Methods and apparatus of video coding in 4:4:4 chroma format

Abstract

An electronic apparatus performs a method of decoding video data comprising: receiving from a bitstream, video data corresponding to a coding unit, the coding unit is coded by intra-prediction mode; obtaining a first syntax element from the bitstream, the first syntax element indicates whether the coding unit has been coded using adaptive color-space transform (ACT); in accordance with a determination that the first syntax element has a zero value: obtaining one or more syntax elements from the bitstream, the one or more syntax elements indicate whether chroma components of the coding unit have been coded using block differential pulse coded modulation (BDPCM); in accordance with a determination that the first syntax element has a non-zero value: assigning default values to the one or more syntax elements associated with the BDPCM; decoding the coding unit from the video data in accordance with the first syntax element associated with the ACT and the one or more syntax elements associated with the BDPCM. Fig. 1

Description

TECHNICAL FIELD

[0002] The present application generally relates to video data coding andcompression, and in particular, to method and system of performing adaptive color-spacetransform (ACT) with chroma residual scaling.BACKGROUND

[0003] Digital video is supported by a variety of electronic devices, such as digitaltelevisions, laptop or desktop computers, tablet computers, digital cameras, digital recordingdevices, digital media players, video gaming consoles, smart phones, video teleconferencingdevices, video streaming devices, etc. The electronic devices transmit, receive, encode,decode, and / or store digital video data by implementing video compression / decompressionstandards as defined by MPEG-4, ITU-T H.263, ITU-T H.264 / MPEG-4, Part 10, AdvancedVideo 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 inherentin the video data. For block-based video coding, a video frame is partitioned into one or moreslices, each slice having multiple video blocks, which may also be referred to as coding treeunits (CTUs). Each CTU may contain one coding unit (CU) or recursively split into smallerThis application is a divisional application of Indian Patent Application no. 202247026145.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 multipleprediction units (PUs). Each CU can be coded in either intra, inter or IBC modes. Videoblocks in an intra coded (I) slice of a video frame are encoded using spatial prediction withrespect to reference samples in neighboring blocks within the same video frame. Videoblocks in an inter coded (P or B) slice of a video frame may use spatial prediction withrespect to reference samples in neighboring blocks within the same video frame or temporalprediction with respect to reference samples in other previous and / or future reference videoframes.

[0004] Spatial or temporal prediction based on a reference block that has beenpreviously encoded, e.g., a neighboring block, results in a predictive block for a current videoblock to be coded. The process of finding the reference block may be accomplished by blockmatching algorithm. Residual data representing pixel differences between the current block tobe coded and the predictive block is referred to as a residual block or prediction errors. Aninter-coded block is encoded according to a motion vector that points to a reference block in areference frame forming the predictive block, and the residual block. The process ofdetermining the motion vector is typically referred to as motion estimation. An intra codedblock is encoded according to an intra prediction mode and the residual block. For furthercompression, 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 bequantized. The quantized transform coefficients, initially arranged in a two-dimensionalarray, may be scanned to produce a one-dimensional vector of transform coefficients, andthen entropy encoded into a video bitstream to achieve even more compression.

[0005] The encoded video bitstream is then saved in a computer-readable storagemedium (e.g., flash memory) to be accessed by another electronic device with digital videocapability or directly transmitted to the electronic device wired or wirelessly. The electronicdevice then performs video decompression (which is an opposite process to the videocompression described above) by, e.g., parsing the encoded video bitstream to obtain syntaxelements from the bitstream and reconstructing the digital video data to its original formatfrom the encoded video bitstream based at least in part on the syntax elements obtained fromthe bitstream, and renders the reconstructed digital video data on a display of the electronicdevice.

[0006] With digital video quality going from high definition, to 4Kx2K or even8Kx4K, the amount of vide data to be encoded / decoded grows exponentially. It is a constantchallenge in terms of how the video data can be encoded / decoded more efficiently whilemaintaining the image quality of the decoded video data.

[0007] Certain video content, e.g., screen content videos, is encoded in 4:4:4 chromaformat in which all three components (the luma component and both chroma components)have the same resolution. Although 4:4:4 chroma format includes more redundanciescompared to that in 4:2:0 chroma format and 4:2:2 chroma format (which is unfriendly toachieving a good compression efficiency), 4:4:4 chroma format is still the preferred encodingformat for many applications where high fidelity is required to preserve color information,such as sharp edges, in the decoded video. Given the redundancies that exist in 4:4:4 chromaformat video, there are evidences that significant coding improvement can be achieved byexploiting the correlations among the three color components of 4:4:4 video (e.g. Y, Cb andCr in YCbCr domain; or G, B and R in RGB domain). Due to these correlations, during thedevelopment of HEVC screen content coding (SCC) extension, the adaptive color-spacetransform (ACT) tool is employed to exploit the correlation among three color components.SUMMARY

[0008] The present application describes implementations related to video dataencoding and decoding and, more particularly, to system and method of performing adaptivecolor-space transform (ACT) with luma mapping and chroma scaling (LMCS).

[0009] According to a first aspect of the present application, a method of decodingvideo data includes: receiving, from bitstream, a first syntax element in a slice header of aslice that indicates whether luma mapping with chroma scaling (LMCS) is applied to acoding unit in the slice; receiving a second syntax element for the coding unit that indicateswhether the coding unit has been coded using adaptive color-space transform (ACT); if thesecond syntax element has a non-zero value, decoding the coding unit by applying inverseACT to convert luma and chroma residuals of the coding unit from a transformed color spaceto an original color space of the video data; and if the first syntax element has a non-zerovalue, decoding the coding unit by performing inverse luma mapping to the luma samplesand inverse scaling to the chroma residuals of the coding unit after performing the inverseACT.

[0010] According to a second aspect of the present application, a method of decodingvideo data includes: receiving, from bitstream, video data corresponding to a coding unit,wherein the coding unit is coded by intra-prediction mode; receiving a first syntax elementfrom the video data, wherein the first syntax element indicates whether the coding unit hasbeen coded using adaptive color-space transform (ACT); if the first syntax element has a zerovalue, receiving one or more syntax elements from the video data, wherein the one or moresyntax elements indicate whether the chroma components of the coding unit has been codedusing block differential pulse coded modulation (BDPCM); if the first syntax element has anon-zero value, assigning default values to the one or more syntax elements associated withthe BDPCM; decoding the coding unit from the video data in accordance with the first syntaxelement associated with the ACT and the one or more syntax elements associated with theBDPCM.

[0011] According to a third aspect of the present application, an electronic apparatusincludes one or more processing units, memory and a plurality of programs stored in thememory. The programs, when executed by the one or more processing units, cause theelectronic apparatus to perform the methods of decoding video data as described above.

[0012] According to a forth aspect of the present application, a non-transitorycomputer readable storage medium stores a plurality of programs for execution by anelectronic apparatus having one or more processing units. The programs, when executed bythe one or more processing units, cause the electronic apparatus to perform the methods ofdecoding video data as described above.BRIEF DESCRIPTION OF DRAWINGS

[0013] The accompanying drawings, which are included to provide a furtherunderstanding of the implementations and are incorporated herein and constitute a part of thespecification, illustrate the described implementations and together with the description serveto explain the underlying principles. Like reference numerals refer to corresponding parts.

[0014] FIG. 1 is a block diagram illustrating an exemplary video encoding anddecoding system in accordance with some implementations of the present disclosure.

[0015] FIG. 2 is a block diagram illustrating an exemplary video encoder inaccordance with some implementations of the present disclosure.

[0016] FIG. 3 is a block diagram illustrating an exemplary video decoder inaccordance with some implementations of the present disclosure.

[0017] FIGS. 4A through 4E are block diagrams illustrating how a frame isrecursively partitioned into multiple video blocks of different sizes and shapes in accordancewith some implementations of the present disclosure.

[0018] FIGS. 5A and 5B are block diagrams illustrating examples of applying thetechnique of adaptive color-space transform (ACT) to transform residuals between RGBcolor space and YCgCo color space in accordance with some implementations of the presentdisclosure.

[0019] FIG. 6 is a block diagram of applying the technique of luma mapping withchroma scaling (LMCS) in an exemplary video data decoding process in accordance withsome implementations of the present disclosure.

[0020] FIG. 7 is a block diagram illustrating an exemplary video decoding process bywhich a video decoder implements the technique of inverse adaptive color-space transform(ACT) in accordance with some implementations of the present disclosure.

