WO2021138476A1

WO2021138476A1 - Coding of chrominance residuals

Info

Publication number: WO2021138476A1
Application number: PCT/US2020/067547
Authority: WO
Inventors: Xianglin Wang; Xiaoyu XIU; Yi-Wen Chen; Tsung-Chuan MA; Hong-Jheng Jhu; Bing Yu
Original assignee: Beijing Dajia Internet Information Technology Co., Ltd.
Priority date: 2019-12-30
Filing date: 2020-12-30
Publication date: 2021-07-08
Also published as: CN114846807A

Abstract

This application is directed to decoding video data. An electronic device reconstructs a plurality of residual blocks of a video frame from a video bitstream. Luma residuals and chroma residuals of the plurality of residual blocks of the video frame are clipped into a first dynamic range and a second dynamic range, respectively. For each residual block, a luma residual for a luma component and chroma residuals for two chroma components are determined from the clipped luma and chroma residuals of the plurality of residual blocks of the video frame. In some embodiments, the electronic device determines whether a joint coding of chroma residuals (JCCR) mode is enabled. In accordance with a determination that the JCCR mode is enabled, the chroma residuals for the two chroma components are determined jointly according to a JCCR scheme.

Description

Coding of Chrominance Residuals

RELATED APPLICATIONS

[0001] The present application claims priority to U.S. Provision Application No. 62/955,

319, entitled “Joint Coding of Chrominance Residuals”, filed on December 30, 2019, which is incorporated by reference in its entirety.

[0002] The present application is related to PCT Application No. PCT/US2020/030743, titled “Methods and Apparatus of Joint Coding of Chroma Residuals”, filed April 30, 2020, which claims priority to U.S. Provisional Patent Application No. 62/841,158, entitled “Joint Coding of Chrominance Residuals”, filed April 30, 2019, all of which are incorporated by reference in their entirety.

TECHNICAL FIELD

[0003] The present application generally relates to video data coding and compression, and in particular, to methods and apparatus of decoding of chroma residuals.

BACKGROUND

[0004] Digital video is supported by a variety of electronic devices, such as digital televisions, laptop or desktop computers, tablet computers, digital cameras, digital recording devices, digital media players, video gaming consoles, smart phones, video teleconferencing devices, video streaming devices, etc. The electronic devices transmit, receive, encode, decode, and/or store digital video data by implementing video compression/decompression standards as defined by MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), High Efficiency Video Coding (HEVC), and Versatile Video Coding (VVC) standard. Video compression typically includes performing spatial (intra frame) prediction and/or temporal (inter frame) prediction to reduce or remove redundancy inherent in the video data. For block-based video coding, a video frame is partitioned into one or more slices, each slice having multiple video blocks, which may also be referred to as coding tree units (CTUs). Each CTU may contain one coding unit (CU) or recursively split into smaller CUs until the predefined minimum CU size is reached. Each CU (also named leaf CU) contains one or multiple transform units (TUs) and each CU also contains one or multiple prediction units (PUs). Each CU can be coded in either intra, inter or IBC modes. Video blocks in an intra coded (I) slice of a video frame are encoded using spatial prediction with respect to reference samples in neighboring blocks within the same video frame. Video blocks in an inter coded (P or B) slice of a video frame may use spatial prediction with respect to reference samples in neighboring blocks within the same video frame or temporal prediction with respect to reference samples in other previous and/or future reference video frames.

[0005] Spatial or temporal prediction based on a reference block that has been previously encoded, e.g., a neighboring block, results in a predictive block for a current video block to be coded. The process of finding the reference block may be accomplished by block matching algorithm. Residual data representing pixel differences between the current block to be coded and the predictive block is referred to as a residual block or prediction errors. An inter- coded block is encoded according to a motion vector that points to a reference block in a reference frame forming the predictive block, and the residual block. The process of determining the motion vector is typically referred to as motion estimation. An intra coded block is encoded according to an intra prediction mode and the residual block. For further compression, the residual block is transformed from the pixel domain to a transform domain, e.g., frequency domain, resulting in residual transform coefficients, which may then be quantized. The quantized transform coefficients, initially arranged in a two-dimensional array, may be scanned to produce a one-dimensional vector of transform coefficients, and then entropy encoded into a video bitstream to achieve even more compression.

[0006] The encoded video bitstream is then saved in a computer-readable storage medium (e.g., flash memory) to be accessed by another electronic device with digital video capability or directly transmitted to the electronic device wired or wirelessly. The electronic device then performs video decompression (which is an opposite process to the video compression described above) by, e.g., parsing the encoded video bitstream to obtain syntax elements from the bitstream and reconstructing the digital video data to its original format from the encoded video bitstream based at least in part on the syntax elements obtained from the bitstream, and renders the reconstructed digital video data on a display of the electronic device. [0007] To maintain the flexibility and scalability, a video coding standard typically defines options for the syntax of an encoded video bitstream, which detail parameters allowed for the syntax in the bitstream. In many cases, the options also provide details about the decoding operations a decoder should perform to derive the syntax parameters from the bitstream and achieve correct results in decoding. With digital video quality going from high definition, to 4Kx2K or even 8Kx4K, the amount of vide data to be encoded/decoded grows exponentially. It is a constant challenge in terms of how the video data can be encoded/decoded more efficiently while maintaining the image quality of the decoded video data.

SUMMARY

[0008] The present application describes implementations related to video data encoding and decoding and, more particularly, to methods and apparatus of decoding of luma and chroma residuals. In one aspect of this application, a method of decoding video data includes reconstructing, from a video bitstream, a plurality of residual blocks of a video frame. The method further includes clipping luma residuals and chroma residuals of the plurality of residual blocks of the video frame into a first dynamic range and a second dynamic range, respectively, and determining a luma residual for a luma component and the chroma residuals for two chroma components of each residual block of the video frame from the clipped luma and chroma residuals of the plurality of residual blocks of the video frame. In some embodiments, the method further includes determining whether a joint coding of chroma residuals (JCCR) mode is enabled. In accordance with a determination that the JCCR mode is enabled, the chroma residuals for the two chroma components of the video frame are determined jointly according to a scheme of joint coding of chroma residuals. In an example, one or both of the first dynamic range and the second dynamic range are defined as [-2^K_1, 2^{K 1} - 1 ] or [-(2^{K 1}-1), 2^{K 1} - 1 ], where K is an integer (e.g., 16). K may correspond to a predefined bit depth or a coding bit depth of the video frame.

[0009] In another aspect of the application, an electronic device includes one or more processing units, memory and a plurality of programs stored in the memory. The programs, when executed by the one or more processing units, cause the electronic device to perform the method of encoding video data as described above.

[0010] In yet another aspect of the application, a non-transitory computer readable storage medium stores a plurality of programs for execution by an electronic device having one or more processing units. The programs, when executed by the one or more processing units, cause the electronic device to perform the method of decoding video data as described above.

BRIEF DESCRIPTION OF DRAWINGS

[0011] The accompanying drawings, which are included to provide a further understanding of the implementations and are incorporated herein and constitute a part of the specification, illustrate the described implementations and together with the description serve to explain the underlying principles. Like reference numerals refer to corresponding parts. [0012] FIG. l is a block diagram illustrating an exemplary video encoding and decoding system in accordance with some implementations of the present disclosure.

[0013] FIG. 2 is a block diagram illustrating an exemplary video encoder in accordance with some implementations of the present disclosure.

[0014] FIG. 3 is a block diagram illustrating an exemplary video decoder in accordance with some implementations of the present disclosure.

[0015] FIGS. 4A through 4E are block diagrams illustrating how a frame is recursively partitioned into multiple video blocks of different sizes and shapes in accordance with some implementations of the present disclosure.

[0016] FIGS. 5A and 5B are flowcharts illustrating an exemplary process by which a video encoder implements the techniques of encoding video data using a joint coding of chroma residuals scheme in accordance with some implementations of the present disclosure.

[0017] FIGS. 6A through 6C are flowcharts illustrating an exemplary process by which a video decoder implements the techniques of decoding video data using the joint coding of chroma residuals scheme in accordance with some implementations of the present disclosure. [0018] FIG. 7 is a block diagram of an exemplary video decoder configured for luma mapping with chroma scaling, in accordance with some embodiments.

[0019] FIG. 8 is a block diagram of an exemplary video decoder configured for implementing an adaptive color transform (ACT), in accordance with some embodiments.

[0020] FIGS. 9A and 9B are block diagrams of two example residual decoding subsystems that can be applied in a video decoder, in accordance with some embodiments.

[0021] FIGS. 10A and 10B are block diagrams of another two example residual decoding subsystems, in accordance with some embodiments.

[0022] FIGS. 11 A and 1 IB are a flowchart of a video decoding method implemented by an electronic device, in accordance with some embodiments.

DETAILED DESCRIPTION

[0023] Reference will now be made in detail to specific implementations, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous non-limiting specific details are set forth in order to assist in understanding the subject matter presented herein. But it will be apparent to one of ordinary skill in the art that various alternatives may be used without departing from the scope of claims and the subject matter may be practiced without these specific details. For example, it will be apparent to one of ordinary skill in the art that the subject matter presented herein can be implemented on many types of electronic devices with digital video capabilities.

[0024] FIG. 1 is a block diagram illustrating an exemplary system 10 for encoding and decoding video blocks in parallel in accordance with some implementations of the present disclosure. As shown in FIG. 1, system 10 includes a source device 12 that generates and encodes video data to be decoded at a later time by a destination device 14. Source device 12 and destination device 14 may comprise any of a wide variety of electronic devices, including desktop or laptop computers, tablet computers, smart phones, set-top boxes, digital televisions, cameras, display devices, digital media players, video gaming consoles, video streaming device, or the like. In some implementations, source device 12 and destination device 14 are equipped with wireless communication capabilities.

[0025] In some implementations, destination device 14 may receive the encoded video data to be decoded via a link 16. Link 16 may comprise any type of communication medium or device capable of moving the encoded video data from source device 12 to destination device 14. In one example, link 16 may comprise a communication medium to enable source device 12 to transmit the encoded video data directly to destination device 14 in real-time. The encoded video data may be modulated according to a communication standard, such as a wireless communication protocol, and transmitted to destination device 14. The communication medium may comprise any wireless or wired communication medium, such as a radio frequency (RF) spectrum or one or more physical transmission lines. The communication medium may form part of a packet-based network, such as a local area network, a wide-area network, or a global network such as the Internet. The communication medium may include routers, switches, base stations, or any other equipment that may be useful to facilitate communication from source device 12 to destination device 14.

