Fixed filter with nonlinear adaptive loop filter in video decoding

Abstract

A video decoder may determine a filter set for a coding tree block (CTB) from a plurality of fixed filter sets. Based on the filter set for the CTB being from the fixed filter set, the video decoder may set a clipping value to a maximum supported value. In addition, the video decoder may determine a clipped input to an adaptive loop filter (ALF) for the filter set based on the clipping value, each of the inputs being an input sample minus a current sample. The video decoder may then apply the ALF to the clipped input.

Description

[0001] This application claims the benefit of U.S. Patent Application No. 16 / 811,913, filed on March 6, 2020; U.S. Provisional Patent Application No. 62 / 823,546, filed on March 25, 2019; and U.S. Provisional Patent Application No. 62 / 837,651, filed on April 23, 2019, the entire contents of each of the above applications are incorporated by reference. Technical Field

[0002] The present disclosure relates to video encoding and video decoding. Background Art

[0003] Digital video capabilities may be incorporated into a wide variety of devices, including digital televisions, digital live 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 game devices, video game consoles, cellular or satellite radio telephones (so-called "smart phones"), video teleconferencing devices, video streaming devices, etc. Digital video devices implement video coding techniques (such as those described in the standards defined by MPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264 / MPEG-4 (Part 10, Advanced Video Coding (AVC)), ITU-T H.265 / High Efficiency Video Coding (HEVC), and extensions of such standards). By implementing such video coding techniques, video devices may more efficiently send, receive, encode, decode, and / or store digital video information.

[0004] Video coding techniques include 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 (e.g., a video picture or a portion of a video picture) may be partitioned into video blocks, which may also be referred to as coding tree units (CTUs), coding units (CUs), and / or coding nodes. Video blocks in an intra-coded (I) slice of a picture are encoded using spatial prediction relative to reference samples in neighboring blocks in the same picture. Video blocks in an inter-coded (P or B) slice of a picture may use spatial prediction relative to reference samples in neighboring blocks in the same picture or temporal prediction relative to reference samples in other reference pictures. Pictures may be referred to as frames, and reference pictures may be referred to as reference frames. Summary of the invention

[0005] In summary, the present disclosure describes techniques for applying an adaptive loop filter (ALF) in video coding. A video encoder or video decoder may use the ALF to improve the quality of reconstructed video data. As part of applying the ALF, a video decoder (e.g., a video encoder or video decoder) may apply a clipping operation to an input value and multiply the resulting clipped value by a filter coefficient. The video decoder may then add the resulting value to the value of the current sample to determine a filtered version of the current sample. The techniques of the present disclosure may speed up the process of applying the ALF by avoiding performing a clipping operation when the filter coefficients are in a predefined fixed filter set. To avoid performing a clipping operation, the video decoder may set the clipping value to a maximum supported value.

[0006] In one example, the present disclosure describes a method for decoding video data, the method comprising: determining a filter set for a decoding tree block (CTB) from a plurality of fixed filter sets, wherein the CTB is in a picture of the video data; setting a clipping value to a maximum supported value based on that the filter set for the CTB is from the fixed filter set; determining a clipped input of an adaptive loop filter (ALF) for the filter set based on the clipping value, each of the inputs being an input sample minus a current sample; and applying the ALF to the clipped input.

[0007] In another example, the present disclosure describes a device for decoding video data, the device comprising: a memory for storing the video data; and one or more processors implemented in a circuit, the one or more processors being configured to: determine a filter set for a coding tree block (CTB) from a plurality of fixed filter sets, wherein the CTB is in a picture of the video data; set a clipping value to a maximum supported value based on that the filter set for the CTB is from the fixed filter set; determine a clipped input of an adaptive loop filter (ALF) for the filter set based on the clipping value, each of the inputs being an input sample minus a current sample; and apply the ALF to the clipped input.

[0008] In another example, the present disclosure describes an apparatus for decoding video data, the apparatus comprising: a unit for determining a filter set for a decoding tree block (CTB) from a plurality of fixed filter sets, wherein the CTB is in a picture of the video data; a unit for setting a clipping value to a maximum supported value based on that the filter set for the CTB is from the fixed filter set; a unit for determining a clipped input of an adaptive loop filter (ALF) for the filter set based on the clipping value, each of the inputs being an input sample minus a current sample; and a unit for applying the ALF to the clipped input.

[0009] In another example, the present disclosure describes a computer-readable storage medium having instructions stored thereon, which when executed cause one or more processors to perform the following operations: determine a filter set for a decoding tree block (CTB) from a plurality of fixed filter sets, wherein the CTB is in a picture of the video data; set a clipping value to a maximum supported value based on that the filter set for the CTB is from the fixed filter set; determine a clipped input to an adaptive loop filter (ALF) for the filter set based on the clipping value, each of the inputs being an input sample minus a current sample; and apply the ALF to the clipped input.

[0010] 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, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is a block diagram illustrating an example video encoding and decoding system that may perform the techniques of this disclosure.

[0012] Figure 2A and Figure 2B is a conceptual diagram illustrating an example quadtree binary tree (QTBT) structure and a corresponding coding tree unit (CTU).

[0013] Figure 3 is a block diagram illustrating an example video encoder that may perform the techniques of this disclosure.

[0014] Figure 4 is a block diagram illustrating an example video decoder that may perform the techniques of this disclosure.

[0015] Figure 5 is a flow chart illustrating an example method for encoding a current block of video data.

[0016] Figure 6is a flow chart illustrating an example method for decoding a current block of video data.

[0017] Figure 7 is a flow diagram illustrating example operations for applying an adaptive loop filter (ALF) filter to a coding tree block (CTB). DETAILED DESCRIPTION

[0018] Adaptive loop filtering is a technique that can be applied during video encoding and video decoding. For example, a video encoder can apply an adaptive loop filter (ALF) to a coding tree block (CTB) as part of a reconstruction loop. A video decoder can apply an ALF to a CTB after reconstructing the CTB from one or more prediction blocks and residual data.

[0019] To apply the ALF to the CTB, a video decoder (e.g., a video encoder or a video decoder) may determine a filter set for the CTB. The filter set for the CTB includes a filter coefficient set. In order to reduce the amount of data signaled in the bitstream, the filter set for the CTB may be signaled at a higher level than the CTB (e.g., at the level of a sequence parameter set (SPS), a picture parameter set (PPS), a slice header, etc.), reused from a filter set for a previously decoded CTB, obtained from a plurality of fixed filter sets pre-configured at the video encoder and the video decoder, or available in some other manner. The fixed filter set is pre-configured at the video encoder and the video decoder so that the filter coefficients of the fixed filter set do not need to be signaled in the bitstream or reconstructed by the video decoder from the data signaled in the bitstream.

[0020] During the application of the ALF to the CTB, the video decoder may clip the input value. Clipping the input value limits the input value to a predefined range. The predefined range may be limited by a pair of clipping values ​​(which may also be referred to as clipping parameters). Clipping the input value may ensure that when the video decoder multiplies the input value by the corresponding filter coefficient, the resulting value is not so large or so negative that more bits are required to represent the resulting value than the bits available to represent the resulting value. Allowing the resulting value to be so large or so negative that more bits are required to represent the resulting value than the bits available to represent the resulting value may result in errors and may reduce picture quality.

[0021] Performing a clipping operation may slow down the process of encoding and decoding video data. This is because the clipping operation may require one or two comparison operations for each input value. In addition, determining the clipping value increases the complexity of the encoder.

[0022] Therefore, according to the technology of the present disclosure, the video decoder can determine the filter set for CTB from multiple fixed filter sets. Based on the filter set for CTB from the fixed filter set, the video decoder can set the clipping value to the maximum support value. Setting the clipping value to the maximum support value can effectively eliminate the need for performing clipping operations, because the input value will never be greater or more negative than the maximum support value. Using an input value with a maximum support value can be acceptable because the fixed filter set can be defined so that any possible input value multiplied by the filter coefficient in the fixed filter set will not result in the following value: the value is so large or so negative that the value cannot be represented using the number of bits that can be used to represent such a value. Therefore, the video decoder can determine the clipped input (which can be the same as the input value itself) of the ALF for the filter set based on the clipping value, wherein each of these inputs is an input sample minus the current sample. Then, the video decoder can apply the ALF to the clipped input. In this way, by avoiding the execution of the clipping operation, the process of encoding and decoding video data can have a technical advantage of being accelerated relative to the process of encoding and decoding video without implementing the technology of the present disclosure.

[0023] Although this disclosure primarily refers to applying ALF to CTBs, the techniques of this disclosure may be applied to sub-blocks of CTBs (such as coding blocks), or even at the level of individual pixels or samples.

[0024] Figure 1 1 is a block diagram illustrating an example video encoding and decoding system 100 that can perform the techniques of the present disclosure. In general, the techniques of the present disclosure relate to decoding (encoding and / or decoding) video data. Generally, video data includes any data used to process video. Thus, video data can include original unencoded video, encoded video, decoded (e.g., reconstructed) video, and video metadata (e.g., signaling data).

[0025] like Figure 1As shown, in this example, system 100 includes a source device 102 that provides encoded video data to be decoded and displayed by a destination device 116. Specifically, source device 102 provides the video data to destination device 116 via computer-readable medium 110. Source device 102 and destination device 116 may include any of a wide variety of devices, including desktop computers, notebook computers (i.e., laptop computers), tablet computers, set-top boxes, telephone handsets such as smart phones, televisions, cameras, computers, mobile devices, broadcast receiver devices, display devices, digital media players, video game consoles, video streaming devices, etc. In some cases, source device 102 and destination device 116 may be equipped for wireless communication and, therefore, may be referred to as wireless communication devices.

[0026] exist Figure 1 In the example of , source device 102 includes video source 104, memory 106, video encoder 200 and output interface 108. Destination device 116 includes input interface 122, video decoder 300, memory 120 and display device 118. According to the present disclosure, the video encoder 200 of source device 102 and the video decoder 300 of destination device 116 can be configured to apply a technology for applying an adaptive loop filter (ALF). Therefore, source device 102 represents an example of a video encoding device, and destination device 116 represents an example of a video decoding device. In other examples, source device and destination device may include other components or arrangements. For example, source device 102 may receive video data from an external video source such as an external camera. Similarly, destination device 116 may be docked with an external display device instead of including an integrated display device.

[0027] like Figure 1The system 100 shown is only an example. In general, any digital video encoding and / or decoding device can perform techniques for applying an adaptive loop filter (ALF). The source device 102 and the destination device 116 are only examples of such decoding devices, wherein the source device 102 generates decoded video data for transmission to the destination device 116. The present disclosure refers to a "decoding" device as a device that performs decoding (e.g., encoding and / or decoding) of data. Therefore, the video encoder 200 and the video decoder 300 represent examples of decoding devices (specifically, video encoders and video decoders), respectively. In some examples, the source device 102 and the destination device 116 can operate in a substantially symmetrical manner, so that each of the source device 102 and the destination device 116 includes a video encoding and decoding component. Therefore, the system 100 can support one-way or two-way video transmission between the source device 102 and the destination device 116, for example, for video streaming, video playback, video broadcasting, or video telephony.

[0028] Typically, the video source 104 represents the source of video data (i.e., the original, undecoded video data), and provides a series of pictures (also referred to as "frames") of the order of the video data to the video encoder 200, which encodes the data for the pictures. The video source 104 of the source device 102 may include a video capture device, such as a camera, a video archive unit containing previously captured original video, and / or a video feed interface for receiving video from a video content provider. As another alternative, the video source 104 may generate computer graphics-based data as the source video, or a combination of real-time video, archived video, and computer-generated video. In each case, the video encoder 200 encodes the captured, pre-captured, or computer-generated video data. The video encoder 200 may rearrange the pictures from the received order (sometimes referred to as "display order") to a decoding order for decoding. The video encoder 200 may generate a bitstream comprising encoded video data. Source device 102 may then output the encoded video data onto computer-readable medium 110 via output interface 108 to be received and / or retrieved by, for example, input interface 122 of destination device 116 .

[0029] The memory 106 of the source device 102 and the memory 120 of the destination device 116 represent general purpose memories. In some examples, the memories 106, 120 can store original video data, for example, original video from the video source 104 and original decoded video data from the video decoder 300. In addition or alternatively, the memories 106, 120 can store software instructions that can be executed by, for example, the video encoder 200 and the video decoder 300, respectively. Although the memory 106 and the memory 120 are shown as separate from the video encoder 200 and the video decoder 300 in this example, it should be understood that the video encoder 200 and the video decoder 300 can also include internal memories for functionally similar or equivalent purposes. In addition, the memories 106, 120 can store, for example, encoded video data output from the video encoder 200 and input to the video decoder 300. In some examples, portions of the memories 106, 120 can be allocated as one or more video buffers, for example, to store original decoded and / or encoded video data.

