) in temporal order. Intra-image prediction (often abbreviated as intra-prediction) exploits spatial correlation within a given image, while inter-image prediction exploits correlation (temporal or otherwise) between images. In one example, a particular image being encoded / decoded, called the current image, is divided into blocks. If a block in the current image is similar to a reference block in a previously encoded and still buffered reference image in the video, the block in the current image can be coded by a vector called a vector vector (MV). The MV points to a reference block within the reference image and may have a third dimension that identifies the reference image if multiple reference images are used.

[0079] In some embodiments, bidirectional prediction techniques can be used in inter-image prediction. According to bidirectional prediction techniques, two reference images, e.g., a first reference image and a second reference image, are used, both of which are earlier in decoding order than a current image in a video (but may be earlier and later, respectively, in display order). A block in the current image can be coded by a first MV that points to a first reference block in the first reference image and a second MV that points to a second reference block in the second reference image. A block can be predicted by a combination of the first reference block and the second reference block.

[0080] Furthermore, in inter-image prediction, merge mode techniques can be used to improve coding efficiency.

[0081] According to some embodiments of the present disclosure, prediction, such as inter-image prediction and intra-image prediction, is performed in units of blocks. For example, according to the HEVC standard, images in a sequence of video images are divided into coding tree units (CTUs) for compression, and the CTUs in an image have the same size, such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. Generally, a CTU includes three coding tree blocks (CTBs), one luma CTB and two chroma CTBs. Each CTU can be recursively quadtree-decomposed into one or more coding units (CUs). For example, a 64×64 pixel CTU can be divided into one 64×64 pixel CU, four 32×32 pixel CUs, or sixteen 16×16 pixel CUs. In one example, each CU is analyzed to determine the prediction type of the CU, such as inter-prediction type or intra-prediction type. The CU is then divided into one or more prediction units (PUs) according to temporal and / or spatial predictability. Generally, each PU includes a luma prediction block (PB) and two chroma PBs. In one embodiment, the prediction operation in encoding (encoding / decoding) is performed in units of a prediction block. Taking a luma prediction block as an example of a prediction block, the prediction block includes a matrix of pixel values (e.g., luma values) of 8x8 pixels, 16x16 pixels, 8x16 pixels, 16x8 pixels, etc.

[0082] 6 shows a diagram of a video encoder (603) according to another embodiment of the present disclosure. The video encoder (603) is configured to receive a processed block (e.g., a predictive block) of sample values in a current video image in a sequence of video images and encode the processed block into an encoded image that is part of an encoded video sequence. In one example, the video encoder (603) is used in place of the video encoder (303) of the example of FIG. 3.

[0083] In an HEVC example, the video encoder (603) receives a matrix of sample values for a processing block, such as a prediction block of 8x8 samples. The video encoder (603) determines whether the processing block is optimally coded using intra mode, inter mode, or bidirectional prediction mode, e.g., using rate-distortion optimization. If the processing block is coded in intra mode, the video encoder (603) can code the processing block into a coded image using intra prediction techniques. If the processing block is coded in inter mode or bidirectional prediction mode, the video encoder (603) can code the processing block into a coded image using inter prediction or bidirectional prediction techniques, respectively. In certain video coding techniques, the merge mode may be an inter-image prediction submode, in which MVs are derived from one or more MV predictors without coding MV components other than the predictors. In certain other video coding techniques, there may be MV components applicable to the current block. In one example, the video encoder (603) includes other components, such as a mode decision module (not shown), for determining the mode of the processing block.

[0084] In the example of Figure 6, the video encoder (603) includes an inter-encoder (630), an intra-encoder (622), a residual calculator (623), a switch (626), a residual encoder (624), a general controller (621), and an entropy encoder (625), all coupled together as shown in Figure 6.

[0085] The inter-encoder (630) is configured to receive samples of a current block (e.g., a processing block), compare the block with one or more reference blocks in a reference image (e.g., blocks in a previous image and a subsequent image), generate inter-prediction information (e.g., a description of redundant information due to inter-coding techniques, MV, merge mode information), 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 image is a decoded reference image decoded based on the coded video information.

[0086] The intra encoder (622) is configured to receive samples of a current block (e.g., a processing block), optionally compare the block with previously coded blocks in the same image, generate quantized coefficients after transformation, and optionally also generate intra prediction information (e.g., intra prediction direction information according to one or more intra coding techniques). In one example, the intra encoder (622) also calculates an intra prediction result (e.g., a predicted block) based on the intra prediction information and a reference block in the same image.

[0087] The general-purpose controller (621) is configured to determine general-purpose control data and control other components of the video encoder (603) based on the general-purpose control data. In one example, the general-purpose controller (621) determines the mode of the block and provides a control signal to the switch (626) based on the mode. For example, if the mode is intra-mode, the general-purpose controller (621) controls the switch (626) to select intra-mode results for use by the residual calculator (623) and controls the entropy encoder (625) to select intra-prediction information and include the intra-prediction information in the bitstream. If the mode is inter-mode, the general-purpose controller (621) controls the switch (626) to select inter-prediction results for use by the residual calculator (623) and controls the entropy encoder (625) to select inter-prediction information and include the inter-prediction information in the bitstream.

[0088] The residual calculator (623) is configured to calculate the difference (residual data) between the received block and a prediction result selected from the intra-encoder (622) or inter-encoder (630). The residual encoder (624) operates on the residual data and is configured to encode the residual data to generate transform coefficients. In one example, the residual encoder (624) is configured to transform the residual data from the spatial domain to the frequency domain to generate transform coefficients. The transform coefficients are then quantized to obtain quantized transform coefficients. In various embodiments, the video encoder (603) also includes a residual decoder (628). The residual decoder (628) is configured to perform an inverse transform to generate decoded residual data. The decoded residual data can be used by the intra-encoder (622) and inter-encoder (630), as appropriate. For example, the inter-encoder (630) can generate decoded blocks based on the decoded residual data and inter-prediction information, and the intra-encoder (622) can generate decoded blocks based on the decoded residual data and intra-prediction information. The decoded blocks are processed appropriately to generate decoded images, which may be buffered in a memory circuit (not shown) and used as reference images in some examples.

