Patent Description:
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-<NUM>, ITU-T H. <NUM>, ITU-T H. <NUM>/MPEG-<NUM>, Part <NUM>, 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.

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.

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.

In-loop filtering is applied on a reconstructed CU before it is put in a reference picture store and used to code other video blocks. Adaptive Loop Filters (ALF) are applied for chroma and luma components of the reconstructed CU, respectively, while a cross component filter can be applied to make use of the luma components to refine the chroma components of the CU. It would be beneficial to have a more efficient coding mechanism to encode and decode these color components while maintaining the image quality of the decoded video data.

In <NPL>, two tests measure the performance of the cross-component adaptive loop filter (CC-ALF) as a loop filter and as a post-processing step, respectively.

In<NPL>, a design for the Cross Component Adaptive Loop Filter (CC-ALF) is proposed. CC-ALF operates as part of the adaptive loop filter process and makes use of luma sample values to refine each chroma component.

In <NPL>, the document provides a source of general tutorial information on the VVC design and also provides an encoder-side description of VTM7.

In <NPL>) an allignment of CC-ALF with ALF is proposed.

This application describes implementations related to video data encoding and decoding and, more particularly, to method and system of improvement in coding of chroma and luma components of a video frame based on cross component adaptive filtering. Each chroma component of the video frame is filtered based on a plurality of surrounding chroma components and refined based on a set of adjacent luma components. Specifically, an anchor luma component is identified and deducted from the set of adj acent luma components to form a set of difference luma components, allowing each chroma component to be refined using a combination of the difference luma components corresponding to the set of adj acent luma components. Enabling disclosure for the protected invention is provided with the embodiments described in relation to <FIG>. The other figures, aspects, and embodiments are provided for illustrative purposes and do not represent embodiments of the invention unless when combined with all of the features respectively defined in the independent claims.

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.

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. 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.

<FIG> is a block diagram illustrating an exemplary system <NUM> for encoding and decoding video blocks in parallel, in accordance with some embodiments. As shown in <FIG>, system <NUM> includes a source device <NUM> that generates and encodes video data to be decoded at a later time by a destination device <NUM>. Source device <NUM> and destination device <NUM> 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 <NUM> and destination device <NUM> are equipped with wireless communication capabilities.

In some implementations, destination device <NUM> may receive the encoded video data to be decoded via a link <NUM>. Link <NUM> may comprise any type of communication medium or device capable of moving the encoded video data from source device <NUM> to destination device <NUM>. In one example, link <NUM> may comprise a communication medium to enable source device <NUM> to transmit the encoded video data directly to destination device <NUM> 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 <NUM>. 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 <NUM> to destination device <NUM>.

In some other implementations, the encoded video data may be transmitted from output interface <NUM> to a storage device <NUM>. Subsequently, the encoded video data in storage device <NUM> may be accessed by destination device <NUM> via input interface <NUM>. Storage device <NUM> 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 <NUM> may correspond to a file server or another intermediate storage device that may hold the encoded video data generated by source device <NUM>. Destination device <NUM> may access the stored video data from storage device <NUM> 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 <NUM>. 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 <NUM> 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 <NUM> may be a streaming transmission, a download transmission, or a combination of both.

As shown in <FIG>, source device <NUM> includes a video source <NUM>, a video encoder <NUM> and an output interface <NUM>. Video source <NUM> 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 <NUM> is a video camera of a security surveillance system, source device <NUM> and destination device <NUM> 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.

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

Destination device <NUM> includes an input interface <NUM>, a video decoder <NUM>, and a display device <NUM>. Input interface <NUM> may include a receiver and/or a modem and receive the encoded video data over link <NUM>. The encoded video data communicated over link <NUM>, or provided on storage device <NUM>, may include a variety of syntax elements generated by video encoder <NUM> for use by video decoder <NUM> 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.

In some implementations, destination device <NUM> may include a display device <NUM>, which can be an integrated display device and an external display device that is configured to communicate with destination device <NUM>. Display device <NUM> 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.

Video encoder <NUM> and video decoder <NUM> may operate according to proprietary or industry standards, such as VVC, HEVC, MPEG-<NUM>, Part <NUM>, 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 <NUM> of source device <NUM> 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 <NUM> of destination device <NUM> may be configured to decode video data according to any of these current or future standards.

Video encoder <NUM> and video decoder <NUM> 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.

<FIG> is a block diagram illustrating an exemplary video encoder <NUM> in accordance with some implementations described in the present application. Video encoder <NUM> 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 adj acent video frames or pictures of a video sequence.

As shown in <FIG>, video encoder <NUM> includes video data memory <NUM>, prediction processing unit <NUM>, decoded picture buffer (DPB) <NUM>, summer <NUM>, transform processing unit <NUM>, quantization unit <NUM>, and entropy encoding unit <NUM>. Prediction processing unit <NUM> further includes motion estimation unit <NUM>, motion compensation unit <NUM>, partition unit <NUM>, intra prediction processing unit <NUM>, and intra block copy (BC) unit <NUM>. In some implementations, video encoder <NUM> also includes inverse quantization unit <NUM>, inverse transform processing unit <NUM>, and summer <NUM> for video block reconstruction. An in-loop filter <NUM> may be positioned between summer <NUM> and DPB <NUM>, and includes a deblocking filter to filter block boundaries and remove blockiness artifacts from reconstructed video. The in-loop filter <NUM> further includes a sample adaptive offset (SAO) and adaptive in-loop filter (ALF) to filter the output of summer <NUM> before the output of summer <NUM> is put into DPB <NUM> and used to code other video blocks. Video encoder <NUM> 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.

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

As shown in <FIG>, after receiving video data, partition unit <NUM> within prediction processing unit <NUM> 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 <NUM> 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 <NUM> may provide the resulting intra or inter prediction coded block to summer <NUM> to generate a residual block and to summer <NUM> to reconstruct the encoded block for use as part of a reference frame subsequently. Prediction processing unit <NUM> also provides syntax elements, such as motion vectors, intra-mode indicators, partition information, and other such syntax information, to entropy encoding unit <NUM>.

In order to select an appropriate intra predictive coding mode for the current video block, intra prediction processing unit <NUM> within prediction processing unit <NUM> 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 <NUM> and motion compensation unit <NUM> within prediction processing unit <NUM> 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 <NUM> may perform multiple coding passes, e.g., to select an appropriate coding mode for each block of video data.

In some implementations, motion estimation unit <NUM> 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 <NUM>, 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 <NUM> 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 <NUM> for inter prediction, or may utilize motion estimation unit <NUM> to determine the block vector.

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 <NUM> may calculate values for sub-integer pixel positions of reference frames stored in DPB <NUM>. For example, video encoder <NUM> 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 <NUM> may perform a motion search relative to the full pixel positions and fractional pixel positions and output a motion vector with fractional pixel precision.

Motion estimation unit <NUM> 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 <NUM>) or a second reference frame list (List <NUM>), each of which identifies one or more reference frames stored in DPB <NUM>. Motion estimation unit <NUM> sends the calculated motion vector to motion compensation unit <NUM> and then to entropy encoding unit <NUM>.

