Block-based Adaptive Loop Filter (ALF) with Adaptive Parameter Set (APS) in video coding

文档序号：214968 发布日期：2021-11-05 浏览：21次中文

阅读说明：本技术 在视频译码中具有自适应参数集（aps）的基于块的自适应环路滤波器（alf） (Block-based Adaptive Loop Filter (ALF) with Adaptive Parameter Set (APS) in video coding ) 是由胡楠 V·谢廖金 M·卡切夫维茨于 2020-03-19 设计创作，主要内容包括：对视频数据进行解码的方法,所述方法包括从针对其为图片、切片、瓦片或瓦片组中的一者或多者中的亮度块启用自适应环路滤波的视频比特流中解码用于指示针对亮度块的自适应参数集的数量的第一语法元素；基于针对亮度块的所述自适应参数集的数量,来对针对亮度块的多个第一自适应参数集索引进行解码；以及从针对其为图片、切片、瓦片或瓦片组中的一者或多者中的色度块启用自适应环路滤波的视频比特流中解码针对色度块的第二自适应参数集索引。(A method of decoding video data, the method comprising decoding, from a video bitstream for which adaptive loop filtering is enabled for luma blocks in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element for indicating a number of adaptive parameter sets for luma blocks; decoding a first plurality of adaptive parameter set indices for a luma block based on a number of the adaptive parameter sets for the luma block; and decoding a second adaptive parameter set index for the chroma block from the video bitstream for which adaptive loop filtering is enabled for the chroma block in one or more of the picture, slice, tile, or tile group.)

1. A method of decoding video data, the method comprising:

decoding, from a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element for indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles;

decoding a plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group;

decoding, from the video bitstream for which adaptive loop filtering is enabled for chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group;

applying a first adaptive loop filter to the luma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the plurality of first adaptive parameter set indices; and

applying a second adaptive loop filter to the chroma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the second adaptive parameter set index.

2. The method of claim 1, further comprising:

receiving the first syntax element in one or more of a picture header, a slice header, a tile header, or a tile group header of the video bitstream for indicating a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group, the plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group, and the second adaptive parameter set index for the chroma block in the one or more of the picture, the slice, the tile, or the tile group.

3. The method of claim 1, wherein each of the number of adaptation parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles comprises a respective luma adaptation loop filter set.

4. The method of claim 3, wherein each of the number of adaptation parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles only includes a respective luma adaptation loop filter set.

5. The method of claim 1, wherein the adaptation parameter set corresponding to the second adaptation parameter set index comprises a chroma adaptive loop filter set.

6. The method of claim 5, wherein the adaptation parameter set corresponding to the second adaptation parameter set index includes only the chroma adaptive loop filter set.

7. The method of claim 1, further comprising:

decoding, from the video bitstream, a second syntax element indicating whether luma adaptive loop filter information for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles is updated; and

decoding, from the video bitstream, a third syntax element indicating whether chroma adaptive loop filter information for the chroma block in the one or more of the picture, the slice, the tile, or the group of tiles is updated.

8. The method of claim 1, further comprising:

displaying a picture that includes filtered luma blocks in the one or more of the picture, the slice, the tile, or the tile group and filtered chroma blocks in the one or more of the picture, the slice, the tile, or the tile group.

9. An apparatus configured to decode video data, the apparatus comprising:

a memory configured to store luma blocks in one or more of a picture, a slice, a tile, or a group of tiles and chroma blocks in the one or more of the picture, the slice, the tile, or the group of tiles; and

one or more processors implemented in circuitry and in communication with the memory, the one or more processors configured to:

decoding, from a video bitstream for which adaptive loop filtering is enabled for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles, a first syntax element for indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles;

decoding, from the video bitstream for which adaptive loop filtering is enabled for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group;

applying a second adaptive loop filter to the chroma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the second adaptive parameter set index.

10. The apparatus of claim 9, wherein the one or more processors are further configured to:

11. The apparatus of claim 9, wherein each of the number of adaptation parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles comprises a respective luma adaptation loop filter set.

12. The apparatus of claim 11, wherein each of the number of adaptation parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles comprises only a respective luma adaptation loop filter set.

13. The apparatus of claim 9, wherein the adaptation parameter set corresponding to the second adaptation parameter set index comprises a chroma adaptive loop filter set.

14. The apparatus of claim 13, wherein the adaptation parameter set corresponding to the second adaptation parameter set index includes only the chroma adaptive loop filter set.

15. The apparatus of claim 9, wherein the one or more processors are further configured to:

16. The apparatus of claim 9, further comprising:

a display configured to display a picture that includes filtered luma blocks in the one or more of the picture, the slice, the tile, or the group of tiles and filtered chroma blocks in the one or more of the picture, the slice, the tile, or the group of tiles.

17. An apparatus configured to decode video data, the apparatus comprising:

means for decoding, from a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element for indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles;

means for decoding a plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group;

means for decoding, from the video bitstream for which adaptive loop filtering is enabled for chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group;

means for applying a first adaptive loop filter to the luma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the plurality of first adaptive parameter set indices; and

means for applying a second adaptive loop filter to the chroma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the second adaptive parameter set index.

18. A non-transitory computer-readable storage medium storing instructions that, when executed, cause one or more processors in a device configured to decode video data to:

applying a second adaptive loop filter to the chroma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the second adaptive parameter set index.

19. A method of encoding video data, the method comprising:

for a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, encoding a first syntax element indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles;

encoding a plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group; and

encoding, for the video bitstream for which adaptive loop filtering is enabled for chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group.

20. The method of claim 19, further comprising:

signaling, in one or more of a picture header, a slice header, a tile header, or a tile group header of the video bitstream, the first syntax element for indicating a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group, the plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group, and the second adaptive parameter set index for the chroma block in the one or more of the picture, the slice, the tile, or the tile group.

21. The method of claim 19, wherein each of the number of adaptation parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles comprises a respective luma adaptation loop filter set.

22. The method of claim 21, wherein each of the number of adaptation parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles only includes a respective luma adaptation loop filter set.

23. The method of claim 19, wherein the adaptation parameter set corresponding to the second adaptation parameter set index comprises a chroma adaptive loop filter set.

24. The method of claim 23, wherein the adaptation parameter set corresponding to the second adaptation parameter set index comprises only a chroma adaptive loop filter set.

25. The method of claim 19, further comprising:

encoding a second syntax element for indicating whether luma adaptive loop filter information for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles is updated; and

encoding a third syntax element for indicating whether chroma adaptive loop filter information for the chroma block in the one or more of the picture, the slice, the tile, or the group of tiles is updated.

26. The method of claim 19, further comprising:

capturing a picture that includes the luma block in the one or more of the picture, the slice, the tile, or the group of tiles and the chroma block in the one or more of the picture, the slice, the tile, or the group of tiles.

27. An apparatus configured to encode video data, the apparatus comprising:

one or more processors implemented in circuitry and in communication with the memory, the one or more processors configured to:

encoding, for a video bitstream for which adaptive loop filtering is enabled for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles, a first syntax element indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles;

encoding, for the video bitstream for which adaptive loop filtering is enabled for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group.

28. The apparatus of claim 27, wherein the one or more processors are further configured to:

29. The apparatus of claim 27, wherein each of the number of adaptation parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles comprises a respective luma adaptation loop filter set.

30. The apparatus of claim 29, wherein each of the number of adaptation parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles comprises only a respective luma adaptation loop filter set.

31. The apparatus of claim 27, wherein an adaptation parameter set corresponding to the second adaptation parameter set index comprises a chroma adaptive loop filter set.

32. The apparatus of claim 31, wherein the adaptation parameter set corresponding to the second adaptation parameter set index comprises only a chroma adaptive loop filter set.

33. The apparatus of claim 27, wherein the one or more processors are further configured to:

34. The apparatus of claim 27, further comprising:

a camera configured to capture a picture that includes the luma block in the one or more of the picture, the slice, the tile, or the group of tiles and the chroma block in the one or more of the picture, the slice, the tile, or the group of tiles.

35. An apparatus configured to encode video data, the apparatus comprising:

means for encoding, for a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element for indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles;

means for encoding a plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group; and

means for encoding, for the video bitstream for which adaptive loop filtering is enabled for chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group.

36. A non-transitory computer-readable storage medium storing instructions that, when executed, cause one or more processors in a device configured to encode video data to:

Technical Field

The present disclosure relates to video encoding and video decoding.

Background

Digital video capabilities may be incorporated into a variety of devices, including digital televisions, digital direct broadcast systems, wireless broadcast systems, Personal Digital Assistants (PDAs), laptop or desktop computers, tablet computers, e-book readers, digital cameras, digital recording devices, digital media players, video gaming devices, video gaming consoles, cellular or satellite radio telephones, so-called "smart phones," video conferencing devices, video streaming devices, and the like. Digital video devices implement video compression techniques such as those described in standards defined by the MPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264/MPEG-4, part 10, Advanced Video Coding (AVC), the recently filed High Efficiency Video Coding (HEVC) standard, and extensions of such standards. Video devices may more efficiently transmit, receive, encode, decode, and/or store digital video information by implementing such video compression techniques.

Video compression techniques perform spatial (intra-picture) prediction and/or temporal (inter-picture) prediction to reduce or remove redundancy inherent in video sequences. For block-based video coding, a video slice (i.e., a video frame or a portion of a video frame) may be divided into video blocks, which may also be referred to as treeblocks, Coding Units (CUs), and/or coding nodes. Video blocks in a slice of intra-coding (I) of a picture are encoded using spatial prediction with respect to reference samples in neighboring blocks in the same picture. Video blocks in inter-coded (P or B) slices of a picture may use spatial prediction with respect to reference samples in neighboring blocks in the same picture or temporal prediction with respect to reference samples in other reference pictures. A picture may be referred to as a frame and a reference picture may be referred to as a reference frame.

Spatial or temporal prediction produces results that are predictive blocks for the block to be coded. The residual data represents the pixel differences between the original block to be coded and the prediction block. An inter-coded block is encoded according to a motion vector pointing to a block of reference samples constituting a prediction block and residual data indicating the difference between the coded block and the prediction block. The intra-coded block is encoded according to an intra-coding mode and residual data. For further compression, the residual data may be transformed from the pixel domain to a transform domain, resulting in residual transform coefficients, which are then quantized. The quantized transform coefficients initially arranged in a two-dimensional array may be scanned in order to produce a one-dimensional vector of transform coefficients, and entropy coding may be applied to achieve even more compression.

Disclosure of Invention

In general, this disclosure describes techniques for coding adaptive block-based Adaptive Loop Filter (ALF) parameters for application of ALF in a video coding process using an adaptive parameter set. In particular, this disclosure describes techniques to signal a number of adaptive parameter sets and an index for each of the number of adaptive parameter sets when ALF is enabled for a picture, slice, tile, or group of tiles. In some examples of the disclosure, the number and index of adaptive parameter sets may be different for luma and chroma blocks of a picture, slice, tile, or group of tiles. In this way, the ALF parameters may be signaled in a more flexible manner for different color components and coding efficiency may be improved.

In one example, a method comprises: decoding, from a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element for indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles; decoding a plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group; decoding, from the video bitstream for which adaptive loop filtering is enabled for chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group; applying a first adaptive loop filter to the luma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the plurality of first adaptive parameter set indices; and applying a second adaptive loop filter to the chroma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the second adaptive parameter set index.

In another example, an apparatus includes a memory and one or more processors implemented in circuitry and in communication with the memory, the memory configured to store luma blocks in one or more of a picture, a slice, a tile, or a group of tiles and chroma blocks in the one or more of the picture, the slice, the tile, or the group of tiles, the one or more processors configured to: decoding, from a video bitstream for which adaptive loop filtering is enabled for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles, a first syntax element for indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles; decoding a plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group; decoding, from the video bitstream for which adaptive loop filtering is enabled for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group; applying a first adaptive loop filter to the luma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the plurality of first adaptive parameter set indices; and applying a second adaptive loop filter to the chroma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the second adaptive parameter set index.

In another example, an apparatus includes: means for decoding, from a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element for indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles; means for decoding a plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group; means for decoding, from the video bitstream for which adaptive loop filtering is enabled for chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group; means for applying a first adaptive loop filter to the luma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the plurality of first adaptive parameter set indices; and means for applying a second adaptive loop filter to the chroma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the second adaptive parameter set index.

In another example, a computer-readable storage medium is encoded with instructions that, when executed, cause a programmable processor to: decoding, from a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element for indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles; decoding a plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group; decoding, from the video bitstream for which adaptive loop filtering is enabled for chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group; applying a first adaptive loop filter to the luma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the plurality of first adaptive parameter set indices; and applying a second adaptive loop filter to the chroma block in the one or more of the picture, the slice, the tile, or the group of tiles based on the second adaptive parameter set index.