[0089] The entropy encoder (625) is configured to format the bitstream to include the coded block. The entropy encoder (625) is configured to include various information in accordance with an appropriate standard, such as HEVC. In one example, the entropy encoder (625) is configured to include general control data, selected prediction information (e.g., intra-prediction information or inter-prediction information), residual information, and other appropriate information in the bitstream. Note that, according to the disclosed subject matter, there is no residual information when coding a block in a merged sub-mode of either the inter mode or the bi-prediction mode.

[0090] 7 shows a diagram of a video decoder (710) according to another embodiment of the present disclosure. The video decoder (710) is configured to receive coded images that are part of a coded video sequence and decode the coded images to generate reconstructed images. In one example, the video decoder (710) is used in place of the video decoder (310) of the example of FIG. 3.

[0091] In the example of Figure 7, the video decoder (710) includes an entropy decoder (771), an inter-decoder (780), a residual decoder (773), a reconstruction module (774), and an intra-decoder (772), which are coupled together as shown in Figure 7.

[0092] The entropy decoder (771) can be configured to reconstruct, from the coded image, specific symbols representing the syntax elements that make up the coded image. Such symbols can include, for example, prediction information (e.g., intra- or inter-prediction information) that can identify the mode in which the block is coded (e.g., intra- or inter-prediction mode, merged submode, or the latter two in another submode), specific samples or metadata used for prediction by the intra- or inter-decoder (772) or inter-decoder (780), respectively, residual information in the form of quantized transform coefficients, etc. In one example, if the prediction mode is an inter- or bi-prediction mode, the inter-prediction information is provided to the inter-decoder (780), and if the prediction type is an intra-prediction type, the intra-prediction information is provided to the intra-decoder (772). The residual information may be dequantized and provided to the residual decoder (773).

[0093] The inter decoder (780) is configured to receive the inter prediction information and generate an inter prediction result based on the inter prediction information.

[0094] The intra decoder (772) is configured to receive intra prediction information and generate a prediction result based on the intra prediction information.

[0095] The residual decoder (773) is 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 (773) may also require certain control information (to include quantization parameters (QPs)), which may be provided by the entropy decoder (771) (this may be only a small amount of control information, so a data path is not shown).

[0096] The reconstruction module (774) is configured to combine, in the spatial domain, the residual output by the residual decoder and the prediction result (possibly output by the inter-prediction module or the intra-prediction module) to form a reconstructed block, which may be part of a reconstructed image, which may be part of a reconstructed video. It should be noted that other suitable operations, such as deblocking operations, may be performed to improve visual quality.

[0097] It should be noted that the video encoders (303), (503), and (603) and the video decoders (310), (410), and (710) may be implemented using any suitable technology. In one embodiment, the video encoders (303), (503), and (603) and the video decoders (310), (410), and (710) may be implemented using one or more integrated circuits. In another embodiment, the video encoders (303), (503), and (603) and the video decoders (310), (410), and (710) may be implemented using one or more processors executing software instructions.

[0098] II. Adaptive Loop Filter

[0099] In some relevant examples, such as VVC Draft 7, an adaptive loop filter (ALF) with block-based filter adaptation can be applied. For the luma component, one of 25 filters can be selected for each 4x4 block based on the orientation and activity of the respective 4x4 block.

[0100] 8A-8B show exemplary diamond filter shapes for ALF according to some embodiments of the present disclosure: Figure 8A shows a 5x5 diamond shape that can be applied to the chroma component, and Figure 8B shows a 7x7 diamond shape that can be applied to the luma component.

[0101] In some relevant examples, for the luma component, each 4x4 block can be classified into one of 25 classes. The classification index C of a 4x4 block is a function of the directionality D and activity of the 4x4 block, as follows:

number

[0102]

number

[0103] D and

number

[0104]

number

[0105] where the indices i and j refer to the coordinates of the top-left sample in a 4x4 block, and R(i,j) denotes the reconstructed sample at coordinate (i,j).

[0106] To reduce the complexity of block classification, a subsampled 1-D Laplacian calculation can be applied. Figures 9A-9D show exemplary subsampled positions used for vertical, horizontal, and two diagonal gradient calculations, respectively.

[0107] Then the maximum and minimum values of the horizontal and vertical gradients can be set as follows:

[0108]

number

[0109] The maximum and minimum values of the two diagonal gradients can be set as follows:

[0110]

number

[0111] These values can be compared with each other and with two thresholds t1 and t2 to derive the value of the directionality D.

[0112] Step 1:

number

[0113] Step 2:

number

[0114] Step 3:

number

[0115] Step 4:

number

[0116] The activity value A is calculated as follows:

[0117]

number

[0118] A is further quantized to the range 0 to 4 (inclusive), and the quantized value is

number

[0119] For the saturation components in an image, no classification method is applied, i.e., a single set of ALF coefficients can be applied for each saturation component.

[0120] Before filtering each 4x4 luminance block, geometric transformations such as rotation, diagonal flip, and vertical flip are applied to the filter coefficients f(k,l) and corresponding filter clipping values c(k,l) according to the gradient values calculated for each block. This is equivalent to applying these transformations to samples within the filter support region in order to make different blocks to which ALF is applied more similar by aligning their orientations.

[0121] Three geometric transformations, including diagonal flip, vertical flip, and rotation, can be written as follows:

[0122]

number

[0123] where K is the filter size, 0≦k, l≦k-1 are the coordinates of the transform coefficients, with position (0,0) at the top-left corner and position (K-1,K-1) at the bottom-right corner. The transform is applied to the filter coefficients f(k,l) and clipping values c(k,l) according to the gradient value calculated for the corresponding block. The relationship between the transform and the gradient in the four directions can be summarized in Table 1.

[0124] [Table 1]

[0125] In some related examples, such as VVC Draft 7, the filter parameters of the ALF are signaled in an adaptive parameter set (APS). One APS can signal up to 25 sets of luma filter coefficients and clipping value indices and up to 8 sets of chroma filter coefficients and clipping value indices. To reduce bit overhead, different classifications of filter coefficients for the luma component can be merged. The slice header signals the index of the APS used for the current slice. The signaling of the ALF is CTU-based in VVC Draft 7.