[0021] FIGS. 8A and 8B are block diagrams illustrating exemplary video decodingprocesses by which a video decoder implements the technique of inverse adaptive color-spacetransform (ACT) and luma mapping with chroma scaling (LMCS) in accordance with someimplementations of the present disclosure.

[0022] FIG. 9 is a block diagram illustrating exemplary decoding logics betweenperforming adaptive color-space transform (ACT) and block differential pulse codedmodulation (BDPCM) in accordance with some implementations of the present disclosure.

[0023] FIG. 10 is a flowchart illustrating an exemplary process by which a videodecoder decodes video data by performing inverse adaptive color-space transform withchroma scaling in accordance with some implementations of the present disclosure.DETAILED DESCRIPTION

[0024] Reference will now be made in detail to specific implementations, examples ofwhich 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 thesubject matter presented herein. But it will be apparent to one of ordinary skill in the art thatvarious alternatives may be used without departing from the scope of claims and the subjectmatter may be practiced without these specific details. For example, it will be apparent to oneof ordinary skill in the art that the subject matter presented herein can be implemented onmany types of electronic devices with digital video capabilities.

[0025] FIG. 1 is a block diagram illustrating an exemplary system 10 for encodingand decoding video blocks in parallel in accordance with some implementations of thepresent disclosure. As shown in FIG. 1, system 10 includes a source device 12 that generatesand encodes video data to be decoded at a later time by a destination device 14. Sourcedevice 12 and destination device 14 may comprise any of a wide variety of electronicdevices, including desktop or laptop computers, tablet computers, smart phones, set-topboxes, digital televisions, cameras, display devices, digital media players, video gamingconsoles, video streaming device, or the like. In some implementations, source device 12 anddestination device 14 are equipped with wireless communication capabilities.

[0026] In some implementations, destination device 14 may receive the encodedvideo data to be decoded via a link 16. Link 16 may comprise any type of communicationmedium or device capable of moving the encoded video data from source device 12 todestination device 14. In one example, link 16 may comprise a communication medium toenable source device 12 to transmit the encoded video data directly to destination device 14in real-time. The encoded video data may be modulated according to a communicationstandard, such as a wireless communication protocol, and transmitted to destination device14. The communication medium may comprise any wireless or wired communicationmedium, 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 areanetwork, a wide-area network, or a global network such as the Internet. The communicationmedium may include routers, switches, base stations, or any other equipment that may beuseful to facilitate communication from source device 12 to destination device 14.

[0027] In some other implementations, the encoded video data may be transmittedfrom output interface 22 to a storage device 32. Subsequently, the encoded video data instorage device 32 may be accessed by destination device 14 via input interface 28. Storagedevice 32 may include any of a variety of distributed or locally accessed data storage mediasuch as a hard drive, Blu-ray discs, DVDs, CD-ROMs, flash memory, volatile or non-volatilememory, or any other suitable digital storage media for storing encoded video data. In afurther example, storage device 32 may correspond to a file server or another intermediatestorage 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 streamingor downloading. The file server may be any type of computer capable of storing encodedvideo data and transmitting the encoded video data to destination device 14. Exemplary fileservers 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 videodata through any standard data connection, including a wireless channel (e.g., a Wi-Ficonnection), a wired connection (e.g., DSL, cable modem, etc.), or a combination of both thatis suitable for accessing encoded video data stored on a file server. The transmission ofencoded video data from storage device 32 may be a streaming transmission, a downloadtransmission, or a combination of both.

[0028] As shown in FIG. 1, source device 12 includes a video source 18, a videoencoder 20 and an output interface 22. Video source 18 may include a source such as a videocapture device, e.g., a video camera, a video archive containing previously captured video, avideo feed interface to receive video from a video content provider, and / or a computergraphics system for generating computer graphics data as the source video, or a combinationof such sources. As one example, if video source 18 is a video camera of a securitysurveillance system, source device 12 and destination device 14 may form camera phones orvideo phones. However, the implementations described in the present application may beapplicable to video coding in general, and may be applied to wireless and / or wiredapplications.

[0029] The captured, pre-captured, or computer-generated video may be encoded byvideo encoder 20. The encoded video data may be transmitted directly to destination device14 via output interface 22 of source device 12. The encoded video data may also (oralternatively) be stored onto storage device 32 for later access by destination device 14 orother devices, for decoding and / or playback. Output interface 22 may further include amodem and / or a transmitter.

[0030] Destination device 14 includes an input interface 28, a video decoder 30, and adisplay device 34. Input interface 28 may include a receiver and / or a modem and receive theencoded video data over link 16. The encoded video data communicated over link 16, orprovided on storage device 32, may include a variety of syntax elements generated by videoencoder 20 for use by video decoder 30 in decoding the video data. Such syntax elementsmay be included within the encoded video data transmitted on a communication medium,stored on a storage medium, or stored a file server.

[0031] In some implementations, destination device 14 may include a display device34, which can be an integrated display device and an external display device that isconfigured to communicate with destination device 14. Display device 34 displays thedecoded video data to a user, and may comprise any of a variety of display devices such as aliquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED)display, or another type of display device.

[0032] Video encoder 20 and video decoder 30 may operate according to proprietaryor 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 applicationis not limited to a specific video coding / decoding standard and may be applicable to othervideo coding / decoding standards. It is generally contemplated that video encoder 20 ofsource device 12 may be configured to encode video data according to any of these current orfuture standards. Similarly, it is also generally contemplated that video decoder 30 ofdestination device 14 may be configured to decode video data according to any of thesecurrent or future standards.

[0033] Video encoder 20 and video decoder 30 each may be implemented as any of avariety of suitable encoder circuitry, such as one or more microprocessors, digital signalprocessors (DSPs), application specific integrated circuits (ASICs), field programmable gatearrays (FPGAs), discrete logic, software, hardware, firmware or any combinations thereof.When implemented partially in software, an electronic device may store instructions for thesoftware in a suitable, non-transitory computer-readable medium and execute the instructionsin hardware using one or more processors to perform the video coding / decoding operationsdisclosed in the present disclosure. Each of video encoder 20 and video decoder 30 may beincluded in one or more encoders or decoders, either of which may be integrated as part of acombined encoder / decoder (CODEC) in a respective device.

[0034] FIG. 2 is a block diagram illustrating an exemplary video encoder 20 inaccordance with some implementations described in the present application. Video encoder20 may perform intra and inter predictive coding of video blocks within video frames. Intrapredictive coding relies on spatial prediction to reduce or remove spatial redundancy in videodata within a given video frame or picture. Inter predictive coding relies on temporalprediction to reduce or remove temporal redundancy in video data within adjacent videoframes or pictures of a video sequence.

[0035] As shown in FIG. 2, video encoder 20 includes video data memory 40,prediction processing unit 41, decoded picture buffer (DPB) 64, summer 50, transformprocessing unit 52, quantization unit 54, and entropy encoding unit 56. Prediction processingunit 41 further includes motion estimation unit 42, motion compensation unit 44, partitionunit 45, intra prediction processing unit 46, and intra block copy (BC) unit 48. In someimplementations, video encoder 20 also includes inverse quantization unit 58, inversetransform processing unit 60, and summer 62 for video block reconstruction. A deblockingfilter (not shown) may be positioned between summer 62 and DPB 64 to filter blockboundaries to remove blockiness artifacts from reconstructed video. An in loop filter (notshown) may also be used in addition to the deblocking filter to filter the output of summer 62.Video encoder 20 may take the form of a fixed or programmable hardware unit or may bedivided among one or more of the illustrated fixed or programmable hardware units.

[0036] Video data memory 40 may store video data to be encoded by the componentsof 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 encodingvideo data by video encoder 20 (e.g., in intra or inter predictive coding modes). Video datamemory 40 and DPB 64 may be formed by any of a variety of memory devices. In variousexamples, video data memory 40 may be on-chip with other components of video encoder 20,or off-chip relative to those components.

[0037] As shown in FIG. 2, after receiving video data, partition unit 45 withinprediction processing unit 41 partitions the video data into video blocks. This partitioningmay 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 associatedwith the video data. The video frame may be divided into multiple video blocks (or sets ofvideo blocks referred to as tiles). Prediction processing unit 41 may select one of a pluralityof possible predictive coding modes, such as one of a plurality of intra predictive codingmodes or one of a plurality of inter predictive coding modes, for the current video blockbased on error results (e.g., coding rate and the level of distortion). Prediction processing unit41 may provide the resulting intra or inter prediction coded block to summer 50 to generate aresidual block and to summer 62 to reconstruct the encoded block for use as part of areference frame subsequently. Prediction processing unit 41 also provides syntax elements,such as motion vectors, intra-mode indicators, partition information, and other such syntaxinformation, to entropy encoding unit 56.