[0026] In some other implementations, the encoded video data may be transmitted from output interface 22 to a storage device 32. Subsequently, the encoded video data in storage device 32 may be accessed by destination device 14 via input interface 28. Storage device 32 may include any of a variety of distributed or locally accessed data storage media such as a hard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing encoded video data. In a further example, storage device 32 may correspond to a file server or another intermediate storage device that may hold the encoded video data generated by source device 12. Destination device 14 may access the stored video data from storage device 32 via streaming or downloading. The file server may be any type of computer capable of storing encoded video data and transmitting the encoded video data to destination device 14. Exemplary file servers include a web server (e.g., for a website), an FTP server, network attached storage (NAS) devices, or a local disk drive. Destination device 14 may access the encoded video data through any standard data connection, including a wireless channel (e.g., a Wi-Fi connection), a wired connection (e.g., DSL, cable modem, etc.), or a combination of both that is suitable for accessing encoded video data stored on a file server. The transmission of encoded video data from storage device 32 may be a streaming transmission, a download transmission, or a combination of both.

[0027] As shown in FIG. 1, source device 12 includes a video source 18, a video encoder

20 and an output interface 22. Video source 18 may include a source such as a video capture device, e.g., a video camera, a video archive containing previously captured video, a video feed interface to receive video from a video content provider, and/or a computer graphics system for generating computer graphics data as the source video, or a combination of such sources. As one example, if video source 18 is a video camera of a security surveillance system, source device 12 and destination device 14 may form camera phones or video phones. However, the implementations described in the present application may be applicable to video coding in general, and may be applied to wireless and/or wired applications.

[0028] The captured, pre-captured, or computer-generated video may be encoded by video encoder 20. The encoded video data may be transmitted directly to destination device 14 via output interface 22 of source device 12. The encoded video data may also (or alternatively) be stored onto storage device 32 for later access by destination device 14 or other devices, for decoding and/or playback. Output interface 22 may further include a modem and/or a transmitter.

[0029] Destination device 14 includes an input interface 28, a video decoder 30, and a display device 34. Input interface 28 may include a receiver and/or a modem and receive the encoded video data over link 16. The encoded video data communicated over link 16, or provided on storage device 32, may include a variety of syntax elements generated by video encoder 20 for use by video decoder 30 in decoding the video data. Such syntax elements may be included within the encoded video data transmitted on a communication medium, stored on a storage medium, or stored a file server.

[0030] In some implementations, destination device 14 may include a display device 34, which can be an integrated display device and an external display device that is configured to communicate with destination device 14. Display device 34 displays the decoded video data to a user, and may comprise any of a variety of display devices such as a liquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED) display, or another type of display device.

[0031] Video encoder 20 and video decoder 30 may operate according to proprietary or industry standards, such as VVC, HEVC, MPEG-4, Part 10, Advanced Video Coding (AVC), or extensions of such standards. It should be understood that the present application is not limited to a specific video coding/decoding standard and may be applicable to other video coding/decoding standards. It is generally contemplated that video encoder 20 of source device 12 may be configured to encode video data according to any of these current or future standards. Similarly, it is also generally contemplated that video decoder 30 of destination device 14 may be configured to decode video data according to any of these current or future standards.

[0032] Video encoder 20 and video decoder 30 each may be implemented as any of a variety of suitable encoder circuitry, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), discrete logic, software, hardware, firmware or any combinations thereof.

When implemented partially in software, an electronic device may store instructions for the software in a suitable, non-transitory computer-readable medium and execute the instructions in hardware using one or more processors to perform the video coding/decoding operations disclosed in the present disclosure. Each of video encoder 20 and video decoder 30 may be included in one or more encoders or decoders, either of which may be integrated as part of a combined encoder/decoder (CODEC) in a respective device.

[0033] FIG. 2 is a block diagram illustrating an exemplary video encoder 20 in accordance with some implementations described in the present application. Video encoder 20 may perform intra and inter predictive coding of video blocks within video frames. Intra predictive coding relies on spatial prediction to reduce or remove spatial redundancy in video data within a given video frame or picture. Inter predictive coding relies on temporal prediction to reduce or remove temporal redundancy in video data within adjacent video frames or pictures of a video sequence.

[0034] As shown in FIG. 2, video encoder 20 includes video data memory 40, prediction processing unit 41, decoded picture buffer (DPB) 64, summer 50, transform processing unit 52, quantization unit 54, and entropy encoding unit 56. Prediction processing unit 41 further includes motion estimation unit 42, motion compensation unit 44, partition unit 45, intra prediction processing unit 46, and intra block copy (BC) unit 48. In some implementations, video encoder 20 also includes inverse quantization unit 58, inverse transform processing unit 60, and summer 62 for video block reconstruction. An in-loop filter 66 may be positioned between summer 62 and DPB 64, and includes a deblocking filter to filter block boundaries and remove blockiness artifacts from reconstructed video. The in-loop filter 66 further includes a sample adaptive offset (S AO) and adaptive in-loop filter (ALF) to filter the output of summer 62 before the output of summer 62 is put into DPB 64 and used to code other video blocks. Video encoder 20 may take the form of a fixed or programmable hardware unit or may be divided among one or more of the illustrated fixed or programmable hardware units.

[0035] Video data memory 40 may store video data to be encoded by the components of video encoder 20. The video data in video data memory 40 may be obtained, for example, from video source 18. DPB 64 is a buffer that stores reference video data for use in encoding video data by video encoder 20 (e.g., in intra or inter predictive coding modes). Video data memory 40 and DPB 64 may be formed by any of a variety of memory devices. In various examples, video data memory 40 may be on-chip with other components of video encoder 20, or off-chip relative to those components.

[0036] As shown in FIG. 2, after receiving video data, partition unit 45 within prediction processing unit 41 partitions the video data into video blocks. This partitioning may also include partitioning a video frame into slices, tiles, or other larger coding units (CUs) according to a predefined splitting structures such as quad-tree structure associated with the video data. The video frame may be divided into multiple video blocks (or sets of video blocks referred to as tiles). Prediction processing unit 41 may select one of a plurality of possible predictive coding modes, such as one of a plurality of intra predictive coding modes or one of a plurality of inter predictive coding modes, for the current video block based on error results (e.g., coding rate and the level of distortion). Prediction processing unit 41 may provide the resulting intra or inter prediction coded block to summer 50 to generate a residual block and to summer 62 to reconstruct the encoded block for use as part of a reference frame subsequently. Prediction processing unit 41 also provides syntax elements, such as motion vectors, intra-mode indicators, partition information, and other such syntax information, to entropy encoding unit 56.

[0037] In order to select an appropriate intra predictive coding mode for the current video block, intra prediction processing unit 46 within prediction processing unit 41 may perform intra predictive coding of the current video block relative to one or more neighboring blocks in the same frame as the current block to be coded to provide spatial prediction. Motion estimation unit 42 and motion compensation unit 44 within prediction processing unit 41 perform inter predictive coding of the current video block relative to one or more predictive blocks in one or more reference frames to provide temporal prediction. Video encoder 20 may perform multiple coding passes, e.g., to select an appropriate coding mode for each block of video data.

[0038] In some implementations, motion estimation unit 42 determines the inter prediction mode for a current video frame by generating a motion vector, which indicates the displacement of a prediction unit (PU) of a video block within the current video frame relative to a predictive block within a reference video frame, according to a predetermined pattern within a sequence of video frames. Motion estimation, performed by motion estimation unit 42, is the process of generating motion vectors, which estimate motion for video blocks. A motion vector, for example, may indicate the displacement of a PU of a video block within a current video frame or picture relative to a predictive block within a reference frame (or other coded unit) relative to the current block being coded within the current frame (or other coded unit). The predetermined pattern may designate video frames in the sequence as P frames or B frames.

Intra BC unit 48 may determine vectors, e.g., block vectors, for intra BC coding in a manner similar to the determination of motion vectors by motion estimation unit 42 for inter prediction, or may utilize motion estimation unit 42 to determine the block vector.

[0039] A predictive block is a block of a reference frame that is deemed as closely matching the PU of the video block to be coded in terms of pixel difference, which may be determined by sum of absolute difference (SAD), sum of square difference (SSD), or other difference metrics. In some implementations, video encoder 20 may calculate values for sub integer pixel positions of reference frames stored in DPB 64. For example, video encoder 20 may interpolate values of one-quarter pixel positions, one-eighth pixel positions, or other fractional pixel positions of the reference frame. Therefore, motion estimation unit 42 may perform a motion search relative to the full pixel positions and fractional pixel positions and output a motion vector with fractional pixel precision.

[0040] Motion estimation unit 42 calculates a motion vector for a PU of a video block in an inter prediction coded frame by comparing the position of the PU to the position of a predictive block of a reference frame selected from a first reference frame list (List 0) or a second reference frame list (List 1), each of which identifies one or more reference frames stored in DPB 64. Motion estimation unit 42 sends the calculated motion vector to motion compensation unit 44 and then to entropy encoding unit 56. [0041] Motion compensation, performed by motion compensation unit 44, may involve fetching or generating the predictive block based on the motion vector determined by motion estimation unit 42. Upon receiving the motion vector for the PU of the current video block, motion compensation unit 44 may locate a predictive block to which the motion vector points in one of the reference frame lists, retrieve the predictive block from DPB 64, and forward the predictive block to summer 50. Summer 50 then forms a residual video block of pixel difference values by subtracting pixel values of the predictive block provided by motion compensation unit 44 from the pixel values of the current video block being coded. The pixel difference values forming the residual vide block may include luma or chroma difference components or both. Motion compensation unit 44 may also generate syntax elements associated with the video blocks of a video frame for use by video decoder 30 in decoding the video blocks of the video frame. The syntax elements may include, for example, syntax elements defining the motion vector used to identify the predictive block, any flags indicating the prediction mode, or any other syntax information described herein. Note that motion estimation unit 42 and motion compensation unit 44 may be highly integrated, but are illustrated separately for conceptual purposes.

