CROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is filed upon and claims priority to U.S. Provisional Application No. 63/399,213, entitled “Methods and Devices for Adaptive Loop Filter,” filed on August 18, 2022, the entirety of which is incorporated by reference for all purposes.

FIELD

[0002] The present disclosure is related to video coding and compression, and in particular but not limited to, methods and apparatus on improving the coding efficiency of adaptive loop filter (ALF).

BACKGROUND

[0003] Various video coding techniques may be used to compress video data. Video coding is performed according to one or more video coding standards. For example, nowadays, some well- known video coding standards include Versatile Video Coding (VVC), High Efficiency Video Coding (HEVC, also known as H.265 or MPEG-H Part2) and Advanced Video Coding (AVC, also known as H.264 or MPEG-4 Part 10), which are jointly developed by ISO/IEC MPEG and ITU-T VCEG. AOMedia Video 1 (AVI) was developed by Alliance for Open Media (AOM) as a successor to its preceding standard VP9. Audio Video Coding (AVS), which refers to digital audio and digital video compression standard, is another video compression standard series developed by the Audio and Video Coding Standard Workgroup of China. Most of the existing video coding standards are built upon the famous hybrid video coding framework i.e., using block-based prediction methods (e.g., inter-prediction, intra-prediction) to reduce redundancy present in video images or sequences and using transform coding to compact the energy of the prediction errors. An important goal of video coding techniques is to compress video data into a form that uses a lower bit rate while avoiding or minimizing degradations to video quality.

[0004] The first generation AVS standard includes Chinese national standard “Information

Technology, Advanced Audio Video Coding, Part 2: Video” (known as AVS1) and “Information Technology, Advanced Audio Video Coding Part 16: Radio Television Video” (known as AVS+). It can offer around 50% bit-rate saving at the same perceptual quality compared to MPEG-2 standard. The AVS1 standard video part was promulgated as the Chinese national standard in February 2006. The second generation AVS standard includes the series of Chinese national standard “Information Technology, Efficient Multimedia Coding” (knows as AVS2), which is mainly targeted at the transmission of extra HD TV programs. The coding efficiency of the AVS2 is double of that of the AVS+. In May 2016, the AVS2 was issued as the Chinese national standard. Meanwhile, the AVS2 standard video part was submitted by Institute of Electrical and Electronics Engineers (IEEE) as one international standard for applications. The AVS3 standard is one new generation video coding standard for UHD video application aiming at surpassing the coding efficiency of the latest international standard HEVC. In March 2019, at the 68-th AVS meeting, the AVS3-P2 baseline was finished, which provides approximately 30% bit-rate savings over the HEVC standard. Currently, there is one reference software, called high performance model (HPM), is maintained by the AVS group to demonstrate a reference implementation of the AVS3 standard.

SUMMARY

[0005] The present disclosure provides examples of techniques relating to improving the coding efficiency of ALF.

[0006] According to a first aspect of the present disclosure, there is provided a method for video decoding. In the method, a decoder may obtain one or more spatial neighboring samples associated with a current sample, where the one or more spatial neighboring samples are from at least one of prediction samples or reconstructed samples, and the reconstructed samples are samples prior to sample adaptive offset (SAO) filtering. Furthermore, the decoder may obtain a filtered sample for the current sample based on the one or more spatial neighboring samples. [0007] According to a second aspect of the present disclosure, there is provided a method for video encoding. In the method, an encoder may obtain one or more spatial neighboring samples associated with a current sample, where the one or more spatial neighboring samples are from at least one of prediction samples or reconstructed samples, and the reconstructed samples are samples prior to SAO filtering. Furthermore, the encoder may obtain a filtered sample for the current sample based on the one or more spatial neighboring samples.

[0008] According to a third aspect of the present disclosure, there is provided a method for video decoding. In the method, a decoder may obtain a first feature based on a first ALF classifier that is edge-based. Additionally, the decoder may obtain a second feature based on a second ALF classifier that is band-based. Furthermore, the decoder may derive a combined classifier for an online ALF based on the first feature and the second feature.

[0009] According to a fourth aspect of the present disclosure, there is provided a method for video encoding. In the method, an encoder may obtain a first feature based on a first ALF classifier that is edge-based. Additionally, the encoder may obtain a second feature based on a second ALF classifier that is band-based. Furthermore, the encoder may derive a combined classifier for an online ALF based on the first feature and the second feature.

[0010] According to a fifth aspect of the present disclosure, there is provided a method for video decoding. In the method, a decoder may obtain a bitstream from an encoder. Furthermore, the decoder may adjust a chroma ALF shape associated with the bitstream based on a luma ALF shape associated with the bitstream.

[0011] According to a sixth aspect of the present disclosure, there is provided a method for video encoding. In the method, an encoder may signal a syntax element that indicates a luma ALF in a bitstream. Furthermore, the encoder may adjust a chroma ALF shape associated with the bitstream based on the luma ALF shape.

[0012] According to a seventh aspect of the present disclosure, there is provided an apparatus for video decoding. The apparatus may include one or more processors and a memory coupled to the one or more processors and configured to store instructions executable by the one or more processors. Furthermore, the one or more processors, upon execution of the instructions, are configured to perform the method according to the first aspect, the third aspect, or the fifth aspect.

[0013] According to an eight aspect of the present disclosure, there is provided an apparatus for video encoding. The apparatus may include one or more processors and a memory coupled to the one or more processors and configured to store instructions executable by the one or more processors. Furthermore, the one or more processors, upon execution of the instructions, are configured to perform the method according to the second aspect, the fourth aspect, or the sixth aspect.

[0014] According to a ninth aspect of the present disclosure, there is provided a non-transitory computer-readable storage medium for storing computer-executable instructions that, when executed by one or more computer processors, cause the one or more computer processors to receive a bitstream, and perform the method according to the first aspect, the third aspect, or the fifth aspect based on the bitstream.

[0015] According to a tenth aspect of the present disclosure, there is provided a non-transitory computer-readable storage medium for storing computer-executable instructions that, when executed by one or more computer processors, cause the one or more computer processors to perform the method according to the second aspect, the fourth aspect, or the sixth aspect to encode the current block into a bitstream, and transmit the bitstream.

[0016] According to an eleventh aspect of the present disclosure, there is provided a non- transitory computer-readable storage medium for storing a bitstream to be decoded by the method according to the first aspect, the third aspect, or the fifth aspect.

[0017] According to a twelfth aspect of the present disclosure, there is provided a non- transitory computer-readable storage medium for storing a bitstream generated by the method according to the second aspect, the fourth aspect, or the sixth aspect.

BRIEF DESCRIPTION OF THE DRAWINGS [0018] A more particular description of the examples of the present disclosure will be rendered by reference to specific examples illustrated in the appended drawings. Given that these drawings depict only some examples and are not therefore considered to be limiting in scope, the examples will be described and explained with additional specificity and details through the use of the accompanying drawings.

[0019] FIG. 1A is a block diagram illustrating a system for encoding and decoding video blocks in accordance with some examples of the present disclosure.

[0020] FIG. IB is a block diagram of an encoder in accordance with some examples of the present disclosure.

[0021] FIGS. 1C-1F are block diagrams illustrating how a frame is recursively partitioned into multiple video blocks of different sizes and shapes in accordance with some examples of the present disclosure.

[0022] FIG. 1G is a block diagram illustrating an exemplary video encoder in accordance with some examples of the present disclosure

[0023] FIG. 2A is a block diagram of a decoder in accordance with some examples of the present disclosure.

[0024] FIG. 2B is a block diagram illustrating an exemplary video decoder in accordance with some examples of the present disclosure.

[0025] FIG. 3A is a diagram illustrating block partitions in a multi-type tree structure in accordance with some examples of the present disclosure.

[0026] FIG. 3B is a diagram illustrating block partitions in a multi-type tree structure in accordance with some examples of the present disclosure.

[0027] FIG. 3C is a diagram illustrating block partitions in a multi-type tree structure in accordance with some examples of the present disclosure.

[0028] FIG. 3D is a diagram illustrating block partitions in a multi-type tree structure in accordance with some examples of the present disclosure. [0029] FIG. 3E is a diagram illustrating block partitions in a multi-type tree structure in accordance with some examples of the present disclosure.

[0030] FIG. 4A and FIG. 4B are showing two fdter shapes 7><7 diamond shape and 5x5 diamond shape supported for luma and chroma components in accordance with some examples of the present disclosure.

[0031] FIG. 5 shows only gradient of every second sample in a 10x 10 window is calculated in accordance with some examples of the present disclosure.

[0032] FIG. 6A shows 90-degree rotation that is applied to filter coefficients in accordance with some examples of the present disclosure.

[0033] FIG. 6B shows diagonal flip that is applied to filter coefficients in accordance with some examples of the present disclosure.

[0034] FIG. 6C shows vertical flip that is applied to filter coefficients in accordance with some examples of the present disclosure.

[0035] FIG. 7 shows the filter shape of a filter F ₂ that is applied to neighboring samples, and samples before deblocking filter (DBF) to derive a filtered sample in accordance with some examples of the present disclosure.

[0036] FIG. 8 shows various filter shapes including or 5x5 used to extract information in prediction signal in accordance with some examples of the present disclosure. [0037] FIG. 9 shows a long cross shape to which a chroma ALF filter shape is changed in accordance with some examples of the present disclosure.

[0038] FIG. 10 is a diagram illustrating a computing environment coupled with a user interface in accordance with some examples of the present disclosure.

[0039] FIG. 11 shows various online ALF filter inputs in accordance with some examples of the present disclosure.

[0040] FIG. 12 shows 1x1 and 3x3 filter shapes that are applied to the prediction samples of the ALF in accordance with some examples of the present disclosure. [0041] FIG. 13 is a flow chart illustrating a method for video decoding in accordance with some examples of the present disclosure.

[0042] FIG. 14 is a flow chart illustrating a method for video encoding corresponding to the method for video decoding as shown in FIG. 13 in accordance with some examples of the present disclosure.

[0043] FIG. 15 is a flow chart illustrating a method for video decoding in accordance with some examples of the present disclosure.

[0044] FIG. 16 is a flow chart illustrating a method for video encoding corresponding to the method for video decoding as shown in FIG. 15 in accordance with some examples of the present disclosure.

[0045] FIG. 17 is a flow chart illustrating a method for video decoding in accordance with some examples of the present disclosure.