In one example, a method comprises: for a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, encoding a first syntax element indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles; encoding a plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group; and encode, for the video bitstream for which adaptive loop filtering is enabled for chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group.

In another example, an apparatus includes a memory and one or more processors implemented in circuitry and in communication with the memory, the memory configured to store luma blocks in one or more of a picture, a slice, a tile, or a group of tiles and chroma blocks in the one or more of the picture, the slice, the tile, or the group of tiles, the one or more processors configured to: encoding, for a video bitstream for which adaptive loop filtering is enabled for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles, a first syntax element indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles; encoding a plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group; and encode, for the video bitstream for which adaptive loop filtering is enabled for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group.

In another example, an apparatus includes: means for encoding, for a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element for indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles; means for encoding a plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group; and means for encoding, for the video bitstream for which adaptive loop filtering is enabled for chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group.

In another example, a computer-readable storage medium is encoded with instructions that, when executed, cause a programmable processor to: for a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, encoding a first syntax element indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles; encoding a plurality of first adaptive parameter set indices for the luma block in the one or more of the picture, the slice, the tile, or the tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group; and encode, for the video bitstream for which adaptive loop filtering is enabled for chroma blocks in the one or more of the picture, the slice, the tile, or the tile group, a second adaptive parameter set index for the chroma blocks in the one or more of the picture, the slice, the tile, or the tile group.

The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.

Drawings

Fig. 1 is a block diagram illustrating an example video encoding and decoding system that may utilize the techniques described in this disclosure.

Fig. 2 shows an example block diagram of an HEVC decoder.

Fig. 3A-3D show four 1-D patterns for Edge Offset (EO) sample classification.

Fig. 4A-4B illustrate example arrays for storing Adaptive Loop Filter (ALF) parameters.

Fig. 5 is a block diagram illustrating an example video encoder that may implement the techniques described in this disclosure.

Fig. 6 is a block diagram illustrating an example video decoder that may implement the techniques described in this disclosure.

Fig. 7 illustrates an example implementation of a filter unit for performing the techniques of this disclosure.

Fig. 8 is a flow chart illustrating an example video encoding method of the present disclosure.

Fig. 9 is a flowchart illustrating an example video decoding method of the present disclosure.

Detailed Description

This disclosure describes techniques related to filtering operations that may be used in a post-processing stage, as part of in-loop coding, or in a prediction stage of video coding. The techniques of this disclosure may be implemented as improvements or extensions to existing video codecs, such as the ITU-T h.265/HEVC (high efficiency video coding) standard, and/or as efficient coding tools for future video coding standards, such as the ITU-T h.266/VVC (multi-functional video coding) standard currently being developed.

Video coding typically includes predicting a block of video data from already coded blocks of video data in the same picture (i.e., intra-prediction) or predicting a block of video data from already coded blocks of video data in a different picture (i.e., inter-prediction). In some examples, the video encoder also calculates residual data by comparing the prediction block to the original block. Thus, the residual data represents the difference between the predicted block and the original block. The video encoder may transform and quantize the residual data, and signal the transformed and quantized residual data in an encoded bitstream.

The video decoder adds residual data to the prediction block to generate a reconstructed video block that matches the original video block more closely than the prediction block alone. To further improve the quality of the decoded video, the video decoder may perform one or more filtering operations on the reconstructed video blocks. Examples of these filtering operations include deblocking filtering, Sample Adaptive Offset (SAO) filtering, and Adaptive Loop Filtering (ALF). The parameters for these filtering operations may be determined by the video encoder and explicitly signaled in the encoded video bitstream, or implicitly determined by the video decoder, without requiring parameters to be explicitly signaled in the encoded video bitstream.

This disclosure describes techniques related to a filtering method called "Adaptive Loop Filter (ALF)". ALF may be used in the post-processing stage, or in the in-loop coding or prediction process. SAO filtering and/or ALF may be used with various existing video codec techniques and extensions to such codecs, or efficient coding tools in any future video coding standard. The Joint Exploration Model (JEM) techniques of HEVC and joint video exploration team (jfet) are discussed below in connection with this disclosure.

As used in this disclosure, the term video coding generally refers to video encoding or video decoding. Likewise, the term video coder may refer generally to a video encoder or a video decoder. Furthermore, certain techniques described in this disclosure with respect to video decoding may also be applicable to video encoding, and vice versa. For example, video encoders and video decoders are often configured to perform the same process or the inverse process. In addition, video encoders typically perform video decoding as part of the process of determining how to encode the video data.

Fig. 1 is a block diagram illustrating an example video encoding and decoding system 10 that may utilize the techniques described in this disclosure. As shown in fig. 1, system 10 includes a source device 12, source device 12 generating encoded video data to be later decoded by a destination device 14. Source device 12 and destination device 14 may be any of a variety of devices, including desktop computers, notebook computers (i.e., laptop computers), tablet computers, set-top boxes, handsets such as so-called "smart" phones, so-called "smart" pads, televisions, cameras, display devices, digital media players, video game consoles, video streaming devices, and so forth. In some cases, source device 12 and destination device 14 may be equipped for wireless communication.

Destination device 14 may receive encoded video data to be decoded via link 16. Link 16 may be any type of medium or device capable of moving encoded video data from source device 12 to destination device 14. For example, encoded video data may be output from output interface 22 to link 16. In one example, link 16 may be a communication medium for enabling source device 12 to transmit encoded video data directly to destination device 14 in real-time. The encoded video data may be modulated according to a communication standard, such as a wireless communication protocol, and transmitted to destination device 14. The communication medium may 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 a router, switch, base station, or any other equipment that may be useful for facilitating communication from source device 12 to destination device 14.

In another example, the encoded video data may be output from the output interface 22 to the storage device 26. Likewise, encoded video data may be accessed from storage device 26 through input interface 28. Storage device 26 may include any of a variety of distributed or locally accessed data storage media such as a hard drive, blu-ray discs, DVDs, CD-ROMs, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing encoded video data. In a further example, storage device 26 may correspond to a file server or another intermediate storage device that may hold the encoded video generated by source device 12. Destination device 14 may access stored video data from storage device 26 via streaming or download. The file server may be any type of server capable of storing encoded video data and transmitting the encoded video data to destination device 14. Example file servers include a web server (e.g., for a website), an FTP (file transfer protocol) server, a Network Attached Storage (NAS) device, or a local disk drive. Destination device 14 may access the encoded video data through any standard data connection, including an internet connection. This may include a wireless channel (e.g., a Wi-Fi connection), a wired connection (e.g., Digital Subscriber Line (DSL), cable modem, etc.), or a combination of both suitable for accessing encoded video data stored on a file server. The transmission of the encoded video data from the storage device 26 may be a streaming transmission, a download transmission, or a combination of both.

The techniques of this disclosure are not necessarily limited to wireless applications or settings. The techniques may be applied to video transcoding in support of any of a variety of multimedia applications, such as over-the-air television broadcasts, cable television transmissions, satellite television transmissions, streaming video transmissions (e.g., via the internet, encoding of digital video for storage on a data storage medium, decoding of digital video stored on a data storage medium), or other applications. In some examples, system 10 may be configured to support one-way or two-way video transmission to support applications such as video streaming, video playback, video broadcasting, and/or video telephony.

In the example of fig. 1, source device 12 includes a video source 18, a video encoder 20, and an output interface 22. In some cases, output interface 22 may include a modulator/demodulator (modem) and/or a transmitter. In source device 12, video source 18 may include a source such as a video capture device (e.g., a video camera), a video archive containing previously captured video, a video feed interface for receiving video from a video content provider, and/or a computer graphics system for generating computer graphics data as source video, or a combination of such sources. As one example, if video source 18 is a video camera, source device 12 and destination device 14 may form so-called camera phones or video phones. However, the techniques described in this disclosure may be generally applicable to video coding, and may be applied to wireless and/or wired applications.

The captured, pre-captured, or computer-generated video may be encoded by a video encoder. The encoded video data may be transmitted directly to destination device 14 via output interface 22 of source device 12. The encoded video data may also (or alternatively) be stored onto storage device 26 for later access by destination device 14 or other devices for decoding and/or playback.

Destination device 14 includes an input interface 28, a video decoder 30, and a display device 32. In some cases, input interface 28 may include a receiver and/or a modem. Input interface 28 of destination device 14 receives the encoded video data over link 16. Encoded video data communicated over link 16 or provided on storage device 26 may include various syntax elements generated by video encoder 20 for use by a video decoder, such as video decoder 30, in decoding the video data. Such syntax elements may be included with encoded video data sent over a communication medium, stored on a storage medium, or stored on a file server.

The display device 32 may be integrated with the destination device 14 or external to the destination device 14. In some examples, destination device 14 may include an integrated display device and also be configured to interface with an external display device. In other examples, destination device 14 may be a display device. In general, display device 32 displays the decoded video data to a user, and may be any of a variety of display devices, such as a Liquid Crystal Display (LCD), a plasma display, an Organic Light Emitting Diode (OLED) display, or another type of display device.

Video encoder 20 and video decoder 30 may operate in accordance with a video compression standard, such as the High Efficiency Video Coding (HEVC) standard. Video encoder 20 and video decoder 30 may additionally operate in accordance with HEVC extensions such as range extensions, multiview extensions (MV-HEVC), or scalable extensions (SHVC) that have been developed by the video coding joint collaboration team of the ITU-T Video Coding Experts Group (VCEG) and the ISO/IEC Moving Picture Experts Group (MPEG) and the 3D video coding extension development joint collaboration team (JCT-3V). Alternatively, video encoder 20 and video decoder 30 may operate in accordance with other proprietary or industry standards, such as the ITU-T h.264 standard (otherwise known as ISO/IEC MPEG-4), part 10, Advanced Video Coding (AVC), or extensions of such standards, such as Scalable Video Coding (SVC) and multi-view video coding (MVC) extensions. However, the techniques of this disclosure are not limited to any particular coding standard. Other examples of video compression standards include ITU-T H.261, ISO/IEC MPEG-1 visualizations, ITU-T H.262 or ISO/IEC MPEG-2 visualizations, ITU-T H.263, and ISO/IEC MPEG-4 visualizations. HEVC (ITU-T h.265), including its range extension, multi-view extension (MV-HEVC) and scalable extension (SHVC), is developed by the video coding joint collaboration team (JCT-VC) of the ITU-T Video Coding Experts Group (VCEG) and the ISO/IEC Moving Picture Experts Group (MPEG) and the 3D video coding extension development joint collaboration team (JCT-3V). The finalized HEVC draft (hereinafter HEVC) is available at http:// phenix. int-evry. fr/jct/doc _ end _ user/documents/14_ Vienna/wg 11/JCTVC-N1003-v 1. zip.

ITU-T VCEG (Q6/16) and ISO/IEC MPEG (JTC 1/SC 29/WG 11) are now studying the potential needs for standardization of future video coding technologies with compression capabilities potentially exceeding those of the current HEVC standard (including current and recent extensions for screen content coding and high dynamic range coding). The team is jointly conducting this exploration activity under a joint collaborative effort called joint video exploration team (jfet) to evaluate the compression technology design proposed by their experts in the field. Jvt meets the first year in 2015, 10 months, 19 days to 21 days. The latest version of the reference software (i.e. joint exploration model 7 (JEM 7)) can be downloaded from the following links: https:// jvet. hhi. fraunhofer. de/svn/svn _ HMJEMSofwar/tags/HM-16.6-7.0/. The description of the algorithm for JEM7 is described in the following: chen, e.alshina, g.j. Sullivan, j. -r.ohm, j.boyce, "algorithmdescription of Joint Exploration Test Model 7 (JEM 7)" jmet-G1001, torhino, july 2017.

In the examples of the present disclosure described below, video encoder 20 and video decoder 30 may operate in accordance with one or more versions of the ITU-T h.266 standard being developed, also referred to as multi-function video coding (VVC). The Draft of the VVC standard is found in Bross et al, "Versatile Video Coding (Draft 7)" ("multifunctional Video Coding (Draft 7)"), Union Video experts team (JFET) of ITU-T SG 16WP 3 and ISO/IEC JTC 1/SC 29/WG 11, conference 16: geneva, Switzerland, 2019, 10/month 1 to 11, JVET-P2001-v14 (hereinafter referred to as "VVC draft 7"). However, the techniques of this disclosure are not limited to any particular coding standard.

Although not shown in fig. 1, in some aspects, video encoder 20 and video decoder 30 may each be integrated with an audio encoder and decoder, and may include appropriate multiplexing-demultiplexing units or other hardware and software to handle encoding of both audio and video in a common data stream or separate data streams. If applicable, in some examples, the mux-demux units may conform to the ITU h.223 multiplexer protocol or other protocols such as the User Datagram Protocol (UDP).

Video encoder 20 and video decoder 30 may each be implemented as any of a variety of suitable encoder circuits or decoder circuits, 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 the techniques are implemented in part in software, the 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 techniques of this disclosure. Each of video encoder 20 and video decoder 30 may be included in one or more encoders or decoders, either of which may be integrated as part of a combined encoder/decoder (CODEC) in the respective device.