Motion compensation, performed by motion compensation unit <NUM>, may involve fetching or generating the predictive block based on the motion vector determined by motion estimation unit <NUM>. Upon receiving the motion vector for the PU of the current video block, motion compensation unit <NUM> may locate a predictive block to which the motion vector points in one of the reference frame lists, retrieve the predictive block from DPB <NUM>, and forward the predictive block to summer <NUM>. Summer <NUM> then forms a residual video block of pixel difference values by subtracting pixel values of the predictive block provided by motion compensation unit <NUM> 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 <NUM> may also generate syntax elements associated with the video blocks of a video frame for use by video decoder <NUM> 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 <NUM> and motion compensation unit <NUM> may be highly integrated, but are illustrated separately for conceptual purposes.

In some implementations, intra BC unit <NUM> may generate vectors and fetch predictive blocks in a manner similar to that described above in connection with motion estimation unit <NUM> and motion compensation unit <NUM>, 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 <NUM> may determine an intra-prediction mode to use to encode a current block. In some examples, intra BC unit <NUM> 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 <NUM> 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 <NUM> 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 <NUM> 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.

In other examples, intra BC unit <NUM> may use motion estimation unit <NUM> and motion compensation unit <NUM>, 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.

Whether the predictive block is from the same frame according to intra prediction, or a different frame according to inter prediction, video encoder <NUM> 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.

Intra prediction processing unit <NUM> may intra-predict a current video block, as an alternative to the inter-prediction performed by motion estimation unit <NUM> and motion compensation unit <NUM>, or the intra block copy prediction performed by intra BC unit <NUM>, as described above. In particular, intra prediction processing unit <NUM> may determine an intra prediction mode to use to encode a current block. To do so, intra prediction processing unit <NUM> may encode a current block using various intra prediction modes, e.g., during separate encoding passes, and intra prediction processing unit <NUM> (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 <NUM> may provide information indicative of the selected intra-prediction mode for the block to entropy encoding unit <NUM>. Entropy encoding unit <NUM> may encode the information indicating the selected intra-prediction mode in the bitstream.

After prediction processing unit <NUM> determines the predictive block for the current video block via either inter prediction or intra prediction, summer <NUM> 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 <NUM>. Transform processing unit <NUM> transforms the residual video data into residual transform coefficients using a transform, such as a discrete cosine transform (DCT) or a conceptually similar transform.

Transform processing unit <NUM> may send the resulting transform coefficients to quantization unit <NUM>. Quantization unit <NUM> 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 <NUM> may then perform a scan of a matrix including the quantized transform coefficients. Alternatively, entropy encoding unit <NUM> may perform the scan.

Following quantization, entropy encoding unit <NUM> 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 <NUM>, or archived in storage device <NUM> for later transmission to or retrieval by video decoder <NUM>. Entropy encoding unit <NUM> may also entropy encode the motion vectors and the other syntax elements for the current video frame being coded.

Inverse quantization unit <NUM> and inverse transform processing unit <NUM> 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 <NUM> may generate a motion compensated predictive block from one or more reference blocks of the frames stored in DPB <NUM>. Motion compensation unit <NUM> may also apply one or more interpolation filters to the predictive block to calculate sub-integer pixel values for use in motion estimation.

Summer <NUM> adds the reconstructed residual block to the motion compensated predictive block produced by motion compensation unit <NUM> to produce a reference block for storage in DPB <NUM>. The reference block may then be used by intra BC unit <NUM>, motion estimation unit <NUM> and motion compensation unit <NUM> as a predictive block to inter predict another video block in a subsequent video frame.

<FIG> is a block diagram illustrating an exemplary video decoder <NUM> in accordance with some implementations of the present application. Video decoder <NUM> includes video data memory <NUM>, entropy decoding unit <NUM>, prediction processing unit <NUM>, inverse quantization unit <NUM>, inverse transform processing unit <NUM>, summer <NUM>, and DPB <NUM>. Prediction processing unit <NUM> further includes motion compensation unit <NUM>, intra prediction processing unit <NUM>, and intra BC unit <NUM>. Video decoder <NUM> may perform a decoding process generally reciprocal to the encoding process described above with respect to video encoder <NUM> in connection with <FIG>. For example, motion compensation unit <NUM> may generate prediction data based on motion vectors received from entropy decoding unit <NUM>, while intra-prediction unit <NUM> may generate prediction data based on intra-prediction mode indicators received from entropy decoding unit <NUM>.

In some examples, a unit of video decoder <NUM> 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 <NUM>. For example, intra BC unit <NUM> may perform the implementations of the present application, alone, or in combination with other units of video decoder <NUM>, such as motion compensation unit <NUM>, intra prediction processing unit <NUM>, and entropy decoding unit <NUM>. In some examples, video decoder <NUM> may not include intra BC unit <NUM> and the functionality of intra BC unit <NUM> may be performed by other components of prediction processing unit <NUM>, such as motion compensation unit <NUM>.

Video data memory <NUM> may store video data, such as an encoded video bitstream, to be decoded by the other components of video decoder <NUM>. The video data stored in video data memory <NUM> may be obtained, for example, from storage device <NUM>, 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 <NUM> may include a coded picture buffer (CPB) that stores encoded video data from an encoded video bitstream. Decoded picture buffer (DPB) <NUM> of video decoder <NUM> stores reference video data for use in decoding video data by video decoder <NUM> (e.g., in intra or inter predictive coding modes). Video data memory <NUM> and DPB <NUM> 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 <NUM> and DPB <NUM> are depicted as two distinct components of video decoder <NUM> in <FIG>. But it will be apparent to one skilled in the art that video data memory <NUM> and DPB <NUM> may be provided by the same memory device or separate memory devices. In some examples, video data memory <NUM> may be on-chip with other components of video decoder <NUM>, or off-chip relative to those components.

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

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 <NUM> of prediction processing unit <NUM> 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.

When the video frame is coded as an inter-predictive coded (i.e., B or P) frame, motion compensation unit <NUM> of prediction processing unit <NUM> 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 <NUM>. Each of the predictive blocks may be produced from a reference frame within one of the reference frame lists. Video decoder <NUM> may construct the reference frame lists, List <NUM> and List <NUM>, using default construction techniques based on reference frames stored in DPB <NUM>.

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

Motion compensation unit <NUM> and/or intra BC unit <NUM> 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 <NUM> 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.

Similarly, intra BC unit <NUM> 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 <NUM>, 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.

Motion compensation unit <NUM> may also perform interpolation using the interpolation filters as used by video encoder <NUM> during encoding of the video blocks to calculate interpolated values for sub-integer pixels of reference blocks.

Inverse quantization unit <NUM> inverse quantizes the quantized transform coefficients provided in the bitstream and entropy decoded by entropy decoding unit <NUM> using the same quantization parameter calculated by video encoder <NUM> for each video block in the video frame to determine a degree of quantization. Inverse transform processing unit <NUM> 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.

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

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.

As shown in <FIG>, video encoder <NUM> (or more specifically partition unit <NUM>) 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 <NUM> in a sequence parameter set, such that all the CTUs in a video sequence have the same size being one of <NUM>×<NUM>, <NUM>×<NUM>, <NUM>×<NUM>, and <NUM>×<NUM>. But it should be noted that the present application is not necessarily limited to a particular size. As shown in <FIG>, 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 <NUM>, 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.