[0030] The computer-readable medium 110 may represent any type of medium or device capable of delivering the encoded video data from the source device 102 to the destination device 116. In one example, the computer-readable medium 110 represents a communication medium that enables the source device 102 to directly send the encoded video data to the destination device 116 in real time, for example, via a radio frequency network or a computer-based network. The output interface 108 may modulate the transmission signal including the encoded video data according to a communication standard such as a wireless communication protocol, and the input interface 122 may demodulate the received transmission signal according to a communication standard such as a wireless communication protocol. The communication medium may include any wireless or wired communication medium, for example, a radio frequency (RF) spectrum or one or more physical transmission lines. The communication medium may form a portion 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, a switch, a base station, or any other device that may be useful for facilitating communication from the source device 102 to the destination device 116.

[0031] In some examples, computer-readable medium 110 may include storage device 112. Source device 102 may output the encoded data from output interface 108 to storage device 112. Similarly, destination device 116 may access the encoded data from storage device 112 via input interface 122. Storage device 112 may include any of a variety of distributed or locally accessed data storage media, such as a hard drive, Blu-ray disc, DVD, CD-ROM, flash memory, volatile or non-volatile memory, or any other suitable digital storage medium for storing encoded video data.

[0032] In some examples, the computer-readable medium 110 may include a file server 114 or another intermediate storage device that can store the encoded video data generated by the source device 102. The source device 102 may output the encoded video data to the file server 114 or another intermediate storage device that can store the encoded video generated by the source device 102. The destination device 116 may access the stored video data from the file server 114 via streaming or downloading. The file server 114 may be any type of server device capable of storing the encoded video data and sending the encoded video data to the destination device 116. The file server 114 may represent a web server (e.g., for a website), a file transfer protocol (FTP) server, a content delivery network device, or a network attached storage (NAS) device. The destination device 116 may access the encoded video data from the file server 114 via 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., a digital subscriber line (DSL), a cable modem, etc.), or a combination of the two that is suitable for accessing the encoded video data stored on the file server 114. The file server 114 and the input interface 122 may be configured to operate according to: a streaming transfer protocol, a download transfer protocol, or a combination thereof.

[0033] Output interface 108 and input interface 122 may represent wireless transmitters / receivers, modems, wired networking components (e.g., Ethernet cards), wireless communication components that operate according to any of the various IEEE 802.11 standards, or other physical components. In examples where output interface 108 and input interface 122 include wireless components, output interface 108 and input interface 122 may be configured to transmit data (such as encoded video data) according to a cellular communication standard (such as 4G, 4G-LTE (Long Term Evolution), Advanced LTE, 5G, etc.). In some examples where output interface 108 includes a wireless transmitter, output interface 108 and input interface 122 may be configured to transmit data (such as encoded video data) according to other wireless standards (such as IEEE 802.11 specifications, IEEE 802.15 specifications (e.g., ZigBee TM ), Bluetooth TMStandards, etc.) to transmit data (such as encoded video data). In some examples, source device 102 and / or destination device 116 may include corresponding system-on-chip (SoC) devices. For example, source device 102 may include a SoC device for performing the functions assigned to video encoder 200 and / or output interface 108, and destination device 116 may include a SoC device for performing the functions assigned to video decoder 300 and / or input interface 122.

[0034] The techniques of the present disclosure may be applied to video decoding to support any of a variety of multimedia applications, such as over-the-air television broadcasting, cable television transmission, satellite television transmission, Internet streaming video transmission (such as Dynamic Adaptive Streaming over HTTP (DASH)), digital video encoded onto a data storage medium, decoding of digital video stored on a data storage medium, or other applications.

[0035] The input interface 122 of the destination device 116 receives the encoded video bitstream from the computer-readable medium 110 (e.g., a communication medium, a storage device 112, a file server 114, etc.). The encoded video bitstream computer-readable medium 110 may include signaling information such as the following syntax elements defined by the video encoder 200 (which is also used by the video decoder 300): the syntax elements have values ​​that describe the characteristics of video blocks or other decoding units (e.g., slices, pictures, groups of pictures, sequences, etc.) and / or modes for processing thereof. The display device 118 displays the decoded pictures of the decoded video data to the user. The display device 118 may represent any of a variety of display devices, such as a cathode ray tube (CRT), a liquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED) display, or another type of display device.

[0036] Despite Figure 1 2. Although not shown in the drawings, in some examples, the video encoder 200 and the video decoder 300 may each be integrated with an audio encoder and / or an audio decoder and may include appropriate MUX-DEMUX units or other hardware and / or software to process multiplexed streams including both audio and video in a common data stream. If applicable, the MUX-DEMUX units may follow the ITU H.223 multiplexer protocol or other protocols such as the User Datagram Protocol (UDP).

[0037] The video encoder 200 and the video decoder 300 can each be implemented as any of a variety of suitable encoder and / 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 combination thereof. When the technology is partially implemented in software, the device can store instructions for the software in a suitable non-transitory computer-readable medium, and use one or more processors to execute the instructions in hardware to perform the technology of the present disclosure. Each of the video encoder 200 and the video decoder 300 can be included in one or more encoders or decoders, and any of the encoders or decoders can be integrated as part of a combined encoder / decoder (CODEC) in the corresponding device. The device including the video encoder 200 and / or the video decoder 300 can include an integrated circuit, a microprocessor, and / or a wireless communication device (such as a cellular phone).

[0038] The video encoder 200 and the video decoder 300 may operate according to a video coding standard, such as the ITU-T H.265 (also known as the High Efficiency Video Coding (HEVC) standard) or an extension thereof, such as the multi-view and / or scalable video coding extension. Alternatively, the video encoder 200 and the video decoder 300 may operate according to other proprietary or industry standards, such as the Joint Exploration Test Model (JEM) or the ITU-T H.266 standard, also known as Versatile Video Coding (VVC). The latest draft of the VVC standard is described in the following document: Bross et al., “Versatile Video Coding (Draft 4)”, Joint Video Experts Group (JVET) of ITU-T SG 16WP 3 and ISO / IEC JTC 1 / SC 29 / WG 11, 13th Meeting: Marrakech, Morocco, January 9-18, 2019, JVET-M1001-v5 (hereinafter referred to as “VVC Draft 4”). However, the techniques of this disclosure are not limited to any particular coding standard.

[0039] Typically, the video encoder 200 and the video decoder 300 can perform block-based decoding of the picture. The term "block" generally refers to a structure that includes data to be processed (e.g., to be encoded, decoded, or otherwise used in the encoding and / or decoding process). For example, a block may include a two-dimensional matrix of samples of luminance and / or chrominance data. Typically, the video encoder 200 and the video decoder 300 can decode video data represented in a YUV (e.g., Y, Cb, Cr) format. That is, instead of decoding red, green, and blue (RGB) data for samples of a picture, the video encoder 200 and the video decoder 300 can decode luminance and chrominance components, wherein the chrominance components may include both red hue and blue hue chrominance components. In some examples, the video encoder 200 converts the received RGB formatted data to a YUV representation before encoding, and the video decoder 300 converts the YUV representation to an RGB format. Alternatively, a pre-processing unit and a post-processing unit (not shown) can perform these conversions.

[0040] In general, the present disclosure may refer to the decoding (e.g., encoding and decoding) of a picture as a process that includes encoding or decoding data for the picture. Similarly, the present disclosure may refer to the decoding of a block of a picture as a process that includes encoding or decoding data for the block (e.g., prediction and / or residual decoding). The encoded video bitstream typically includes a series of values ​​for syntax elements that represent decoding decisions (e.g., decoding modes) and partitioning of the picture into blocks. Therefore, references to decoding a picture or block should generally be understood as decoding the values ​​of the syntax elements used to form the picture or block.

[0041] HEVC defines various blocks, including coding units (CUs), prediction units (PUs), and transform units (TUs). According to HEVC, a video decoder (such as the video encoder 200) partitions a coding tree unit (CTU) into CUs according to a quadtree structure. That is, the video decoder partitions the CTU and CU into four equal, non-overlapping squares, and each node of the quadtree has zero or four child nodes. A node without child nodes may be referred to as a "leaf node", and the CU of such a leaf node may include one or more PUs and / or one or more TUs. The video decoder may further partition the PU and TU. For example, in HEVC, a residual quadtree (RQT) represents the partitioning of a TU. In HEVC, a PU represents inter-frame prediction data, and a TU represents residual data. An intra-predicted CU includes intra-frame prediction information, such as an intra-frame mode indication.

[0042] As another example, the video encoder 200 and the video decoder 300 may be configured to operate according to JEM or VVC. According to JEM or VVC, a video decoder (such as the video encoder 200) partitions a picture into a plurality of coding tree units (CTUs). The video encoder 200 may partition the CTUs according to a tree structure (such as a quadtree-binary tree (QTBT) structure or a multi-type tree (MTT) structure). The QTBT structure removes the concept of multiple partition types, such as the distinction between CU, PU, ​​and TU of HEVC. The QTBT structure includes two levels: a first level partitioned according to quadtree partitioning, and a second level partitioned according to binary tree partitioning. The root node of the QTBT structure corresponds to the CTU. The leaf nodes of the binary tree correspond to the coding units (CUs).

[0043] In the MTT segmentation structure, blocks can be segmented using quadtree (QT) segmentation, binary tree (BT) segmentation, and one or more types of ternary tree (TT) segmentation. A ternary tree segmentation is a segmentation in which a block is split into three sub-blocks. In some examples, the ternary tree segmentation divides the block into three sub-blocks without dividing the original block through the center. The segmentation types in MTT (e.g., QT, BT, and TT) can be symmetric or asymmetric.

[0044] In some examples, the video encoder 200 and the video decoder 300 may use a single QTBT or MTT structure to represent each of the luma component and the chroma components, while in other examples, the video encoder 200 and the video decoder 300 may use two or more QTBT or MTT structures, such as one QTBT / MTT structure for the luma component and another QTBT / MTT structure for the two chroma components (or two QTBT / MTT structures for the respective chroma components).

[0045] The video encoder 200 and the video decoder 300 may be configured to use quadtree segmentation, QTBT segmentation, MTT segmentation, or other segmentation structures according to HEVC. For the purpose of explanation, a description of the technology of the present disclosure is given with respect to QTBT segmentation. However, it should be understood that the technology of the present disclosure may also be applied to a video decoder configured to use quadtree segmentation or also use other types of segmentation.

[0046] This disclosure may use "NxN" and "N by N" interchangeably to refer to the sample size of a block (such as a CU or other video block) in terms of vertical and horizontal dimensions, for example, 16x16 samples or 16 by 16 samples. Typically, a 16x16 CU will have 16 samples in the vertical direction (y=16) and 16 samples in the horizontal direction (x=16). Similarly, an NxNCU typically has N samples in the vertical direction and N samples in the horizontal direction, where N represents a non-negative integer value. The samples in a CU may be arranged in rows and columns. In addition, a CU does not necessarily need to have the same number of samples in the horizontal direction as in the vertical direction. For example, a CU may include NxM samples, where M is not necessarily equal to N.

[0047] The video encoder 200 encodes video data representing prediction and / or residual information and other information for a CU. The prediction information indicates how the CU will be predicted in order to form a prediction block for the CU. The residual information typically represents the sample-by-sample difference between the samples of the CU before encoding and the prediction block.

[0048] In order to predict a CU, the video encoder 200 can generally form a prediction block for the CU by inter-frame prediction or intra-frame prediction. Inter-frame prediction generally refers to predicting a CU based on data of a previously decoded picture, while intra-frame prediction generally refers to predicting a CU based on previously decoded data of the same picture. In order to perform inter-frame prediction, the video encoder 200 can use one or more motion vectors to generate a prediction block. The video encoder 200 can generally perform motion search to identify a reference block that closely matches the CU, for example, in terms of the difference between the CU and the reference block. The video encoder 200 can use the sum of absolute differences (SAD), the sum of squared differences (SSD), the mean absolute difference (MAD), the mean square difference (MSD), or other such difference calculations to calculate a difference metric to determine whether the reference block closely matches the current CU. In some examples, the video encoder 200 can use unidirectional prediction or bidirectional prediction to predict the current CU.

[0049] Some examples of JEM and VVC also provide an affine motion compensation mode, which can be considered an inter-frame prediction mode. In the affine motion compensation mode, the video encoder 200 can determine two or more motion vectors representing non-translational motion (such as zooming in or out, rotation, perspective motion, or other irregular motion types).

