Extended multi-transform selection for video coding

Abstract

An example apparatus for coding video data includes a memory configured to store video data; and one or more processors implemented in a circuit and configured to code a first codeword representing a selected transform scheme of a transform candidate set of a multiple transform selection (MTS) scheme for a current block of video data, the selected transformation scheme is a secondary transformation to be applied in a set of available secondary transforms in addition to the primary transformation; coding a second codeword representing the secondary transform from the set of available secondary transforms; and applying the primary transform and the secondary transform during decoding the residual data of the current block. The second codeword may be a value of a low frequency non-separable transform (LFNST) syntax element.

Description

[0001] This application is a divisional application of an application with a filing date of April 3, 2020, an application number of 202080023522.7, and an invention name of “Extended multiple transform selection for video decoding”.

[0002] This application claims the benefit of U.S. Application No. 16 / 838,553, filed on April 2, 2020, U.S. Provisional Application No. 62 / 830,125, filed on April 5, 2019, and U.S. Provisional Application No. 62 / 855,398, filed on May 31, 2019, the entire contents of each of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to video coding, including video encoding and video decoding. Background Art

[0004] Digital video functionality 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 radio telephones, so-called "smart phones", video teleconferencing devices, video streaming devices, etc. Digital video devices implement video decoding techniques, such as those described in standards defined by MPEG-2, MPEG-4, ITU-T H.263, ITU-T H.264 / MPEG-4 Part 10, Advanced Video Coding (AVC), High Efficiency Video Coding (HEVC) standards, ITU-T H.265 / High Efficiency Video Coding (HEVC), and extensions of such standards. By implementing such video decoding techniques, video devices can more efficiently send, receive, encode, decode and / or store digital video information.

[0005] Video coding techniques include spatial (intra-frame) prediction and / or temporal (inter-frame) prediction to reduce or eliminate redundancy inherent in video sequences. For block-based video coding, a video segment (e.g., a video picture or a portion of a video picture) may 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) segment of a picture are encoded using spatial prediction relative to reference samples in neighboring blocks in the same picture. Video blocks in an inter-coded (P or B) segment of a picture may use spatial prediction relative to reference samples in neighboring blocks in the same picture, or temporal prediction relative to reference samples in other reference pictures. Pictures may be referred to as frames, and reference pictures may be referred to as reference frames. Summary of the invention

[0006] In general, the present disclosure describes techniques related to transform coding in video coding. Transform coding is an important element of modern video compression standards. The present disclosure describes a multiple transform selection (MTS) design that extends other MTS tools, such as the MTS tool of Versatile Video Coding (VVC) / ITU-T H.266. Since the design described in the present disclosure allows the encoder to select a transform from more transform candidates, these techniques can improve coding efficiency. The present disclosure also describes various simplified versions of the low-frequency non-separable transform (LFNST), which can reduce the complexity of the encoder and decoder without significantly reducing coding efficiency. Therefore, these techniques can be used in advanced video codecs and next-generation video coding standards, such as VVC.

[0007] In one example, a method of coding (encoding or decoding) video data includes: decoding a first codeword representing a selected transform scheme in a transform candidate set of a multiple transform selection (MTS) scheme for a current block of video data, the selected transform scheme being: a secondary transform in an available secondary transform set to be applied in addition to a primary transform; decoding a second codeword representing a secondary transform from the available secondary transform set; and applying the primary transform and the secondary transform during coding of residual data of the current block.

[0008] In another example, an apparatus for decoding video data includes: a memory configured to store the video data; and one or more processors implemented in circuits and configured to: decode a first codeword representing a selected transform scheme in a transform candidate set of a multiple transform selection (MTS) scheme for a current block of the video data, the selected transform scheme being: a secondary transform in an available secondary transform set to be applied in addition to a primary transform; decode a second codeword representing a secondary transform from the available secondary transform set; and apply the primary transform and the secondary transform during decoding of residual data of the current block.

[0009] In another example, an apparatus for decoding video data includes: a unit for decoding a first codeword, the first codeword representing a selected transform scheme in a transform candidate set of a multiple transform selection (MTS) scheme for a current block of the video data, the selected transform scheme being: a secondary transform in an available secondary transform set to be applied in addition to a primary transform; a unit for decoding a second codeword representing a secondary transform from the available secondary transform set; and a unit for applying the primary transform and the secondary transform during decoding of residual data of the current block.

[0010] In another example, a computer-readable storage medium has instructions stored thereon that, when executed, cause a processor to perform the following operations: decode a first codeword representing a selected transform scheme from a transform candidate set of a multiple transform selection (MTS) scheme for a current block of video data, the selected transform scheme being a secondary transform from a set of available secondary transforms to be applied in addition to a primary transform; decode a second codeword representing a secondary transform from the set of available secondary transforms; and apply the primary transform and the secondary transform during decoding of residual data for the current block.

[0011] The details of one or more examples are set forth in the accompanying drawings and the description that follows. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a block diagram illustrating an example video encoding and decoding system that may perform the techniques of this disclosure.

[0013] Figure 2A and Figure 2B is a conceptual diagram illustrating an example quadtree binary tree (QTBT) structure and a corresponding coding tree unit (CTU).

[0014] Figure 3A and 3B is a conceptual diagram illustrating an example transform scheme based on a residual quadtree for High Efficiency Video Coding (HEVC).

[0015] Figure 4 is a block diagram illustrating an example system for hybrid video coding with adaptive transform selection.

[0016] Figure 5A and 5B This is a conceptual diagram illustrating horizontal transform and vertical transform as separate transform implementations.

[0017] Figure 6 is a conceptual diagram representing an example of Multiple Transform Selection (MTS) signaling for identifying two transforms.

[0018] Figure 7 is a conceptual diagram showing example transform assignments and corresponding unary codewords.

[0019] Figure 8 is a conceptual diagram illustrating an example MTS design supporting secondary transforms.

[0020] Fig. 9 is a conceptual diagram illustrating an example of a low frequency non-separable transform (LFNST) that a video coder (video encoder or video decoder) can apply.

[0021] Fig.10 is a conceptual diagram showing an example of LFNST applied to a subset of coefficients of an HxW block (at the upper left).

[0022] Fig.11A and 11B is a conceptual diagram illustrating an example two-step LFNST process implementation.

[0023] Fig.12 is a block diagram illustrating an example video encoder that may perform the techniques of this disclosure.

[0024] Fig.13 is a block diagram illustrating an example video decoder that may perform the techniques of this disclosure.

[0025] Fig.14 is a flowchart illustrating an example method for encoding a current block according to techniques of this disclosure.

[0026] Fig.15 is a flow chart illustrating an example method for decoding a current block of video data, in accordance with techniques of this disclosure.

[0027] Fig.16 is a flow chart illustrating an example video encoding method according to techniques of this disclosure.

[0028] Fig.17 is a flow chart illustrating an example video decoding method according to techniques of this disclosure. DETAILED DESCRIPTION

[0029] This disclosure describes techniques related to transform coding, which is an important element of modern video compression standards, for example, as discussed in M. Wien, “High Effciency Video Coding: Coding Tools and Specification”, Springer-Verlag, Berlin, 2015. This disclosure describes an extended Multiple Transform Selection (MTS) technique.

[0030] Typically, video data is represented as a series of consecutive pictures. A video decoder divides a picture into blocks and decodes each block. Coding typically includes prediction and residual coding. During prediction, a video decoder may use intra-frame prediction (where a prediction block is formed by adjacent previously coded blocks in the same picture) or inter-frame prediction (where a prediction block is formed by previously coded blocks of previously coded pictures) to form a prediction block. The residual block represents the pixel-by-pixel difference between the prediction block and the original uncoded block. A video encoder may apply a transform to a residual block to generate a transform block including transform coefficients, and a video decoder may apply an inverse transform to a transform block to reproduce a version of the residual block.

[0031] Assume that the input N-point vector is represented as x=[x0,x1,…,x N-1 ] T , and it is transformed into y=[y0,y1,…,y N-1 ] T Another N-point transformation coefficient vector, the process can be further described according to one of the following transformation formulas, where k ranges from 0 to N-1, and these transformation formulas include:

[0032] DCT Type-I (DCT-1):

[0033]

[0034] in

[0035] DCT Type-II (DCT-2):

[0036]

[0037] in

[0038] DCT Type-III (DCT-3):

[0039]

[0040] in

[0041] DCT Type-IV (DCT-4):

[0042]

[0043] DCT Type-V (DCT-5):

[0044]

[0045] in

[0046] DCT Type-VI (DCT-6):

[0047]

[0048] in

[0049] DCT Type-VII (DCT-7):

[0050]

[0051] in

[0052] DCT Type-VIII (DCT-8):

[0053]

[0054] DST Type-I (DST-1):

[0055]

[0056] DST Type-II (DST-2):

[0057]

[0058] in

[0059] DST Type-III (DST-3):

[0060]

[0061] in

[0062] DST Type-IV (DST-4):

[0063]

[0064] DST Type-V (DST-5):

[0065]

[0066] DST Type-VI (DST-6):

[0067]

[0068] DST Type-VII (DST-7):

[0069]

[0070] DST Type-VIII (DST-8):

[0071]

[0072] in

[0073] The transform type is specified by the mathematical formula of the transform basis function. For example, 4-point DST-VII and 8-point DST-VII have the same transform type regardless of the value of N.

[0074] Without loss of generality, all the above transformation types can be expressed using the following generalized formula:

[0075]

[0076] where T is the transform matrix specified by the definition of a particular transform (e.g., DCT Type-I to DCT Type-VIII or DST Type-I to DST Type-VIII), and the row vectors of T, such as [T i,0 ,T i,1 ,T i,2 ,…,T i,N-1 ] is the i-th transformation basis vector. A transformation applied to an N-point input vector is called an N-point transformation.

[0077] It should also be noted that the above transformation formula applied to 1-D input data x can be expressed in matrix multiplication form as shown below

[0078] y=T·x

[0079] Where T represents the transformation matrix, x represents the input data vector, and y represents the output transformation coefficient vector.

[0080] The transformations described above are applied to 1-D input data, and the transformations can also be extended for 2-D input data sources. Assume X is the input MxN data array. Typical methods of applying transformations to 2-D input data include separable 2-D transformations and non-separable 2-D transformations.

[0081] The separable 2-D transform applies a 1-D transform to the horizontal and vertical vectors of X in sequence, as follows:

[0082] Y=C·X·R T

[0083] Where C and R represent the given MxM and NxN transformation matrices, respectively. As can be seen from the formula, C applies a 1-D transformation to the column vectors of X, while R applies a 1-D transformation to the row vectors of X. In the latter part of the present disclosure, for simplicity, C and R may represent the left (vertical) transformation and the right (horizontal) transformation and may be considered to form a transformation pair. There is a case where C is equal to R and is an orthogonal matrix. In this case, a separable 2-D transformation is determined by only one transformation matrix.

[0084] The non-separable 2-D transformation first reorganizes all elements of X into a single vector, X', by performing the following mathematical mapping as an example:

[0085] X ( ' i·N+j) =X i,j

[0086] Then, a 1-D transformation T' is applied to X' as follows:

[0087] Y=T′·X

[0088] Where T' is a (M*N)x(M*N) transformation matrix.

[0089] In video coding, separable 2-D transforms are often applied because they generally require less amount of operations (additions and multiplications) compared to 1-D transforms.

