Low-Frequency Non-Separable Transform (LFNST) with Reduced Zeros in Video Coding

Abstract

An example device for decoding video data includes: a memory configured to store video data; and one or more processors implemented in circuitry and configured to: determine that a transform block of the video data has a size of 8x8 coefficients and that the transform block is transformed using a low-frequency non-separable transform (LFNST); decode at least nine non-zero transform coefficients of the transform block; perform an inverse transform on the transform block using the inverse LFNST to reproduce a residual block corresponding to the transform block; and use the residual block to reconstruct a block of the video data.

Description

[0001] This application claims the benefit of U.S. Application No. 17 / 125,778, filed on Dec. 17, 2020, and U.S. Provisional Application No. 62 / 951,984, filed on Dec. 20, 2019, the entire contents of each of which are hereby incorporated by reference. Technical Field

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

[0003] Digital video capabilities can be incorporated into a variety of devices, including digital televisions, digital direct broadcast systems, wireless broadcast systems, personal digital assistants (PDAs), laptop or desktop computers, tablet computers, e-book readers, digital cameras, digital recording devices, digital media players, video game devices, video game consoles, cellular or satellite radiotelephones (so-called “smart phones”), video teleconferencing devices, video streaming devices, etc. Digital video devices implement video decoding 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. Video devices can more efficiently transmit, receive, encode, decode, and / or store digital video information by implementing such video decoding techniques.

[0004] Video decoding techniques include spatial (intra-picture) prediction and / or temporal (inter-picture) prediction to reduce or remove redundancy inherent in a video sequence. For block-based video decoding, a video slice (e.g., a video picture or a portion of a video picture) can be divided 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 with respect to reference samples in neighboring blocks within the same picture. Video blocks in an inter-coded (P or B) slice of a picture can use spatial prediction with respect to reference samples in neighboring blocks within the same picture or temporal prediction with respect to reference samples in other reference pictures. A picture may be referred to as a frame, and a reference picture may be referred to as a reference frame. Summary of the Invention

[0005] Generally speaking, the present disclosure describes techniques for transform coding in video coding, such as in video compression standards. For example, the present disclosure describes various examples of low-frequency non-separable transform designs that can improve coding efficiency. The example techniques can be utilized in Versatile Video Coding (VVC / H.266). The example techniques can be used with other improved video codecs, including extensions of HEVC and next-generation video coding standards. Specifically, certain techniques of the present disclosure relate to explicitly coding (encoding or decoding) a certain number of non-zero transform coefficients of an 8x8 transform block that is transformed using a low-frequency non-separable transform (LFNST), and not coding the remaining transform coefficients. For example, a video coder can be configured to explicitly code up to ten transform coefficients of a transform block, and the video coder can infer zero values for the remaining transform coefficients.

[0006] In one example, a method for decoding video data includes: determining that a transform block of the video data has a size of 8x8 coefficients and that the transform block is transformed using a low-frequency non-separable transform (LFNST); decoding at least nine non-zero transform coefficients of the transform block; using an inverse LFNST to inverse-transform the transform block to reproduce a residual block corresponding to the transform block; and using the residual block to reconstruct a block of the video data.

[0007] In another example, a device for decoding video data includes: a memory configured to store the video data; and one or more processors implemented in a circuit and configured to: determine that a transform block of the video data has a size of 8x8 coefficients and that the transform block is transformed using a low-frequency non-separable transform (LFNST); decode at least nine non-zero transform coefficients of the transform block; use an inverse LFNST to inverse-transform the transform block to reproduce a residual block corresponding to the transform block; and use the residual block to reconstruct a block of the video data.

[0008] In another example, a computer-readable storage medium has instructions stored thereon that, when executed, cause a processor to: determine that a transform block of the video data has a size of 8x8 coefficients and that the transform block is transformed using a low-frequency non-separable transform (LFNST); decode at least nine non-zero transform coefficients of the transform block; use an inverse LFNST to inverse-transform the transform block to reproduce a residual block corresponding to the transform block; and use the residual block to reconstruct a block of the video data.

[0009] In another example, a device for decoding video data includes: a unit configured to determine that a transform block of the video data has a size of 8x8 coefficients and that the transform block is transformed using a low-frequency non-separable transform (LFNST); a unit configured to decode at least nine non-zero transform coefficients of the transform block; a unit configured to inverse-transform the transform block using an inverse LFNST to reproduce a residual block corresponding to the transform block; and a unit configured to reconstruct a block of the video data using the residual block.

[0010] Details of one or more examples are set forth in the accompanying drawings and the following description. Other features, objects, and advantages will be apparent from the description, the drawings, and the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is a block diagram showing an example video encoding and decoding system that can execute the techniques of the present disclosure.

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

[0013] Figure 3 is a block diagram showing an example video encoder that can execute the techniques of the present disclosure.

[0014] Figure 4 is a block diagram showing an example video decoder that can execute the techniques of the present disclosure.

[0015] Figure 5 is a block diagram showing an example set of components that can perform a low-frequency non-separable transform (LFNST).

[0016] Figure 6 is a conceptual diagram showing applying an inverse LFNST to a transform block to reproduce a residual block.

[0017] Figure 7 is a conceptual diagram showing applying an inverse LFNST to a 4x4 block.

[0018] Figure 8 is a conceptual diagram showing applying an inverse LFNST to a 4x4 block to produce an 8x8 block.

[0019] Figure 9 is a flowchart showing an example of encoding video data according to the techniques of the present disclosure.

[0020] Figure 10 is a flowchart showing an example of decoding video data according to the techniques of the present disclosure. DETAILED DESCRIPTION

[0021] Figure 1 FIG. 1 is a block diagram showing an example video encoding and decoding system 100 that can implement the techniques of the present disclosure. The techniques of the present disclosure generally relate to decoding (encoding and / or decoding) video data. Generally, video data includes any data for processing video. Thus, video data can include raw unencoded video, encoded video, decoded (e.g., reconstructed) video, and video metadata, such as signaling data.

[0022] As Figure 1 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. In particular, source device 102 provides the video data to destination device 116 via a computer-readable medium 110. Source device 102 and destination device 116 can include any one of a variety of devices, including desktop computers, notebook computers (i.e., laptop computers), tablet computers, set-top boxes, cellular phones such as smart phones, televisions, cameras, display devices, digital media players, video game consoles, video streaming devices, etc. In some cases, source device 102 and destination device 116 can be equipped for wireless communication and can thus be referred to as wireless communication devices.

[0023] In Figure 1 the example of FIG. 1, source device 102 includes a video source 104, a memory 106, a video encoder 200, and an output interface 108. Destination device 116 includes an input interface 122, a video decoder 300, a memory 120, and a display device 118. According to the present disclosure, video encoder 200 of source device 102 and video decoder 300 of destination device 116 can be configured to apply techniques for low-frequency non-separable transform (LFNST) with reduced return-to-zero, as described in the present disclosure. 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 and destination devices can include other components or arrangements. For example, source device 102 can receive video data from an external video source such as an external camera. Similarly, destination device 116 can interface with an external display device rather than include an integrated display device.

[0024] As Figure 1The system 100 shown is merely an example. In general, any digital video encoding and / or decoding device may perform techniques for LFNST with reduced run length, as described in this disclosure. The source device 102 and the destination device 116 are merely examples of such decoding devices, where the source device 102 generates encoded video data for transmission to the destination device 116. This disclosure refers to a "decoding" device as a device that performs decoding (encoding and / or decoding) of data. Thus, the video encoder 200 and the video decoder 300 represent examples of decoding devices (and in particular, a video encoder and a video decoder). In some examples, the source device 102 and the destination device 116 may operate in a substantially symmetric manner such that each of the source device 102 and the destination device 116 includes video encoding and decoding components. Thus, the system 100 may support unidirectional or bidirectional video transmission between the source device 102 and the destination device 116, such as for video streaming, video playback, video broadcast, or video telephony.

[0025] In general, the video source 104 represents a source of video data (i.e., raw, unencoded video data), and provides a sequential series of pictures (also referred to as "frames") 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 containing previously captured raw video, and / or a video feed interface for receiving video from a video content provider. As a further alternative, the video source 104 may generate computer graphics-based data as the source video, or a combination of live video, archived video, and computer-generated video. In each case, the video encoder 200 may encode the captured, pre-captured, or computer-generated video data. The video encoder 200 may reorder the pictures from the received order (sometimes referred to as the "display order") into a decoding order for decoding. The video encoder 200 may generate a bitstream including the encoded video data. The source device 102 may then output the encoded video data via the output interface 108 onto a computer-readable medium 110 for reception and / or retrieval by, for example, the input interface 122 of the destination device 116.

[0026] The memories 106 of the source device 102 and 120 of the destination device 116 represent general memories. In some examples, the memories 106, 120 may store raw video data, such as raw video from the video source 104 and raw decoded video data from the video decoder 300. Additionally or alternatively, the memories 106, 120 may store software instructions executable by, for example, the video encoder 200 and the video decoder 300, respectively. Although the memories 106 and 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 may also include internal memories for functionally similar or equivalent purposes. Further, the memories 106, 120 may 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 may be allocated as one or more video buffers, such as to store raw decoded and / or encoded video data.

[0027] The computer-readable medium 110 may represent any type of medium or device capable of transferring 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 for enabling the source device 102 to directly send the encoded video data to the destination device 116 in real time, such as via a radio frequency network or a computer-based network. In accordance with a communication standard such as a wireless communication protocol, the output interface 108 may modulate a transmission signal including the encoded video data, and the input interface 122 may demodulate the received transmission signal. The communication medium may include any wireless or wired communication medium, such as the radio frequency (RF) spectrum or one or more physical transmission lines. The communication medium may form part of a packet-based network, such as a local area network, a wide area network, or a global network such as the Internet. The communication medium may include routers, switches, base stations, or any other equipment that may be useful for facilitating communication from the source device 102 to the destination device 116.

[0028] In some examples, the source device 102 may output the encoded data from the output interface 108 to the storage device 112. Similarly, the destination device 116 may access the encoded data from the storage device 112 via the input interface 122. The storage device 112 may include any one of a variety of distributed or locally accessible data storage media, such as a hard disk drive, a Blu-ray disc, a DVD, a CD-ROM, flash memory, volatile or non-volatile memory, or any other suitable digital storage media for storing the encoded video data.

[0029] In some examples, the source device 102 may output the encoded video data to a file server 114 or to another intermediate storage device that stores 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., Wi-Fi connection), a wired connection (e.g., Digital Subscriber Line (DSL), cable modem, etc.), or a combination of both, 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 protocol, a download protocol, or a combination thereof.

[0030] The output interface 108 and the input interface 122 may represent a wireless transmitter / receiver, a modem, a wired networking component (e.g., an Ethernet card), a wireless communication component operating according to any of the various IEEE 802.11 standards, or other physical components. In examples where the output interface 108 and the input interface 122 include wireless components, the output interface 108 and the 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), enhanced LTE, 5G, etc.). In some examples where the output interface 108 includes a wireless transmitter, the output interface 108 and the input interface 122 may be configured to transmit data (such as encoded video data) according to other wireless standards (such as the IEEE 802.11 specifications, the IEEE 802.15 specifications (e.g., ZigBee TM (ZigBee TM ))), Bluetooth TM (Bluetooth TM ) standards, etc.). In some examples, the source device 102 and / or the destination device 116 may include respective System-on-Chip (SoC) devices. For example, the source device 102 may include an SoC device for performing the functions attributed to the video encoder 200 and / or the output interface 108, and the destination device 116 may include an SoC device for performing the functions attributed to the video decoder 300 and / or the input interface 122.