To achieve a better performance, video encoder <NUM> 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>, the 64x64 CTU <NUM> is first divided into four smaller CU, each having a block size of 32x32. Among the four smaller CUs, CU <NUM> and CU <NUM> are each divided into four CUs of 16x16 by block size. The two 16x16 CUs <NUM> and <NUM> are each further divided into four CUs of 8x8 by block size. <FIG> depicts a quad-tree data structure illustrating the end result of the partition process of the CTU <NUM> as depicted in <FIG>, 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>, 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 <FIG> and <FIG> is only for illustrative purposes and one CTU can be split into CUs to adapt to varying local characteristics 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 further partitioned by a binary and ternary tree structure. As shown in <FIG>, there are five partitioning types, i.e., quaternary partitioning, horizontal binary partitioning, vertical binary partitioning, horizontal ternary partitioning, and vertical ternary partitioning.

In some implementations, video encoder <NUM> 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 <NUM> may generate predictive luma, Cb, and Cr blocks for luma, Cb, and Cr prediction blocks of each PU of the CU.

Video encoder <NUM> may use intra prediction or inter prediction to generate the predictive blocks for a PU. If video encoder <NUM> uses intra prediction to generate the predictive blocks of a PU, video encoder <NUM> may generate the predictive blocks of the PU based on decoded samples of the frame associated with the PU. If video encoder <NUM> uses inter prediction to generate the predictive blocks of a PU, video encoder <NUM> 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.

After video encoder <NUM> generates predictive luma, Cb, and Cr blocks for one or more PUs of a CU, video encoder <NUM> 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 <NUM> 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.

Furthermore, as illustrated in <FIG>, video encoder <NUM> 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.

Video encoder <NUM> 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 <NUM> 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 <NUM> may apply one or more transforms to a Cr transform block of a TU to generate a Cr coefficient block for the TU.

After generating a coefficient block (e.g., a luma coefficient block, a Cb coefficient block or a Cr coefficient block), video encoder <NUM> 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 <NUM> quantizes a coefficient block, video encoder <NUM> may entropy encode syntax elements indicating the quantized transform coefficients. For example, video encoder <NUM> may perform Context-Adaptive Binary Arithmetic Coding (CABAC) on the syntax elements indicating the quantized transform coefficients. Finally, video encoder <NUM> 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 <NUM> or transmitted to destination device <NUM>.

After receiving a bitstream generated by video encoder <NUM>, video decoder <NUM> may parse the bitstream to obtain syntax elements from the bitstream. Video decoder <NUM> 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 <NUM>. For example, video decoder <NUM> 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 <NUM> 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 <NUM> may reconstruct the frame.

As noted above, video coding achieves video compression using primarily two modes, 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 many video coding standards. In palette-based coding, which may be particularly suitable for screen-generated content coding, a video coder (e.g., video encoder <NUM> or video decoder <NUM>) forms a palette table of colors representing the video data of a given block. The palette table includes the most dominant (e.g., frequently used) pixel values in the given block. Pixel values that are not frequently represented in the video data of the given block are either not included in the palette table or included in the palette table as escape colors.

Each entry in the palette table includes an index for a corresponding pixel value that in the palette table. The palette indices for samples in the block may be coded to indicate which entry from the palette table is to be used to predict or reconstruct which sample. This palette mode starts with the process of generating a palette predictor for a first block of a picture, slice, tile, or other such grouping of video blocks. As will be explained below, the palette predictor for subsequent video blocks is typically generated by updating a previously used palette predictor. For illustrative purpose, it is assumed that the palette predictor is defined at a picture level. In other words, a picture may include multiple coding blocks, each having its own palette table, but there is one palette predictor for the entire picture.

To reduce the bits needed for signaling palette entries in the video bitstream, a video decoder may utilize a palette predictor for determining new palette entries in the palette table used for reconstructing a video block. For example, the palette predictor may include palette entries from a previously used palette table or even be initialized with a most recently used palette table by including all entries of the most recently used palette table. In some implementations, the palette predictor may include fewer than all the entries from the most recently used palette table and then incorporate some entries from other previously used palette tables. The palette predictor may have the same size as the palette tables used for coding different blocks or may be larger or smaller than the palette tables used for coding different blocks. In one example, the palette predictor is implemented as a first-in-first-out (FIFO) table including <NUM> palette entries.

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 entry of the palette predictor. The one-bit flag may have a first value (e.g., a binary one) indicating that the associated entry of the palette predictor is to be included in the palette table or a second value (e.g., a binary zero) indicating that the associated entry of the palette predictor is not to be included in the palette table. If the size of palette predictor is larger than the palette table used for a block of video data, then the video decoder may stop receiving more flags once a maximum size for the palette table is reached.

In some implementations, some entries in a palette table may be directly signaled in the encoded video bitstream instead of being determined using the palette predictor. 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 chroma components 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, those palette 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 the number of bits needed to signal the entries of a new palette table, thereby improving the overall coding efficiency of palette mode coding.

In many instances, the palette predictor for one block is determined based on the palette table used to code one or more previously coded blocks. But when coding the first coding tree unit in a picture, a slice or a tile, the palette table of a previously coded block may not be available. Therefore a palette predictor cannot be generated using entries of the previously used palette tables. In such case, a sequence of palette predictor initializers may be signaled in a sequence parameter set (SPS) and/or a picture parameter set (PPS), which are values used to generate a palette predictor when a previously used palette table is not available. An SPS generally refers to a syntax structure of syntax elements that apply to a series of consecutive coded video pictures called a coded video sequence (CVS) as determined by the content of a syntax element found in the PPS referred to by a syntax element found in each slice segment header. A PPS generally refers to a syntax structure of syntax elements that apply to one or more individual pictures within a CVS as determined by a syntax element found in each slice segment header. Thus, an SPS is generally considered to be a higher level syntax structure than a PPS, meaning the syntax elements included in the SPS generally change less frequently and apply to a larger portion of video data compared to the syntax elements included in the PPS.

<FIG> illustrates a portion of a video frame <NUM> in a bitstream, in accordance with some embodiments. The video frame <NUM> includes a plurality of pixels, and each pixel is made of a plurality of color elements (e.g., blue, green and red). In video encoding and decoding, color information of the plurality of pixels is represented by a plurality of luma samples <NUM> and a plurality of chroma samples <NUM>. Each of the plurality of pixels corresponds to a respective luma sample <NUM>, and each luma sample <NUM> also corresponds to a respective pixel in the video frame <NUM>. Each chroma sample <NUM> corresponds to a respective set of luma samples <NUM> according to a subsampling scheme. Each luma sample has a luma component Y', and each chroma sample <NUM> has a blue-difference chroma component Cb and a red-difference chroma component Cr. The subsampling scheme of the luma and chroma components (Y':Cb:Cr) has a three-part ratio, e.g., <NUM>:<NUM>:<NUM>, <NUM>:<NUM>:<NUM>, <NUM>:<NUM>:<NUM>, <NUM>:<NUM>:<NUM>, and <NUM>:<NUM>:<NUM>. Specifically, the luma samples <NUM> and chroma samples <NUM> of the video frame <NUM> comply with the subsampling scheme having the three-part ratio equal to <NUM>:<NUM>:<NUM>, and on average, every four luma samples <NUM> correspond to one chroma sample <NUM> having the blue-difference chroma component Cb and the red-difference chroma component Cr.