[0050] In order to perform intra prediction, the video encoder 200 can select an intra prediction mode to generate a prediction block. Some examples of JEM and VVC provide sixty-seven intra prediction modes, including various directional modes, as well as planar modes and DC modes. Typically, the video encoder 200 selects an intra prediction mode that describes the neighboring samples of the current block according to which the samples of the current block (e.g., the block of the CU) are to be predicted. Assuming that the video encoder 200 decodes the CTU and CU in a raster scan order (from left to right, from top to bottom), such samples can typically be above, above left, or to the left of the current block in the same picture as the current block.

[0051] The video encoder 200 encodes data representing a prediction mode for the current block. For example, for an inter-frame prediction mode, the video encoder 200 may encode data representing which of the various available inter-frame prediction modes is used and motion information for the corresponding mode. For unidirectional or bidirectional inter-frame prediction, for example, the video encoder 200 may encode motion vectors using advanced motion vector prediction (AMVP) or merge mode. The video encoder 200 may use a similar mode to encode motion vectors for affine motion compensation mode.

[0052] After a prediction such as intra prediction or inter prediction of a block, the video encoder 200 may calculate residual data for the block. The residual data (such as a residual block) represents the sample-by-sample difference between the block and a prediction block for the block, which is formed using a corresponding prediction mode. The video encoder 200 may apply one or more transforms to the residual block to produce transformed data in a transform domain rather than in a sample domain. For example, the video encoder 200 may apply a discrete cosine transform (DCT), an integer transform, a wavelet transform, or a conceptually similar transform to the residual video data. In addition, the video encoder 200 may apply a secondary transform after the first transform, such as a mode-dependent non-separable secondary transform (MDNSST), a signal-dependent transform, a Karhunen-Loeve transform (KLT), etc. The video encoder 200 generates transform coefficients after applying one or more transforms.

[0053] As described above, after any transform to produce transform coefficients, the video encoder 200 may perform quantization of the transform coefficients. Quantization generally refers to a process in which transform coefficients are quantized to potentially reduce the amount of data used to represent the transform coefficients, thereby providing further compression. By performing the quantization process, the video encoder 200 may reduce the bit depth associated with some or all transform coefficients. For example, the video encoder 200 may round down an n-bit value to an m-bit value during quantization, where n is greater than m. In some examples, to perform quantization, the video encoder 200 may perform a bitwise right shift of the value to be quantized.

[0054] After quantization, the video encoder 200 may scan the transform coefficients to generate a one-dimensional vector from a two-dimensional matrix including the quantized transform coefficients. The scan may be designed to place the transform coefficients of higher energy (and therefore lower frequency) in front of the vector and the transform coefficients of lower energy (and therefore higher frequency) in the back of the vector. In some examples, the video encoder 200 may scan the quantized transform coefficients using a predefined scan order to generate a serialized vector, and then entropy encode the quantized transform coefficients of the vector. In other examples, the video encoder 200 may perform adaptive scanning. After scanning the quantized transform coefficients to form a one-dimensional vector, the video encoder 200 may entropy encode the one-dimensional vector, for example, according to context adaptive binary arithmetic coding (CABAC). The video encoder 200 may also entropy encode the values ​​of the syntax elements used to describe the metadata associated with the encoded video data for use by the video decoder 300 when decoding the video data.

[0055] To perform CABAC, the video encoder 200 may assign context within a context model to a symbol to be transmitted. The context may relate to, for example, whether neighboring values ​​of the symbol are zero values. The probability determination may be based on the context assigned to the symbol.

[0056] The video encoder 200 may also generate syntax data (such as block-based syntax data, picture-based syntax data, and sequence-based syntax data) for the video decoder 300, for example, in a picture header, a block header, a slice header, or other syntax data (such as a sequence parameter set (SPS), a picture parameter set (PPS), or a video parameter set (VPS). Likewise, the video decoder 300 may decode such syntax data to determine how to decode the corresponding video data.

[0057] In this way, the video encoder 200 can generate a bitstream that includes encoded video data, such as syntax elements describing the partitioning of a picture into blocks (e.g., CUs) and prediction and / or residual information for these blocks. Finally, the video decoder 300 can receive the bitstream and decode the encoded video data.

[0058] In general, the video decoder 300 performs a process that is the reverse of the process performed by the video encoder 200 to decode the encoded video data of the bitstream. For example, the video decoder 300 may use CABAC to decode the values ​​of the syntax elements for the bitstream in a manner substantially similar to, but reverse to, the CABAC encoding process of the video encoder 200. The syntax elements may define partitioning information for partitioning a picture into CTUs, and partitioning each CTU according to a corresponding partitioning structure (such as a QTBT structure) to define CUs of the CTUs. The syntax elements may also define prediction and residual information for a block (e.g., CU) of video data.

[0059] The residual information may be represented by, for example, quantized transform coefficients. The video decoder 300 may inverse quantize and inverse transform the quantized transform coefficients of the block to regenerate a residual block for the block. The video decoder 300 uses the signaled prediction mode (intra-frame prediction or inter-frame prediction) and related prediction information (e.g., motion information for inter-frame prediction) to form a prediction block for the block. The video decoder 300 may then combine the prediction block and the residual block (on a sample-by-sample basis) to regenerate the original block. The video decoder 300 may perform additional processing, such as performing a deblocking process to reduce visual artifacts along the boundaries of the blocks.

[0060] The bitstream may include a sequence of network abstraction layer (NAL) units. A NAL unit is a syntax structure that includes an indication of a data type and bytes containing data in the form of a raw byte sequence payload (RBSP) in the NAL unit, the RBSP being interspersed with contention prevention bits as necessary. Each NAL unit in the NAL unit may include a NAL unit header and may encapsulate an 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. The RBSP may be a syntax structure containing an integer number of bytes encapsulated within the NAL unit. In some cases, the RBSP includes zero bits.

[0061] As described above, the bitstream may include representations of coded pictures of video data and associated data. The associated data may include parameter sets. The NAL unit may encapsulate RBSPs for video parameter sets (VPS), sequence parameter sets (SPS), and picture parameter sets (PPS). VPS is a syntax structure that includes syntax elements applicable to zero or more fully decoded video sequences (CVSs). SPS is also a syntax structure that includes syntax elements applicable to zero or more entire CVSs. SPS may include syntax elements that identify a VPS that is valid when the SPS is valid. Therefore, the syntax elements of VPS may be more generally applicable than the syntax elements of SPS. PPS is a syntax structure that includes syntax elements applicable to zero or more decoded pictures. PPS may include syntax elements that identify an SPS that is valid when the PPS is valid. The slice header of a slice segment may include syntax elements indicating a PPS that is valid when the slice segment is being decoded.

[0062] In addition, in VVC draft 4, parameter sets may include an adaptation parameter set (APS). An APS is a syntax structure that contains syntax elements applicable to zero or more slices (as determined by zero or more syntax elements found in a slice header). The APS may be stored in memory in a first-in, first-out manner. The APS may be used for syntax elements that change more frequently than syntax elements in a VPS, SPS, or PPS but less frequently than syntax elements in a slice header.

[0063] As described above, the video encoder 200 and the video decoder 300 may apply CABAC encoding and decoding to the values ​​of the syntax elements. In order to apply CABAC encoding to the syntax elements, the video encoder 200 may binarize the values ​​of the syntax elements to form a series of one or more bits, which are referred to as "binary symbols (bin)". In addition, the video encoder 200 may identify a decoding context. The decoding context may identify the probability that a binary symbol has a specific value. For example, the decoding context may indicate a 0.7 probability of decoding a binary symbol of a 0 value and a 0.3 probability of decoding a binary symbol of a 1 value. After identifying the decoding context, the video encoder 200 may divide the interval into a lower sub-interval and an upper sub-interval. One of the sub-intervals may be associated with a value of 0, and the other sub-interval may be associated with a value of 1. The width of the sub-interval may be proportional to the probability indicated by the identified decoding context for the associated value. If the binary symbol of the syntax element has a value associated with the lower sub-interval, the encoded value may be equal to the lower boundary of the lower sub-interval. If the same binary symbol of the syntax element has a value associated with an upper subinterval, the encoded value may be equal to the lower boundary of the upper subinterval. To encode the next binary symbol of the syntax element, the video encoder 200 may repeat these steps, where the interval is a subinterval associated with the value of the encoded bit. When the video encoder 200 repeats these steps for the next binary symbol, the video encoder 200 may use a probability that is modified based on the probability indicated by the identified coding context and the actual value of the encoded binary symbol.

[0064] When the video decoder 300 performs CABAC decoding on the value of the syntax element, the video decoder 300 can identify the decoding context. Then, the video decoder 300 can divide the interval into a lower sub-interval and an upper sub-interval. One of the sub-intervals can be associated with a value of 0, and the other sub-interval can be associated with a value of 1. The width of the sub-interval can be proportional to the probability indicated by the identified decoding context for the associated value. If the encoded value is within the lower sub-interval, the video decoder 300 can decode the binary symbol with the value associated with the lower sub-interval. If the encoded value is within the upper sub-interval, the video decoder 300 can decode the binary symbol with the value associated with the upper sub-interval. In order to decode the next binary symbol of the syntax element, the video decoder 300 can repeat these steps, wherein the interval is a sub-interval containing the encoded value. When the video decoder 300 repeats these steps for the next binary symbol, the video decoder 300 can use the probability modified based on the probability indicated by the identified decoding context and the decoded binary symbol. The video decoder 300 may then binarize the binary symbols to restore the values ​​of the syntax elements.

[0065] A slice of a picture may include an integer number of blocks of the picture. For example, a slice of a picture may include an integer number of CTUs of the picture. A CTB is an NxN block of samples (for a given value of N); dividing the components of a picture (e.g., color components) into CTBs is a partitioning of the picture. A superblock is the top level of a block quadtree within a tile. In addition, in some examples, all superblocks within a picture are of the same size and are square. For example, a superblock may be 128x128 luma samples or 64x64 luma samples. A superblock contains 1 or 2 or 4 mode information blocks, or may be bisected in each direction to create 4 subblocks, which themselves may be further subdivided to form a block quadtree. CTBs and superblocks may be roots of partitions that become smaller blocks. Therefore, the discussion of CTBs in the present disclosure may also apply to superblocks.

[0066] The CTUs of a slice may be ordered consecutively in a scan order, such as a raster scan order. Tile scanning is a specific sequential ordering of CTBs that partition a picture, where the CTBs are ordered consecutively in a raster scan of CTBs in a tile, and tiles in a picture are ordered consecutively in a raster scan of tiles in the picture. In some examples, a tile is a rectangular area of ​​CTBs within a specific tile column and a specific tile row in a picture.

[0067] Adaptive loop filtering is a technique that can improve quality and / or compression in video decoding. In some implementations of ALF, a video decoder (e.g., video encoder 20 or video decoder 30) may determine a classification index for each CTB of the current picture. The classification index for the CTB indicates the category to which the CTB belongs. The video decoder may determine the classification index for the CTB based on the directionality of the block and the quantized value of the activity of the block. For example, the video decoder may determine the classification index for the CTB in a manner described in Section 8.6.4.3 of VVC Draft 4. After determining the classification index for the block of the current picture, the video decoder may apply a filter to each sample of the block. In some examples, the video decoder may use up to three diamond filters for the luminance component. Different classification indexes correspond to different sets of filter coefficients.

[0068] In VVC Test Model 4 (VTM-4.0), the adaptive loop filter for a pixel is applied as follows:

[0069] O(x,y)=I(x,y)+∑ (i,j)≠(0,0) w(i,j)·(I(x+i,y+j)-I(x,y)) (1)

[0070] Wherein, the sample I(x+i,y+j) is an input sample, O(x,y) is a filtered output sample (i.e., the filter result), and w(i,j) represents the filter coefficient. The input sample I(x,y) can be a reconstructed sample of the CTB. The present disclosure may refer to the value I(x+i,y+j)-I(x,y) as the input value of the ALF because the ALF filter coefficients are applied to these values.

[0071] In "CE5: Results of tests CE5-3.1 to CE5-3.4 on Non-Linear Adaptive Loop Filter" by Taquet et al., Joint Video Experts Team (JVET) of ITU-T SG 16 WP 3 and ISO / IEC JTC 1 / SC 29 / WG 11, 14th meeting, Geneva, Switzerland, March 19 - 27, 2019, Document No. JVET-N0242 (hereinafter referred to as "JVET-N0242"), Equation (1) is modified by introducing a clipping operation to produce Equation (2) as follows:

[0072] O′(x,y) = I(x,y) + ∑ (i,j)≠(0,0) w(i,j)·K(I(x+i,y+j)-I(x,y),k(i,j)), (2)

[0073] Wherein, K(d,b) = min(b,max(-b,d)) is a clipping function, and k(i,j) is a clipping parameter that depends on the (i,j) filter coefficient. The clipping parameter k(i,j) indicates the maximum and minimum values that the input value I(x+i,y+j)-I(x,y) may have.