[0031] The techniques of the present disclosure may be applied to video coding to support any of a variety of multimedia applications, such as over-the-air television broadcasts, cable television transmissions, satellite television transmissions, Internet streaming video transmissions (such as HTTP-based 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.

[0032] The input interface 122 of the destination device 116 receives an encoded video bitstream from a computer-readable medium 110 (e.g., a communication medium, a storage device 112, a file server 114, etc.). The encoded video bitstream may include signaling information defined by the video encoder 200 and is also used by the video decoder 300, such as values that describe the characteristics and / or processing of video blocks or other coding units (e.g., slices, pictures, groups of pictures, sequences, etc.). 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.

[0033] Although not shown in Figure 1 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 (multiplexing-demultiplexing) units or other hardware and / or software to process a multiplexed stream that includes both audio and video in a common data stream. If applicable, the MUX-DEMUX unit may follow the ITU H.223 multiplexer protocol or other protocols, such as the User Datagram Protocol (UDP).

[0034] Video encoder 200 and video decoder 300 may each be implemented as any one 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 implemented partially in software, the device may store instructions for the software in a suitable non-transitory computer-readable medium and execute the instructions with one or more processors in hardware to perform the techniques of the present disclosure. Each of video encoder 200 and video decoder 300 may be included in one or more encoders or decoders, and any one of the encoders or decoders may be integrated as part of a combined encoder / decoder (CODEC) in the respective device. Devices including video encoder 200 and / or video decoder 300 may include integrated circuits, microprocessors, and / or wireless communication devices, such as cellular telephones.

[0035] As described above, one video coding standard is HEVC. HEVC is described in the following document: M. Wien, High Efficiency Video Coding: Coding Tools and Specification, Springer, Berlin, 2015.

[0036] Video encoder 200 and video decoder 300 may operate according to other proprietary or industry standards, such as the Joint Exploration Test Model (JEM) or ITU-T H.266 (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 7)”, Joint Video Team (JVET) of ITU-T SG 16 WP 3 and ISO / IEC JTC 1 / SC 29 / WG 11, 16th meeting: Geneva, Switzerland, October 1-11, 2019, JVET-P2001-v14 (hereinafter referred to as “VVC Draft 7”). However, the techniques of the present disclosure are not limited to any particular coding standard.

[0037] Generally, video encoder 200 and video decoder 300 may perform block - based coding of pictures. The term "block" generally refers to a structure that includes data to be processed (e.g., data that is encoded, decoded, or otherwise used during the encoding and / or decoding process). For example, a block may include a two - dimensional matrix of samples of luminance and / or chrominance data. Generally, video encoder 200 and video decoder 300 may code video data represented in the YUV (e.g., Y, Cb, Cr) format. That is, instead of coding the red, green, and blue (RGB) data of the samples for a picture, video encoder 200 and video decoder 300 may code the luminance and chrominance components, where the chrominance components may include both the red - hue and blue - hue chrominance components. In some examples, video encoder 200 converts the received RGB - formatted data to a YUV representation before encoding, and video decoder 300 converts the YUV representation to the RGB format. Alternatively, pre - processing and post - processing units (not shown) may perform these conversions.

[0038] The present disclosure may generally relate to coding (e.g., encoding and decoding) of pictures to include processes of encoding or decoding data of pictures. Similarly, the present disclosure may relate to coding of blocks of pictures to include processes of encoding or decoding data for blocks (e.g., prediction and / or residual coding). An encoded video bitstream generally includes a series of values for syntax elements that represent coding decisions (e.g., coding modes) and the partitioning of a picture into blocks. Thus, a reference to coding a picture or a block should generally be understood as coding the values of the syntax elements used to form the picture or the block.

[0039] HEVC defines various blocks, including coding units (CUs), prediction units (PUs), and transform units (TUs). According to HEVC, a video coder (such as video encoder 200) divides a coding tree unit (CTU) into CUs according to a quadtree structure. That is, the video coder divides the CTU and CUs 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 coder may further divide PUs and TUs. For example, in HEVC, a residual quadtree (RQT) represents the division of TUs. In HEVC, a PU represents inter - frame prediction data, while a TU represents residual data. An intra - predicted CU includes intra - prediction information, such as an intra - mode indication.

[0040] As another example, video encoder 200 and video decoder 300 may be configured to operate according to JEM or VVC. According to JEM or VVC, a video coder (such as video encoder 200) divides a picture into multiple coding tree units (CTUs). Video encoder 200 may divide a CTU 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 partitioning types, such as the separation between CUs, PUs, and TUs in HEVC. The QTBT structure includes two levels: a first level divided according to quadtree partitioning, and a second level divided 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 coding units (CUs).

[0041] In the MTT partitioning structure, a block may be partitioned using quadtree (QT) partitioning, binary tree (BT) partitioning, and one or more types of ternary tree (TT) (also referred to as trinary tree (TT)) partitioning. Ternary tree or trinary tree partitioning is a partitioning in which a block is divided into three sub-blocks. In some examples, the ternary tree or trinary tree partitioning divides a block into three sub-blocks without dividing the original block through the center. The partitioning types in the MTT (e.g., QT, BT, and TT) may be symmetric or asymmetric.

[0042] In some examples, video encoder 200 and video decoder 300 may use a single QTBT or MTT structure to represent each of the luminance component and the chrominance components, while in other examples, video encoder 200 and video decoder 300 may use two or more QTBT or MTT structures, such as one QTBT / MTT structure for the luminance component and another QTBT / MTT structure for the two chrominance components (or two QTBT / MTT structures for the respective chrominance components).

[0043] Video encoder 200 and video decoder 300 may be configured to use quadtree partitioning, QTBT partitioning, MTT partitioning, or other partitioning structures per HEVC. For purposes of explanation, the description of the techniques of the present disclosure is given with respect to QTBT partitioning. However, it should be understood that the techniques of the present disclosure may also be applied to video coders configured to use quadtree partitioning or also other types of partitioning.

[0044] Blocks (e.g., CTUs or CUs) can be grouped in a picture in various ways. As an example, a tile can refer to a rectangular region of CTU rows within a particular tile in a picture. A tile can be a rectangular region of CTUs within a particular tile column and a particular tile row in a picture. A tile column refers to a rectangular region of CTUs having a height equal to the height of the picture and a width specified by a syntax element (e.g., such as in a picture parameter set). A tile row refers to a rectangular region of CTUs having a height specified by a syntax element (e.g., such as in a picture parameter set) and a width equal to the width of the picture.

[0045] In some examples, a tile can be divided into multiple tiles, and each of the tiles can include one or more CTU rows within the tile. A tile that is not divided into multiple tiles can also be referred to as a tile. However, a tile that is a proper subset of a tile cannot be referred to as a tile.

[0046] Tiles in a picture can also be arranged in slices. A slice can be an integer number of tiles of a picture, which can be uniquely contained within a single network abstraction layer (NAL) unit. In some examples, a slice includes multiple complete tiles or a continuous sequence of complete tiles of only one tile.

[0047] This disclosure can interchangeably use "NxN" and "N times N" to refer to the sample size of a block (such as a CU or other video block) in the vertical and horizontal dimensions. For example, 16x16 samples or 16 times 16 samples. Generally, a 16x16 CU will have 16 samples in the vertical direction (y = 16) and 16 samples in the horizontal direction (x = 16). Similarly, an NxN CU generally has N samples in the vertical direction and N samples in the horizontal direction, where N represents a non-negative integer value. Samples in a CU can be arranged in rows and columns. Additionally, 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 can include NxM samples, where M does not necessarily equal N.

[0048] Video encoder 200 encodes video data for a CU's representation prediction and / or residual information and other information. The prediction information indicates how the CU is to be predicted to form a prediction block for the CU. The residual information generally represents the per-sample difference between the samples of the CU before encoding and the prediction block.

[0049] To predict a CU, the video encoder 200 can generally form a prediction block for the CU through inter - prediction or intra - prediction. Inter - prediction generally refers to predicting the CU based on the data of previously decoded pictures, while intra - prediction generally refers to predicting the CU based on the previously decoded data of the same picture. To perform inter - prediction, the video encoder 200 can use one or more motion vectors to generate the prediction block. The video encoder 200 can generally perform a 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 calculate a difference metric using the sum of absolute differences (SAD), sum of squared differences (SSD), mean absolute difference (MAD), mean squared difference (MSD), or other such difference calculations to determine whether the reference block closely matches the current CU. In some examples, the video encoder 200 can use uni - directional prediction or bi - directional prediction to predict the current CU.

[0050] Some examples of JEM and VVC also provide an affine motion compensation mode, which can be considered an inter - 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).

[0051] To perform intra - prediction, the video encoder 200 can select an intra - prediction mode to generate the prediction block. Some examples of JEM and VVC provide sixty - seven intra - prediction modes, including various directional modes, as well as a planar mode and a DC mode. Generally, the video encoder 200 selects an intra - prediction mode that describes the neighboring samples of the current block (e.g., the block of the CU) according to which the samples of the current block are predicted. Assuming the video encoder 200 decodes the CTUs and CUs in raster scan order (from left to right, top to bottom), such samples can generally be above, top - left, or to the left of the current block in the same picture as the current block.

[0052] The video encoder 200 encodes data representing the prediction mode for the current block. For example, for an inter - prediction mode, the video encoder 200 can encode data representing which inter - prediction mode among various available inter - prediction modes is used and the motion information for the corresponding mode. For uni - directional or bi - directional inter - prediction, for example, the video encoder 200 can use advanced motion vector prediction (AMVP) or the merge mode to encode the motion vectors. The video encoder 200 can use a similar mode to encode the motion vectors for the affine motion compensation mode.

[0053] After prediction such as intra prediction or inter prediction of a block, video encoder 200 may compute residual data for the block. Residual data, such as a residual block, represents the sample-by-sample difference between the block and a predicted block for the block, which is formed using a corresponding prediction mode. 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, video encoder 200 may apply a discrete cosine transform (DCT) or a discrete sine transform (DST), an integer transform, a wavelet transform, or a conceptually similar transform to the residual video data. Additionally, 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), a low-frequency non-separable transform (LFNST), etc. Video encoder 200 produces transform coefficients after the application of one or more transforms.

[0054] As described above, after any transform used to produce transform coefficients, video encoder 200 may perform quantization of the transform coefficients. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the transform coefficients, providing further compression. By performing the quantization process, video encoder 200 may reduce the bit depth associated with some or all of the transform coefficients. For example, video encoder 200 may round an n-bit value down to an m-bit value during quantization, where n is greater than m. In some examples, to perform quantization, video encoder 200 may perform a bitwise right shift of the value to be quantized.