[0046] FIG. 18 is a flow chart illustrating a method for video encoding corresponding to the method for video decoding as shown in FIG. 17 in accordance with some examples of the present disclosure.

DETAILED DESCRIPTION

[0047] Reference will now be made in detail to specific implementations, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous non-limiting specific details are set forth in order to assist in understanding the subject matter presented herein. But it will be apparent to one of ordinary skill in the art that various alternatives may be used. For example, it will be apparent to one of ordinary skill in the art that the subject matter presented herein can be implemented on many types of electronic devices with digital video capabilities.

[0048] Terms used in the disclosure are only adopted for the purpose of describing specific embodiments and not intended to limit the disclosure. “A/an,” “said,” and “the” in a singular form in the disclosure and the appended claims are also intended to include a plural form, unless other meanings are clearly denoted throughout the disclosure. It is also to be understood that term “and/or” used in the disclosure refers to and includes one or any or all possible combinations of multiple associated items that are listed.

[0049] Reference throughout this specification to “one embodiment,” “an embodiment,” “an example,” “some embodiments,” “some examples,” or similar language means that a particular feature, structure, or characteristic described is included in at least one embodiment or example. Features, structures, elements, or characteristics described in connection with one or some embodiments are also applicable to other embodiments, unless expressly specified otherwise. [0050] Throughout the disclosure, the terms “first,” “second,” “third,” etc. are all used as nomenclature only for references to relevant elements, e.g., devices, components, compositions, steps, etc., without implying any spatial or chronological orders, unless expressly specified otherwise. For example, a “first device” and a “second device” may refer to two separately formed devices, or two parts, components, or operational states of a same device, and may be named arbitrarily.

[0051] The terms “module,” “sub-module,” “circuit,” “sub-circuit,” “circuitry,” “sub-circuitry,” “unit,” or “sub-unit” may include memory (shared, dedicated, or group) that stores code or instructions that can be executed by one or more processors. A module may include one or more circuits with or without stored code or instructions. The module or circuit may include one or more components that are directly or indirectly connected. These components may or may not be physically attached to, or located adjacent to, one another.

[0052] As used herein, the term “if’ or “when” may be understood to mean “upon” or “in response to” depending on the context. These terms, if appear in a claim, may not indicate that the relevant limitations or features are conditional or optional. For example, a method may comprise steps of: i) when or if condition X is present, function or action X’ is performed, and ii) when or if condition Y is present, function or action Y’ is performed. The method may be implemented with both the capability of performing function or action X’, and the capability of performing function or action Y’ . Thus, the functions X’ and Y’ may both be performed, at different times, on multiple executions of the method.

[0053] A unit or module may be implemented purely by software, purely by hardware, or by a combination of hardware and software. In a pure software implementation, for example, the unit or module may include functionally related code blocks or software components, that are directly or indirectly linked together, so as to perform a particular function.

[0054] FIG. 1A is a block diagram illustrating an exemplary system 10 for encoding and decoding video blocks in parallel in accordance with some implementations of the present disclosure. As shown in FIG. 1A, the system 10 includes a source device 12 that generates and encodes video data to be decoded at a later time by a destination device 14. The source device 12 and the destination device 14 may include 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. Tn some implementations, the source device 12 and the destination device 14 are equipped with wireless communication capabilities.

[0055] In some implementations, the destination device 14 may receive the encoded video data to be decoded via a link 16. The link 16 may include any type of communication medium or device capable of moving the encoded video data from the source device 12 to the destination device 14. In one example, the link 16 may include a communication medium to enable the source device 12 to transmit the encoded video data directly to the destination device 14 in real time. The encoded video data may be modulated according to a communication standard, such as a wireless communication protocol, and transmitted to the destination device 14. The communication medium may include 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 the source device 12 to the destination device 14.

[0056] In some other implementations, the encoded video data may be transmitted from an output interface 22 to a storage device 32. Subsequently, the encoded video data in the storage device 32 may be accessed by the destination device 14 via an input interface 28. The storage device 32 may include any of a variety of distributed or locally accessed data storage media such as a hard drive, Blu-ray discs, Digital Versatile Disks (DVDs), Compact Disc Read-Only Memories (CD-ROMs), flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing the encoded video data. In a further example, the storage device 32 may correspond to a file server or another intermediate storage device that may hold the encoded video data generated by the source device 12. The destination device 14 may access the stored video data from the storage device 32 via streaming or downloading. The file server may be any type of computer capable of storing the encoded video data and transmitting the encoded video data to the destination device 14 Exemplary file servers include a web server (e.g., for a website), a File Transfer Protocol (FTP) server, Network Attached Storage (NAS) devices, or a local disk drive. The destination device 14 may access the encoded video data through any standard data connection, including a wireless channel (e.g., a Wireless Fidelity (Wi-Fi) connection), a wired connection (e.g., Digital Subscriber Line (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 the encoded video data from the storage device 32 may be a streaming transmission, a download transmission, or a combination of both.

[0057] As shown in FIG. 1A, the source device 12 includes a video source 18, a video encoder 20 and the output interface 22. The video source 18 may include a source such as a video capturing device, e.g., a video camera, a video archive containing previously captured video, a video feeding 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 the video source 18 is a video camera of a security surveillance system, the source device 12 and the destination device 14 may form camera phones or video phones. However, the implementations described in the present application may be applicable to video coding in general, and may be applied to wireless and/or wired applications.

[0058] The captured, pre-captured, or computer-generated video may be encoded by the video encoder 20. The encoded video data may be transmitted directly to the destination device 14 via the output interface 22 of the source device 12. The encoded video data may also (or alternatively) be stored onto the storage device 32 for later access by the destination device 14 or other devices, for decoding and/or playback The output interface 22 may further include a modem and/or a transmitter.

[0059] The destination device 14 includes the input interface 28, a video decoder 30, and a display device 34. The input interface 28 may include a receiver and/or a modem and receive the encoded video data over the link 16. The encoded video data communicated over the link 16, or provided on the storage device 32, may include a variety of syntax elements generated by the video encoder 20 for use by the video decoder 30 in decoding the video data. Such syntax elements may be included within the encoded video data transmitted on a communication medium, stored on a storage medium, or stored on a fde server.

[0060] In some implementations, the destination device 14 may include the display device 34, which can be an integrated display device and an external display device that is configured to communicate with the destination device 14. The display device 34 displays the decoded video data to a user, and may include 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.

[0061] The video encoder 20 and the video decoder 30 may operate according to proprietary or industry standards, such as VVC, HEVC, MPEG-4, Part 10, AVC, or extensions of such standards. It should be understood that the present application is not limited to a specific video encoding/decoding standard and may be applicable to other video encoding/decoding standards. It is generally contemplated that the video encoder 20 of the source device 12 may be configured to encode video data according to any of these current or future standards. Similarly, it is also generally contemplated that the video decoder 30 of the destination device 14 may be configured to decode video data according to any of these current or future standards.

[0062] The video encoder 20 and the video decoder 30 each may be implemented as any of a variety of suitable encoder and/or decoder 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 encoding/decoding operations disclosed in the present disclosure. Each of the video encoder 20 and the video decoder 30 may be included in one or more encoders or decoders, either of which may be integrated as part of a combined encoder/decoder (CODEC) in a respective device.

[0063] Like HEVC, VVC is built upon the block-based hybrid video coding framework. FIG. IB is a block diagram illustrating a block-based video encoder in accordance with some implementations of the present disclosure. In the encoder 100, the input video signal is processed block by block, called coding units (CUs). The encoder 100 may be the video encoder 20 as shown in FIG. 1A. In VTM-1.0, a CU can be up to 128x128 pixels. However, different from the HEVC which partitions blocks only based on quad-trees, in VVC, one coding tree unit (CTU) is split into CUs to adapt to varying local characteristics based on quad/binary/ternary-tree. Additionally, the concept of multiple partition unit type in the HEVC is removed, i.e., the separation of CU, prediction unit (PU) and transform unit (TU) does not exist in the VVC anymore; instead, each CU is always used as the basic unit for both prediction and transform without further partitions. In the multi-type tree structure, one CTU is firstly partitioned by a quad-tree structure. Then, each quad-tree leaf node can be further partitioned by a binary and ternary tree structure.

[0064] FIGS. 3A-3E are schematic diagrams illustrating multi-type tree splitting modes in accordance with some implementations of the present disclosure. FIGS. 3A-3E respectively show five splitting types including quaternary partitioning (FIG. 3A), vertical binary partitioning (FIG. 3B), horizontal binary partitioning (FIG. 3C), vertical ternary partitioning (FIG. 3D), and horizontal ternary partitioning (FIG. 3E).

[0065] For each given video block, spatial prediction and/or temporal prediction may be performed. Spatial prediction (or “intra prediction”) uses pixels from the samples of already coded neighboring blocks (which are called reference samples) in the same video picture/slice to predict the current video block. Spatial prediction reduces spatial redundancy inherent in the video signal. Temporal prediction (also referred to as “inter prediction” or “motion compensated prediction”) uses reconstructed pixels from the already coded video pictures to predict the current video block. Temporal prediction reduces temporal redundancy inherent in the video signal. Temporal prediction signal for a given CU is usually signaled by one or more motion vectors (MVs) which indicate the amount and the direction of motion between the current CU and its temporal reference. Also, if multiple reference pictures are supported, one reference picture index is additionally sent, which is used to identify from which reference picture in the reference picture store the temporal prediction signal comes.

[0066] After spatial and/or temporal prediction, an intra/inter mode decision circuitry 121 in the encoder 100 chooses the best prediction mode, for example based on the rate-distortion optimization method. The block predictor 120 is then subtracted from the current video block; and the resulting prediction residual is de-correlated using the transform circuitry 102 and the quantization circuitry 104. The resulting quantized residual coefficients are inverse quantized by the inverse quantization circuitry 116 and inverse transformed by the inverse transform circuitry 118 to form the reconstructed residual, which is then added back to the prediction block to form the reconstructed signal of the CU. Further, in-loop filtering 115, such as a deblocking filter, a sample adaptive offset (SAO), and/or an adaptive in-loop filter (ALF) may be applied on the reconstructed CU before it is put in the reference picture store of the picture buffer 117 and used to code future video blocks. To form the output video bitstream 114, coding mode (inter or intra), prediction mode information, motion information, and quantized residual coefficients are all sent to the entropy coding unit 106 to be further compressed and packed to form the bitstream.