[0090] In traditional video codecs (such as H.264 / AVC), integer approximations of 4-point and 8-point discrete cosine transform (DCT) type II are always applied to intra-frame prediction residuals and inter-frame prediction residuals. In order to better adapt to various statistics of residual samples, more flexible transform types other than DCT type II are utilized in new generation video codecs. For example, in HEVC, integer approximations of 4-point type-VII discrete sine transform (DST) are used for intra-frame prediction residuals, which are theoretically proved and experimentally verified (in J. Han, A. Saxena and K. Rose, "Towards jointly optimal spatial prediction and adaptive transform in video / image coding", IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), March 2010, pages 726-729), and DST type-VII is more effective than DCT type-II for residual vectors generated along the intra-frame prediction direction. For example, for row residual vectors generated by the horizontal intra prediction direction, DST type-VII is more efficient than DCT type-II. In HEVC, the integer approximation of 4-point DST type-VII is only applied to 4×4 luma intra prediction residual blocks. The 4-point DST-VII for HEVC is shown below,

[0091] 4x4 DST-VII:

[0092] {29,55,74,84}

[0093] {74,74,0,-74}

[0094] {84,-29,-74,55}

[0095] {55,-84,74,-29}

[0096] In HEVC, for residual blocks that are not 4×4 luma intra prediction residual blocks, integer approximations of 4-point, 8-point, 16-point, and 32-point DCT type-II can also be applied, as shown below:

[0097] 4-point DCT-II:

[0098] {64,64,64,64}

[0099] {83,36,-36,-83}

[0100] {64,-64,-64,64}

[0101] {36,-83,83,-36}

[0102] 8-point DCT-II:

[0103] {64,64,64,64,64,64,64,64}

[0104] {89,75,50,18,-18,-50,-75,-89}

[0105] {83,36,-36,-83,-83,-36,36,83}

[0106] {75,-18,-89,-50,50,89,18,-75}

[0107] {64,-64,-64,64,64,-64,-64,64}

[0108] {50,-89,18,75,-75,-18,89,-50}

[0109] {36,-83,83,-36,-36,83,-83,36}

[0110] {18,-50,75,-89,89,-75,50,-18}

[0111] 16-point DCT-II:

[0112] {64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64}

[0113] {90,87,80,70,57,43,25,9,-9,-25,-43,-57,-70,-80,-87,-90}

[0114] {89,75,50,18,-18,-50,-75,-89,-89,-75,-50,-18,18,50,75,89}

[0115] {87,57,9,-43,-80,-90,-70,-25,25,70,90,80,43,-9,-57,-87}

[0116] {83,36,-36,-83,-83,-36,36,83,83,36,-36,-83,-83,-36,36,83}

[0117] {80,9,-70,-87,-25,57,90,43,-43,-90,-57,25,87,70,-9,-80}

[0118] {75,-18,-89,-50,50,89,18,-75,-75,18,89,50,-50,-89,-18,75}

[0119] {70,-43,-87,9,90,25,-80,-57,57,80,-25,-90,-9,87,43,-70}

[0120] {64,-64,-64,64,64,-64,-64,64,64,-64,-64,-64,64,-64,64,-64,64}

[0121] {57,-80,-25,90,-9,-87,43,70,-70,-43,87,9,-90,25,80,-57}

[0122] {50,-89,18,75,-75,-18,89,-50,-50,89,-18,-75,75,18,-89,50}

[0123] {43,-90,57,25,-87,70,9,-80,80,-9,-70,87,-25,-57,90,-43}

[0124] {36,-83,83,-36,-36,83,-83,36,36,-83,83,-36,-36,83,-83,36}

[0125] {25,-70,90,-80,43,9,-57,87,-87,57,-9,-43,80,-90,70,-25}

[0126] {18,-50,75,-89,89,-75,50,-18,-18,50,-75,89,-89,75,-50,18}

[0127] {9,-25,43,-57,70,-80,87,-90,90,-87,80,-70,57,-43,25,-9}

[0128] 32-point DCT-II:

[0129] {64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64,64}

[0130] {90,90,88,85,82,78,73,67,61,54,46,38,31,22,13,4,-4,-13,-22,-31,-38,-46,-54,-61,-67,-73,-78,-82,-85,-88,-90,-90}

[0132] {90,87,80,70,57,43,25,9,-9,-25,-43,-57,-70,-80,-87,-90,-90,-87,-80,-70,-57,-43,-25,-9,9,25,43,57,70,80,87,90}

[0134] {90,82,67,46,22,-4,-31,-54,-73,-85,-90,-88,-78,-61,-38,-13,13,38,61,78,88,90,85,73,54,31,4,-22,-46,-67,-82,-90}

[0136] {89,75,50,18,-18,-50,-75,-89,-89,-75,-50,-18,18,50,75,89,89,75,50,18,-18,-50,-75,-89,-89,-75,-50,-18,18,50,75,89}

[0138] {88,67,31,-13,-54,-82,-90,-78,-46,-4,38,73,90,85,61,22,-22,-61,-85,-90,-73,-38,4,46,78,90,82,54,13,-31,-67,-88}

[0140] {87,57,9,-43,-80,-90,-70,-25,25,70,90,80,43,-9,-57,-87,-87,-57,-9,43,80,90,70,25,-25,-70,-90,-80,-43,9,57,87}

[0141] {85,46,-13,-67,-90,-73,-22,38,82,88,54,-4,-61,-90,-78,-31,31,78,90,61,4,-54,-88,-82,-38,22,73,90,67,13,-46,-85}

[0144] {83,36,-36,-83,-83,-36,36,83,83,36,-36,-83,-83,-36,36,83,83,36,-36,-83,-83,-36,36,83,83,36,-36,-83,-83,-36,36,83}

[0146] {82,22,-54,-90,-61,13,78,85,31,-46,-90,-67,4,73,88,38,-38,-88,-73,-4,67,90,46,-31,-85,-78,-13,61,90,54,-22,-82}

[0148] {80,9,-70,-87,-25,57,90,43,-43,-90,-57,25,87,70,-9,-80,-80,-9,70,87,25,-57,-90,-43,43,90,57,-25,-87,-70,9,80}

[0150] {78,-4,-82,-73,13,85,67,-22,-88,-61,31,90,54,-38,-90,-46,46,90,38,-54,-90,-31,61,88,22,-67,-85,-13,73,82,4,-78}

[0152] {75,-18,-89,-50,50,89,18,-75,-75,18,89,50,-50,-89,-18,75,75,-18,-89,-50,50,89,18,-75,-75,18,89,50,-50,-89,-18,75}

[0154] {73,-31,-90,-22,78,67,-38,-90,-13,82,61,-46,-88,-4,85,54,-54,-85,4,88,46,-61,-82,13,90,38,-67,-78,22,90,31,-73}

[0156] {70,-43,-87,9,90,25,-80,-57,57,80,-25,-90,-9,87,43,-70,-70,43,87,-9,-90,-25,80,57,-57,-80,25,90,9,-87,-43,70}

[0158] {67,-54,-78,38,85,-22,-90,4,90,13,-88,-31,82,46,-73,-61,61,73,-46,-82,31,88,-13,-90,-4,90,22,-85,-38,78,54,-67}

[0160] {64,-64,-64,64,64,-64,-64,64,64,-64,-64,64,64,-64,-64,64,64,-64,-64,64,64,-64,-64,64,64,-64,-64,64,64,-64,-64,64}

[0162] {61,-73,-46,82,31,-88,-13,90,-4,-90,22,85,-38,-78,54,67,-67,-54,78,38,-85,-22,90,4,-90,13,88,-31,-82,46,73,-61}

[0164] {57,-80,-25,90,-9,-87,43,70,-70,-43,87,9,-90,25,80,-57,-57,80,25,-90,9,87,-43,-70,70,43,-87,-9,90,-25,-80,57}

[0166] {54,-85,-4,88,-46,-61,82,13,-90,38,67,-78,-22,90,-31,-73,73,31,-90,22,78,-67,-38,90,-13,-82,61,46,-88,4,85,-54}

[0168] {50,-89,18,75,-75,-18,89,-50,-50,89,-18,-75,75,18,-89,50,50,-89,18,75,-75,-18,89,-50,-50,89,-18,-75,75,18,-89,50}

[0170] {46,-90,38,54,-90,31,61,-88,22,67,-85,13,73,-82,4,78,-78,-4,82,-73,-13,85,-67,-22,88,-61,-31,90,-54,-38,90,-46}

[0172] {43,-90,57,25,-87,70,9,-80,80,-9,-70,87,-25,-57,90,-43,-43,90,-57,-25,87,-70,-9,80,-80,9,70,-87,25,57,-90,43}

[0174] {38,-88,73,-4,-67,90,-46,-31,85,-78,13,61,-90,54,22,-82,82,-22,-54,90,-61,-13,78,-85,31,46,-90,67,4,-73,88,-38}

[0176] {36,-83,83,-36,-36,83,-83,36,36,-83,83,-36,-36,83,-83,36,36,-83,83,-36,-36,83,-83,36,36,-83,83,-36,-36,83,-83,36}

[0178] {31,-78,90,-61,4,54,-88,82,-38,-22,73,-90,67,-13,-46,85,-85,46,13,-67,90,-73,22,38,-82,88,-54,-4,61,-90,78,-31}

[0180] {25,-70,90,-80,43,9,-57,87,-87,57,-9,-43,80,-90,70,-25,-25,70,-90,80,-43,-9,57,-87,87,-57,9,43,-80,90,-70,25}

[0182] {22,-61,85,-90,73,-38,-4,46,-78,90,-82,54,-13,-31,67,-88,88,-67,31,13,-54,82,-90,78,-46,4,38,-73,90,-85,61,-22}

[0184] {18,-50,75,-89,89,-75,50,-18,-18,50,-75,89,-89,75,-50,18,18,-50,75,-89,89,-75,50,-18,-18,50,-75,89,-89,75,-50,18}

[0186] {13,-38,61,-78,88,-90,85,-73,54,-31,4,22,-46,67,-82,90,-90,82,-67,46,-22,-4,31,-54,73,-85,90,-88,78,-61,38,-13}

[0188] {9,-25,43,-57,70,-80,87,-90,90,-87,80,-70,57,-43,25,-9,-9,25,-43,57,-70,80,-87,90,-90,87,-80,70,-57,43,-25,9}

[0190] {4,-13,22,-31,38,-46,54,-61,67,-73,78,-82,85,-88,90,-90,90,-90,88,-85,82,-78,73,-67,61,-54,46,-38,31,-22,13,-4}

[0192] Figure 1 1 is a block diagram illustrating an exemplary video encoding and decoding system 100 that can perform the techniques of the present disclosure. The techniques of the present disclosure are generally directed to decoding (encoding and / or decoding) video data. Generally, video data includes any data used to process video. Thus, video data may include original unencoded video, encoded video, decoded (e.g., reconstructed) video, and video metadata (such as signaling data).

[0193] like Figure 1As shown, in this example, system 100 includes a source device 102 that provides encoded video data to be decoded and displayed by a destination device 116. Specifically, source device 102 provides the video data to destination device 116 via computer-readable medium 110. Source device 102 and destination device 116 may include any of a variety of devices, including desktop computers, notebook (i.e., laptop) computers, tablet computers, set-top boxes, telephone handsets (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 may be equipped for wireless communication and, therefore, may be referred to as wireless communication devices.