[0042] In some implementations, intra BC unit 48 may generate vectors and fetch predictive blocks in a manner similar to that described above in connection with motion estimation unit 42 and motion compensation unit 44, but with the predictive blocks being in the same frame as the current block being coded and with the vectors being referred to as block vectors as opposed to motion vectors. In particular, intra BC unit 48 may determine an intra prediction mode to use to encode a current block. In some examples, intra BC unit 48 may encode a current block using various intra-prediction modes, e.g., during separate encoding passes, and test their performance through rate-distortion analysis. Next, intra BC unit 48 may select, among the various tested intra-prediction modes, an appropriate intra-prediction mode to use and generate an intra-mode indicator accordingly. For example, intra BC unit 48 may calculate rate-distortion values using a rate-distortion analysis for the various tested intra prediction modes, and select the intra-prediction mode having the best rate-distortion characteristics among the tested modes as the appropriate intra-prediction mode to use. Rate- distortion analysis generally determines an amount of distortion (or error) between an encoded block and an original, unencoded block that was encoded to produce the encoded block, as well as a bitrate (i.e., a number of bits) used to produce the encoded block. Intra BC unit 48 may calculate ratios from the distortions and rates for the various encoded blocks to determine which intra-prediction mode exhibits the best rate-distortion value for the block.

[0043] In other examples, intra BC unit 48 may use motion estimation unit 42 and motion compensation unit 44, in whole or in part, to perform such functions for Intra BC prediction according to the implementations described herein. In either case, for Intra block copy, a predictive block may be a block that is deemed as closely matching the block to be coded, in terms of pixel difference, which may be determined by sum of absolute difference (SAD), sum of squared difference (SSD), or other difference metrics, and identification of the predictive block may include calculation of values for sub-integer pixel positions.

[0044] Whether the predictive block is from the same frame according to intra prediction, or a different frame according to inter prediction, video encoder 20 may form a residual video block by subtracting pixel values of the predictive block from the pixel values of the current video block being coded, forming pixel difference values. The pixel difference values forming the residual video block may include both luma and chroma component differences.

[0045] Intra prediction processing unit 46 may intra-predict a current video block, as an alternative to the inter-prediction performed by motion estimation unit 42 and motion compensation unit 44, or the intra block copy prediction performed by intra BC unit 48, as described above. In particular, intra prediction processing unit 46 may determine an intra prediction mode to use to encode a current block. To do so, intra prediction processing unit 46 may encode a current block using various intra prediction modes, e.g., during separate encoding passes, and intra prediction processing unit 46 (or a mode select unit, in some examples) may select an appropriate intra prediction mode to use from the tested intra prediction modes. Intra prediction processing unit 46 may provide information indicative of the selected intra-prediction mode for the block to entropy encoding unit 56. Entropy encoding unit 56 may encode the information indicating the selected intra-prediction mode in the bitstream.

[0046] After prediction processing unit 41 determines the predictive block for the current video block via either inter prediction or intra prediction, summer 50 forms a residual video block by subtracting the predictive block from the current video block. The residual video data in the residual block may be included in one or more transform units (TUs) and is provided to transform processing unit 52. Transform processing unit 52 transforms the residual video data into residual transform coefficients using a transform, such as a discrete cosine transform (DCT) or a conceptually similar transform. [0047] Transform processing unit 52 may send the resulting transform coefficients to quantization unit 54. Quantization unit 54 quantizes the transform coefficients to further reduce bit rate. The quantization process may also reduce the bit depth associated with some or all of the coefficients. The degree of quantization may be modified by adjusting a quantization parameter. In some examples, quantization unit 54 may then perform a scan of a matrix including the quantized transform coefficients. Alternatively, entropy encoding unit 56 may perform the scan.

[0048] Following quantization, entropy encoding unit 56 entropy encodes the quantized transform coefficients into a video bitstream using, e.g., context adaptive variable length coding (CAVLC), context adaptive binary arithmetic coding (CABAC), syntax -based context-adaptive binary arithmetic coding (SBAC), probability interval partitioning entropy (PIPE) coding or another entropy encoding methodology or technique. The encoded bitstream may then be transmitted to video decoder 30, or archived in storage device 32 for later transmission to or retrieval by video decoder 30. Entropy encoding unit 56 may also entropy encode the motion vectors and the other syntax elements for the current video frame being coded.

[0049] Inverse quantization unit 58 and inverse transform processing unit 60 apply inverse quantization and inverse transformation, respectively, to reconstruct the residual video block in the pixel domain for generating a reference block for prediction of other video blocks. As noted above, motion compensation unit 44 may generate a motion compensated predictive block from one or more reference blocks of the frames stored in DPB 64. Motion compensation unit 44 may also apply one or more interpolation filters to the predictive block to calculate sub integer pixel values for use in motion estimation.

[0050] Summer 62 adds the reconstructed residual block to the motion compensated predictive block produced by motion compensation unit 44 to produce a reference block for storage in DPB 64. The reference block may then be used by intra BC unit 48, motion estimation unit 42 and motion compensation unit 44 as a predictive block to inter predict another video block in a subsequent video frame.

[0051] FIG. 3 is a block diagram illustrating an exemplary video decoder 30 in accordance with some implementations of the present application. Video decoder 30 includes video data memory 79, entropy decoding unit 80, prediction processing unit 81, inverse quantization unit 86, inverse transform processing unit 88, summer 90, and DPB 92. Prediction processing unit 81 further includes motion compensation unit 82, intra prediction processing unit 84, and intra BC unit 85. Video decoder 30 may perform a decoding process generally reciprocal to the encoding process described above with respect to video encoder 20 in connection with FIG. 2. For example, motion compensation unit 82 may generate prediction data based on motion vectors received from entropy decoding unit 80, while intra-prediction unit 84 may generate prediction data based on intra-prediction mode indicators received from entropy decoding unit 80.

[0052] In some examples, a unit of video decoder 30 may be tasked to perform the implementations of the present application. Also, in some examples, the implementations of the present disclosure may be divided among one or more of the units of video decoder 30. For example, intra BC unit 85 may perform the implementations of the present application, alone, or in combination with other units of video decoder 30, such as motion compensation unit 82, intra prediction processing unit 84, and entropy decoding unit 80. In some examples, video decoder 30 may not include intra BC unit 85 and the functionality of intra BC unit 85 may be performed by other components of prediction processing unit 81, such as motion compensation unit 82. [0053] Video data memory 79 may store video data, such as an encoded video bitstream, to be decoded by the other components of video decoder 30. The video data stored in video data memory 79 may be obtained, for example, from storage device 32, from a local video source, such as a camera, via wired or wireless network communication of video data, or by accessing physical data storage media (e.g., a flash drive or hard disk). Video data memory 79 may include a coded picture buffer (CPB) that stores encoded video data from an encoded video bitstream. Decoded picture buffer (DPB) 92 of video decoder 30 stores reference video data for use in decoding video data by video decoder 30 (e.g., in intra or inter predictive coding modes). Video data memory 79 and DPB 92 may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magneto resistive RAM (MRAM), resistive RAM (RRAM), or other types of memory devices. For illustrative purpose, video data memory 79 and DPB 92 are depicted as two distinct components of video decoder 30 in FIG. 3. But it will be apparent to one skilled in the art that video data memory 79 and DPB 92 may be provided by the same memory device or separate memory devices. In some examples, video data memory 79 may be on-chip with other components of video decoder 30, or off-chip relative to those components.

[0054] During the decoding process, video decoder 30 receives an encoded video bitstream that represents video blocks of an encoded video frame and associated syntax elements. Video decoder 30 may receive the syntax elements at the video frame level and/or the video block level. Entropy decoding unit 80 of video decoder 30 entropy decodes the bitstream to generate quantized coefficients, motion vectors or intra-prediction mode indicators, and other syntax elements. Entropy decoding unit 80 then forwards the motion vectors and other syntax elements to prediction processing unit 81.

[0055] When the video frame is coded as an intra predictive coded (I) frame or for intra coded predictive blocks in other types of frames, intra prediction processing unit 84 of prediction processing unit 81 may generate prediction data for a video block of the current video frame based on a signaled intra prediction mode and reference data from previously decoded blocks of the current frame.

[0056] When the video frame is coded as an inter-predictive coded (i.e., B or P) frame, motion compensation unit 82 of prediction processing unit 81 produces one or more predictive blocks for a video block of the current video frame based on the motion vectors and other syntax elements received from entropy decoding unit 80. Each of the predictive blocks may be produced from a reference frame within one of the reference frame lists. Video decoder 30 may construct the reference frame lists, List 0 and List 1, using default construction techniques based on reference frames stored in DPB 92.

[0057] In some examples, when the video block is coded according to the intra BC mode described herein, intra BC unit 85 of prediction processing unit 81 produces predictive blocks for the current video block based on block vectors and other syntax elements received from entropy decoding unit 80. The predictive blocks may be within a reconstructed region of the same picture as the current video block defined by video encoder 20.

[0058] Motion compensation unit 82 and/or intra BC unit 85 determines prediction information for a video block of the current video frame by parsing the motion vectors and other syntax elements, and then uses the prediction information to produce the predictive blocks for the current video block being decoded. For example, motion compensation unit 82 uses some of the received syntax elements to determine a prediction mode (e.g., intra or inter prediction) used to code video blocks of the video frame, an inter prediction frame type (e.g., B or P), construction information for one or more of the reference frame lists for the frame, motion vectors for each inter predictive encoded video block of the frame, inter prediction status for each inter predictive coded video block of the frame, and other information to decode the video blocks in the current video frame.

[0059] Similarly, intra BC unit 85 may use some of the received syntax elements, e.g., a flag, to determine that the current video block was predicted using the intra BC mode, construction information of which video blocks of the frame are within the reconstructed region and should be stored in DPB 92, block vectors for each intra BC predicted video block of the frame, intra BC prediction status for each intra BC predicted video block of the frame, and other information to decode the video blocks in the current video frame.

[0060] Motion compensation unit 82 may also perform interpolation using the interpolation filters as used by video encoder 20 during encoding of the video blocks to calculate interpolated values for sub-integer pixels of reference blocks. In this case, motion compensation unit 82 may determine the interpolation filters used by video encoder 20 from the received syntax elements and use the interpolation filters to produce predictive blocks.

[0061] Inverse quantization unit 86 inverse quantizes the quantized transform coefficients provided in the bitstream and entropy decoded by entropy decoding unit 80 using the same quantization parameter calculated by video encoder 20 for each video block in the video frame to determine a degree of quantization. Inverse transform processing unit 88 applies an inverse transform, e.g., an inverse DCT, an inverse integer transform, or a conceptually similar inverse transform process, to the transform coefficients in order to reconstruct the residual blocks in the pixel domain.

[0062] After motion compensation unit 82 or intra BC unit 85 generates the predictive block for the current video block based on the vectors and other syntax elements, summer 90 reconstructs decoded video block for the current video block by summing the residual block from inverse transform processing unit 88 and a corresponding predictive block generated by motion compensation unit 82 and intra BC unit 85. An in-loop filter 94 may be positioned between summer 90 and DPB 92, and includes a deblocking filter to filter block boundaries and remove blockiness artifacts from the decoded video block. The in-loop filter 94 further includes a SAO filter and an ALF to filter the decoded video block outputted by summer 90. The decoded video blocks in a given frame are then stored in DPB 92, which stores reference frames used for subsequent motion compensation of next video blocks. DPB 92, or a memory device separate from DPB 92, may also store decoded video for later presentation on a display device, such as display device 34 of FIG. 1.