In video encoding or encoding, each of the luma samples <NUM> and chroma samples <NUM> are reconstructed from residual blocks of the video frame <NUM> and filtered by a deblocking filter, an SAO filter, and an ALF filter of an in-loop filter <NUM> or <NUM> to remove artifacts. The filtered luma samples <NUM> and chroma samples <NUM> are stored into a decoded picture buffer <NUM> or <NUM> and used to code or decode other video blocks in the video frame <NUM>. In some embodiments, each of the deblocking, SAO and ALF filters is configured to filter the luma samples <NUM> or chroma samples <NUM> based on the same type of samples, e.g., filter each luma sample <NUM> based on a respective set of adjacent luma samples <NUM> and filter each luma sample <NUM> based on a respective set of adjacent chroma samples <NUM>. In some embodiments, the in-loop filter <NUM> or <NUM> further includes a cross component filter configured to filter each chroma sample <NUM> based on one or more luma samples <NUM> that are adjacent to the respective chroma sample <NUM>. Conversely, in some embodiments, the in-loop filter <NUM> or <NUM> includes an alternative cross component filter configured to filter each luma sample <NUM> based on one or more luma samples <NUM> that are adjacent to the respective luma sample <NUM>.

Specifically, a cross component filter includes a cross component ALF that is configured to refine each chroma sample <NUM> based on the one or more luma samples <NUM> that are adjacent to the respective chroma sample <NUM>. For example, the cross component ALF is a linear, diamond-shaped filter. For each chroma sample <NUM>, one or more adjacent luma samples <NUM> includes eight luma samples <NUM> corresponding to a pixel group <NUM> having eight pixels. Six of the eight luma samples <NUM> form a hexagon that encloses the respective chroma sample <NUM> and two remainder luma samples <NUM>. Each chroma sample <NUM> corresponds to a chroma refinement value that is a linear combination of luma values of the eight luma samples <NUM> according to the linear and diamond-shaped cross component ALF. In accordance with such a diamond-shaped filter, each luma sample <NUM> of the video frame <NUM> can be used for cross component filtering of more than one chroma sample <NUM>. More details of specific examples of cross component ALFs are discussed below with reference to <FIG> and <FIG>.

Each of the deblocking, SAO, and ALF filters of the in-loop filter <NUM> or <NUM> includes one or more in-loop filter coefficients, and the cross component ALF also includes a plurality of cross component filter coefficients. The in-loop and cross component filter coefficients are signaled in an Adaptation Parameter Set (APS). In an example, an APS carries and signals multiple sets (e.g., up to <NUM> sets) of luma filter coefficients and clipping value indexes, and multiple sets (e.g., up to <NUM> sets) of chroma filter coefficients and clipping value indexes. The APS is transferred with the video frame <NUM> in the bitstream from the video encoder <NUM> to the video encoder <NUM>, i.e., the APS is an overhead of the transfer of the bitstream. In some embodiments, filter coefficients of different classification for luma components of the luma samples <NUM> are merged to reduce the overhead of the transfer of the bitstream. In an example, the indices of the APS used for an image slice are signaled in a corresponding slice header.

In some embodiments, the cross component filter coefficients are determined and applied on a block level (e.g., on a slice level, on a CTB level) and signaled as a context-coded flag (i.e., a CCALF filer index) for each block of luma and chroma samples. Each block of luma and chroma samples optionally has a variable size. That said, in some embodiments, a plurality of predefined cross component filter coefficient sets are stored in the video encoder <NUM> and video decoder <NUM> separately. The video frame <NUM> is transferred from the video encoder <NUM> to the video decoder <NUM> with the context-coded flag. The video decoder <NUM> identifies a set of predefined cross component filter coefficients from the cross component filter coefficient sets that are stored locally according to the context-coded flag. Conversely, in some embodiments, the cross component filter coefficients are transmitted directly with the APS, and are scaled by a predefined factor (e.g., <NUM><NUM>) and rounded to a fixed point representation.

<FIG> is a block diagram of an in-loop filter <NUM> that is applied in a video encoder <NUM> or decoder <NUM> and includes a cross component filter <NUM>, in accordance with some embodiments, and <FIG> is a diagram of a pixel group <NUM> grouping luma samples <NUM> according to a diamond shape, in accordance with some embodiments. <FIG> is a flowchart of a cross component filtering process <NUM> based on difference luma values of the luma samples <NUM>, in accordance with some embodiments. The video encoder <NUM> or decoder <NUM> obtains a plurality of luma samples <NUM> and a plurality of chroma samples <NUM> corresponding to a plurality of pixel groups <NUM> of a video frame <NUM>. Each luma sample <NUM> has a respective luminance value, and each chroma sample <NUM> has a respective chrominance value. Each of the plurality of pixel groups <NUM> includes a respective chroma sample <NUM> and a set of luma samples <NUM> in the respective pixel group <NUM>. The cross component filter <NUM> is configured to generate a chroma refinement value <NUM> for the respective chroma sample <NUM> based on the set of luma samples <NUM>. The respective chroma sample <NUM> is then updated using the chroma refinement value <NUM>, i.e., a chrominance value of the respective chroma sample <NUM> is refined with the chroma refinement value <NUM>. The updated respective chroma sample of each pixel group <NUM> is stored in association with the video frame <NUM>. As such, a refined chrominance value <NUM> is generated based on the set of luma samples surrounding each chroma sample <NUM> and stored for the respective chroma sample <NUM> in the picture buffer <NUM> or <NUM>.

Each pixel group <NUM> corresponds to a respective chroma sample <NUM>, and the corresponding set of luma samples <NUM> are identified for the respective pixel group <NUM> based on a filter configuration of the cross component filter <NUM>. In an example, the luma samples <NUM> and chroma samples <NUM> are arranged according to a subsampling scheme having a ratio equal to <NUM>: <NUM>:<NUM> as shown in <FIG>. Referring to <FIG>, in some embodiments, the cross component filter <NUM> is a linear diamond-shaped filter involving six luma samples <NUM> that are located at six angles of a hexagon enclosing the chroma sample and two luma samples <NUM>. For each chroma sample <NUM>, each of the set of corresponding luma samples <NUM> used for its cross component filtering is also used for cross component filtering of one or two chroma samples <NUM> that are immediately adjacent to the respective chroma sample. Luma sample pairs 502A, 502B, 502C and 502D are also used by chroma samples that are above, below, to the left of, and to the right of the respective chroma sample <NUM> for cross component filtering, respectively. Alternatively, in some embodiments (<FIG>), the cross component filter <NUM> is a 3x3 filter involving a set of nine luma samples <NUM>. Each luma sample <NUM> has a location overlapping or immediately adjacent to that of the respective chroma sample <NUM>. Eight of the nine luma samples surround the respective chroma sample <NUM>. Additionally, in some embodiments (<FIG>), the cross component filter <NUM> is a cross-shaped filter involving a set of five luma samples <NUM>. Each luma sample <NUM> has a location at a center or a tip of a cross shape, and the respective chroma sample <NUM> overlaps the center of the cross shape.

In some embodiments, the cross component filter <NUM> includes a first cross component filter 602A and a second cross component filter 602B configured to generate a first refinement value 604A and a second refinement value 604B. Each chroma sample <NUM> includes a blue-difference chroma component 608A and a red-difference chroma component 608B that are separately updated using the first and second refinement values 604A and 604B to output a first refined chrominance value 606A and a second refined chrominance value 606B, respectively.