[0194] exist Figure 1 In the example of , source device 102 includes video source 104, memory 106, video encoder 200 and output interface 108. Destination device 116 includes input interface 122, video decoder 300, memory 120 and display device 118. According to the present disclosure, the video encoder 200 of source device 102 and the video decoder 300 of destination device 116 can be configured to apply the technology to decode MTS data. Therefore, source device 102 represents an example of a video encoding device, and destination device 116 represents an example of a video decoding device. In other examples, source device and destination device may include other components or arrangements. For example, source device 102 may receive video data from an external video source such as an external camera. Similarly, destination device 116 may be connected to an external display device interface instead of including an integrated display device.

[0195] like Figure 1 The system 100 shown is only an example. In general, any digital video encoding and / or decoding device can perform the technology for decoding MTS data. The source device 102 and the destination device 116 are only examples of such decoding devices, wherein the source device 102 generates encoded video data for transmission to the destination device 116. The present disclosure refers to a "decoding" device as a device that performs the decoding (encoding and / or decoding) of data. Therefore, the video encoder 200 and the video decoder 300 represent examples of decoding devices, specifically video encoders and video decoders, respectively. In some examples, the devices 102, 116 can operate in a substantially symmetrical manner so that each of the devices 102, 116 includes a video encoding and decoding component. Therefore, the system 100 can support one-way or two-way video transmission between video devices 102, 116, for example, for video streaming, video playback, video broadcasting or video telephony.

[0196] Typically, the video source 104 represents a source of video data (i.e., raw, unencoded video data) and provides a sequence of pictures (also referred to as "frames") of the video data to the video encoder 200, which encodes the data of 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 another alternative, the video source 104 may generate computer graphics-based data as the source video, or a combination of real-time video, archived video, and computer-generated video. In each case, the video encoder 200 encodes captured, pre-captured, or computer-generated video data. The video encoder 200 may rearrange the pictures from the order in which they are received (sometimes referred to as the "display order") to the decoding order for decoding. The video encoder 200 may generate a bitstream comprising encoded video data. Source device 102 may then output the encoded video data onto computer-readable medium 110 via output interface 108 for receipt and / or retrieval by, for example, input interface 122 of destination device 116 .

[0197] The memory 106 of the source device 102 and the memory 120 of the destination device 116 represent general purpose memory. In some examples, the memory 106, 120 can store original video data, for example, the original video from the video source 104 and the original decoded video data from the video decoder 300. In addition or alternatively, the memory 106, 120 can store software instructions that can be executed by, for example, the video encoder 200 and the video decoder 300, respectively. Although shown separately from the video encoder 200 and the video decoder 300 in this example, it should be understood that the video encoder 200 and the video decoder 300 can also include internal memory to achieve functionally similar or equivalent purposes. In addition, the memory 106, 120 can store, for example, encoded video data output from the video encoder 200 and input to the video decoder 300. In some examples, portions of the memory 106, 120 can be allocated as one or more video buffers, for example, to store original, decoded and / or encoded video data.

[0198] The computer-readable medium 110 may represent any type of medium or device capable of transmitting 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 send the encoded video data directly to the destination device 116 in real time, for example, via a radio frequency network or a computer-based network. According to 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 a radio frequency (RF) spectrum or one or more physical transmission lines. The communication medium may form part of a packet-based network such as a local area network, a wide area network, or a global network such as the Internet. The communication medium may include a router, a switch, a base station, or any other device that can be used to facilitate communication from the source device 102 to the destination device 116.

[0199] In some examples, source device 102 may output the encoded data from output interface 108 to storage device 112. Similarly, destination device 116 may access the encoded data from storage device 112 via input interface 122. Storage device 116 may include any of a variety of distributed or locally accessed data storage media, such as a hard drive, Blu-ray disc, DVD, CD-ROM, flash memory, volatile or non-volatile memory, or any other suitable digital storage medium for storing encoded video data.

[0200] In some examples, source device 102 may output the encoded video data to file server 114 or another intermediate storage device that may store the encoded video generated by source device 102. Destination device 116 may access the stored video data from file server 114 by means of streaming or downloading. File server 114 may be any type of server device capable of storing the encoded video data and sending the encoded video data to destination device 116. File server 114 may represent a network server (e.g., for a website), a file transfer protocol (FTP) server, a content delivery network device, or a network attached storage (NAS) device. Destination device 116 may access the encoded video data from file server 114 through any standard data connection including an Internet connection. This may include a wireless channel (e.g., a Wi-Fi connection), a wired connection (e.g., a DSL, a cable modem, etc.), or a combination of both that is suitable for accessing the encoded video data stored on file server 114. File server 114 and input interface 122 may be configured to operate according to a streaming protocol, a downloading transmission protocol, or a combination thereof.

[0201] Output interface 108 and input interface 122 may represent wireless transmitters / receivers, modems, wired networking components (e.g., Ethernet cards), wireless communication components that operate according to any of the various IEEE 802.11 standards, or other physical components. In examples where output interface 108 and input interface 122 include wireless components, output interface 108 and input interface 122 may be configured to transmit data, such as encoded video data, according to a cellular communication standard such as 4G, 4G-LTE (Long Term Evolution), LTE Advanced, 5G, etc. In some examples where output interface 108 includes a wireless transmitter, output interface 108 and input interface 122 may be configured to transmit data, such as encoded video data, according to other wireless standards such as IEEE 802.11 specifications, IEEE 802.15 specifications (e.g., ZigBee 5G, etc.). TM ),Bluetooth TM Standards, etc.) to transmit data such as encoded video data. In some examples, source device 102 and / or destination device 116 may include respective system-on-chip (SoC) devices. For example, source device 102 may include an SoC device to perform functions attributed to video encoder 200 and / or output interface 108, and destination device 116 may include an SoC device to perform functions attributed to video decoder 300 and / or input interface 122.

[0202] The techniques of the present disclosure may be applied to video decoding to support any of a variety of multimedia applications, such as over-the-air television broadcasts, cable television transmissions, satellite television transmissions, Internet streaming video transmissions (such as Dynamic Adaptive Streaming over HTTP (DASH)), digital video encoded onto a data storage medium, decoding of digital video stored on a data storage medium, or other applications.

[0203] The input interface 122 of the destination device 116 receives an encoded video bitstream from the computer-readable medium 110 (e.g., the storage device 112, the file server 114, etc.). The encoded video bitstream of the computer-readable medium 110 may include signaling information defined by the video encoder 200, which is also used by the video decoder 300, such as syntax elements with values ​​describing characteristics and / or processing of video blocks or other decoding units (e.g., slices, pictures, groups of pictures, sequences, etc.). The display device 118 displays decoded pictures that decode the video data to a 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.

[0204] Although not in Figure 1, but in some examples, the video encoder 200 and the video decoder 300 may each be integrated with an audio encoder and / or an audio decoder and may include appropriate MUX-DEMUX units or other hardware and / or software to process a multiplexed stream including both audio and video in a common data stream. If applicable, the MUX-DEMUX unit may conform to the ITU H.223 multiplexer protocol or other protocols, such as the User Datagram Protocol (UDP).

[0205] The video encoder 200 and the video decoder 300 can each be implemented as any of a variety of suitable encoder and / or decoder circuits, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), discrete logic, software, hardware, firmware, or any combination thereof. When the technology is partially implemented in software, the device may store instructions for the software in a suitable non-transitory computer-readable medium and use one or more processors to execute the instructions in hardware to perform the technology of the present disclosure. Each of the video encoder 200 and the video decoder 300 can be included in one or more encoders or decoders, any of which can be integrated as part of a combined encoder / decoder (CODEC) in a corresponding device. The device including the video encoder 200 and / or the video decoder 300 may include an integrated circuit, a microprocessor, and / or a wireless communication device, such as a cellular phone.

[0206] The video encoder 200 and the video decoder 300 may operate in accordance with a video coding standard such as ITU-T H.265, also known as High Efficiency Video Coding (HEVC), or an extension thereof such as multi-view and / or scalable video coding extensions. Alternatively, the video encoder 200 and the video decoder 300 may operate in accordance with other proprietary or industry standards, such as the upcoming Versatile Video Coding (VVC) standard (which is planned to become ITU-T H.266). A working draft for VVC is Bross et al., “Versatile Video Coding (Draft 5)” Joint Video Experts Group (JVET), ITU-T SG16 WP3 and ISO / IEC JTC1 / SC29 / WG11, 14th Meeting, Geneva, Switzerland, March 19-27, 2019, document JVET-N1001-v5. However, the techniques of the present disclosure are not limited to any particular coding standard.

[0207] Typically, the video encoder 200 and the video decoder 300 can perform block-based decoding of a picture. The term "block" generally refers to a structure that includes data to be processed (e.g., encoded, decoded, or otherwise used in an encoding and / or decoding process). For example, a block may include a two-dimensional matrix of samples of luminance and / or chrominance data. Typically, the video encoder 200 and the video decoder 300 can decode video data represented in a YUV (e.g., Y, Cb, Cr) format. That is, instead of decoding the red, green, and blue (RGB) data of the samples of the picture, the video encoder 200 and the video decoder 300 can decode the luminance component and the chrominance component, wherein the chrominance component may include a red-tone chrominance component and a blue-tone chrominance component. In some examples, the video encoder 200 converts the received RGB format data into a YUV representation before encoding, and the video decoder 300 converts the YUV representation into an RGB format. Alternatively, a pre-processing and post-processing unit (not shown) can perform these conversions.

[0208] The present disclosure may generally relate to the decoding (e.g., encoding and decoding) of a picture to include the process of encoding or decoding the data of the picture. Similarly, the present disclosure may relate to the decoding of a block of a picture to include the process of encoding or decoding the data of the block, for example, prediction and / or residual decoding. The encoded video bitstream typically includes a series of values ​​for syntax elements that represent decoding decisions (e.g., decoding modes) and the partitioning of the picture into blocks. Therefore, references to decoding a picture or a block should generally be understood as decoding the values ​​of the syntax elements that form the picture or block.

[0209] HEVC defines various blocks, including coding units (CUs), prediction units (PUs), and transform units (TUs). According to HEVC, a video decoder (such as the video encoder 200) divides a coding tree unit (CTU) into CUs according to a quadtree structure. That is, the video decoder divides the CTU and the CU into four equal non-overlapping squares, and each node of the quadtree has zero or four child nodes. A node without child nodes may be referred to as a "leaf node", and the CU of such a leaf node may include one or more PUs and / or one or more TUs. The video decoder may further divide the PU and TU. For example, in HEVC, the residual quadtree (RQT) represents the division of TUs. In HEVC, PU represents inter-frame prediction data, and TU represents residual data. The intra-predicted CU includes intra-frame prediction information, such as an intra-frame mode indication.

[0210] As another example, the video encoder 200 and the video decoder 300 may be configured to operate according to VVC. According to VVC, a video decoder (such as the video encoder 200) divides a picture into a plurality of coding tree units (CTUs). The video encoder 200 may divide the CTU according to a tree structure such as a quadtree-binary tree (QTBT) structure. The QTBT structure of VVC eliminates the concept of multiple partition types, such as the separation between CU, PU, ​​and TU of HEVC. The QTBT structure of VVC 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).

[0211] In some examples, the video encoder 200 and the video decoder 300 may use a single QTBT structure to represent each of the luma component and the chroma components, while in other examples, the video encoder 200 and the video decoder 300 may use two or more QTBT structures, such as one QTBT structure for the luma component and another QTBT structure for the two chroma components (or two QTBT structures for each chroma component).