[0063] In a typical video coding process, a video sequence typically includes an ordered set of frames or pictures. Each frame may include three sample arrays, denoted SL, SCb, and SCr. SL is a two-dimensional array of luma samples. SCb is a two-dimensional array of Cb chroma samples. SCr is a two-dimensional array of Cr chroma samples. In other instances, a frame may be monochrome and therefore includes only one two-dimensional array of luma samples. [0064] As shown in FIG. 4A, video encoder 20 (or more specifically partition unit 45) generates an encoded representation of a frame by first partitioning the frame into a set of coding tree units (CTUs). A video frame may include an integer number of CTUs ordered consecutively in a raster scan order from left to right and from top to bottom. Each CTU is a largest logical coding unit and the width and height of the CTU are signaled by the video encoder 20 in a sequence parameter set, such that all the CTUs in a video sequence have the same size being one of 128x128, 64x64, 32x32, and 16x16. But it should be noted that the present application is not necessarily limited to a particular size. As shown in FIG. 4B, each CTU may comprise one coding tree block (CTB) of luma samples, two corresponding coding tree blocks of chroma samples, and syntax elements used to code the samples of the coding tree blocks. The syntax elements describe properties of different types of units of a coded block of pixels and how the video sequence can be reconstructed at the video decoder 30, including inter or intra prediction, intra prediction mode, motion vectors, and other parameters. In monochrome pictures or pictures having three separate color planes, a CTU may comprise a single coding tree block and syntax elements used to code the samples of the coding tree block. A coding tree block may be an NxN block of samples.

[0065] To achieve a better performance, video encoder 20 may recursively perform tree partitioning such as binary-tree partitioning, ternary-tree partitioning, quad-tree partitioning or a combination of both on the coding tree blocks of the CTU and divide the CTU into smaller coding units (CUs). As depicted in FIG. 4C, the 64x64 CTU 400 is first divided into four smaller CU, each having a block size of 32x32. Among the four smaller CUs, CU 410 and CU 420 are each divided into four CUs of 16x16 by block size. The two 16x16 CUs 430 and 440 are each further divided into four CUs of 8x8 by block size. FIG. 4D depicts a quad-tree data structure illustrating the end result of the partition process of the CTU 400 as depicted in FIG. 4C, each leaf node of the quad-tree corresponding to one CU of a respective size ranging from 32x32 to 8x8. Like the CTU depicted in FIG. 4B, each CU may comprise a coding block (CB) of luma samples and two corresponding coding blocks of chroma samples of a frame of the same size, and syntax elements used to code the samples of the coding blocks. In monochrome pictures or pictures having three separate color planes, a CU may comprise a single coding block and syntax structures used to code the samples of the coding block. It should be noted that the quad-tree partitioning depicted in FIGS. 4C and 4D is only for illustrative purposes and one CTU can be split into CUs to adapt to varying local characteristics based on quad/temary/binary-tree partitions. In the multi-type tree structure, one CTU is partitioned by a quad-tree structure and each quad-tree leaf CU can be further partitioned by a binary and ternary tree structure. As shown in FIG. 4E, there are five partitioning types, i.e., quaternary partitioning, horizontal binary partitioning, vertical binary partitioning, horizontal ternary partitioning, and vertical ternary partitioning.

[0066] In some implementations, video encoder 20 may further partition a coding block of a CU into one or more MxN prediction blocks (PB). A prediction block is a rectangular (square or non-square) block of samples on which the same prediction, inter or intra, is applied. A prediction unit (PU) of a CU may comprise a prediction block of luma samples, two corresponding prediction blocks of chroma samples, and syntax elements used to predict the prediction blocks. In monochrome pictures or pictures having three separate color planes, a PU may comprise a single prediction block and syntax structures used to predict the prediction block. Video encoder 20 may generate predictive luma, Cb, and Cr blocks for luma, Cb, and Cr prediction blocks of each PU of the CU.

[0067] Video encoder 20 may use intra prediction or inter prediction to generate the predictive blocks for a PU. If video encoder 20 uses intra prediction to generate the predictive blocks of a PU, video encoder 20 may generate the predictive blocks of the PU based on decoded samples of the frame associated with the PU. If video encoder 20 uses inter prediction to generate the predictive blocks of a PU, video encoder 20 may generate the predictive blocks of the PU based on decoded samples of one or more frames other than the frame associated with the PU.

[0068] After video encoder 20 generates predictive luma, Cb, and Cr blocks for one or more PUs of a CU, video encoder 20 may generate a luma residual block for the CU by subtracting the CU’s predictive luma blocks from its original luma coding block such that each sample in the CU’s luma residual block indicates a difference between a luma sample in one of the CU's predictive luma blocks and a corresponding sample in the CU's original luma coding block. Similarly, video encoder 20 may generate a Cb residual block and a Cr residual block for the CU, respectively, such that each sample in the CU's Cb residual block indicates a difference between a Cb sample in one of the CU's predictive Cb blocks and a corresponding sample in the CU's original Cb coding block and each sample in the CU's Cr residual block may indicate a difference between a Cr sample in one of the CU's predictive Cr blocks and a corresponding sample in the CU's original Cr coding block.

[0069] Furthermore, as illustrated in FIG. 4C, video encoder 20 may use quad-tree partitioning to decompose the luma, Cb, and Cr residual blocks of a CU into one or more luma, Cb, and Cr transform blocks. A transform block is a rectangular (square or non-square) block of samples on which the same transform is applied. A transform unit (TU) of a CU may comprise a transform block of luma samples, two corresponding transform blocks of chroma samples, and syntax elements used to transform the transform block samples. Thus, each TU of a CU may be associated with a luma transform block, a Cb transform block, and a Cr transform block. In some examples, the luma transform block associated with the TU may be a sub-block of the CU's luma residual block. The Cb transform block may be a sub-block of the CU's Cb residual block. The Cr transform block may be a sub-block of the CU's Cr residual block. In monochrome pictures or pictures having three separate color planes, a TU may comprise a single transform block and syntax structures used to transform the samples of the transform block. [0070] Video encoder 20 may apply one or more transforms to a luma transform block of a TU to generate a luma coefficient block for the TU. A coefficient block may be a two- dimensional array of transform coefficients. A transform coefficient may be a scalar quantity. Video encoder 20 may apply one or more transforms to a Cb transform block of a TU to generate a Cb coefficient block for the TU. Video encoder 20 may apply one or more transforms to a Cr transform block of a TU to generate a Cr coefficient block for the TU.

[0071] After generating a coefficient block (e.g., a luma coefficient block, a Cb coefficient block or a Cr coefficient block), video encoder 20 may quantize the coefficient block. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the transform coefficients, providing further compression. After video encoder 20 quantizes a coefficient block, video encoder 20 may entropy encode syntax elements indicating the quantized transform coefficients. For example, video encoder 20 may perform Context- Adaptive Binary Arithmetic Coding (CAB AC) on the syntax elements indicating the quantized transform coefficients. Finally, video encoder 20 may output a bitstream that includes a sequence of bits that forms a representation of coded frames and associated data, which is either saved in storage device 32 or transmitted to destination device 14.

[0072] After receiving a bitstream generated by video encoder 20, video decoder 30 may parse the bitstream to obtain syntax elements from the bitstream. Video decoder 30 may reconstruct the frames of the video data based at least in part on the syntax elements obtained from the bitstream. The process of reconstructing the video data is generally reciprocal to the encoding process performed by video encoder 20. For example, video decoder 30 may perform inverse transforms on the coefficient blocks associated with TUs of a current CU to reconstruct residual blocks associated with the TUs of the current CU. Video decoder 30 also reconstructs the coding blocks of the current CU by adding the samples of the predictive blocks for PUs of the current CU to corresponding samples of the transform blocks of the TUs of the current CU. After reconstructing the coding blocks for each CU of a frame, video decoder 30 may reconstruct the frame.

[0073] Recent research suggests that there seems to be a correlation relationship between the Cb residual and the Cr residual of a CU. In some cases, these two chroma residuals appear to be inversely correlated to each other. In this case, a mode for joint coding of chroma residuals is proposed to signal only one chroma residual block (e.g., the Cb residual block) of a CU to improve the coding efficiencies along with a flag indicating that the joint coding of chroma residuals is enabled. In some embodiments, the average of positive Cb residual and negative Cr residual are used as the joint residual of the two components to improve the accuracy when the two chroma residuals are not exactly inversely correlated as follows: res Joint = (resCb - resCr) / 2, where resCb represents the Cb residual block of a CU and resCr represents the Cr residual block of the CU.

[0074] In some implementations, video coder calculates both the averaged sum block and averaged difference block between the two chroma residuals are as follows: resJointCb = (resCb - resCr) / 2; resJointCr = (resCb + resCr) / 2

[0075] Compared to the two residual blocks, resCb and resCr, values in the averaged difference block resJointCr have smaller magnitudes and can be quantified with few bits with the same or similar level of accuracy.

[0076] In some implementations, different modes for the j oint coding of chroma residuals are proposed, with each mode targeting a specific correlation relationship between Cb residuals and Cr residuals as follows:

Mode 1 : Cb is coded and Cr is derived according to Cr = CSign * Cb/2;

Mode 2: Cb is coded and Cr is derived according to Cr = CSign * Cb;

Mode 3: Cr is coded and Cb is derived according to Cb = CSign * Cr/2, where CSign represents the sign used for deriving the second chroma residual block from the first chroma residual block. CSign is signalled as a tile group header syntax element and has a value of either -1 or 1. [0077] In some implementations, the mode of joint coding of chroma residuals is signaled by a TU-level flag, i.e., tu cb cr joint residual. If tu cb cr joint residual is equal to 1, one of the three modes above is used. The specific mode used is derived from the signaled chroma coded block flags (CBFs) according to the following table:

Table 1: CBF-based joint chroma residual coding mode derivation

[0078] In some implementations, if a joint chroma coding mode is chosen, the quantization parameter (QP) for coding the joint chroma residual component is decreased by 1 (for modes 1 and 3) or 2 (for mode 2).