The in-loop filter <NUM> further includes sample adaptive offset (SAO) filters <NUM> and adaptive loop filters (ALF) <NUM> coupled to the SAO filters <NUM>. Before the set of luma samples <NUM> are applied to generate the chroma refinement value <NUM> for each chroma sample <NUM>, the SAO filters <NUM> compensates each of the plurality of luma samples <NUM> and the plurality of chroma samples <NUM>. Specifically, the SAO filters 610A, 620B and 610C compensate the luma samples <NUM>, the blue blue-difference chroma components Cb of the chroma samples <NUM>, and the red-difference chroma components Cr of the chroma samples <NUM>, respectively. Each of the compensated luma samples <NUM> is updated using a luma ALF 612A based on a set of adj acent luma samples <NUM>, while each of the compensated chroma samples 616A and 616B is updated using a chroma ALF 612B based on a set of adjacent chroma samples <NUM>. In some embodiments, the chroma ALF 612B and the cross component filter <NUM> are controlled jointly, i.e., are enabled or disabled jointly to generate the chroma components <NUM> and chroma refinement value <NUM> concurrently.

Referring to <FIG>, in some embodiments, for cross component filtering, an anchor luma sample <NUM> is determined (<NUM>) from the set of luma samples <NUM> in each pixel group <NUM> corresponding to a respective chroma sample <NUM> according to a predefined anchoring rule. The chroma refinement value <NUM> is generated by differencing (<NUM>) the respective luminance value of each luma sample <NUM> in the set of luma samples <NUM> by the anchor luminance value and applying (<NUM>) the cross component filter <NUM> to the difference luminance values of the set of luma samples <NUM>. For each chroma sample <NUM>, cross component filtering is represented by the following equation: <MAT> where ChromaR is a cross component filtering result of the respective chroma sample <NUM>, f(i) is a cross component filter coefficient corresponding to each of the set of luma samples <NUM> corresponding to the respective chroma sample <NUM>, Luma(i) is a luminance value of each luma sample <NUM>, and LumaAnchor is the anchor luminance value of the anchor luma sample <NUM>. In some embodiments, a non-linear clipping operation is performed (<NUM>) on the difference luminance value of the set of luma samples <NUM> after the cross component filter <NUM> is applied. The cross component filtering result (i.e., the refined chrominance value <NUM>) is represented by the following equation: <MAT> where DR is a dynamic range of the non-linear clipping operation. The dynamic range of the non-linear clipping operation is associated with a bit depth.

Referring to <FIG>, in some embodiments, each pixel group <NUM> has a predefined shape (e.g., a hexagon) that is symmetric with respect to two orthogonal axes <NUM> and <NUM> passing a center <NUM> of the predefined shape. In accordance with the predefined anchoring rule, the anchor luma sample <NUM> for each pixel group <NUM> is selected from the set of luma samples <NUM> and has the closest distance to a center of the respective pixel group than a remainder of the set of luma samples <NUM>. The anchor luma sample <NUM> is one of two luma samples 502A-<NUM> and 502B-<NUM> that are fully enclosed in the hexagon in <FIG>. Alternatively, in some embodiments, in accordance with the predefined anchoring rule, the anchor luma sample <NUM> for each chroma sample is distinct from the set of luma samples <NUM>, and the anchor luminance value of the anchor luma sample <NUM> is an average of the luminance values of two or more luma samples <NUM> in the pixel group <NUM>. For example, the anchor luminance value is an average of the luminance values of the two luma samples 502A-<NUM> and 502B-<NUM> that are closest to the center <NUM> in the luma samples <NUM> of the pixel group <NUM>. In another example, the anchor luminance value is an average of the luminance values of all of the luma samples <NUM> in the pixel group <NUM>.

<FIG> illustrates an image block <NUM> stored in a line buffer for video processing, in accordance with some embodiments. An example of the image block <NUM> is a coding tree unit. Modified block classification and filtering are employed for samples near horizontal boundaries. For this purpose, a virtual boundary <NUM> is defined as a line by shifting a horizontal boundary <NUM> with a number (N) of lines of samples. The number is <NUM> for luma components and <NUM> for chroma components. For cross component filtering, the virtual boundary <NUM> is shifted from the horizontal boundary <NUM> according to a cross component filtering scheme of a cross component filter <NUM>.

Specifically, the image block <NUM> includes a top virtual boundary 702A and a bottom virtual boundary 702B. Luma and chroma samples between each virtual boundary <NUM> and the corresponding horizontal boundary <NUM> are not stored in the line buffer and have to be reproduced from luma and chroma samples that are enclosed between two boundary lines 702A and 702B. Stated another way, a plurality of pixel groups <NUM> of a video frame <NUM> includes at least a first subset of pixels <NUM> and a second subset of pixels <NUM> immediately adjacent to the first subset of pixels <NUM>. The first subset of pixels <NUM> and the second subset of pixels <NUM> are divided by the virtual boundary <NUM> of the block <NUM>. The first subset of pixels <NUM> is enclosed in the block by the virtual boundary <NUM>. Luma and chroma samples of the first subset of pixels <NUM> are available, while luma and chroma samples of the second subset of pixels <NUM> are not available. Luminance values of the luma samples corresponding to the second subset of pixels <NUM> are replaced with luminance values of the luma samples corresponding to the first subset of pixels <NUM>.

In some embodiments, the luma sample corresponding to each of the second subset of pixels (e.g., pixels 708A and 708B) is replaced with the luma sample corresponding to a respective pixel (e.g., pixels 706A and 706B) in the first subset of pixels. The respective pixel 706A or 706B of the first subset of pixels and the respective pixel 708A or 708B of the second subset of pixels are symmetric with respect to the virtual boundary <NUM> of the block <NUM>, respectively. Alternatively, in some embodiments, the luma sample corresponding to each of the second subset of pixels (e.g., pixel 708C) is replaced with the luma sample corresponding to a respective pixel (e.g., pixel 706C) in the first subset of pixels. The respective pixel 706C of the first subset of pixels is the closest pixel to the respective one 708C of the second subset of pixels among the first subset of pixels <NUM>. More details on reproducing the luma and chroma samples of the second subset of pixels <NUM> are discussed in the context of adaptive loop filtering and cross component filtering below with reference to <FIG>, <FIG> and <FIG>.

<FIG> is an example ALF filtering scheme <NUM> in which a luma sample <NUM> is processed from a set of neighboring luma samples <NUM> by a luma ALF 612A, and <FIG> are ALF filtering schemes for six luma samples adjacent to a virtual boundary <NUM>, in accordance with some embodiments. The luma ALF 612A has a diamond filter shape (e.g., a 7x7 diamond shape) and is selected from a plurality of predefined filters (e.g., <NUM> filters) for each 4x4 block based on a direction and activity of local gradients. Each square in <FIG> represents a luma sample labelled with a corresponding filter coefficient (C0-C12) of the luma ALF 612A having the diamond shape. For the luma sample <NUM>, a total <NUM> filter coefficients (C0-C12) are symmetrically applied to combine <NUM> luma samples using the luma ALF 612A. Modified block classification is applied for the Luma sample <NUM>. For one-dimensional (1D) Laplacian gradient calculation of the 4x4 block below the virtual boundary 702A, only the luma samples <NUM> below the virtual boundary 702A are used. Similarly, for 1D Laplacian gradient calculation of a 4x4 block above the virtual boundary 702B, only the luma samples above the virtual boundary 702B are used. Quantization of activity value is scaled based on a reduced number of luma samples <NUM> used in 1D Laplacian gradient calculation.