As will be explained in more detail below, video encoder 20 may be configured to: for a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, encoding a first syntax element for indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles; encoding a plurality of first adaptive parameter set indices for a luma block in one or more of a picture, a slice, a tile, or a tile group based on a number of the adaptive parameter sets for luma blocks in one or more of the picture, the slice, the tile, or the tile group; and for a video bitstream for which adaptive loop filtering is enabled for chroma blocks in one or more of a picture, a slice, a tile, or a tile group, encoding a second adaptive parameter set index for chroma blocks in one or more of the picture, slice, tile, or tile group.

In a reciprocal manner, video decoder 30 may be configured to: decoding, from a video bitstream for which adaptive loop filtering is enabled for luma blocks in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element for indicating a number of adaptive parameter sets for luma blocks in one or more of a picture, a slice, a tile, or a group of tiles; decoding a plurality of first adaptive parameter set indices for a luma block in one or more of a picture, a slice, a tile, or a tile group based on a number of the adaptive parameter sets for luma blocks in one or more of the picture, the slice, the tile, or the tile group; decoding, from a video bitstream for which adaptive loop filtering is enabled for chroma blocks in one or more of a picture, slice, tile, or group of tiles, a second adaptive parameter set index for chroma blocks in one or more of a picture, slice, tile, or group of tiles; applying a first adaptive loop filter to a luma block in one or more of a picture, a slice, a tile, or a group of tiles based on a plurality of first adaptive parameter set indices; and applying a second adaptive loop filter to chroma blocks in one or more of the picture, slice, tile, or group of tiles based on the second adaptive parameter set index.

In HEVC, VVC, and other video coding specifications, considerA frequency sequence typically comprises a series of pictures. Pictures may also be referred to as "frames". In one example approach, a picture may include three sample arrays, denoted as S_L、S_CbAnd S_Cr. In such an exemplary manner, S_LIs a two-dimensional array (i.e., block) of luminance samples. S_CbIs a two-dimensional array of Cb chroma samples. S_CrIs a two-dimensional array of Cr chroma samples. Chroma samples may also be referred to herein as "chroma" samples. In other examples, the picture may be monochrome and may include only an array of luma samples.

To generate an encoded representation of a picture, video encoder 20 may generate a set of Coding Tree Units (CTUs). Each of the CTUs may include a coding tree block of luma samples, two corresponding coding tree blocks of chroma samples, and syntax structures for coding the samples of the coding tree blocks. In a monochrome picture or a picture with three separate color planes, a CTU may include a single coding tree block and syntax structures used to code the samples of the coding tree block. The coding tree block may be an NxN block of samples. A CTU may also be referred to as a "treeblock" or a "largest coding unit" (LCU). The CTU of HEVC may be substantially similar to macroblocks of other standards, such as h.264/AVC. However, a CTU is not necessarily limited to a particular size and may include one or more Coding Units (CUs). A slice may include an integer number of CTUs arranged consecutively in raster scan order.

To generate a coded CTU, video encoder 20 may recursively perform quadtree partitioning on the coding tree blocks of the CTU to divide the coding tree blocks into coding blocks, hence the name "coding tree unit". The coded block may be an NxN block of samples. A CU may include a coding block of luma samples and two corresponding coding blocks of chroma samples in a picture having a luma sample array, a Cb sample array, and a Cr sample array, and syntax structures for coding samples of the coding block. In a monochrome picture or a picture with three separate color planes, a CU may include a single coding block and syntax structures used to code the samples of the coding block.

Video encoder 20 may divide the coding block of a CU into one or more prediction blocks. A prediction block is a rectangular (i.e., square or non-square) block of samples on which the same prediction is applied. A Prediction Unit (PU) in a CU may include a prediction block of luma samples, two corresponding prediction blocks of chroma samples, and a syntax structure for predicting the prediction block. In a monochrome picture or a picture with three separate color planes, a PU may include a single prediction block and syntax structures for predicting the prediction block. Video encoder 20 may generate predictive luma, Cb, and Cr blocks for the luma, Cb, and Cr prediction blocks for each PU in the CU.

Video encoder 20 may generate the prediction block for the PU using intra prediction or inter prediction. If video encoder 20 uses intra prediction to generate the prediction block for the PU, video encoder 20 may generate the prediction block for the PU based on decoded samples of the picture associated with the PU. If video encoder 20 uses inter prediction to generate the prediction block for the PU, video encoder 20 may generate the prediction block for the PU based on decoded samples associated with the PU for one or more pictures other than the picture.

After video encoder 20 generates predictive luma, Cb, and Cr blocks for one or more PUs in a CU, video encoder 20 may generate a luma residual block for the CU. Each sample in the luma residual block of the CU indicates a difference between a luma sample in one of the predictive luma blocks of the CU and a corresponding sample in the original luma coding block of the CU. Further, video encoder 20 may generate a Cb residual block for the CU. Each sample in the Cb residual block of the CU may indicate a difference between the Cb sample in one of the predictive Cb blocks of the CU and the corresponding sample in the original Cb coding block of the CU. Video encoder 20 may also generate a Cr residual block for the CU. Each sample in the Cr residual block of the CU may indicate a difference between a Cr sample in one of the predictive Cr blocks of the CU and a corresponding sample in the original Cr coding block of the CU.

In addition, video encoder 20 may use quadtree partitioning to decompose the luma, Cb, Cr residual block of a CU into one or more luma, Cb, and Cr transform blocks. A transform block is a rectangular (e.g., square or non-square) block of samples on which the same transform is applied. A Transform Unit (TU) of a CU may include a transform block of luma samples, two corresponding transform blocks of chroma samples, and syntax structures for transforming 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. The luma transform block associated with a TU may be a sub-block of a luma residual block of a CU. The Cb transform block may be a sub-block of a Cb residual block of the CU. The Cr transform block may be a sub-block of the Cr residual block of the CU. In a monochrome picture or a picture with three separate color planes, a TU may include a single transform block and syntax structures used to transform the samples of the transform block.

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. The coefficient block may be a two-dimensional array of transform coefficients. The transform coefficients may be scalars. Video encoder 20 may apply one or more transforms to Cb transform blocks of a TU to generate Cb coefficient blocks for the TU. Video encoder 20 may apply one or more transforms to a Cr transform block of a TU to generate a Cr coefficient block for the TU.

Block structures with CTUs, CUs, PUs, and TUs have been written above to generally describe the block structure used in HEVC. However, other video coding standards may use different block structures. As one example, while HEVC allows PUs and TUs to have different sizes or shapes, other video coding standards may require that prediction blocks and transform blocks have the same size. The techniques of this disclosure are not limited to block structures of HEVC or VVC, and may be compatible with other block structures.

After generating the coefficient block (e.g., a luminance coefficient block, a Cb coefficient block, or a Cr coefficient block), video encoder 20 may quantize the coefficient block. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the transform coefficients, thereby providing further compression. After video encoder 20 quantizes the coefficient block, video encoder 20 may entropy encode syntax elements indicating the quantized transform coefficients. For example, video encoder 20 may perform Context Adaptive Binary Arithmetic Coding (CABAC) on syntax elements used to indicate the quantized transform coefficients.

Video encoder 20 may output a bitstream that includes a sequence of bits that constitute a representation of the coded pictures and associated data. The bitstream may include a series of Network Abstraction Layer (NAL) units. A NAL unit is a syntax structure that contains an indication of the type of data in the NAL unit and bytes that contain the data in the form of a Raw Byte Sequence Payload (RBSP) with emulation prevention bits interspersed as necessary. Each of the NAL units includes a NAL unit header and an encapsulated RBSP. The NAL unit header may include a syntax element indicating a NAL unit type code. The NAL unit type code specified by the NAL unit header of the NAL unit indicates the type of the NAL unit. An RBSP may be a syntax structure that contains an integer number of bytes encapsulated within a NAL unit. In some cases, the RBSP includes zero bits.

Different types of NAL units may encapsulate different types of RBSPs. For example, a first type of NAL unit may encapsulate RBSPs for a Picture Parameter Set (PPS), a second type of NAL unit may encapsulate RBSPs for a coded slice, a third type of NAL unit may encapsulate RBSPs for a Supplemental Enhancement Information (SEI) message, and so on. NAL units that encapsulate RBSPs for video coding data (as opposed to RBSPs for parameter sets and SEI messages) may be referred to as Video Coding Layer (VCL) NAL units.

Video decoder 30 may receive the bitstream generated by video encoder 20. In addition, video decoder 30 may parse the bitstream to obtain syntax elements from the bitstream. Video decoder 30 may reconstruct pictures of the video data based at least in part on syntax elements obtained from the bitstream. The process for reconstructing the video data may be generally reciprocal to the process performed by video encoder 20. Furthermore, video decoder 30 may inverse quantize coefficient blocks associated with TUs of the current CU. Video decoder 30 may perform an inverse transform on the coefficient blocks to reconstruct transform blocks associated with TUs of the current CU. Video decoder 30 may reconstruct the coding block of the current CU by adding samples of a prediction block for a PU of the current CU to corresponding samples of a transform block of a TU of the current CU. By reconstructing the coded blocks for each CU of a picture, video decoder 30 may reconstruct the picture.

Aspects of HEVC, VVC and JEM techniques will now be discussed. Fig. 2 shows an example block diagram of an HEVC decoder 31. The video decoder 31 shown in fig. 2 may correspond to the video decoder 30 described above, the video decoder 30 being described in more detail below. HEVC employs two in-loop filters, including a deblocking filter (DBF) and SAO. Additional details regarding HEVC decoding and SAO are described in c.fu, e.alshina, a.alshin, y.huang, c.chen, chia.tsai, c.hsu, s.lei, j.park, w.han, "Sample adaptive offset in the HEVC standard" IEEE trans. Circuits system for video (IEEE academic systems video technology), 22(12): 1755-.

As shown in fig. 2, the input to the DBF may be a reconstructed image after intra-prediction or inter-prediction, as shown with the output from the reconstruction block. The DBF performs detection of artifacts at coded block boundaries and attenuates the artifacts by applying selected filters. Compared to the h.264/AVC deblocking filter, the HEVC deblocking filter has lower computational complexity and better parallel processing capability, while still achieving significant reduction of visual artifacts. For further examples, see a. Norkin, g.bjontegard, a.fuldseth, m.narroschke, m.ikeda, k.andersson, Minhua Zhou, g.van der auwer a, "HEVC Deblocking Filter," IEEE trans. circuits system of video technology, 22(12): 1746-.

In HEVC, deblocking filter decisions are made separately for each boundary of four sample lengths that lies on a grid that divides a picture into blocks of 8x8 samples. Deblocking is performed on block boundaries if the following conditions are true: (1) a block boundary is a Prediction Unit (PU) or Transform Unit (TU) boundary; (2) a boundary strength (Bs) as defined in table 1 below is greater than zero; and (3) the change in signal on both sides of the block boundary as defined in equation (1) below is below a specified threshold.

Table 1. Intensity (Bs) value for a boundary between two adjacent luminance blocks

If Bs > 0 for a luma block boundary, then deblock filtering is applied to the boundary if the following conditions are valid:

|p_2，0-2p_1，0+p_0，0|+|p_2，3-2p_1，3+p_0，3|+|q_2，0-2q_1，0+q_0，0|+ |q_2，3-2q_1，3+q_0，3|＜β (1)

HEVC considers two types of luma deblocking filters, namely: (i) a normal filter and (ii) a strong filter. The selection of a deblocking filter depends on whether a particular signal variation term is less than certain thresholds (see the "HEVC deblocking filter" by Norkin et al (2012) cited above). Although the filtering decision is based only on two rows (columns) of a vertical (or horizontal, as the case may be) boundary that is four pixels long, the filter is applied to each row (or column, as the case may be) in the boundary. The number of pixels used in the filtering process and the number of pixels that can be modified with each type of filtering are summarized in table 2 below.

TABLE 2 number of pixels used/modified per boundary in HEVC deblocking

Chroma deblocking is performed only if Bs equals two (2). Only one type of chroma deblocking filter is used. Chroma deblocking filter use imagePrime p₀，p₁，q₀，q₁And the pixels p in each row can be modified₀And q is₀(for the sake of brevity, the second subscript used to indicate the row index is omitted in the above description because the filter is applied to each row). In JEM, deblocking is performed at the CU level. The size of CUs on either side of the boundary may be larger than 8x 8. The minimum CU size in JEM is 4 × 4. Therefore, the deblocking filter can also be applied to the boundary of the 4x4 block.

The input to the SAO may be the reconstructed picture after applying the deblocking filter, as shown with the output from the deblocking filter in fig. 2. The concept/idea of SAO is to reduce the average sample distortion of a region by first classifying the region samples into a plurality of classes with a selected classifier, obtaining an offset for each class, and then adding an offset to each sample of a class, wherein the classifier index and the offset of the region are decoded in the bitstream. In HEVC, a region (unit for SAO parameter signaling) is defined as a CTU.