[0079] In sum, video encoder 20 derives a joint chroma residual by a corresponding mixing operation of the Cb and Cr residuals, and selects one of the three supported chroma coding modes (including the CSign) based on distortion analysis (e.g., the distortion obtained by first mixing the Cb and Cr residuals into joint chroma residuals and then reconstructing the Cb and Cr residuals from the joint chroma residual, without quantization). The selected mode is then tested in the additional mode decision process (i.e., using transform, quantization, and entropy coding). In some implementations, the tile group header syntax element that indicates the sign (CSign) for deriving the second chroma component is determined by analyzing the correlation between high-pass filtered versions of the original Cb and Cr components for the tile group.

[0080] In some implementations, the correlation between the first chroma residual and the second chroma residual indicates that the signaling of tu cb cr joint residual flag is dependent upon the signaling of one, not two, chroma coded block flag. For example, if the first signaled chroma coded block flag has a value of one, the tu cb cr joint residual flag will be signaled, and there is no need for signaling the second chroma coded block flag due to the correlation between the first and second chroma residual blocks. The second chroma coded block flag is signaled only if the tu cb cr joint residual flag has a value of zero, i.e., there is no correlation between the first and second chroma residual blocks.

[0081] In some implementations, one or two contexts are utilized for the CABAC coding of tu cb cr joint residual flag. For example, one of the two contexts is selected based on the value of the Cr coded block flag. When the Cr coded block flag is 1, one context is used; otherwise (i.e. the Cr coded block flag is equal to 0), the other context is used. If the Cb coded block flag is equal to 1, the TU-level flag tu cb cr joint residual is signaled and the two contexts are utilized for coding the tu cb cr joint residual flag.

[0082] In some implementations, the TU-level flag tu cb cr joint residual is signaled only if both of the two chroma CBFs are 1. When the tu cb cr joint residual flag has a value of 1, one additional syntax element is signaled to indicate which of the three modes is selected. For the CABAC coding of the mode syntax, different codeword binarization may be used. One exemplary codeword binarization may be the truncated unary codeword, with a maximum codeword index of 2, as shown in Table 2 below.

Table 2: Codeword binarization for joint chroma coding mode signaling

[0083] In some implementations, an additional syntax element is proposed to control the syntax signalling of the mode of joint coding of chroma residual at different levels. For example, the syntax element may be signalled at a video sequence level, a picture level, or a tile group level, a tile level, or a slice level. When this syntax element is signalled with a value of 1 at a particular level, the TU-level control flags, i.e., tu cb cr joint residual, at or below the level are also signalled to indicate the usage of the joint coding of chroma residual. When this syntax is signalled with a value of 0, the joint coding of chroma residual is disabled at the level and the TU-level controls flags are not to be coded when coding the CUs at or below the level where this flag is signalled with the value of 0.

[0084] FIGS. 5A and 5B are flowcharts illustrating an exemplary process 500 by which a video encoder 20 implements the techniques of encoding video data using a joint coding of chroma residuals scheme in accordance with some implementations of the present disclosure. Video encoder 20 obtains, from a video bitstream having a hierarchical structure, a first syntax element associated with a first level of the hierarchical structure (510). As explained above, there are multiple options for the first level and accordingly, the first element may be in one of a sequence parameter set (SPS), a picture parameter set (PPS), a tile group header, a tile group header, a tile header, a slice header, etc. The multiple chroma components of each of the one or more blocks correspond to a transform unit of the video data, which in turn is associated with a particular coding unit.

[0085] Next, video encoder 20 checks the value of the first syntax element (e.g., in the form of a one-bit flag) and determines whether the syntax element indicates that a joint coding of the chroma residuals mode is enabled or not (530). For example, a value of 1 indicates that the joint coding of the chroma residuals mode is enabled and a value of 0 indicates that the joint coding of the chroma residuals mode is disabled. If the joint coding of the chroma residuals mode is enabled (530-yes), video encoder 20 then encodes, into a video bitstream, chroma residuals for multiple chroma components of one or more blocks under the first level jointly according to a predefined scheme of joint coding of chroma residuals (550). As noted above, at least three different schemes of joint coding of chroma residuals are proposed for handling different types of video data. Various syntax elements and CABAC contexts are used accordingly for processing one of the multiple chroma components while the other chroma components is derived from the processed chroma component according to a correlation relationship under the predefined scheme of joint coding of chroma residuals. If the joint coding of the chroma residuals mode is disabled (530-no), video encoder 20 then encodes, into the video bitstream, chroma residuals for multiple chroma components of one or more blocks under the first level separately (570). In other words, each of the multiple chroma components of one or more blocks is encoded into the video bitstream and the TU-level control flag tu cb cr joint residual is set to be zero for each CU.

[0086] Finally, video encoder 20 outputs the video bitstream including the encoded chroma residuals for the multiple chroma components of the one or more blocks and the first syntax element (590).

[0087] In some implementations, as depicted in FIG. 5B, after the first syntax element indicates that the joint coding of chroma residuals mode is enabled, video encoder 20 obtains a second syntax element associated with each of the one or more blocks (550-1) and determines whether the second syntax element indicates that a block-level joint coding of chroma residuals mode is enabled (550-3). If so (550-3, yes), video encoder 20 encodes, into the video bitstream, the chroma residuals for the multiple chroma components of the block jointly according to the predefined scheme of joint coding of chroma residuals (550-5); otherwise (550-3, no), video encoder 20 encodes, into the video bitstream, the chroma residuals for the multiple chroma components of the block separately (550-7). In other words, a value of 0 at the first syntax element can disable the application of the joint coding of chroma residuals for all the blocks under the first level such that there is no need for signaling of the second syntax element at the block level. But a value of 1 at the first syntax element does not dictate that every block under the first level will have to be encoded using one of the schemes of joint coding of chroma residuals. Each individual block still has its own control through the choice of the second syntax element, thereby enhancing the flexibility of the implementation of the video encoder.

[0088] In some implementations, video encoder 20 selects, from the multiple modes

(see, e.g., Table 1 above), a mode according to values of the chroma coding flags of the multiple chroma components of the block, which may require a rate distortion analysis. Video encoder 20 then encodes, into the video bitstream, the chroma residuals for one of the multiple chroma components of the block according to the selected mode and the values of the chroma coding flags of the multiple chroma components of the block, respectively.

[0089] FIGS. 6A through 6C are flowcharts illustrating an exemplary process 600 by which a video decoder implements the techniques of decoding video data using the joint coding of chroma residuals scheme in accordance with some implementations of the present disclosure. Video decoder 30 receives, from a video bitstream having a hierarchical structure, a first syntax element associated with a first level of the hierarchical structure (610) and then checks whether the first syntax element indicates that a joint coding of chroma residuals mode is enabled or not (630). If so (630-yes), video decoder 30 reconstructs, from the video bitstream, chroma residuals for multiple chroma components of one or more blocks under the first level jointly according to a predefined scheme of joint coding of chroma residuals (650). Otherwise (630-no), video decoder 30 reconstructs, from the video bitstream, the chroma residuals for the multiple chroma components of the one or more blocks under the first level separately (670). As explained above, there are multiple options for the first level and accordingly, the first element may be in one of a sequence parameter set (SPS), a picture parameter set (PPS), a tile group header, a tile group header, a tile header, a slice header, etc. The multiple chroma components of each of the one or more blocks correspond to a transform unit of the video data, which in turn is associated with a particular coding unit.

[0090] In some implementations, as depicted in FIG. 6B, after the first syntax element indicates that the joint coding of chroma residuals mode is enabled, video decoder 30 receives, from the video bitstream, a second syntax element associated with each of the one or more blocks (650-1) and determines whether the second syntax element indicates that a block-level joint coding of chroma residuals mode is enabled (650-3). If so (650-3, yes), video decoder 30 reconstructs, from the video bitstream, the chroma residuals for the multiple chroma components of the block jointly according to the predefined scheme of joint coding of chroma residuals (650- 5); otherwise (650-3, no), video decoder 30 reconstructs, from the video bitstream, the chroma residuals for the multiple chroma components of the block separately (650-7). In other words, a value of 0 at the first syntax element can disable the application of the joint coding of chroma residuals for all the blocks under the first level such that there is no need for signaling of the second syntax element at the block level. But a value of 1 at the first syntax element does not dictate that every block under the first level will have to be encoded using one of the schemes of joint coding of chroma residuals. Each individual block still has its own control through the choice of the second syntax element, thereby enhancing the flexibility of the implementation of the video decoder.

[0091] In some implementations, as depicted in FIG. 6C and described above in connection with Table 1, each of the multiple chroma components of a block has a chroma coding flag and the predefined scheme of joint coding of chroma residuals has multiple modes (650-11). Video decoder 30 selects, from the multiple modes (see, e.g., Table 1 above), a mode according to values of the chroma coding flags of the multiple chroma components of the block (650-13) and then reconstructs, from the video bitstream, the chroma residuals for the multiple chroma components of the block according to the selected mode (650-15). Assuming that the multiple chroma components of the block include a first chroma component (e.g., Cb component) and a second chroma component (e.g., Cr component) (650-15-1), video decoder 30 reconstructs, from the video bitstream, the chroma residuals for the first chroma component of the block (650-15-3) and directly derives the chroma residuals for the second chroma component from the chroma residuals for the first chroma component of the block (650-15-5) as described above about the multiple modes of joint coding of chroma residuals.

[0092] As depicted in the Table 1 above, when the TU-level tu cb cr joint residual flag is 1, and the Cb CBF is 1 and the Cr CBF is 0, mode 1 is selected. But it is still possible that both the two chroma blocks, Cb and Cr blocks, actually have non-zero residuals, causing discrepancy between a signaled chroma block CBF value and the actual corresponding chroma block residuals. Such discrepancy may impact coding performance when such chroma CBF values are subsequently used for other purposes, e.g. used as contexts for coding other syntaxes. As depicted in FIG. 6C, video decoder 30 may reset the chroma coding flags of the multiple chroma components of the block to a predefined value (650-17). For example, in the scheme of joint chroma residual coding with multiple modes, when the TU-level flag tu cb cr joint residual is signaled as 1, the Cb and Cr chroma coded block flag (CBF) syntax elements are reset to 1 after the current block is reconstructed, regardless of which of the three modes is used. For example, under mode 1, even if the signaled Cr CBF is 0, it would be reset to 1 after the current block is reconstructed.