[0212] The video encoder 200 and the video decoder 300 may be configured to use quadtree partitioning according to HEVC, QTBT partitioning according to VVC, or other partitioning structures. For illustrative purposes, a 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 decoders configured to use quadtree partitioning or other types of partitioning.

[0213] This disclosure may use "N×N" and "N by N" interchangeably to refer to the sample size of a block (such as a CU or other video block) in terms of vertical and horizontal dimensions, for example, 16×16 samples or 16 by 16 samples. Typically, a 16×16 CU has 16 samples in the vertical direction (y=16) and 16 samples in the horizontal direction (x=16). Similarly, an N×N CU typically has N samples in the vertical direction and N samples in the horizontal direction, where N represents a non-negative integer value. The samples in a CU may be arranged in rows and columns. In addition, a CU does not necessarily have the same number of samples in the horizontal direction as in the vertical direction. For example, a CU may include N×M samples, where M is not necessarily equal to N.

[0214] The video encoder 200 encodes the video data of the CU representing the prediction and / or residual information and other information. The prediction information indicates how the CU will be predicted in order to form a prediction block for the CU. The residual information generally represents the sample-by-sample difference between the samples of the CU before encoding and the prediction block.

[0215] In order to predict a CU, the video encoder 200 may generally form a prediction block for the CU by inter-frame prediction or intra-frame prediction. Inter-frame prediction generally refers to predicting a CU based on data of a previously decoded picture, while intra-frame prediction generally refers to predicting a CU based on previously decoded data of the same picture. In order to perform inter-frame prediction, the video encoder 200 may use one or more motion vectors to generate a prediction block. The video encoder 200 may generally perform a motion search to identify a reference block that closely matches the CU, for example, according to the difference between the CU and the reference block. The video encoder 200 may calculate a difference metric using the sum of absolute differences (SAD), the sum of squared differences (SSD), the mean absolute difference (MAD), the 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 may use unidirectional prediction or bidirectional prediction to predict the current CU.

[0216] VVC also provides an affine motion compensation mode, which can be considered an inter-frame prediction mode. In the affine motion compensation mode, the video encoder 200 can determine two or more motion vectors representing non-translational motion (such as zooming in or out, rotation, perspective motion, or other irregular motion types).

[0217] To perform intra prediction, the video encoder 200 may select an intra prediction mode to generate a prediction block. VVC provides 67 intra prediction modes, including various directional modes, as well as planar mode and DC mode. Typically, the video encoder 200 selects an intra prediction mode that describes neighboring samples of a current block (e.g., a block of a CU), and predicts samples of the current block from the neighboring samples. Assuming that the video encoder 200 decodes CTUs and CUs in a raster scan order (from left to right, from top to bottom), such samples may typically be above, above left, or to the left of the current block in the same picture as the current block).

[0218] The video encoder 200 encodes data representing the prediction mode of the current block. For example, for an inter-frame prediction mode, the video encoder 200 may encode data representing which inter-frame prediction mode of various available inter-frame prediction modes is used and motion information for the corresponding mode. For unidirectional or bidirectional inter-frame prediction, for example, the video encoder 200 may encode motion vectors using advanced motion vector prediction (AMVP) or merge mode. The video encoder 200 may use a similar mode to encode motion vectors for affine motion compensation mode.

[0219] After a prediction, such as intra prediction or inter prediction of a block, the video encoder 200 may calculate residual data for the block. The residual data, such as a residual block, represents the sample-by-sample difference between the block and a prediction block for the block formed using a corresponding prediction mode. The video encoder 200 may apply one or more transforms to the residual block to produce transformed data in a transform domain rather than a sample domain. For example, the video encoder 200 may apply a discrete cosine transform (DCT), an integer transform, a wavelet transform, or a conceptually similar transform to the residual video data. In addition, the video encoder 200 may apply a secondary transform after the first transform, such as a mode-dependent non-separable secondary transform (MDNSST), a signal-dependent transform, a Karhunen-Loeve transform (KLT), etc. The video encoder 200 generates transform coefficients after applying one or more transforms.

[0220] According to the techniques of the present disclosure, the video encoder 200 may determine a specific transform type (or multiple transforms) to apply to the residual block of the current block. The determined transform type may include a primary transform, which may be a separable transform including a horizontal transform and a vertical transform. In some examples, the determined transform type may also include a secondary transform (e.g., a non-separable transform). The video encoder 200 may encode a first codeword representing a selected transform type, which indicates whether the primary transform and the selected transform type include a secondary transform. In the event that the first codeword indicates that the selected transform type includes a secondary transform, the video encoder 200 may further encode a second codeword representing a selected secondary transform in a set of available secondary transforms. In addition, the video encoder 200 may apply both the primary transform and the secondary transform. Reference will be made to Tables 1-12 and Figure 6-8 An example of such a combination of codewords is explained in more detail.

[0221] As described above, after any transforms used to produce transform coefficients, the video encoder 200 may perform quantization of the transform coefficients. Quantization generally refers to the process of quantizing transform coefficients to possibly reduce the amount of data used to represent the coefficients, thereby providing further compression. By performing the quantization process, the video encoder 200 may reduce the bit depth associated with some or all coefficients. For example, the video encoder 200 may round down an n-bit value to an m-bit value during quantization, where n is greater than m. In some examples, to perform quantization, the video encoder 200 may perform a bitwise right shift of the value to be quantized.

[0222] After quantization, the video encoder 200 may scan the transform coefficients to generate a one-dimensional vector from a two-dimensional matrix including the quantized transform coefficients. The scan may be designed to place coefficients of higher energy (and therefore lower frequency) at the front of the vector and transform coefficients of lower energy (and therefore higher frequency) at the back of the vector. In some examples, the video encoder 200 may scan the quantized transform coefficients using a predefined scan order to generate a serialized vector, and then entropy encode the quantized transform coefficients of the vector. In other examples, the video encoder 200 may perform adaptive scanning. After scanning the quantized transform coefficients to form a one-dimensional vector, the video encoder 200 may entropy encode the one-dimensional vector, for example, according to context adaptive binary arithmetic coding (CABAC). The video encoder 200 may also entropy encode the values ​​of syntax elements that describe metadata associated with the encoded video data for use by the video decoder 300 when decoding the video data.

[0223] To perform CABAC, the video encoder 200 may assign context within a context model to a symbol to be transmitted. The context may relate to, for example, whether neighboring values ​​of the symbol are zero values. The probability determination may be based on the context assigned to the symbol.

[0224] The video encoder 200 may also generate syntax data, such as block-based syntax data, picture-based syntax data, and sequence-based syntax data (e.g., in a picture header, a block header, a slice header), or other syntax data, such as a sequence parameter set (SPS), a picture parameter set (PPS), or a video parameter set (VPS), to the video decoder 300. The video decoder 300 may similarly decode such syntax data to determine how to decode the corresponding video data.

[0225] In this way, the video encoder 200 can generate a bitstream including encoded video data, such as a syntax element describing the division of a picture into blocks (e.g., CUs) and prediction and / or residual information of the blocks. Finally, the video decoder 300 can receive the bitstream and decode the encoded video data.

[0226] In general, the video decoder 300 performs a reciprocal process of the process performed by the video encoder 200 to decode the encoded video data of the bitstream. For example, the video decoder 300 may use CABAC to decode the values ​​of the syntax elements of the bitstream in a manner that is substantially similar to the CABAC encoding process of the video encoder 200, although reciprocal. The syntax elements may define: partition information of the picture into CTUs, and the partition of each CTU according to a corresponding partition structure (such as a QTBT structure) to define CUs of the CTU. The syntax elements may further define prediction and residual information for a block (e.g., CU) of video data.

[0227] The residual information may be represented by, for example, a quantized transform coefficient. The video decoder 300 may inverse quantize and inverse transform the quantized transform coefficient of the block to reproduce a residual block of the block.

[0228] According to the techniques of the present disclosure, video decoder 300 may decode a first codeword representing a transform type to be applied to decoded transform coefficients of a current block of video data. As discussed above, the transform type may represent a primary transform, which may be a separable transform including a horizontal transform and a vertical transform. The transform type may further include a secondary transform. If the transform type includes a secondary transform, video decoder 300 may decode a second codeword representing the secondary transform that may be included in an available secondary transform set. Video decoder 300 may then apply the secondary transform to the decoded transform coefficients to produce a set of intermediate transform coefficients, and then apply the primary transform to the set of intermediate transform coefficients to reproduce a residual block for the current block.

[0229] The video decoder 300 uses the signaled prediction mode (intra-frame or inter-frame prediction) and related prediction information (e.g., motion information for inter-frame prediction) to form a prediction block for the block. The video decoder 300 can then combine the prediction block and the residual block (on a sample-by-sample basis) to reproduce the original block. The video decoder 300 may perform additional processing, such as performing a deblocking process to reduce visual artifacts along the boundaries of blocks.

[0230] As mentioned above, the video encoder 200 and the video decoder 300 may apply CABAC encoding and decoding to the values ​​of the syntax elements. In order to apply CABAC encoding to the syntax elements, the video encoder 200 may binarize the values ​​of the syntax elements to form a series of one or more bits, which are referred to as "bins". In addition, the video encoder 200 may identify a coding context. The coding context may identify the probability of a binary symbol having a specific value. For example, the coding context may indicate a 0.7 probability of coding a 0-value binary symbol and a 0.3 probability of coding a 1-value binary symbol. After identifying the coding context, the video encoder 200 may divide the interval into a lower sub-interval and an upper sub-interval. One of these sub-intervals may be associated with a value of 0, while another sub-interval may be associated with a value of 1.

[0231] The width of the sub-interval may be proportional to the probability indicated by the identified coding context for the associated value. If a binary symbol of a syntax element has a value associated with a lower sub-interval, the encoded value may be equal to the lower boundary of the lower sub-interval. If the same binary symbol of a syntax element has a value associated with an upper sub-interval, the encoded value may be equal to the lower boundary of the upper sub-interval. To encode the next binary symbol of the syntax element, the video encoder 200 may repeat these steps using the interval as the sub-interval associated with the value of the encoded bit. When the video encoder 200 repeats these steps for the next binary symbol, the video encoder 200 may use a modified probability based on the probability indicated by the identified coding context and the actual value of the encoded binary symbol.

[0232] When the video decoder 300 performs CABAC decoding on the value of the syntax element, the video decoder 300 may identify the decoding context. The video decoder 300 may then divide the interval into a lower sub-interval and an upper sub-interval. One of these sub-intervals may be associated with a value of 0, while the other sub-interval may be associated with a value of 1. The width of the sub-interval may be proportional to the probability indicated by the identified decoding context for the associated value. If the encoded value is within the lower sub-interval, the video decoder 300 may decode the binary symbol with the value associated with the lower sub-interval. If the encoded value is within the upper sub-interval, the video decoder 300 may decode the binary symbol with the value associated with the upper sub-interval. In order to decode the next binary symbol of the syntax element, the video decoder 300 may repeat these steps using the interval as the sub-interval containing the encoded value. When the video decoder 300 repeats these steps for the next binary symbol, the video decoder 300 may use the modified probability based on the probability indicated by the identified decoding context and the decoded binary symbol. Video decoder 300 may then debinarize the binary symbols to recover the values ​​of the syntax elements.