[0074] The clipping operation can be defined as:

[0075]

[0076] In Equation 3 above, z is the input value. Additionally, in Equation 3 above, x and y are clipping parameters corresponding to the maximum and minimum values. The values x and y for position (i,j) can be represented as k(i,j). The estimation of Equation 3 may require performing one or two comparison operations (i.e., z < x; and if z ≥ x, then z > y).

[0077] In Hu et al., “CE5: Coding tree block based adaptive loop filter (CE5-4)”, Joint Video Experts Group (JVET) of ITU-T SG 16WP 3 and ISO / IEC JTC 1 / SC 29 / WG 11, 14th Meeting, Geneva, Switzerland, March 19-27, 2019, Document No. JVET-N0415 (hereinafter referred to as “JVET-N0415”), the video encoder 200 may select a filter set for a coding tree block (CTB) from some candidate sets. In other words, the video encoder 200 may select a filter set for a CTB from multiple candidate sets. In JVET-N0415, when a video decoder (e.g., video encoder 200 or video decoder 300) applies an ALF to a CTB, a filter set index is signaled for each CTB to indicate which set is applied. The candidate filter sets that can be applied to the CTB include the signaled ALF filter sets from previously decoded pictures, tile groups, tiles, fixed filter sets (also called predefined filter sets), and the filter sets signaled for the current tile group, picture, or slice (if any).

[0078] In JVET-N0415, fixed filter sets can also be used as predictors of signaled filter sets. Specifically, JVET-N0415 indicates that for luma components, when ALF is applied to luma CTBs, it is indicated to select among 16 fixed filter sets, 5 temporal filter sets, or 1 signaled filter set. In JVET-N0415, only the filter set index is signaled. In addition, in JVET-N0415, for a slice, only a new set of 25 filters can be signaled. According to JVET-N0415, if a new set is signaled for a slice, all luma CTBs in the same slice share the filter set. The fixed filter set can be used to predict a new slice level filter set and can also be used as a candidate filter set for luma CTBs. According to JVET-N0415, the total number of filters is 64. For chroma components, when ALF is applied to chroma CTB, if a new filter is signaled for the slice, the CTB uses the new filter; otherwise, the latest temporal chroma filter that satisfies the temporal scalability constraint is applied. In the context of JVET-N0415, a temporal scalability constraint refers to a constraint that a picture cannot use coded information (such as filters) from a higher temporal layer.

[0079] When combining these two methods (i.e., the methods of JVET-N0242 and JVET-N0415), it is unclear how to apply clipping with a fixed filter set. This disclosure describes example techniques that can be used to perform clipping with a fixed filter set. The examples and techniques of this disclosure can be used alone or in combination.

[0080] According to a first example technique of the present disclosure, a flag is signaled at a sequence parameter set or a picture parameter set (e.g., by the video encoder 200) to indicate whether a fixed filter set can be used to predict filter coefficients in a nonlinear ALF filter. A nonlinear ALF filter is an ALF filter in which one or more nonlinear operations (such as a clipping operation) are applied. In contrast, a linear ALF filter is an ALF filter in which only linear operations are applied, such as the ALF filters expressed in Equations 1 and 2. In this first example, if a fixed filter set can be used to predict filter coefficients of a nonlinear ALF filter, the video encoder 200 can signal data indicating differential values ​​of filter coefficients of one or more fixed filter sets in the fixed filter set. The video decoder 300 can obtain the flag from the bitstream. If the flag indicates that the fixed filter set can be used to predict filter coefficients of a nonlinear ALF filter, the video decoder 300 can obtain the differential value from the bitstream and add the differential value to the filter coefficients of one or more fixed filter sets in the fixed filter set to reconstruct the filter coefficients of the nonlinear ALF filter.

[0081] According to a second example technique of the present disclosure, some fixed clipping parameter sets are applied to predict clipping parameters in a new nonlinear ALF filter set. As described above, clipping parameters can define upper and lower limits for input values ​​of the ALF. Fixed clipping parameter sets can be predefined. In other words, in this second example, fixed clipping parameter sets can be defined at the video encoder 200 and the video decoder 300 so that the fixed clipping parameter sets do not need to be signaled in the bitstream by the video encoder 200. The video decoder 300 does not need to obtain or process any syntax elements from the bitstream to determine the fixed clipping parameter sets.

[0082] In this second example, the video encoder 200 signals a flag at a sequence, picture, slice, tile group or APS level to determine whether these predefined clipping parameter sets can be used. In addition, if the predefined clipping parameter set can be used to predict the filter coefficient of a nonlinear ALF filter, the video encoder 200 can signal the data indicating the differential value of the predefined clipping parameters of one or more predefined clipping parameter sets in the predefined clipping parameter set. If the flag indicates that the predefined clipping parameter set can be used to predict the clipping parameters of a nonlinear ALF filter, the video decoder 300 can obtain the differential value from the bitstream, and the differential value is added to the clipping parameters of one or more predefined clipping parameter sets in the predefined clipping parameter set to reconstruct the clipping parameters.

[0083] In the third example technique, when a fixed filter set is applied to a CTB, clipping may also be applied. For example, in one example of the third technique, one or more default clipping values ​​(e.g., by the video encoder 200 or the video decoder 300) are used for the fixed filter set. In one such example, when the fixed filter set is applied to the CTB, clipping is not applied. In other words, when one of the fixed filter sets is used to apply the ALF to the CTB, neither the video encoder 200 nor the video decoder 300 performs a clipping operation as part of applying the ALF to the CTB. Therefore, in such an example, based on the clipping value being the maximum supported value, neither the video encoder 200 nor the video decoder 300 applies clipping to the input. In this case, the default clipping value is the maximum value that can be supported. Because the default clipping value is the maximum value that can be supported, the video encoder 200 and the video decoder 300 can be implemented to avoid performing clipping operations because the input value is never greater than the maximum supported value. The maximum supported value itself can be signaled in the bitstream (e.g., by the video encoder 200).

[0084] In another example where a fixed filter set and clipping can be applied, all classes (e.g., filters) of the fixed filter set use the same set of clipping values. For example, in VVC, each filter set can contain 25 filters, which can also be referred to as a class. In another example where a fixed filter set and clipping can be applied (e.g., by the video encoder 200 and the video decoder 300), each class (e.g., filter) has its own set of clipping values.

[0085] In an example where a fixed filter set and clipping can be applied, the video encoder 200 can signal a flag for a sequence / picture / tile group to indicate (e.g., enable) whether clipping can be applied when the fixed filter set is applied to a CTB or other type of block. Signaling a flag for a sequence / picture / tile group means that the video encoder 200 can signal the flag in an SPS, PPS, or tile group header. In some examples of the third technology, when clipping can be applied to a CTB (or block), the video encoder 200 can signal a flag for a sequence / picture / tile group to indicate that all CTBs (or blocks) in the picture / tile group use the same clipping parameters. These clipping parameters can be explicitly signaled (e.g., by the video encoder 200), or can be the same as the clipping parameters of the clipping parameter set from one or more previously decoded clipping parameters.

[0086] In addition, in one example of the third technique, when clipping can be applied to a CTB, the video encoder 200 can signal a flag to indicate whether clipping is applied to the CTB. The video decoder 300 can obtain the flag from the bitstream and determine whether clipping is applied to the CTB (or block) based on the flag. Clipping information from a previously decoded CTB (or block) or predefined clipping parameters can be used (e.g., by the video encoder 200 and the video decoder 300) to establish a context model to signal the flag. In other words, the video encoder 200 and the video decoder 300 can select a context model for use when CABAC decoding a flag indicating whether clipping is applied to the CTB (or block) based on clipping information from a previously decoded CTB or predefined clipping parameters. For example, a mapping from each predefined clipping parameter set to a context model can be defined. The video encoder 200 and the video decoder 300 can use the mapping to select a context model. Similarly, in some examples, the values ​​of the upper and lower limits expressed in the clipping information used with a previously decoded CTB (or block) may be associated with different context models.

[0087] In some examples of the third technique, when clipping is applied to a CTB (or block), the video encoder 200 may signal a flag to indicate whether the clipping parameters are explicitly signaled or the clipping parameters are the same as the clipping parameters in a previously decoded non-linear ALF filter set. The previously decoded non-linear ALF filter set may be signaled earlier in the bitstream than the data representing the CTB (or block). The video decoder 300 may obtain the flag from the bitstream. The video decoder 300 may then determine based on the flag whether the clipping parameters to be used when applying the ALF to the CTB (or block) are explicitly signaled or the same as the clipping parameters in the previously decoded non-linear ALF filter set.

[0088] In addition, in some examples of the third technique, when a CTB (or block) reuses clipping parameters or predefined clipping parameters in a previously decoded nonlinear ALF filter set, the video encoder 200 can signal an index to indicate the previously decoded nonlinear ALF filter set or predefined clipping parameter set from which the clipping parameters of the current CTB (or block) were obtained. In such an example, the video decoder 300 can obtain the index from the bitstream. The video decoder 300 can determine which previously decoded nonlinear ALF filter set or predefined clipping parameter set the index indicates. The video decoder 300 can use the indicated filter set or clipping parameter set when applying the ALF to the current CTB (or block).

[0089] In general, the present disclosure may involve "signaling" certain information (such as syntax elements). The term "signaling" may generally refer to the transmission of values ​​for syntax elements and / or other data for decoding encoded video data by the video encoder 200. That is, the video encoder 200 may signal values ​​for syntax elements in a bitstream. Generally, signaling refers to generating values ​​in a bitstream. As described above, the source device 102 may transmit the bitstream to the destination device 116 in substantially real time or not in real time (such as may occur when the syntax elements are stored to the storage device 112 for later retrieval by the destination device 116). The video decoder 300 may obtain the signaled information (e.g., syntax elements) from the bitstream. Thus, the video encoder 200 may encode the syntax elements signaled in the bitstream, and the video decoder 300 may decode the syntax elements signaled in the bitstream; thus, the video decoder may decode the syntax elements signaled in the bitstream.

[0090] Figure 2A and 2B1 is a conceptual diagram showing an example quadtree binary tree (QTBT) structure 130 and a corresponding coding tree unit (CTU) 132. Solid lines represent quadtree splitting, while dashed lines indicate binary tree splitting. In each split (i.e., non-leaf) node of the binary tree, a flag is signaled to indicate which split type (i.e., horizontal or vertical) is used, where, in this example, 0 indicates horizontal splitting and 1 indicates vertical splitting. For quadtree splitting, since the quadtree node splits the block horizontally and vertically into 4 sub-blocks of equal size, there is no need to indicate the split type. Therefore, the video encoder 200 can encode the following, and the video decoder 300 can decode the following: syntax elements (such as split information) for the regional tree level (i.e., the first level) of the QTBT structure 130 (i.e., solid lines), and syntax elements (such as split information) for the prediction tree level (i.e., the second level) of the QTBT structure 130 (i.e., dashed lines). The video encoder 200 may encode video data (such as prediction and transform data) for a CU represented by a terminal leaf node of the QTBT structure 130 , and the video decoder 300 may decode the video data.

[0091] generally, Figure 2B The CTU 132 may be associated with parameters defining the size of blocks corresponding to nodes at the first and second levels of the QTBT structure 130. These parameters may include a CTU size (indicating the size of the CTU 132 in samples), a minimum quadtree size (MinQTSize, which indicates the minimum allowed quadtree leaf node size), a maximum binary tree size (MaxBTSize, which indicates the maximum allowed binary tree root node size), a maximum binary tree depth (MaxBTDepth, which indicates the maximum allowed binary tree depth), and a minimum binary tree size (MinBTSize, which indicates the minimum allowed binary tree leaf node size).

[0092] The root node of the QTBT structure corresponding to the CTU may have four child nodes at the first level of the QTBT structure, each of which may be segmented according to a quadtree segmentation. That is, the node of the first level is a leaf node (without child nodes) or has four child nodes. The example of the QTBT structure 130 represents such a node as including a parent node and a child node with a solid line branch. If the node of the first level is not larger than the maximum allowed binary tree root node size (MaxBTSize), they may be further segmented by the corresponding binary tree. The binary tree splitting of a node may be iterated until the node generated from the split reaches the minimum allowed binary tree leaf node size (MinBTSize) or the maximum allowed binary tree depth (MaxBTDepth). The example of the QTBT structure 130 represents such a node as having a dotted line branch. The binary tree leaf node is called a coding unit (CU), which is used for prediction (e.g., intra-picture or inter-picture prediction) and transformation without any further segmentation. As discussed above, a CU may also be referred to as a "video block" or "block".