[0055] After quantization, video encoder 200 may scan the transform coefficients to produce a one-dimensional vector from a two-dimensional matrix including the quantized transform coefficients. The scan may be designed to place higher-energy (and thus lower-frequency) transform coefficients at the front of the vector and lower-energy (and thus higher-frequency) transform coefficients at the back of the vector. In some examples, video encoder 200 may use a predefined scan order to scan the quantized transform coefficients to produce a serialized vector, and then entropy code the quantized transform coefficients of the vector. In other examples, video encoder 200 may perform an adaptive scan. After scanning the quantized transform coefficients to form a one-dimensional vector, video encoder 200 may entropy code the one-dimensional vector, for example, according to context-adaptive binary arithmetic coding (CABAC). Video encoder 200 may also entropy code the values of syntax elements for describing metadata associated with the encoded video data for use by video decoder 300 when decoding the video data.

[0056] To perform CABAC, video encoder 200 may assign a context within a context model to a symbol to be sent. The context may relate to, for example, whether neighboring values of the symbol are zero values. Probability determination may be based on the context assigned to the symbol.

[0057] Video encoder 200 may further generate syntax data for video decoder 300 (such as block-based syntax data, picture-based syntax data, and sequence-based syntax data) in, for example, 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)). Video decoder 300 may similarly decode such syntax data to determine how to decode the corresponding video data.

[0058] In this way, video encoder 200 may generate a bitstream including the encoded video data, such as syntax elements that describe the segmentation of a picture into blocks (e.g., CUs) and prediction and / or residual information for the blocks. Ultimately, video decoder 300 may receive the bitstream and decode the encoded video data.

[0059] Generally, video decoder 300 performs a process reverse to that performed by video encoder 200 to decode the encoded video data of the bitstream. For example, video decoder 300 may use CABAC to decode the values of the syntax elements for the bitstream in a manner that is substantially similar to, but reverse of, the CABAC encoding process of video encoder 200. The syntax elements may define the segmentation information of a picture into CTUs and the segmentation of each CTU according to a corresponding segmentation structure (such as a QTBT structure) to define the CUs of the CTUs. The syntax elements may further define prediction and residual information for blocks (e.g., CUs) of the video data.

[0060] The residual information may be represented by, for example, quantized transform coefficients. Video decoder 300 may inverse-quantize and inverse-transform the quantized transform coefficients of a block to reproduce the residual block for the block. Video decoder 300 uses the signaled prediction mode (intra prediction or inter prediction) and related prediction information (such as motion information for inter prediction) to form a prediction block for the block. Video decoder 300 may then combine the prediction block and the residual block (on a sample-by-sample basis) to reproduce the original block. Video decoder 300 may perform additional processing, such as performing a deblocking process to reduce visual artifacts along the boundaries of the blocks.

[0061] According to the techniques of the present disclosure, video encoder 200 and video decoder 300 may be configured to perform operations related to LFNST. In LFNST, as described in more detail, video encoder 200 may perform DCT or DST on residual values to generate transform coefficients, and perform LFNST on the resultant transform coefficients of the DCT or DST to generate LFNST coefficients, and then quantize, entropy code, and signal the LFNST coefficients. In turn, video decoder 300 may receive the quantized, entropy-coded LFNST coefficients, perform entropy decoding on the quantized, entropy-coded LFNST coefficients, and perform inverse quantization on the quantized LFNST coefficients to generate LFNST coefficients. Video decoder 300 may perform inverse LFNST on the LFNST coefficients to generate inverse-transformed LFNST coefficients (which may be the same as the transform coefficients generated by video encoder 200), and perform inverse DCT or DST on the inverse-transformed LFNST coefficients to generate the residual values of the reconstructed residual block.

[0062] Video encoder 200 may be configured to encode only a certain number of quantized LFNST coefficients (hereinafter simply referred to as "LFNST coefficients" for brevity, but it should be understood that LFNST coefficients may also be quantized). For example, video encoder 200 may explicitly encode up to ten LFNST coefficients among the LFNST coefficients. Thus, video encoder 200 may explicitly encode any number of LFNST coefficients within the range [1, 2, …, 9, 10] of an 8x8 transform block.

[0063] Video encoder 200 may use, for example, the complexity of the video data (such as the profile, tier, and / or level of the video coding standard to which the video data conforms) to determine the number of LFNST coefficients to be explicitly encoded. Additionally or alternatively, video encoder 200 may use a rate-distortion optimization (RDO) process to determine the number of LFNST coefficients to be explicitly encoded. In some examples, the number of LFNST coefficients to be explicitly encoded may be predefined, for example, by the applicable video coding standard.

[0064] Video encoder 200 may encode a value representing the number of LFNST coefficients that are explicitly encoded, for example, in a video parameter set (VPS), sequence parameter set (SPS), picture parameter set (PPS), adaptive parameter set (APS), picture header, slice header, block header, or other syntax data. Video encoder 200 may avoid encoding the values of transform coefficients that exceed the number of non-zero transform coefficients. Video encoder 200 may be configured according to constraints imposed by the video coding standard that sets the maximum number of non-zero LFNST coefficients to be explicitly decoded, for example, up to ten.

[0065] The video decoder 300 may also determine that the number of non-zero LFNST coefficients of the encoded 8x8 block is within the range [1, 2, …, 9, 10]. For example, the video decoder 300 may determine that the number is a predefined (i.e., predetermined) value or from the data signaled (such as the value signaled in the VPS, SPS, PPS, APS, picture header, slice header, block header, or other such syntax data). The video decoder 300 may then decode the number of non-zero LFNST coefficients from the bitstream including the video data, and infer (i.e., without decoding) that the remaining transform coefficients (e.g., 64 minus the number of encoded LFNST coefficients) are zero values. Thus, the video decoder 300 may determine that the encoded video data of the video bitstream after the last explicitly encoded non-zero LFNST coefficient of the transform block corresponds to syntax elements other than the transform coefficients (e.g., other than the syntax elements representing whether the transform coefficients are significant, greater than one, greater than two, remaining levels, signs, etc.).

[0066] Thus, in one example, the 8x8 transform block may be transformed using LFNST and may include nine non-zero explicitly decoded transform coefficients, with fifty-five undecoded transform coefficients inferred to have zero values. That is, in this example, the bitstream will not include any data for the fifty-five undecoded transform coefficients. As another example, the 8x8 transform block may be transformed using LFNST and may include ten non-zero explicitly decoded transform coefficients, with fifty-four undecoded transform coefficients inferred to have zero values. That is, in this example, the bitstream will not include any data for the fifty-four undecoded transform coefficients.

[0067] In this way, there may be a constraint on the number of allowed non-zero coefficients (e.g., the number of non-zero coefficients after DCT or DST transform from the video encoder 200 or the number of non-zero coefficients in the LFNST coefficients). Having this constraint can reduce the computational overhead for the video encoder 200 and the video decoder 300, which can improve the operation of the video encoder 200 and the video decoder 300.

[0068] In one example, the video encoder 200 may be configured to perform a transform on residual values to generate a TU, determine that the TU size is 4x4, set all coefficients of the TU (except for eight coefficients) to be equal to zero, perform LFNST on the TU having at most eight non-zero coefficients to generate LFNST coefficients, and signal information indicative of the LFNST coefficients. In one example, the video encoder 200 may be configured to perform a transform on residual values to generate a TU, determine that the TU size is 8x8, set all coefficients of the TU (except for ten coefficients) to be equal to zero, perform LFNST on the TU having at most ten non-zero coefficients to generate LFNST coefficients, and signal information indicative of the LFNST coefficients. In one example, the video encoder 200 may be configured to perform a transform on residual values to generate a TU, determine that the TU size is greater than 8x8, set all coefficients of the TU (except for sixteen coefficients) to be equal to zero, perform LFNST on the TU having at most sixteen non-zero coefficients to generate LFNST coefficients, and signal information indicative of the LFNST coefficients.

[0069] In one example, the video decoder 300 may be configured to generate a 4x4 sized TU having a plurality of coefficients, where the TU includes at most eight non-zero coefficients, perform inverse LFNST on the TU to generate inverse-transformed LFNST coefficients, perform an inverse transform on the inverse-transformed LFNST coefficients to generate residual values for a current block, and reconstruct the current block based on the residual values. In one example, the video decoder 300 may be configured to generate an 8x8 sized TU having a plurality of coefficients, where the TU includes at most ten non-zero coefficients, perform inverse LFNST on the TU to generate inverse-transformed LFNST coefficients, perform an inverse transform on the inverse-transformed LFNST coefficients to generate residual values for a current block, and reconstruct the current block based on the residual values. In one example, the video decoder 300 may be configured to generate a TU having a size greater than 8x8 and having a plurality of coefficients, where the TU includes at most sixteen non-zero coefficients, perform inverse LFNST on the TU to generate inverse-transformed LFNST coefficients, perform an inverse transform on the inverse-transformed LFNST coefficients to generate residual values for a current block, and reconstruct the current block based on the residual values.

[0070] The present disclosure may generally relate to "signaling" certain information, such as syntax elements. The term "signaling" may generally refer to the conveyance of values for syntax elements and / or other data used to decode the encoded video data. That is, the video encoder 200 may signal the values for syntax elements in the bitstream. Generally, signaling refers to generating values in the bitstream. As described above, the source device 102 may convey the bitstream to the destination device 116 substantially in real time or not in real time (such as may occur when storing the syntax elements to the storage device 112 for later retrieval by the destination device 116).

[0071] Figure 2A and Figure 2B FIG. 6 is a conceptual diagram showing an example quadtree binary tree (QTBT) structure 130 and corresponding coding tree units (CTUs) 132. Solid lines represent quadtree splits, and dashed lines indicate binary tree splits. In each split (i.e., non-leaf) node of the binary tree, a flag is signaled to indicate which split type is used (i.e., horizontal or vertical), where in this example, 0 indicates a horizontal split, and 1 indicates a vertical split. For quadtree splits, since quadtree nodes split the block horizontally and vertically into 4 sub-blocks of equal size, there is no need to indicate the split type. Thus, the video encoder 200 may encode, and the video decoder 300 may decode, syntax elements (such as split information) for the region tree level (i.e., solid lines) of the QTBT structure 130, and syntax elements (such as split information) for the prediction tree level (i.e., dashed lines) of the QTBT structure 130. The video encoder 200 may encode video data (such as prediction and transform data) for the CUs represented by the terminal leaf nodes of the QTBT structure 130, and the video decoder 300 may decode video data (such as prediction and transform data) for the CUs represented by the terminal leaf nodes of the QTBT structure 130.

[0072] Generally Figure 2B The CTU 132 may be associated with parameters that define the size of the blocks corresponding to the nodes at the first and second levels of the QTBT structure 130. These parameters may include the CTU size (representing the size of the CTU 132 in samples), the minimum quadtree size (MinQTSize, representing the minimum allowable quadtree leaf node size), the maximum binary tree size (MaxBTSize, representing the maximum allowable binary tree root node size), the maximum binary tree depth (MaxBTDepth, representing the maximum allowable binary tree depth), and the minimum binary tree size (MinBTSize, representing the minimum allowable binary tree leaf node size).