[0233] In video coding standards before HEVC, only fixed separable transforms were used, in which DCT-2 was used vertically and horizontally. In HEVC, in addition to DCT-2, DST-7 is also used for 4×4 blocks as a fixed separable transform.

[0234] U.S. Patent 10,306,229, U.S. Patent Publication 2018 / 0020218, and U.S. Provisional Patent Application 62 / 679,570 describe the Multi-Transform Selection (MTS) technique. MTS was previously known as Adaptive Multi-Transform (AMT). An example of MTS in U.S. Provisional Patent Application 62 / 679,570 has been adopted in the Joint Experimental Model (JEM-7.0) of the Joint Video Experts Group (JVET), and a simplified version of MTS was later adopted in VVC.

[0235] The present disclosure may generally relate to "signaling" certain information, such as syntax elements. The term "signaling" may generally refer to the communication of values ​​of syntax elements and / or other data used to decode encoded video data. That is, video encoder 200 may signal the values ​​of syntax elements in a bitstream. Generally, signaling refers to generating values ​​in a bitstream. As described above, source device 102 may transmit the bitstream to destination device 116 substantially in real time or in non-real time (such as may occur when storing syntax elements to storage device 112 for later retrieval by destination device 116).

[0236] Figure 2A and Figure 2B is a conceptual diagram showing an exemplary quadtree binary tree (QTBT) structure 130 and a corresponding coding tree unit (CTU) 132. Solid lines represent quadtree splits, while dashed lines represent binary tree splits. In each split (i.e., non-leaf) node of the binary tree, a flag is signaled to indicate which split type (i.e., horizontal or vertical) is used, in which, in this example, 0 indicates horizontal splitting and 1 indicates vertical splitting. For quadtree splitting, since the quadtree node splits a block horizontally and vertically into 4 sub-blocks of equal size, there is no need to indicate the split type. Therefore, the video encoder 200 can encode and the video decoder 300 can decode syntax elements (such as split information) at the regional tree level (i.e., solid lines) of the QTBT structure 130 and syntax elements (such as split information) at the prediction tree level (e.g., dashed lines) of the QTBT structure 130. The video encoder 200 may encode and the video decoder 300 may decode video data (such as prediction data and transform data) for a CU represented by a terminal leaf node of the QTBT structure 130 .

[0237] Generally speaking, Figure 2B The CTU 132 may be associated with parameters defining the size of the blocks corresponding to the nodes of the QTBT structure 130 at the first and second levels. These parameters may include the CTU size (indicating the size of the CTU 132 in samples), the minimum quadtree size (MinQTSize, indicating the minimum allowed quadtree leaf node size), the maximum binary tree size (MaxBTSize, indicating the maximum allowed binary tree root node size), the maximum binary tree depth (MaxBTDepth, indicating the maximum allowed binary tree depth), and the minimum binary tree size (MinBTSize, indicating the minimum allowed binary tree leaf node size).

[0238] The root node of the QTBT structure corresponding to the CTU may have four child nodes at the first level of the QTBT structure, each of which may be divided according to a quadtree partition. That is, the nodes of the first level are leaf nodes (without child nodes) or have four child nodes. The example of the QTBT structure 130 represents such a node as a parent node and a child node including a solid line for branching. The nodes of the first level that are not greater than the maximum allowed binary tree root node size (MaxBTSize) may be further divided by the corresponding binary tree. The binary tree splitting of a node may be iterated until the node generated by the split reaches the minimum allowed binary tree leaf node size (MinBTSize) or the maximum allowed binary tree depth (MaxBTDepth). The example of the QTBT structure 130 represents such a node as having a dotted line for branching. The binary tree leaf node is referred to as a coding unit (CU), which is used for prediction (e.g., intra-frame prediction or inter-frame prediction) and transformation without any further partitioning. As described above, a CU may also be referred to as a "video block" or "block".

[0239] In one example of a QTBT partitioning structure, the CTU size is set to 128×128 (luminance sample and two corresponding 64×64 chrominance samples), MinQTSize is set to 16×16, MaxBTSize is set to 64×64, MinBTSize (for width and height) is set to 4, and MaxBTDepth is set to 4. Quadtree partitioning is first applied to the CTU to generate quadtree leaf nodes. The size of the quadtree leaf node can range from 16×16 (i.e., MinQTSize) to 128×128 (i.e., CTU size). If the leaf quadtree node is 128×128, it will not be further split by the binary tree because its size exceeds MaxBTSize (i.e., 64×64 in this example). Otherwise, the leaf quadtree node will be further split by the binary tree. Therefore, the quadtree leaf node is also the root node of the binary tree and has a binary tree depth of 0. When the binary tree depth reaches MaxBTDepth (4 in this example), further splitting is not allowed. When the width of the binary tree node is equal to MinBTSize (4 in this example), it means that no further horizontal splitting is allowed. Similarly, a binary tree node with a height equal to MinBTSize means that no further vertical splitting is allowed for the binary tree node. As described above, the leaf nodes of the binary tree are called CUs and are further processed according to prediction and transformation without further division.

[0240] Figure 3A and 3Bis a conceptual diagram showing an example transform scheme based on HEVC's residual quadtree. In HEVC, a transform coding structure using a residual quadtree (RQT) is applied to adapt to various characteristics of residual blocks, which is briefly described as follows, adapted from www.hhi.fraunhofer.de / fields-of-competence / image-processing / research-groups / image-video-coding / hevc-high-efficiency-video-coding / transform-coding-using-the-residual-quadtree-rqt.html.

[0241] In HEVC, each picture is divided into coding tree units (CTUs), which are decoded in raster scan order for a particular tile or fragment. A CTU is a square block and represents the root of a quadtree (i.e., a coding tree). The size of a CTU can range from 8×8 to 64×64 luminance samples, but 64×64 is commonly used. Each CTU can be further split into smaller square blocks called coding units (CUs).

[0242] After recursively splitting the CTU into CUs, each CU is further split into prediction units (PUs) and transform units (TUs). The partitioning of CUs into TUs is recursively performed based on a quadtree method, so the residual signal of each CU is decoded by a tree structure, i.e., residual quadtree (RQT). RQT allows the size of TUs to be from 4×4 to 32×32 luma samples.

[0243] Figure 3A An example is shown in which a CU includes 10 TUs labeled with letters a to j and corresponding block partitions. Figure 3B Each node of the RQT shown in is actually corresponding to Figure 3A Each TU is processed in a depth-first tree traversal order. Figure 3A , which is shown in alphabetical order after a recursive Z-scan with a depth-first traversal. The quadtree approach enables the transform to be adapted to the varying spatial-frequency characteristics of the residual signal.

[0244] Generally, the larger the transform block size, the larger the spatial support it has and the better the frequency resolution it provides. However, the smaller the transform block size, the smaller the spatial support it has and the better the spatial resolution it provides. The trade-off between spatial resolution and frequency resolution is selected by the encoder mode decision, for example, based on rate-distortion optimization techniques. The rate-distortion optimization technique calculates the weighted sum of decoded bits and reconstruction distortion, i.e., rate-distortion cost, for each decoding mode (e.g., a specific RQT split structure), and selects the decoding mode with the minimum rate-distortion cost as the best mode.

[0245] Three parameters are defined in the RQT according to HEVC: the maximum depth of the tree, the minimum allowed transform size and the maximum allowed transform size. The minimum transform size and the maximum transform size can vary in the range from 4×4 to 32×32 samples, which corresponds to the supported block transforms mentioned in the previous paragraph. The maximum allowed depth of the RQT limits the number of TUs. The maximum depth equal to zero means that if each included TB reaches the maximum allowed transform size (e.g. 32×32), the CB can no longer be split.

[0246] All these parameters interact and affect the RQT structure in HEVC. Consider the case where the root CB size is 64x64, the maximum depth is equal to zero, and the maximum transform size is equal to 32x32. In this case, the CB must be split at least once, because otherwise it will result in an unallowed 64x64 TB. According to HEVC, the RQT parameters, i.e., the maximum RQT depth, the minimum and maximum transform sizes, are sent in the bitstream at the sequence parameter set level. Regarding the RQT depth, different values ​​may be specified and signaled for intra-coded and inter-coded CUs.

[0247] In HEVC, the quadtree transform is applied to both intra residual blocks and inter residual blocks. Typically, the DCT-II transform of the same size as the current residual quadtree partition is applied to the residual block. However, if the current residual quadtree block is 4×4 and generated by intra prediction, the above-mentioned 4×4 DST-VII transform is applied.

[0248] In HEVC, larger transform sizes, such as 64×64 transforms, are not used, mainly because the benefits of larger transform sizes are limited considering the relatively high complexity for relatively small resolution videos.

[0249] Figure 44 is a block diagram illustrating an example system 400 for hybrid video coding with adaptive transform selection. The techniques of the present disclosure may be performed by such a system or a corresponding decoding system. In general, the techniques of the present disclosure are applicable to adaptive transform coding schemes, where, for each block of prediction residual, a different transform may be selected by a video encoder, signaled as auxiliary information, and determined by a video decoder using the auxiliary information.

[0250] Figure 4 The system 140 includes a block separation unit 142, a block prediction unit 144, a residual generation unit 146, a block transform unit 148, a transform bank 150, a quantization unit 152, an entropy coding unit 154, an inverse quantization unit 156, an inverse block transform unit 158, a block reconstruction unit 160, and a frame buffer 162. The block separation unit 142 generally receives the original undecoded video data and divides the picture of the video data into blocks. The block prediction unit 144 generates a prediction block for the current block of the video data to be encoded. The block separation unit 142 provides the current block to the residual generation unit 146, and the block prediction unit 144 provides the prediction block to the residual generation unit 146. The residual generation unit 146 generates a residual block (r) and provides the residual block to the block transform unit 148.

[0251] Block transform unit 148 selects one or more transforms from transform bank 150. For example, in accordance with the techniques of this disclosure, transform bank 150 may include one or more primary transforms (e.g., separable transforms) and one or more secondary transforms (e.g., non-separable transforms). Block transform unit 148 may then apply the primary transform and, if applicable, the secondary transform to generate transform coefficients. In addition, block transform unit 148 may send an indication (t) of the transform(s) to entropy encoding unit 154. Block transform unit 148 converts the transform coefficients (T (t) r) is provided to the quantization unit 152.

[0252] Quantization unit 152 quantizes the transform coefficients, for example by reducing the bit depth of the transform coefficients according to a quantization parameter (QP) of the current block. Quantization unit 152 provides the quantized transform coefficients to entropy encoding unit 154 and inverse quantization unit 156.

[0253] The entropy encoding unit 154 performs entropy encoding of the values ​​of the syntax elements, which include an indication of the transform (t) and the quantized transform coefficients. According to the techniques of the present disclosure, the entropy encoding unit 154 may encode a first codeword that represents a selected transform scheme in a transform candidate set of a multiple transform selection (MTS) scheme for a current block of video data. The selected transform scheme may include a primary transform, and in some examples, a secondary transform to be applied in addition to the primary transform. In the case where the selected transform scheme includes a secondary transform, the entropy encoding unit 154 may encode a second codeword representing the secondary transform in the set of available secondary transforms. The entropy encoding unit 154 may include entropy encoded data (e.g., the first and / or second codewords and the entropy encoded syntax elements for the quantized transform coefficients) in an encoded video bitstream.