[0093] In one example of a QTBT partitioning structure, the CTU size is set to 128x128 (luminance sample and two corresponding 64x64 chrominance samples), MinQTSize is set to 16x16, MaxBTSize is set to 64x64, MinBTSize (for both width and height) is set to 4, and MaxBTDepth is set to 4. Quadtree partitioning is first applied to the CTU to generate a quadtree leaf node. The quadtree leaf node can have a size from 16x16 (i.e., MinQTSize) to 128x128 (i.e., CTU size). If the quadtree leaf node is 128x128, the quadtree leaf node will not be further split by the binary tree because the size exceeds MaxBTSize (i.e., 64x64 in this example). Otherwise, the quadtree leaf node will be further split by the binary tree. Therefore, the quadtree leaf node is also the root node for the binary tree and has a binary tree depth of 0. When the binary tree depth reaches MaxBTDepth (4 in this example), no further splitting is allowed. When a binary tree node has a width equal to MinBTSize (4 in this example), it means that no further vertical splitting is allowed. Similarly, a binary tree node with a height equal to MinBTSize means that no further horizontal splitting is allowed for the binary tree node. As described above, the leaf nodes of the binary tree are called CUs and are further processed according to prediction and transformation without further segmentation.

[0094] Figure 3 is a block diagram illustrating an example video encoder 200 that may perform the techniques of this disclosure. Figure 3The above description is provided for the purpose of explanation and should not be considered to limit the techniques generally exemplified and described in this disclosure. For the purpose of explanation, this disclosure describes the video encoder 200 in the context of video coding standards such as the HEVC video coding standard and the developing H.266 (VVC) video coding standard. However, the techniques of this disclosure are not limited to these video coding standards and are generally applicable to video encoding and decoding.

[0095] exist Figure 3 In the example of , the video encoder 200 includes a video data memory 230, a mode selection unit 202, a residual generation unit 204, a transform processing unit 206, a quantization unit 208, an inverse quantization unit 210, an inverse transform processing unit 212, a reconstruction unit 214, a filter unit 216, a decoded picture buffer (DPB) 218, and an entropy coding unit 220. Any one or all of the video data memory 230, the mode selection unit 202, the residual generation unit 204, the transform processing unit 206, the quantization unit 208, the inverse quantization unit 210, the inverse transform processing unit 212, the reconstruction unit 214, the filter unit 216, the DPB 218, and the entropy coding unit 220 may be implemented in one or more processors or in processing circuits. In addition, the video encoder 200 may include additional or alternative processors or processing circuits to perform these and other functions.

[0096] The video data memory 230 may store video data to be encoded by the components of the video encoder 200. The video encoder 200 may receive video data from, for example, the video source 104 ( Figure 1 ) receives the video data stored in the video data memory 230. DPB 218 can act as a reference picture memory, which stores reference video data for use when subsequent video data is predicted by the video encoder 200. Video data memory 230 and DPB 218 can be formed by 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 230 and DPB 218 can be provided by the same memory device or a separate memory device. In various examples, video data memory 230 can be on chip with other components of the video encoder 200 (as shown), or off chip relative to those components.

[0097] In the present disclosure, references to the video data memory 230 should not be interpreted as limited to memory internal to the video encoder 200 (unless specifically described as such), or to memory external to the video encoder 200 (unless specifically described as such). Rather, references to the video data memory 230 should be understood as a reference memory that stores video data received by the video encoder 200 for encoding (e.g., video data for a current block to be encoded). Figure 1 The memory 106 may also provide temporary storage for outputs from the various units of the video encoder 200 .

[0098] Explained Figure 3 The various units of the video encoder 200 are provided to help understand the operations performed by the video encoder 200. These units may be implemented as fixed-function circuits, programmable circuits, or a combination thereof. Fixed-function circuits refer to circuits that provide specific functions and are pre-set with respect to the operations that can be performed. Programmable circuits refer to circuits that can be programmed to perform various tasks and provide flexible functions with the operations that can be performed. For example, a programmable circuit may execute software or firmware that causes the programmable circuit to operate in a manner defined by the instructions of the software or firmware. Fixed-function circuits may execute software instructions (e.g., to receive parameters or output parameters), but the types of operations performed by fixed-function circuits are generally immutable. In some examples, one or more of these units may be different circuit blocks (fixed-function or programmable), and in some examples, one or more units may be integrated circuits.

[0099] The video encoder 200 may include an arithmetic logic unit (ALU), an elementary function unit (EFU), a digital circuit, an analog circuit, and / or a programmable core formed by a programmable circuit. In an example where the operation of the video encoder 200 is performed using software executed by a programmable circuit, the memory 106 ( Figure 1 ) may store object code for software received and executed by the video encoder 200, or another memory (not shown) within the video encoder 200 may store such instructions.

[0100] The video data memory 230 is configured to store the received video data. The video encoder 200 may retrieve the picture of the video data from the video data memory 230 and provide the video data to the residual generation unit 204 and the mode selection unit 202. The video data in the video data memory 230 may be the original video data to be encoded.

[0101] The mode selection unit 202 includes a motion estimation unit 222, a motion compensation unit 224, and an intra prediction unit 226. The mode selection unit 202 may include additional functional units that perform video prediction according to other prediction modes. As examples, the mode selection unit 202 may include a palette decoding unit, an intra block copy decoding unit (which may be part of the motion estimation unit 222 and / or the motion compensation unit 224), an affine decoding unit, a linear model (LM) decoding unit, etc.

[0102] The mode selection unit 202 generally coordinates multiple encoding passes to test combinations of encoding parameters and the resulting rate-distortion values ​​for such combinations. The encoding parameters may include partitioning of a CTU into CUs, a prediction mode for a CU, a transform type for residual data of a CU, a quantization parameter for residual data of a CU, etc. The mode selection unit 202 may ultimately select a combination of encoding parameters that has a better rate-distortion value than other tested combinations.

[0103] The video encoder 200 may partition the picture retrieved from the video data memory 230 into a series of CTUs, and encapsulate one or more CTUs in a slice. The mode selection unit 202 may partition the CTUs of the picture according to a tree structure (such as the QTBT structure of VCC or the quadtree structure of HEVC). As described above, the video encoder 200 may partition the CTUs according to the tree structure to form one or more CUs. Such a CU may also be generally referred to as a "video block" or "block".

[0104] Typically, the mode selection unit 202 also controls its components (e.g., the motion estimation unit 222, the motion compensation unit 224, and the intra prediction unit 226) to generate a prediction block for the current block (e.g., the current CU, or the overlapping portion of the PU and TU in HEVC). In order to inter-predict the current block, the motion estimation unit 222 may perform a motion search to identify one or more closely matching reference blocks in one or more reference pictures (e.g., one or more previously decoded pictures stored in the DPB 218). Specifically, the motion estimation unit 222 may calculate a value representing the degree of similarity of the potential reference block to the current block, for example, based on the sum of absolute differences (SAD), the sum of squared differences (SSD), the mean absolute difference (MAD), the mean squared difference (MSD), etc. The motion estimation unit 222 may typically perform these calculations using the sample-by-sample difference between the current block and the reference block under consideration. The motion estimation unit 222 may identify the reference block with the lowest value resulting from these calculations, which indicates the reference block that most closely matches the current block.

[0105] The motion estimation unit 222 may form one or more motion vectors (MVs) that define the position of the reference block in the reference picture relative to the position of the current block in the current picture. The motion estimation unit 222 may then provide the motion vectors to the motion compensation unit 224. For example, for unidirectional inter prediction, the motion estimation unit 222 may provide a single motion vector, while for bidirectional inter prediction, the motion estimation unit 222 may provide two motion vectors. The motion compensation unit 224 may then use the motion vectors to generate a prediction block. For example, the motion compensation unit 224 may use the motion vectors to retrieve data for the reference block. As another example, if the motion vectors have fractional sample precision, the motion compensation unit 224 may interpolate the values ​​for the prediction block according to one or more interpolation filters. In addition, for bidirectional inter prediction, the motion compensation unit 224 may retrieve data for two reference blocks identified by the corresponding motion vectors and combine the retrieved data, for example, by sample-by-sample averaging or weighted averaging.

[0106] As another example, for intra prediction or intra prediction decoding, the intra prediction unit 226 can generate a prediction block based on samples adjacent to the current block. For example, for directional mode, the intra prediction unit 226 can generally mathematically combine the values ​​of adjacent samples and fill these calculated values ​​across the current block in a defined direction to produce a prediction block. As another example, for DC mode, the intra prediction unit 226 can calculate the average of adjacent samples of the current block and generate a prediction block to include the resulting average for each sample of the prediction block.

[0107] The mode selection unit 202 provides the prediction block to the residual generation unit 204. The residual generation unit 204 receives the original undecoded version of the current block from the video data memory 230 and receives the prediction block from the mode selection unit 202. The residual generation unit 204 calculates the sample-by-sample difference between the current block and the prediction block. The resulting sample-by-sample difference defines a residual block for the current block. In some examples, the residual generation unit 204 may also determine the difference between the sample values ​​in the residual block to generate the residual block using residual differential pulse coding modulation (RDPCM). In some examples, the residual generation unit 204 may be formed using one or more subtractor circuits that perform binary subtraction.

[0108] In an example where the mode selection unit 202 partitions the CU into PUs, each PU may be associated with a luma prediction unit and a corresponding chroma prediction unit. The video encoder 200 and the video decoder 300 may support PUs of various sizes. As noted above, the size of the CU may refer to the size of the luma coding block of the CU, and the size of the PU may refer to the size of the luma prediction unit of the PU. Assuming that the size of a particular CU is 2Nx2N, the video encoder 200 may support PU sizes of 2Nx2N or NxN for intra-frame prediction, and 2Nx2N, 2NxN, Nx2N, NxN or similar symmetrical PU sizes for inter-frame prediction. The video encoder 200 and the video decoder 300 may also support asymmetric partitioning for PU sizes of 2NxnU, 2NxnD, nLx2N, and nRx2N for inter-frame prediction.

[0109] In an example where the mode selection unit does not further split the CU into PUs, each CU may be associated with a luma coding block and a corresponding chroma coding block. As described above, the size of a CU may refer to the size of the luma coding block of the CU. The video encoder 200 and the video decoder 300 may support a CU size of 2Nx2N, 2NxN, or Nx2N.

[0110] For other video coding techniques (such as intra block copy mode coding, affine mode coding, and linear model (LM) mode coding, to name a few examples), the mode selection unit 202 generates a prediction block for the current block being encoded via a corresponding unit associated with the coding technique. In some examples (such as palette mode coding), the mode selection unit 202 may not generate a prediction block, but instead generate syntax elements indicating how to reconstruct the block based on the selected palette. In such a mode, the mode selection unit 202 may provide these syntax elements to the entropy coding unit 220 for encoding.

[0111] As described above, the residual generation unit 204 receives video data for a current block and a corresponding prediction block. The residual generation unit 204 then generates a residual block for the current block. To generate the residual block, the residual generation unit 204 calculates the sample-by-sample difference between the prediction block and the current block.

[0112] The transform processing unit 206 applies one or more transforms to the residual block to generate a block of transform coefficients (referred to herein as a "transform coefficient block"). The transform processing unit 206 can apply various transforms to the residual block to form a transform coefficient block. For example, the transform processing unit 206 can apply a discrete cosine transform (DCT), a directional transform, a Karhunen-Loeve transform (KLT), or a conceptually similar transform to the residual block. In some examples, the transform processing unit 206 can perform a variety of transforms on the residual block, for example, a primary transform and a secondary transform (such as a rotation transform). In some examples, the transform processing unit 206 does not apply a transform to the residual block.

[0113] The quantization unit 208 may quantize the transform coefficients in the transform coefficient block to produce a quantized transform coefficient block. The quantization unit 208 may quantize the transform coefficients of the transform coefficient block according to a quantization parameter (QP) value associated with the current block. The video encoder 200 (e.g., via the mode selection unit 202) may adjust the degree of quantization applied to the coefficient block associated with the current block by adjusting the QP value associated with the CU. Quantization may cause a loss of information, and therefore, the quantized transform coefficients may have a lower precision than the original transform coefficients produced by the transform processing unit 206.

[0114] The inverse quantization unit 210 and the inverse transform processing unit 212 may apply inverse quantization and inverse transform, respectively, to the quantized transform coefficient block to reconstruct a residual block from the transform coefficient block. The reconstruction unit 214 may generate a reconstructed block corresponding to the current block (although potentially with some degree of distortion) based on the reconstructed residual block and the prediction block generated by the mode selection unit 202. For example, the reconstruction unit 214 may add samples of the reconstructed residual block to corresponding samples of the prediction block generated by the mode selection unit 202 to generate a reconstructed block.