[0067] For example, a deblocking filter is available in AVC, HEVC as well as the now-current version of VVC. In HEVC, an additional in-loop filter called SAO is defined to further improve coding efficiency. In the now-current version of the VVC standard, yet another in-loop filter called ALF is being actively investigated, and it has a good chance of being included in the final standard.

[0068] These in-loop filter operations are optional. Performing these operations helps to improve coding efficiency and visual quality. They may also be turned off as a decision rendered by the encoder 100 to save computational complexity.

[0069] It should be noted that intra prediction is usually based on unfiltered reconstructed pixels, while inter prediction is based on filtered reconstructed pixels if these filter options are turned on by the encoder 100.

[0070] FIG. 2A is a block diagram illustrating a block-based video decoder 200 which may be used in conjunction with many video coding standards. This decoder 200 is similar to the reconstruction-related section residing in the encoder 100 of FIG. IB. The block-based video decoder 200 may be the video decoder 30 as shown in FIG. 1 A. In the decoder 200, an incoming video bitstream 201 is first decoded through an Entropy Decoding 202 to derive quantized coefficient levels and prediction-related information. The quantized coefficient levels are then processed through an Inverse Quantization 204 and an Inverse Transform 206 to obtain a reconstructed prediction residual. A block predictor mechanism, implemented in an Intra/inter Mode Selector 212, is configured to perform either an Intra Prediction 208, or a Motion Compensation 210, based on decoded prediction information. A set of unfiltered reconstructed pixels are obtained by summing up the reconstructed prediction residual from the Inverse Transform 206 and a predictive output generated by the block predictor mechanism, using a summer 214.

[0071] The reconstructed block may further go through an In-Loop Filter 209 before it is stored in a Picture Buffer 213 which functions as a reference picture store. The reconstructed video in the Picture Buffer 213 may be sent to drive a display device, as well as used to predict future video blocks. In situations where the In-Loop Filter 209 is turned on, a filtering operation is performed on these reconstructed pixels to derive a final reconstructed Video Output 222.

[0072] FIG. 1G is a block diagram illustrating another exemplary video encoder 20 in accordance with some implementations described in the present application. The video encoder 20 may perform intra and inter predictive coding of video blocks within video frames. Intra predictive coding relies on spatial prediction to reduce or remove spatial redundancy in video data within a given video frame or picture. Inter predictive coding relies on temporal prediction to reduce or remove temporal redundancy in video data within adjacent video frames or pictures of a video sequence. It should be noted that the term “frame” may be used as synonyms for the term “image” or “picture” in the field of video coding.

[0073] As shown in FIG. 1 G, the video encoder 20 includes a video data memory 40, a prediction processing unit 41, a Decoded Picture Buffer (DPB) 64, a summer 50, a transform processing unit 52, a quantization unit 54, and an entropy encoding unit 56. The prediction processing unit 41 further includes a motion estimation unit 42, a motion compensation unit 44, a partition unit 45, an intra prediction processing unit 46, and an intra Block Copy (BC) unit 48. In some implementations, the video encoder 20 also includes an inverse quantization unit 58, an inverse transform processing unit 60, and a summer 62 for video block reconstruction. An inloop filter 63, such as a deblocking filter, may be positioned between the summer 62 and the DPB 64 to filter block boundaries to remove blockiness artifacts from reconstructed video.

Another in-loop filter, such as Sample Adaptive Offset (SAO) filter and/or Adaptive in-Loop Filter (ALF), may also be used in addition to the deblocking filter to filter an output of the summer 62. In some examples, the in-loop filters may be omitted, and the decoded video block may be directly provided by the summer 62 to the DPB 64. The video encoder 20 may take the form of a fixed or programmable hardware unit or may be divided among one or more of the illustrated fixed or programmable hardware units.

[0074] The video data memory 40 may store video data to be encoded by the components of the video encoder 20. The video data in the video data memory 40 may be obtained, for example, from the video source 18 as shown in FIG. 1A. The DPB 64 is a buffer that stores reference video data (for example, reference frames or pictures) for use in encoding video data by the video encoder 20 (e.g., in intra or inter predictive coding modes). The video data memory 40 and the DPB 64 may be formed by any of a variety of memory devices. In various examples, the video data memory 40 may be on-chip with other components of the video encoder 20, or off- chip relative to those components.

[0075] As shown in FIG. 1G, after receiving the video data, the partition unit 45 within the prediction processing unit 41 partitions the video data into video blocks. This partitioning may also include partitioning a video frame into slices, tiles (for example, sets of video blocks), or other larger Coding Units (CUs) according to predefined splitting structures such as a Quad-Tree (QT) structure associated with the video data. The video frame is or may be regarded as a two- dimensional array or matrix of samples with sample values. A sample in the array may also be referred to as a pixel or a pel. A number of samples in horizontal and vertical directions (or axes) of the array or picture define a size and/or a resolution of the video frame. The video frame may be divided into multiple video blocks by, for example, using QT partitioning. The video block again is or may be regarded as a two-dimensional array or matrix of samples with sample values, although of smaller dimension than the video frame. A number of samples in horizontal and vertical directions (or axes) of the video block define a size of the video block. The video block may further be partitioned into one or more block partitions or sub-blocks (which may form again blocks) by, for example, iteratively using QT partitioning, Binary-Tree (BT) partitioning or Triple-Tree (TT) partitioning or any combination thereof It should be noted that the term “block” or “video block” as used herein may be a portion, in particular a rectangular (square or non- square) portion, of a frame or a picture. With reference, for example, to HEVC and VVC, the block or video block may be or correspond to a Coding Tree Unit (CTU), a CU, a Prediction Unit (PU) or a Transform Unit (TU) and/or may be or correspond to a corresponding block, e.g., a Coding Tree Block (CTB), a Coding Block (CB), a Prediction Block (PB) or a Transform Block (TB) and/or to a sub-block.

[0076] The prediction processing unit 41 may select one of a plurality of possible predictive coding modes, such as one of a plurality of intra predictive coding modes or one of a plurality of inter predictive coding modes, for the current video block based on error results (e.g., coding rate and the level of distortion). The prediction processing unit 41 may provide the resulting intra or inter prediction coded block to the summer 50 to generate a residual block and to the summer 62 to reconstruct the encoded block for use as part of a reference frame subsequently. The prediction processing unit 41 also provides syntax elements, such as motion vectors, intra-mode indicators, partition information, and other such syntax information, to the entropy encoding unit 56.

[0077] In order to select an appropriate intra predictive coding mode for the current video block, the intra prediction processing unit 46 within the prediction processing unit 41 may perform intra predictive coding of the current video block relative to one or more neighbor blocks in the same frame as the current block to be coded to provide spatial prediction. The motion estimation unit 42 and the motion compensation unit 44 within the prediction processing unit 41 perform inter predictive coding of the current video block relative to one or more predictive blocks in one or more reference frames to provide temporal prediction. The video encoder 20 may perform multiple coding passes, e.g., to select an appropriate coding mode for each block of video data.

[0078] In some implementations, the motion estimation unit 42 determines the inter prediction mode for a current video frame by generating a motion vector, which indicates the displacement of a 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 the motion estimation unit 42, is the process of generating motion vectors, which estimate motion for video blocks. A motion vector, for example, may indicate the displacement of a video block within a current video frame or picture relative to a predictive block within a reference frame relative to the current block being coded within the current frame. The predetermined pattern may designate video frames in the sequence as P frames or B frames. The intra BC unit 48 may determine vectors, e.g., block vectors, for intra BC coding in a manner similar to the determination of motion vectors by the motion estimation unit 42 for inter prediction, or may utilize the motion estimation unit 42 to determine the block vector.

[0079] A predictive block for the video block may be or may correspond to a block or a reference block of a reference frame that is deemed as closely matching 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, the video encoder 20 may calculate values for sub-integer pixel positions of reference frames stored in the DPB 64. For example, the video encoder 20 may interpolate values of one-quarter pixel positions, one-eighth pixel positions, or other fractional pixel positions of the reference frame. Therefore, the motion estimation unit 42 may perform a motion search relative to the full pixel positions and fractional pixel positions and output a motion vector with fractional pixel precision.

[0080] The motion estimation unit 42 calculates a motion vector for a video block in an inter prediction coded frame by comparing the position of the video block to the position of a predictive block of a reference frame selected from a first reference frame list (List 0) or a second reference frame list (List 1), each of which identifies one or more reference frames stored in the DPB 64. The motion estimation unit 42 sends the calculated motion vector to the motion compensation unit 44 and then to the entropy encoding unit 56.

[0081] Motion compensation, performed by the motion compensation unit 44, may involve fetching or generating the predictive block based on the motion vector determined by the motion estimation unit 42. Upon receiving the motion vector for the current video block, the motion compensation unit 44 may locate a predictive block to which the motion vector points in one of the reference frame lists, retrieve the predictive block from the DPB 64, and forward the predictive block to the summer 50. The summer 50 then forms a residual video block of pixel difference values by subtracting pixel values of the predictive block provided by the motion compensation unit 44 from the pixel values of the current video block being coded. The pixel difference values forming the residual video block may include luma or chroma component differences or both. The motion compensation unit 44 may also generate syntax elements associated with the video blocks of a video frame for use by the video decoder 30 in decoding the video blocks of the video frame. The syntax elements may include, for example, syntax elements defining the motion vector used to identify the predictive block, any flags indicating the prediction mode, or any other syntax information described herein. Note that the motion estimation unit 42 and the motion compensation unit 44 may be highly integrated, but are illustrated separately for conceptual purposes.

[0082] In some implementations, the intra BC unit 48 may generate vectors and fetch predictive blocks in a manner similar to that described above in connection with the motion estimation unit 42 and the motion compensation unit 44, but with the predictive blocks being in the same frame as the current block being coded and with the vectors being referred to as block vectors as opposed to motion vectors. In particular, the intra BC unit 48 may determine an intra-prediction mode to use to encode a current block. In some examples, the intra BC unit 48 may encode a current block using various intra-prediction modes, e g., during separate encoding passes, and test their performance through rate-distortion analysis. Next, the intra BC unit 48 may select, among the various tested intra-prediction modes, an appropriate intra-prediction mode to use and generate an intra-mode indicator accordingly. For example, the intra BC unit 48 may calculate rate-distortion values using a rate-distortion analysis for the various tested intra-prediction modes, and select the intra-prediction mode having the best rate-distortion characteristics among the tested modes as the appropriate intra-prediction mode to use. Rate-distortion analysis generally determines an amount of distortion (or error) between an encoded block and an original, unencoded block that was encoded to produce the encoded block, as well as a bitrate (i.e., a number of bits) used to produce the encoded block. Intra BC unit 48 may calculate ratios from the distortions and rates for the various encoded blocks to determine which intra-prediction mode exhibits the best rate-distortion value for the block.