In some embodiments, a symmetric padding operation at the virtual boundaries <NUM> is used for both luma and chroma samples <NUM> and <NUM> for filtering processing and ALF block classification. When the luma sample <NUM> being filtered is located below and adjacent to the virtual boundary 702A (e.g., separated from the virtual boundary 702A by two or less lines of samples), one or more neighboring samples <NUM> that are located above the virtual boundary 702A are padded. Likewise, when the luma sample <NUM> being filtered is located below the virtual boundary 702B (e.g., separated from the virtual boundary 702B by two or less lines of samples), one or more neighboring samples <NUM> that are located below the virtual boundary 702B are padded. In some embodiments, this padding process is applied for slice, brick and/or tile boundaries. In some embodiments, for ALF block classification, only the luma samples which are in the same slice, brick or tile are used, and a corresponding activity value is scaled accordingly. In some embodiments, for ALF filtering and ALF block classification, repetitive padding is applied on all boundaries (i.e., picture/sub-picture/slice/tile) excluding ALF virtual boundary.

Stated another way, the plurality of pixel groups <NUM> of the video frame <NUM> includes a first subset of pixels and a second subset of pixels immediately adjacent to the first subset of pixels. The first subset of pixels and the second subset of pixels are divided by a virtual boundary <NUM> of a block <NUM>. The first subset of pixels <NUM> is enclosed in the block <NUM> by the virtual boundary <NUM>. Luma samples <NUM> of the first subset of pixels are available, while the luma samples <NUM> of the second subset of pixels are not available and need to be generated from the luma samples <NUM>. The luminance values of the luma samples <NUM> corresponding to the second subset of pixels are replaced with (i.e., duplicated from) luminance values of the luma samples <NUM> corresponding to the first subset of pixels. Each of the luma samples <NUM> is duplicated from or replaced by a respective luma sample <NUM>. Optionally, the respective replacing luma sample <NUM> (e.g., sample 808A) is symmetric with the respective replaced luma sample <NUM> (e.g., sample 806A) with respect to the virtual boundary <NUM>. Optionally, the respective replacing luma sample <NUM> (e.g., sample 808B) is closest to the respective replaced luma sample <NUM> (e.g., sample 806A) among the luma samples <NUM> corresponding to the first subset of pixels.

Referring to <FIG>, in some embodiments, the respective replacing luma sample <NUM> (e.g., sample 808C) is symmetric with the respective replaced luma sample <NUM> (e.g., sample 806A) with respect to a center line <NUM> of the ALF filtering scheme. Alternatively, in some embodiments, the respective replacing luma sample <NUM> (e.g., sample 808D) is symmetric with the respective replaced luma sample <NUM> (e.g., sample 806A) with respect to a center (i.e., sample <NUM>) of the ALF filtering scheme.

<FIG> is an example ALF filtering scheme <NUM> in which a chroma sample <NUM> is processed from a set of neighboring samples <NUM> by a chroma ALF 612B, and <FIG> are ALF filtering schemes for four chroma samples adjacent to a virtual boundary <NUM>, in accordance with some embodiments. The chroma ALF 612B has a diamond filter shape (e.g., a 5x5 diamond shape). Each square in <FIG> represents a chroma sample labelled with a corresponding filter coefficient (C0-C6) of the chroma ALF 612B having the diamond shape. For the chroma sample <NUM>, a total <NUM> filter coefficients (C0-C6) are symmetrically applied to combine <NUM> chroma samples in the chroma ALF 612B.

As explained above, in some embodiments, symmetric padding operation at the virtual boundaries <NUM> are used for the chroma samples <NUM> for filtering processing and ALF block classification. When the chroma sample <NUM> being filtered is located below and adjacent to the virtual boundary 702A (e.g., separated from the virtual boundary 702A by zero or one line of chroma samples), one or more neighboring samples <NUM> that are located above the virtual boundary 702A are padded. Likewise, when the chroma sample <NUM> being filtered is located above and adjacent to the virtual boundary 702B (e.g., separated from the virtual boundary 702B by zero or one line of chroma samples), one or more neighboring samples <NUM> that are located below the virtual boundary 702B are padded. In some embodiments, this padding process is applied for slice, brick and/or tile boundaries. In some embodiments, for ALF block classification, only the chroma samples which are in the same slice, brick or tile are used, and a corresponding activity value is scaled accordingly. In some embodiments, for ALF filtering and ALF block classification, repetitive padding is applied on all boundaries (i.e., picture/sub-picture/slice/tile) excluding ALF virtual boundary.

Chroma samples <NUM> of a first subset of pixels are available, and the chroma samples <NUM> of the second subset of pixels are not available and need to be generated from the chroma samples <NUM>. The chrominance values of the chroma samples <NUM> corresponding to the second subset of pixels are replaced with chrominance values of the chroma samples <NUM> corresponding to the first subset of pixels. Each of the chroma samples <NUM> is duplicated from or replaced by a respective chroma sample <NUM>. Optionally, the respective replacing chroma sample <NUM> (e.g., sample 908A) is symmetric with the respective replaced chroma sample <NUM> (e.g., sample 906A) with respect to the virtual boundary <NUM>. Optionally, the respective replacing chroma sample <NUM> (e.g., sample 908B) is closest to the respective replaced chroma sample <NUM> (e.g., sample 906A) among the chroma samples <NUM> corresponding to the first subset of pixels.

Referring to <FIG>, in some embodiments, the respective replacing luma sample <NUM> (e.g., sample 908C) is symmetric with the respective replaced luma sample <NUM> (e.g., sample 906C) with respect to a center line <NUM> of the ALF filtering scheme. Alternatively, in some embodiments, the respective replacing luma sample <NUM> (e.g., sample 908D) is symmetric with the respective replaced luma sample <NUM> (e.g., sample 906C) with respect to a center (i.e., sample <NUM>) of the ALF filtering scheme.

<FIG> illustrates example boundary pixel groups <NUM>, <NUM>, <NUM> and <NUM> that apply sample padding for cross component filtering, in accordance with some embodiments. Each open circle represents a luma sample <NUM> applied to generate a chroma refinement value for a chroma sample <NUM> represented by a solid circle. In each boundary pixel group, each luma sample <NUM> is associated with a corresponding filter coefficient (C0-C7) of a cross component filter <NUM> having a diamond shape. For each chroma sample <NUM>, a total <NUM> filter coefficients (C0-C7) are applied to combine <NUM> luma samples in the cross component filer <NUM>. A video frame <NUM> corresponding to the luma and chroma samples <NUM> and <NUM> includes a first subset of pixels and a second subset of pixels that are divided by a virtual boundary <NUM> of a block <NUM>. The first subset of pixels and the second subset of pixels are divided by a virtual boundary <NUM> of a block <NUM>. In each pixel group, a first subset of luma samples 502A correspond to the first subset of pixels and are available for use, and a second subset of luma samples 502B correspond to the second subset of pixels and are not available. The virtual boundary <NUM> is parallel with one of the two orthogonal axes <NUM> and <NUM> of each pixel group. For cross component filtering, luminance values of the luma samples 502B corresponding to the second subset of pixels are replaced with or duplicated from luminance values of the luma samples 502A corresponding to the first subset of pixels.