[0126] The clipping value index decoded from the APS allows the clipping values to be determined using a table of clipping values for luma and chroma components. These clipping values depend on the internal bit depth. For example, the clipping value table can be obtained by the following formula:

[0127]

number

[0128] where B is equal to the internal bit depth, α is a predefined constant value equal to 2.35, and N is equal to 4, which is the number of clipping values allowed in VVC Draft 7. Table 2 shows an example of the output of equation (14).

[0129] [Table 2]

[0130] The slice header can signal up to seven APS indices to specify the luma filter set to be used for the current slice. The filtering process can be further controlled at the CTB level. A flag can be signaled to indicate whether the ALF is applied to the luma CTB. The luma CTB can select one filter set from 16 fixed filter sets and a filter set from the APS. A filter set index is signaled to the luma CTB to indicate which filter set is applied. The 16 fixed filter sets can be predefined and hard-coded in both the encoder and decoder.

[0131] For the chroma component, an APS index can be signaled in the slice header to indicate the chroma filter set used for the current slice. At the CTB level, if multiple chroma filter sets exist within an APS, a filter index can be signaled for each luma CTB.

[0132] The filter coefficients can be quantized with a norm equal to 128. To limit the complexity of the multiplications, bitstream adaptation can be applied to ensure that coefficient values for non-center positions are in the range of -27 to 27-1 (inclusive). Center position coefficients are not signaled in the bitstream and are assumed to be equal to 128.

[0133] In some relevant examples, such as VVC Draft 7, the syntax and semantics of clipping indexes and corresponding values can be defined as follows:

[0134] alf_luma_clip_idx[sfIdx][j] specifies the clipping index of the clipping value used before multiplying the j-th coefficient of the signaled luma filter indicated by sfIdx. It is a bitstream conformance requirement that the alf_luma_clip_idx[sfIdx][j] value for sfIdx=0..alf_luma_num_filters_signalled_minus1 and j=0..11 be in the range 0 to 3 (inclusive).

[0135] The luma filter clipping values AlfClipL[adaptation_parameter_set_id][filtIdx] with elements AlfClipL[adaptation_parameter_set_id][filtIdx][j], with filtIdx=0..NumAlfFilters-1 and j=0..11, are derived in Table 2 according to bitDepth set equal to BitDepthY and clipIdx set equal to alf_luma_clip_idx[alf_luma_coeff_delta_idx[filtIdx]][j].

[0136] alf_chroma_clip_idx[altIdx][j] specifies the clipping index of the clipping value used before multiplying the j-th coefficient of the alternative chroma filter with index altIdx. It is a bitstream conformance requirement that the value of alf_chroma_clip_idx[altIdx][j] for altIdx=0..alf_chroma_num_alt_filters_minus1, j=0..5 be in the range 0..3 (inclusive).

[0137] The chroma filter clipping value AlfClipC[adaptation_parameter_set_id][altIdx] with element AlfClipC[adaptation_parameter_set_id][altIdx][j], with altIdx=0..alf_chroma_num_alt_filters_minus1, j=0..5, is derived in Table 2 according to bitDepth set equal to BitDepthC and clipIdx set equal to alf_chroma_clip_idx[altIdx][j].

[0138] On the decoder side, when ALF is enabled for a CTB, each sample R(i,j) in a CU is filtered to generate a corresponding sample value R'(i,j) as shown below.

[0139]

number

[0140] where f(k,l) denotes the decoded filter coefficients, K(x,y) is the clipping function, and c(k,l) denotes the decoded clipping parameters. The variables k and l are

number

[0141] 10 illustrates an exemplary modified block classification applied to the luma component according to one embodiment of the present disclosure. The modified block classification and filtering employed for samples near horizontal CTU boundaries can reduce the line buffer requirements of ALF. As shown in FIG. 10, a virtual boundary is defined as a line by shifting the horizontal CTU boundary by “N” samples, where N is equal to 4 for the luma component and 2 for the chroma component, respectively.

[0142] The 1D Laplacian gradient computation for a 4x4 block above the virtual boundary uses only the samples above the virtual boundary. Similarly, the 1D Laplacian gradient computation for a 4x4 block below the virtual boundary uses only the samples below the virtual boundary. The quantization of the activity value A is scaled accordingly to account for the reduced number of samples used in the 1D Laplacian gradient computation.

[0143] 11 shows an exemplary modified ALF for the luma component at the virtual boundary according to some embodiments. In the filtering process, a symmetric padding operation at the virtual boundary can be used for both the luma component and the chroma component. As shown in FIG. 11, if the sample being filtered is located below the virtual boundary, the adjacent samples located above the virtual boundary are padded. Meanwhile, the corresponding samples on the opposite side are also padded symmetrically.

[0144] To improve coding efficiency, a coding unit-synchronized image quadtree-based ALP is used in some related examples. The luma image can be divided into multiple multi-level quadtree partitions, and the boundaries of each partition are aligned with the boundaries of the largest coding unit (LCU). Each partition has its own filtering process, so it can be called a filter unit (FU).

[0145] The two-pass encoding flow is described as follows: In the first pass, the quadtree division pattern and optimal filter of each FU are determined. The filtering distortion is estimated by fast filtering distortion estimation (FFDE) during the decision process. The reconstructed image is filtered according to the determined quadtree division patterns and selected filters of all FUs. In the second pass, CU-synchronous ALF on / off control is performed. According to the ALF on / off result, the filtered image from the first pass is partially restored by the reconstructed image.

[0146] FIG. 12 illustrates an exemplary LCU-aligned image quadtree partitioning according to one embodiment. A top-down partitioning strategy is adopted to divide an image into multi-level quadtree partitions using a rate-distortion criterion. Each partition is called a filter unit. The partitioning process aligns the quadtree partitions to LCU boundaries. The coding order of the FUs follows the z-scan order. For example, as shown in FIG. 12, an image is divided into 10 FUs, and the coding order is FU0, FU1, FU2, FU3, FU4, FU5, FU6, FU7, FU8, and FU9.

[0147] Figure 13 shows an exemplary quadtree division pattern corresponding to Figure 12. To indicate the quadtree division pattern of an image, division flags can be coded and transmitted in z-order.