[0083] In other examples, the intra BC unit 48 may use the motion estimation unit 42 and the motion compensation unit 44, in whole or in part, to perform such functions for Intra BC prediction according to the implementations described herein. In either case, for Intra block copy, a predictive block may be a block that is deemed as closely matching the block to be coded, in terms of pixel difference, which may be determined by SAD, SSD, or other difference metrics, and identification of the predictive block may include calculation of values for subinteger pixel positions.

[0084] Whether the predictive block is from the same frame according to intra prediction, or a different frame according to inter prediction, the video encoder 20 may form a residual video block by subtracting pixel values of the predictive block from the pixel values of the current video block being coded, forming pixel difference values. The pixel difference values forming the residual video block may include both luma and chroma component differences.

[0085] The intra prediction processing unit 46 may intra-predict a current video block, as an alternative to the inter-prediction performed by the motion estimation unit 42 and the motion compensation unit 44, or the intra block copy prediction performed by the intra BC unit 48, as described above. In particular, the intra prediction processing unit 46 may determine an intra prediction mode to use to encode a current block. To do so, the intra prediction processing unit 46 may encode a current block using various intra prediction modes, e.g., during separate encoding passes, and the intra prediction processing unit 46 (or a mode selection unit, in some examples) may select an appropriate intra prediction mode to use from the tested intra prediction modes. The intra prediction processing unit 46 may provide information indicative of the selected intra-prediction mode for the block to the entropy encoding unit 56. The entropy encoding unit 56 may encode the information indicating the selected intra-prediction mode in the bitstream.

[0086] After the prediction processing unit 41 determines the predictive block for the current video block via either inter prediction or intra prediction, the summer 50 forms a residual video block by subtracting the predictive block from the current video block. The residual video data in the residual block may be included in one or more TUs and is provided to the transform processing unit 52. The transform processing unit 52 transforms the residual video data into residual transform coefficients using a transform, such as a Discrete Cosine Transform (DCT) or a conceptually similar transform.

[0087] The transform processing unit 52 may send the resulting transform coefficients to the quantization unit 54. The quantization unit 54 quantizes the transform coefficients to further reduce the 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, the quantization unit 54 may then perform a scan of a matrix including the quantized transform coefficients. Alternatively, the entropy encoding unit 56 may perform the scan.

[0088] Following quantization, the entropy encoding unit 56 entropy encodes the quantized transform coefficients into a video bitstream using, e.g., Context Adaptive Variable Length Coding (CAVLC), Context Adaptive Binary Arithmetic Coding (CAB AC), 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 the video decoder 30 as shown in FIG. 1 A, or archived in the storage device 32 as shown in FIG. 1 A for later transmission to or retrieval by the video decoder 30. The entropy encoding unit 56 may also entropy encode the motion vectors and the other syntax elements for the current video frame being coded.

[0089] The inverse quantization unit 58 and the inverse transform processing unit 60 apply inverse quantization and inverse transformation, respectively, to reconstruct the residual video block in the pixel domain for generating a reference block for prediction of other video blocks. As noted above, the motion compensation unit 44 may generate a motion compensated predictive block from one or more reference blocks of the frames stored in the DPB 64. The motion compensation unit 44 may also apply one or more interpolation fdters to the predictive block to calculate sub-integer pixel values for use in motion estimation.

[0090] The summer 62 adds the reconstructed residual block to the motion compensated predictive block produced by the motion compensation unit 44 to produce a reference block for storage in the DPB 64. The reference block may then be used by the intra BC unit 48, the motion estimation unit 42 and the motion compensation unit 44 as a predictive block to inter predict another video block in a subsequent video frame.

[0091] FIG. 2B is a block diagram illustrating another exemplary video decoder 30 in accordance with some implementations of the present application. The video decoder 30 includes a video data memory 79, an entropy decoding unit 80, a prediction processing unit 81, an inverse quantization unit 86, an inverse transform processing unit 88, a summer 90, and a DPB 92. The prediction processing unit 81 further includes a motion compensation unit 82, an intra prediction unit 84, and an intra BC unit 85. The video decoder 30 may perform a decoding process generally reciprocal to the encoding process described above with respect to the video encoder 20 in connection with FIG. 1G. For example, the motion compensation unit 82 may generate prediction data based on motion vectors received from the entropy decoding unit 80, while the intra-prediction unit 84 may generate prediction data based on intra-prediction mode indicators received from the entropy decoding unit 80.

[0092] In some examples, a unit of the video decoder 30 may be tasked to perform the implementations of the present application. Also, in some examples, the implementations of the present disclosure may be divided among one or more of the units of the video decoder 30. For example, the intra BC unit 85 may perform the implementations of the present application, alone, or in combination with other units of the video decoder 30, such as the motion compensation unit 82, the intra prediction unit 84, and the entropy decoding unit 80. In some examples, the video decoder 30 may not include the intra BC unit 85 and the functionality of intra BC unit 85 may be performed by other components of the prediction processing unit 81, such as the motion compensation unit 82.

[0093] The video data memory 79 may store video data, such as an encoded video bitstream, to be decoded by the other components of the video decoder 30. The video data stored in the video data memory 79 may be obtained, for example, from the storage device 32, from a local video source, such as a camera, via wired or wireless network communication of video data, or by accessing physical data storage media (e.g., a flash drive or hard disk). The video data memory 79 may include a Coded Picture Buffer (CPB) that stores encoded video data from an encoded video bitstream. The DPB 92 of the video decoder 30 stores reference video data for use in decoding video data by the video decoder 30 (e.g., in intra or inter predictive coding modes). The video data memory 79 and the DPB 92 may be formed by any of a variety of memory devices, such as dynamic random access memory (DRAM), including Synchronous DRAM (SDRAM), Magneto-resistive RAM (MRAM), Resistive RAM (RRAM), or other types of memory devices. For illustrative purpose, the video data memory 79 and the DPB 92 are depicted as two distinct components of the video decoder 30 in FIG. 2B. But it will be apparent to one skilled in the art that the video data memory 79 and the DPB 92 may be provided by the same memory device or separate memory devices. In some examples, the video data memory 79 may be on-chip with other components of the video decoder 30, or off-chip relative to those components.

[0094] During the decoding process, the video decoder 30 receives an encoded video bitstream that represents video blocks of an encoded video frame and associated syntax elements. The video decoder 30 may receive the syntax elements at the video frame level and/or the video block level. The entropy decoding unit 80 of the video decoder 30 entropy decodes the bitstream to generate quantized coefficients, motion vectors or intra-prediction mode indicators, and other syntax elements. The entropy decoding unit 80 then forwards the motion vectors or intraprediction mode indicators and other syntax elements to the prediction processing unit 81.

[0095] When the video frame is coded as an intra predictive coded (I) frame or for intra coded predictive blocks in other types of frames, the intra prediction unit 84 of the prediction processing unit 81 may generate prediction data for a video block of the current video frame based on a signaled intra prediction mode and reference data from previously decoded blocks of the current frame.

[0096] When the video frame is coded as an inter-predictive coded (i.e., B or P) frame, the motion compensation unit 82 of the prediction processing unit 81 produces one or more predictive blocks for a video block of the current video frame based on the motion vectors and other syntax elements received from the entropy decoding unit 80 Each of the predictive blocks may be produced from a reference frame within one of the reference frame lists. The video decoder 30 may construct the reference frame lists, List 0 and List 1, using default construction techniques based on reference frames stored in the DPB 92.

[0097] In some examples, when the video block is coded according to the intra BC mode described herein, the intra BC unit 85 of the prediction processing unit 81 produces predictive blocks for the current video block based on block vectors and other syntax elements received from the entropy decoding unit 80. The predictive blocks may be within a reconstructed region of the same picture as the current video block defined by the video encoder 20.

[0098] The motion compensation unit 82 and/or the intra BC unit 85 determines prediction information for a video block of the current video frame by parsing the motion vectors and other syntax elements, and then uses the prediction information to produce the predictive blocks for the current video block being decoded. For example, the motion compensation unit 82 uses some of the received syntax elements to determine a prediction mode (e.g., intra or inter prediction) used to code video blocks of the video frame, an inter prediction frame type (e.g., B or P), construction information for one or more of the reference frame lists for the frame, motion vectors for each inter predictive encoded video block of the frame, inter prediction status for each inter predictive coded video block of the frame, and other information to decode the video blocks in the current video frame.

[0099] Similarly, the intra BC unit 85 may use some of the received syntax elements, e.g., a flag, to determine that the current video block was predicted using the intra BC mode, construction information of which video blocks of the frame are within the reconstructed region and should be stored in the DPB 92, block vectors for each intra BC predicted video block of the frame, intra BC prediction status for each intra BC predicted video block of the frame, and other information to decode the video blocks in the current video frame.

[00100] The motion compensation unit 82 may also perform interpolation using the interpolation filters as used by the video encoder 20 during encoding of the video blocks to calculate interpolated values for sub-integer pixels of reference blocks. In this case, the motion compensation unit 82 may determine the interpolation filters used by the video encoder 20 from the received syntax elements and use the interpolation filters to produce predictive blocks. [00101] The inverse quantization unit 86 inverse quantizes the quantized transform coefficients provided in the bitstream and entropy decoded by the entropy decoding unit 80 using the same quantization parameter calculated by the video encoder 20 for each video block in the video frame to determine a degree of quantization. The inverse transform processing unit 88 applies an inverse transform, e.g., an inverse DCT, an inverse integer transform, or a conceptually similar inverse transform process, to the transform coefficients in order to reconstruct the residual blocks in the pixel domain.