Two SAO types that can meet the requirements of low complexity are employed in HEVC. These two types are Edge Offset (EO) and Band Offset (BO), which will be discussed in further detail below. The index of the SAO type is decoded. For EO, the sample classification is based on a comparison between the current sample and neighboring samples according to the following 1-D pattern: horizontal, vertical, 135 diagonal and 45 diagonal.

3A-3D show four 1-D patterns for EO sample classification: horizontal (fig. 3A, EO-type 0), vertical (fig. 3B, EO-type 1), 135 ° diagonal (fig. 3C, EO-type 2), and 45 ° diagonal (fig. 3D, EO-type 3). Additional details related to SAO are c.fu, e.alshina, a.alshin, y.huang, c.chen, chia.tsai, c.hsu, s.lei, j.park, w.han, "Sample adaptive offset in the HEVC standard" IEEE trans.circuits system of video technology (IEEE proceedings video technology), 22(12):1755 and 1764 (2012).

Five categories, represented in Table 3 as edge indices (edgeIdx), are further defined according to the selected EO style. For edge indices equal to 0-3, the magnitude of the offset may be signaled when the sign flag is implicitly coded (i.e., a negative offset for the edge index is equal to 0 or 1 and a positive offset for the edge index is equal to 2 or 3). For an edge index equal to 4, the offset is always set to 0, which means that no operation is required in this case.

Table 3: classification for EO

For BO, the sample classification is based on the sample value. Each color component may have its own SAO parameter for classification for BO type SAO filtering. BO means adding one offset to all samples of the same band. The sample value range is divided equally into 32 bands. For 8 bit samples ranging from 0 to 255, the width of the frequency band is 8, and sample values from 8k to 8k +7 belong to the frequency band k, where k ranges from 0 to 31. An offset is added to all samples of the same frequency band. The average difference between the original and reconstructed samples in the band (i.e., the offset of the band) is signaled to a decoder (e.g., video decoder 30). There is no constraint on the offset sign. Only the offsets and starting band positions of four (4) consecutive bands are signaled to a decoder, e.g., video decoder 30.

Video encoder 20 and video decoder 30 may be configured to implement the various ALF filtering techniques set forth in JEM. Aspects of these JEM filtering techniques (e.g., ALF) will now be described. In addition to the modified deblocking and HEVC SAO methods, JEM includes another filtering method, referred to as geometric transform-based adaptive loop filtering (GALF). The input to the ALF/GALF may be a reconstructed image after application of the SAO (e.g., the output of the sample adaptive offset in fig. 2). Aspects of GALF are described in Tsai, c.y., Chen, c.y., Yamakage, t., Chong, i.s., Huang, y.w., Fu, c.m., Itoh, t., Watanabe, t., Chujoh, t., karcewicz, m.and Lei, s.m., "Adaptive loop filtering for video Coding" IEEE Journal of Selected Topics in Signal Processing Journal, 7(6), pages 934-945, 2013 and in m.karcewicz, l.zhang, w.j. Chien and x.li, "Geometry transform-in-block-filter" (Adaptive filter for Picture Coding) in geometric Coding (PCS 2016), in geometric Coding based on image transformation).

The ALF technique attempts to minimize the mean square error between the original samples and the decoded samples by using an adaptive wiener filter. The input image is denoted as p, the source image as S and the FIR (finite impulse response) filter as h. The following expression for the Sum of Squared Errors (SSE) should then be minimized, where (x, y) represents an arbitrary pixel location in p or S.

SSE＝∑_x，y(∑_i，jh(i，j)p(x-i，y-j)-S(x，y))²

Is denoted by h_optCan be obtained by setting the partial derivative of SSE with respect to h (i, j) equal to 0, as follows:

this results in the wiener-hopplev equation shown below, which gives the optimal filter h_opf：

∑_i，jh_opt(i，j)(∑_x，yp(x-i，y-j)p(x-m，y-n))＝∑_x，yS(x，y)p(x- m，y-n)

In some examples of JEM, samples in a picture are classified into twenty-five (25) classes based on local gradients, rather than using one filter for the entire picture. A separate optimal wiener filter is derived for the pixels in each class. Several techniques have been employed to improve the effectiveness of ALF by reducing signaling overhead and computational complexity. The following lists some of the techniques that have been used to improve ALF effectiveness by reducing signaling overhead and/or computational complexity:

1. prediction from fixed filter: the optimal filter coefficients for each class are predicted using a prediction pool of fixed filters that includes 16 candidate filters for each class. The optimal prediction candidate is selected for each class, and only the prediction error is sent.

2. Class merging: rather than using twenty-five (25) different filters (one for each class), pixels in multiple classes may share one filter in order to reduce the number of filter parameters to be decoded. Merging the two classes may result in higher accumulated SSE, but lower rate-distortion (R-D) cost.

3. Variable number of taps: the number of filter taps is adaptive at the frame level. Theoretically, filters with more taps may achieve lower SSE, but may not be a good choice in terms of rate-distortion (R-D) cost due to the bit overhead associated with more filter coefficients.

4. Block level on/off control: the ALF can be turned on and off on a block basis. The block size at which the on/off control flag is signaled is adaptively selected at the frame level. The filter coefficients may be recalculated using pixels from those blocks for which only ALF is on.

5. And (3) time prediction: filters derived for previously coded frames are stored in a buffer. If the current frame is a P-frame or a B-frame, then one of the stored filter sets may be used to filter the frame if it results in a better RD cost. The flag is signaled to indicate the use of temporal prediction. If temporal prediction is used, an index indicating which set of stored filters to use is signaled. No additional signaling of the ALF coefficients is required. The block level ALF on/off control flag may also be signaled for a frame using temporal prediction

Details of some aspects of ALF are briefly summarized in this and the following paragraphs. Some aspects of ALF relate to pixel classification and geometric transformation. The sum of the absolute values of the vertical, horizontal and diagonal laplacian at all pixels within the 6x6 window covering each pixel in the reconstructed frame (before ALF) is calculated. The reconstructed frame is then divided into non-overlapping 2x2 blocks. Four pixels in these blocks are classified into twenty-five (25) classes (denoted as C) based on the total Laplacian activity and directionality of the block_k(k ═ 0, 1.., 24)). Furthermore, one of the four geometric transformations (no transformation, diagonal flip, vertical flip, or rotation) is also applied to the filter based on the gradient directionality of the block. Details can be found in m.karczewicz, l.zhang, w.j.chien, and x.li, "Geometry transformation-based adaptive in-loop filter (geometric transform-based adaptive in-loop filter)" Picture Coding Symposium (PCS), 2016.

Some aspects of ALF involve filter derivation and prediction from fixed filters. For each class C_kThe optimal prediction filter is first derived for C based on the SSE given by the filter_kIs selected from the pool of (1), denoted as h_pred，k. C to be minimized_kThe SSE of (A) can be written as follows:

SSE_k＝∑_x，y(∑_i，j(h_pred，k(i，j)+h_Δ，k(i，j))p(x-i，y-j)-S(x，y))²，k＝ 0，...，24，(x，y)∈C_k

wherein is h_Δ，kIs directed to C_kAnd h is the optimum filter_pred，kThe difference between them. Let p' (x, y) be Σ_i，jh_pred，k(i, j) p (x-i, y-j) is represented by h_pred，kThe result of filtering the pixel p (x, y). Then for SSE_kThe expression of (c) can be rewritten as:

by making SSE_kRelative to h_Δ，k(i, j) partial derivative equal to 0, obtaining a modified wiener-HoThe Poff equation, as follows:

k＝0，...，24，(x，y)∈C_k

for the sake of simplicity of expression, by R respectively_pp，k(i-m, j-n) and R'_ps，k(m, n) represents a group having (x, y) ∈ C_kSigma of_x，yp (x-i, y-j) p (x-m, y-n) and ∑_x，y(S (x, y) -p' (x, y)) p (x-m, y-n). Then, the above equation can be written as:

∑_i，jh_Δ，k(i，j)R_pp，k(i-m，j-n)＝R′_ps，k(m，n)k＝0，...，24 (1)

for each C_kAt all (x, y) ∈ C_kUpper computation autocorrelation matrix R_pp，k(i-m, j-n) and a cross-correlation vector R'_ps，k(m，n)。

In one example of ALF, the difference between only the optimal filter and the fixed prediction filter is calculated and sent. If none of the candidate filters available in the pool is a good predictor, the identified filter (i.e., the filter with only one non-zero coefficient equal to 1 at the center making the input and output identical) will be used as the predictor.

Some aspects of ALF involve the merging of pixel classes. Classes are combined to reduce the overhead of signaling filter coefficients. The cost of merging two classes is increased relative to SSE. Consider having passed SSE_mAnd SSE_nTwo classes C of SSE given separately_mAnd C_n. Is provided with C_m+nIs shown by mixing C_mAnd C_nMerging with SSE (denoted as SSE)_m+n) And the obtained class. SSE_m+nIs always greater than or equal to SSE_m+ SSE_n. Let Δ SSE_m+nIs shown by mixing C_mAnd C_nIncrease in SSE caused by incorporation, Δ SSE_m+nIs equal to SSE_m+n-(SSE_m+SSE_n). To calculate SSE_m+nThose skilled in the art need to derive h using the following expression similar to (1)_Δ，m+n(for C)_m+nFilter prediction error of): sigma_i，jh_Δ，m+n(i，j)(R_pp，m(i-u，j-v)+R_pp，n(i-u，j-v))＝R′_ps，m(u，v)+ R′_ps，n(u，v) (2)

For merged class C_m+nThe SSE of (A) can be calculated as:

SSE_m+n＝-∑_u，vh_Δ，m+n(u，v)(R′_ps，m(u，v)+R′_ps，n(u，v))+(R_ss，m+R_ss，n)

to reduce the number of classes from N to N-1, it may be necessary to find two classes C_mAnd C_nSuch that combining them results in a minimum Δ SSE compared to any other combination_m+n. Some ALF designs examine each pair of available classes for merging to find the pair with the smallest merging cost.

If merge C_mAnd C_n(having m < n), then C_nMarked as unavailable for further merging, and for C_mIs modified into a combined autocorrelation and cross-correlation as follows:

R_pp，m＝R_pp，m+R_pp，n

R′_ps，m＝R′_ps，m+R′_ps，n

R_ss，m＝R_ss，m+R_ss，n

the optimal number of ALF classes after merging needs to be decided for each frame based on the RD cost. This is done by starting with twenty-five (25) classes and merging pairs of classes (from the set of available classes) in turn until only one class remains. For the number of each possible class (1, 2...., 25) remaining after merging, a map is stored indicating which classes are merged together. The optimal number of classes is then chosen such that the RD cost is minimized, as follows:

wherein D < u >_NIs a total SSE using N classesR|_NIs the total number of bits used to code the N filters, and λ is a weighting factor determined by a Quantization Parameter (QP). Sending is for N_optA merge map of the number of classes indicating which classes are merged together.

Aspects of the signaling of the ALF parameters are described below. A brief step-wise description of the ALF parameter encoding process is given below:

1. the frame level ALF on/off flag is signaled.

2. If the ALF is ON, a temporal prediction flag is signaled.

3. If temporal prediction is used, the index of the frame from which the current frame is filtered using the corresponding ALF parameters is signaled.

4. If temporal prediction is not used, then the auxiliary ALF information and filter coefficients are signaled as follows:

a. the following ancillary ALF information is signaled before the filter coefficients are signaled.

i. The number of unique filters used after class merging.

The number of filter taps.

Merging information indicating which classes share filter prediction errors.

indices of the fixed filter predictor for each class.

b. After signaling the auxiliary information, the filter coefficient prediction error is signaled as follows:

i. a flag is signaled to indicate whether the filter prediction error is forced to zero (0) for some of the remaining classes after merging.

A flag is signaled to indicate (if the number of classes left after combining is greater than one (1)) whether differential decoding is used to signal the filter prediction error.

The filter coefficient prediction error is then signaled using a kth order exponential golomb code, where the k values for different coefficient positions are empirically selected.

c. The filter coefficients for the chroma components, if available, are coded directly without any prediction method.

5. Finally, the block level ALF on/off control flag is signaled.

The ALF filter parameters are signaled in the APS in the VVC. In ALF APS in VVC, the signaling may include the following:

1. signaling whether ALF APS includes a luminance ALF filter

2. If the ALF APS includes a luminance ALF filter, then

a. Signaling the number of unique brightness filters used after class merging

b. Signaling class merging information indicating which classes share the same filter

c. Signaling a marker if clipping is applied to a luminance ALF filter

d. After a-c, the filter coefficients are signaled and if clipping is applied to the luminance ALF filter, the clipping parameters are signaled.