[0115] The filter unit 216 may perform one or more filter operations on the reconstructed block. For example, the filter unit 216 may perform a deblocking operation to reduce blocking artifacts along the edge of the CU. In some examples, the filter unit 216 may apply the ALF. For example, according to one or more techniques of the present disclosure, the filter unit 216 may determine whether the filter set to be used with the CTB is from a plurality of fixed filter sets. If the filter set to be used with the CTB is from a plurality of fixed filter sets, the filter unit 216 may set the clipping value to the maximum supported clipping value. The filter unit 216 may then determine the clipped input for the ALF based on the clipping value. The filter unit 216 may then apply the ALF to the clipped input.

[0116] The video encoder 200 stores the reconstructed blocks in the DPB 218. For example, in an example where the operation of the filter unit 216 is not required, the reconstruction unit 214 can store the reconstructed blocks in the DPB 218. In an example where the operation of the filter unit 216 is required, the filter unit 216 can store the filtered reconstructed blocks in the DPB 218. The motion estimation unit 222 and the motion compensation unit 224 can retrieve the reference picture formed by the reconstructed (and potentially filtered) blocks from the DPB 218 to perform inter-frame prediction on the blocks of the subsequently encoded pictures. In addition, the intra-frame prediction unit 226 can use the reconstructed blocks of the current picture in the DPB 218 to perform intra-frame prediction on other blocks in the current picture.

[0117] In general, the entropy coding unit 220 may entropy encode syntax elements received from other functional components of the video encoder 200. For example, the entropy coding unit 220 may entropy encode a quantized transform coefficient block from the quantization unit 208. As another example, the entropy coding unit 220 may entropy encode a prediction syntax element (e.g., motion information for inter-frame prediction or intra-frame mode information for intra-frame prediction) from the mode selection unit 202. The entropy coding unit 220 may perform one or more entropy coding operations on syntax elements as another example of video data to generate entropy-encoded data. For example, the entropy coding unit 220 may perform a context-adaptive variable length coding (CAVLC) operation, a CABAC operation, a variable-to-variable (V2V) length coding operation, a syntax-based context-adaptive binary arithmetic coding (SBAC) operation, a probability interval partitioning entropy (PIPE) coding operation, an exponential Golomb coding operation, or another type of entropy coding operation on the data. In some examples, the entropy coding unit 220 may operate in a bypass mode in which the syntax elements are not entropy encoded.

[0118] The video encoder 200 may output a bitstream including entropy-encoded syntax elements required for reconstructing a block of a slice or picture. Specifically, the entropy encoding unit 220 may output a bitstream.

[0119] The above operations are described with respect to blocks. Such descriptions should be understood as operations for luma coding blocks and / or chroma coding blocks. As described above, in some examples, the luma coding blocks and chroma coding blocks are the luma components and chroma components of a CU. In some examples, the luma coding blocks and chroma coding blocks are the luma components and chroma components of a PU.

[0120] In some examples, operations performed with respect to luma coding blocks need not be repeated for chroma coding blocks. As an example, operations for identifying motion vectors (MVs) and reference pictures for luma coding blocks need not be repeated to identify MVs and reference pictures for chroma blocks. Specifically, the MVs for luma coding blocks may be scaled to determine MVs for chroma blocks, and the reference pictures may be the same. As another example, the intra prediction process may be the same for luma coding blocks and chroma coding blocks.

[0121] Video encoder 200 represents an example of a device configured to encode video data, the device comprising: a memory configured to store the video data; and one or more processing units implemented in circuits and configured to: encode a syntax element at a sequence parameter set (SPS) or picture parameter set (PPS) level of a bitstream comprising an encoded representation of the video data, the syntax element indicating whether a fixed filter set can be used to predict filter coefficients in a non-linear ALF filter; and apply the non-linear ALF filter to a block of the video data based on the syntax element.

[0122] In some examples, the video encoder 200 represents an example of a device configured to encode video data, the device comprising: a memory configured to store video data; and one or more processing units implemented in circuits and configured to: apply a fixed clipping parameter set to predict clipping parameters of a nonlinear ALF filter set; and apply a nonlinear ALF filter to a block of video data based on the nonlinear ALF filter set. In some examples, the video encoder 200 represents an example of a device configured to encode video data, the device comprising: a memory configured to store video data; and one or more processing units implemented in circuits and configured to: determine a filter set for a CTB from a plurality of fixed filter sets; clip an input to an adaptive loop filter for the filter set, the input being a sample of a CTB; and apply the ALF to the clipped input. The motion estimation unit 222, the motion compensation unit 224, and / or the intra prediction unit 226 may generate a prediction block using a filtered CTB generated by applying the ALF to the clipped input.

[0123] Figure 4 is a block diagram illustrating an example video decoder 300 that may perform the techniques of this disclosure. Figure 4 It is provided for the purpose of explanation and does not limit the techniques generally illustrated and described in this disclosure. For the purpose of explanation, this disclosure describes the video decoder 300 according to the techniques of JEM, VVC and HEVC. However, the techniques of this disclosure can be performed by a video decoding device configured for other video decoding standards.

[0124] exist Figure 4 In the example of , the video decoder 300 includes a coded picture buffer (CPB) memory 320, an entropy decoding unit 302, a prediction processing unit 304, an inverse quantization unit 306, an inverse transform processing unit 308, a reconstruction unit 310, a filter unit 312, and a decoded picture buffer (DPB) 134. Any or all of the CPB memory 320, the entropy decoding unit 302, the prediction processing unit 304, the inverse quantization unit 306, the inverse transform processing unit 308, the reconstruction unit 310, the filter unit 312, and the DPB 134 may be implemented in one or more processors or in processing circuitry. Furthermore, the video decoder 300 may include additional or alternative processors or processing circuitry to perform these and other functions.

[0125] The prediction processing unit 304 includes a motion compensation unit 316 and an intra prediction unit 318. The prediction processing unit 304 may include an addition unit that performs predictions according to other prediction modes. As an example, the prediction processing unit 304 may include a palette unit, an intra block copy unit (which may form part of the motion compensation unit 316), an affine unit, a linear model (LM) unit, etc. In other examples, the video decoder 300 may include more, fewer, or different functional components.

[0126] CPB memory 320 may store video data, such as an encoded video bitstream, to be decoded by components of video decoder 300. For example, the video bitstream may be obtained from computer readable medium 110 ( Figure 1 ) obtains video data stored in CPB memory 320. CPB memory 320 may include a CPB that stores encoded video data (e.g., syntax elements) from an encoded video bitstream. In addition, CPB memory 320 may store video data other than syntax elements of decoded pictures, such as temporary data representing outputs from various units of video decoder 300. DPB 314 typically stores decoded pictures, and video decoder 300 may output decoded pictures and / or use decoded pictures as reference video data when decoding subsequent data or pictures of the encoded video bitstream. CPB memory 320 and DPB 314 may be formed by 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. CPB memory 320 and DPB 314 may be provided by the same memory device or separate memory devices. In various examples, CPB memory 320 can be on-chip with other components of video decoder 300 , or off-chip relative to those components.

[0127] Additionally or alternatively, in some examples, video decoder 300 may retrieve the video from memory 120 ( Figure 1 ) to retrieve the decoded video data. That is, memory 120 may utilize CPB memory 320 to store data as discussed above. Likewise, when some or all of the functionality of video decoder 300 is implemented in software to be executed by processing circuitry of video decoder 300, memory 120 may store instructions to be executed by video decoder 300.

[0128] It explains Figure 4 The various units shown in FIG. 3 are used to help understand the operations performed by the video decoder 300. These units may be implemented as fixed function circuits, programmable circuits, or a combination thereof. Figure 3 , fixed-function circuits refer to circuits that provide specific functions and are pre-set with respect to the operations that can be performed. Programmable circuits refer to circuits that can be programmed to perform various tasks and provide flexible functions with respect to the operations that can be performed. For example, a programmable circuit can execute software or firmware that causes the programmable circuit to operate in a manner defined by the instructions of the software or firmware. Fixed-function circuits can execute software instructions (e.g., to receive parameters or output parameters), but the types of operations performed by fixed-function circuits are generally immutable. In some examples, one or more of these units may be different circuit blocks (fixed-function or programmable), and in some examples, one or more units may be integrated circuits.

[0129] The video decoder 300 may include an ALU, an EFU, a digital circuit, an analog circuit, and / or a programmable core formed by a programmable circuit. In an example where the operation of the video decoder 300 is performed by software executed on the programmable circuit, an on-chip or off-chip memory may store instructions (e.g., object code) of the software received and executed by the video decoder 300.

[0130] The entropy decoding unit 302 may receive the encoded video data from the CPB and entropy decode the video data to regenerate the syntax elements. The prediction processing unit 304, the inverse quantization unit 306, the inverse transform processing unit 308, the reconstruction unit 310, and the filter unit 312 may generate decoded video data based on the syntax elements extracted from the bitstream.

[0131] Generally, the video decoder 300 reconstructs a picture block by block. The video decoder 300 may perform a reconstruction operation on each block individually (wherein a block currently being reconstructed (ie, decoded) may be referred to as a "current block").

[0132] The entropy decoding unit 302 may entropy decode syntax elements defining the quantized transform coefficients of the quantized transform coefficient block and transform information such as a quantization parameter (QP) and / or a transform mode indication. The inverse quantization unit 306 may use the QP associated with the quantized transform coefficient block to determine a degree of quantization and, likewise, determine a degree of inverse quantization for the inverse quantization unit 306 to apply. The inverse quantization unit 306 may, for example, perform a bitwise left shift operation to inverse quantize the quantized transform coefficients. The inverse quantization unit 306 may thereby form a transform coefficient block including the transform coefficients.

[0133] After the inverse quantization unit 306 forms the transform coefficient block, the inverse transform processing unit 308 may apply one or more inverse transforms to the transform coefficient block to generate a residual block associated with the current block. For example, the inverse transform processing unit 308 may apply an inverse DCT, an inverse integer transform, an inverse Karhunen-Loeve transform (KLT), an inverse rotational transform, an inverse directional transform, or another inverse transform to the coefficient block.

[0134] In addition, prediction processing unit 304 generates a prediction block based on the prediction information syntax elements entropy decoded by entropy decoding unit 302. For example, if the prediction information syntax elements indicate that the current block is inter-predicted, motion compensation unit 316 can generate a prediction block. In this case, the prediction information syntax elements can indicate a reference picture in DPB 314 from which to retrieve the reference block, and a motion vector that identifies the position of the reference block in the reference picture relative to the position of the current block in the current picture. Motion compensation unit 316 can generally generate a prediction block in the same manner as described for motion compensation unit 224 ( Figure 3 )The inter-frame prediction process is performed in a manner substantially similar to that described in detail above.

[0135] As another example, if the prediction information syntax element indicates that the current block is intra-predicted, the intra-prediction unit 318 may generate a prediction block according to the intra-prediction mode indicated by the prediction information syntax element. Again, the intra-prediction unit 318 may generally generate a prediction block in the same manner as described with respect to the intra-prediction unit 226 ( Figure 3 The intra prediction process is performed in a substantially similar manner as described in the above description. The intra prediction unit 318 may retrieve data of neighboring samples of the current block from the DPB 314.

[0136] The reconstruction unit 310 may reconstruct the current block using the prediction block and the residual block. For example, the reconstruction unit 310 may reconstruct the current block by adding samples of the residual block to corresponding samples of the prediction block.

[0137] The filter unit 312 may perform one or more filter operations on the reconstructed block. For example, the filter unit 312 may perform a deblocking operation to reduce block artifacts along the edge of the reconstructed block. In some examples, the filter unit 312 applies ALF. For example, according to one or more techniques of the present disclosure, the filter unit 312 may determine whether the filter set to be used with the CTB is from a plurality of fixed (i.e., predefined) filter sets. If the filter set to be used with the CTB is from a plurality of fixed filter sets, the filter unit 312 may set the clipping value to the maximum supported clipping value, which may be equivalent to not applying any clipping. Therefore, in such an example, the filter unit 312 may not apply clipping to the input based on the clipping value being the maximum supported value. Then, the filter unit 312 may determine the clipped input for the ALF based on the clipping value. Then, the filter unit 312 may apply the ALF to the clipped input.

[0138] The video decoder 300 may store the reconstructed block in the DPB 314. For example, in examples where the operation of the filter unit 312 is not performed, the reconstruction unit 310 may store the reconstructed block in the DPB 314. In examples where the operation of the filter unit 312 is performed, the filter unit 312 may store the filtered reconstructed block in the DPB 314. As discussed above, the DPB 314 may provide reference information (such as samples of the current picture for intra-frame prediction and previously decoded pictures for subsequent motion compensation) to the prediction processing unit 304. In addition, the video decoder 300 may output the decoded picture from the DPB for use in, for example, Figure 1 Subsequent presentation on a display device such as display device 118.