[0073] The root node corresponding to the CTU in the QTBT structure can have four child nodes at the first level of the QTBT structure, and each of the child nodes can be divided according to the quadtree division. That is, the nodes at the first level are leaf nodes (without child nodes) or have four child nodes. An example of the QTBT structure 130 represents such a node as including a parent node and child nodes with solid lines for the branches. If the node at the first level is not greater than the maximum allowed binary tree root node size (MaxBTSize), the node can be further divided by its respective binary tree division. The binary tree splitting of a node can be iterative until the nodes obtained from the splitting reach the minimum allowed binary tree leaf node size (MinBTSize) or the maximum allowed binary tree depth (MaxBTDepth). An example of the QTBT structure 130 represents such a node as having dashed lines for the branches. The binary tree leaf nodes are called coding units (CUs), which are used for prediction (e.g., intra-picture or inter-picture prediction) and transformation without any further division. As discussed above, the CU can also be referred to as a "video block" or a "block".

[0074] In an example of the QTBT division structure, the CTU size is set to 128x128 (luminance samples 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. The quadtree division is first applied to the CTU to generate quadtree leaf nodes. The quadtree leaf nodes can have sizes ranging from 16x16 (i.e., MinQTSize) to 128x128 (i.e., CTU size). If the leaf quadtree leaf node is 128x128, since the size exceeds MaxBTSize (i.e., in this example, 64x64), the leaf quadtree node will not be further split by the binary tree. Otherwise, the leaf quadtree node will be further divided by the binary tree. Therefore, the quadtree leaf node is still the root node for the binary tree and has a binary tree depth of 0. When the binary tree depth reaches MaxBTDepth (in this example, 4), further splitting is not permitted. When a binary tree node has a width equal to MinBTSize (in this example, 4), this means that further horizontal splitting is not permitted. Similarly, a binary tree node with a height equal to MinBTSize means that further vertical splitting is not permitted for this 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 division.

[0075] The following describes transform-related tools. In video coding standards prior to HEVC, only fixed separable transforms were used in the case of vertically and horizontally using DCT-2. In HEVC, in addition to DCT-2, DST-7 is also adopted for 4x4 blocks as a fixed separable transform.

[0076] U.S. Patent No. 10,306,229, U.S. Patent Publication No. 2018 / 0020218, and U.S. Patent Publication No. 2019 / 0373261 describe various transform selection (MTS) methods. MTS was previously called adaptive multi-transform (AMT), which is just a name change, and the technology is the same. An example of MTS in U.S. Patent Publication No. 2019 / 0373261 has been adopted in the joint experimental model (JEM-7.0) of the Joint Video Team (JVET), and a simplified version of MTS has subsequently been adopted in VVC.

[0077] Figure 3 is a block diagram showing an example video encoder 200 that can perform the techniques of the present disclosure. Figure 3 is provided for purposes of explanation and should not be considered a limitation on the techniques broadly illustrated and described in the present disclosure. For purposes of explanation, the present disclosure describes the video encoder 200 in the context of video coding standards such as the HEVC video coding standard and the H.266 video coding standard under development. However, the techniques of the present disclosure are not limited to these video coding standards and generally apply to video encoding and decoding.

[0078] In Figure 3 the example, 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 can be implemented in one or more processors or in processing circuitry. For example, the units of the video encoder 200 can be implemented as one or more circuits or logic elements, as part of a hardware circuit, or as part of a processor, ASIC, or FPGA. Additionally, the video encoder 200 can include additional or alternative processors or processing circuitry to perform these and other functions.

[0079] The video data memory 230 may store video data to be encoded by components of the video encoder 200. The video encoder 200 may receive the video data stored in the video data memory 230 from, for example, a video source 104( Figure 1 ). The DPB 218 may act as a reference picture memory that stores reference video data for use by the video encoder 200 in predicting subsequent video data. The video data memory 230 and the DPB 218 may be formed of any one 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. The video data memory 230 and the DPB 218 may be provided by the same memory device or separate memory devices. In various examples, the video data memory 230 may be on-chip (as shown) with other components of the video encoder 200 or off-chip relative to those components.

[0080] In this disclosure, a reference to the video data memory 230 should not be construed as limited to a memory inside the video encoder 200 (unless specifically described as such), or a memory outside the video encoder 200 (unless specifically described as such). Rather, a reference 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 of outputs from various units of the video encoder 200.

[0081] illustrates Figure 3 various units to assist in understanding the operations performed by the video encoder 200. The units may be implemented as fixed function circuits, programmable circuits, or a combination thereof. Fixed function circuits refer to circuits that provide a specific function 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 functionality with respect to 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 instructions in 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 distinct circuit blocks (fixed function or programmable), and in some examples, one or more of these units may be integrated circuits.

[0082] Video encoder 200 may include an arithmetic logic unit (ALU), a basic function unit (EFU), digital circuitry, analog circuitry, and / or a programmable core formed from programmable circuitry. In an example where software executed by the programmable circuitry is used to perform the operations of video encoder 200, memory 106( Figure 1 ) may store the instructions (e.g., object code) of the software that video encoder 200 receives and executes, or another memory (not shown) within video encoder 200 may store such instructions.

[0083] Video data memory 230 is configured to store received video data. Video encoder 200 may retrieve pictures of video data from video data memory 230 and provide the video data to residual generation unit 204 and mode selection unit 202. The video data in video data memory 230 may be the original video data to be encoded.

[0084] Mode selection unit 202 includes motion estimation unit 222, motion compensation unit 224, and intra prediction unit 226. Mode selection unit 202 may include additional functional units that perform video prediction according to other prediction modes. As an example, mode selection unit 202 may include a palette unit, a block copy unit (which may be part of motion estimation unit 222 and / or motion compensation unit 224), an affine unit, a linear model (LM) unit, etc.

[0085] Mode selection unit 202 generally coordinates multiple encoding paths to test combinations of encoding parameters and the resulting rate-distortion values for such combinations. The encoding parameters may include the partitioning of CTUs into CUs, the prediction mode for a CU, the transform type for the residual data of a CU, the quantization parameter for the residual data of a CU, etc. Mode selection unit 202 may ultimately select the combination of encoding parameters that has a better rate-distortion value than other tested combinations.

[0086] Video encoder 200 may partition a picture retrieved from video data memory 230 into a series of CTUs and encapsulate one or more CTUs within a slice. Mode selection unit 202 may partition the CTUs of a picture according to a tree structure (such as the QTBT structure or quadtree structure of HEVC described above). As described above, video encoder 200 may form one or more CUs from partitioning CTUs according to the tree structure. Such CUs may generally also be referred to as "video blocks" or "blocks".

[0087] 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 a current block (e.g., a current CU, or in HEVC, the overlapping part of a PU and a TU). For the inter prediction of the current block, the motion estimation unit 222 may perform a motion search to identify one or more reference blocks in one or more reference pictures (e.g., one or more previously decoded pictures stored in the DPB 218) that closely match. In particular, the motion estimation unit 222 may calculate a value representing how closely a potential reference block will match the current block, for example, according to 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 typically may perform these calculations using the per-sample differences between the current block and the reference block under consideration. The motion estimation unit 222 may identify the reference block having the lowest value resulting from these calculations, indicating the reference block that most closely matches the current block.

[0088] The motion estimation unit 222 may form one or more motion vectors (MVs) defining the position of the reference block in the reference picture relative to 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 uni-directional inter prediction, the motion estimation unit 222 may provide a single motion vector, while for bi-directional 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 the data of the reference block. As another example, if the motion vectors have fractional sample accuracy, the motion compensation unit 224 may interpolate the values for the prediction block according to one or more interpolation filters. Additionally, for bi-directional inter prediction, the motion compensation unit 224 may retrieve the data for the two reference blocks identified by the respective motion vectors, and combine the retrieved data, for example, by per-sample averaging or weighted averaging.

[0089] As another example, for intra prediction or intra prediction coding, the intra prediction unit 226 may generate a prediction block from samples adjacent to the current block. For example, for the directional mode, the intra prediction unit 226 may typically mathematically combine the values of the adjacent samples and fill in the calculated values in a defined direction across the current block to produce a prediction block. As another example, for the DC mode, the intra prediction unit 226 may calculate the average value of the adjacent samples for the current block and generate a prediction block to include this resulting average value for each sample of the prediction block.

[0090] The mode selection unit 202 provides a prediction block to the residual generation unit 204. The residual generation unit 204 receives an original, unencoded 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 per-sample difference between the current block and the prediction block. The resultant per-sample difference defines the residual block for the current block. In some examples, the residual generation unit 204 may also determine the differences between the sample values in the residual block to generate the residual block using residual differential pulse code modulation (RDPCM). In some examples, the residual generation unit 204 may be formed using one or more subtractor circuits that perform binary subtraction.

[0091] In examples in which the mode selection unit 202 divides a CU into PUs, each PU may be associated with a luminance prediction unit and a corresponding chrominance prediction unit. The video encoder 200 and the video decoder 300 may support PUs of various sizes. As indicated above, the size of a CU may refer to the size of the luminance coding block of the CU, and the size of a PU may refer to the size of the luminance 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 prediction and PU sizes of 2Nx2N, 2NxN, Nx2N, NxN, or similar symmetric PU sizes for inter 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 prediction.

[0092] In examples in which the mode selection unit 202 does not further divide a CU into PUs, each CU may be associated with a luminance coding block and a corresponding chrominance coding block. As described above, the size of a CU may refer to the size of the luminance coding block of the CU. The video encoder 200 and the video decoder 300 may support CU sizes of 2Nx2N, 2NxN, or Nx2N.

[0093] For other video coding techniques (to name a few examples, such as intra-block copy mode coding, affine mode coding, and linear model (LM) mode coding), the mode selection unit 202 generates a prediction block for the current block being encoded via respective units 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 the manner in which a block is to be reconstructed based on a selected palette. In such a mode, the mode selection unit 202 may provide these syntax elements to the entropy coding unit 220 for coding.

[0094] As described above, the residual generation unit 204 receives video data for a current block and a corresponding predicted 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 predicted block and the current block.

[0095] 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 may apply various transforms to the residual block to form a transform coefficient block. For example, the transform processing unit 206 may apply a discrete cosine transform (DCT), a discrete sine transform (DST), an orientation transform, a Karhunen-Loève transform (KLT), or a conceptually similar transform to the residual block. In some examples, the transform processing unit 206 may perform multiple transforms on the residual block, e.g., 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.

[0096] In one or more examples described in the present disclosure, the transform processing unit 206 may include a separable and LFNST block marked as "encoder side" below in Figure 5 . For example, the transform processing unit 206 may perform a separable transform (e.g., DCT-2) on the residual to generate transform coefficients, and then perform LFNST on the transform coefficients to generate LFNST coefficients. The LFNST coefficients may then be quantized and entropy encoded for transmission.

[0097] 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 the 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 transform coefficient block associated with the current block by adjusting the QP value associated with the CU. Quantization may cause loss of information, and thus, the quantized transform coefficients may have lower precision compared to the original transform coefficients generated by the transform processing unit 206.

[0098] The inverse quantization unit 210 and the inverse transform processing unit 212 may apply inverse quantization and inverse transform to the quantized transform coefficient block, respectively, to reconstruct the 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 predicted block generated by the mode selection unit 202. For example, the reconstruction unit 214 may add the samples of the reconstructed residual block to the corresponding samples of the predicted block generated by the mode selection unit 202 to produce the reconstructed block.