3. Signaling whether ALF APS includes chroma ALF filter

a. Signaling the number of unique chrominance filters

b. Signaling a marker if clipping is applied to a chroma ALF filter

c. After a-b, the filter coefficients are signaled and if clipping is applied to the luminance ALF filter, the clipping parameters are signaled

In VVC, the ALF APS index is used in the picture or signaled in the picture and/or slice header.

Brightness:

1. signaling picture/slice level luminance ALF on/off flag

2. If the ALF is on for luma, the number of APSs applied to the luma component of the current picture/slice is signaled.

3. If the number of APS is greater than 0, each APS index applied to the luma component of the current picture/slice is signaled.

Chroma:

1. signaling picture/slice level chroma ALF on/off information

2. If the ALF is on for any chroma component, the APS index applied to the chroma component of the current picture/slice is signaled.

3. If the ALF is on for chroma CTB, then the filter index is signaled.

A Coding Tree Block (CTB) level on/off flag and filter index information are also signaled. Brightness:

an ALF on/off flag for luminance CTB is signaled.

If ALF is on for luminance CTB, then either the fixed filter set index or APS index is signaled.

Chroma:

an ALF on/off flag for chroma CTBs is signaled.

In some examples of JEM, the design of ALF may present one or more potential problems. As one example, some example ALF designs perform multiple passes over each frame to design one set of filters for the entire frame (one filter for each class of pixels or one filter shared among multiple classes in the frame). This introduces high encoder latency. This is particularly problematic in low latency applications (such as video conferencing), where it may be important to send even partially encoded frames to the channel as quickly as possible.

As another example, according to some ALF designs, one filter set is used for the entire picture. The local statistics in the small blocks of the original and reconstructed pictures may be different from the cumulative statistics obtained using the entire picture. Thus, an ALF filter that is optimal for the entire picture may not be optimal for a given block.

As another example, a potential problem in the case of using small blocks of a picture to design a new wiener filter set for better local adaptivity is that the number of pixels available in a small block may not be sufficient to get a good estimate of the correlation matrix and vector. This may lead to ill-posed wiener-hopplev equations, which in turn may not give good ALF coefficients.

As another example, some example ALF designs define sixteen (16) fixed filters for each of twenty-five (25) classes, resulting in a total of four hundred (400) filters. These filters can be used as prediction filters for the final filter coefficients of each class. The indices of the prediction filters used are signaled for each class. This may result in high signaling overhead and lower overall coding gain.

Another potential drawback of using a fixed set of filters as predictors is that the set of predictors is not modified based on new filters designed for previous frames. Since temporally adjacent frames are likely to have similar statistics, using an optimal filter for a previous frame may result in an efficient prediction of the optimal filter for the current frame.

As another example, some example ALF designs require two passes over the current frame to make block level filter on/off decisions. This introduces additional encoder latency. The ALF on/off flag is signaled block misaligned with the Coding Unit (CU). Therefore, CU information such as mode, Coded Block Flag (CBF), etc. cannot be considered in the ALF on/off control decision. Using this information may reduce the on/off signaling overhead.

To address one or more of the problems discussed above, this disclosure describes techniques for further improving coding gain and visual quality obtained by ALF. Video encoder 20 and/or video decoder 30 may apply any of the following itemized techniques separately. Alternatively, video encoder 20 and/or video decoder 30 may apply any combination of the itemized techniques discussed below.

In accordance with some techniques of this disclosure, video encoder 20 may signal the set of ALF filters for each block in one picture/slice/tile group. In one example, an ALF set (e.g., the ALF filter set mentioned above) may be indicated by a set index in a list of filter sets. In some examples, the index identifies a particular ALF filter set from among a plurality of ALF filter sets included in a list.

a. In one example, the block may be a Coding Tree Unit (CTU) or any other block. The blocks may be decoupled from the partitions.

b. In one example, a list of multiple sets of filters per picture/slice/tile group is provided, where each set may contain each type of assigned filter. The set index may be signaled per block. The flag may be signaled per block to indicate that the ALF is not used.

c. The list of filter sets may contain a fixed set of pre-trained filters and a set of filters derived using previous frames or filters signaled in the bitstream.

According to some examples of the disclosure, video encoder 20 and/or video decoder 30 may share a list of filter sets that span different pictures. In one example, the filter set list may be initialized with pre-trained filters. After coding the picture, video encoder 20 may derive a new filter set based on the encoded picture and add the new filter set to the filter set list. Alternatively, the new filter set may replace an existing filter set in the list. After coding another picture, video encoder 20 may derive another filter set and include the derived filter set in the filter set list. In this example, the filter set list is common to all pictures and may be updated after coding a picture. From the decoder side, video decoder 30 may detect signaling of a new filter set after decoding a picture or before decoding a picture.

According to some examples of the present disclosure, video encoder 20 may allow for signaling of a new set of filters per picture/slice/tile/group of tiles.

a. In one example, a new filter set may be added to the filter set list. The updated list may then be used to filter blocks in the next picture or pictures.

b. In another example, the updated list (containing the new filter set derived using the current picture/slice/tile group) may be used to filter the blocks in the current picture/slice/tile group.

According to some examples of the present disclosure, the filter set list may be updated with filters derived using previous picture/slice/tile groups. The order in which filter sets are added or ordered in the filter set list may be fixed, predefined, or flexible. The list may be reordered per picture based on information about the current picture and information about the picture from which the corresponding filter in the list is derived. Video encoder 20 may use the index in the filter set list to indicate the filter set to video decoder 30. In some examples, video encoder 20 may assign smaller index values to more frequently used filters or to newly added filters.

a. In one example, the newly derived filter set may be added to the beginning of the list. In another example, a set of filters derived using a previous frame may be placed in a list before an existing set of filters in the list (e.g., a fixed set of filters).

b. The ordering of the filter sets in the list may depend on other picture-related information. For example, a filter derived from a picture in the same temporal layer may be placed in a list before a filter derived using a picture in another temporal layer.

c. In one example, the index of a filter set in a list may depend on whether the corresponding picture from which the filter set is derived is a reference picture for prediction of the current picture. Filters corresponding to more frequently used reference pictures may be placed before filters derived from other reference pictures.

d. In one example, a filter derived using a picture coded with a similar QP as the current picture may be placed before a filter derived from a previous picture coded with a different QP.

e. The maximum number of filters in the list may be limited. In one example, up to thirty-two (32) filter sets may be retained in the list. The maximum number of filter sets in the list may be signaled in the slice header, sequence parameter set, picture parameter set, or other high level syntax information, or elsewhere.

f. Video encoder 20 may signal different set indices using different numbers of bits. Fewer bits may be used to indicate lower index positions in the list than higher index positions (since the filter set near the top of the list is more likely to be selected).

According to some examples of the disclosure, some blocks may share the same ALF information, e.g., a merge of ALF information across two or more blocks. In one example, the index of the filter set and/or the ALF on/off flag (which indicates whether ALF applies to a block) may be shared across multiple blocks. The ALF merge indicator may indicate which blocks are merged and which ALF information is associated with the ALF merge indicator. The merge indicator may be an index, a flag, or any other syntax element.

a. The ALF information of a block may be merged with the upper block or with the left block.

b. More flexible ALF information merging, which allows merging of one block with any other block (i.e., not necessarily adjacent blocks) in a picture, may also be used.

In some examples of the present disclosure, the ALF on/off flag may be derived based on other existing block information. In one example, video decoder 30 may derive the ALF on/off flag based on existing block information and, as such, video encoder 20 may not signal the ALF on/off flag. Video encoder 20 may signal an ALF on/off signal for a set of blocks represented as ALF blocks. For example, blocks sharing the same ALF on/off flag may represent ALF blocks. In another example, an ALF block may be equal to a block.

a. In one example, the ALF on/off flag may be derived based on the number of blocks in an ALF block that share the same ALF on/off flag, with a non-zero CBF flag. If the number of non-zero CBF flags is less than a certain threshold, then ALF may be disabled or a default ALF filter applied to the blocks.

b. In the above example of the sub-bullets 'a', the number of non-zero transform coefficients may be counted instead of the CBF flag. A threshold may be introduced for the counted coefficients and if the number of non-zero transform coefficients is less than the threshold, then ALF may be disabled for blocks included in the ALF blocks or a default ALF filter may be applied to these blocks.

c. In another example, if the number of blocks in an ALF block that are coded with skip mode is greater than a certain threshold, ALF may be disabled for those blocks.

d. In the example of the sub-bullets 'c' above, the skip mode is used as an example, and other decoding modes may be utilized in deriving the ALF on/off flag.

As discussed above, when using ALF, the block-based signaling mechanism may improve coding efficiency. For example, video encoder 20 may signal a filter set index for each block to indicate which of the candidate sets in the filter set is used for the block. The candidate set may include various filter sets, such as a newly signaled filter set for a current picture/slice/tile group and/or a filter set from a previously coded picture/slice/tile group.

In U.S. patent application No. 16/567,966, filed on 9, 11, 2019, methods and systems for storing a filter set of previously coded pictures are discussed. In the example described in us patent application 16/567,966, luminance and chrominance may have separate buffers. In one example, ALF coefficients of previously coded pictures are stored and allowed to be reused as ALF coefficients for a current picture. For the current picture, video encoder 20 may choose to use the ALF coefficients stored for the reference picture and then bypass ALF coefficient signaling. In this case, video encoder 20 signals the index to only one of the reference pictures, and the stored ALF coefficients of the indicated reference picture are simply inherited (e.g., reused) for the current picture. To indicate the use of temporal prediction, video encoder 20 first codes the flag before sending the index.

In some examples of JEM (e.g., JEM7), ALF parameters from up to six previous pictures/slices are stored in separate arrays for each temporal layer. For example, if there are 5 temporal layers in the layered B/P coding structure (which is the case in the random access setting used in current video coding standardization), then both video encoder 20 and video decoder 30 use a 5x6 memory array so that there are 30 memory cells in total to store the previously obtained ALF parameters.

The design of JEM7 effectively deletes stored ALF parameters when encoding/decoding intra random access pictures (IRAP or I-frames). To avoid repetition, the ALF parameters are stored in memory only when they are newly obtained by reception at video decoder 30 (for video encoder 20, the new parameters are obtained via estimation/training). The storage of parameters operates in a first-in-first-out (FIFO) manner; thus, if the array is full, the new set of ALF parameter values overwrites the oldest parameter in decoding order.

The main purpose of using a 2D array (e.g., in memory) to store ALF parameters is to preserve temporal scalability in the layered B/P frame coding structure. For thei<k at T_iThe frame at a layer cannot depend on the frame at layer T_kFrame (e.g., cannot be selected from at layer T)_kPredicted from the frame of (a). In other words, at a lower temporal level (e.g., T)₂) Cannot depend on the frame/slice being at a higher layer (e.g., T)₃And T₄) Frame/slice at (c). Current temporal prediction in ALF preserves temporal scalability by simply storing ALF parameters obtained from different temporal layers in different rows of the 2-D array, and ensuring that these parameters are used without disrupting the dependency structure in the layered B/P frames.

One of the problems in the design of time prediction in JEM7 is that it requires a large amount of memory. In particular, a 5 × 6 array having 30 storage units is required to store the ALF parameters at both the video encoder 20 and the video decoder 30. In view of this problem, the examples described below can reduce memory requirements by using 1D arrays, while still maintaining temporal scalability. The following sections give a description of examples for storing and using 1D arrays in ALF for temporal prediction.

FIG. 4A illustrates a method for storing ALF parameters (P)₁，P₂，……，P_N) And associated temporal layer ID (tld)₁，tld₂，……，tld_n) A single array 120 of size N. Each storage unit of the array 120 stores (i) ALF parameters and (ii) temporal layer ids (tid) indicating from which layer the corresponding ALF parameters are estimated. Temporal layer information is used to ensure that ALF parameters obtained from a higher temporal layer (e.g., tId-4) are not used to encode/decode frames/slices at a lower temporal layer (e.g., tId-3).

Combinations of the methods listed below may be applied to load, store, and use of ALF parameters for temporal prediction.

1) For use in temporal prediction of ALF parameters, the array may store parameters from either B-slices or P-slices.

2) In the array, entries for ALF parameters should be stored in a certain order (e.g., in decoding order). When all N entries are used for storage (i.e., when the array is full), the newly obtained parameters may be stored by deleting an entry and then adding the new parameters to the array.

This may be done, as an example, in a FIFO (first in first out) fashion, where the last entry in the array (i.e., the oldest parameter set) is deleted when the array is full,

and the new parameter is stored in the first element of the array.

In another example, the ALF parameters replace certain stored parameters having the same temporal layer ID in the buffer; for example, some of the parameters may be the oldest parameters in the buffer, or

Less frequently used, or any other rule may be applied.

3) A non-negative index value, referred to as prevIdx, may be signaled to identify which set of ALF parameters to load/use from the buffer for encoding/decoding.