[0139] In this manner, the video decoder 300 represents an example of a device configured to decode video data, the device comprising: a memory (e.g., DPB 314) configured to store video data; and one or more processing units implemented in circuits and configured to: decode a syntax element indicating whether a fixed filter set can be used to predict filter coefficients in a non-linear ALF filter at an SPS or PPS level of a bitstream comprising an encoded representation of the video data. In some examples, the entropy decoding unit 302 decodes the syntax element. In addition, the one or more processing units of the video decoder 300 can apply a non-linear ALF filter to a block of video data based on the syntax element. For example, the filter unit 312 can apply a non-linear ALF filter to the block.

[0140] In some examples, the video decoder 300 represents an example of a device configured to decode video data, the device comprising: a memory (e.g., DPB 314) configured to store the video data; and one or more processing units implemented in circuits and configured to: apply a fixed clipping parameter set to predict clipping parameters of a non-linear ALF filter set. In addition, the one or more processing units of the video decoder 300 can apply a non-linear ALF filter to a block of the video data based on the non-linear ALF filter set. For example, the filter unit 312 of the video decoder 300 can apply a fixed clipping parameter set to predict clipping parameters of a non-linear ALF filter set, and apply a non-linear ALF filter based on the non-linear ALF filter set.

[0141] In some examples, video decoder 300 represents an example of a device configured to decode video data, the device including: a memory configured to store video data (e.g., DPB 314); and one or more processing units implemented in circuits and configured to: determine a filter set for CTB from a plurality of fixed filter sets; clip an input to an adaptive loop filter for the filter set, the input being a sample of the CTB; and apply an ALF to the clipped input. Filter unit 312 may determine a filter set for the CTB, clip the input, and apply an ALF to the clipped input.

[0142] In some examples, the video decoder 300 represents an example of a device configured to decode video data, the device comprising: a memory (e.g., DPB 314) configured to store video data; and one or more processing units implemented in a circuit and configured to: determine a filter set for a CTB from a plurality of fixed filter sets, wherein the CTB is in a picture of the video data. In this example, based on the filter set for the CTB being from the fixed filter set, the video decoder 300 sets the clipping value to a maximum supported value. In addition, the video decoder 300 may determine a clipped input for an ALF of the filter set based on the clipping value, each of the inputs being an input sample minus a current sample. The video decoder 300 may apply the ALF to the clipped input. In this example, the filter unit 312 of the video decoder 300 may determine the filter set, set the clipping value, determine the clipped input, and apply the ALF to the clipped input. The motion compensation unit 316 and / or the intra prediction unit 318 may generate a prediction block using the filtered CTB generated by applying the ALF to the clipped input.

[0143] Figure 5 2 is a flowchart illustrating an example method for encoding a current block. The current block may include a current CU. Although with respect to the video encoder 200 ( Figure 1 and 2), but it should be understood that other devices may be configured to perform the same Figure 5 A similar approach to the one used in this paper.

[0144] In this example, the video encoder 200 initially predicts a current block (350). For example, the video encoder 200 may form a prediction block for the current block. The video encoder 200 may then calculate a residual block for the current block (352). To calculate the residual block, the video encoder 200 may calculate the difference between the original uncoded block and the prediction block for the current block. The video encoder 200 may then transform and quantize the transform coefficients of the residual block (354). Next, the video encoder 200 may scan the quantized transform coefficients of the residual block (356). During or after the scan, the video encoder 200 may entropy encode the transform coefficients (358). For example, the video encoder 200 may encode the transform coefficients using CAVLC or CABAC. The video encoder 200 may then output the entropy encoded data of the block (360).

[0145] Despite Figure 5 Although not shown in the example of FIG. 1 , the video encoder 200 may inverse quantize the transform coefficients and apply inverse transform to the inverse quantized transform coefficients to reconstruct residual data. The video encoder 200 may reconstruct the current block based on the prediction block and the reconstructed residual data. The video encoder 200 may apply the ALF to the CTB including the current block after combining the prediction block and the residual block. Figure 7 Example operations for applying an ALF filter to a CTB are included in accordance with one or more techniques of this disclosure. Video encoder 200 may use the filter CTB for future reference (eg, as part of a reference picture).

[0146] Figure 6 is a flowchart illustrating an example method for decoding a current block of video data. The current block may include a current CU. Although with respect to the video decoder 300 ( Figure 1 and 3 ), but it should be understood that other devices may be configured to perform the same Figure 6 A similar approach to the one used in this paper.

[0147] The video decoder 300 may receive entropy encoded data for a current block (e.g., entropy encoded prediction information and entropy encoded data for transform coefficients of a residual block corresponding to the current block) (370). The video decoder 300 may entropy decode the entropy encoded data to determine the prediction information for the current block and regenerate the transform coefficients of the residual block (372). The video decoder 300 may predict the current block (374), for example, by calculating a prediction block for the current block using an intra-frame or inter-frame prediction mode indicated by the prediction information for the current block. The video decoder 300 may then inverse scan the regenerated transform coefficients (376) to create a block of quantized transform coefficients. The video decoder 300 may then inverse quantize the transform coefficients and apply an inverse transform to the transform coefficients to generate a residual block (378). Ultimately, the video decoder 300 may decode the current block by combining the prediction block and the residual block (380). Although in Figure 6 Although not shown in the example of , the video decoder 300 may also apply ALF to the CTB including the current block after combining the prediction block and the residual block. Figure 7 Included are example operations for applying an ALF filter to a CTB in accordance with one or more techniques of this disclosure.

[0148] Figure 7 is a flow diagram illustrating example operations for applying an ALF filter to a CTB, in accordance with one or more techniques of this disclosure. Figure 7 The operations may be performed by a video decoder (such as the video encoder 200 or the video decoder 300).

[0149] exist Figure 7 In an example, a video decoder may determine whether a filter set for a CTB is from a plurality of fixed filter sets (400). The CTB is in a picture of video data. The video decoder may determine whether the filter set for the CTB is from a plurality of fixed filter sets in one of a plurality of ways. For example, in some examples, a flag signaled in a bitstream may indicate whether the filter set for the CTB is from a plurality of fixed filter sets. The flag may be signaled in a sequence parameter set, a picture parameter set, a slice header, a tile group header, an adaptive parameter set, or in another syntax structure in the bitstream. In other examples, the video decoder may determine whether the filter set for the CTB is from a plurality of fixed filter sets based on one or more other factors (such as the position of the CTB in a slice, tile, or picture). For example, it may be required that the filter set for the first occurrence of a CTB in a slice, tile, or picture must be from a plurality of fixed filter sets. In some examples, an index signaled in a bitstream may indicate which fixed filter set in the fixed filter set is the filter set for the CTB.

[0150] In response to determining that the filter set for the CTB is from a plurality of fixed filter sets ("yes" branch of 400), the video decoder may set the clipping value to a maximum supported value (402). The maximum supported value may be based on the bit depth of samples for the video data. For example, the maximum supported value may be equal to 2^bit depth.

[0151] In response to determining that the filter set for the CTB is not from a plurality of fixed filter sets (“NO” branch of 400), the video decoder may set the clipping value to a value other than a maximum supported value (404). For example, in some examples, a flag signaled in the bitstream may indicate whether the clipping value (i.e., clipping parameter) for the CTB is explicitly signaled or whether the clipping value for the CTB is the same as (i.e., reused from) a clipping value in a previously decoded non-linear ALF filter set. In some examples, when the clipping value for the CTB is reused from a previously decoded non-linear ALF filter set or one of a plurality of fixed filter sets, an index signaled in the bitstream may indicate which of the previously decoded non-linear ALF filters or fixed filter sets is the filter set for the CTB.

[0152] exist Figure 7 In an example of , after setting the clipping value (404), the video decoder may determine a clipped input (406) for the ALF of the filter set based on the clipping value. Each of these inputs is an input sample minus a current sample. For example, in Equation 2 above, each of these inputs has the form of I(x+i,y+j)-I(x,y), and the video decoder may clip the input based on the clipping value k(i,j). If the clipping value is set to the maximum supported value, clipping will never occur. Therefore, after setting the clipping value to the maximum supported value based on the filter set for the CTB being from a fixed filter set, in some examples, the video decoder may not apply clipping to the input. However, the present disclosure may refer to the input from this stage as a clipped input, regardless of whether the video decoder actually applies clipping.

[0153] The video decoder may apply the ALF to the clipped input (408). For example, as part of applying the ALF to the clipped input, the video decoder may determine a product for the clipped input for each of the clipped inputs. In this example, the product for the clipped input is the result of multiplying the filter coefficients for the clipped input by the clipped input. Additionally, in this example, the video decoder may sum the products for the clipped input to determine a summed value. The video decoder may add the summed value to the current sample. Thus, the video decoder may apply equation 2 above. Subsequently, the video decoder may generate a prediction block based on the CTB after applying the ALF to the clipped input.

[0154] In some examples, the video coder may code (i.e., video encoder 200 may encode or video decoder 300 may decode) a syntax element indicating whether a fixed filter set may be used to predict filter coefficients in a non-linear ALF filter at an SPS or PPS level of a bitstream that includes an encoded representation of the video data. If the flag indicates that a fixed filter set may be used to predict filter coefficients, the video coder may use the fixed filter set to predict the filter coefficients and use the filter coefficients when applying the ALF.

[0155] In some examples, the video decoder may decode a flag indicating whether clipping is enabled when the filter set is applied to the CTB. In other words, the video encoder 200 may encode a flag and / or the video decoder 300 may decode the flag, the flag indicating whether clipping is enabled when the filter set-based ALF is applied to the CTB. In the case where the flag indicates that clipping is enabled when the filter set is applied to the CTB, the video decoder may perform Figure 7 Otherwise, the video decoder may apply ALF based on the filter set without performing any clipping operation.

[0156] although Figure 7 The examples are described with reference to CTB, but may be applied with respect to other block types or individual pixels or samples. Figure 7 For example, in the case of a single sample application Figure 7In an example of operation, a video decoder (e.g., video encoder 200 or video decoder 300) may determine a filter set for a sample from a plurality of fixed filter sets, wherein the sample is in a picture of video data. In this example, based on the filter set for the sample being from the fixed filter set, the video decoder may set the clipping value to a maximum supported value. In addition, the video decoder may determine a clipped input to an ALF for the filter set based on the clipping value, each of the inputs being an input sample minus the sample. The video decoder may apply the ALF to the clipped input to determine an updated value for the sample. The video decoder may perform these actions according to examples provided elsewhere in the present disclosure.

[0157] The following paragraphs provide a non-limiting set of examples based on techniques of the present disclosure.

[0158] Example 1. A method for decoding video data, the method comprising: decoding a syntax element at a sequence parameter set (SPS) or picture parameter set (PPS) level of a bitstream including an encoded representation of the video data, the syntax element indicating whether a fixed filter set can be used to predict filter coefficients in a nonlinear adaptive loop filter (ALF) filter; and applying the nonlinear ALF filter to a block of the video data based on the syntax element.

[0159] Example 2. A method of decoding video data, the method comprising: applying a fixed clipping parameter set to predict clipping parameters of a nonlinear adaptive loop filter (ALF) filter set; and applying a nonlinear ALF filter to a block of the video data based on the nonlinear ALF filter set.

[0160] Example 3. A method for decoding video data, the method comprising: determining a filter set for a decoding tree block (CTB) from a plurality of fixed filter sets; limiting the input of an adaptive loop filter (ALF) for the filter set, the input being samples of the CTB; and applying the ALF to the limited input.

[0161] Example 4. The method according to Example 3 further includes: decoding a flag indicating whether limiting is enabled when applying the filter set to the CTB.

[0162] Example 5. The method according to any one of Examples 3-4 further includes: decoding a flag indicating whether all CTBs in a picture or tile group use the same set of limiting parameters to limit the input to the ALF.

[0163] Example 6. The method according to any one of Examples 3-5 further includes: based on enabling clipping for the CTB, decoding a flag indicating whether clipping is applied to the CTB.

[0164] Example 7. A method according to any one of Examples 3-6, wherein: the method further includes, based on applying limiting to the CTB, decoding a flag indicating whether a limiting parameter is explicitly signaled, and limiting the input includes: limiting the input based on the limiting parameter.

[0165] Example 8. A method according to any one of Examples 3-7, wherein: the method further comprises: based on applying limiting to the CTB, decoding a flag indicating whether a limiting parameter is the same as a limiting parameter in a previously decoded nonlinear ALF filter set, and limiting the input comprises: limiting the input based on the limiting parameter.

[0166] Example 9. A method according to any one of Examples 3-8, wherein: the method further comprises: based on the CTB reusing the limiting parameters or predefined limiting parameters in the previously decoded nonlinear ALF filter set, decoding an index indicating whether the limiting parameters reused by the CTB are from the previously decoded nonlinear ALF filter set or the predefined limiting parameters, and limiting the input comprises: limiting the input based on the limiting parameters.