Referring to <FIG>, in some embodiments, each of the second subset of luma samples (e.g., 502B-<NUM>) is replaced with a respective luma sample (e.g., 502A-<NUM>) that is symmetric with the respective luma sample (e.g., 502B-<NUM>) with respect to the virtual boundary <NUM>. Alternatively, in some embodiments, each of the second subset of luma samples (e.g., 502B-<NUM>) is replaced with a respective luma sample (e.g., 502A-<NUM>) that is closest to the respective luma sample (e.g., 502B-<NUM>) among the first subset of luma samples 502A. In <FIG>, the luma sample 502A-<NUM> that is symmetric with the respective luma sample 502B-<NUM> is distinct from the luma sample 502A-<NUM> that is closest to the respective luma sample 502B-<NUM>. Conversely, each of the luma samples 502B-<NUM> and 502B-<NUM> is duplicated from the same respective luma sample, independently of whether the same respective luma sample is chosen based on a symmetry with respect to the virtual boundary <NUM> or a distance from the respective luma sample 502B-<NUM> or 502B-<NUM>.

Alternatively, in some embodiments, the respective replacing luma sample 502A (e.g., sample 502A-<NUM> in <FIG>) is symmetric with the respective replaced luma sample 502B (e.g., sample 502B-<NUM> in <FIG>) with respect to a center line <NUM> of the pixel group. Alternatively, in some embodiments, the respective replacing luma sample 502A (e.g., sample 502A-<NUM>) is symmetric with the respective replaced luma sample 502B (e.g., sample 502B-<NUM>) with respect to a center (i.e., chroma sample <NUM>) of the pixel group.

After each of the second subset of luma samples 502B is replaced with a respective luma sample 502A, an anchor luma sample is determined from the luma samples 502A and 502B according to a predefined anchoring rule. The chroma refinement value of the chroma sample <NUM> is generated by differencing the respective luminance value of each luma sample 502A or 502B by an anchor luminance value of the anchor luma sample and applying the cross component filter <NUM> to the difference luminance values of the set of luma samples <NUM> to generate the chroma refinement value <NUM>. As such, the chroma sample <NUM> is updated using the chroma refinement value <NUM>.

In some embodiments, the above padding process for cross component filtering is applied for slice, brick and/or tile boundaries. In some embodiments, only the chroma samples which are in the same slice, brick or tile are used, and a corresponding activity value is scaled accordingly. In some embodiments, repetitive padding is applied on all boundaries (i.e., picture/sub-picture/slice/tile) excluding ALF virtual boundary.

<FIG> are schematic diagrams of two additional example cross component filtering schemes <NUM> and <NUM> of a cross component filter <NUM>, in accordance with some embodiments. Each open circle represents a luma sample <NUM> applied to generate a chroma refinement value <NUM> for a chroma sample <NUM> represented by a solid circle. In each cross component filtering scheme <NUM> or <NUM>, each luma sample <NUM> is associated with a corresponding filter coefficient of the cross component filter <NUM>, and the chroma sample <NUM> modified based on a linear combination of the luma samples <NUM> in the same pixel group. Specifically, difference luminance values of the luma samples <NUM> in the pixel group are combined to generate a chroma refinement value <NUM> to update the chroma sample <NUM>.

Referring to <FIG>, the chroma sample <NUM> overlaps a central luma sample 1102A and is surrounded by eight peripheral luma samples 1102B. The solid circle overlaps with one of the open circles at a center of the pixel group. An anchor luma sample is selected as one of the luma samples <NUM> (e.g., the central luma sample 1102A) or derived from a subset or all of the luma samples <NUM>. For example, the anchor luma sample is an average of all nine luma samples <NUM> or an average of eight peripheral luma samples 1102B. An anchor luminance value is deducted from a respective luminance value of each of the luma samples <NUM> to provide the difference luminance values of the luma samples <NUM>. The cross component filter <NUM> applies a set of filter coefficients to filter the difference luminance values of the luma samples <NUM> and generate the chroma refinement value <NUM> for the chroma sample <NUM>.

In some situations, the chroma sample <NUM> is adjacent to a virtual boundary <NUM> of a block <NUM> stored in a line buffer, and the virtual boundary <NUM> divides the luma samples <NUM> in the pixel group into a first subset of luma samples and a second subset of luma samples. For example, a first row <NUM> of the luma samples <NUM> is above a top virtual boundary 702A and not stored in the line buffer, while a second row <NUM> and a third row <NUM> of luma samples <NUM> are stored therein. The first row <NUM> of luma samples are replaced by the second row <NUM> of luma samples for the purposes of generating the chroma refinement value <NUM>. Alternatively, in another example, the third row <NUM> of the luma samples <NUM> is below a bottom virtual boundary 702B and not stored in the line buffer, while the first row <NUM> and the second row <NUM> of luma samples <NUM> are stored therein. The third row <NUM> of luma samples are replaced by the second row <NUM> of luma samples for the purposes of generating the chroma refinement value <NUM>. Additionally, in some embodiments, one of the first and third rows of luma samples is beyond the virtual boundary <NUM> and not stored in the line buffer, but is replaced by the other one of these two rows that is stored in the line buffer, because the first and third rows <NUM> and <NUM> are symmetric with respect to a symmetry axis <NUM> of the cross component filtering scheme <NUM>.

Referring to <FIG>, the chroma sample <NUM> overlaps a central luma sample 1102A and is surrounded by four peripheral luma samples 1102B that are above, below, to the left of, and to the right of the central luma sample 1102A. An anchor luma sample is selected as one of the luma samples <NUM> (e.g., the central luma sample 1102A) or derived from a subset or all of the luma samples <NUM>. For example, the anchor luma sample is an average of all five luma samples <NUM> or an average of four peripheral luma samples 1102B. In some situations, the chroma sample <NUM> is adj acent to a virtual boundary <NUM> of a block <NUM> stored in a line buffer. For example, a top luma sample 1102B-<NUM> is above a top virtual boundary 702A and not stored in the line buffer, and is reproduced from one of the central luma sample 1102A and a bottom luma sample 1102B-<NUM>. In another example, the bottom luma sample 1102B-<NUM> is below a bottom virtual boundary 702B and not stored in the line buffer, and therefore, is reproduced from one of the central luma sample 1102A and the top luma sample 1102B-<NUM>. After all luma samples <NUM> are available in the pixel group <NUM>, an anchor luma sample is identified and difference luminance values are determined for the luma samples <NUM>, thereby allowing the chroma sample <NUM> to be modified based on these luma samples <NUM> using the cross component filter <NUM>.