[0038] In order to select an appropriate intra predictive coding mode for the currentvideo block, intra prediction processing unit 46 within prediction processing unit 41 mayperform intra predictive coding of the current video block relative to one or more neighboringblocks 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 unit41 perform inter predictive coding of the current video block relative to one or morepredictive blocks in one or more reference frames to provide temporal prediction. Videoencoder 20 may perform multiple coding passes, e.g., to select an appropriate coding modefor each block of video data.

[0039] In some implementations, motion estimation unit 42 determines the interprediction mode for a current video frame by generating a motion vector, which indicates thedisplacement of a prediction unit (PU) of a video block within the current video framerelative to a predictive block within a reference video frame, according to a predeterminedpattern within a sequence of video frames. Motion estimation, performed by motionestimation unit 42, is the process of generating motion vectors, which estimate motion forvideo blocks. A motion vector, for example, may indicate the displacement of a PU of avideo block within a current video frame or picture relative to a predictive block within areference frame (or other coded unit) relative to the current block being coded within thecurrent frame (or other coded unit). The predetermined pattern may designate video frames inthe sequence as P frames or B frames. Intra BC unit 48 may determine vectors, e.g., blockvectors, for intra BC coding in a manner similar to the determination of motion vectors bymotion estimation unit 42 for inter prediction, or may utilize motion estimation unit 42 todetermine the block vector.

[0040] A predictive block is a block of a reference frame that is deemed as closelymatching the PU of the video block to be coded in terms of pixel difference, which may bedetermined by sum of absolute difference (SAD), sum of square difference (SSD), or otherdifference 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 20may interpolate values of one-quarter pixel positions, one-eighth pixel positions, or otherfractional pixel positions of the reference frame. Therefore, motion estimation unit 42 mayperform a motion search relative to the full pixel positions and fractional pixel positions andoutput a motion vector with fractional pixel precision.

[0041] Motion estimation unit 42 calculates a motion vector for a PU of a video blockin an inter prediction coded frame by comparing the position of the PU to the position of apredictive block of a reference frame selected from a first reference frame list (List 0) or asecond reference frame list (List 1), each of which identifies one or more reference framesstored in DPB 64. Motion estimation unit 42 sends the calculated motion vector to motioncompensation unit 44 and then to entropy encoding unit 56.

[0042] Motion compensation, performed by motion compensation unit 44, mayinvolve fetching or generating the predictive block based on the motion vector determined bymotion estimation unit 42. Upon receiving the motion vector for the PU of the current videoblock, motion compensation unit 44 may locate a predictive block to which the motion vectorpoints in one of the reference frame lists, retrieve the predictive block from DPB 64, andforward the predictive block to summer 50. Summer 50 then forms a residual video block ofpixel difference values by subtracting pixel values of the predictive block provided by motioncompensation unit 44 from the pixel values of the current video block being coded. The pixeldifference values forming the residual vide block may include luma or chroma differencecomponents or both. Motion compensation unit 44 may also generate syntax elementsassociated with the video blocks of a video frame for use by video decoder 30 in decoding thevideo blocks of the video frame. The syntax elements may include, for example, syntaxelements defining the motion vector used to the predictive block, any flagsindicating the prediction mode, or any other syntax information described herein. Note thatmotion estimation unit 42 and motion compensation unit 44 may be highly integrated, but areillustrated separately for conceptual purposes.

[0043] In some implementations, intra BC unit 48 may generate vectors and fetchpredictive blocks in a manner similar to that described above in connection with motionestimation unit 42 and motion compensation unit 44, but with the predictive blocks being inthe same frame as the current block being coded and with the vectors being referred to asblock vectors as opposed to motion vectors. In particular, intra BC unit 48 may determine anintra-prediction mode to use to encode a current block. In some examples, intra BC unit 48may encode a current block using various intra-prediction modes, e.g., during separateencoding passes, and test their performance through rate-distortion analysis. Next, intra BCunit 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, intraBC unit 48 may calculate rate-distortion values using a rate-distortion analysis for the varioustested 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 touse. Rate-distortion analysis generally determines an amount of distortion (or error) betweenan encoded block and an original, unencoded block that was encoded to produce the encodedblock, as well as a bitrate (i.e., a number of bits) used to produce the encoded block. Intra BCunit 48 may calculate ratios from the distortions and rates for the various encoded blocks todetermine which intra-prediction mode exhibits the best rate-distortion value for the block.

[0044] In other examples, intra BC unit 48 may use motion estimation unit 42 andmotion compensation unit 44, in whole or in part, to perform such functions for Intra BCprediction according to the implementations described herein. In either case, for Intra blockcopy, a predictive block may be a block that is deemed as closely matching the block to becoded, 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 thepredictive block may include calculation of values for sub-integer pixel positions.

[0045] Whether the predictive block is from the same frame according to intraprediction, or a different frame according to inter prediction, video encoder 20 may form aresidual video block by subtracting pixel values of the predictive block from the pixel valuesof the current video block being coded, forming pixel difference values. The pixel differencevalues forming the residual video block may include both luma and chroma componentdifferences.

[0046] Intra prediction processing unit 46 may intra-predict a current video block, asan alternative to the inter-prediction performed by motion estimation unit 42 and motioncompensation unit 44, or the intra block copy prediction performed by intra BC unit 48, asdescribed above. In particular, intra prediction processing unit 46 may determine an intraprediction mode to use to encode a current block. To do so, intra prediction processing unit46 may encode a current block using various intra prediction modes, e.g., during separateencoding passes, and intra prediction processing unit 46 (or a mode select unit, in someexamples) may select an appropriate intra prediction mode to use from the tested intraprediction modes. Intra prediction processing unit 46 may provide information indicative ofthe selected intra-prediction mode for the block to entropy encoding unit 56. Entropyencoding unit 56 may encode the information indicating the selected intra-prediction mode inthe bitstream.

[0047] After prediction processing unit 41 determines the predictive block for thecurrent video block via either inter prediction or intra prediction, summer 50 forms a residualvideo block by subtracting the predictive block from the current video block. The residualvideo data in the residual block may be included in one or more transform units (TUs) and isprovided to transform processing unit 52. Transform processing unit 52 transforms theresidual video data into residual transform coefficients using a transform, such as a discretecosine transform (DCT) or a conceptually similar transform.

[0048] Transform processing unit 52 may send the resulting transform coefficients toquantization unit 54. Quantization unit 54 quantizes the transform coefficients to furtherreduce bit rate. The quantization process may also reduce the bit depth associated with someor all of the coefficients. The degree of quantization may be modified by adjusting aquantization parameter. In some examples, quantization unit 54 may then perform a scan of amatrix including the quantized transform coefficients. Alternatively, entropy encoding unit 56may perform the scan.

[0049] Following quantization, entropy encoding unit 56 entropy encodes thequantized transform coefficients into a video bitstream using, e.g., context adaptive variablelength coding (CAVLC), context adaptive binary arithmetic coding (CABAC), syntax-basedcontext-adaptive binary arithmetic coding (SBAC), probability interval partitioning entropy(PIPE) coding or another entropy encoding methodology or technique. The encoded bitstreammay then be transmitted to video decoder 30, or archived in storage device 32 for latertransmission to or retrieval by video decoder 30. Entropy encoding unit 56 may also entropyencode the motion vectors and the other syntax elements for the current video frame beingcoded.

[0050] Inverse quantization unit 58 and inverse transform processing unit 60 applyinverse quantization and inverse transformation, respectively, to reconstruct the residualvideo block in the pixel domain for generating a reference block for prediction of other videoblocks. As noted above, motion compensation unit 44 may generate a motion compensatedpredictive block from one or more reference blocks of the frames stored in DPB 64. Motioncompensation unit 44 may also apply one or more interpolation filters to the predictive blockto calculate sub-integer pixel values for use in motion estimation.

[0051] Summer 62 adds the reconstructed residual block to the motion compensatedpredictive block produced by motion compensation unit 44 to produce a reference block forstorage in DPB 64. The reference block may then be used by intra BC unit 48, motionestimation unit 42 and motion compensation unit 44 as a predictive block to inter predictanother video block in a subsequent video frame.