[0254] The inverse quantization unit 156 may inverse quantize the quantized transform coefficients and pass the resulting transform coefficients to the inverse block transform unit 158. The inverse block transform unit 158 ​​may apply a primary transform and, if applicable, a secondary transform to the transform coefficients to reproduce a residual block. The inverse block transform unit 158 ​​may provide the residual block to a block reconstruction unit 160, which may combine the residual block with a prediction block to generate a reconstructed block and store the reconstructed block in a frame buffer 162. The frame buffer 162 may also be referred to as a decoded picture buffer (DPB).

[0255] Figure 4 Each of the various components of can be implemented in hardware, software, firmware, or a combination thereof. When implemented in software or firmware, instructions for various operations can be stored in a memory and executed by one or more processing units. The processing unit and the memory can be implemented in a circuit. The processing unit may include, for example, one or more digital signal processors (DSPs), general-purpose microprocessors, application-specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or other equivalent integrated or discrete logic circuits, in any combination.

[0256] In this way, Figure 4 The system 140 represents an example of a video encoder, which includes a memory configured to store video data; and one or more processors implemented in circuits and configured to: decode a first codeword, the first codeword representing a selected transform scheme from a transform candidate set of a multiple transform selection (MTS) scheme for a current block of video data, the selected transform scheme being: a secondary transform from a set of available secondary transforms to be applied in addition to a primary transform; decode a second codeword representing a secondary transform from the set of available secondary transforms; and apply the primary transform and the secondary transform during decoding of residual data for the current block.

[0257] Figure 5A and5B is a conceptual diagram illustrating horizontal and vertical transforms as separate transform implementations. In particular, horizontal and vertical transforms may be used to independently transform horizontal and vertical rows of residual values ​​(eg, to reduce computational complexity, block transforms may be computed in a separable manner).

[0258] In video coding standards before HEVC, only fixed separable transforms were used, where DCT-2 was used vertically and horizontally. In HEVC, in addition to DCT-2, DST-7 is also used for 4×4 blocks as a fixed separable transform. Adaptive extensions to these fixed transforms are described in U.S. patent applications Nos. 15 / 005,736 and 15 / 649,612, and examples of MTS (also known as adaptive multi-transform (AMT)) are described in U.S. patent applications Nos. 15 / 005,736, filed on January 25, 2016; 15 / 649,612, filed on July 13, 2017; and 62 / 679,570, filed on June 1, 2018, which have been adopted in the Joint Experimental Model (JEM) of the Joint Video Experts Group (JVET) (Joint Video Experts Group (JVET) of ITU-T SG16 WP3 and ISO / IEC JTC1 / SC29 / WG11, JEM software, available at jvet.hhi.fraunhofer.de / svn / svn_HMJEMSoftware / tags / HM-16.6-JEM-7.0).

[0259] Figure 6 is a conceptual diagram showing an example of MTS signaling for identifying two transforms. In the current version of VTM (Versatile Video Coding (Draft 4), Joint Video Experts Group (JVET), ITU-T SG16 WP3 and ISO / IEC JTC1 / SC29 / WG11, 13th Meeting: Marrakech, Morocco, January 9-18, 2019, document JVET-M1001, available at phenix.it-sudparis.eu / jvet / doc_end_user / documents / 13_Marrakech / wg11 / JVET-M1001-v7.zip), multiple transform candidates are signaled based on truncated unary binarization, which can be achieved by Figure 6 and 7 The transform candidates are then associated with the codewords obtained by concatenation.

[0260] Figure 7is a conceptual diagram showing an example transform assignment and the corresponding unary codeword. MTS signaling in the current version of VVC includes assigning transforms to Figure 6 The codeword is obtained by concatenating the binary trees in , where "H: DCT-8, V: DST-7" means that for separable transform, DCT-8 is applied horizontally and DST-7 is applied vertically, and IDT represents 1-D identity transform (scaling is performed).

[0261] The MTS (Multiple Transform Selection) design of VVC uses six transform candidates (such as Figure 7 ), and in addition to using a single type of transform in both horizontal and vertical directions (i.e., applying IDT, DCT-2, and DST-7 horizontally and vertically), it supports combinations with DST-7 and DCT-8. In fact, by allowing a larger number of transform candidates, better decoding efficiency can be achieved. The present disclosure describes various extensions to the current MTS design that can improve decoding efficiency.

[0262] The MTS scheme can be defined by assigning transforms to codewords that specify a signaling method. The video encoder 200 and / or the video decoder 300 can be configured according to the techniques of the present disclosure, as discussed above and discussed in more detail below. Specifically, the MTS scheme according to the present disclosure can be defined by assigning transforms to codewords that specify a signaling method. Thus, the MTS scheme can be fully defined by specifying: (i) a single transform set or multiple transform sets (i.e., transform candidates), and (ii) associated signaling methods. Thus, the video encoder 200 and the video decoder 300 can be configured to use any of the techniques of the present disclosure, alone or in any combination, to decode an indication of the MTS scheme.

[0263] The indication of the MTS scheme may be a codeword. In some examples, the MTS scheme may include both a primary transform, such as a separable transform (e.g., a horizontal transform and a vertical transform) and a secondary transform. In these examples, the video encoder 200 and the video decoder 300 may decode a second codeword representing the secondary transform, where the second codeword may identify the secondary transform in the set of available secondary transforms.

[0264] The MTS design in VVC uses a single transform set consisting of 6 separable transform candidates, as shown in Table 1 below:

[0265] Table 1 - Figure 7 The allowed transform candidates in VVC and the corresponding codewords used to signal the candidates are shown in FIG.

[0266] Candidate Horizontal (H) Vertical (V) Codeword 1 IDT IDT 0 2 DCT-2 DCT-2 10 3 DST-7 DST-7 110 4 DCT-8 DST-7 1110 5 DST-7 DCT-8 11110 6 DCT-8 DCT-8 11111

[0267] You can use the Figure 7 The binary tree shown on the right Figure 6 ) are concatenated to generate a codeword to signal the above exemplary six transformation candidates. For each codeword, Figure 7 (Left) Transformation candidates are allocated as shown.

[0268] An alternative MTS design may be defined based on one or more combinations of the following techniques. That is, the video encoder 200 and the video decoder 300 may perform any of the techniques described below, alone or in any combination.

[0269] 1. The MTS design can be extended by including new transform candidates, which replace or do not replace some of the candidates in the current candidate set in VVC shown in Table 1.

[0270] 2. A combination of DCT-2 and DST-7 may be included as an additional transform candidate.

[0271] a. In one example, two additional transform candidates can be added on top of the current VVC, allowing a total of 8 separable transform candidates, as shown in Table 2:

[0272] Table 2 - Transformation form candidates and associated codewords

[0273] Candidate Horizontal (H) Vertical (V) Codeword 1 IDT IDT 0 2 DCT-2 DCT-2 10 3 DST-7 DST-7 110 4 DCT-8 DST-7 1110 5 DST-7 DCT-8 11110 6 DCT-8 DCT-8 111110 7 DCT-2 DST-7 1111110 8 DST-7 DCT-2 1111111

[0274] b. In another example, two more transform candidates may be added by removing the "H: DCT-8, V: DCT-8" combination, so that a total of 7 separable transform candidates are allowed, as shown in Table 3:

[0275] Table 3 - Example transform candidates including a combination of DCT2 and DST7 without a H:DCT-8, V:DCT-8 combination

[0276] Candidate Horizontal (H) Vertical (V) Codeword 1 IDT IDT 0 2 DCT-2 DCT-2 10 3 DST-7 DST-7 110 4 DCT-8 DST-7 1110 5 DST-7 DCT-8 11110 6 DCT-2 DST-7 111110 7 DST-7 DCT-2 111111

[0277] 3. A combination of IDT and DST-7 may be included as an additional transform candidate.

[0278] a. For example, the following seven transformation candidates can be used in MTS, as shown in Table 4.

[0279] Table 4 - Example transform candidates including a combination of IDT and DST7 without a combination of H:DCT-8, V:DCT-8

[0280] Candidate Horizontal (H) Vertical (V) Codeword 1 IDT IDT 0 2 DCT-2 DCT-2 10 3 DST-7 DST-7 110 4 DCT-8 DST-7 1110 5 DST-7 DCT-8 11110 6 IDT DST-7 111110 7 DST-7 IDT 111111

[0281] b. In another example, the following 10 transform candidates may be used in MTS by adding a combination of DCT-2 and DST-7 and a combination of IDT and DST-7.

[0282] Table 5 - Example transform candidates including combinations of DCT-2 and DST-7 and IDT and DST-7 by retaining the H:DCT-8, V:DCT-8 combination as the 6th candidate

[0283]

[0284]

[0285] c. In another example, by replacing DCT-8 and DCT-8 combinations in the above list, the following 9-transform skipping (which is equivalent to applying the identity candidate) can be used in MTS as follows:

[0286] Table 6 - Example transform candidates including combinations of DCT-2 and DST-7 and IDT and DST-7 by removing the H:DCT-8, V:DCT-8 combination

[0287] Candidate Horizontal (H) Vertical (V) Codeword 1 IDT IDT 0 2 DCT-2 DCT-2 10 3 DST-7 DST-7 110 4 DCT-8 DST-7 1110 5 DST-7 DCT-8 11110 6 IDT DST-7 111110 7 DST-7 IDT 1111110 8 DCT-2 DST-7 11111110 9 DST-7 DCT-2 11111111

[0288] 4. Candidates and their associated binarizations (ie, codewords) may have different orderings.

[0289] a. The ranking may be predefined and may be a fixed design based on the statistics / frequency of each transformation candidate.

[0290] b. For example, the ranking can be achieved by ranking the frequency of each transformation candidate used.

[0291] c. For example, it can be designed to reduce the average codeword length used to signal the transform candidates (eg, a Huffman code generated based on the probability of each candidate being used).

[0292] d. For example, in actual codecs, H:DST-7, V:DST-7 and H:DCT-2, V:DCT-2 combinations are frequently used. Therefore, in order to reduce signaling overhead, the MTS designs in Table 1 can be sorted as in the following example in Table 7:

[0293] Table 7 - Example of reordering transform candidates in Table 1, where the 1st and 3rd transform candidates are swapped.

[0294] Candidate Horizontal (H) Vertical (V) Codeword 1 DST-7 DST-7 0 2 DCT-2 DCT-2 10 3 IDT IDT 110 4 DCT-8 DST-7 1110 5 DST-7 DCT-8 11110 6 DCT-8 DCT-8 11111

[0295] 5. Depending on the prediction mode and / or block size, different MTS designs may be used to code a block, where a block may be a transform unit (TU) or a coding unit (CU).

[0296] a. Different MTS designs can include:

[0297] i. different sets of transformation candidates;

[0298] ii. Different signaling and binarization (i.e., codeword for each candidate);

[0299] iii. Both i) and ii) above.

[0300] b. Depending on the intra prediction mode and / or inter prediction mode, multiple MTS designs can be used to determine the transform selection:

[0301] i. Different types of prediction methods (e.g., intra prediction and inter prediction) can use different MTS designs. For example, for decoding of inter-frame prediction blocks, the MTS defined in Table 1 can be used, while for intra-frame prediction blocks, the MTS defined in Table 5 can be used to determine the transformation.