<FIG> is a flow chart of a video coding method <NUM>, in accordance with some embodiments. The video coding method <NUM> is implemented in an electronic device having a video encoder or decoder. The electronic device obtains (<NUM>) a plurality of luma samples <NUM> and a plurality of chroma samples <NUM> corresponding to a plurality of pixel groups <NUM> of a video frame <NUM> in a bitstream. For each of the plurality of pixel groups <NUM>, a respective chroma sample <NUM> and a set of luma samples <NUM> are identified (<NUM>) in the pixel group. Each luma sample <NUM> has a respective luminance value. An anchor luma sample <NUM> is determined (<NUM>) from the set of luma samples <NUM> in each pixel group <NUM> according to a predefined anchoring rule. The anchor luma sample <NUM> has an anchor luminance value. The electronic device generates (<NUM>) a chroma refinement value <NUM> based on the set of luma samples <NUM> by differencing (<NUM>) the respective luminance value of each luma sample <NUM> in the set by the anchor luminance value and applying (<NUM>) a cross component filter <NUM> to the difference luminance values of the set of luma samples <NUM> to generate the chroma refinement value. In some embodiments, a non-linear clipping operation is performed on the difference luminance values of the set of luma samples <NUM> prior to applying the cross component filter <NUM>. The respective chroma sample is updated (<NUM>) using the chroma refinement value. The electronic device stores (<NUM>) the updated respective chroma sample <NUM> of each pixel group <NUM> in association with the video frame <NUM>.

In some embodiments, the plurality of pixel groups <NUM> includes a first subset of pixels <NUM> and a second subset of pixels <NUM> immediately adjacent to the first subset of pixels <NUM>. The electronic device determines (<NUM>) that the first subset of pixels <NUM> and the second subset of pixels <NUM> are divided by a virtual boundary <NUM> of a block <NUM> (e.g., a coding tree unit). The first subset of pixels and the second subset of pixels are divided by a virtual boundary <NUM> of a block <NUM>. Luma samples of the first subset of pixels <NUM> are available (e.g., stored with the block <NUM>) and immediately adjacent to the virtual boundary <NUM> of the block <NUM>, and luma samples of the second subset of pixels <NUM> are not available (e.g., not stored with the block <NUM>). The luminance values of the luma samples corresponding to the second subset of pixels <NUM> are replaced (<NUM>) with luminance values of the luma samples corresponding to the first subset of pixels <NUM>. In some embodiments, the luma sample corresponding to each of the second subset of pixels <NUM> is replaced with the luma sample corresponding to a respective pixel in the first subset of pixels <NUM>, and the respective pixel of the first subset of pixels <NUM> is the closest pixel to the respective one of the second subset of pixels <NUM> among the first subset of pixels <NUM>. Alternatively, in some embodiments, the luma sample corresponding to each of the second subset of pixels <NUM> is replaced with the luma sample corresponding to a respective pixel in the first subset of pixels <NUM>, and the respective pixel of the first subset of pixels <NUM> and the respective one of the second subset of pixels <NUM> are symmetric with respect to the virtual boundary <NUM> of the block <NUM>. For at least one pixel group <NUM>, the set of luma samples <NUM> corresponds (<NUM>) to at least one of the second subset of pixels. That said, in some embodiments, for the at least one pixel group <NUM>, the chroma refinement value is generated based on the luminance value of the replaced luma sample corresponding to at least one of the second subset of pixels <NUM>.

Further, in some embodiments, each pixel group <NUM> includes a set of pixels located according to a predefined shape that is symmetric with respect to two orthogonal axes <NUM> and <NUM> passing a center <NUM> of the predefined shape. For each pixel group <NUM>, each of the set of luma samples <NUM> in the pixel group <NUM> corresponds to a respective pixel in the pixel group. The virtual boundary <NUM> of the block <NUM> is parallel with one of the two orthogonal axes <NUM> and <NUM>.

In some embodiments, in accordance with the predefined anchoring rule, the anchor luma sample <NUM> for each pixel group <NUM> is selected from the set of luma samples <NUM> and has the closest distance to a center <NUM> of the respective pixel group <NUM> than a remainder of the set of luma samples <NUM>. Alternatively, in some embodiments, in accordance with the predefined anchoring rule, the anchor luma sample <NUM> for each pixel group <NUM> is distinct from the set of luma samples <NUM>, and the anchor luminance value of the anchor luma sample <NUM> is an average of the luminance values of two or more luma samples <NUM>.

In some embodiments, for each chroma sample <NUM>, the chroma refinement value <NUM> includes a first refinement value 604A and a second refinement value 604B. Each chroma sample <NUM> includes a blue-difference chroma component 608A and a red-difference chroma component 608B that are separately updated using the first and second refinement values 604A and 604B, respectively.

In some embodiments, each pixel group <NUM> includes a set of pixels located according to a predefined shape, and each of the set of luma samples <NUM> corresponds to a respective pixel in the respective pixel group <NUM>. Further, in some embodiments, for each pixel group <NUM>, the chroma sample <NUM> corresponding to the pixel group <NUM> is presumed to be located at a center <NUM> of the predefined shape. Additionally, in some embodiments, the predefined shape is a diamond shape, and the set of luma samples <NUM> includes eight luma samples <NUM> that are organized according to the diamond shape.

In some embodiments, the luma samples <NUM> and chroma samples <NUM> comply with a subsampling scheme having a three-part ratio equal to <NUM>:<NUM>:<NUM>. For each pixel group <NUM>, the respective chroma sample <NUM> corresponds to four whole luma samples <NUM> on average, and has a blue-difference chroma component Cb and a red-difference chroma component Cr.

In some embodiments, the cross component filter <NUM> includes a linear, diamond shaped filter configured to combine the difference luminance values of the set of luma samples <NUM> surrounding each chroma sample <NUM> in a linear manner.

In some embodiments, prior to applying the luma samples <NUM> to generate the chroma refinement value for each chroma sample <NUM>, the electronic device compensates each of the plurality of luma samples <NUM> and the plurality of chroma samples <NUM> using a sample adaptive offset (SAO) filter and updates each of the compensated chroma samples <NUM> using a chroma adaptive in-loop filter 612B. Further, in some embodiments, the cross component filter <NUM> and the chroma adaptive in-loop filter 612B are controlled jointly.

In some embodiments, for each of the plurality of luma samples <NUM>, a filtered luma sample <NUM> is generated from the respective luma sample <NUM> using a luma adaptive in-loop filter 612A.

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. 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.

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.

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.

Claim 1:
A method for coding video data, comprising:
obtaining (<NUM>), from a bitstream, a plurality of luma samples and a plurality of chroma samples corresponding to a plurality of pixel groups of a video frame; and
for each pixel group of the plurality of pixel groups:
identifying (<NUM>) a respective chroma sample and a set of luma samples corresponding to the pixel group, the respective chroma sample having a chrominance value, and each luma sample having a respective luminance value;
determining (<NUM>) an anchor luma sample according to a predefined rule, the anchor luma sample having an anchor luminance value;
generating (<NUM>) a chroma refinement value based on the set of luma samples, further including (<NUM>) obtaining (<NUM>) a difference between the respective luminance value of each luma sample in the set of luma samples and the anchor luminance value, and (<NUM>) applying (<NUM>) a cross component adaptive loop filter to differences corresponding to the set of luma samples to generate the chroma refinement value; and
deriving (<NUM>) an updated chrominance value of the respective chroma sample using the chroma refinement value and the chrominance value of the respective chroma sample,
wherein the set of luma samples includes six luma samples located at six angles of a hexagon, the hexagon enclosing the respective chroma sample and two luma samples.