[0099] Filter unit 216 may perform one or more filtering operations on the reconstructed block. For example, filter unit 216 may perform a deblocking operation to reduce block effect artifacts along the edges of the CU. In some examples, the operation of filter unit 216 may be skipped.

[0100] Video encoder 200 stores the reconstructed block in DPB 218. For example, in an example where the operation of filter unit 216 is not required, reconstruction unit 214 may store the reconstructed block into DPB 218. In an example where the operation of filter unit 216 is required, filter unit 216 may store the filtered reconstructed block into DPB 218. Motion estimation unit 222 and motion compensation unit 224 may retrieve reference pictures formed from the reconstructed (and potentially filtered) blocks from DPB 218 to perform inter prediction on blocks of subsequently encoded pictures. Additionally, intra prediction unit 226 may use the reconstructed blocks of the current picture in DPB 218 to perform intra prediction on other blocks in the current picture.

[0101] Generally, entropy coding unit 220 may perform entropy coding on syntax elements received from other functional components of video encoder 200. For example, entropy coding unit 220 may perform entropy coding on the quantized transform coefficient blocks from quantization unit 208. As another example, entropy coding unit 220 may perform entropy coding on prediction syntax elements (e.g., motion information for inter prediction or intra mode information for intra prediction) from mode selection unit 202. Entropy coding unit 220 may perform one or more entropy coding operations on the syntax elements (which are another example of video data) to generate entropy-coded data. For example, entropy coding unit 220 may perform context adaptive variable length coding (CAVLC) operations, CABAC operations, variable-to-variable (V2V) length coding operations, syntax-based context adaptive binary arithmetic coding (SBAC) operations, probability interval partitioning entropy (PIPE) coding operations, exponential Golomb coding operations, or another type of entropy coding operation on the data. In some examples, entropy coding unit 220 may operate in a bypass mode where the syntax elements are not entropy coded.

[0102] According to the technology of the present disclosure, the entropy coding unit 220 may perform entropy coding on a predetermined number of non-zero LFNST coefficients (e.g., nine or ten LFNST coefficients) of an 8x8 transform block. The entropy coding unit 220 may determine that the current transform block has a size of 8x8 coefficients and is transformed using LFNST. Thus, the entropy coding unit 220 may perform entropy coding on a predetermined number of non-zero LFNST coefficients (e.g., nine or ten non-zero LFNST coefficients), and then skip the coding of the remaining LFNST coefficients. The inverse transform processing unit 212 may treat the skipped LFNST coefficients as having a value of zero.

[0103] The entropy coding unit 220 may be pre-configured with a predetermined number of non-zero LFNST coefficients to be encoded. Alternatively, the entropy coding unit 220 may receive data regarding the predetermined number from, for example, the mode selection unit 202. For example, the mode selection unit 202 may determine rate-distortion optimization (RDO) values for encoding up to ten non-zero LFNST coefficients, and select the number of non-zero LFNST coefficients that yields the best RDO value as the predetermined number. The entropy coding unit 220 may be further configured with a predetermined maximum number of non-zero LFNST coefficients to be encoded, e.g., ten. The mode selection unit 202 may determine RDO values during the encoding process, e.g., to encode a transform block (and blocks containing the transform block, such as a CU). Additionally or alternatively, the mode selection unit 202 may determine the predetermined number according to the profile, layer, and / or level of the applicable video coding standard that the bitstream including the transform block conforms to. The entropy coding unit 220 may also perform entropy coding on data representing the predetermined number, such as in a VPS, SPS, PPS, APS, picture header, slice header, block header, or other such syntax structures.

[0104] Thus, in one example, the entropy coding unit 220 may receive an 8x8 block of LFNST coefficients, i.e., a total of sixty-four (64) LFNST coefficients. The entropy coding unit 220 may perform entropy coding on the LFNST coefficients such that the entropy coding unit 220 performs entropy coding on nine or ten non-zero LFNST coefficients, and skips the coding of the remaining LFNST coefficients (sixty-four minus the number of non-zero encoded LFNST coefficients, e.g., fifty-five or fifty-four).

[0105] The video encoder 200 may output a bitstream including entropy-coded syntax elements required to reconstruct blocks of a slice or picture. In particular, the entropy coding unit 220 may output the bitstream. The bitstream may conform to an applicable video coding standard, such as HEVC or VVC, and in particular, may be applied to a specific profile, layer, and / or level combination of the corresponding bitstream.

[0106] The inverse transform processing unit 212 may be configured to perform an inverse transform on a transform block. For example, the inverse transform processing unit 212 may initially determine that only a predetermined number of LFNST coefficients have non-zero values, and that the remaining LFNST coefficients are zero-valued. The inverse transform processing unit 212 may apply an inverse LFNST to the inverse-quantized LFNST coefficients received from the inverse quantization unit 210, and then apply an inverse primary transform to reproduce the residual block. The reconstruction unit 214 may reconstruct the original block (e.g., the original decoded block). In this way, the video encoder 200 may also be considered a device for decoding video data, since the video encoder 200 includes a decoding loop represented by the inverse quantization unit 210, the inverse transform processing unit 212, the reconstruction unit 214, the filter unit 216, and the DPB 218.

[0107] The operations described above are described with respect to blocks of video data. Such a description should be understood as operations on luminance decoded blocks and / or chrominance decoded blocks. As described above, in some examples, the luminance decoded block and the chrominance decoded block are the luminance and chrominance components of a CU. In some examples, the luminance decoded block and the chrominance decoded block are the luminance and chrominance components of a PU.

[0108] In some examples, the operations performed on the luminance decoded block need not be repeated for the chrominance decoded block. As an example, the operations for identifying a motion vector (MV) and a reference picture for the luminance decoded block need not be repeated for identifying the MV and reference picture for the chrominance block. Rather, the MV for the luminance decoded block may be scaled to determine the MV for the chrominance block, and the reference picture may be the same. As another example, the intra prediction process may be the same for both the luminance decoded block and the chrominance decoded block.

[0109] Video encoder 200 represents an example of a device configured to encode video data, the device including: a memory configured to store video data; and one or more processing units implemented in circuitry and configured to transform residual values to generate TUs; determine that the TU size is 4x4; set all coefficients of the TU except for eight coefficients to be equal to zero; perform LFNST on the TU having at most eight non-zero coefficients to generate LFNST coefficients; and signal information indicating the LFNST coefficients. The one or more processing units implemented in circuitry may be configured to: transform residual values to generate TUs; determine that the TU size is 8x8; set all coefficients of the TU except for ten coefficients to be equal to zero; perform LFNST on the TU having at most ten non-zero coefficients to generate LFNST coefficients; and signal information indicating the LFNST coefficients. The one or more processing units implemented in circuitry may be configured to: transform residual values to generate TUs; determine that the TU size is greater than 8x8; set all coefficients of the TU except for sixteen coefficients to be equal to zero; perform LFNST on the TU having at most sixteen non-zero coefficients to generate LFNST coefficients; and signal information indicating the LFNST coefficients.

[0110] Video encoder 200 also represents an example of a device for decoding video data, the device including: a memory configured to store video data; and one or more processors implemented in circuitry and configured to: determine that a transform block of the video data has a size of 8x8 coefficients and the transform block is transformed using a low-frequency non-separable transform (LFNST); decode at least nine non-zero transform coefficients of the transform block; use inverse LFNST to inverse-transform the transform block to reproduce a residual block corresponding to the transform block; and use the residual block to reconstruct a block of the video data.

[0111] Figure 4 is a block diagram showing an example video decoder 300 that may execute the techniques of the present disclosure. Figure 4 is provided for explanatory purposes and does not limit the techniques extensively illustrated and described in the present disclosure. For explanatory purposes, the present disclosure describes video decoder 300 according to the techniques of JEM, VVC, and HEVC. However, the techniques of the present disclosure may be performed by video coding devices configured for other video coding standards.

[0112] In Figure 4In the example, 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 one 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. For example, the units of video decoder 300 may be implemented as one or more circuits or logic elements, as part of a hardware circuit, or as part of a processor, ASIC, or FPGA. Additionally, video decoder 300 may include additional or alternative processors or processing circuitry to perform these and other functions.

[0113] The prediction processing unit 304 includes a motion compensation unit 316 and an intra prediction unit 318. The prediction processing unit 304 may include additional units for performing prediction according to other prediction modes. As an example, the prediction processing unit 304 may include a palette unit, a 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, video decoder 300 may include more, fewer, or different functional components.

[0114] The CPB memory 320 may store video data to be decoded by components of video decoder 300, such as an encoded video bitstream. The video data stored in the CPB memory 320 may be obtained, for example, from a computer-readable medium 110 ( Figure 1 ). The CPB memory 320 may include a CPB that stores encoded video data (e.g., syntax elements) from the encoded video bitstream. Additionally, the CPB memory 320 may store video data other than the syntax elements of decoded pictures, such as temporary data representing the outputs from the various units of video decoder 300. The DPB 314 generally stores decoded pictures, and video decoder 300 may output the decoded pictures and / or use the decoded pictures as reference video data when decoding subsequent data or pictures of the encoded video bitstream. The CPB memory 320 and the DPB 314 may be formed of any of a variety of memory devices, such as DRAM, including SDRAM, MRAM, RRAM, or other types of memory devices. The CPB memory 320 and the DPB 314 may be provided by the same memory device or separate memory devices. In various examples, the CPB memory 320 may be on-chip or off-chip relative to the other components of video decoder 300.

[0115] Additionally or alternatively, in some examples, the video decoder 300 may retrieve the decoded video data from the memory 120( Figure 1 ). That is, the memory 120 may store data as discussed above with respect to the CPB memory 320. Similarly, when some or all of the functions of the video decoder 300 are implemented in software to be executed by the processing circuitry of the video decoder 300, the memory 120 may store the instructions to be executed by the video decoder 300.

[0116] Description Figure 4 The various units shown in Figure 3 are provided to assist in understanding the operations performed by the video decoder 300. The units may be implemented as fixed-function circuitry, programmable circuitry, or a combination thereof. Similar to

[0117] a fixed-function circuit refers to circuitry that provides a specific function and is pre-set with respect to the operations that can be performed. A programmable circuit refers to circuitry that can be programmed to perform various tasks and provides flexible functionality with respect to 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 instructions in the software or firmware. A fixed-function circuit may execute software instructions (e.g., to receive parameters or output parameters), but the types of operations performed by the fixed-function circuit are generally immutable. In some examples, one or more of the units may be distinct circuit blocks (fixed-function or programmable), and in some examples, one or more of the units may be integrated circuits.

[0117] The video decoder 300 may include an ALU, EFU, digital circuitry, analog circuitry, and / or programmable cores formed from programmable circuitry. In examples in which the operations of the video decoder 300 are performed by software executed on programmable circuitry, on-chip or off-chip memory may store the instructions (e.g., object code) of the software that the video decoder 300 receives and executes.

[0118] The entropy decoding unit 302 may receive the encoded video data from the CPB and perform entropy decoding on the video data to reproduce the syntax elements. The prediction processing unit 304, inverse quantization unit 306, inverse transform processing unit 308, reconstruction unit 310, and filter unit 312 may generate the decoded video data based on the syntax elements extracted from the bitstream.