[0052] FIG. 3 is a block diagram illustrating an exemplary video decoder 30 inaccordance with some implementations of the present application. Video decoder 30 includesvideo data memory 79, entropy decoding unit 80, prediction processing unit 81, inversequantization unit 86, inverse transform processing unit 88, summer 90, and DPB 92.Prediction processing unit 81 further includes motion compensation unit 82, intra predictionprocessing unit 84, and intra BC unit 85. Video decoder 30 may perform a decoding processgenerally reciprocal to the encoding process described above with respect to video encoder20 in connection with FIG. 2. For example, motion compensation unit 82 may generateprediction 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 indicatorsreceived from entropy decoding unit 80.

[0053] In some examples, a unit of video decoder 30 may be tasked to perform theimplementations of the present application. Also, in some examples, the implementations ofthe 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 compensationunit 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 85may be performed by other components of prediction processing unit 81, such as motioncompensation unit 82.

[0054] Video data memory 79 may store video data, such as an encoded videobitstream, to be decoded by the other components of video decoder 30. The video data storedin video data memory 79 may be obtained, for example, from storage device 32, from a localvideo 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 datamemory 79 may include a coded picture buffer (CPB) that stores encoded video data from anencoded video bitstream. Decoded picture buffer (DPB) 92 of video decoder 30 storesreference video data for use in decoding video data by video decoder 30 (e.g., in intra or interpredictive coding modes). Video data memory 79 and DPB 92 may be formed by any of avariety of memory devices, such as dynamic random access memory (DRAM), includingsynchronous DRAM (SDRAM), magneto-resistive RAM (MRAM), resistive RAM (RRAM),or other types of memory devices. For illustrative purpose, video data memory 79 and DPB92 are depicted as two distinct components of video decoder 30 in FIG. 3. But it will beapparent to one skilled in the art that video data memory 79 and DPB 92 may be provided bythe same memory device or separate memory devices. In some examples, video data memory79 may be on-chip with other components of video decoder 30, or off-chip relative to thosecomponents.

[0055] During the decoding process, video decoder 30 receives an encoded videobitstream that represents video blocks of an encoded video frame and associated syntaxelements. Video decoder 30 may receive the syntax elements at the video frame level and / orthe video block level. Entropy decoding unit 80 of video decoder 30 entropy decodes thebitstream to generate quantized coefficients, motion vectors or intra-prediction modeindicators, and other syntax elements. Entropy decoding unit 80 then forwards the motionvectors and other syntax elements to prediction processing unit 81.

[0056] When the video frame is coded as an intra predictive coded (I) frame or forintra coded predictive blocks in other types of frames, intra prediction processing unit 84 ofprediction processing unit 81 may generate prediction data for a video block of the currentvideo frame based on a signaled intra prediction mode and reference data from previouslydecoded blocks of the current frame.

[0057] 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 morepredictive blocks for a video block of the current video frame based on the motion vectorsand other syntax elements received from entropy decoding unit 80. Each of the predictiveblocks may be produced from a reference frame within one of the reference frame lists. Videodecoder 30 may construct the reference frame lists, List 0 and List 1, using defaultconstruction techniques based on reference frames stored in DPB 92.

[0058] In some examples, when the video block is coded according to the intra BCmode described herein, intra BC unit 85 of prediction processing unit 81 produces predictiveblocks for the current video block based on block vectors and other syntax elements receivedfrom entropy decoding unit 80. The predictive blocks may be within a reconstructed region ofthe same picture as the current video block defined by video encoder 20.

[0059] Motion compensation unit 82 and / or intra BC unit 85 determines predictioninformation for a video block of the current video frame by parsing the motion vectors andother syntax elements, and then uses the prediction information to produce the predictiveblocks for the current video block being decoded. For example, motion compensation unit 82uses some of the received syntax elements to determine a prediction mode (e.g., intra or interprediction) 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 predictionstatus for each inter predictive coded video block of the frame, and other information todecode the video blocks in the current video frame.

[0060] 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 reconstructedregion and should be stored in DPB 92, block vectors for each intra BC predicted video blockof 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.

[0061] Motion compensation unit 82 may also perform interpolation using theinterpolation filters as used by video encoder 20 during encoding of the video blocks tocalculate interpolated values for sub-integer pixels of reference blocks. In this case, motioncompensation unit 82 may determine the interpolation filters used by video encoder 20 fromthe received syntax elements and use the interpolation filters to produce predictive blocks.

[0062] Inverse quantization unit 86 inverse quantizes the quantized transformcoefficients provided in the bitstream and entropy decoded by entropy decoding unit 80 usingthe same quantization parameter calculated by video encoder 20 for each video block in thevideo frame to determine a degree of quantization. Inverse transform processing unit 88applies an inverse transform, e.g., an inverse DCT, an inverse integer transform, or aconceptually similar inverse transform process, to the transform coefficients in order toreconstruct the residual blocks in the pixel domain.

[0063] After motion compensation unit 82 or intra BC unit 85 generates the predictiveblock for the current video block based on the vectors and other syntax elements, summer 90reconstructs decoded video block for the current video block by summing the residual blockfrom inverse transform processing unit 88 and a corresponding predictive block generated bymotion compensation unit 82 and intra BC unit 85. An in-loop filter (not pictured) may bepositioned between summer 90 and DPB 92 to further process the decoded video block. Thedecoded video blocks in a given frame are then stored in DPB 92, which stores referenceframes used for subsequent motion compensation of next video blocks. DPB 92, or a memorydevice separate from DPB 92, may also store decoded video for later presentation on adisplay device, such as display device 34 of FIG. 1.

[0064] In a typical video coding process, a video sequence typically includes anordered 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-dimensionalarray of Cb chroma samples. SCr is a two-dimensional array of Cr chroma samples. In otherinstances, a frame may be monochrome and therefore includes only one two-dimensionalarray of luma samples.

[0065] 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 ofcoding tree units (CTUs). A video frame may include an integer number of CTUs orderedconsecutively in a raster scan order from left to right and from top to bottom. Each CTU is alargest logical coding unit and the width and height of the CTU are signaled by the videoencoder 20 in a sequence parameter set, such that all the CTUs in a video sequence have thesame size being one of 128x128, 64x64, 32x32, and 16x16. But it should be noted that thepresent application is not necessarily limited to a particular size. As shown in FIG. 4B, eachCTU may comprise one coding tree block (CTB) of luma samples, two corresponding codingtree blocks of chroma samples, and syntax elements used to code the samples of the codingtree blocks. The syntax elements describe properties of different types of units of a codedblock 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 otherparameters. In monochrome pictures or pictures having three separate color planes, a CTUmay comprise a single coding tree block and syntax elements used to code the samples of thecoding tree block. A coding tree block may be an NxN block of samples.

[0066] To achieve a better performance, video encoder 20 may recursively performtree partitioning such as binary-tree partitioning, ternary-tree partitioning, quad-treepartitioning or a combination of both on the coding tree blocks of the CTU and divide theCTU into smaller coding units (CUs). As depicted in FIG. 4C, the 64x64 CTU 400 is firstdivided into four smaller CU, each having a block size of 32x32. Among the four smallerCUs, CU 410 and CU 420 are each divided into four CUs of 16x16 by block size. The two16x16 CUs 430 and 440 are each further divided into four CUs of 8x8 by block size. FIG. 4Ddepicts a quad-tree data structure illustrating the end result of the partition process of theCTU 400 as depicted in FIG. 4C, each leaf node of the quad-tree corresponding to one CU ofa respective size ranging from 32x32 to 8x8. Like the CTU depicted in FIG. 4B, each CUmay comprise a coding block (CB) of luma samples and two corresponding coding blocks ofchroma samples of a frame of the same size, and syntax elements used to code the samples ofthe coding blocks. In monochrome pictures or pictures having three separate color planes, aCU may comprise a single coding block and syntax structures used to code the samples of thecoding block. It should be noted that the quad-tree partitioning depicted in FIGS. 4C and 4Dis only for illustrative purposes and one CTU can be split into CUs to adapt to varying localcharacteristics based on quad / ternary / 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 furtherpartitioned by a binary and ternary tree structure. As shown in FIG. 4E, there are fivepartitioning types, i.e., quaternary partitioning, horizontal binary partitioning, vertical binarypartitioning, horizontal ternary partitioning, and vertical ternary partitioning.