[0302] ii. Different subsets of intra prediction modes may use different MTS designs. Different subsets of modes may be defined by mutually exclusive and jointly exhaustive selections of a subset of angle modes, planar modes, and DC modes. For example, for planar mode (0), DC mode (1), and diagonal mode (34), the MTS design with 3 candidates in Table 8 may be used. For angle modes from (2) to (33), Table 9 may be used. For angle modes from (35) to (66) or the remaining angle modes, Table 10 may be used.

[0303] Table 8 - Example of MTS design with 3 candidates

[0304] Candidate Horizontal (H) Vertical (V) Codeword 1 IDT IDT 0 2 DCT-2 DCT-2 10 3 DST-7 DST-7 11

[0305] Table 9 - Example of MTS design with 5 candidates

[0306]

[0307]

[0308] Table 10 - Example of an MTS design with 5 candidates, where the 4th and 5th candidates in Table 9 are swapped

[0309] Candidate Horizontal (H) Vertical (V) Codeword 1 IDT IDT 0 2 DCT-2 DCT-2 10 3 DST-7 DST-7 110 4 DST-7 DCT-8 1110 5 DCT-8 DST-7 1111

[0310] c. Multiple MTS designs can be used to determine the transformation selection based on block shape and block size.

[0311] i. Different MTS designs are available for blocks of different sizes and / or shapes.

[0312] ii. For example, to decode a small block, an MTS design with fewer candidates may be used, while for a larger block, another MTS design with more transform candidates may be used. Thus, the transform signaling overhead for small blocks may be reduced.

[0313] iii. Small blocks can be defined based on their width and / or height. For example, blocks with a width or height less than 8 can be considered small blocks, while the remaining blocks can be considered large blocks (for example, if the minimum value of the width and height of a block is less than 16, the block can be classified as a small block).

[0314] iv. Blocks can also be classified based on square / rectangular shape, where the ratio between width and height can be used to classify blocks with different shapes (e.g., 4×8 and 8×4 blocks can belong to one category, while blocks of size 4×16 and 16×4 can belong to another category).

[0315] d. A single (unified) MTS design can also be used for signaling.

[0316] 6. The context derivation for signaling the transformation candidate may also be performed according to one or a combination of the following:

[0317] a. Block size;

[0318] b. Block shape;

[0319] c. Intra-frame mode;

[0320] d. Inter-frame mode.

[0321] - Separate contexts may be defined for intra-predicted and inter-predicted CUs / TUs, respectively.

[0322] - Separate contexts can be defined based on the minimum values ​​of the width and height of the block respectively.

[0323] 7. In addition to separable transforms, the MTS design may also include non-separable transforms as transform candidates. An example is shown in Table 11.

[0324] Table 11 - Example transform candidates including non-separable transforms in addition to separable transforms in MTS design

[0325]

[0326]

[0327] Furthermore, in addition to the separable transforms, sub-transforms can also be included in the MTS design. Table 12 gives an example of MTS, where the H:DCT-8, V:DCT-8 combination is replaced by a sub-transform set.

[0328] The secondary transform may include various aspects described in U.S. Application Nos. 15 / 270,455, filed on September 20, 2016, and 16 / 364,007, filed on March 25, 2019. Specifically, at the encoder side, the secondary transform may be applied to a subset of the primary transform coefficients (e.g., which are obtained from a 2-D DCT-2), where the order is reversed at the decoder (the inverse secondary transform is applied first, followed by the primary transform).

[0329] Secondary transitions may require additional signaling to determine if Figure 8 Note that if there is only a single secondary transform candidate (ie, the set may be only a single secondary transform), no additional signaling is required on top of the MTS signaling in Table 12.

[0330] For secondary transforms, transform candidates may also depend on one or a combination of prediction mode, block size, and block shape.

[0331] Table 12: Transformation candidates that include secondary transformations in addition to separable transformations in MTS design

[0332]

[0333] 8. Separable transforms in MTS designs can be constructed using combinations of other types of DST and DCT (e.g., DST-4 and DCT-4) besides IDT, DST-7, DCT-8, and DCT-2.

[0334] 9. One or a combination of the above methods may be used for intra-predicted CUs.

[0335] 10. One or a combination of the above methods can be used for inter-predicted CUs.

[0336] 11. One or a combination of the above methods can be used for both intra-predicted CUs and inter-predicted CUs.

[0337] 12. One or a combination of the above methods can be used for the luminance channel or the chrominance channel or both.

[0338] Figure 8 is a conceptual diagram showing an example MTS design supporting secondary transforms. If a secondary transform is signaled / selected, additional signaling may be used to indicate the secondary transform among the N possible secondary transforms. That is, the video encoder 200 may encode a first codeword indicating a primary transform and a secondary transform to be applied in addition to the primary transform, and further encode a codeword indicating a secondary transform in a transform set (e.g., Figure 8) of the N available transforms shown in . Similarly, video decoder 300 may decode the first codeword and determine that the first codeword indicates a primary transform and that a secondary transform is to be applied. Therefore, video decoder 300 may further decode the second codeword in response to the first codeword and use the second codeword to determine the secondary transform. Video encoder 200 and video decoder 300 may further apply both the primary transform and the secondary transform.

[0339] In some examples, as discussed in more detail below, the secondary transform may be a low frequency non-separable transform (LFNST).Thus, the first codeword may be referred to as an MST syntax element and the second codeword may be referred to as an LFNST syntax element.

[0340] Fig. 9 and 10 is a conceptual diagram showing the use of a low frequency non-separable transform (LFNST). LFNST is used in JEM-7.0 to further improve the decoding efficiency of MTS, where the implementation of LFNST is based on the Hypercube-Givens transform (HyGT), which is described in U.S. Patent Publication No. 2017 / 0238013, U.S. Patent Publication No. 2017 / 0094313, 2017 / 0238014, U.S. Patent Application 16 / 364,007, and U.S. Provisional Patent Applications 62 / 668,105 and 62 / 849,689 (describing alternative designs and further details). LFNST was previously referred to as a non-separable secondary transform (NSST) or secondary transform, but LFNST, NSST, and secondary transform may generally refer to the same technology. Recently, LFNST was adopted into the VVC draft standard, as described in Koo et al., “CE6: Reduced Secondary Transform (RST) (CE6-3.1),” Joint Video Experts Group (JVET), ITU-T SG16 WP3 and ISO / IEC JTC1 / SC29 / WG11, 14th Meeting, Geneva, Switzerland, March 19-27, 2019, document JVET-N0193).

[0341] Fig. 9 is a conceptual diagram illustrating the LFNST transform applied by the video encoder 200 and the video decoder 300. LFNST introduces a new stage between the separable transform and quantization in the codec. Fig.10 is a conceptual diagram showing LFNST applied to a subset of coefficients of an HxW block (in the upper left).

[0342] VVC draft 5 includes the following specifications, which introduce some encoder / decoder complexity without significant decoding benefits:

[0343] 1) LFNST can be used with any MTS transform except transform skip (TS) mode,

[0344] 2) The context model used to signal the LFNST index depends on the MTS index. For example, the video decoder may select the context model used when CABAC decoding (or other decoding techniques) the LFNST index based on the MTS index. The context model may indicate the probability that the first bit of the LFNST index has a specific value.

[0345] 3) LFNST can be used to decode the chroma channels, although the specification disables MTS for chroma.

[0346] 4) LFNST applied to 4×4 and 8×8 blocks can be implemented using a single stage (ie, using a single inseparable transform), however the current implementation is based on a two-stage process.

[0347] This disclosure describes techniques that can simplify LFNST designs by addressing the above issues.The LFNST designs described in this disclosure can be used alone or in any combination.

[0348] In VVC draft 5, LFNST includes 3 modes, which are signaled using LFNST index values ​​0, 1, and 2, where:

[0349] LFNST index 0 corresponds to skipping LFNST processing (e.g., using only MTS),

[0350] LFNST indexes 1 and 2 are used to determine a non-separable transform from a set of two transforms selected according to the mode and size of a block (eg, CU, TU, etc.).

[0351] Based on this design, LFNST can be used under certain conditions:

[0352] LFNST may be applied with a predefined set of transforms (i.e., certain MTS candidates). Thus, if a transform from the predefined set is selected, the LFNST index may be signaled, and the set may depend on the block size (width and height). Otherwise, i.e., if a transform outside the predefined set is selected, the LFNST index may be inferred to be zero, thereby skipping LFNST (i.e., not applying).

[0353] o The use of LFNST may be restricted based on transform type and / or MTS index / flag and / or block size.

[0354] LFNST can be enabled when using predefined transform types and / or MTS indexes / flags.

[0355] • If a separable 2-D DCT-2 is used (ie, if DCT-2 is applied horizontally and vertically), LFNST may be enabled.

[0356] o In VVC, this corresponds to: if the MTS index / flag is zero (ie, 2-D DCT-2 is used), then the LFNST index is signaled; and if the MTS index / flag is different from 0, then the LFNST index / flag is not signaled and is inferred to be zero by the video decoder 300.

[0357] o In this case, the context model used to code the LFNST index / flag does not depend on the MTS index.

[0358] For transform skip mode, LFNST can be disabled.

[0359] When transform skipping is enabled, LFNST processing is skipped and the LFNST index / flag is inferred to be 0.

[0360] o The context model used for coding the signaling of the LFNST index may depend on the MTS index. For each MTS index, a separate context may be defined for coding the LFNST index.

[0361] LFNST can be used for luma blocks and can be disabled for chroma channels. Therefore, for chroma channels, the LFNST index is not signaled and is inferred to be 0.

[0362] Therefore, in an example according to one or more techniques of the present disclosure, if one or more restrictions on the signaling of the LFNST index do not apply to the current block, the video encoder 200 may add the LFNST index for the current block of video data to the bitstream including the encoded representation of the video data. In addition, in this example, the video encoder 200 may apply a transform to the residual data of the current block to generate intermediate data of the current block. In this example, based on the value of the LFNST index, the video encoder 200 may apply the LFNST to the intermediate data to generate the transform coefficients of the current block. The video encoder 200 may include data representing the transform coefficients of the current block in the bitstream.

[0363] In one example of one or more techniques according to the present disclosure, if one or more restrictions on the signaling of the LFNST index do not apply to the current block, the video decoder 300 may obtain the LFNST index for the current block of video data from a bitstream including an encoded representation of the video data. In this example, the video decoder 300 may determine the transform coefficient block based on the data in the bitstream. Based on the value of the LFNST index, the video decoder 300 may apply an inverse LFNST to the transform coefficient block to generate intermediate data for the current block. The video decoder 300 may apply an inverse transform of the transform to the intermediate data of the current block to generate residual data for the current block. In this example, the video decoder 300 may reconstruct samples of the current block based on the residual data of the current block.

[0364] Fig.11A and 11B is a conceptual diagram showing an example two-step LFNST process implementation according to the VVC Test Model (VTM) of May 30, 2019. In this example, LFNST is applied to the top of a subset of separable transform coefficients (e.g., MTS coefficients) within the darker shaded sub-block at the upper left region. Fig.11A This two-step process may be unavoidable for block shapes / sizes in . However, for 4×4 and 8×8 blocks, as Fig. 11B As shown, the LFNST and separable transform sizes are aligned (i.e., support for LFNST and separable transforms may include the same coefficient positions / locations within the darker shaded blocks). In this case, the transform process can be simplified to a single-stage non-separable transform as follows:

[0365] Instead of applying LFNST in two stages (eg, instead of applying LFNST with a separable transform), the video encoder 200 and the video decoder 300 may obtain coefficients directly from a non-separable transform in one stage. For example:

[0366] o For the 4×4 case, a 16-length non-separable transform is used, which can be implemented as a matrix-vector multiplication.