[0093] FIG. 7 is a block diagram of an exemplary video decoder 30 configured for luma mapping with chroma scaling (LMCS), in accordance with some embodiments. In the video decoder 30, a first plurality of decoding modules (shaded) are implemented in a mapped domain and include an entropy encoding unit 80, inverse quantization unit 86, inverse transform processing unit 88, luma intra prediction unit 84A and luma sample reconstruction unit 90A. Luma prediction samples and luma residual samples are added up in the mapped domain. A second plurality of decoding modules (unshaded) are implemented in a non-mapped domain (also called an original domain) and include a motion compensated prediction unit 82, chroma intra prediction unit 84B, chroma sample reconstruction unit 90B and in-loop filter 94 that further includes one or more of deblocking, SAO and ALF filters. Chroma prediction samples and chroma residual samples are added in the non-mapped domain. Luma and chroma components of reference pictures are outputted in the non-mapped domain and stored in a decoded picture buffer (DPB) 92. LMCS is applied before the in-loop filters 94A and 94B configured to process luma and chroma components of reconstructed video data, respectively. LMCS is implemented jointly by an in-loop mapping module 96 of the luma components and a luma-dependent chroma residual scaling unit 98. The in-loop mapping module 96 is implemented based on an adaptive piecewise linear model and further includes a forward mapping unit 96 A and a backward mapping unit 96B.

[0094] The in-loop mapping module 96 is configured to adjust a dynamic range of an input video data to improve a coding efficiency. The forward mapping unit 96A is based on a forward mapping function FwdMap , and the backward mapping unit 96B is based on a corresponding inverse mapping function InvMap. The forward mapping function FwdMap is signaled from the video encoder 20 to the video decoder 30 using a luma mapping model 702, e.g., a piecewise linear model with 16 equal-size pieces. In some embodiments, the inverse mapping function InvMap is derived directly from the forward mapping function FwdMap and does not need to be signaled.

[0095] In some embodiments, one or more parameters of the mapping unit 96 (e.g., parameters of the luma mapping model 702) are signaled at a picture level. For example, a presence flag is signaled to indicate if the luma mapping model 702 is present for a current image slice. If the luma mapping model 702 is present for the current image slice, a plurality of piecewise linear model parameters are further signaled to facilitate implementation of a piecewise linear model. Based on the piecewise linear model, the input video data’s dynamic range is partitioned into 16 segments with equal size in the original domain, and each segment of the dynamic range of the input video data is mapped to a corresponding segment in the original domain. For a given segment in the original domain, its corresponding segment in the mapped domain may have the same size as or a different size from that of the given segment in the original domain. The size of each segment in the mapped domain is indicated by a number of codewords (i.e., that store mapped sample values) of that respective segment. For each segment in the original domain, the piecewise linear mapping parameters can be derived based on the number of codewords in its corresponding segment in the mapped domain. In an example, the input video data has a resolution of 10 bits. If each of the segments in the mapped domain has 64 codewords assigned to the respective segment, each of the 16 segments in the original domain has 64 pixel values according to a one-to-one mapping (i.e. a mapping with each sample value unchanged). The signaled number of codewords for each segment in the mapped domain is used to calculate a scaling factor and adjust the forward or backward mapping function accordingly for that segment. Further, in some embodiments, another LMCS control flag is signaled to enable/disable LMCS for the image slice at the picture level.

[0096] In an example, for an i- th segment (/ 0 ... 75), the corresponding piece-wise linear model applied by the forward mapping function FwdMap of the forward mapping unit 96A is defined by two input pivot points InputPivot[i] and InputPivot[i+ 1] , and two output (mapped) pivot points MappedPivot[i] and MappedPivot[i+ 1] . Further, if the input video data has a resolution of 10 bits, the values of InputPivot[i] an d Mappedl’i vol/i / (/ = 0 ... 15) are calculated as follows:

1. Set the variable OrgCW= 64

2. For 7 = 0:16, lnputPivot[ i ] = i * OrgCW

3. For / 0:16. MappedPivot[i] is calculated as follows :

MappedPivot[ 0 ] = 0; for( i = 0; i <16 ; i++)

MappedPivot[ i + 1 ] = MappedPivot [ i ] + SignaledCW[ i ] where SignaledCW[i] is the signaled number of codewords for the i- th segment.

[0097] The chroma residual scaling (CRS) unit 98 is coupled to an inverse transform processing unit 88 and configured to compensate for interaction of quantization precision between luma components and corresponding chroma components when in-loop mapping is applied to the luma components. A first CRS flag is signaled in a picture header to indicate whether chroma residual scaling is enabled or disabled. If luma mapping is enabled and a dual- tree partition of luma and chroma components is disabled for a current picture, a second CRS flag is signaled to indicate whether luma-dependent chroma residual scaling is applied or not. If luma mapping is not used or the dual-tree partition is enabled for the current picture, the chroma residual scaling is disabled via the first CRS flag. Additionally, in some embodiments, chroma residual scaling is disabled for CUs that contain less than or equal to four chroma samples.

[0098] For both intra and inter CUs, scaling parameters that are used to scale chroma residuals are determined based on an average of corresponding mapped luma prediction samples. Assume avg’r is the average of the luma prediction samples in the mapped domain.

The scaling parameter Cscaieim is determined according to the following steps:

1. Find the segment index Yi_dx of the piecewise linear model to which avg’r belongs to in the mapped domain. Here Yi_dx has an integer value ranging from 0 to 15.

2. Cs_caieinv = cScalelnv [Y i_dx] , where cScalelnv [i] , i = 0 ... 15, is a pre-computed 16-piece look up table (LUT).

[0099] Intra prediction is performed in the mapped domain in LMCS. When CUs are coded in an intra prediction mode, a combined intra and inter prediction (CIIP) mode, or an intra block copy (IBC) mode, and avg’r is computed as an average of luma prediction samples. Otherwise, avg’r is computed as an average of forward mapped inter predicted luma samples. Moreover, unlike luma mapping which is performed on a sample basis, an original chroma residual sample value Cscaieim is fixed for an entire chroma CU. Given the original chroma residual sample value Cscaieim , chroma residual scaling is applied as follows:

Encoder side. CResScale CRes/Cscalelnv Decode side. Cues CResScaleXCscalelnv where C_/?ra represents a scaled chroma residual sample value.

[00100] FIG. 8 is a block diagram of an exemplary video decoder 30 configured for implementing an inverse adaptive color transform (ACT), in accordance with some embodiments. In HEVC SCC extension, ACT is applied to reduce redundancy between three color components in 444 chroma format. ACT is also adopted into a VVC standard to enhance a coding efficiency of 444 chroma format coding. For example, ACT performs in-loop color space conversion in a prediction residual domain by adaptively converting residuals from an input color space to a YCgCo color space. Referring to FIG. 8, in some embodiments, two color spaces are adaptively selected by signaling an ACT flag at a CU level. When the ACT flag is equal to a first value (“1”), residuals of the CUs are coded in the YCgCo color space, and decoded using an inverse ACT unit 802. When the ACT flag is equal to a second value (“0”), the residuals of the CUs are coded in an original color space of input video data, and decoded without using the inverse ACT unit 802. Additionally, in some embodiments, ACT is only enabled for a CU in an inter prediction or IBC mode when there is at least one non-zero coefficient in the CU, and the inverse ACT unit 802 is enabled to decode the residuals of the CU in the inter prediction or IBC mode. In some embodiments, ACT is only enabled for a CU in an intra prediction mode when chroma components select the same intra prediction mode of luma components, and the inverse ACT unit 802 and intra prediction unit 84 are enabled to decode the residuals of the CU in the intra prediction mode.

[00101] In some embodiments, color space conversion for ACT are based on YCgCo transform matrices. Specifically, forward and inverse YCgCo color transform matrices are applied to convert a GBR vector to a YCgCo vector and vice versa as follows:

[00102] In some embodiments, ACT transform matrices (e.g., forward and inverse

YCgCo color transform matrices in the above equations) are not normalized. QP adjustments of

(-5, -5, -3) are applied to transform residuals of Y, Cg and Co components to compensate a dynamic range change of residuals signals before and after color transform. The adjusted QP affects the quantization and inverse quantization of the residuals in the CU. For other coding processes (such as deblocking), original and unadjusted QP is still applied. Additionally, in some embodiments, forward and inverse color transforms need to access the residuals of all three color components, ACT is always disabled for separate-tree partition and an ISP mode where prediction block sizes of different color components are different.

[00103] FIGS. 9A and 9B are block diagrams of two example residual decoding subsystems 900 and 950 that can be applied in a video decoder 30, in accordance with some embodiments, respectively. Each of the residual decoding subsystems 900 and 950 includes an inverse quantization unit 86, an inverse transform processing unit 88, an inverse JCCR unit 902, an inverse ACT unit 802, and inverse CRS unit 98. The inverse JCCR unit 902 and inverse ACT unit 802 are optionally placed between the inverse transform processing unit 88 and CRS unit 98 in FIG. 7. Quantized transform coefficients are provided to the video decoder 30 in a video bitstream and entropy decoded by an entropy decoding unit 80. The inverse quantization unit 86 inverse quantizes the decoded quantized transform coefficients using the same quantization parameter calculated by video encoder 20 for each video block in the video frame to determine a degree of quantization. The inverse transform processing unit 88 applies an inverse transform, e.g., an inverse DCT, an inverse integer transform, or a conceptually similar inverse transform process, to the transform coefficients in order to reconstruct the residual blocks in a pixel domain. The reconstructed residual blocks include luma and chroma components of a plurality of pixels in a video frame. The inverse JCCR unit 902 is enabled in a JCCR mode to determine chroma residuals for the chroma components of the video frame jointly according to a scheme of joint coding of chroma residuals. For example, in the JCCR mode, only one chroma residual block (e.g., a Cb residual block) of a CU is signaled along with a flag indicating that the joint coding of chroma residuals is enabled, while two chroma residuals (e.g., Cb and Cr residuals) are determined jointly from the only one chroma residual block. The inverse ACT unit 802 is coupled to the inverse JCCR unit 902 to apply adaptive color transform on the chroma residuals (e.g., both the Cb residual and the Cr residual) of the CU. The inverse CRS unit 98 is coupled to the inverse ACT unit 802 and configured to compensate for interaction of quantization precision between luma components and corresponding chroma components of the CU when in-loop mapping is applied to the luma components of the CU.