[0119] Generally, the video decoder 300 reconstructs pictures on a block-by-block basis. The video decoder 300 may perform the reconstruction operation on each block individually (where the block currently being reconstructed (i.e., decoded) may be referred to as the "current block").

[0120] The entropy decoding unit 302 may perform entropy decoding on the syntax elements of the quantized transform coefficients that define the quantized transform coefficient block and transform information such as quantization parameter (QP) and / or transform mode indication. The inverse quantization unit 306 may use the QP associated with the quantized transform coefficient block to determine the degree of quantization, and likewise, determine the degree of inverse quantization to be applied by the inverse quantization unit 306. 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 thus form a transform coefficient block including the transform coefficients.

[0121] 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 or inverse DST, inverse integer transform, inverse Karhunen - Loève transform (KLT), inverse rotation transform, inverse orientation transform, or another inverse transform to the transform coefficient block.

[0122] In one or more examples described in the present disclosure, the inverse transform processing unit 308 may include the inverse LFNST and inverse separable block shown on the Figure 5 decoder side. For example, the inverse transform processing unit 308 may receive the decoded coefficients (e.g., decoded LFNST coefficients) and perform an inverse LFNST to generate inverse-transformed LFNST coefficients (which may be similar or identical to the transform coefficients generated after the separable transform performed by the video encoder 200). The inverse transform processing unit 308 may perform an inverse separable transform (e.g., DCT-2) to generate the residual values.

[0123] In addition, the prediction processing unit 304 generates a prediction block based on the prediction information syntax elements entropy decoded by the entropy decoding unit 302. For example, if the prediction information syntax element indicates that the current block is inter - frame predicted, the motion compensation unit 316 may generate the prediction block. In such a case, the prediction information syntax element may indicate the reference picture in the DPB 314 from which the reference block is to be retrieved, and a motion vector for identifying the position of the reference block in the reference picture relative to the position of the current block in the current picture. The motion compensation unit 316 may generally perform the inter - frame prediction process in a manner substantially similar to the manner described with respect to the motion compensation unit 224 ( Figure 3 ).

[0124] As another example, if the prediction information syntax element indicates that the current block is intra - frame predicted, the intra - frame prediction unit 318 may generate a prediction block according to the intra - frame prediction mode indicated by the prediction information syntax element. Again, the intra - frame prediction unit 318 may generally perform in a manner substantially similar to the manner described with respect to the intra - frame prediction unit 226 ( Figure 3)Execute the intra prediction process in a manner substantially similar to the described manner. The intra prediction unit 318 can retrieve data for neighboring samples of the current block from the DPB 314.

[0125] The reconstruction unit 310 can use the prediction block and the residual block to reconstruct the current block. For example, the reconstruction unit 310 can add the samples of the residual block to the corresponding samples of the prediction block to reconstruct the current block.

[0126] The filter unit 312 can perform one or more filter operations on the reconstructed block. For example, the filter unit 312 can perform a deblocking operation to reduce block effect artifacts along the edges of the reconstructed block. The operation of the filter unit 312 is not necessarily performed in all examples.

[0127] The video decoder 300 can store the reconstructed block in the DPB 314. For example, in an example where the operation of the filter unit 312 is not performed, the reconstruction unit 310 can store the reconstructed block in the DPB 314. In an example where the operation of the filter unit 312 is performed, the filter unit 312 can store the filtered reconstructed block in the DPB 314. As discussed above, the DPB 314 can provide reference information (such as the current picture for intra prediction and the samples of the previously decoded pictures for subsequent motion compensation) to the prediction processing unit 304. In addition, the video decoder 300 can output the decoded picture (e.g., the decoded video) from the DPB 314 for subsequent presentation on a display device such as Figure 1 the display device 118.

[0128] In this way, video decoder 300 represents an example of a video decoding device that includes: a memory configured to store video data; and one or more processing units implemented in circuitry and configured to generate a 4x4 transform unit (TU) having a plurality of coefficients, where the TU includes at most eight non-zero coefficients; perform an inverse low-frequency non-separable transform (LFNST) on the TU to generate inverse-transformed LFNST coefficients; perform an inverse transform on the inverse-transformed LFNST coefficients to generate a residual value for a current block; and reconstruct the current block based on the residual value. The one or more processing units implemented in circuitry may be configured to generate an 8x8 TU having a plurality of coefficients, where the TU includes at most ten non-zero coefficients; perform an inverse LFNST on the TU to generate inverse-transformed LFNST coefficients; perform an inverse transform on the inverse-transformed LFNST coefficients to generate a residual value for a current block; and reconstruct the current block based on the residual value. The one or more processing units implemented in circuitry may be configured to generate a TU having a size greater than 8x8 and having a plurality of coefficients, where the TU includes at most sixteen non-zero coefficients; perform an inverse LFNST on the TU to generate inverse-transformed LFNST coefficients; perform an inverse transform on the inverse-transformed LFNST coefficients to generate a residual value for a current block; and reconstruct the current block based on the residual value.

[0129] Video decoder 300 also represents an example of a device for decoding video data, the device including: a memory configured to store video data; and one or more processors implemented in circuitry and configured to: determine that a transform block of the video data has a size of 8x8 coefficients and that the transform block is transformed using a low-frequency non-separable transform (LFNST); decode at least nine non-zero transform coefficients of the transform block; use an inverse LFNST to inverse-transform the transform block to reproduce a residual block corresponding to the transform block; and use the residual block to reconstruct a block of the video data.

[0130] Figure 5 is a block diagram showing an example set of components that can perform a low-frequency non-separable transform (LFNST). Figure 5 The example depicts both encoder-side LFNST and decoder-side LFNST. On the encoder side, an encoder (such as video encoder 200) may apply a primary (separable) transform, then apply LFNST, and then quantize the LFNST coefficients. On the decoder side, a decoder (such as video decoder 300) may apply inverse quantization, apply inverse LFNST, and then apply an inverse primary (separable) transform.

[0131] As Figure 5As shown, the LFNST is used in JEM-7.0 to further improve the decoding efficiency of the MTS. The LFNST was previously known as the Non-Separable Secondary Transform (NSST) or the Secondary Transform, where all these abbreviations refer to the same process. The LFNST is implemented based on U.S. Patent No. 10,448,053. In addition, U.S. Patent No. 10,491,922, U.S. Patent Publication No. 2017 / 0094314, U.S. Patent No. 10,349,085, U.S. Provisional Application No. 62 / 668,105, and U.S. Patent Publication No. 2019 / 0297351 describe alternative designs and further details. Recently, the LFNST has been adopted in the VVC standard based on the following document: Koo et al., "CE6: Reduced Secondary Transform (RST) (CE6-3.1)", Joint Video Exploration Team (JVET) of ITU-T SG 16 WP 3 and ISO / IEC JTC 1 / SC 29 / WG 11, 14th meeting: March 19 - 27, 2019, Geneva, Switzerland, JVET-N0193 (hereinafter referred to as "JVET-N0193").

[0132] Figure 6 is a conceptual diagram showing the application of the inverse LFNST to a transform block to reproduce a residual block. The decoding process using the LFNST is described below. The inverse transform using the LFNST involves the following example steps shown in Figure 6 . The decoded transform coefficients (e.g., sub-block 400 in Figure 6 ) are used as the input to the inverse LFNST by first converting the 2-D block to a 1-D list (or vector) of coefficients via a predefined scan / sorting. The inverse LFNST is applied to the 1-D list of input coefficients, and the output coefficients are reorganized into a 2-D block via a predefined scan / sorting (e.g., sub-block 402 in Figure 6 ). The inverse-transformed LFNST coefficients are used as the input to the separable inverse DCT-2 to obtain the reconstructed residual.

[0133] Figure 7 is a conceptual diagram showing the application of the inverse LFNST to a 4x4 block. In the current version of VVC draft 7, the LFNST can be applied to 4x4 and 8x8 sub-blocks. In both cases, the sixteen decoded coefficients in the 4x4 sub-block (some of which may be canonically zeroed) are input to the inverse LFNST. For the 4x4 case, a 16x16 inverse LFNST is used to construct sixteen intermediate coefficients before the separable inverse DCT-2, as shown in Figure 7 .

[0134] Figure 8 It is a conceptual diagram showing the application of the inverse LFNST to a 4x4 block to produce an 8x8 block. In particular, in this example, the inverse LFNST is used to reconstruct forty-eight intermediate coefficients from sixteen input coefficients, and then the intermediate coefficients are rearranged into an L-shaped pattern. For the 8x8 case, a 16x48 inverse LFNST is used to construct forty-eight intermediate coefficients before the separable inverse DCT-2, as Figure 8 shown. In Figure 8 , the forty-eight intermediate coefficients are reorganized in an L-shaped pattern.

[0135] The inverse LFNST process can be fully defined based on (i) a transformation matrix (e.g., the LFNST transformation matrix) and (ii) a reorganization pattern / scan for the intermediate coefficients. U.S. Provisional Application No. 62 / 849,689 describes an example of the zeroing process in VVC Draft 7. Another example of using zeroing to group an ordered list of coefficients is described in U.S. Provisional Application No. 62 / 799,410.

[0136] For the 4x4 LFNST, the following two patterns / scans are used depending on the intra mode:

[0137]

[0138] In the above, the two patterns / scans indicate the reordering of the intermediate coefficients. For example, g_lfnstRGScan4x4 does not change the row-major reordering of the coefficients. However, lfnstRGTranScan4x4 reorders by transposing the order of the coefficients (e.g., the coefficients at 1, 2, 3, 6, 7, and 11 are exchanged with the coefficients at 4, 8, 12, 9, 13, and 14 respectively).

[0139] For the 4x4 LFNST, in the current VVC, eight 16x16 matrices are used as candidates. These are listed in Section 8.7.4.3 of JVET-O2001: Bross et al., "Versatile Video Coding (Draft 6)", Joint Video Exploration Team (JVET) of ITU-T SG 16 WP 3 and ISO / IEC JTC 1 / SC 29 / WG 11, 15th Meeting: Gothenburg, Sweden, July 3-12, 2019.

[0140] For the 8x8 LFNST, the following two patterns / scans are used depending on the intra mode:

[0141]

[0142] In the above, two modes / scanning indications reorder the intermediate coefficients. Specifically, g_lfnstRGScan8x8 reorganizes 48 intermediate coefficients in an L-shaped pattern (e.g., the 48th coefficient is mapped to the position 59 in Figure 8 ). The scanning lfnstRGTranScan4x4 reorders the L-shaped pattern by transposing the coefficients (e.g., the 48th coefficient is mapped to the position 31 in Figure 8 ). For 8x8 LFNST, in the current VVC, eight 16x48 matrices are used as candidates. These are listed in Section 8.7.4.3 of JVET-O2001.

[0143] There may be some problems in the case of the LFNST technique. For example, the zeroing process may be undesirable unless necessary (e.g., the zeroing process may be necessary for reducing the worst-case number of multiplications to reduce complexity). For the LFNST design in the current VVC, the worst-case number of multiplications per coefficient is equal to 8 (due to the case where 4x4 LFNST is applied to a 4x4 TU, where each coefficient requires (8x16) / 16 = 8 multiplications). The second-worst case is due to 8x8 LFNST being applied to an 8x8 TU, where each coefficient requires (8x48) / 64 = 6 multiplications.