[0148] The filter for each FU can be selected from two filter sets based on a rate-distortion criterion. The first set contains newly derived ½-symmetric square and diamond filters for the current FU. The second set is from a time-delay filter buffer that stores previously derived filters for FUs of previous images. The filter with the smallest rate-distortion cost among these two sets can be selected for the current FU. Similarly, if the current FU is not the smallest FU and can be further divided into four child FUs, the rate-distortion costs of the four child FUs are calculated. By recursively comparing the rate-distortion costs of the divided and undivided cases, a quadtree division pattern for the image can be determined.

[0149] In some related examples, the maximum quadtree division level is 2, which means the maximum number of FUs is 16. During the quadtree division determination, correlation values for deriving Wiener coefficients of 16 FUs at the lowest quadtree level (smallest FU) can be reused. For the remaining FUs, Wiener filters can be derived from the correlations of the 16 FUs at the lowest quadtree level. Therefore, only one frame buffer access is required to derive filter coefficients for all FUs.

[0150] After the quadtree division pattern is determined, CU-synchronized ALF on / off control can be performed to further reduce filtering distortion. By comparing the filtering distortion with the non-filtering distortion, leaf CUs can explicitly switch ALF on / off in their local regions. Coding efficiency can be further improved by redesigning the filter coefficients according to the ALF on / off results. However, the redesign process requires additional frame buffer accesses. In some relevant examples, there is no redesign process after the CU-synchronized ALF on / off determination to minimize the number of frame buffer accesses.

[0151] III. Cross-component adaptive loop filter

[0152] In some related examples, a cross-component adaptive loop filter (CC-ALF) is used, which uses luma sample values to refine each chroma component.

[0153] 14A shows an exemplary arrangement of CC-ALF according to one embodiment of the present disclosure. FIG. 14B shows an exemplary linear diamond-shaped filter applied to the luma channel of each chroma component during CC-ALF operation. The filter coefficients can be transmitted in APS, e.g., 2 10 The input signal is scaled by a factor of 16x16 and rounded for fixed-point representation. The application of the filter is controlled by variable block sizes and is signaled by context coding flags received for each block of samples. Block sizes are received at the slice level for each chroma component along with the CC-ALF enable flag. In one example, block sizes (in chroma samples) of 16x16, 32x32, and 64x64 are supported.

[0154] Table 3 shows the syntax elements related to CC-ALF.

[0155] [Table 3]

[0156] In Table 3, the syntax element alf_ctb_cross_component_cb_idc[xCtb>>CtbLog2SizeY][yCtb>>CtbLog2SizeY] equal to 0 indicates that a cross-component Cb filter is not applied to the block of samples in the Cb color component at the luma location (xCtb, yCtb). The syntax element alf_ctb_cross_component_cb_idc[xCtb>>CtbLog2SizeY][yCtb>>CtbLog2SizeY] not equal to 0 indicates that alf_ctb_cross_component_cb_idc[xCtb>>CtbLog2SizeY][yCtb>>CtbLog2SizeY]-th cross-component Cb filter is applied to the block of samples in the Cb color component at the luma location (xCtb, yCtb).

[0157] The syntax element alf_ctb_cross_component_cr_idc[xCtb>>CtbLog2SizeY][yCtb>>CtbLog2SizeY] equal to 0 indicates that the cross-component Cr filter is not applied to the block of samples in the Cr color component at the luma location (xCtb, yCtb). The syntax element alf_ctb_cross_component_cr_idc[xCtb>>CtbLog2SizeY][yCtb>>CtbLog2SizeY] not equal to 0 indicates that the alf_ctb_cross_component_cr_idc[xCtb>>CtbLog2SizeY][yCtb>>CtbLog2SizeY]-th cross-component Cr filter is applied to the block of samples in the Cr color component at the luma location (xCtb, yCtb).

[0158] IV. Constrained Directional Enhancement Filter

[0159] One goal of the in-loop constrained directionality enhancement filter (CDEF) is to filter out coding artifacts while preserving image details. In some related examples, such as HEVC, the sample adaptive offset (SAO) algorithm can achieve a similar goal by defining signal offsets for different classes of pixels. Unlike SAO, the CDEF is a nonlinear spatial filter. The filter design is constrained to be easily vectorizable so that it can be implemented with single instruction multiple data (SIMD) operations, which is not the case for other nonlinear filters, such as median filters or bilateral filters.

[0160] The CDEF design arises from the following observations: The amount of ringing artifacts in a coded image tends to be roughly proportional to the quantization step size. Although the amount of detail is a property of the input image, the smallest detail retained in the quantized image also tends to be proportional to the quantization step size. For a given quantization step size, the amplitude of the ringing is generally smaller than the amplitude of the detail.

[0161] CDEF works by identifying the orientation of each block and then adaptively filtering along the identified orientation and, to a lesser extent, along orientations rotated 45 degrees from the identified orientation. The strength of the filters is explicitly signaled, allowing for a high degree of control over the blurring. An efficient encoding search is designed for the filter strength. CDEF is based on two previously proposed in-loop filters, a combined filter being adopted in the new AOMedia Video1 (AV) codec.

[0162] FIG. 15 shows an exemplary direction search for an 8x8 block according to one embodiment of the present disclosure. The direction search is performed on the reconstructed pixels immediately after the deblocking filter. Because these pixels are available to the decoder, no signaling of direction is required. The search is performed on 8x8 blocks, which are small enough to properly handle non-linear edges but large enough to reliably estimate direction when applied to the quantized image. Consistent direction across the 8x8 region also facilitates vectorization of the filter. For each block, the direction that best matches the pattern within the respective block is determined by minimizing the sum of squared differences (SSD) between the quantized block and the nearest fully directional block. A fully directional block is one in which all pixels along a line in one direction have the same value.