[00102] After the motion compensation unit 82 or the intra BC unit 85 generates the predictive block for the current video block based on the vectors and other syntax elements, the summer 90 reconstructs decoded video block for the current video block by summing the residual block from the inverse transform processing unit 88 and a corresponding predictive block generated by the motion compensation unit 82 and the intra BC unit 85. An in-loop filter 91 such as deblocking filter, SAO filter and/or ALF may be positioned between the summer 90 and the DPB 92 to further process the decoded video block. In some examples, the in-loop filter 91 may be omitted, and the decoded video block may be directly provided by the summer 90 to the DPB 92. The decoded video blocks in a given frame are then stored in the DPB 92, which stores reference frames used for subsequent motion compensation of next video blocks. The DPB 92, or a memory device separate from the DPB 92, may also store decoded video for later presentation on a display device, such as the display device 34 of FIG. 1A.

[00103] In the current VVC and AVS3 standards, motion information of the current coding block is either copied from spatial or temporal neighboring blocks specified by a merge candidate index or obtained by explicit signaling of motion estimation. The focus of the present disclosure is to improve the accuracy of the motion vectors for affine merge mode by improving the derivation methods of affine merge candidates. To facilitate the description of the present disclosure, the existing affine merge mode design in the VVC standard is used as an example to illustrate the proposed ideas. Please note that though the existing affine mode design in the VVC standard is used as the example throughout the present disclosure, to a person skilled in the art of modem video coding technologies, the proposed technologies can also be applied to a different design of affine motion prediction mode or other coding tools with the same or similar design spirit.

[00104] 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.

[00105] As shown in FIG. 1C, the video encoder 20 (or more specifically a partition unit in a prediction processing unit of the video encoder 20) generates an encoded representation of a frame by first partitioning the frame into a set of CTUs. A video frame may include an integer number of CTUs ordered consecutively in a raster scan order from left to right and from top to bottom. Each CTU is a largest logical coding unit and the width and height of the CTU are signaled by the video encoder 20 in a sequence parameter set, such that all the CTUs in a video sequence have the same size being one of 128x 128, 64x64, 32x32, and 16x ]6. But it should be noted that the present application is not necessarily limited to a particular size. As shown in FIG. ID, each CTU may include one CTB of luma samples, two corresponding coding tree blocks of chroma samples, and syntax elements used to code the samples of the coding tree blocks. The syntax elements describe properties of different types of units of a coded block of pixels and how the video sequence can be reconstructed at the video decoder 30, including inter or intra prediction, intra prediction mode, motion vectors, and other parameters. In monochrome pictures or pictures having three separate color planes, a CTU may include 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.

[00106] To achieve a better performance, the video encoder 20 may recursively perform tree partitioning such as binary -tree partitioning, ternary-tree partitioning, quad-tree partitioning or a combination thereof on the coding tree blocks of the CTU and divide the CTU into smaller CUs. As depicted in FIG. IE, the 64x64 CTU 400 is first divided into four smaller CUs, each having a block size of 32x32. Among the four smaller CUs, CU 410 and CU 420 are each divided into four CUs of 16x16 by block size. The two 16x16 CUs 430 and 440 are each further divided into four CUs of 8x8 by block size. FIG. IF depicts a quad-tree data structure illustrating the end result of the partition process of the CTU 400 as depicted in FIG. IE, each leaf node of the quadtree corresponding to one CU of a respective size ranging from 32x32 to 8x8. Like the CTU depicted in FIG. ID, each CU may include a 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 include a single coding block and syntax structures used to code the samples of the coding block. It should be noted that the quad-tree partitioning depicted in FIGS. 1E-1F 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 FIGS. 3A-3E, there are five possible partitioning types of a coding block having a width W and a height H, i.e., quaternary partitioning, horizontal binary partitioning, vertical binary partitioning, horizontal ternary partitioning, and vertical ternary partitioning.

[00107] In some implementations, the video encoder 20 may further partition a coding block of a CU into one or more MxN PBs. A PB is a rectangular (square or non-square) block of samples on which the same prediction, inter or intra, is applied. A PU of a CU may include a PB of luma samples, two corresponding PBs of chroma samples, and syntax elements used to predict the PBs. In monochrome pictures or pictures having three separate color planes, a PU may include a single PB and syntax structures used to predict the PB. The video encoder 20 may generate predictive luma, Cb, and Cr blocks for luma, Cb, and Cr PBs of each PU of the CU. [00108] The video encoder 20 may use intra prediction or inter prediction to generate the predictive blocks for a PU. If the video encoder 20 uses intra prediction to generate the predictive blocks of a PU, the video encoder 20 may generate the predictive blocks of the PU based on decoded samples of the frame associated with the PU. If the video encoder 20 uses inter prediction to generate the predictive blocks of a PU, the video encoder 20 may generate the predictive blocks of the PU based on decoded samples of one or more frames other than the frame associated with the PU.

[00109] After the video encoder 20 generates predictive luma, Cb, and Cr blocks for one or more PUs of a CU, the video encoder 20 may generate a luma residual block for the CU by subtracting the CU’s predictive luma blocks from its original luma coding block such that each sample in the CU’s luma residual block indicates a difference between a luma sample in one of the CU's predictive luma blocks and a corresponding sample in the CU's original luma coding block. Similarly, the video encoder 20 may generate a Cb residual block and a Cr residual block for the CU, respectively, such that each sample in the CU's Cb residual block indicates a difference between a Cb sample in one of the CU's predictive Cb blocks and a corresponding sample in the CU's original Cb coding block and each sample in the CU's Cr residual block may indicate a difference between a Cr sample in one of the CU's predictive Cr blocks and a corresponding sample in the CU's original Cr coding block.

[00110] Furthermore, as illustrated in FIG. IE, the video encoder 20 may use quad-tree partitioning to decompose the luma, Cb, and Cr residual blocks of a CU into one or more luma, Cb, and Cr transform blocks respectively. A transform block is a rectangular (square or nonsquare) block of samples on which the same transform is applied. A TU of a CU may include 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 include a single transform block and syntax structures used to transform the samples of the transform block.

[00111] The video encoder 20 may apply one or more transforms to a luma transform block of a TU to generate a luma coefficient block for the TU. A coefficient block may be a two- dimensional array of transform coefficients. A transform coefficient may be a scalar quantity. The video encoder 20 may apply one or more transforms to a Cb transform block of a TU to generate a Cb coefficient block for the TU. The video encoder 20 may apply one or more transforms to a Cr transform block of a TU to generate a Cr coefficient block for the TU.

[00112] After generating a coefficient block (e.g., a luma coefficient block, a Cb coefficient block or a Cr coefficient block), the video encoder 20 may quantize the coefficient block. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the transform coefficients, providing further compression. After the video encoder 20 quantizes a coefficient block, the video encoder 20 may entropy encode syntax elements indicating the quantized transform coefficients. For example, the video encoder 20 may perform CAB AC on the syntax elements indicating the quantized transform coefficients. Finally, the video encoder 20 may output a bitstream that includes a sequence of bits that forms a representation of coded frames and associated data, which is either saved in the storage device 32 or transmitted to the destination device 14.

[00113] After receiving a bitstream generated by the video encoder 20, the video decoder 30 may parse the bitstream to obtain syntax elements from the bitstream. The video decoder 30 may reconstruct the frames of the video data based at least in part on the syntax elements obtained from the bitstream. The process of reconstructing the video data is generally reciprocal to the encoding process performed by the video encoder 20. For example, the video decoder 30 may perform inverse transforms on the coefficient blocks associated with TUs of a current CU to reconstruct residual blocks associated with the TUs of the current CU. The video decoder 30 also reconstructs the coding blocks of the current CU by adding the samples of the predictive blocks for PUs of the current CU to corresponding samples of the transform blocks of the TUs of the current CU. After reconstructing the coding blocks for each CU of a frame, video decoder 30 may reconstruct the frame.

[00114] 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). It is noted that IBC could be regarded as either intra-frame prediction or a third mode. Between the two modes, inter-frame prediction contributes more to the coding efficiency than intra-frame prediction because of the use of motion vectors for predicting a current video block from a reference video block.

[00115] But with the ever improving video data capturing technology and more refined video block size for preserving details in the video data, the amount of data required for representing motion vectors for a current frame also increases substantially One way of overcoming this challenge is to benefit from the fact that not only a group of neighboring CUs in both the spatial and temporal domains have similar video data for predicting purpose but the motion vectors between these neighboring CUs are also similar. Therefore, it is possible to use the motion information of spatially neighboring CUs and/or temporally co-located CUs as an approximation of the motion information (e g., motion vector) of a current CU by exploring their spatial and temporal correlation, which is also referred to as “Motion Vector Predictor (MVP)” of the current CU.

[00116] Instead of encoding, into the video bitstream, an actual motion vector of the current CU determined by the motion estimation unit as described above in connection with FIG. IB, the motion vector predictor of the current CU is subtracted from the actual motion vector of the current CU to produce a Motion Vector Difference (MVD) for the current CU. By doing so, there is no need to encode the motion vector determined by the motion estimation unit for each CU of a frame into the video bitstream and the amount of data used for representing motion information in the video bitstream can be significantly decreased.

[00117] Like the process of choosing a predictive block in a reference frame during inter-frame prediction of a code block, a set of rules need to be adopted by both the video encoder 20 and the video decoder 30 for constructing a motion vector candidate list (also known as a “merge list”) for a current CU using those potential candidate motion vectors associated with spatially neighboring CUs and/or temporally co-located CUs of the current CU and then selecting one member from the motion vector candidate list as a motion vector predictor for the current CU. By doing so, there is no need to transmit the motion vector candidate list itself from the video encoder 20 to the video decoder 30 and an index of the selected motion vector predictor within the motion vector candidate list is sufficient for the video encoder 20 and the video decoder 30 to use the same motion vector predictor within the motion vector candidate list for encoding and decoding the current CU.

ALF in VVC

Filter Shapes, Linear Filtering and Adaptive Clipping

[00118] In VVC, ALF is applied to the output samples of SAO. Two filter shapes, 7x7 diamond shape and 5x5 diamond shape are supported for luma and chroma components, respectively, as shown in FIGS. 4A-4B. In FIGS. 4A-4B, each square corresponds to a luma or a chroma sample and the center square corresponds to a current to-be-filtered sample. The filter coefficients use point-symmetry and each integer filter coefficient is represented with 7-bit fractional precision. In addition, the sum of coefficients of one filter is equal to 128, which is the fixed-point representation of 1.0 with 7-bit fractional precision: where the number of coefficients N is equal to 13 and 7 for 7x7 and 5x5 filter shape, respectively.