[0067] In some implementations, video encoder 20 may further partition a codingblock of a CU into one or more MxN prediction blocks (PB). A prediction block is arectangular (square or non-square) block of samples on which the same prediction, inter orintra, is applied. A prediction unit (PU) of a CU may comprise a prediction block of lumasamples, two corresponding prediction blocks of chroma samples, and syntax elements usedto predict the prediction blocks. In monochrome pictures or pictures having three separatecolor planes, a PU may comprise a single prediction block and syntax structures used topredict the prediction block. Video encoder 20 may generate predictive luma, Cb, and Crblocks for luma, Cb, and Cr prediction blocks of each PU of the CU.

[0068] Video encoder 20 may use intra prediction or inter prediction to generate thepredictive blocks for a PU. If video encoder 20 uses intra prediction to generate the predictiveblocks of a PU, video encoder 20 may generate the predictive blocks of the PU based ondecoded samples of the frame associated with the PU. If video encoder 20 uses interprediction to generate the predictive blocks of a PU, video encoder 20 may generate thepredictive blocks of the PU based on decoded samples of one or more frames other than theframe associated with the PU.

[0069] After video encoder 20 generates predictive luma, Cb, and Cr blocks for oneor more PUs of a CU, video encoder 20 may generate a luma residual block for the CU bysubtracting the CU's predictive luma blocks from its original luma coding block such thateach sample in the CU's luma residual block indicates a difference between a luma sample inone of the CU's predictive luma blocks and a corresponding sample in the CU's original lumacoding block. Similarly, video encoder 20 may generate a Cb residual block and a Cr residualblock for the CU, respectively, such that each sample in the CU's Cb residual block indicatesa difference between a Cb sample in one of the CU's predictive Cb blocks and acorresponding sample in the CU's original Cb coding block and each sample in the CU's Crresidual block may indicate a difference between a Cr sample in one of the CU's predictiveCr blocks and a corresponding sample in the CU's original Cr coding block.

[0070] Furthermore, as illustrated in FIG. 4C, video encoder 20 may use quad-treepartitioning to decompose the luma, Cb, and Cr residual blocks of a CU into one or moreluma, 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 maycomprise a transform block of luma samples, two corresponding transform blocks of chromasamples, and syntax elements used to transform the transform block samples. Thus, each TUof a CU may be associated with a luma transform block, a Cb transform block, and a Crtransform block. In some examples, the luma transform block associated with the TU may bea sub-block of the CU's luma residual block. The Cb transform block may be a sub-block ofthe CU's Cb residual block. The Cr transform block may be a sub-block of the CU's Crresidual block. In monochrome pictures or pictures having three separate color planes, a TUmay comprise a single transform block and syntax structures used to transform the samples ofthe transform block.

[0071] Video encoder 20 may apply one or more transforms to a luma transformblock of a TU to generate a luma coefficient block for the TU. A coefficient block may be atwo-dimensional array of transform coefficients. A transform coefficient may be a scalarquantity. Video encoder 20 may apply one or more transforms to a Cb transform block of aTU to generate a Cb coefficient block for the TU. Video encoder 20 may apply one or moretransforms to a Cr transform block of a TU to generate a Cr coefficient block for the TU.

[0072] After generating a coefficient block (e.g., a luma coefficient block, a Cbcoefficient block or a Cr coefficient block), video encoder 20 may quantize the coefficientblock. Quantization generally refers to a process in which transform coefficients arequantized to possibly reduce the amount of data used to represent the transform coefficients,providing further compression. After video encoder 20 quantizes a coefficient block, videoencoder 20 may entropy encode syntax elements indicating the quantized transformcoefficients. For example, video encoder 20 may perform Context-Adaptive BinaryArithmetic Coding (CABAC) on the syntax elements indicating the quantized transformcoefficients. Finally, video encoder 20 may output a bitstream that includes a sequence of bitsthat forms a representation of coded frames and associated data, which is either saved instorage device 32 or transmitted to destination device 14.

[0073] After receiving a bitstream generated by video encoder 20, video decoder 30may parse the bitstream to obtain syntax elements from the bitstream. Video decoder 30 mayreconstruct the frames of the video data based at least in part on the syntax elements obtainedfrom the bitstream. The process of reconstructing the video data is generally reciprocal to theencoding process performed by video encoder 20. For example, video decoder 30 mayperform inverse transforms on the coefficient blocks associated with TUs of a current CU toreconstruct residual blocks associated with the TUs of the current CU. Video decoder 30 alsoreconstructs the coding blocks of the current CU by adding the samples of the predictiveblocks for PUs of the current CU to corresponding samples of the transform blocks of theTUs of the current CU. After reconstructing the coding blocks for each CU of a frame, videodecoder 30 may reconstruct the frame.

[0074] As noted above, video coding achieves video compression using primarily twomodes, i.e., intra-frame prediction (or intra-prediction) and inter-frame prediction (or inter-prediction). Palette-based coding is another coding scheme that has been adopted by manyvideo coding standards. In palette-based coding, which may be particularly suitable forscreen-generated content coding, a video coder (e.g., video encoder 20 or video decoder 30)forms a palette table of colors representing the video data of a given block. The palette tableincludes the most dominant (e.g., frequently used) pixel values in the given block. Pixelvalues that are not frequently represented in the video data of the given block are either notincluded in the palette table or included in the palette table as escape colors.

[0075] Each entry in the palette table includes an index for a corresponding pixelvalue that in the palette table. The palette indices for samples in the block may be coded toindicate which entry from the palette table is to be used to predict or reconstruct whichsample. This palette mode starts with the process of generating a palette predictor for a firstblock of a picture, slice, tile, or other such grouping of video blocks. As will be explainedbelow, the palette predictor for subsequent video blocks is typically generated by updating apreviously used palette predictor. For illustrative purpose, it is assumed that the palettepredictor is defined at a picture level. In other words, a picture may include multiple codingblocks, each having its own palette table, but there is one palette predictor for the entirepicture.

[0076] To reduce the bits needed for signaling palette entries in the video bitstream, avideo decoder may utilize a palette predictor for determining new palette entries in the palettetable used for reconstructing a video block. For example, the palette predictor may includepalette entries from a previously used palette table or even be initialized with a most recentlyused palette table by including all entries of the most recently used palette table. In someimplementations, the palette predictor may include fewer than all the entries from the mostrecently used palette table and then incorporate some entries from other previously usedpalette tables. The palette predictor may have the same size as the palette tables used forcoding different blocks or may be larger or smaller than the palette tables used for codingdifferent blocks. In one example, the palette predictor is implemented as a first-in-first-out(FIFO) table including 64 palette entries.

[0077] To generate a palette table for a block of video data from the palette predictor,a video decoder may receive, from the encoded video bitstream, a one-bit flag for each entryof the palette predictor. The one-bit flag may have a first value (e.g., a binary one) indicatingthat the associated entry of the palette predictor is to be included in the palette table or asecond value (e.g., a binary zero) indicating that the associated entry of the palette predictoris not to be included in the palette table. If the size of palette predictor is larger than thepalette table used for a block of video data, then the video decoder may stop receiving moreflags once a maximum size for the palette table is reached.

[0078] In some implementations, some entries in a palette table may be directlysignaled in the encoded video bitstream instead of being determined using the palettepredictor. For such entries, the video decoder may receive, from the encoded video bitstream,three separate m-bit values indicating the pixel values for the luma and two chromacomponents associated with the entry, where m represents the bit depth of the video data.Compared with the multiple m-bit values needed for directly signaled palette entries, thosepalette entries derived from the palette predictor only require a one-bit flag. Therefore,signaling some or all palette entries using the palette predictor can significantly reduce thenumber of bits needed to signal the entries of a new palette table, thereby improving theoverall coding efficiency of palette mode coding.

[0079] In many instances, the palette predictor for one block is determined based onthe palette table used to code one or more previously coded blocks. But when coding the firstcoding tree unit in a picture, a slice or a tile, the palette table of a previously coded block maynot be available. Therefore a palette predictor cannot be generated using entries of thepreviously used palette tables. In such case, a sequence of palette predictor initializers may besignaled in a sequence parameter set (SPS) and / or a picture parameter set (PPS), which arevalues used to generate a palette predictor when a previously used palette table is notavailable. An SPS generally refers to a syntax structure of syntax elements that apply to aseries of consecutive coded video pictures called a coded video sequence (CVS) asdetermined by the content of a syntax element found in the PPS referred to by a syntaxelement found in each slice segment header. A PPS generally refers to a syntax structure ofsyntax elements that apply to one or more individual pictures within a CVS as determined bya syntax element found in each slice segment header. Thus, an SPS is generally considered tobe a higher level syntax structure than a PPS, meaning the syntax elements included in theSPS generally change less frequently and apply to a larger portion of video data compared tothe syntax elements included in the PPS.