[0163] One reason for identifying a direction is to align filter taps along that direction to reduce ringing while preserving directional edges or patterns. However, directional filtering alone may not be enough to reduce ringing. It is also desirable to use filter taps for pixels that are not aligned with the primary direction. These extra taps are processed more conservatively to reduce the risk of blurring. For this reason, CDEF defines primary and secondary taps. The complete 2-D CDEF filter is expressed as follows:

[0164]

number

[0165] where D is the damping parameter and S (p) and S (s) are the intensities of the primary and secondary taps, respectively, round(·) rounds away from zero,

number

[0166] Loop recovery in V.AV1

[0167] In addition to traditional deblocking operations, a set of in-loop restoration methods can be used in post-deblocking video coding, typically to remove noise and improve edge quality. These methods are switchable within a frame for tiles of appropriate size. The specific method described is based on a separable symmetric Wiener filter and a dual self-induced filter with subspace projection. Because content statistics can change significantly within a frame, these tools are integrated within a switchable framework, where different tools can be triggered in different regions of the frame.

[0168] In the case of a Wiener filter, every pixel in the degraded frame can be reconstructed as a non-causal filtered version of each pixel in a w × w window around each pixel, where w = 2r + 1 is odd with respect to the integer r. The 2D filter taps are expressed as w in column vectorized form. 2× 1 element vector F, a simple linear minimum mean square error (LMMSE) optimization finds the filter parameters F=H -1 M, where H=E[XX T ] is the autocovariance of x and w in a w × w window around the pixel 2 A column vectorized version of the sample, M=E[YX T ] is the cross-correlation of x and the scalar source sample y to be estimated. The encoder can estimate H and M from the deblocked frame and its realization in the source, and send the resulting filter F to the decoder. However, this also reduces the 2 Not only does transmitting these taps incur a significant bitrate cost, but non-separable filtering also significantly complicates decoding. Therefore, several additional constraints are imposed on the properties of F. First, F is constrained to be separable, so that the filtering can be implemented as separable horizontal and vertical w-tap convolutions. Second, each of the horizontal and vertical filters is constrained to be symmetric. Third, the sum of both the horizontal and vertical filter coefficients is assumed to be 1.

[0169] The locally linear model of guided filtering can be expressed as follows:

[0170]

number

[0171] A locally linear model is used to calculate the filtered output y from the unfiltered sample x, where F and G are determined based on the statistics of the degraded image and the guided image in the neighborhood of the filtered pixel. If the guided image is the same as the degraded image, the resulting so-called self-guided filtering has the effect of edge-preserving smoothing. The specific form of self-guided filtering depends on two parameters: the radius r and the noise parameter e, and can be enumerated as follows:

[0172] (1) The mean μ and variance σ of the pixels in a (2r+1)(2r+1) window around each pixel 2 This can be efficiently achieved by box filtering based on integral imaging.

[0173] (2) For each pixel, f = σ 2 / (σ 2 +e), and calculate g = (1 - f)μ.

[0174] (3) For use, calculate F and G for each pixel as the average of the f and g values within a 3x3 window around the pixel.

[0175] The filtering is controlled by r and e, where higher r results in greater spatial variance and higher e results in greater range variance.

[0176] 16 shows an example subspace projection according to one embodiment. Although neither of the inexpensive restorations X1 and X2 are close to the source Y, a suitable multiplier {α, β} can bring them much closer to the source, as long as they are somewhat moved in the right direction.

[0177] VI. Cross-component filtering for block-level reconstruction

[0178] In some related examples, such as VVC, a cross-component filtering (CCF) process has been proposed and adopted as an additional filtering process after SAO. That is, the CCF process is applied outside of the transform, quantization, inverse quantization, and reconstruction loops. Therefore, the CCF process applied to the current block does not improve the prediction of the subsequent coding block because the reconstructed samples of the current block used as reference samples for the subsequent block are not affected by the CCF process. In such examples, the CCF process is applied at the image level. In order for the CCF process to affect the prediction of the subsequent coding block, in embodiments of the present disclosure, the CCF process can be applied at the block level.

[0179] This disclosure includes methods for applying a CCF process to block-level reconstruction. A CCF process can be defined as a filtering process that uses reconstructed samples of a first color component (e.g., Y or Cb or Cr) as input, and the output can be applied to a second color component different from the first color component. One example of a CCF process is the CC-ALF process described in Section III.

[0180] According to aspects of the present disclosure, a CCF process can be applied before SAO is completed. For example, the CCF process can be applied after inverse quantization and / or inverse transform. The filtered reconstructed samples of the current block output by the CCF process can be used as reference samples for subsequent blocks and / or can be used to generate predicted samples for the subsequent blocks.

[0181] 17 illustrates an exemplary CCF applied after the inverse transform of a block, according to one embodiment of the present disclosure. The filtered samples can be used for intra-prediction and / or inter-prediction of a subsequent block, and in-loop filtering can be applied on top of the filtered samples output from the CCF process.

[0182] In one embodiment, CCF may be applied only to the chroma color component. The input of the CCF process may be the reconstructed luma samples, and the output may be an offset value that is added on top of the chroma samples of the current block.

[0183] In one embodiment, a CCF process may be applied to the residual, whose input may be the luma residual values derived from the inverse transform, and whose output may be an offset value that is added to the chroma residual values of the current block.

[0184] In one embodiment, a CCF process may be applied to predicted sample values, where the input of the CCF process may be applied to predicted luma sample values, and the output may be an offset value that is added to the predicted chroma sample values of the current block.

[0185] In one embodiment, the filter coefficients used in the CCF process may be signaled, for example, in a video parameter set (VPS), a sequence parameter set (SPS), a picture parameter set (PPS), an adaptation parameter set (APS), a slice header, or a tile header.

[0186] In one embodiment, the filter coefficients used in the CCF process are predefined constants.

[0187] In one embodiment, the filter coefficients used in the CCF process for the current image are derived from the filter coefficients used in another CCF process, for example, the filter coefficients used in the CCF process for the current image can be derived from the filter coefficients used in the CCF process of a different image and an offset value signaled for the current image.

[0188] In one embodiment, the filter coefficients used in the CCF process of one color component (eg, the chroma component) are derived from the filter coefficients used in the CCF process of another color component (eg, the luma component).

[0189] In one embodiment, filter coefficients are selected for each sample according to some statistics derived from the sample values determined before the CCF process is applied.

[0190] In some embodiments, a determination is made as to whether the CCF process is enabled. The CCF process is applied based on a determination that the CCF process is enabled.

[0191] In one embodiment, the CCF process may be enabled or disabled for specific block sizes. In one example, the CCF process is disabled for blocks whose width and / or height are smaller than a given threshold.