[0144] If up to 10 non-zero coefficients are allowed in 8x8 LFNST (i.e., each coefficient requires (10x48) / 64 = 7.5 multiplications), the worst case may still be maintained in the VVC. Therefore, the number of coefficients that are zeroed unnecessarily can be reduced by allowing more than two coefficients in the transform block (e.g., two more non-zero coefficients). That is, the worst-case scenario can allow up to 10 non-zero coefficients.

[0145] To allow up to 10 coefficients for the VVC design, the present disclosure describes the following example techniques that can be used together or separately. If LFNST is used and the TU size is 4x4, up to eight non-zero coefficients are allowed in the decoded coefficients (e.g., the box 400 of Figure 6 ), and the remaining coefficients are canonically zeroed. If LFNST is used and the TU size is 8x8, up to 10 non-zero coefficients are allowed in the decoded coefficients (e.g., the box 400 of Figure 6 ), and the remaining coefficients are canonically zeroed. If LFNST is used (e.g., in the example where the TU size is greater than 8x8), up to 16 non-zero coefficients are allowed in the decoded coefficients (e.g., the box 400 of Figure 6 ), and the remaining coefficients are canonically zeroed.

[0146] For the above example of zeroing depending on the block size for LFNST, the update to VVC Draft 7 can be reflected as follows: <delete>and< / delete> Indicates deletion, and <add>and< / add> Is used to indicate addition.

[0147] The variables predModeIntra, nLfnstOutSize, log2LfnstSize, nLfnstSize, and nonZeroSize are derived as follows:

[0148] predModeIntra =

[0149] (cIdx == 0)? IntraPredModeY[xTbY][yTbY] : IntraPredModeC[xTbY][yTbY] (1149)

[0150] nLfnstOutSize = (nTbW >= 8 && nTbH >= 8)? 48 : 16 (1150)

[0151] log2LfnstSize = (nTbW >= 8 && nTbH >= 8)? 3 : 2 (1151)

[0152] nLfnstSize = 1 << log2LfnstSize (1152)

[0153] <delete>nonZeroSize = ((nTbW == 4 && nTbH == 4) ||

[0154] (nTbW == 8 && nTbH == 8))? 8 : 16 (1153) < / delete>

[0155] – <add>If (nTbW == 4 && nTbH == 4) is true, then the following applies:

[0156] nonZeroSize = 8

[0157] – Otherwise, if (nTbW == 8 && nTbH == 8) is true, then the following applies:

[0158] nonZeroSize = 10

[0159] – Otherwise, the following applies:

[0160] nonZeroSize = 16< / add>

[0161] Figure 9 Is a flowchart showing an example method for encoding a current block. The current block may include a current CU. Although described with respect to video encoder 200( Figure 1 and Figure 3 ), it should be understood that other devices may be configured to perform methods similar to those of Figure 9 .

[0162] In this example, video encoder 200 initially predicts the current block (350). For example, video encoder 200 may form a prediction block for the current block. Video encoder 200 may then calculate a residual block for the current block (352). To calculate the residual block, video encoder 200 may calculate the difference between the original unencoded block and the prediction block for the current block.

[0163] The video encoder 200 can then perform a transform on the residual block and quantize the transform coefficients of the residual block (354). Specifically, according to the techniques of the present disclosure, the video encoder 200 can apply a primary (separable) transform to the residual block and then apply LFNST to the resulting transform coefficients to generate LFNST coefficients. The video encoder 200 can then quantize the LFNST coefficients. The video encoder 200 also determines the number of non-zero LFNST coefficients to be encoded (356). For example, the video encoder 200 can perform an RDO test and / or determine the number of non-zero LFNST coefficients according to the profile, layer, and / or level of the corresponding video coding standard to which the video data corresponds.

[0164] Next, the video encoder 200 can scan the quantized LFNST coefficients (358). During or after the scan, the video encoder 200 can perform entropy coding on the determined number of non-zero LFNST coefficients (360). For example, the video encoder 200 can use CAVLC or CABAC to encode the LFNST coefficients. In addition, the video encoder 200 can skip the encoding of subsequent LFNST coefficients, thereby treating the remaining LFNST coefficients of the transform block as having zero values. The video encoder 200 can then output the entropy-coded data of the block (362).

[0165] The video encoder 200 can also perform inverse quantization and inverse transform on the LFNST coefficients (364). In particular, the video encoder 200 can reconstruct the transform block, which includes the encoded pre-determined number of non-zero LFNST coefficients and zero-valued coefficients for the remaining coefficients of the transform block. By performing an inverse transform on the transform block (using inverse LFNST and inverse primary / separable transform), the video encoder 200 can reproduce the residual block. The video encoder 200 can then reconstruct the original block (366), for example, by combining the residual block with the prediction block.

[0166] In this way, Figure 9 The method of represents an example of a method for decoding video data, which includes: determining that a transform block of the video data has a size of 8x8 coefficients and that the transform block is transformed using a low-frequency non-separable transform (LFNST); decoding at least nine non-zero transform coefficients of the transform block; using inverse LFNST to perform an inverse transform on the transform block to reproduce the residual block corresponding to the transform block; and using the residual block to reconstruct a block of the video data.

[0167] Figure 10 is a flowchart showing an example method for decoding a current block of video data. The current block can include a current CU. Although with respect to the video decoder 300 ( Figure 1 and 4) is described, but it should be understood that other devices may be configured to perform methods similar to those of Figure 10 the method.

[0168] Video decoder 300 may receive entropy-coded data for a current block (such as entropy-coded prediction information and entropy-coded data of coefficients for a residual block corresponding to the current block) (370). Video decoder 300 may determine the number of non-zero LFNST coefficients for the current block (372), assuming the current block includes an 8x8 LFNST transform block.

[0169] Video decoder 300 may entropy-decode the entropy-coded data to determine the prediction information for the current block and the coefficients to reproduce the residual block (374). When entropy-decoding the entropy-coded data, video decoder 300 may entropy-decode only the determined number of non-zero LFNST coefficients for the current block (e.g., nine or ten non-zero LFNST coefficients).

[0170] Video decoder 300 may predict the current block, for example, using an intra or inter prediction mode indicated by the prediction information for the current block (376), to calculate a prediction block for the current block.

[0171] Video decoder 300 may then inverse-scan the number of non-zero LFNST coefficients (378) to create a transform block of quantized non-zero LFNST coefficients and set the remaining coefficients in the transform block to be equal to zero (380). Video decoder 300 may then inverse-quantize and inverse-transform the transform coefficients (using inverse LFNST and inverse primary / separable transform) to produce a residual block (382). Video decoder 300 may finally decode and reconstruct the current block, including combining the prediction block with the residual block (384).

[0172] In this way, Figure 10 the method represents an example of a method for decoding video data, which includes: determining that a transform block of the video data has a size of 8x8 coefficients and the transform block is transformed using a low-frequency non-separable transform (LFNST); decoding at least nine non-zero transform coefficients of the transform block; using inverse LFNST to inverse-transform the transform block to reproduce a residual block corresponding to the transform block; and using the residual block to reconstruct a block of the video data.

[0173] Certain example techniques of the present disclosure are summarized in the following clauses:

[0174] Clause 1: A method for decoding video data, the method comprising: generating a transform unit (TU) of size 4x4 having a plurality of coefficients, wherein the TU includes at most eight non-zero coefficients; performing an inverse low-frequency non-separable transform (LFNST) on the TU to generate inverse-transformed LFNST coefficients; performing an inverse transform on the inverse-transformed LFNST coefficients to generate a residual value for a current block; and reconstructing the current block based on the residual value.

[0175] Clause 2: A method for decoding video data, the method comprising: generating a transform unit (TU) of size 8x8 having a plurality of coefficients, wherein the TU includes at most ten non-zero coefficients; performing an inverse low-frequency non-separable transform (LFNST) on the TU to generate inverse-transformed LFNST coefficients; performing an inverse transform on the inverse-transformed LFNST coefficients to generate a residual value for a current block; and reconstructing the current block based on the residual value.

[0176] Clause 3: A method for decoding video data, the method comprising: generating a transform unit (TU) of size greater than 8x8 having a plurality of coefficients, wherein the TU includes at most sixteen non-zero coefficients; performing an inverse low-frequency non-separable transform (LFNST) on the TU to generate inverse-transformed LFNST coefficients; performing an inverse transform on the inverse-transformed LFNST coefficients to generate a residual value for a current block; and reconstructing the current block based on the residual value.

[0177] Clause 4: The method according to any one of Clauses 1-3, wherein the inverse transform includes a separable inverse transform.

[0178] Clause 5: The method according to Clause 4, wherein the separable inverse transform includes an inverse discrete cosine transform (DCT)-2.

[0179] Clause 6: The method according to any combination of Clauses 1-5.

[0180] Clause 7: A method for encoding video data, the method comprising: performing a transform on a residual value to generate a transform unit (TU); determining that the TU size is 4x4; setting all coefficients of the TU other than eight coefficients to be equal to zero such that the TU has at most eight non-zero coefficients; performing a low-frequency non-separable transform (LFNST) on the TU to generate LFNST coefficients; and signaling information indicative of the LFNST coefficients.

[0181] Clause 8: A method for encoding video data, the method comprising: performing a transform on a residual value to generate a transform unit (TU); determining that the TU size is 8x8; setting all coefficients of the TU except for ten coefficients to be equal to zero such that the TU has at most ten non-zero coefficients; performing a low-frequency non-separable transform (LFNST) on the TU to generate LFNST coefficients; and signaling information indicative of the LFNST coefficients.

[0182] Clause 9: A method for encoding video data, the method comprising: performing a transform on a residual value to generate a transform unit (TU); determining that the TU size is greater than 8x8; setting all coefficients of the TU except for sixteen coefficients to be equal to zero such that the TU has at most sixteen non-zero coefficients; performing a low-frequency non-separable transform (LFNST) on the TU to generate LFNST coefficients; and signaling information indicative of the LFNST coefficients.

[0183] Clause 10: The method according to any one of Clauses 7-9, wherein the transform is a separable transform.

[0184] Clause 11: The method according to Clause 10, wherein the transform includes a discrete cosine transform (DCT)-2.

[0185] Clause 12: The method according to any combination of Clauses 7-11.

[0186] Clause 13: A device for decoding video data, the device comprising: a memory configured to store video data; and a processing circuit configured to perform the method according to any one clause or combination of Clauses 1-6.

[0187] Clause 14: The device according to Clause 13, wherein the device further comprises: a display configured to display the decoded video data.

[0188] Clause 15: A device for encoding video data, the device comprising: a memory configured to store video data; and a processing circuit configured to perform the method according to any one clause or combination of Clauses 7-12.

[0189] Clause 16: The device according to any one of Clauses 13-15, wherein the device comprises one or more of a camera, a computer, a mobile device, a broadcast receiver device, or a set-top box.

[0190] Clause 17: A computer-readable storage medium having instructions stored thereon that, when executed, cause one or more processors to perform the method according to any one clause or combination of Clauses 1-6 or 7-12.

[0191] Clause 18: An apparatus for decoding video data, the apparatus comprising units for performing the method according to any one clause or combination of clauses 1-6 or 7-12.