[0080] FIGS. 5A through 5B are block diagrams illustrating examples of applying thetechnique of adaptive color-space transform (ACT) to transform residuals between RGBcolor space and YCgCo color space in accordance with some implementations of the presentdisclosure.

[0081] In the HEVC screen content coding extension, ACT is applied to adaptivelytransform residuals from one color space (e.g., RGB) into another color space (e.g., YCgCo)such that the correlation (e.g., redundancy) between three color components (e.g., R, G, andB) are significantly reduced in the YCgCo color space. Further, in the existing ACT design,the adaptation of different color spaces is carried out at transform unit (TU) level by signalingone flag tu_act_enabled_flag for each TU. When the flag tu_act_enabled_flag is equal toone, it indicates that the residuals of the current TU is coded in the YCgCo space; otherwise(i.e., the flag is equal to 0), it indicates that the residuals of the current TU is coded in theoriginal color space (i.e., without color space conversion). Additionally, depending onwhether the current TU is coded in lossless mode or in lossy mode, different color spacetransform formulas are applied. Specifically, the forward and the inverse color spacetransform formulas between the RGB color space and the YCgCo color space for lossymodes are defined in FIG. 5A.

[0082] For lossless mode, the reversible version of RGB-YCgCo transform (alsoknown as YCgCo-LS) is used. The reversible version of RGB-YCgCo transform isimplemented based on lifting operations depicted in FIG. 5B and the related description.

[0083] As shown in FIG. 5A, the forward and inverse color transform matrices usedin lossy mode are not normalized. Therefore, the magnitude of the YCgCo signal is smallerthan that of the original signal after the color transform is applied. In order to compensate thedecrease in magnitude caused by the forward color transform, an adjusted quantizationparameter is applied to the residuals in the YCgCo domain. Specifically, when the colorspace transform is applied, the QP values QPY, QPCg and QPCo, which are used to quantizethe YCgCo domain residuals, are set to be QP - 5, QP - 5 and QP - 3 respectively, where QPis the quantization parameter used in the original color space.

[0084] FIG. 6 is a block diagram of applying the technique of luma mapping withchroma scaling (LMCS) in an exemplary video data decoding process in accordance withsome implementations of the present disclosure.

[0085] In the VVC, LMCS is used as a new coding tool applied before the in-loopfilters (e.g., the de-blocking filter, the SAO and the ALF). In general, LMCS has two mainmodules: 1) in-loop mapping of the luma component based on adaptive piecewise linearmodels; 2) luma-dependent chroma residual scaling. FIG. 6 shows a modified decodingprocess with LMCS being applied. In FIG. 6, decoding modules that are conducted in themapped domain include the entropy decoding module, the inverse quantization module, theinverse transform module, the luma intra prediction module, and the luma samplereconstruction module (i.e., the addition of the luma prediction samples and the luma residualsamples). The decoding modules that are conducted in the original (i.e., non-mapped) domaininclude the motion compensated prediction module, the chroma intra prediction module, thechroma sample reconstruction module (i.e., the addition of the chroma prediction samplesand the chroma residual samples), and all the in-loop filter modules such as the deblockingmodule, the SAO module, and the ALF module. The new operational modules introduced byLMCS include the forward mapping module 610 of luma samples, the inverse mappingmodule 620 of luma samples, and the chroma residual scaling module 630.

[0086] The in-loop mapping of LMCS can adjust the dynamic range of the inputsignal to improve the coding efficiency. The in-loop mapping of the luma samples in theexisting LMCS design is built upon two mapping functions: one forward mapping functionFwdMap and one corresponding inverse mapping function InvMap. The forward mappingfunction is signaled from encoder to decoder using one piecewise linear model with sixteenequal-size pieces. The inverse mapping function can be directly derived from the forwardmapping function and therefore does not need to be signaled.

[0087] The parameters of luma mapping model are signaled at slice level. A presenceflag is firstly signaled to indicate if luma mapping model is to be signaled for a current slice.If luma mapping model is present in the current slice, the corresponding piecewise linearmodel parameters are further signaled. Additionally, at slice level, another LMCS control flagis signaled to enable / disable LMCS for the slice.

[0088] Chroma residual scaling module 630 is designed to compensate for theinteraction of quantization precision between the luma signal and its corresponding chromasignals when the in-loop mapping is applied to the luma signal. It is also signaled in the sliceheader whether chroma residual scaling is enabled or disabled for the current slice. If lumamapping is enabled, an additional flag is signaled to indicate if luma-dependent chromaresidual scaling is applied or not. When luma mapping is not used, luma-dependent chromaresidual scaling is always disabled and no additional flag is required. Additionally, thechroma residual scaling is always disabled for the CUs that contain less than or equal to fourchroma samples.

[0089] FIG. 7 is a block diagram illustrating an exemplary video decoding process bywhich a video decoder implements the technique of inverse adaptive color-space transform(ACT) in accordance with some implementations of the present disclosure.

[0090] Similar to ACT design in the HEVC SCC, ACT in VVC converts theintra / inter prediction residuals of one CU in 4:4:4 chroma format from an original color space(e.g., RGB color space) into YCgCo color space. As a result, the redundancy between threecolor components can be reduced for better coding efficiency. FIG. 7 depicts the decodingflowchart in which how the inverse ACT is applied in the VVC framework through theaddition of the inverse ACT module 710. When processing a CU encoded with ACT beingenabled, the entropy decoding, the inverse quantization and the inverse DCT / DST-basedtransform first apply to the CU. After that, as depicted in FIG. 7, the inverse ACT is invokedto convert the decoded residuals from YCgCo color space to the original color space (e.g.,RGB and YCbCr). Additionally, because ACT in lossy mode are not normalized, the QPadjustments of (-5, -5, -3) are applied to Y, Cg and Co components to compensate thechanged magnitude of transformed residuals.

[0091] In some embodiments, ACT method reuses the same ACT core transforms ofthe HEVC to do the color conversion between different color spaces. Specifically, twodifferent versions of color transforms are applied depending on whether a current CU iscoded in lossy or lossless manner. The forward and inverse color transforms for lossy caseuse the irreversible YCgCo transform matrix as depicted in FIG. 5A. For the lossless case, thereversible color transform YCgCo-LS as shown in FIG. 5B is applied. Moreover, differentfrom the existing ACT design, the following changes are introduced to the proposed ACTscheme to handle its interaction with the other coding tools in the VVC standard.

[0092] For example, because the residuals of one CU in the HEVC may be partitionedinto multiple TUs, ACT control flag is separately signaled for each TU to indicate whetherthe color space conversion needs to be applied or not. However, as described above inconnection with FIG. 4E, one quadtree nested with binary and ternary partition structure isapplied in the VVC to replace the multiple partition type concept, thus removing the separateCU, PU and TU partitions in the HEVC. This means that in most cases one CU leaf node isalso used as the unit of prediction and transform processing without further partition, unlessthe maximum supported transform size is smaller than the width or height of one componentof the CU. Based on such partition structure, it is proposed in this disclosure to adaptivelyenable and disable ACT at CU level. Specifically, one flag cu_act_enabled_flag is signaledfor each CU to select between the original color space and the YCgCo color space for codingthe residuals of the CU. If the flag is equal to 1, it indicates that the residuals of all the TUswithin the CU are coded in the YCgCo color space. Otherwise, if the flagcu_act_enabled_flag is equal to 0, all the residuals of the CU are coded in the original colorspace.

[0093] In some embodiments, a syntax element, e.g., sps_act_enabled_flag, is addedto the sequence parameter set (SPS) to indicate if ACT is enabled or not at the sequence level.In addition, as the color-space conversion is applied to video content whose luma and chromacomponents have the same resolution (e.g., 4:4:4 chroma format 4:4:4), one bitstreamconformance requirement needs to be added such that ACT can be enabled only for 4:4:4chroma format. Table 1 illustrates the modified SPS syntax table with the above syntaxadded.Table 1 Modified SPS syntax table

[0094] Specifically, sps_act_enabled_flag equal to 1 indicates ACT is enabled andsps_act_enabled_flag equal to 0 indicates that ACT is disabled such that the flagcu_act_enabled_flag is not signaled for the CUs that refers to the SPS but inferred to be 0.When ChromaArrayType is not equal to 3, it is a requirement of the bitstream conformancethat the value of sps_act_enabled_flag shall be equal to 0.