A filtered sample value R(x, y) at coordinates (x, y) is derived by applying coefficient c _t to the reconstructed sample values R(x,y) as follows: where are the coordinates of the reconstructed samples corresponding to i-th coefficient Due to the constraint in equation (1), equation (2) can be written as:

[00119] In VVC, the possibility to clip the difference between the neighboring sample value and the current to-be-filtered sample is added to equation (3) as follows: where bf is the clipping parameter for a coefficient determined by a clipping index is derived as follows: where BD is the sample bit depth and can be 0, 1, 2 or 3. Luma Sub-Block Level Filter Adaptation

[00120] In VVC, sub-block level filter adaption is only applied to luma component. Each 4x4 luma block is classified based on its directionality and 2D Laplacian activity. First, the values of sample gradients for horizontal, vertical and two diagonal directions are calculated: (7)

[00121] Based on the sample gradients, sub-block horizontal gradient, g _h , vertical gradient, g _v , and two diagonal gradients, g _dQ and g _dl , are calculated as

Indices i and j refer to the coordinates of the upper left sample in the 4x4 luma block. As it can be seen from equation (8), the sum of sample gradients within a 10x10 luma window that covers the target 4x4 block is used for classifying that block. To reduce the complexity, only gradient of every second sample in a 10x 10 window is calculated as illustrated in FIG. 5. The values of other sample gradients are set to 0.

[00122] Second, to assign the directionality D, the ratio of the maximum and the minimum of the sub-block horizontal and vertical gradients and the ratio of the maximum and the minimum of two sub-block diagonal gradients are compared against each other with a set of thresholds tq and t ₂ : Step 1: If both set to 0.

Step 2: If -, the directionality D is calculated in Step 3, otherwise in Step 4. Step 3: If g is set to 2, otherwise D is set to 1. Step 4: If is set to 4, otherwise D is set to 3.

[00123] Each subsequent step in the above calculation of D is only executed if there is no value assigned to D in the previous steps. Third, an activity value A is calculated as

A is further mapped to the range of 0 to 4: , where Finally, each 4x4 luma block is categorized into one of the 25 classes:

Each class can have its own filter assigned.

[00124] Before filtering each 4x4 luma block, a geometric transformation, such as 90-degree rotation, diagonal or vertical flip, is applied to the filter coefficients, as illustrated in FIGS. 6A- 6C, depending on the sub-block gradient value as specified in Table 1.

Table 1 Geometric transformation based on sub-block gradient values

Coding Tree Block Level Filter Adaptation

[00125] In addition to the luma 4x4 block-level filter adaptation, ALF supports CTB-level filter adaptation. A luma CTB can use a filter set calculated for the current slice or one of the filter sets calculated for the already coded slices. It can also use one of the 16 offline trained filter sets. Within each luma CTB, which filter from the chosen filter set should be applied to each 4 >< 4 block, is determined by the class C calculated in equation (12) for this block.

[00126] Chroma uses only CTB-level filter adaptation. Up to 8 filters can be used for chroma components in a slice. Each CTB can select one of these filters.

Syntax Design

[00127] Filter coefficients and clipping indices are carried in ALF APSs. An ALF APS can include up to 8 chroma filters and one luma filter set with up to 25 filters. An index i _c is also included for each of the 25 luma classes. Classes having the same index i _c share the same filter. By merging different classes, the number of bits required to represent the filter coefficients is reduced. The absolute value of a filter coefficient is represented using a Oth order Exp-Golomb code followed by a sign bit for a non-zero coefficient. When clipping is enabled, a clipping index is also signaled for each filter coefficient using a two-bit fixed-length code. The storage needed for ALF coefficients and clipping indices within an APS is at most 3480 bits. Up to 8 ALF APSs can be used by the decoder at the same time

[00128] Filter control syntax elements include two types of information. First, ALF on/off flags are signaled at sequence, picture, slice and CTB levels. Chroma ALF can be enabled at picture and slice level only if luma ALF is enabled at the corresponding level. Second, filter usage information is signaled at picture, slice and CTB level, if ALF is enabled at that level. Referenced ALF APSs IDs are coded at a slice level or at a picture level if all the slices within the picture use the same APSs. Luma component can reference up to 7 ALF APSs and chroma components can reference 1 ALF APS. For a luma CTB, an index is signaled indicating which ALF APS or offline trained luma filter set is used. For a chroma CTB, the index indicates which filter in the referenced APS is used.

Line Buffer Reduction

[00129] To reduce the storage requirement for ALF, VVC employs line buffer boundary processing. In VVC, line buffer boundaries are placed 4 luma samples and 2 chroma samples above horizontal CTU boundaries. When applying ALF to a sample on one side of a line buffer boundary, samples on the other side of the line buffer boundary cannot be used.

ALF in ECM

ALF simplification removal

[00130] ALF gradient subsampling and ALF virtual boundary processing are removed. Block size for classification is reduced from 4x4 to 2x2. Filter size for both luma and chroma, for which ALF coefficients are signaled, is increased to 9x9.

ALF with fixed filters

[00131] To filter a luma sample, three different classifiers and and three different sets of filters and ) are used. Sets and contain fixed filters, with coefficients trained for classifiers and . Coefficients of filters in are signalled. Which filter from a set Ff is used for a given sample is decided by a class assigned to this sample using classifier

Filtering

[00132] At first, two 13x13 diamond shape fixed filters F _o and F ₁ are applied to derive two intermediate samples and . After that, is applied to neighboring samples, and samples before deblocking filter (DBF) to derive a filtered sample as where is the clipped difference between a neighboring sample and current sample is the clipped difference between and current sample is the clipped difference between a neighboring sample before DBF and current sample The filter coefficients , are signaled. The filter shape of is presented in FIG. 7.

Classification

[00133] Based on directionality D _t and activity a class is assigned to each 2x2 block: where represents the total number of directionalities [00134] As in VVC, values of the horizontal, vertical, and two diagonal gradients are calculated for each sample using 1-D Laplacian. The sum of the sample gradients within a 4><4 window that covers the target 2x2 block is used for classifier C _o and the sum of sample gradients within a 12x12 window is used for classifiers and ₂ . The sums of horizontal, vertical and two diagonal gradients are denoted, respectively, as g and g The directionality is determined by comparing with a set of thresholds. The directionality D ₂ is derived as in VVC using thresholds 2 and 4.5. For and horizontal/vertical edge strength and diagonal edge strength are calculated first. Thresholds are used. Edge strength is 0 if r otherwise, is the maximum integer such that r , Edge strength is 0 if ; otherwise, is the maximum integer such that , When horizontal/vertical edges are dominant, the is derived by using Table 2 (a); otherwise, diagonal edges are dominant, the is derived by using Table 2 (b). [00135] To obtain A _t , the sum of vertical and horizontal gradients A _L is mapped to the range of 0 to n, where n is equal to 4 for 4 ₂ and 15 for A _o and A _± .

[00136] In an ALF_APS, up to 4 luma filter sets are signalled, each set may have up to 25 filters.

Alternative 2x2 ALF classifier

[00137] Classification in ALF is extended with an additional alternative classifier. For a signalled luma filter set, a flag is signalled to indicate whether the alternative classifier is applied. Geometrical transformation is not applied to the alternative band classifier. When the band-based classifier is applied, the sum of sample values of a 2x2 luma block is calculated at first. Then the class index is calculated as below, class index = (sum * 25) » (sample bit depth + 2). (16)

[00138] Although ALF has been improved in ECM, there is room to further improve its performance.

[00139] First, online ALF filter in ECM takes spatial neighboring pixels, fixed ALF filter results and spatial neighboring pixels before deblocking filter as input. However, besides these information, other information such as spatial neighboring pixels in prediction signal, spatial neighboring pixels before SAO can also be used as online ALF filter equation input, which may benefit the coding performance.

[00140] Second, edge based classifier and band based classifier are used adaptively for online ALF filter in ECM. However, these two classifiers may be further combined to provide other classifiers, which may benefit the coding performance.

[00141] Third, the filter shape for chroma ALF is diamond in ECM, while the filter shape for luma ALF is long cross shape, such non-unified design may not be optimal from standardization point of view.

[00142] In this disclosure, to address the issues as pointed out above, methods are provided to further improve the existing design of the ALF. In general, the main features of the examples provided in this disclosure are summarized as follows. 1. Online ALF filter takes spatial neighboring pixels in prediction signal, spatial neighboring pixels before SAO as additional input.

2. The classifiers which combine the features of edge based classifier and band based classifier are used as additional classifier for online ALF filter.

3. The filter shape for chroma ALF is changed from diamond shape to long cross shape to unify with the filter shape for luma ALF.

[00143] It is noted that the disclosed methods may be applied independently or jointly.

Information in prediction and before SAO used as additional ALF input

[00144] According to the one or more embodiments of the disclosure, information in prediction and before SAO are used as additional ALF equation input. Different methods may be used to achieve this goal.

[00145] In the first method, it is proposed to take the spatial neighboring pixels in prediction signal as additional ALF equation input. Various filter shapes may be used to extract the information in prediction signal. For example, the filter shape can be 1 * 1, 3*3 or 5*5 as shown in Figure 8. Various equation forms can be used to extract the information in prediction signal. In one example, the clipping differences between the surrounding pixels in prediction signal and current pixel are used as ALF equation input. In another example, the clipping differences between the surrounding pixels in prediction signal and the collocated pixel in prediction signal, the clipping difference between the collocated pixel in prediction signal and current pixel are used as ALF equation input.

[00146] In the second method, it is proposed to take the spatial neighboring pixels in before SAO signal as additional ALF equation input. Various filter shapes can be used to extract the information in before SAO signal. For example, the filter shape can be 1 x 1, 3x3 or 5x5 as shown in Figure 8. Various equation forms can be used to extract the information in before SAO signal. In one example, the clipping differences between the surrounding pixels in before SAO signal and current pixel are used as ALF equation input. In another example, the clipping differences between the surrounding pixels in before SAO signal and the collocated pixel in before SAO signal, the clipping difference between the collocated pixel in before SAO signal and current pixel are used as ALF equation input.

[00147] In the third method, it is proposed to take both information in prediction and before SAO signal as ALF equation input. The utilization method proposed in the first and second method can be combined to achieve the third method.