[0192] In one embodiment, the CCF process may be enabled or disabled for a particular partitioning scheme. In one example, the CCF process is disabled when the luma and chroma components have different block sizes, e.g., when the luma and chroma components have different partitioning schemes or semi-separate partitioning schemes.

[0193] In one embodiment, the CCF process may be enabled or disabled for a particular prediction mode. In one example, the CCF process is disabled in DC mode. In one example, the CCF process is disabled in planar mode. In one example, the CCF process is disabled for one or more of SMOOTH, SMOOTH_H, and SMOOTH_V modes. In one example, the CCF process is disabled in Paeth prediction mode. In one example, the CCF process is disabled in SKIP mode. In one example, the CCF process is enabled in directional mode. In one example, the CCF process is enabled in inter-intra decoding mode.

[0194] In one embodiment, the filter shape of the CCF process may depend on the block width and / or height.

[0195] In one embodiment, whether the CCF process is enabled depends on the transform coefficients of the inverse transform process. In one example, if the transform coefficients (before or after inverse quantization) are all zero, the CCF process is not applied. In one example, if only the DC transform coefficients (before or after inverse quantization) are non-zero, the CCF process is not applied. In one example, if only the low-frequency transform coefficients (before or after inverse quantization) are non-zero, the CCF process is not applied. A low-frequency transform coefficient is a transform coefficient located at coordinates (x, y), where x and / or y are less than a predetermined threshold.

[0196] In one embodiment, whether the CCF process is enabled depends on the quantization parameter of the inverse quantization process.

[0197] VII. Flowchart

[0198] 18 shows a flowchart outlining an exemplary process (1800) according to one embodiment of the present disclosure. In various embodiments, the process (1800) is performed by processing circuitry within the terminal devices (210), (220), (230), and (240), a processing circuitry performing the functions of the video encoder (303), a processing circuitry performing the functions of the video decoder (310), a processing circuitry performing the functions of the video decoder (410), a processing circuitry performing the functions of the intra prediction module (452), a processing circuitry performing the functions of the video encoder (503), a processing circuitry performing the functions of the predictor (535), a processing circuitry performing the functions of the intra encoder (622), a processing circuitry performing the functions of the intra decoder (772), or the like. In some embodiments, the process (1800) is implemented with software instructions, and thus, the processing circuitry performs the process (1800) when the processing circuitry executes the software instructions.

[0199] The process (1800) may generally begin at step (S1810), where the process (1800) decodes prediction information for a current block in a current image that is part of a coded video sequence. The prediction information indicates a CCF process for the current block. The process (1800) then proceeds to step (S1820).

[0200] In step (S1820), the process (1800) generates filtered reconstructed samples of the current block by applying a CCF process to at least one of the predicted samples, residual values, or reconstructed samples of the current block. The filtered reconstructed samples of the current block are used to reconstruct the subsequent block. The process (1800) then proceeds to step (S1830).

[0201] In step S1830, the process 1800 reconstructs the current block and subsequent blocks based on the filtered reconstructed samples of the current block, after which the process 1800 ends.

[0202] In one embodiment, the process (1800) generates filtered reconstructed samples of the current block by applying a CCF process to the output of at least one of the inverse quantization process and the inverse transform process of the current block.

[0203] In one embodiment, the process (1800) reconstructs the current block by adding the output of the CCF process, which includes the offset values of the current block, to the chroma reconstructed samples of the current block.

[0204] In one embodiment, the process (1800) reconstructs the current block by adding the output of the CCF process, which includes the offset value of the current block, to the chroma residual value of the current block.

[0205] In one embodiment, the process (1800) reconstructs the current block by adding the output of the CCF process, which includes the offset values of the current block, to the chroma prediction samples of the current block.

[0206] In one embodiment, the filter coefficients of the CCF process are included in the prediction information.

[0207] In one embodiment, the filter coefficients of the CCF process are predefined constants.

[0208] In one embodiment, the filter coefficients of the CCF process used in the current image are determined based on the filter coefficients of the CCF process used in another image and an offset value of the filter coefficients of the CCF process used in the current image, and the offset value may be included in the prediction information.

[0209] In one embodiment, the filter coefficients of the CCF process used for one color component are determined based on the filter coefficients of the CCF process used for another color component.

[0210] In one embodiment, the filter coefficients of the CCF process are determined based on the sample values of the current block determined before the CCF process is applied.

[0211] In one embodiment, whether the CCF process is enabled is determined based on one of the partitioning scheme, prediction mode, block width, block height, transform coefficients, and quantization parameters.

[0212] VIII. Computer Systems

[0213] The techniques described above can be implemented as computer software using computer-readable instructions and physically stored on one or more computer-readable media. For example, Figure 19 illustrates a computer system (1900) suitable for implementing certain embodiments of the disclosed subject matter.

[0214] Computer software can be coded using any suitable machine or computer language that can be assembled, compiled, linked, or similar mechanisms to create code containing instructions that can be executed by one or more computer central processing units (CPUs), graphics processing units (GPUs), etc., directly, or through interpretation, microcode execution, etc.

[0215] The instructions may be executed on various types of computers or components thereof, including, for example, personal computers, tablet computers, servers, smartphones, gaming devices, Internet of Things devices, and the like.

[0216] 19 for computer system (1900) are exemplary in nature and are not intended to suggest any limitation on the scope of use or functionality of the computer software implementing 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 (1900).

[0217] The computer system (1900) 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 (e.g., keystrokes, swipes, data glove movements), audio input (e.g., voice, clapping), visual input (e.g., gestures), or olfactory input (not shown). Human interface devices may also be used to capture certain media not necessarily directly associated with conscious human input, such as audio (e.g., voice, music, ambient sounds), images (e.g., scanned images, photographic images obtained from a still image camera), and video (e.g., two-dimensional video, three-dimensional video, including stereoscopic video).

[0218] The input human interface devices may include one or more of a keyboard (1901), a mouse (1902), a trackpad (1903), a touchscreen (1910), a data glove (not shown), a joystick (1905), a microphone (1906), a scanner (1907), and a camera (1908) (only one of each is shown).