[0167] Example 10. A method according to any one of Examples 3-9, wherein: the method includes: setting a default limiting value to a maximum supported value based on applying the fixed filter set to the CTB, and limiting the input includes: limiting the input based on the default limiting value.

[0168] Example 11. A method according to any one of Examples 3-9, wherein: for at least one of the fixed filter sets, all classes of the fixed filter set use the same set of limiting values, and limiting the input includes: limiting the input based on the limiting value.

[0169] Example 12. A method according to any one of Examples 3-9, wherein: for at least one of the fixed filter sets, each class of the fixed filter set uses a different set of limiting values, and limiting the input includes: limiting the input based on the limiting value used for the class of the fixed filter set.

[0170] Example 13. A method comprising the method according to any one of Examples 1-12.

[0171] Example 14. A method according to any one of Examples 1-13, wherein decoding includes decoding.

[0172] Example 15. A method according to any one of Examples 1-14, wherein decoding includes encoding.

[0173] Example 16. An apparatus for decoding video data, the apparatus comprising one or more units for performing the method according to any one of Examples 1-15.

[0174] Example 17. An apparatus according to Example 13, wherein the one or more units include one or more processors implemented in a circuit.

[0175] Example 18. The device according to any one of Examples 16 and 17 further includes: a memory for storing the video data.

[0176] Example 19. The apparatus of any of Examples 16-18, further comprising: a display configured to display the decoded video data.

[0177] Example 20. A device according to any of Examples 16-19, wherein the device includes one or more of a camera, a computer, a mobile device, a broadcast receiver device, or a set-top box.

[0178] Example 21. A device according to any of Examples 16-20, wherein the device includes a video decoder.

[0179] Example 22. A device according to any of Examples 16-21, wherein the device includes a video encoder.

[0180] Example 23. A computer-readable storage medium having instructions stored thereon, which, when executed, cause one or more processors to perform a method according to any one of Examples 1-15.

[0181] Example 24. An apparatus for encoding video data, the apparatus comprising means for performing the method according to any one of Examples 1-15.

[0182] It is to be appreciated that, according to examples, certain actions or events of any of the techniques described herein may be performed in a different order, may be added, combined, or omitted entirely (e.g., not all described actions or events are necessary to implement the techniques). In addition, in some examples, actions or events may be performed concurrently rather than sequentially, such as through multithreading, interrupt handling, or multiple processors.

[0183] 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 through a computer-readable medium as one or more instructions or codes and executed by a hardware-based processing unit. A computer-readable medium may include a computer-readable storage medium corresponding to a tangible medium such as a data storage medium or a communication medium, the communication medium including, for example, any medium that facilitates the transfer of a computer program from one place to another according to a communication protocol. In this manner, a computer-readable medium may generally correspond to (1) a non-temporary tangible computer-readable storage medium, or (2) a communication medium such as a signal or carrier wave. A data storage medium may be any available medium that can be accessed by one or more computers or one or more processors to obtain instructions, codes, and / or data structures for implementing the techniques described in the present disclosure. A computer program product may include a computer-readable medium.

[0184] By way of example and not limitation, such computer-readable storage media may include RAM, ROM, EEPROM, CD-ROM or other optical disk storage, 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 can be accessed by a computer. In addition, any connection is appropriately referred to as a computer-readable medium. For example, if a coaxial cable, optical fiber cable, twisted pair, digital subscriber line (DSL) or wireless technology (e.g., infrared, radio and microwave) is used to transmit instructions from a website, server or other remote source, then the coaxial cable, optical fiber cable, twisted pair, DSL or wireless technology (e.g., infrared, radio and microwave) is included in the definition of the medium. However, it should be understood that computer-readable storage media and data storage media do not include connections, carrier waves, signals or other temporary media, but are instead directed to non-temporary tangible storage media. As used herein, disks and optical disks include compact disks (CDs), laser optical disks, optical disks, digital versatile disks (DVDs), floppy disks and blue-ray disks, wherein disks usually copy data magnetically, while optical disks use lasers to optically copy data. Combinations of the above should also be included within the scope of computer-readable media.

[0185] 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 gate arrays (FPGAs), or other equivalent integrated or discrete logic circuits. Therefore, the terms "processor" and "processing circuitry" as used herein may refer to any of the aforementioned structures or any other structure suitable for implementing the techniques described herein. In addition, 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. In addition, the techniques may be implemented entirely in one or more circuits or logic elements.

[0186] The techniques of the present disclosure may be implemented in a wide variety of devices or apparatuses, including a wireless handset, an integrated circuit (IC), or a set of ICs (e.g., a chipset). Various components, modules, or units are described in the present disclosure to emphasize the functional aspects of a device configured to perform the disclosed techniques, but do not necessarily need to be implemented by different hardware units. Specifically, as described above, the various units may be combined in a codec hardware unit, or provided by a collection of interoperable hardware units (including one or more processors as described above) in combination with appropriate software and / or firmware.

[0187] Various examples have been described. These and other examples are within the scope of the following claims.

Claims

1. A method for decoding video data, the method comprising: Determining a filter set for a coding tree block (CTB) and determining whether the filter set is from a plurality of fixed filter sets, the filter set for the CTB comprising a set of filter coefficients, wherein the CTB is in a picture of the video data; setting a clipping value to a maximum supported value based on the filter set for the CTB being from the fixed filter set; determining a clipped input for an adaptive loop filter ALF of the filter set based on the clipping value, the clipping value limiting the input to a predefined range and each of the inputs being an input sample minus a current sample, wherein determining the clipped input comprises: not applying clipping to the input based on the clipping value being the maximum supported value; and The ALF is applied to the clipped input.

2. The method according to claim 1, wherein: Applying the ALF to the clipped input comprises: for each of the clipped inputs, determining a product for the clipped input, the product for the clipped input being a result of multiplying the filter coefficient for the clipped input by the clipped input; summing the products for the clipped inputs to determine a summed value; and The summed value is added to the current sample.

3. The method according to claim 1, wherein: The ALF is a non-linear ALF, and the method further comprises: A syntax element is coded at a sequence parameter set (SPS) level or a picture parameter set (PPS) level of a bitstream comprising an encoded representation of the video data, the syntax element indicating whether a fixed filter set can be used to predict filter coefficients in a non-linear adaptive loop filter (ALF).

4. The method according to claim 1, further comprising: A flag is decoded that indicates whether clipping is enabled when the filter set is applied to the CTB.

5. The method according to claim 1, wherein: Determining the clipped input includes not applying clipping to the input based on the set of filters for the CTB being from the fixed set of filters.

6. The method according to claim 1, wherein: Decoding includes decoding.

7. The method according to claim 1, wherein: Decoding includes encoding.

8. The method according to claim 1, wherein: The method further comprises: After applying the ALF to the clipped input, a prediction block is generated based on the CTB.

9. A device for decoding video data, the device comprising: A memory for storing the video data; as well as One or more processors implemented in circuitry, the one or more processors configured to: Determining a filter set for a coding tree block (CTB) and determining whether the filter set is from a plurality of fixed filter sets, the filter set for the CTB comprising a set of filter coefficients, wherein the CTB is in a picture of the video data; setting a clipping value to a maximum supported value based on the filter set for the CTB being from the fixed filter set; determining a clipped input for an adaptive loop filter ALF of the filter set based on the clipping value, the clipping value limiting the input to a predefined range and each of the inputs being an input sample minus a current sample, wherein the one or more processors are configured such that the one or more processors do not apply clipping to the input based on the clipping value being the maximum supported value; and The ALF is applied to the clipped input.

10. The device according to claim 9, wherein: The one or more processors are configured such that, as part of applying the ALF to the clipped input, the one or more processors perform the following operations: for each of the clipped inputs, determining a product for the clipped input, the product for the clipped input being a result of multiplying the filter coefficient for the clipped input by the clipped input; summing the products for the clipped inputs to determine a summed value; as well as The summed value is added to the current sample.

11. The device according to claim 9, wherein: The ALF is a non-linear ALF, and the one or more processors are further configured to: A syntax element is coded at a sequence parameter set (SPS) level or a picture parameter set (PPS) level of a bitstream comprising an encoded representation of the video data, the syntax element indicating whether a fixed filter set can be used to predict filter coefficients in a non-linear adaptive loop filter (ALF).

12. The device according to claim 9, wherein: The one or more processors are further configured to decode a flag indicating whether clipping is enabled when the filter set is applied to the CTB.

13. The apparatus according to claim 9, wherein: The one or more processors are further configured to generate a prediction block based on the CTB after applying the ALF to the clipped input.

14. The apparatus according to claim 9, wherein: The one or more processors are configured such that the one or more processors do not apply limiting to the input based on the filter set for the CTB being from the fixed filter set.

15. The apparatus according to claim 9, further comprising: A display configured to display the decoded video data.

16. The apparatus according to claim 9, wherein: The device includes one or more of a camera, a computer, or a broadcast receiver device.

17. The apparatus according to claim 9, wherein: The apparatus comprises a video decoder.

18. The apparatus according to claim 9, wherein: The apparatus includes a video encoder.

19. A device for decoding video data, the device comprising: means for determining a filter set for a coding tree block (CTB) and determining whether the filter set is from a plurality of fixed filter sets, the filter set for the CTB comprising a set of filter coefficients, wherein the CTB is in a picture of the video data; means for setting a clipping value to a maximum supported value based on the filter set for the CTB being from the fixed filter set; means for determining a clipped input to an adaptive loop filter ALF of the filter set based on the clipping value, the clipping value limiting the input to a predefined range and each of the inputs being an input sample minus a current sample, wherein the means for determining a clipped input to an adaptive loop filter ALF of the filter set based on the clipping value does not apply clipping to the input based on the clipping value being the maximum supported value; and Means for applying the ALF to the clipped input.

20. The apparatus of claim 19, wherein: The means for applying the ALF to the clipped input comprises: means for determining, for each of the clipped inputs, a product for the clipped input, the product for the clipped input being a result of multiplying the filter coefficient for the clipped input by the clipped input; means for summing the products for the clipped inputs to determine a summed value; and means for adding the summed value to the current sample.

21. The apparatus of claim 19, wherein: The ALF is a non-linear ALF, and the apparatus further comprises: Means for coding, at a sequence parameter set (SPS) level or a picture parameter set (PPS) level of a bitstream comprising an encoded representation of the video data, a syntax element indicating whether a fixed filter set can be used to predict filter coefficients in a non-linear adaptive loop filter (ALF).

22. The apparatus of claim 19, further comprising: Means for decoding a flag indicating whether clipping is enabled when the filter set is applied to the CTB.

23. The apparatus of claim 19, wherein: The means for determining a clipped input to the ALF for the filter set based on the clipping value does not apply clipping to the input based on the filter set for the CTB being from the fixed filter set.

24. A computer-readable storage medium having stored thereon instructions that, when executed, cause one or more processors to: Determine a filter set for decoding a tree block (CTB) and determine whether the filter set is from a plurality of fixed filter sets, the filter set for the CTB comprising a filter coefficient set, wherein, The CTB is in a picture of the video data; setting a clipping value to a maximum supported value based on the filter set for the CTB being from the fixed filter set; determining a clipped input for an adaptive loop filter ALF of the filter set based on the clipping value, the clipping value limiting the input to a predefined range and each of the inputs being an input sample minus a current sample, wherein execution of the instructions causes the one or more processors to: not apply clipping to the input based on the clipping value being the maximum supported value; as well as The ALF is applied to the clipped input.

25. The computer-readable storage medium of claim 24, wherein: Execution of the instructions for applying the ALF to the clipped input causes the one or more processors to: for each of the clipped inputs, determining a product for the clipped input, the product for the clipped input being a result of multiplying the filter coefficient for the clipped input by the clipped input; summing the products for the clipped inputs to determine a summed value; as well as The summed value is added to the current sample.

26. The computer-readable storage medium of claim 24, wherein: The ALF is a non-linear ALF, and executing the instructions further causes the one or more processors to perform the following operations: A syntax element is coded at a sequence parameter set (SPS) level or a picture parameter set (PPS) level of a bitstream comprising an encoded representation of the video data, the syntax element indicating whether a fixed filter set can be used to predict filter coefficients in a non-linear adaptive loop filter (ALF).

27. The computer-readable storage medium of claim 24, wherein: Execution of the instructions causes the one or more processors to decode a flag indicating whether clipping is enabled when the filter set is applied to the CTB.

28. The computer-readable storage medium of claim 24, wherein: Execution of the instructions for determining the clipped input causes the one or more processors to not apply clipping to the input based on the set of filters used for the CTB being from the fixed set of filters.

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