[00104] In some embodiments, residuals generated by each unit of the residual decoding subsystems 900 and 950 are controlled within one or more respective dynamic ranges (e.g., corresponding to a 16-bit resolution including a sign bit). In the example residual decoding subsystems 900 and 950, a first clipping operation is applied, i.e., a first clipping unit 904 is coupled between the inverse transform processing unit 88 and the inverse JCCR unit 902, to maintain a first dynamic range DRA and a second dynamic range DR_B for luma and chroma residuals, respectively. In an example, one or both of the dynamic ranges DRA and DR_B correspond to a first bit depth equal to 16 bits, and are defined as [-2¹⁵, 2¹⁵-1] inclusively. The first clipping unit 904 clips corresponding residuals of the luma and chroma residuals of the residual blocks M outputted by the inverse transform processing unit 88 to the respective dynamic range DRA and/or DR_B according to a clipping function of Clip(- 2¹⁵, 2¹⁵-1 ,M). In yet another example, one or both of the dynamic ranges DRA and DR_B correspond to a first bit depth equal to 16 bits, and are defined as [-2¹⁵-1, 2¹⁵-1] inclusively. The first clipping unit 904 clips corresponding residuals of the luma and chroma residuals of the residual blocks outputted by the inverse transform processing unit 88 to the respective dynamic range DRA and/or DR_B according to another clipping function of Clip{-{ 2¹⁵-1), 2¹⁵- 1 , M). Additionally, in another example, one or both of the dynamic ranges DRA and DR_B are defined as [-2^B, 2^B-1] inclusively. The first clipping unit 904 clips corresponding residuals of the luma and chroma residuals of the residual blocks M outputted by the inverse transform processing unit 88 to the respective dynamic range DRA and/or DR_B using a clipping function of Clip(- 2^B, 2^B- 1 , M) or C/ip(-(2^B- \ ), 2^B-1, A/). The integer B is obtained with the video bitstream and applied by the inverse transform processing unit 88 to reconstruct the luma residuals of the residual blocks of the CUs of the corresponding video frame. The integer B refers to a coding bit depth of input video data. [00105] Additionally, in some embodiments, a bitstream conformance constraint is applied to ensure that residuals (i.e., residuals fed into the inverse quantization unit 86) are within a 16-bit dynamic range, i.e., [-2¹⁵ _, 2¹⁵-1] Referring to FIG. 9A, no clipping unit is applied after the inverse JCCR unit 902, and decoded residuals outputted the JCCR unit 902 exceeds the 16-bit dynamic range when the JCCR mode 2 with a signaled negative sign is selected (i.e., Cr = -Cb) and the decoded Cb residual sample is equal to the minimum of 16-bit sign integer (i.e., - 2¹⁵). In these situations, the Cr residual sample is equal to 2¹⁵ which is outside the 16-bit range [- 2¹⁵ _, 2¹⁵-1].

[00106] In the example residual decoding subsystem 950, two distinct clipping operations are applied. In addition to the first clipping unit 904, a second clipping unit 906 is coupled to an output of the inverse JCCR unit 902, e.g., between the inverse JCCR unit 902 and inverse ACT unit 802, to maintain a chroma dynamic range DRC_R. The chroma dynamic range DRC_R corresponds to a chroma bit depth that is optionally equal to or less than a second bit depth corresponding to the second dynamic range DR_B. The second dynamic range DR_B is identical to or encloses the chroma dynamic range DRC_R. For example, both the second and chroma bit depths are equal to K, where K is an integer (e.g., 16, or a coding bit depth received with the video bitstream), and the second and chroma dynamic ranges DR_B and DRC_R are identical and selected from [-(2^K-1), 2^K -1], [-2^K, 2^K-1], [-(2^K-1), 2^K], and [-2^K, 2^K]

[00107] FIGS. 10A and 10B are block diagrams of another two example residual decoding subsystems 1000 and 1050 that can be applied in a video decoder 30, in accordance with some embodiments, respectively. Each of the residual decoding subsystems 1000 and 1050 includes an inverse quantization unit 86, an inverse transform processing unit 88, an inverse JCCR unit 902, an inverse ACT unit 802, and inverse CRS unit 98. In both of the example residual decoding subsystems 1000 and 1050, at least three clipping operations are applied. A first clipping unit 904 is coupled between the inverse transform processing unit 88 and the inverse JCCR unit 902 to maintain a first dynamic range DRA and a second dynamic range DR_B for luma and chroma residuals corresponding to a first bit depth and a second bit depth, respectively. A third clipping unit 1002 is coupled between the inverse quantization unit 86 and inverse transform processing unit 88 to maintain a third dynamic range DRc of residual information corresponding to a third bit depth. A fourth clipping unit 1004 is coupled between the inverse ACT unit 802 and inverse CRS unit 98 to maintain a fourth dynamic range DR_D of residual information corresponding to a fourth bit depth.

[00108] In some embodiments associated with the example residual decoding subsystem 1050, a second clipping unit 906 is coupled at an output of the inverse JCCR unit 902, e.g., between the inverse JCCR unit 902 and inverse ACT unit 802, to maintain a chroma dynamic range DR_CR corresponding to a chroma bit depth in addition to the first, third and fourth clipping units 904, 1002 and 1004. In an example, the chroma dynamic range DR_CR corresponds to a 16- bit bit depth, and the second clipping unit 906 is applied to protect the reconstructed JCCR chroma residuals outputted by the JCCR unit 902 from exceeding the 16-bit dynamic range DRCR.

[00109] The clipping units 1002, 904, 906 and 1004 are arranged in an ordered sequence on a video decoding path. For luma or chroma residuals, an upstream clipping unit on the video decoding path corresponds to a dynamic range that is equal to or encloses that of a downstream clipping unit on the video decoding path. Specifically, for luma or chroma residuals, the third bit depth is optionally equal to or greater than the first or second bit depth, and the third dynamic range DRc is identical to or encloses the first or second dynamic range DR_A or DR_B, respectively. The second bit depth is optionally equal to or greater than the fourth bit depth, and the second dynamic range DR_B is identical to or encloses the fourth dynamic range DR_D. In the residual decoding subsystem 1050, the second bit depth of chroma residuals is optionally equal to or greater than the chroma bit depth, and the second dynamic range DR_B is identical to or encloses the chroma dynamic range DR_CR. The chroma bit depth is optionally equal to or greater than the fourth bit depth, and the chroma dynamic range DR_CR is identical to or encloses the fourth dynamic range DR_D.

[00110] Referring to FIG. 10B, in an example, the first, second and third clipping unit 904, 906 and 1002 have the same dynamic range defined by a clipping function of Clip(- 2¹⁵, 2¹⁵- 1, AT), i.e., have the same bit depth of 16 bits, while the fourth clipping unit 1004 has a distinct dynamic range DR_D defined by a distinct clipping function of Clip(- 2^B, 2^B- 1 , AT) It is noted that a clipping function of clip(min, max, M) clips an input value M into a dynamic range of [min, max] and that B is an coding bit depth of the input video data. Additionally, in some embodiments, there is a bitstream conformance constraint to ensure the parsed residuals (i.e., the residuals fed into the inverse quantization unit 86) are within a 16-bit dynamic range, i.e., [-2¹⁵, 2¹⁵-1]). In some embodiments, in FIG. 10A, there is no clipping operation applied to an output of the inverse JCCR unit 902. Such design could cause the decoded residuals after the JCCR unit 902 to exceed the 16-bit dynamic range when a JCCR mode 2 having a signaled negative sign is selected (i.e., Cr = -Cb) and the decoded Cb residual sample is equal to the minimum of 16-bit sign integer (i.e., -2¹⁵). In such a case, the calculated Cr residual sample is equal to 2¹⁵, which falls outside the 16-bit dynamic range of [-2¹⁵, 2¹⁵-1]

[00111] In some embodiments, the clipping operation of the first clipping unit 904 applied before the inverse JCCR unit 902 is symmetric, e.g., has a clipping function of Clip{-{ 2¹⁵-1), 2¹⁵- 1, M) which is configured to clip the residuals outputted by the inverse transform processing unit 88 to a dynamic range of [-(2¹⁵-1), 2¹⁵-1], which is symmetric. In an example, this symmetric clipping function clips both luma and chroma residuals after the inverse transform processing unit 88. Both the first and second dynamic ranges DR_A and DR_B are symmetric, e.g., [-(2¹⁵-1), 2¹⁵-1] In another example, such a symmetric clipping function is applied to clip chroma residuals to the second dynamic range DR_B, and an asymmetric clipping function (e.g., Clip(- 2¹⁵, 2¹⁵-1, M)) is applied to clip associated luma residuals to the first dynamic range DR_A. The first and second dynamic ranges DR_A and DR_B are [-2¹⁵, 2¹⁵-1] and [-(2¹⁵-1), 2¹⁵-1], respectively.

[00112] As explained above, in some embodiments, the first, second and third clipping units 904, 906 and 1002 are configured to implement the same clipping operation that clips respective input residuals to the same dynamic range associated with the same bit depth, while the fourth clipping unit 1004 is configured implement a distinct clipping operation that clips respective input residuals to a distinct dynamic range associated with a distinct bit depth. Additionally, in some embodiments, the first, second, third and fourth clipping units 904, 906, 1002 and 1004 are unified to implement the same clipping operation that clips respective input residuals to the same dynamic range associated with the same bit depth. For example, the first, second, third and fourth clipping units 904, 906, 1002 and 1004 are unified to apply a clipping function of Clip(- 2¹⁵, 2¹⁵- 1 , M).

[00113] FIGS. 11 A and 1 IB are a flowchart of a video decoding method 1100 implemented by an electronic device (e.g., including a video decoder 30), in accordance with some embodiments. The electronic device reconstructs (1102), from a video bitstream, a plurality of residual blocks of a video frame, clips (1104) luma residuals and chroma residuals of the plurality of residual blocks of the video frames into a first dynamic range DR_A and a second dynamic range DR_B, respectively, and determines (1106) a luma residual for a luma component and chroma residuals for two chroma components of each residual block of the video frame from the clipped luma and chroma residual of the plurality of residual blocks of the video frame. In some embodiments, the clipped luma and chroma residuals of the plurality of residual blocks are stored in a cache according to the first dynamic range DRA and the second dynamic range DR_B, respectively. The clipping operation helps conserving a storage space of the cache and expediting a corresponding video decoding rate. In an example, at least one of the first dynamic range DRA and second dynamic range DR_B is asymmetric and defined as [-2¹⁵, 2¹⁵-1], i.e., one or both of the dynamic ranges DRA and DR_B are [-2¹⁵, 2¹⁵-1] In another example, at least one of the first dynamic range DRA and second dynamic range DR_B is symmetric and is defined as [- (2¹⁵-1), 2¹⁵-1] , i.e., one or both of the dynamic ranges DRA and DR_B are [-(2¹⁵-1), 2¹⁵-1] In yet another example, at least one of the first dynamic range DRA and second dynamic range DR_B is asymmetric and is defined as [-2^B, 2^B-1], i.e., one or both of the dynamic ranges DRA and DR_B are [-2^B, 2^B-1], and B is a coding bit depth of the video frame and obtained with the video bitstream. In yet another example, at least one of the first dynamic range DRA and second dynamic range DR_B is symmetric and is defined as [-(2^B-1), 2^B-1], i.e., one or both of the dynamic ranges DRA and DR_B are [-2^B, 2^B-1], and B is the coding bit depth obtained with the video bitstream.