[0219] The computer system (1900) may also include certain human interface output devices. Such human interface output devices may stimulate one or more of the human user's senses, for example, through tactile output, sound, light, and smell / taste. Such human interface output devices may include haptic output devices (e.g., haptic feedback via a touchscreen (1910), data gloves (not shown), or joystick (1905), although some haptic feedback devices may not function as input devices), audio output devices (speakers (1909), headphones (not shown), etc.), visual output devices (screens (1910), including CRT screens, LCD screens, plasma screens, OLED screens, etc., each with or without touchscreen input capability, each with or without haptic feedback capability, and some capable of outputting two-dimensional visual output or three-dimensional or higher-dimensional output via means such as stereographic output, virtual reality glasses (not shown), holographic displays, and smoke tanks (not shown), and printers (not shown). These visual output devices (such as a screen (1910)) can be connected to the system bus (1948) via a graphics adapter (1950).

[0220] The computer system (1900) may also include human-accessible storage and associated media such as optical or similar media (1921), including CD / DVD ROM / RW (1920) with CD / DVD, thumb drives (1922), removable hard drives or solid state drives (1923), legacy magnetic media (not shown) such as tape and floppy disks, and specialized ROM / ASIC / PLD-based devices (not shown) such as security dongles.

[0221] 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.

[0222] The computer system 1900 may also include a network interface 1954 to one or more communications networks 1955. The one or more communications networks 1955 may be, for example, wireless, wired, or optical. The one or more communications networks 1955 may further be local, wide area, metropolitan, automotive and industrial, real-time, delay tolerant, etc. Examples of the one or more communications networks 1955 include local area networks such as Ethernet, cellular networks including WLAN, GSM, 3G, 4G, 5G, LTE, etc., TV wired or wireless wide area digital networks including cable TV, satellite TV, and terrestrial broadcast TV, automotive and industrial networks including CANBus, etc. Certain networks typically require an external network interface adapter connected to a specific general-purpose data port or peripheral bus 1949 (e.g., a USB port on the computer system 1900), while other networks are typically integrated into the core of the computer system 1900 by connecting to the system bus, as 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, the computer system 1900 can communicate with other entities. Such communications can be one-way, for example, to other computer systems using local or wide-area digital networks, receive-only (e.g., broadcast TV), one-way transmit-only (e.g., CANbus to a specific CANbus device), or bidirectional. As noted above, specific protocols and protocol stacks can be used with each of these networks and network interfaces.

[0223] The aforementioned human interface devices, human-accessible storage devices, and network interfaces may be connected to the core (1940) of the computer system (1900).

[0224] A core (1940) may include one or more central processing units (CPUs) (1941), graphics processing units (GPUs) (1942), specialized programmable processing units (1943) in the form of field programmable gate arrays (FPGAs), hardware accelerators for specific tasks (1944), graphics adapters (1950), etc. These devices may be connected via a system bus (1948), along with read-only memory (ROM) (1945), random access memory (1946), and internal mass storage devices (1947) such as internal hard drives or SSDs that are not user-accessible. In some computer systems, the system bus (1948) is accessible in the form of one or more physical plugs to allow expansion with additional CPUs, GPUs, etc. Peripheral devices may be connected directly to the core's system bus (1948) or via a peripheral bus (1949). In one example, a screen (1910) may be connected to the graphics adapter (1950). Peripheral bus architectures include PCI, USB, etc.

[0225] The CPU (1941), GPU (1942), FPGA (1943), and accelerator (1944) can execute certain instructions, which in combination can constitute the aforementioned computer code. The computer code can be stored in ROM (1945) or RAM (1946). Transient data can also be stored in RAM (1946), while permanent data can be stored, for example, in internal mass storage (1947). Cache memory, which can be closely associated with one or more of the CPU (1941), GPU (1942), mass storage (1947), ROM (1945), RAM (1946), etc., can be used to enable fast storage and retrieval of data from any memory device.

[0226] The computer-readable medium may 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 kind well known and available to those skilled in the computer software arts.

[0227] By way of example and not limitation, the architecture (1900), and in particular a computer system having a core (1940), can provide functionality as a result of the processor (including a CPU, GPU, FPGA, accelerator, etc.) executing software embodied in one or more tangible computer-readable media. Such computer-readable media can be media associated with the user-accessible mass storage devices introduced above, as well as specific storage devices of the core (1940) that are non-transitory in nature, such as the core's internal mass storage device (1947) or ROM (1945). Software implementing various embodiments of the present disclosure can be stored in such devices and executed by the core (1940). The computer-readable media can include one or more memory devices or chips, as appropriate. The software can cause the core (1940), and in particular the processor (including a CPU, GPU, FPGA, etc.) therein, to perform specific processes or portions of specific processes described herein, including defining data structures stored in RAM (1946) and modifying such data structures in accordance with the software-defined processes. Additionally or alternatively, a computer system may provide functionality as a result of logic hardwired or otherwise embodied in circuitry (e.g., accelerators (1944)) that can operate in place of or in conjunction with software to perform specific processes or portions of specific processes described herein. References to software may encompass logic, and vice versa, as appropriate. References to computer-readable media may encompass circuitry (such as integrated circuits (ICs)) that stores software for execution, circuitry that embodies logic for execution, or both, as appropriate. The present disclosure encompasses any suitable combination of hardware and software.

[0228] While this disclosure has described several exemplary embodiments, there are alterations, modifications, and various substitute equivalents that fall 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 that, although not explicitly shown or described herein, embody the principles of the present disclosure and are therefore within its spirit and scope.