[0095] In another embodiment, instead of always signaling sps_act_enabed_flag, thesignaling of the flag is conditioned on the chroma type of the input signal. Specifically, giventhat ACT can only be applied when luma and chroma components are in the same resolution,the flag sps_act_enabled_flag is only signaled when the input video is captured in the 4:4:4chroma format. With such change, the modified SPS syntax table is:Table 2 Modified SPS syntax table with Signaling Condition

[0096] In some embodiments, the syntax design specification for decoding video datausing ACT is illustrated in the following table.Table 3 Specification for Signaling ACT Mode

[0097] The flag cu_act_enabled_flag equal to 1 indicates that the residuals of thecoding unit are coded in the YCgCo color space and the flag cu_act_enabled_flag equal to 0indicates that the residuals of the coding unit are coded in the original color space (e.g., RGBor YCbCr). When the flag cu_act_enabled_flag is not present, it is inferred to be equal to 0.

[0098] FIGS. 8A and 8B are block diagrams illustrating exemplary video decodingprocesses by which a video decoder implements the technique of inverse adaptive color-spacetransform (ACT) and luma mapping with chroma scaling in accordance with someimplementations of the present disclosure. In some embodiments, the video bitstream iscoded using both ACT (e.g., inverse ACT 710 in FIG. 7) and chroma residual scaling (e.g.,chroma residual scaling 630 in FIG. 6). In some other embodiments, the video bitstream iscoded using chroma residual scaling but not both ACT, thereby not requiring the inverseACTT 710.

[0099] More specifically, FIG. 8A depicts an embodiment in which the video coderperforms chroma residual scaling 630 before inverse ACT 710. As a result, the video coderperforms luma mapping with chroma residual scaling 630 in the color-space transformeddomain. For example, assuming that the input video is captured in RGB format and istransformed into YCgCo color space, the video coder performs chroma residual scaling 630on the chroma residuals Cg and Co according to the luma residuals Y in the YCgCo colorspace.

[00100] FIG. 8B depicts an alternative embodiment in which the video coder performschroma residual scaling 630 after the inverse ACT 710. As a result, the video coder performsluma mapping with chroma residual scaling 630 in the original color-space domain. Forexample, assuming the input video is captured in RGB format, the video coder applieschroma residual scaling on the B and R components.

[00101] FIG. 9 is a block diagram illustrating exemplary decoding logics betweenperforming adaptive color-space transform (ACT) and block differential pulse codedmodulation (BDPCM) in accordance with some implementations of the present disclosure.

[00102] BDPCM is a coding tool for screen content coding. In some embodiments, aBDPCM enable flag is signaled in the SPS at the sequence level. The BDPCM enable flag issignaled only if the transform skip mode is enabled in the SPS.

[00103] When BDPCM is enabled, a flag is transmitted at the CU level if the CU sizeis smaller than or equal to MaxTsSize x MaxTsSize in terms of luma samples and if the CUis intra coded, where MaxTsSize is the maximum block size for which the transform skipmode is allowed. This flag indicates whether regular intra coding or BDPCM is used. IfBDPCM is used, another BDPCM prediction direction flag is further transmitted to indicatewhether the prediction is horizontal or vertical. Then, the block is predicted using the regularhorizontal or vertical intra prediction process with unfiltered reference samples. The residualis quantized and the difference between each quantized residual and its predictor, i.e. thepreviously coded residual of the horizontal or vertical (depending on BDPCM predictiondirection) neighbouring position, is coded.

[00104] For a block of size M (height) x N (width), let ri,j, 0 ≤ i ≤ M - 1, 0 ≤ j ≤N - 1 be the prediction residual. Let Q(ri,j), 0 ≤ i ≤ M - 1,0 ≤ j ≤ N - 1 denote thequantized version of the residual ri,j. BDPCM is applied to the quantized residual values,resulting in a modified M x N array R with elements ri,j, where ri,j is predicted from itsneighboring quantized residual value. For vertical BDPCM prediction mode, for 0 ≤ j ≤(N - 1), the following is used to derive ri,j:Equation (1)

[00105] For horizontal BDPCM prediction mode, for 0 ≤ i ≤ (M - 1), the following is used to derive ri,j:Equation (2)

[00106] At the decoder side, the above process is reversed to compute Q(ri,j), 0 ≤i ≤ M - 1,  0 ≤ j ≤ N - 1, as follows:Equation (3)Equation (4)

[00107] The inverse quantized residuals, Q-1 (Q(ri,j)), are added to the intra blockprediction values to produce the reconstructed sample values.

[00108] The predicted quantized residual values ri,j are sent to the decoder using thesame residual coding process as that in transform skip mode residual coding. In terms of theMPM mode for future intra mode coding, horizontal or vertical prediction mode is stored fora BDPCM-coded CU if BDPCM prediction direction is horizontal or vertical, respectively.For deblocking, if both blocks on the sides of a block boundary are coded using BDPCM,then that particular block boundary is not deblocked. According to the latest VVC workingdraft, when the input video is 4:4:4 chroma format, BDPCM can be applied to both luma andchroma components by signaling two separate flags, i.e., intra_bdpcm_luma_flag andintra_bdpcm_chroma_flag, for luma and chroma channel at CU-level.

[00109] In some embodiments, the video coder performs different logics to betterhandle the interaction between ACT and BDPCM. For example, when ACT is applied to oneintra CU, BDPCM is enabled for luma component but disabled for chroma components(910). The corresponding modified syntax table of coding unit is illustrated as follows:Table 4 BDPCM enabled for luma component only

[00110] In some embodiments, BDPCM is enabled for both luma and chromacomponents when ACT is applied to one intra CU (920). The corresponding modified syntaxtable of coding unit is illustrated as follows:Table 5 BDPCM enabled for both luma and chroma components

[00111] In some embodiments, BDPCM is disabled for both luma and chromacomponents when ACT is applied to one intra CU (930). In such case, there is no need forsignaling the BDPCM related syntax element. The corresponding modified syntax table ofcoding unit is illustrated as follows:Table 6 BDPCM is disabled for both luma and chroma components

[00112] FIG. 10 is a flowchart 1000 illustrating an exemplary process by which avideo coder decodes video data by conditionally performing the technique of inverse adaptivecolor-space transform (ACT) based on the interaction between ACT and other coding tools(e.g., LMCS and BDPCM) in accordance with some implementations of the presentdisclosure.

[00113] As the first step, the video decoder receives, from bitstream, a first syntaxelement in a slice header of a slice that indicates whether luma mapping with chroma scaling(LMCS) is applied to a coding unit in the slice (1010).

[00114] The video decoder then receives, from the bitstream, a second syntax element(e.g., cu_act_enabled_flag) for the coding unit in the slice (1020). The second syntax elementindicates whether the coding unit has been coded using adaptive color-space transform(ACT).

[00115] If the second syntax element has a non-zero value (e.g., indicating that thecoding unit has been coded using ACT), the video decoder decodes the video datacorresponding to the coding unit by applying inverse ACT to convert luma and chromaresiduals of the coding unit from a transformed color space to an original color space of thevideo data (1030).

[00116] After applying the inverse ACT to convert the luma and chroma residuals ofthe coding unit, if the first syntax element has a non-zero value (e.g., indicating that thecoding unit has been coded with LMCS), the video decoder decodes the video datacorresponding to the coding unit by performing inverse luma mapping to the luma samplesand inverse scaling to the chroma residuals of the coding unit (1040).

[00117] In some embodiments, before receiving the second syntax element, the videodecoder decodes receives a third syntax element (e.g., chroma_format_idc) from thebitstream, wherein the third syntax element indicates whether the video data has a predefinedchroma format. As noted above, the ACT is only enabled for certain predefined chromaformat, e.g., 4:4:4 chroma format.

[00118] In some embodiments, the second syntax element (e.g., cu_act_enabled_flag)is present in the bitstream only when the video data has the predefined chroma format (e.g.,the ACT is disabled when the video data is not 4:4:4 chroma format).

[00119] In some embodiments, the transformed color space is YCgCo color spacewhile the original color space can be RGB color space or YCbCr color space.

[00120] In some embodiments, after performing inverse ACT to convert luma andchroma residuals of the coding unit, the video decoder performs inverse luma mapping toluma samples and inverse scaling to chroma residuals of the coding unit by: generatingintermediate luma samples in a mapped domain of the coding unit by adding the lumaresiduals in the original color space to predicted luma samples, converting the intermediateluma samples in the mapped domain to reconstructed luma components in a pre-mappeddomain using one or more adaptive piecewise linear models; inverse scaling the chromaresiduals according to the corresponding intermediate luma samples in the mapped domain;and reconstructing chroma components of the coding unit using the scaled chroma residualsand predicted chroma samples.