A variable length code (such as a unary code) may be used to signal prevIdx. The total available number of parameters for a certain temporal layer Id may be counted in a stored buffer and truncated binarization may be used to signal prevIdx with the total available number of filters minus 1 as the maximum index. However, truncated coding may introduce a mismatch between the encoder and decoder, for example, when some pictures are lost in transmission.

prevIdx can take values from 0 up to N-1. Depending on the type of layered frame structure used for decoding, the maximum value of prevIdx may be smaller.

When coding slices/pictures, the possible candidates for temporal prediction may be decided by traversing the sets included in the array, and all or part of the parameter sets with equal or smaller tId are considered as valid candidates.

The signaling of the array entry (e.g., determining the ALF parameters for decoding) may depend on the temporal layer ID of the current frame being decoded. In particular, prevIdx may correspond to a different entry in the array depending on the temporal layer ID of the current frame being encoded/decoded.

i. FIG. 4B illustrates a method for storing data from different temporal layers (tId)₁＝1，tId₂＝2，tId₃＝2，tId₄1 and tId₅3) of the ALF parameters obtained, N-5 set of the array 130. By way of example, as shown in fig. 4B, prevIdx ═ 1 can point to the following two different entries in the array that depend on tId of the current frame being decoded:

prevIdx-1 corresponds to entry 4 in the array, and when decoding a frame with tId-1, ALF (P) is stored₄1) because it is the second possible option allowed for decoding to preserve temporal scalability, where ALF (P)₄1) is the first candidate signaled with prevIdx ═ 0.

prevIdx-1 corresponds to entry 2 in the array, storing ALF (P) when decoding a frame with tId-2₂2) because it is the second possible option allowed for decoding, where ALF (P) is the first option₁And 1) is the first option corresponding to prevIdx ═ 0.

In case of frame loss (e.g. due to packet loss when sending a video bitstream over a network), the video decoder may choose not to add any entries to the array, and may introduce dummy entries, so even when a picture is lost, dummy entries are added to the buffer. In either case, temporal scalability remains unchanged as long as the methods listed above are applied. In other words, when a frame is lost at a higher level (e.g., T)₃) Frames at lower layers are still decodable (e.g., T)₁And T₂)。

In another example, a picture or slice with a lower temporal layer ID may carry information on the ALF parameters of the higher temporal layer ID. In this case, if a picture with a higher temporal layer ID is lost, the parameter may be obtained from a picture with a lower temporal layer ID. These parameters may also indicate whether the higher temporal layer ID pictures carry such ALF parameters, so that these parameters or dummy parameters may be added to the buffer.

In another example, a Decoder Picture Buffer (DPB) management method may be applied to temporal ALF parameter processing, since DPB management includes processing of lost pictures.

4) Depending on the importance of the ALF parameters (e.g., importance may be measured based on how frequently they are used or based on their temporal layer information), some of these important ALF parameters may be fixed and retained in the buffer until the next I slice is decoded. Such an importance metric may be used to rank and reorder the entries to reduce the signaling overhead (e.g., unary decoding) of signaling prevIdx.

5) Further decisions or restrictions on loading and storing ALF parameters (management of buffers) may be made based on any other side information (in addition to temporal layer IDs) or on importance measures that may also be stored with ALF parameters.

6) Separate buffers may be used for separate coding of the luminance channel and the chrominance channel. As a result, each buffer may have different signaling of prevIdx to determine the ALF parameters for the luma and chroma channels, respectively.

An Adaptive Parameter Set (APS) was proposed in the Joint video experts team (JFET) of Y.Wang et al, "AHG 17: On Header Parameter Set (HPS)" ITU-T SG 16WP 3 and ISO/IEC JTC 1/SC 29/WG 11, conference 13: Malashi, Morocco, 1 month, 9 days-18 days 2019, (hereinafter "JFET-M0132"). In some examples, the adaptive parameter set is also referred to as an adaptive parameter set. APS is used to carry ALF parameters (e.g., both luminance filter parameters and chrominance filter parameters). When the ALF is applied to the picture/slice/tile group, video encoder 20 signals the APS index such that video decoder 30 will use the ALF parameters in the respective APS to apply the ALF to the picture/slice/tile group. However, since the signaling of the APS itself is decoupled from the picture/slice/tile group, it would be desirable to modify the above examples to align with such APS in order to improve coding efficiency. In other words, in some examples, ALF applies to blocks at the slice level, and the particular ALF parameters used apply at the block level. APS, however, are signaled at a higher level. This disclosure describes examples of APS to indicate luma and chroma components that may be used for a slice. In the following example, the previously coded filter may be an ALF or other type of filter.

In some examples, video encoder 20 may be configured to encode and signal a variable (or syntax element, in the following examples, the terms are used interchangeably) in the sequence parameter set/picture parameter set/slice header/tile group header to indicate how many previously coded filter sets may be used for each block corresponding to the sequence parameter set/picture parameter set/slice header/tile group header. Video decoder 30 may be configured to decode such variables/syntax elements and use one or more of the previously coded filter sets for the various blocks.

In one such example, video encoder 20 may encode and signal a variable (e.g., a syntax element in the encoded bitstream) that defines the maximum number of previously signaled filters that video decoder 30 may use for blocks in all components in a sequence/picture/slice/tile group.

In another example, video encoder 20 may encode and signal a variable (e.g., a syntax element in the encoded bitstream) to indicate the maximum number of previously signaled filters that video decoder 30 may use for a luma block in a sequence/picture/slice/tile group. Video encoder 20 may encode and signal another variable (e.g., a syntax element in the encoded bitstream) to indicate the maximum number of previously signaled filters that video decoder 30 may use for a luma block in a sequence/picture/slice/tile group.

In another example, the maximum number of previously signaled filters that a video decoder may use for blocks from all color components (e.g., luma and chroma components) may be fixed and determined without signaling. In another example, the maximum number of previously signaled filters that video decoder 30 may use for blocks from the same color component (e.g., luma and chroma) may be fixed and determined without signaling. In another example, video decoder 30 may use the maximum number of previously signaled filters for blocks from the same color component (e.g., luma or chroma) and having the same coding information, which may be fixed and determined without signaling. The coding information may be a slice type, quantization parameter, prediction type, motion information, intra mode, and/or other coding information.

In some examples (which may be combined with the examples of the previous paragraph, as will be appreciated by those skilled in the art), video encoder 20 and video decoder 30 may use a first-in-first-out (FIFO) buffer to store parameters of previously coded ALF filters. In one example, video encoder 20 and video decoder 30 may be configured to use the same buffer for the luma component and the chroma components such that one element in the buffer is used to store a set of luma and chroma filters from the same picture/slice/tile group. In another example, video encoder 20 and video decoder 30 may be configured to use separate FIFO buffers for the luma component and the chroma components. When the filter parameters for a component are signaled, video decoder 30 will push the signaled filter parameters into the buffer for the corresponding color component.

To align block-based ALF filter set index signaling with the use of one or more APSs that include respective sets of ALF parameters, video encoder 20 and video decoder 30 may be configured to, when ALF is enabled, code a variable (e.g., a syntax element in the encoded bitstream) at each picture, slice, tile, and/or tile group header that indicates a number of APSs for the current picture, slice, tile, and/or tile group header. For example, at the slice level, video encoder 20 and video decoder 30 may be configured to code a variable that indicates the number of APSs for a block of the current slice when ALF is enabled for that slice. Further, after coding the variable indicating the number of APSs, video encoder 20 and video decoder 30 may code an index for each APS available for use by the current picture/slice/tile group header. Then, for each respective block in the slice, video encoder 20 and video decoder 30 may code an index of a particular APS that includes the ALF parameters to be used for the respective block. In this example, video encoder 20 and video decoder 30 may code a set of APS indices for a luminance component and a set of APS indices for a chrominance component.

In one example, to reduce complexity, video encoder 20 and video decoder 30 may be configured to use the same number of APS and the same index of APS for both the luma component and the chroma component. In another example, to better adapt to the characteristics of the luma and chroma components and achieve improved compression efficiency, the luma and chroma components have their own number of APS and APS indices for the blocks of the picture, slice, tile, and/or tile group header. In other words, video encoder 20 and video decoder 30 may be configured to code APS and indices of APS for different numbers of luma and chroma components for a block of a picture, slice, tile, and/or tile group header. In other words, the number of APSs and the indices of the APSs (and thus the ALF parameters) are independent for the luminance component and the chrominance component. In some examples, for simplicity, the number of APSs for a component may be fixed without signaling, which may depend on the coding information. For example, for a picture/slice/tile/group of tiles that is intra coded, no filter sets from other pictures/slices/tiles/group of tiles are allowed. In one example, two chroma components in a picture/slice/tile group may use only one APS. Another example is that each chroma component in a picture/slice/tile group may use only one APS.

According to one example of the present disclosure, video encoder 20 may be configured to encode, for a video bitstream for which adaptive loop filtering is enabled for luminance blocks in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element for indicating a number of adaptive parameter sets for luminance blocks in one or more of a picture, a slice, a tile, or a group of tiles. Video encoder 20 may be further configured to encode a plurality of first adaptive parameter set indices for a luma block in one or more of a picture, a slice, a tile, or a tile group based on a number of the adaptive parameter sets for luma blocks in one or more of the picture, the slice, the tile, or the tile group. For example, if five APSs are used to apply ALF to luma blocks of a slice, video encoder 20 will also encode indices for the five APSs used. Of course, other numbers of APSs may be used for the slices. Video encoder 20 is further configured to encode, for a video bitstream for which adaptive loop filtering is enabled for chroma blocks in one or more of a picture, a slice, a tile, or a tile group, a second adaptive parameter set index for chroma blocks in the one or more of the picture, the slice, the tile, or the tile group. In this case, the chrominance block has only a single APS, and as such, only the index of the APS is encoded and signaled.

Video decoder 30 may be configured to perform techniques reciprocal to those described above for video encoder 20. For example, video decoder 30 may be configured to decode, from a video bitstream for which adaptive loop filtering is enabled for luma blocks in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element indicating a number of adaptive parameter sets for luma blocks in one or more of a picture, a slice, a tile, or a group of tiles. Video decoder 30 may be further configured to decode a plurality of first adaptive parameter set indices for a luma block in one or more of a picture, a slice, a tile, or a tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group. Video decoder 30 may also decode, from the video bitstream for which adaptive loop filtering is enabled for chroma blocks in one or more of the pictures, slices, tiles, or tile groups, a second adaptive parameter set index for the chroma blocks in the one or more of the pictures, slices, tiles, or tile groups. Video decoder 30 may then apply a first adaptive loop filter to the luma block in one or more of the picture, slice, tile, or group of tiles based on the plurality of first adaptive parameter set indices. For example, video decoder 30 may decode syntax elements indicating a particular APS used for a particular luma block in one or more of a picture, slice, tile, or group of tiles, and then apply ALF parameters from the indicated APS to the particular luma block. Likewise, video decoder 30 may apply a second adaptive loop filter to chroma blocks in one or more of the picture, slice, tile, or tile group based on a second adaptive parameter set index. For example, video decoder 30 may apply the indicated ALF parameters of the APS from the second adaptive parameter set index to the chroma block.

In the above examples, video encoder 20 may signal and video decoder 30 may receive a first syntax element in a picture/slice/tile or tile group header that indicates a number of adaptive parameter sets for a luma block in one or more of a picture, slice, tile, or tile group, a plurality of first adaptive parameter set indices for luma blocks in one or more of a picture, slice, tile, or tile group, and a second adaptive parameter set index for chroma blocks in one or more of a picture, slice, tile, or tile group.

In some examples, a video bitstream may be defined to separate the information of the APS such that the APS (luma _ APS) for the luma component is used to carry only the luma filter set and the APS (chroma _ APS) for the chroma component is used to carry only the chroma filter parameters.

Thus, in one example, each of the plurality of adaptation parameter sets for a luma block in one or more of a picture, a slice, a tile, or a group of tiles includes a respective luma adaptive loop filter set. In other examples, each of the plurality of adaptation parameter sets for a luma block in one or more of a picture, a slice, a tile, or a group of tiles includes only a respective luma adaptive loop filter set. Likewise, in one example, the adaptive parameter set corresponding to the second adaptive parameter set index includes a chroma adaptive loop filter set. In another example, the adaptation parameter set corresponding to the second adaptation parameter set index includes only the chroma adaptive loop filter set.

As described above, instead of using one APS index for both the luminance component and the chrominance component, a separate APS index is used so that the APS can be partially updated. For example, video encoder 20 and video decoder 30 may code an index for the Luma APS index (Luma _ APS _ index) to indicate the APS for which Luma filter information is updated. The video encoder 20 and the video decoder 30 may code the chrominance APS index (Chroma _ APS _ index) to indicate an APS whose chrominance filter information is updated.