[0229] Appendix A: Acronyms ALF: Adaptive Loop Filter AMVP: Advanced Motion Vector Prediction APS: Adaptive Parameter Set ASIC: Application Specific Integrated Circuit ATMVP: Alternative / Advanced Temporal Motion Vector Prediction AV1:AOMedia Video 1 AV2:AOMedia Video 2 BMS: Benchmark Set BV: Block Vector CANBus: Controller Area Network Bus CB: coding block CC-ALF: Cross-component adaptive loop filter CD: Compact Disc CDEF: A constrained directionality enhancement filter CPR: Current image reference CPU: Central Processing Unit CRT: cathode ray tube CTB: coding tree block CTU: Coding Tree Unit CU: Coding Unit DPB: Decoder Picture Buffer DPS: Decoding Parameter Set DVD: Digital Video Disc FPGA: Field Programmable Gate Array JCCR: Joint CbCr Residual Coding JVET: Joint Video Exploration Team GOP: Group of Pictures GPU: Graphics Processing Unit GSM: Global System for Mobile Communications HDR: High Dynamic Range HEVC: High Efficiency Video Coding HRD: Hypothetical Reference Decoder IBC: Intra-block copy IC: Integrated Circuit ISP: Intra-subpartition JEM: Joint Exploration Model LAN: Local Area Network LCD: Liquid crystal display LR: Loop Recovery Filter LTE: Long Term Evolution MPM: Most Probable Mode MV: Motion Vector OLED: Organic Light Emitting Diode PB: Predicted Block PCI: Peripheral Component Interconnect PDPC: Position-dependent prediction combination PLD: Programmable Logic Device PPS: Image Parameter Set PU: Prediction Unit RAM: Random Access Memory ROM: Read-Only Memory SAO: Sample Adaptive Offset SCC: Screen Content Coding SDR: Standard Dynamic Range SEI:Supplementary Enhancement Information SNR: Signal to Noise Ratio SPS: Sequence Parameter Set SSD: Solid State Drive TU: Conversion unit USB: Universal Serial Bus VPS: Video Parameter Set VUI: Video Usability Information VVC: Versatile Video Coding WAIP: Wide-angle Intra Prediction [Explanation of symbols]

[0230] 200 Communication Systems 210, 220, 230, 240 Terminal equipment 250 Network 300 Streaming System 301 Video Sources 302 Video Images 303 Video Encoder 304 Encoded Video Data (Encoded Video Bitstream) 305 Streaming Server 306,308 Client Subsystem 307,309 Copying of coded video data 310 Video Decoder 311 Video Images 312 Display 313 Capture Subsystem 320,330 Electronic equipment 401 Channel 410 Video Decoder 412 Rendering Devices 415 Buffer Memory 420 Entropy Decoder / Parser 421 Symbol 430 Electronic equipment 431 Receiver 451 Scaler / Descaler Unit 452 intra prediction units 453 Motion Compensation Prediction Unit 455 Aggregator 456 Loop Filter Unit 457 Reference Image Memory 458 Current Image Buffer 501 Video Sources 503 Video Encoder 520 Electronic equipment 530 Source Coder 532 encoding engine 533 (local) decoder 534 Reference Image Memory (Reference Image Cache) 535 Predictors 540 Transmitter 543 coded video sequence 545 Entropy Coder 550 Controller 560 Communication Channels 603 Video Encoder 621 General-purpose controller 622 Intra Encoder 623 Residual Calculator 624 Residual Encoder 625 Entropy Encoder 626 Switch 628 Residual Decoder 630 Interencoder 710 Video Decoder 771 Entropy Decoder 772 Intra Decoder 773 Residual Decoder 774 Reconstruction Module 780 Interdecoder 1900 Computer System 1901 keyboard 1902 Mouse 1903 Trackpad 1905 Joystick 1906 Microphone 1907 Scanner 1908 Camera 1909 Speaker 1910 screen (touch screen) 1920 CD / DVD / ROM / RW 1921 Optical media 1922 thumb drive 1923 Removable Hard Disk Drive or Solid State Drive 1940 Core 1941 Central Processing Unit (CPU) 1942 Graphics Processing Unit (GPU) 1943 Field Programmable Gate Array (FPGA) 1944 Hardware Accelerator 1945 Read-Only Memory (ROM) 1946 Random Access Memory 1947 Internal mass storage 1948 System Bus 1949 General Purpose Data Port or Peripheral Bus 1950 Graphics Adapter 1954 Network Interface 1955 Communications Network

Claims

1. A decoding method in a decoder, comprising: decoding prediction information for a current block in a current image that is part of a coded video sequence; determining whether a cross-component filtering (CCF) process is enabled for the current block based on one of a partitioning scheme, a prediction mode, a transform coefficient, and a quantization parameter; generating filtered reconstructed samples of the current block by applying the CCF process to at least one of predicted samples, residual values, or reconstructed samples of the current block based on the CCF process being enabled, wherein the filtered reconstructed samples of the current block are used to reconstruct a subsequent block; reconstructing the current block and a subsequent block based on the filtered reconstructed samples of the current block.

2. The method described in claim 1, wherein the generating step includes a step of generating the filtered reconstructed sample of the current block by applying the CCF process to the output of at least one of an inverse quantization process and an inverse transform process of the current block.

3. The method described in claim 1, wherein the reconstructing step includes a step of reconstructing the current block by adding the output of the CCF process, which includes an offset value for the current block, to the saturation reconstruction samples of the current block.

4. The method described in claim 1, wherein the reconstructing step includes a step of reconstructing the current block by adding the output of the CCF process, which includes an offset value for the current block, to the saturation residual value of the current block.

5. The method described in claim 1, wherein the reconstructing step includes a step of reconstructing the current block by adding the output of the CCF process, which includes an offset value for the current block, to the chroma prediction sample of the current block.

6. The method of claim 1, wherein the filter coefficients of the CCF process are included in the prediction information.

7. The method of claim 1, wherein the filter coefficients of the CCF process are predefined constants.

8. The method described in claim 1, wherein the filter coefficients of the CCF process used in the current image are determined based on the filter coefficients of the CCF process used in another image and an offset value of the filter coefficients of the CCF process used in the current image.

9. The method described in claim 1, wherein the filter coefficients of the CCF process used in one color component are determined based on the filter coefficients of the CCF process used in another color component.

10. The method described in claim 1, wherein the filter coefficients of the CCF process are determined based on sample values of the current block determined before the CCF process is applied.

11. The method described in claim 1, wherein in the step of reconstructing the current block and the subsequent block based on the filtered reconstructed sample of the current block, the filtered reconstructed sample of the current block is used as a reference sample for the subsequent block or is used to generate a predicted sample of the subsequent block.

12. An apparatus comprising a processing circuit configured to perform a method according to any one of claims 1 to 11.

13. A computer program comprising instructions which, when executed by at least one processor, cause the at least one processor to perform a method according to any one of claims 1 to 11.

Images