[00121] In some embodiments, the video decoder receives, from bitstream, video datacorresponding to a coding unit, wherein the coding unit is coded by intra-prediction mode.The video coder receives a first syntax element (e.g., cu_act_enabled_flag) from the videodata, wherein the first syntax element indicates whether the coding unit has been coded usingadaptive color-space transform (ACT). If the first syntax element has a zero value, the videocoder receives one or more syntax elements (e.g., intra_bdpcm_chroma_flag,intra_bdpcm_chroma_dir_flag) from the video data, wherein the one or more syntaxelements indicate whether the chroma components of the coding unit has been coded usingblock differential pulse coded modulation (BDPCM). If the first syntax element has a nonzerovalue, the video coder assigns default values to the one or more syntax elementsassociated with the BDPCM. The default values may indicate that the chroma components ofthe coding unit is decoded using the inverse BDPCM or otherwise. The video coder thendecodes the coding unit from the video data in accordance with the first syntax elementassociated with the ACT and the one or more syntax elements associated with the BDPCM.

[00122] In some embodiments, before receiving the first syntax element, the videodecoder receives a second syntax element from the video data, wherein the second syntaxelement indicates whether the video data has a predefined chroma format. As noted above,the ACT is only enabled for certain predefined chroma format, e.g., 4:4:4 chroma format.

[00123] In some embodiments, the first syntax element is present in the bitstream onlywhen the video data has the predefined chroma format.

[00124] In some embodiments, the coding unit is decoded using the inverse BDPCMwhen the coding unit is coded without applying color-space transforms.

[00125] In one or more examples, the functions described may be implemented inhardware, software, firmware, or any combination thereof. If implemented in software, thefunctions may be stored on or transmitted over, as one or more instructions or code, acomputer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to atangible medium such as data storage media, or communication media including any mediumthat facilitates transfer of a computer program from one place to another, e.g., according to acommunication protocol. In this manner, computer-readable media generally may correspondto (1) tangible computer-readable storage media which is non-transitory or (2) acommunication medium such as a signal or carrier wave. Data storage media may be anyavailable media that can be accessed by one or more computers or one or more processors toretrieve instructions, code and / or data structures for implementation of the implementationsdescribed in the present application. A computer program product may include a computer-readable medium.

[00126] The terminology used in the description of the implementations herein is forthe purpose of describing particular implementations only and is not intended to limit thescope 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, unlessthe context clearly indicates otherwise. It will also be understood that the term "and / or" asused herein refers to and encompasses any and all possible combinations of one or more ofthe 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 moreother features, elements, components, and / or groups thereof.

[00127] It will also be understood that, although the terms first, second, etc. may beused 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 firstelectrode could be termed a second electrode, and, similarly, a second electrode could betermed a first electrode, without departing from the scope of the implementations. The firstelectrode and the second electrode are both electrodes, but they are not the same electrode.

[00128] The description of the present application has been presented for purposes ofillustration and description, and is not intended to be exhaustive or limited to the invention inthe form disclosed. Many modifications, variations, and alternative implementations will beapparent to those of ordinary skill in the art having the benefit of the teachings presented inthe foregoing descriptions and the associated drawings. The embodiment was chosen anddescribed in order to best explain the principles of the invention, the practical application, andto enable others skilled in the art to understand the invention for various implementations andto best utilize the underlying principles and various implementations with variousmodifications as are suited to the particular use contemplated. Therefore, it is to beunderstood that the scope of claims is not to be limited to the specific examples of theimplementations disclosed and that modifications and other implementations are intended tobe included within the scope of the appended claims.

Claims

1. A method of decoding video data, characterized in that the method comprising: receiving, by a video decoder (30), from a bitstream, video data corresponding to a coding unit, wherein the coding unit is coded by intra-prediction mode; obtaining, by the video decoder (30), a first syntax element from the bitstream, wherein the first syntax element indicates whether the coding unit has been coded using adaptive color-space transform (ACT); in accordance with a determination that the first syntax element has a zero value: obtaining, by the video decoder (30), one or more syntax elements from the bitstream, wherein the one or more syntax elements indicate whether chroma components of the coding unit have been coded using block differential pulse coded modulation (BDPCM); in accordance with a determination that the first syntax element has a non-zero value: assigning, by the video decoder (30), default values to the one or more syntax elements associated with the BDPCM; decoding, by the video decoder (30), the coding unit from the video data in accordance with the first syntax element associated with the ACT and the one or more syntax elements associated with the BDPCM.

2. The method as claimed in claim 1, further comprising: before obtaining the first syntax element, obtaining, by the video decoder (30), a second syntax element from the bitstream, wherein the second syntax element indicates whether the video data has a predefined chroma format.

3. The method as claimed in claim 2, wherein the predefined chroma format is 4:4:4 chroma format.

4. The method as claimed in claim 2, wherein the first syntax element is present in the bitstream only when the video data has the predefined chroma format.

5. The method as claimed in claim 1, wherein the default values indicate that the chroma components of the coding unit are decoded without using the inverse BDPCM.

6. The method as claimed in claim 1, wherein the default values indicate that the chroma components of the coding unit are decoded using the inverse BDPCM.

7. The method as claimed in claim 1, wherein the coding unit is decoded, by the video decoder(30), using the inverse BDPCM when the coding unit is coded without applying transforms.

8. An electronic apparatus (30) comprising: a processing unit (80, 86, 88, 81, 90); and a memory (79) coupled to the processing unit(80, 86, 88, 81, 90); characterized in that wherein the electronic apparatus is configured to perform the method as claimed in claims 1-7.

9. A method of storing a bitstream, comprising: storing the bitstream on a digital storage medium, wherein the bitstream comprises: video information to be decoded by the method for decoding the video data according to any one of claims 1-7.

10. A method for receiving a bitstream to be decoded by the electronic apparatus according to claim 8 for decoding.

11. A method of video encoding, characterized in that the method comprising: determining, by a video encoder (20), whether a coding unit has been coded using adaptive color-space transform (ACT), wherein the coding unit is coded by intra-prediction mode; in accordance with a determination that the coding unit has not been coded using ACT: determining, by the video encoder (20), whether chroma components of the coding unit have been coded using block differential pulse coded modulation (BDPCM); and decoding, by the video encoder (20), the coding unit based on the determination of whether the chroma components of the coding unit have been coded using BDPCM; and in accordance with a determination that the coding unit has been coded using ACT: decoding, by the video encoder (20), the coding unit by applying inverse ACT.

12. The method as claimed in claim 11, further comprising: signaling, by the video encoder (20), a first syntax element, wherein the first syntax element indicates whether the coding unit has been coded using adaptive color-space transform (ACT); in accordance with a determination that the first syntax element has a zero value: signaling, by the video encoder (20), one or more syntax elements, wherein the one or more syntax elements indicate whether the chroma components of the coding unit have been coded using block differential pulse coded modulation (BDPCM); in accordance with a determination that the first syntax element has a non-zero value: not signaling the one or more syntax elements, wherein when the one or more syntax elements are not signaled, default values are assigned to the one or more syntax elements.

13. The method as claimed in claim 12, further comprising: before signaling the first syntax element, signaling, by the video encoder (20), a second syntax element, wherein the second syntax element indicates whether the video data has a predefined chroma format.

14. The method as claimed in claim 13, wherein the predefined chroma format is 4:4:4 chroma format.

15. The method as claimed in claim 13, wherein the first syntax element is signaled only when the video data has the predefined chroma format.

16. The method as claimed in claim 11, wherein the default values indicate that the chroma components of the coding unit are decoded without using the inverse BDPCM.

17. The method as claimed in claim 11, wherein the default values indicate that the chroma components of the coding unit are decoded using the inverse BDPCM.

18. The method as claimed in claim 11, wherein the coding unit is decoded using the inverse BDPCM when the coding unit is coded without applying transforms.

19. An electronic apparatus (20) characterized in that the apparatus (20) comprising: a processing unit(41, 50, 52, 54,56); a memory (40) coupled to the processing unit(41, 50, 52, 54,56); and wherein the electronic apparatus (20) is configured to perform the method as claimed in claims 11-18.

20. A method of storing a bitstream, comprising: storing the bitstream on a digital storage medium, wherein the bitstream comprises: video information generated by the method for encoding the video data according to any one of claims 11-18.

21. A method for transmitting a bitstream generated by the electronic apparatus according to claim 19 for encoding.