[0192] It should be recognized that, depending on the example, certain actions or events of any of the techniques described herein may be performed in a different order, may be added, combined, or entirely excluded (e.g., not all described actions or events are necessary for the practice of the techniques). Additionally, in certain examples, the actions or events may be performed, for example, by multithreaded processing, interrupt processing, or by multiple processors concurrently rather than sequentially.

[0193] In one or more examples, the described functionality may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functionality may be stored on or transmitted via a computer-readable medium as one or more instructions or code and executed by a hardware-based processing unit. The 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 including any medium that facilitates transfer of a computer program from one place to another according to a communication protocol. In this manner, the computer-readable medium generally may correspond to (1) a tangible computer-readable storage medium (which is non-transitory) or (2) a communication medium such as a signal or carrier wave. The data storage medium may be any available medium that can be accessed by one or more computers or one or more processors to retrieve instructions, code, and / or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium.

[0194] By way of example and not limitation, such a computer-readable storage medium can include RAM, ROM, EEPROM, CD-ROM, or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store the desired program code in the form of instructions or data structures and that can be accessed by a computer. Additionally, any connection is properly termed a computer-readable medium. For example, if the instructions are transmitted using coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of 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 transient media, but instead are directed to non-transitory, tangible storage media. As used herein, disk and disc include compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk, and Blu-ray disc, where disks typically reproduce data magnetically, while discs reproduce data optically using lasers. Combinations of the above should also be included within the scope of computer-readable media.

[0195] The instructions can be executed by one or more processors, such as one or more digital signal processors (DSPs), general microprocessors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Thus, as used herein, the terms "processor" and "processing circuitry" can refer to any one of the foregoing structures or any other structure suitable for implementation of the techniques described herein. Additionally, in some aspects, the functions described herein can be provided within dedicated hardware and / or software modules configured for encoding and decoding, or incorporated into a combined codec. Further, the techniques can be implemented entirely within one or more circuits or logic elements.

[0196] The techniques of the present disclosure can be implemented in a variety of devices or apparatuses, including wireless handsets, integrated circuits (ICs), or a group of ICs (e.g., a chip set). Various components, modules, or units are described in the present disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but are not necessarily required to be implemented by distinct hardware units. Rather, as described above, the various units can be combined in a codec hardware unit, or provided by a collection of interoperating hardware units, including one or more processors as described above, in conjunction with appropriate software and / or firmware.

[0197] Various examples have been described. These and other examples are within the scope of the appended claims.

Claims

1. A method for decoding video data, the method comprising: Determining that a transform block of the video data has a size of 8x8 coefficients and that the transform block is transformed using a low-frequency non-separable transform (LFNST); Decoding at least nine non-zero transform coefficients of the transform block; Using an inverse LFNST to inverse-transform the transform block to generate at least nine inverse-transformed LFNST coefficients to reproduce a residual block corresponding to the transform block; And Using the residual block to reconstruct a block of the video data.

2. The method according to claim 1, wherein Decoding the at least nine non-zero transform coefficients includes: Decoding at most ten non-zero transform coefficients of the transform block; and Inferring that the remaining transform coefficients of the transform block are zero-valued without decoding the values for the remaining transform coefficients.

3. The method according to claim 1, wherein, The transform block includes sixty-four total transform coefficients, a number of the total transform coefficients are zero-valued, the number of the total transform coefficients that are zero-valued is equal to sixty-four minus the number of non-zero transform coefficients, and the number of the total transform coefficients that are zero-valued is at most fifty-five.

4. The method according to claim 1, wherein, Decoding the at least nine non-zero transform coefficients includes: Decoding at most a predetermined maximum number of non-zero transform coefficients for the transform block.

5. The method according to claim 4 further comprises: Setting the values of the remaining transform coefficients of the transform block to be equal to zero.

6. The method according to claim 4 further comprises: Decoding a value for representing the predetermined number from at least one of a video parameter set (VPS), a sequence parameter set (SPS), a picture parameter set (PPS), a slice header, a coding tree unit (CTU) header, or a block header.

7. The method according to claim 1, further comprising: Encoding the transform block before decoding the transform block; When encoding the transform block, determining a number of non-zero transform coefficients to be encoded for the transform block, the number being nine or ten; and Encoding the number of non-zero transform coefficients of the transform block without encoding other transform coefficients of the transform block.

8. The method according to claim 7, wherein, Determining the number of non-zero transform coefficients includes: Determining that the number of non-zero transform coefficients is nine or ten according to a profile, layer, or level of a video coding standard that the video data conforms to.

9. An apparatus for decoding video data, the apparatus comprising: A memory configured to store video data; And One or more processors implemented in circuitry and configured to: Determine that a transform block of the video data has a size of 8x8 coefficients and that the transform block is transformed using a low-frequency non-separable transform (LFNST); Decoding at least nine non-zero transform coefficients of the transform block; Using an inverse LFNST to inverse-transform the transform block to generate at least nine inverse-transformed LFNST coefficients to reproduce a residual block corresponding to the transform block; And Using the residual block to reconstruct a block of the video data.

10. The apparatus according to claim 9, wherein, The one or more processors are further configured to: Decoding at most ten non-zero transform coefficients of the transform block; and Inferring that the remaining transform coefficients of the transform block are zero-valued without decoding the values for the remaining transform coefficients.

11. The device according to claim 9, wherein, The transform block includes sixty-four total transform coefficients, several of the total transform coefficients are zero-valued, the number of the zero-valued total transform coefficients is equal to sixty-four minus the number of non-zero transform coefficients, and the number of the zero-valued total transform coefficients is at most fifty-five.

12. The device according to claim 9, wherein The one or more processors are further configured to: decode at most a predetermined maximum number of non-zero transform coefficients for the transform block.

13. The device according to claim 12, wherein, The one or more processors are further configured to: set the values of the remaining transform coefficients of the transform block to be equal to zero.

14. The device according to claim 12, wherein The one or more processors are further configured to: decode a value for representing the predetermined number from at least one of a video parameter set (VPS), a sequence parameter set (SPS), a picture parameter set (PPS), a slice header, a coding tree unit (CTU) header, or a block header.

15. The apparatus according to claim 9, wherein, The one or more processors are further configured to: Before decoding the transform block, encode the transform block; When encoding the transform block, determine the number of non-zero transform coefficients to be encoded for the transform block, the number being nine or ten; and Encode the number of non-zero transform coefficients of the transform block without encoding the other transform coefficients of the transform block.

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

17. The device according to claim 9, wherein, The device includes one or more of a camera, a computer, a mobile device, a broadcast receiver device, or a set-top box.

18. A computer-readable storage medium having instructions stored thereon that, when executed, cause a processor to perform the following operations: Determine that a transform block of video data has a size of 8x8 coefficients and the transform block is transformed using a low-frequency non-separable transform (LFNST); Decode at least nine non-zero transform coefficients of the transform block; Perform an inverse transform on the transform block using an inverse LFNST to generate at least nine inverse-transformed LFNST coefficients to reproduce a residual block corresponding to the transform block; and Use the residual block to reconstruct a block of the video data.

19. The computer-readable storage medium according to claim 18, wherein, The instructions that cause the processor to decode the at least nine non-zero transform coefficients include instructions that cause the processor to perform the following operations: Decode at most ten non-zero transform coefficients of the transform block; and Infer that the remaining transform coefficients of the transform block are zero-valued without decoding the values for the remaining transform coefficients.

20. The computer-readable storage medium according to claim 18, wherein The transform block includes sixty-four total transform coefficients, several of the total transform coefficients are zero-valued, the number of the zero-valued total transform coefficients is equal to sixty-four minus the number of non-zero transform coefficients, and the number of the zero-valued total transform coefficients is at most fifty-five.

21. The computer-readable storage medium according to claim 18, wherein, The instructions that cause the processor to decode the at least nine non-zero transform coefficients include instructions that cause the processor to perform the following operations: decode at most a predetermined maximum number of non-zero transform coefficients for the transform block.

22. The computer-readable storage medium according to claim 21, further comprising: Set the values of the remaining transform coefficients of the transform block to be equal to zero.

23. The computer-readable storage medium according to claim 21 further comprises instructions that cause the processor to perform the following operations: decoding a value representing the predetermined quantity from at least one of a video parameter set (VPS), a sequence parameter set (SPS), a picture parameter set (PPS), a slice header, a coding tree unit (CTU) header, or a block header.

24. The computer-readable storage medium according to claim 18 further comprises instructions that cause the processor to perform the following operations: encoding the transform block before decoding the transform block; when encoding the transform block, determining a number of non-zero transform coefficients to be encoded for the transform block, the number being nine or ten; and encoding the number of non-zero transform coefficients of the transform block and not encoding other transform coefficients of the transform block.

25. The computer-readable storage medium according to claim 24, wherein Determining the number of non-zero transform coefficients comprises: determining that the number of non-zero transform coefficients is nine or ten according to a profile, layer, or level of a video coding standard that the video data conforms to.

26. An apparatus for decoding video data, the apparatus comprising: a unit configured to determine that a transform block of video data has a size of 8x8 coefficients and that the transform block is transformed using a low-frequency non-separable transform (LFNST); a unit configured to decode at least nine non-zero transform coefficients of the transform block; a unit configured to perform an inverse transform on the transform block using an inverse LFNST to generate at least nine inverse-transformed LFNST coefficients to reconstruct a residual block corresponding to the transform block; and and a unit configured to reconstruct a block of the video data using the residual block.

27. The apparatus according to claim 26, wherein, The unit configured to decode the at least nine non-zero transform coefficients comprises: a unit configured to decode at most ten non-zero transform coefficients of the transform block; and a unit configured to infer that remaining transform coefficients of the transform block are zero values and not decode values for the remaining transform coefficients.

28. The apparatus according to claim 26, wherein, The transform block comprises sixty-four total transform coefficients, a number of the total transform coefficients are zero values, a number of the total transform coefficients that are zero values is equal to sixty-four minus the number of non-zero transform coefficients, and a number of the total transform coefficients that are zero values is at most fifty-five.

29. The apparatus according to claim 26, wherein The unit configured to decode the at least nine non-zero transform coefficients comprises: a unit configured to decode at most a predetermined maximum number of non-zero transform coefficients for the transform block.

30. The apparatus according to claim 29, further comprising: A unit configured to set values of remaining transform coefficients of the transform block to be equal to zero.

31. The device according to claim 29, further comprising: A unit configured to decode a value representing the predetermined quantity from at least one of a video parameter set (VPS), a sequence parameter set (SPS), a picture parameter set (PPS), a slice header, a coding tree unit (CTU) header, or a block header.

32. The apparatus according to claim 26 further comprises: a unit configured to encode the transform block before decoding the transform block; A unit for determining the number of non-zero transform coefficients to be encoded for the transform block when encoding the transform block, the number being nine or ten; and A unit for encoding the number of non-zero transform coefficients of the transform block without encoding other transform coefficients of the transform block.

33. The device according to claim 32, wherein, The unit for determining the number of non-zero transform coefficients includes: a unit for determining that the number of non-zero transform coefficients is nine or ten according to the profile, layer or level of the video coding standard that the video data conforms to.

Images