New classifiers combined the features of edge based classifier and band based classifier [00148] According to the one or more embodiments of the disclosure, the features of edge based classifier and band based classifier are combined to derive new classifiers for online ALF filter. Different methods may be used to achieve this goal.

[00149] In the first method, it is proposed to first compute the directionality D of the sub-block of luma component, then the sum of sample values of the sub-block is calculated and it is mapped to the index referring to the band based classifier, and the class index for the sub-block is calculated as where B is the index calculated referring to the band based classifier, M _D represents the total number of directionalities D. In one example, for the 2x2 luma block, the directionality D is calculated in the same manner as D ₂ in ECM, and B is calculated as

[00150] In the second method, it is proposed to first compute the activity value A of the subblock of luma component, then the sum of sample values of the sub-block is calculated and it is mapped to the index referring to the band based classifier, and the class index for the sub-block is calculated as where B is the index calculated referring to the band based classifier, M _A represents the total number of the activity value A. In one example, for the 2x2 luma block, the activity value A is calculated in the same manner as d ₂ in ECM, and B is calculated as

[00151] In the third method, it is proposed to first compute the index of the sub-block of luma component referring to the edge based classifier, then the sum of sample values of the sub-block is calculated and it is mapped to the index referring to the band based classifier, and the class index for the sub-block is calculated as where B is the index calculated referring to the band based classifier, M _E represents the total number of the index calculated referring to the edge based classifier, E is the index calculated referring to the edge based classifier. Tn one example, for the 2x2 luma block, the index E is calculated in the same manner as C ₂ in ECM, and B is calculated as

Adjust the chroma ALF filter shape to unify with luma ALF filter shape

[00152] In the third aspect of this disclosure, it is provided to change the chroma ALF filter shape from diamond shape to long cross shape as shown in FIG. 9, which is unified with the luma ALF filter shape.

[00153] FIG. 10 shows a computing environment (or a computing device) 1610 coupled with a user interface 1660. The computing environment 1610 can be part of a data processing server. In some embodiments, the computing device 1610 can perform any of various methods or processes (such as encoding/decoding methods or processes) as described hereinbefore in accordance with various examples of the present disclosure. The computing environment 1610 may include a processor 1620, a memory 1640, and an I/O interface 1650.

[00154] The processor 1620 typically controls overall operations of the computing environment 1610, such as the operations associated with the display, data acquisition, data communications, and image processing. The processor 1620 may include one or more processors to execute instructions to perform all or some of the steps in the above-described methods. Moreover, the processor 1620 may include one or more modules that facilitate the interaction between the processor 1620 and other components. The processor may be a Central Processing Unit (CPU), a microprocessor, a single chip machine, a GPU, or the like.

[00155] The memory 1640 is configured to store various types of data to support the operation of the computing environment 1610. Memory 1640 may include predetermine software 1642. Examples of such data include instructions for any applications or methods operated on the computing environment 1610, video datasets, image data, etc. The memory 1640 may be implemented by using any type of volatile or non-volatile memory devices, or a combination thereof, such as a static random access memory (SRAM), an electrically erasable programmable read-only memory (EEPROM), an erasable programmable read-only memory (EPROM), a programmable read-only memory (PROM), a read-only memory (ROM), a magnetic memory, a flash memory, a magnetic or optical disk.

[00156] The VO interface 1650 provides an interface between the processor 1620 and peripheral interface modules, such as a keyboard, a click wheel, buttons, and the like. The buttons may include but are not limited to, a home button, a start scan button, and a stop scan button. The I/O interface 1650 can be coupled with an encoder and decoder.

[00157] In some embodiments, there is also provided a non -transitory computer-readable storage medium including a plurality of programs, such as included in the memory 1640, executable by the processor 1620 in the computing environment 1610, for performing the above-described methods. For example, the non-transitory computer-readable storage medium may be a ROM, a RAM, a CD-ROM, a magnetic tape, a floppy disc, an optical data storage device or the like.

[00158] The non-transitory computer-readable storage medium has stored therein a plurality of programs for execution by a computing device having one or more processors, where the plurality of programs when executed by the one or more processors, cause the computing device to perform the above-described method for motion prediction.

[00159] In some embodiments, the computing environment 1610 may be implemented with one or more application-specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field-programmable gate arrays (FPGAs), graphical processing units (GPUs), controllers, micro-controllers, microprocessors, or other electronic components, for performing the above methods.

[00160] FIG. 11 shows the online ALF filter inputs where the fixed filter output samples are obtained by feeding the reconstructed samples right after SAO into the offline trained fixed filters. Online ALF filter can take reconstructed samples right prior to SAO, i.e., right before SAO as additional inputs, or take prediction samples as additional inputs, or take both reconstructed samples right before SAO and prediction samples as additional inputs. As shown in FIG. 11, the various inputs of the online ALF filter may include reconstructed samples right before SAO and prediction samples, in addition to reconstructed samples right after SAO, fixed filter output samples, and reconstructed samples before DBF.

[00161] FIG. 12 shows 1x1 and 3x3 filter shapes that are applied to the prediction samples of the ALF in accordance with some examples of the present disclosure. In some examples, assuming that the prediction samples are used as additional inputs for online ALF filter, a filtered sample is derived as where R(x, y) indicates the current sample; indicates the clipped difference between a neighboring sample right after SAO and indicates the clipped difference between a fixed filter output sample and indicates the clipped difference between a neighboring sample right before DBF and is the clipped difference between a neighboring prediction sample and current sample R(x,y). The filter coefficients q, i = 0, ... N, are signalled. Different filter shapes, for example, 1x1 and 3x3 diamond shapes as shown in FIG. 12, can be used.

[00162] When the reconstructed samples right before SAG are used as additional inputs for online ALF filter, the prediction samples in above equation can be directly replaced with the reconstructed samples right before SAG.

[00163] FIG. 13 is a flow chart illustrating a method for video decoding in accordance with some examples of the present disclosure.

[00164] In Step 1301, the processor 1620, at the side of a decoder, may obtain one or more spatial neighboring samples associated with a current sample, where the one or more spatial neighboring samples are from at least one of prediction samples or reconstructed samples, and the reconstructed samples are samples prior to SAO filtering.

[00165] In some examples, as shown in FIG. 11, in addition to the reconstructed samples right after SAO, fixed filter output samples, and the reconstructed samples right before DBF, the one or more spatial neighboring samples may further include prediction samples and reconstructed samples right prior to SAO.

[00166] In Step 1302, the processor 1620, at the side of the decoder, may obtain a filtered sample for the current sample based on the one or more spatial neighboring samples.

[00167] In some examples, the processor 1620 may obtain the filtered sample in Step 1302 based on the one or more spatial neighboring samples and one or more filter coefficients, where the one or more filter coefficients are associated with different filter shapes. For example, various filter shapes can be used to extract the information in before SAO signal or prediction signal. The filter shape may be 1 x 1, 3x3 or 5x5 as shown in FIG. 8 or FIG. 12.

[00168] In some examples, the processor 1620 may obtain clipped difference based on the one or more spatial neighboring samples and the current sample, where the clipped difference may be shown as in equation (13) or equation (23) above. [00169] For example, the clipped difference may include one of the following differences: clipped difference between one or more surrounding samples and the current sample, or clipped difference between the one or more surrounding samples and a collocated sample, where the one or more surrounding samples and the collocated sample are from the one or more spatial neighboring samples.

[00170] In some examples, the one or more coefficients may be signaled in a bitstream by an encoder and received by the decoder.

[00171] In some examples, the processor 1620 may obtain the one or more spatial neighboring samples including: one or more neighboring output samples after the SAO filtering, one or more fixed filter output samples, one or more neighboring samples prior to DBF, prediction samples and/or reconstructed samples, and then derive the filtered sample based on the one or more spatial neighboring samples obtained above, as represented in equation (23).

[00172] FIG. 14 is a flow chart illustrating a method for video encoding corresponding to the method for video decoding as shown in FIG. 13 in accordance with some examples of the present disclosure.

[00173] In Step 1401, the processor 1620, at the side of an encoder, may obtain one or more spatial neighboring samples associated with a current sample, where the one or more spatial neighboring samples are from at least one of prediction samples or reconstructed samples, and the reconstructed samples are samples prior to SAO filtering.

[00174] In some examples, as shown in FIG. 11, in addition to the reconstructed samples right after SAO, fixed filter output samples, and the reconstructed samples right before DBF, the one or more spatial neighboring samples may further include prediction samples and reconstructed samples right prior to SAO.

[00175] In Step 1302, the processor 1620, at the side of the encoder, may obtain a filtered sample for the current sample based on the one or more spatial neighboring samples.

[00176] In some examples, the processor 1620 may obtain the filtered sample in Step 1302 based on the one or more spatial neighboring samples and one or more filter coefficients, where the one or more filter coefficients are associated with different filter shapes. For example, various filter shapes can be used to extract the information in before SAO signal or prediction signal. The filter shape may be 1 >< 1, 3><3 or 5><5 as shown in FIG. 8 or FIG. 12.

[00177] In some examples, the processor 1620 may obtain clipped difference based on the one or more spatial neighboring samples and the current sample, where the clipped difference may be shown as in equation (13) or equation (23) above.

[00178] For example, the clipped difference may include one of the following differences: clipped difference between one or more surrounding samples and the current sample, or clipped difference between the one or more surrounding samples and a collocated sample, where the one or more surrounding samples and the collocated sample are from the one or more spatial neighboring samples.

[00179] In some examples, the one or more coefficients may be signaled in a bitstream by an encoder and received by the decoder.

[00180] In some examples, the processor 1620 may obtain the one or more spatial neighboring samples including: one or more neighboring output samples after the SAO filtering, one or more fixed filter output samples, one or more neighboring samples prior to DBF, prediction samples and/or reconstructed samples, and then derive the filtered sample based on the one or more spatial neighboring samples obtained above, as represented in equation (23).

[00181] FIG. 15 is a flow chart illustrating a method for video decoding in accordance with some examples of the present disclosure.

[00182] In Step 1501, the processor 1620, at the side of a decoder, may obtain a first feature based on a first ALF classifier that is edge-based.

[00183] In Step 1502, the processor 1620, at the side of the decoder, may obtain a second feature based on a first ALF classifier that is band-based.