Thus, in another example of the disclosure, video encoder 20 and video decoder 30 may encode/decode a second syntax element that indicates whether luma adaptive loop filter information for luma blocks in one or more of a picture, a slice, a tile, or a group of tiles is updated, and encode/decode a third syntax element that indicates whether chroma adaptive loop filter information for chroma blocks in one or more of a picture, a slice, a tile, or a group of tiles is updated.

According to some examples, when APS is used, video encoder 20 and video decoder 30 may use a first-in-first-out (FIFO) buffer to store previously coded ALF filters. For each picture/slice/tile group, video encoder 20 may signal a new _ filter _ set _ flag (new _ filter _ set _ flag) such that the new filter set is derived from having the signaled APS index; otherwise, no filter parameters are derived from any APS. After decoding of the picture/slice/tile group, if new _ filter _ set _ flag is true, video decoder 30 may push filter set information from the APS to the FIFO buffer. Otherwise, the FIFO buffer is not modified. The FIFO buffer may be fixed length or variable length. The techniques described above for the FIFO buffer may also be applied.

In some examples, the FIFO buffer may be shared among all tile groups/slices in a picture. The index of the APS to be added to the FIFO buffer after decoding a picture may be signaled in some point in the middle of the bitstream of the picture. For example, video encoder 20 may signal such information at the beginning, end, or elsewhere in a picture. In another example, parameters from filter sets of one or more fixed tile groups/slices may be used to update the FIFO buffer without signaling which filter set is to be added to the buffer. For example, after the entire picture is reconstructed, the first n signaled filter sets from a tile group/slice in the picture will be added to the FIFO buffer. In another example, the last n signaled filter sets from a tile group/slice in a picture will be added to the FIFO buffer after the entire picture is reconstructed. In the above example, n is a positive integer. The value of n may be fixed or signaled. In another example, some rules for selecting more than one filter set in a picture may be defined and applied at both video encoder 20 and video decoder 30.

Fig. 5 is a block diagram illustrating an example video encoder 20 that may implement techniques for ALF using the APS described in this disclosure. Video encoder 20 may perform intra-coding and inter-coding of video blocks within a video slice. Intra-coding relies on spatial prediction to reduce or remove spatial redundancy in video within a given video frame or picture. Inter-coding relies on temporal prediction to reduce or remove temporal redundancy in video within adjacent frames or pictures of a video sequence. The intra mode (I mode) may refer to any of several space-based compressed modes. An inter mode, such as unidirectional prediction (P-mode) or bidirectional prediction (B-mode), may refer to any of several time-based compressed modes.

In the example of fig. 5, video encoder 20 includes a video data memory 33, a partition unit 35, a prediction processing unit 41, a summer 50, a transform processing unit 52, a quantization unit 54, and an entropy encoding unit 56. The prediction processing unit 41 includes a Motion Estimation Unit (MEU)42, a Motion Compensation Unit (MCU)44, and an intra prediction processing unit 46. For video block reconstruction, video encoder 20 also includes inverse quantization unit 58, inverse transform processing unit 60, summer 62, filter unit 64, and Decoded Picture Buffer (DPB) 66.

As shown in fig. 5, video encoder 20 receives video data and stores the received video data in video data memory 33. Video data memory 33 may store video data to be encoded by components in video encoder 20. The video data stored in video data storage 33 may be, for example, obtained from video source 18. DPB 66 may be a reference picture memory that stores reference video data for use when encoding video data by video encoder 20, e.g., in an intra-coding mode or an inter-coding mode. Video data memory 33 and DPB 66 may be formed from any of a variety of memory devices, such as Dynamic Random Access Memory (DRAM) (including synchronous DRAM (sdram)), magnetoresistive ram (mram), resistive ram (rram), or other types of memory devices. Video data memory 33 and DPB 66 may be provided by the same memory device or separate memory devices. In various examples, video data memory 33 may be on-chip with other components in video encoder 20 or off-chip with respect to these components.

Partition unit 35 retrieves video data from video data memory 33 and divides the video data into video blocks. Such partitioning may also include, for example, partitioning into slices, tiles, or other larger units, and video block partitioning according to the quad-tree structure of the LCUs and CUs. Video encoder 20 generally shows components that encode video blocks within a video slice to be encoded. A slice may be divided into multiple video blocks (and possibly into a set of video blocks called tiles). Prediction processing unit 41 may select one of a plurality of possible coding modes, such as one of a plurality of intra coding modes or one of a plurality of inter coding modes, for the current video block based on the error result (e.g., coding rate and degree of distortion). Prediction processing unit 41 may provide the resulting intra-coded or inter-coded block to summer 50 to generate residual block data, and to summer 62 to reconstruct the encoded block for use as a reference picture.

Intra prediction unit process 46 within prediction processing unit 41 may perform intra-predictive coding of the current video block relative to one or more neighboring blocks in the same frame or slice as the current block to be coded to provide spatial compression. Motion estimation unit 42 and motion compensation unit 44 within prediction processing unit 41 perform inter-predictive coding of the current video block relative to one or more prediction blocks in one or more reference pictures to provide temporal compression.

The motion estimation unit 42 may be configured to determine an inter prediction mode for a video slice according to a predetermined pattern for a video sequence. The predetermined pattern may designate video slices in the sequence as P-slices or B-slices. The motion estimation unit 42 and the motion compensation unit 44 may be highly integrated, but are shown separately for conceptual purposes. Motion estimation performed by motion estimation unit 42 is the process of generating motion vectors, which estimate motion for video blocks. For example, a motion vector may indicate the displacement of a PU of a video block within a current video frame or picture relative to a prediction block within a reference picture.

A prediction block is a block that is found to closely match a PU of a video block to be coded in terms of pixel differences, which may be determined by Sum of Absolute Differences (SAD), Sum of Squared Differences (SSD), or other difference metrics. In some examples, video encoder 20 may calculate values for sub-integer pixel positions of reference pictures stored in DPB 66. For example, video encoder 20 may interpolate values for a quarter-pixel position, an eighth-pixel position, or other fractional-pixel positions of a reference picture. Accordingly, the motion estimation unit 42 may perform a motion search with respect to the full pixel position and the fractional pixel position, and output a motion vector having a fractional pixel precision.

Motion estimation unit 42 calculates motion vectors for PUs of video blocks in the inter-coded slice by comparing the locations of the PUs to locations of prediction blocks of the reference picture. The reference picture may be selected from a first reference picture list (list 0) or a second reference picture list (list 1), each of list 0 and list 1 identifying one or more reference pictures stored in the DPB 66. The motion estimation unit 42 sends the calculated motion vector to the entropy encoding unit 56 and the motion compensation unit 44.

The motion compensation performed by the motion compensation unit 44 may involve retrieving or generating a prediction block based on a motion vector determined by motion estimation, possibly performing interpolation to sub-pixel accuracy. Upon receiving the motion vector for the PU of the current video block, motion compensation unit 44 may locate the prediction block to which the motion vector points in one of the reference picture lists. Video encoder 20 forms a residual video block by subtracting pixel values of the prediction block from pixel values of the current video block being coded, forming a pixel difference value. The pixel difference values form residual data for the block and may include a luminance difference component and a chrominance difference component. Summer 50 represents the component or components that perform this subtraction operation. Motion compensation unit 44 may also generate syntax elements associated with the video blocks and the video slice for use by video decoder 30 in decoding the video blocks of the video slice.

After prediction processing unit 41 generates a prediction block for the current video block via intra prediction or inter prediction, video encoder 20 forms a residual video block by subtracting the prediction block from the current video block. The residual video data in the residual block may be included in one or more TUs and applied 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. Transform processing unit 52 may convert the residual video data from the pixel domain to a transform domain, such as the frequency domain.

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 reduce the bit depth associated with some or all of the coefficients. The degree of quantization may be modified by adjusting a quantization parameter. In some examples, quantization unit 54 may then perform a scan of a matrix including quantized transform coefficients. In another example, entropy encoding unit 56 may perform the scanning.

After quantization, entropy encoding unit 56 entropy encodes the quantized transform coefficients. For example, entropy encoding unit 56 may perform Context Adaptive Variable Length Coding (CAVLC), Context Adaptive Binary Arithmetic Coding (CABAC), syntax-based context adaptive binary arithmetic coding (SBAC), Probability Interval Partition Entropy (PIPE) coding, or another entropy encoding methodology or technique. Following entropy encoding by entropy encoding unit 56, the encoded bitstream may be sent to video decoder 30 or archived for later transmission or retrieval by video decoder 30. Entropy encoding unit 56 may also entropy encode motion vectors and other syntax elements for the current video slice being coded.

Inverse quantization unit 58 and inverse transform processing unit 60 apply inverse quantization and inverse transform, respectively, to reconstruct the residual block in the pixel domain for later use as a reference block for a reference picture. Motion compensation unit 44 may calculate the reference block by adding the residual block to a predicted block of one of the reference pictures within one of the reference picture lists. Motion compensation unit 44 may also apply one or more interpolation filters to the reconstructed residual block to calculate sub-integer pixel values for use in motion estimation. Summer 62 adds the reconstructed residual block to the motion compensated prediction block produced by motion compensation unit 44 to produce a reconstructed block.

The filter unit 64 filters the reconstructed block (e.g., the output of the summer 62) and stores the filtered reconstructed block in the DPB 66 for use as a reference block. The reference block may be used by motion estimation unit 42 and motion compensation unit 44 as a reference block to inter-predict a block in a subsequent video frame or picture. Filter unit 64 may perform any type of filtering (such as deblocking filtering, SAO filtering, peak SAO filtering, ALF, and/or GALF) and/or other types of loop filters. The deblocking filter may, for example, apply deblocking filtering to filter block boundaries to remove blockiness artifacts from the reconstructed video. The peak SAO filter may apply an offset to the reconstructed pixel values in order to improve the overall decoding quality. Additional loop filters (in-loop or after-loop) may also be used.

The filter unit 64 or other structural components of the video encoder 20 may be configured to perform the techniques of this disclosure. For example, the filter unit 64 may be configured to: for a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, encoding a first syntax element indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, slice, tile, or group of tiles. Filter unit 64 may be further configured to encode a plurality of first adaptive parameter set indices for a luma block in one or more of a picture, a slice, a tile, or a group of tiles based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the group of tiles. For example, if five APSs are used to apply ALF to luma blocks of a slice, filter unit 64 will also encode the indices of the five APSs used. Of course, other numbers of APSs may be used for the slices. The filter unit 64 may be further configured to: for a video bitstream for which adaptive loop filtering is enabled for chroma blocks in one or more of a picture, a slice, a tile, or a tile group, encoding a second adaptive parameter set index for chroma blocks in one or more of the picture, slice, tile, or tile group. In this case, the chrominance block has only a single APS, and as such, only the index of the APS is encoded and signaled.

Fig. 6 is a block diagram illustrating an example video decoder 30 that may implement the techniques described in this disclosure. For example, video decoder 30 of fig. 6 may, for example, be configured to receive the signaling described above with respect to video encoder 20 of fig. 5. In the example of fig. 6, video decoder 30 includes video data memory 78, entropy decoding unit 80, prediction processing unit 81, inverse quantization unit 86, inverse transform processing unit 88, summer 90, DPB 94, and filter unit 92. The prediction processing unit 81 includes a motion compensation unit 82 and an intra prediction processing unit 84. Video decoder 30 may, in some examples, perform a decoding path that is generally reciprocal to the encoding path described with respect to video encoder 20 from fig. 5.

During the decoding process, video decoder 30 receives an encoded video bitstream from video encoder 20 that represents video blocks and associated syntax elements in an encoded video slice. Video decoder 30 stores the received encoded video bitstream in video data memory 78. The video data memory 78 may store video data, such as an encoded video bitstream, to be decoded by components in the video decoder 30. The video data stored in the video data memory 78 may be obtained, for example, from the storage device 26 via the link 16 or from a local video source (such as a camera) or by accessing a physical data storage medium. The video data memory 78 may form a Coded Picture Buffer (CPB) that stores encoded video data from an encoded video bitstream. DPB 94 may be a reference picture memory that stores reference video data for use when decoding video data by video decoder 30, e.g., in an intra-coding mode or an inter-coding mode. The video data memory 78 and DPB 94 may be comprised of any of a variety of memory devices, such as DRAM, SDRAM, MRAM, RRAM, or other types of memory devices. Video data memory 78 and DPB 94 may be provided by the same memory device or separate memory devices. In various examples, video data memory 78 may be on-chip with other components in video decoder 30 or off-chip with respect to these components.

Entropy decoding unit 80 in video decoder 30 decodes the video data stored in video data memory 78 to generate quantized coefficients, motion vectors, and other syntax elements. The entropy decoding unit 80 forwards the motion vectors and other syntax elements to the prediction processing unit 81. Video decoder 30 may receive syntax elements at the video slice level and/or the video block level.