[00114] In some embodiments, the plurality of residual blocks are reconstructed (1102) by dequantizing (1108) a plurality of quantized transform coefficients in the video bitstream to a plurality of transform coefficients for the video frame and applying (1110) an inverse transform to the plurality of transform coefficients to reconstruct the plurality of residual blocks. The plurality of residual clocks are then clipped to the first dynamic range. Further, in some embodiments, prior to applying the inverse transform, each of the plurality of transform coefficients is clipped (1112) into a third dynamic range DRc.

[00115] In some embodiments, the electronic device determines (1114) whether a joint coding of chroma residuals (JCCR) mode is enabled. In accordance with a determination that the JCCR mode is enabled, the chroma residuals for the two chroma components of the video frame are determined jointly (1116) according to a scheme of joint coding of chroma residuals, e.g., by an inverse JCCR unit 902. In some embodiments, the chroma residuals are then clipped (1118) for the two chroma components of the video frame into a chroma dynamic range DRC_R associated with a chroma bit depth. Further, in some embodiments, the video bitstream has a hierarchical structure, and the electronic device obtains (1120) a syntax element associated with a first level of the hierarchical structure, such that the chroma residuals for two chroma components of the video frame are determined jointly under the first level. Conversely, in some embodiments, in accordance with a determination that the syntax element indicates that the JCCR mode is disabled, the electronic device reconstructs (1122) the chroma residuals for the two chroma components of the video frame separately based on the clipped chroma residuals of the plurality of residual blocks.

[00116] In some embodiments, an inverse adaptive color transform is further applied (1124) to the two chroma components of the video frame to obtain alternative chroma residuals for the two chroma components. The alternative chroma residuals for the two chroma components of the video frame are clipped (1126) into a fourth dynamic range DRc, and scaled (1128) for the two chroma components of the video frame.

[00117] In some embodiments, each residual block includes one or more respective luma residuals and one or more respective chroma residuals. The first dynamic range DRA is equal to the second dynamic range DR_B and is symmetric with respect to zero. For each residual block, the one or more respective luma residuals are clipped into the first dynamic range DRA, and the one or more respective chroma residuals are clipped into the second dynamic range DR_B. Conversely, in some embodiments, each residual block includes one or more respective luma residuals and one or more respective chroma residuals. The first dynamic range DRA is symmetric with respect to zero, and the second dynamic range DR_B is asymmetric with respect to zero. The one or more respective luma residuals of each residual block are clipped into the first dynamic range DRA, and the one or more respective chroma residuals of each residual block are clipped into the second dynamic range DR_B.

[00118] In some embodiments associated with unified clipping, the first and second dynamic ranges are identical. The plurality of residual blocks are reconstructed by dequantizing a plurality of quantized transform coefficients in the video bitstream to a plurality of transform coefficients for the video frame and applying an inverse transform to the plurality of transform coefficients to reconstruct the plurality of residual blocks. Prior to applying the inverse transform, the electronic device clips each of the plurality of transform coefficients into the first dynamic range. Subsequently to determining the chroma residuals, the electronic device clips the chroma residuals for the two chroma components of the video frame into the first dynamic range, applies an inverse adaptive color transform to the clipped chroma residuals of the two chroma components of the video frame to obtain alternative chroma residuals for the two chroma components, and clips the alternative chroma residuals for the two chroma components of the video frame into a fourth dynamic range. In some embodiments, the fourth dynamic range is identical to the first dynamic range. In some embodiments, the first dynamic range is [-2¹⁵, 2¹⁵-1], and the fourth dynamic range is [-2^B, 2^B-1], where B (i.e., a coding bit depth) is an integer less than 15.

[00119] In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over, as one or more instructions or code, a computer- readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer- readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the implementations described in the present application. A computer program product may include a computer-readable medium.

[00120] The terminology used in the description of the implementations herein is for the purpose of describing particular implementations only and is not intended to limit the scope of claims. As used in the description of the implementations and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term “and/or” as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, elements, and/or components, but do not preclude the presence or addition of one or more other features, elements, components, and/or groups thereof.

[00121] It will also be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first electrode could be termed a second electrode, and, similarly, a second electrode could be termed a first electrode, without departing from the scope of the implementations. The first electrode and the second electrode are both electrodes, but they are not the same electrode. [00122] The description of the present application has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications, variations, and alternative implementations will be apparent to those of ordinary skill in the art having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others skilled in the art to understand the invention for various implementations and to best utilize the underlying principles and various implementations with various modifications as are suited to the particular use contemplated. Therefore, it is to be understood that the scope of claims is not to be limited to the specific examples of the implementations disclosed and that modifications and other implementations are intended to be included within the scope of the appended claims.

Claims

What is claimed is:

1. A method for decoding video data, comprising: reconstructing, from a video bitstream, a plurality of residual blocks of a video frame; clipping luma residuals and chroma residuals of the plurality of residual blocks of the video frame into a first dynamic range and a second dynamic range, respectively; and for each residual block, determining a luma residual for a luma component and chroma residuals for two chroma components from the clipped luma residuals and chroma residuals of the plurality of residual blocks.

2. The method of claim 1, wherein at least one of the first dynamic range and the second dynamic range is asymmetric with respective to zero and defined as [-2¹⁵, 2¹⁵-1]

3. The method of claim 1, wherein at least one of the first dynamic range and the second dynamic range is symmetric with respective to zero and defined as [-(2¹⁵-1), 2¹⁵-1]

4. The method of claim 1, wherein at least one of the first dynamic range and the second dynamic range is asymmetric with respective to zero and defined as [-2^B, 2^B-1], wherein B is a coding bit depth of the video frame, the method further comprising: obtaining the coding bit depth with the video bitstream.

5. The method of claim 1, wherein at least one of the first dynamic range and the second dynamic range is symmetric with respective to zero and defined as [-(2^B-1), 2^B-1], wherein B is a coding bit depth of the video frame, the method further comprising: obtaining the coding bit depth with the video bitstream.

6. The method of any of claims 1-5, wherein: each residual block includes one or more respective luma residuals and one or more respective chroma residuals; the first dynamic range is equal to the second dynamic range and is symmetric with respect to zero; and clipping the luma residuals and chroma residuals of the plurality of residual blocks of the video frame further comprises: for each residual block, clipping the one or more respective luma residuals into the first dynamic range and the one or more respective chroma residuals into the second dynamic range.

7. The method of any of claims 1-5, wherein: each residual block includes one or more respective luma residuals and one or more respective chroma residuals; the first dynamic range is asymmetric with respect to zero, and the second dynamic range is symmetric with respect to zero; and clipping the luma residuals and chroma residuals of the plurality of residual blocks of the video frame further comprises: clipping the one or more respective luma residuals of each residual block into the first dynamic range and clipping the one or more respective chroma residuals of each residual block into the second dynamic range.

8. The method of any of the preceding claims, wherein the second dynamic range is smaller than and enclosed within the first dynamic range.

9. The method of any of the preceding claims, wherein reconstructing, from the video bitstream, the plurality of residual blocks includes: dequantizing a plurality of quantized transform coefficients in the video bitstream to a plurality of transform coefficients for the video frame; and applying an inverse transform to the plurality of transform coefficients to reconstruct the plurality of residual blocks.

10. The method of claim 9, further comprising: prior to applying the inverse transform, clipping each of the plurality of transform coefficients into a third dynamic range.

11. The method of any of the preceding claims, further comprising: storing, in a cache, the clipped luma residuals and chroma residuals of the plurality of residual blocks according to the first dynamic range and the second dynamic range, respectively.

12. The method of any of the preceding claims, further comprising: applying an inverse adaptive color transform to the two chroma components of the video frame to obtain alternative chroma residuals for the two chroma components; clipping the alternative chroma residuals for the two chroma components of the video frame into a fourth dynamic range; and scaling the clipped alternative chroma residuals for the two chroma components of the video frame.

13. The method of any of the preceding claims, further comprising: determining whether a joint coding of chroma residuals (JCCR) mode is enabled, wherein in accordance with a determination that the JCCR mode is enabled, the chroma residuals for the two chroma components of the video frame are determined jointly according to a scheme of joint coding of chroma residuals.

14. The method of claim 13, further comprising: for each residual block, clipping the chroma residuals for the two chroma components of the video frame into a chroma dynamic range.

15. The method of claim 13, wherein the video bitstream has a hierarchical structure, the method further comprising: obtaining a syntax element associated with a first level of the hierarchical structure; wherein the chroma residuals for two chroma components of the video frame are determined jointly under the first level.

16. The method of claim 15, determining the luma residual for the luma component and the chroma residuals for the two chroma components further comprising: in accordance with a determination that the syntax element indicates that the JCCR mode is disabled, reconstructing the chroma residuals for the two chroma components of the video frame separately based on the clipped chroma residuals of the plurality of residual blocks.

17. The method of claim 1, wherein: the first and second dynamic ranges are identical; reconstructing, from the video bitstream, the plurality of residual blocks includes: dequantizing a plurality of quantized transform coefficients in the video bitstream to a plurality of transform coefficients for the video frame; and applying an inverse transform to the plurality of transform coefficients to reconstruct the plurality of residual blocks; the method further comprises: prior to applying the inverse transform, clipping each of the plurality of transform coefficients into the first dynamic range; clipping the chroma residuals for the two chroma components of the video frame into the first dynamic range; applying an inverse adaptive color transform to the clipped chroma residuals of the two chroma components of the video frame to obtain alternative chroma residuals for the two chroma components; and clipping the alternative chroma residuals for the two chroma components of the video frame into a fourth dynamic range.

18. The method of claim 17, wherein the fourth dynamic range is identical to the first dynamic range.

19. The method of claim 17, wherein the first dynamic range is [-2¹⁵, 2¹⁵-1], and the fourth dynamic range is [-2^B, 2^B-1], where B is an integer less than 15.

20. An electronic device, comprising: one or more processors; and memory having instructions stored thereon, which when executed by the one or more processors cause the processors to perform a method of any of claims 1-19.

21. A non-transitory computer-readable medium, having instructions stored thereon, which when executed by one or more processors cause the processors to perform a method of any of claims 1-19.