[00184] In Step 1503, the processor 1620, at the side of the decoder, may derive a combined classifier for an online ALF based on the first feature and the second feature. [00185] In some examples, the features of edge-based classifier and band-based classifier are combined to derive new classifiers for online ALF filter, so that features from different aspects preserved by different classifiers (e.g., the edge-based classifier and the band-based classifier) can be maintained. In some examples, the first ALF classifier may be the edge-based classifier and the second ALF classifier may be the band-based classifier, or the first ALF classifier may be the band-based classifier and the second ALF classifier may be the edge-based classifier.

[00186] In some examples, the processor 1620 may obtain the first feature based on the first ALF classifier by computing directionality of a sub-block of a luma component and obtaining a total number of the directionality and obtain the second feature based on the second ALF classifier by calculating a sum of sample values of the sub-block and calculating an index referring to the second ALF classifier. For example, as discussed above, it is proposed to first compute the directionality D of the sub-block of luma component, then the sum of sample values of the sub-block is calculated and it is mapped to the index referring to the band-based classifier, and the class index for the subblock is calculated as C = B * M _D + £), where B is the index calculated referring to the band based classifier, M _D represents the total number of directionalities D. In one example, for the 2x2 luma block, the directionality D is calculated in the same manner as D ₂ in ECM, and B is calculated as B = (sum * 5) » (sample bit depth + 2).

[00187] In some examples, the processor 1620 may obtain the first feature based on the first ALF classifier by computing an activity value of a sub-block of a luma component and obtaining a total number of the activity value and obtain the second feature based on the second ALF classifier by calculating a sum of sample values of the sub-block and calculating an index referring to the second ALF classifier. For example, as discussed above, it is proposed to first compute the activity value A of the sub-block of luma component, then the sum of sample values of the sub-block is calculated and it is mapped to the index referring to the band-based classifier, and the class index for the sub-block is calculated as C = B * M _A + A, where B is the index calculated referring to the band based classifier, M _A represents the total number of the activity value A. In one example, for the 2x2 luma block, the activity value A is calculated in the same manner as A ₂ in ECM, and B is calculated as B = (sum * 5) » (sample bit depth + 2).

[00188] In some examples, the processor 1620 may obtain the first feature based on the first ALF classifier by computing an index of a sub-block of a luma component referring to the first ALF classifier and obtaining a total number of the index and obtain the second feature based on the second ALF classifier by calculating a sum of sample values of the sub-block and calculating an index referring to the second ALF classifier. For example, as discussed above, it is proposed to first compute the index of the sub-block of luma component referring to the edge based classifier, then the sum of sample values of the sub-block is calculated and it is mapped to the index referring to the band based classifier, and the class index for the sub-block is calculated as C = B * M _E + E, where B is the index calculated referring to the band based classifier, M _E represents the total number of the index calculated referring to the edge based classifier, E is the index calculated referring to the edge based classifier. In one example, for the 2x2 luma block, the index E is calculated in the same manner as C ₂ in ECM, and B is calculated as B = (sum * 2) » (sample bit depth + 2).

[00189] FIG. 16 is a flow chart illustrating a method for video encoding corresponding to the method for video decoding as shown in FIG. 15 in accordance with some examples of the present disclosure.

[00190] In Step 1601 , the processor 1620, at the side of an encoder, may obtain a first feature based on a first ALF classifier that is edge-based.

[00191] In Step 1602, the processor 1620, at the side of the encoder, may obtain a second feature based on a first ALF classifier that is band-based.

[00192] In Step 1603, the processor 1620, at the side of the encoder, may derive a combined classifier for an online ALF based on the first feature and the second feature.

[00193] In some examples, the features of edge-based classifier and band-based classifier are combined to derive new classifiers for online ALF filter, so that features from different aspects preserved by different classifiers (e.g., the edge-based classifier and the band-based classifier) can be maintained. In some examples, the first ALF classifier may be the edge-based classifier and the second ALF classifier may be the band-based classifier, or the first ALF classifier may be the band-based classifier and the second ALF classifier may be the edge-based classifier.

[00194] In some examples, the processor 1620 may obtain the first feature based on the first ALF classifier by computing directionality of a sub-block of a luma component and obtaining a total number of the directionality and obtain the second feature based on the second ALF classifier by calculating a sum of sample values of the sub-block and calculating an index referring to the second ALF classifier. For example, as discussed above, it is proposed to first compute the directionality D of the sub-block of luma component, then the sum of sample values of the sub-block is calculated and it is mapped to the index referring to the band-based classifier, and the class index for the subblock is calculated as C = B * M _D + D, where B is the index calculated referring to the band based classifier, M _D represents the total number of directionalities D. In one example, for the 2x2 luma block, the directionality D is calculated in the same manner as D ₂ in ECM, and B is calculated as B = (sum * 5) » (sample bit depth + 2).

[00195] In some examples, the processor 1620 may obtain the first feature based on the first ALF classifier by computing an activity value of a sub-block of a luma component and obtaining a total number of the activity value and obtain the second feature based on the second ALF classifier by calculating a sum of sample values of the sub-block and calculating an index referring to the second ALF classifier. For example, as discussed above, it is proposed to first compute the activity value A of the sub-block of luma component, then the sum of sample values of the sub-block is calculated and it is mapped to the index referring to the band-based classifier, and the class index for the sub-block is calculated as C = B * M _A + A, where B is the index calculated referring to the band based classifier, M _A represents the total number of the activity value A. In one example, for the 2x2 luma block, the activity value A is calculated in the same manner as A ₂ in ECM, and B is calculated as B = (sum * 5) » (sample bit depth + 2). [00196] In some examples, the processor 1620 may obtain the first feature based on the first ALF classifier by computing an index of a sub-block of a luma component referring to the first ALF classifier and obtaining a total number of the index and obtain the second feature based on the second ALF classifier by calculating a sum of sample values of the sub-block and calculating an index referring to the second ALF classifier. For example, as discussed above, it is proposed to first compute the index of the sub-block of luma component referring to the edge based classifier, then the sum of sample values of the sub-block is calculated and it is mapped to the index referring to the band based classifier, and the class index for the sub-block is calculated as where B is the index calculated referring to the band based classifier, M _E represents the total number of the index calculated referring to the edge based classifier, E is the index calculated referring to the edge based classifier. In one example, for the 2x2 luma block, the index E is calculated in the same manner as C ₂ in ECM, and B is calculated as B = (sum * 2) » (sample bit depth + 2).

[00197] FIG. 17 is a flow chart illustrating a method for video decoding in accordance with some examples of the present disclosure.

[00198] In Step 1701, the processor 1620, at the side of a decoder, may obtain a bitstream from an encoder.

[00199] In Step 1702, the processor 1620, at the side of the decoder, may adjust a chroma ALF shape associated with the bitstream based on a luma ALF shape associated with the bitstream. [00200] In some examples, the processor 1620, at the side of the decoder, may adjust the chroma ALF shape associated with the bitstream by changing the chroma ALF shape from a diamond shape to a long cross shape, where the luma ALF shape is the long cross shape as shown in FIG. 9, so as to be unified with the luma ALF filter shape.

[00201] FIG. 18 is a flow chart illustrating a method for video encoding corresponding to the method for video decoding as shown in FIG. 17 in accordance with some examples of the present disclosure. [00202] In Step 1801, the processor 1620, at the side of an encoder, may signal a syntax element that indicates a luma ALF in a bitstream.

[00203] In Step 1802, the processor 1620, at the side of the encoder, may adjust a chroma ALF shape associated with the bitstream based on the luma ALF shape.

[00204] In some examples, only one luma filter shape may be enabled in encoder and a flag which identifies the luma filters shape and is transmitted into bitstream is always set to true. Correspondingly, only one chroma filter shape is enabled in encoder and it is aligned with the luma filter shape.

[00205] In some examples, the processor 1620, at the side of the encoder, may adjust the chroma ALF shape associated with the bitstream by changing the chroma ALF shape from a diamond shape to a long cross shape, where the luma ALF shape is the long cross shape as shown in FIG.

9, so as to be unified with the luma ALF filter shape.

[00206] In some examples, there is provided an apparatus for video coding. The apparatus includes a processor 1620 and a memory 1640 configured to store instructions executable by the processor; where the processor, upon execution of the instructions, is configured to perform any method as illustrated in FIGS. 13-18.

[00207] In some other examples, there is provided a non-transitory computer readable storage medium, having instructions stored therein. When the instructions are executed by a processor 1620, the instructions cause the processor to perform any method as illustrated in FIGS. 13-18. In one example, the plurality of programs may be executed by the processor 1620 in the computing environment 1610 to receive (for example, from the video encoder 20 in FIG. 1G) a bitstream or data stream including encoded video information (for example, video blocks representing encoded video frames, and/or associated one or more syntax elements, etc.), and may also be executed by the processor 1620 in the computing environment 1610 to perform the decoding method described above according to the received bitstream or data stream. In another example, the plurality of programs may be executed by the processor 1620 in the computing environment 1610 to perform the encoding method described above to encode video information (for example, video blocks representing video frames, and/or associated one or more syntax elements, etc.) into a bitstream or data stream, and may also be executed by the processor 1620 in the computing environment 1610 to transmit the bitstream or data stream (for example, to the video decoder 30 in FIG. 2B). Alternatively, the non-transitory computer-readable storage medium may have stored therein a bitstream or a data stream including encoded video information (for example, video blocks representing encoded video frames, and/or associated one or more syntax elements etc.) generated by an encoder (for example, the video encoder 20 in FIG. 1G) using, for example, the encoding method described above for use by a decoder (for example, the video decoder 30 in FIG. 2B) in decoding video data. The non-transitory computer-readable storage medium may be, for example, a ROM, a Random Access Memory (RAM), a CD-ROM, a magnetic tape, a floppy disc, an optical data storage device or the like.

[00208] Other examples of the disclosure will be apparent to those skilled in the art from consideration of the specification and practice of the disclosure disclosed here. This application is intended to cover any variations, uses, or adaptations of the disclosure following the general principles thereof and including such departures from the present disclosure as come within known or customary practice in the art. It is intended that the specification and examples be considered as exemplary only.

[00209] It will be appreciated that the present disclosure is not limited to the exact examples described above and illustrated in the accompanying drawings, and that various modifications and changes can be made without departing from the scope thereof.