When a video slice is coded as an intra-coded (I) slice, intra-prediction processing unit 84 in prediction processing unit 81 may generate prediction data for the video block of the current video slice based on the signaled intra-prediction mode and data from previously decoded blocks of the current frame or picture. When a video frame is coded as an inter-coded slice (e.g., a B-slice or a P-slice), motion compensated prediction 82 in prediction processing unit 81 generates a prediction block for a video block of the current video slice based on the motion vectors and other syntax elements received from entropy decoding unit 80. The prediction block may be generated from one of the reference pictures within one of the reference picture lists. Video decoder 30 may use default construction techniques to construct reference frame lists, list 0 and list 1, based on the reference pictures stored in DPB 94.

Motion compensation unit 82 determines prediction information for video blocks of the current video slice by parsing motion vectors and other syntax elements and uses the prediction information to generate a prediction block for the current video block being coded. For example, motion compensation unit 82 uses some of the received syntax elements to determine a prediction mode (e.g., intra-prediction or inter-prediction) for coding video blocks of the video slice, an inter-prediction slice type (e.g., B-slice or P-slice), construction information for one or more of the reference picture lists of the slice, a motion vector for each inter-coded video block of the slice, an inter-prediction state for each inter-coded video block of the slice, and other information for decoding video blocks in the current video slice.

The motion compensation unit 82 may also perform interpolation based on the interpolation filter. Motion compensation unit 82 may calculate interpolated values for sub-integer pixels of the reference block using interpolation filters as used by video encoder 20 during encoding of the video block. In this case, motion compensation unit 82 may determine the interpolation filters used by video encoder 20 from the received syntax elements and use the interpolation filters to generate the prediction blocks.

The inverse quantization unit 86 inversely quantizes (i.e., dequantizes) the quantized transform coefficients provided in the bitstream and decoded by the entropy decoding unit 80. The inverse quantization process may include the use of quantization parameters calculated by video encoder 20 for each video block in a video slice to determine the degree of quantization and, likewise, the degree of inverse quantization that should be applied. 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 produce a block of residues in the pixel domain.

After the prediction processing unit generates a prediction block for the current video block using, for example, intra-prediction or inter-prediction, video decoder 30 forms a reconstructed video block by summing the residual block from inverse transform processing unit 88 with the corresponding prediction block generated by motion compensation unit 82. Summer 90 represents the component or components that perform the summation operation.

The filter unit 92 filters the reconstructed block (e.g., the output of the summer 90) and stores the filtered reconstructed block in the DPB 94 for use as a reference block. The reference block may be used by motion compensation unit 82 as a reference block to inter-predict a block in a subsequent video frame or picture. Filter unit 92 may perform any type of filtering (such as deblocking filtering, SAO filtering, peak SAO filtering, ALF, and/or GALF) and/or other types of loop filters. The deblocking filter may, for example, apply deblocking filtering to filter block boundaries to remove blockiness artifacts from the reconstructed video. The peak SAO filter may apply an offset to the reconstructed pixel values in order to improve the overall decoding quality. Additional loop filters (in-loop or after-loop) may also be used.

Filter unit 92 and/or components of other structures of video decoder 30 may be configured to perform techniques reciprocal to those described above for video encoder 20 and filter unit 64. For example, video decoder 30 may be configured to decode, from a video bitstream for which adaptive loop filtering is enabled for luma blocks in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element indicating a number of adaptive parameter sets for luma blocks in one or more of a picture, a slice, a tile, or a group of tiles. Video decoder 30 may be further configured to decode a plurality of first adaptive parameter set indices for a luma block in one or more of a picture, a slice, a tile, or a tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group. Video decoder 30 may also decode, from the video bitstream for which adaptive loop filtering is enabled for chroma blocks in one or more of the pictures, slices, tiles, or tile groups, a second adaptive parameter set index for the chroma blocks in the one or more of the pictures, slices, tiles, or tile groups. Filter unit 92 may then apply a first adaptive loop filter to the luma block in one or more of the picture, slice, tile, or group of tiles based on the plurality of first adaptive parameter set indices. For example, video decoder 30 may decode syntax elements for indicating a particular APS used for a particular luma block of the slice, and then filter unit 92 applies the ALF parameters from the indicated APS to the particular luma block. Likewise, filter unit 92 may apply a second adaptive loop filter to chroma blocks in one or more of the picture, slice, tile, or group of tiles based on a second adaptive parameter set index. For example, filter unit 92 may apply the indicated ALF parameters of the APS from the second adaptive parameter set index to the chroma block.

Fig. 7 shows an example implementation of the filter unit 92. The filter unit 64 may be implemented in the same manner. Filter unit 64 and filter unit 92 may perform the techniques of this disclosure, possibly in conjunction with other components in video encoder 20 or video decoder 30. In the example of fig. 7, the filter unit 92 includes a deblocking filter 102, an SAO filter 104, and an ALF/GALF filter 106. The SAO filter 104 may, for example, be configured to determine offset values for samples of a block in the manner described in this disclosure.

The filter unit 92 may include fewer filters and/or may include additional filters. Furthermore, the particular filters shown in FIG. 7 may be implemented in a different order. Other loop filters (in or after the coding loop) may also be used to smooth pixel transitions or otherwise improve video quality. The decoded video blocks in a given frame or picture are then stored in the DPB 94, and the DPB 94 stores reference pictures for subsequent motion compensation. DPB 94 may be part of or separate from a further memory that stores decoded video for later presentation on a display device, such as display device 32 in fig. 1.

Fig. 8 is a flow chart illustrating an example video encoding method of the present disclosure. The techniques of fig. 8 may be performed by one or more components (including filter unit 64) in video encoder 20.

According to one example of the present disclosure, video encoder 20 may be configured to: for a video bitstream for which adaptive loop filtering is enabled for a luma block in one or more of a picture, a slice, a tile, or a group of tiles, a first syntax element indicating a number of adaptive parameter sets for the luma block in the one or more of the picture, slice, tile, or group of tiles is encoded (800). Video encoder 20 may be further configured to encode a plurality of first adaptive parameter set indices for a luma block in one or more of a picture, a slice, a tile, or a tile group based on a number of the adaptive parameter sets for the luma block in the one or more of the picture, the slice, the tile, or the tile group (802). For example, if five APSs are used to apply ALF to luma blocks of a slice, video encoder 20 will also encode indices for the five APSs used. Of course, other numbers of APSs may be used for the slices. Video encoder 20 is further configured to: for a video bitstream for which adaptive loop filtering is enabled for chroma blocks in one or more of a picture, a slice, a tile, or a tile group, encoding a second adaptive parameter set index for chroma blocks in one or more of the picture, slice, tile, or tile group (804). In this case, the chrominance block has only a single APS, and as such, only the index of the APS is encoded and signaled.

In the above example, video encoder 20 may signal, in a picture/slice/tile group header, a first syntax element for indicating a number of adaptive parameter sets for a luma block in one or more of a picture, slice, tile, or tile group, a plurality of first adaptive parameter set indices for luma blocks in one or more of a picture, slice, tile, or tile group, and a second adaptive parameter set index for chroma blocks in one or more of a picture, slice, tile, or tile group.

Fig. 9 is a flowchart illustrating an example video decoding method of the present disclosure. The technique of fig. 9 may be performed by one or more components (including filter unit 92) in video decoder 30.

Video decoder 30 may also decode, from the video bitstream for which adaptive loop filtering is enabled for chroma blocks in one or more of the pictures, slices, tiles, or tile groups, a second adaptive parameter set index for the chroma blocks in the one or more of the pictures, slices, tiles, or tile groups (904). Video decoder 30 may then apply a first adaptive loop filter to the luma block in one or more of the picture, slice, tile, or group of tiles based on the plurality of first adaptive parameter set indices (906). For example, video decoder 30 may decode syntax elements for a particular APS that indicate a particular luma block used for one or more of a picture, slice, tile, or group of tiles, and then apply ALF parameters from the indicated APS to the particular luma block. Likewise, video decoder 30 may apply a second adaptive loop filter to chroma blocks in one or more of the picture, slice, tile, or tile group based on a second adaptive parameter set index (908). For example, video decoder 30 may apply the indicated ALF parameters of the APS from the second adaptive parameter set index to the chroma block.

In the above examples, video decoder 30 may receive a first syntax element in a picture/slice/tile group header to indicate a number of adaptive parameter sets for a luma block in one or more of a picture, slice, tile, or tile group, a plurality of first adaptive parameter set indices for luma blocks in one or more of a picture, slice, tile, or tile group, and a second adaptive parameter set index for chroma blocks in one or more of a picture, slice, tile, or tile group.

Illustrative examples of the present disclosure include:

example 1: a method of coding video data, the method comprising: a set of Adaptive Loop Filter (ALF) filters is determined from among a plurality of sets of ALF information for a block of a picture.

Example 2: the method of example 1, wherein the block represents a Coding Tree Unit (CTU) of the picture.

Example 3: the method of example 1 or example 2, wherein each respective set of ALF information includes filters assigned to a class.

Example 4: a method of coding video data, the method comprising allocating a list of Adaptive Loop Filter (ALF) sets across two or more pictures.

Example 5: the method of example 4, further comprising deriving a new filter set after coding the picture, the new filter set not included in the assigned list.

Example 6: the method of example 5, further comprising adding the new filter set to the list.

Example 7: the method of example 5, further comprising replacing one of the ALF sets in the list with the new filter set.

Example 8: the method of any combination of examples 5-7, wherein deriving the new filter set comprises deriving the new filter set using data from one of a previously coded picture, a previously coded slice, or a previously coded tile.

Example 9: a method of coding video data, the method comprising merging Adaptive Loop Filter (ALF) information across multiple blocks of a picture.

Example 10: the method of example 9, wherein the plurality of blocks includes a current block of the picture and an upper adjacent block of the current block.

Example 11: the method of example 9, wherein the plurality of blocks includes a current block of the picture and a left-adjacent block of the current block.

Example 12: the method of example 9, wherein the plurality of blocks includes a current block of the picture and a block included in a different picture than the picture.

Example 13: a method of decoding video data, the method comprising deriving a value of an Adaptive Loop Filter (ALF) on/off flag from previously decoded video data.

Example 14: the method of example 13, wherein deriving the value of the ALF on/off flag comprises deriving the value based on a number of blocks included in ALF blocks sharing the ALF on/off flag.

Example 15: the method of example 14, wherein deriving the value of the ALF on/off flag comprises deriving the value based further on a non-zero Coded Block Flag (CBF) or non-zero transform coefficient in the ALF block

Example 16: a video coding apparatus, comprising: a video data memory storing the video data; and processing circuitry in communication with the video data memory, the processing circuitry configured to perform the method of any of examples 1-15.

Example 17: the video coding device of example 16, wherein the video coding device comprises a video decoding device.

Example 18: the video coding device of example 16, wherein the video coding device comprises a video encoding device.

Example 19: the video coding device of example 18, wherein the video coding device is configured to decode the encoded video bitstream.

Example 20: an apparatus comprising means for performing the method of any of examples 1-15.

Example 21: a computer-readable storage medium encoded with instructions that, when executed, cause a processor of a video coding device to perform the method of any of examples 1-15.

Example 22: any combination of the techniques described in this disclosure.

In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media corresponding to tangible media such as data storage media or communication media including, for example, any media that facilitates transfer of a computer program from one place to another in accordance with a communication protocol. In this manner, the computer-readable medium may generally correspond to: (1) a tangible computer-readable storage medium that is non-transitory; or (2) a communication medium such as a signal or carrier wave. The data storage medium may be any available medium that can be accessed by one or more computers or one or more processors to retrieve instructions, code and/or data structures for implementation of the techniques described in this disclosure. The computer program product may include a computer-readable medium.

By way of example, and not limitation, such computer-readable storage media can be any of RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage or other magnetic storage devices, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if the instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, Digital Subscriber Line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transitory media, but are instead directed to non-transitory, tangible storage media. Disk and disc, as used herein, includes Compact Disc (CD), laser disc, optical disc, Digital Versatile Disc (DVD), floppy disk and blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media.

The instructions may be executed by one or more processors, such as one or more Digital Signal Processors (DSPs), general purpose microprocessors, Application Specific Integrated Circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Thus, the term "processor" as used herein refers to any of the structures described above or any other structure suitable for implementation of the techniques described herein. Further, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules configured for encoding and decoding, or incorporated into a combined codec. Furthermore, the techniques may be fully implemented in one or more circuits or logic elements.

The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including a wireless handheld device, an Integrated Circuit (IC), or a group of ICs (e.g., a chipset). Various components, modules, or units are described in this disclosure to emphasize aspects of the functionality of devices configured to perform the disclosed techniques, but do not necessarily require implementation by different hardware units. Rather, as described above, the various units may be combined in a codec hardware unit, or provided by a collection of interoperating hardware units including one or more processors as described above, in combination with suitable software and/or firmware.

Various examples have been described. These examples and other examples are within the scope of the following claims.

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Block-based Adaptive Loop Filter (ALF) with Adaptive Parameter Set (APS) in video coding

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