[0367] o For the 8×8 case, a 64-length non-separable transform is used, which can also be implemented as a matrix-vector multiplication.

[0368] - Additionally, a zero-out scheme (eg, as described in US Provisional Patent Application 62 / 849,689) may be used to reduce the number of multiplications required for matrix-based implementations.

[0369] o In a zeroing scheme, the first K lowest frequency coefficients may need to be calculated, and the remaining transform coefficients may be normalized to zero (ie, assumed to be zero at both the video encoder 200 and the video decoder 300).

[0370] The value of K can depend on the block size. For example:

[0371] • For 4x4 blocks, K may be 8, thus canonically zeroing the remaining 8 coefficients.

[0372] • For 8x8 blocks, K may be 8, thus canonically zeroing the remaining 56 coefficients.

[0373] • For 8x8 blocks, K may be 16, thus canonically zeroing the remaining 48 coefficients.

[0374] - For 4x4 and 8x8, LFNST can be implemented as a single-stage non-separable transform, and for other cases, LFNST can be implemented as a two-step process as described in US Provisional Patent 62 / 337,736.

[0375] In an example of a technique according to the present disclosure, the video encoder 200 may determine residual data for a first block of video data. Additionally, the video encoder 200 may determine residual data for a second block of video data. Based on the width of the first block being equal to the height of the first block: the video encoder 200 may apply a non-separable transform to the residual data of the first block to generate transform coefficients for the first block; and include data representing the transform coefficients of the first block in a bitstream including an encoded representation of the video data. In this example, based on the width of the second block not being equal to the height of the second block, the video encoder 200 may apply a transform to the residual data of the second block to generate intermediate data for the second block; apply a LFNST to the intermediate data of the second block to generate transform coefficients for the second block; and include data representing the transform coefficients of the second block in the bitstream.

[0376] In another example of the technology according to the present disclosure, the video decoder 300 may determine transform coefficients for a first block of video based on first data in a bitstream that includes an encoded representation of the video data. In addition, the video decoder 300 may determine transform coefficients for a second block of video data based on second data in the bitstream. Based on the width of the first block being equal to the height of the first block, the video decoder 300 may apply an inverse transform of the inseparable transform to the transform coefficients of the first block to generate residual data for the first block; and reconstruct samples of the first block based on the residual data of the first block. In this example, based on the width of the second block not being equal to the height of the second block: the video decoder 300 may apply an inverse transform to the transform coefficients of the second block to generate intermediate data for the second block; apply an inverse transform of the LFNST to the intermediate data of the second block to generate residual data for the second block; and reconstruct samples of the second block based on the residual data of the second block.

[0377] Fig.12is a block diagram illustrating an example video encoder 200 that may perform the techniques of this disclosure. Fig.12 For purposes of illustration, and should not be considered limiting of the techniques broadly illustrated and described in this disclosure. For purposes of illustration, this disclosure describes the video encoder 200 in the context of video coding standards such as the HEVC video coding standard and the developing H.266 / VCC video coding standard. However, the techniques of this disclosure are not limited to these video coding standards, but are generally applicable to video encoding and decoding.

[0378] exist Fig.12 In 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 encoding unit 220. Fig.12 A transform library may also be included, and the transform processing unit 206 and the inverse transform processing unit 212 select transforms from the transform library according to the technology of the present disclosure, as described above. Figure 4 Similarly, Figure 4 As shown in , transform processing unit 206 may provide an indication of the selected transform to entropy encoding unit 220, which may encode data indicating which of multiple transforms of the MTS scheme is selected for a current block of video data in accordance with the techniques of this disclosure.

[0379] The video data memory 230 may store video data to be encoded by the components of the video encoder 200. The video encoder 200 may receive video data from, for example, the video source 104 ( Figure 1 ) receives video data stored in video data memory 230. DPB218 can be used as a reference picture memory, which stores reference video data for use by video encoder 200 when predicting subsequent video data. Video data memory 230 and DPB 218 can be formed by any of a variety of storage devices, such as dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM), resistive RAM (RRAM) or other types of storage devices. Video data memory 230 and DPB 218 can be provided by the same storage device or a separate storage device. In various examples, video data memory 230 can be on-chip with other components of video encoder 200, as shown, or off-chip relative to those components.

[0380] In the present disclosure, references to the video data memory 230 should not be interpreted as limited to memory internal to the video encoder 200 (unless specifically described as such), or memory external to the video encoder 200 (unless specifically described as such). Rather, references to the video data memory 230 should be understood as a reference memory that stores video data received by the video encoder 200 for encoding (e.g., video data of a current block to be encoded). Figure 1 The memory 106 may also provide temporary storage for the outputs of the various units of the video encoder 200 .

[0381] Show Fig.12 The various units of the video encoder 200 are described to help understand the operations performed by the video encoder 200. These units can be implemented as fixed-function circuits, programmable circuits, or a combination thereof. Fixed-function circuits refer to circuits that provide specific functions and pre-set the operations that can be performed. Programmable circuits refer to circuits that can be programmed to perform various tasks and provide flexible functions in the operations that can be performed. For example, a programmable circuit can execute software or firmware that causes the programmable circuit to operate in a manner defined by the instructions of the software or firmware. Fixed-function circuits can execute software instructions (for example, to receive parameters or output parameters), but the type of operation performed by the fixed-function circuit is generally immutable. In some examples, one or more units may be different circuit blocks (fixed function or programmable), and in some examples, one or more units may be integrated circuits.

[0382] The video encoder 200 may include an arithmetic logic unit (ALU), an elementary function unit (EFU), a digital circuit, an analog circuit, and / or a programmable core formed by a programmable circuit. In an example where the operation of the video encoder 200 is performed using software executed by a programmable circuit, the memory 106 ( Figure 1 ) may store object code for software received and executed by the video encoder 200, or another memory (not shown) within the video encoder 200 may store such instructions.

[0383] The video data memory 230 is configured to store received video data. The video encoder 200 may extract a picture of video data from the video data memory 230 and provide the video data to the residual generation unit 204 and the mode selection unit 202. The video data in the video data memory 230 may be original video data to be encoded.

[0384] The mode selection unit 202 includes a motion estimation unit 222, a motion compensation unit 224, and an intra prediction unit 226. The mode selection unit 202 may include other functional units to perform video prediction according to other prediction modes. As an example, the mode selection unit 202 may include a palette unit, an intra-block copy unit (which may be a component of the motion estimation unit 222 and / or the motion compensation unit 224), an affine unit, a linear model (LM) unit, etc.

[0385] The mode selection unit 202 generally coordinates multiple encoding operations to test combinations of encoding parameters and the rate-distortion values ​​obtained for such combinations. The encoding parameters may include: CTU to CU partitioning, prediction mode for CU, transform type for residual data of CU, quantization parameter for residual data of CU, etc. The mode selection unit 202 may ultimately select a combination of encoding parameters with a better rate-distortion value than other tested combinations.

[0386] The video encoder 200 may divide the picture extracted from the video data memory 230 into a series of CTUs and encapsulate one or more CTUs in a fragment. The mode selection unit 202 may divide the CTU of the picture according to a tree structure (such as the QTBT structure or quadtree structure of the HEVC described above). As described above, the video encoder 200 may form one or more CUs by dividing the CTU according to the tree structure. Such CUs are also generally referred to as "video blocks" or "blocks".

[0387] Typically, the mode selection unit 202 also controls its components (e.g., the motion estimation unit 222, the motion compensation unit 224, and the intra prediction unit 226) to generate a prediction block for the current block (e.g., the current CU, or in HEVC, the overlapping portion of the PU and TU). In order to inter-predict the current block, the motion estimation unit 222 may perform a motion search to identify one or more closely matching reference blocks in one or more reference pictures (e.g., one or more previously decoded pictures stored in the DPB 218). Specifically, the motion estimation unit 222 may calculate a value representing the degree of similarity of the potential reference block to the current block, for example, based on the sum of absolute differences (SAD), the sum of squared differences (SSD), the mean absolute difference (MAD), the mean squared difference (MSD), etc. The motion estimation unit 222 may generally perform these calculations using the sample-by-sample difference between the current block and the reference block under consideration. The motion estimation unit 222 may identify the reference block having the lowest value resulting from these calculations, which indicates the reference block that most closely matches the current block.

[0388] The motion estimation unit 222 may form one or more motion vectors (MVs) that define the position of a reference block in a reference picture relative to the position of a current block in a current picture. The motion estimation unit 222 may then provide the motion vectors to the motion compensation unit 224. For example, for unidirectional inter prediction, the motion estimation unit 222 may provide a single motion vector, while for bidirectional inter prediction, the motion estimation unit 222 may provide two motion vectors. The motion compensation unit 224 may then use the motion vectors to generate a prediction block. For example, the motion compensation unit 224 may use the motion vectors to extract data for the reference block. As another example, if the motion vectors have fractional sample accuracy, the motion compensation unit 224 may interpolate the values ​​of the prediction block according to one or more interpolation filters. In addition, for bidirectional inter prediction, the motion compensation unit 224 may extract data for two reference blocks identified by corresponding motion vectors, for example, by sample-by-sample averaging or weighted averaging, and combine the extracted data.

[0389] As another example, for intra prediction or intra prediction decoding, the intra prediction unit 226 can generate a prediction block based on samples adjacent to the current block. For example, for directional mode, the intra prediction unit 226 can generally mathematically combine the values ​​of adjacent samples and fill these calculated values ​​across the current block in a defined direction to produce a prediction block. As another example, for DC mode, the intra prediction unit 226 can calculate the average value of adjacent samples of the current block and generate a prediction block to include the resulting average value for each sample of the prediction block.

[0390] The mode selection unit 202 provides the prediction block to the residual generation unit 204. The residual generation unit 204 receives the original undecoded version of the current block from the video data memory 230 and the prediction block from the mode selection unit 202. The residual generation unit 204 calculates the sample-by-sample difference between the current block and the prediction block. The resulting sample-by-sample difference defines a residual block for the current block. In some examples, the residual generation unit 204 may also determine the difference between the sample values ​​in the residual block to generate the residual block using residual differential pulse code modulation (RDPCM). In some examples, the residual generation unit 204 may be formed using one or more subtractor circuits that perform binary subtraction.

[0391] In the example where the mode selection unit 202 divides the CU into PUs, each PU may be associated with a luma prediction unit and a corresponding chroma prediction unit. The video encoder 200 and the video decoder 300 may support PUs of various sizes. As described above, the size of the CU may refer to the size of the luma coding block of the CU, and the size of the PU may refer to the size of the luma prediction unit of the PU. Assuming that the size of a particular CU is 2N×2N, the video encoder 200 may support PU sizes of 2N×2N or N×N for intra-frame prediction, and symmetrical PU sizes of 2N×2N, 2N×N, N×2N, N×N or the like for inter-frame prediction. The video encoder 200 and the video decoder 300 may also support asymmetric partitioning of PU sizes of 2N×nU, 2N×nD, nL×2N and nR×2N for inter-frame prediction.

[0392] In an example where the mode selection unit does not further divide the CU into PUs, each CU may be associated with a luma coding block and a corresponding chroma coding block. As described above, the size of a CU may refer to the size of the luma coding block of the CU. The video encoder 200 and the video decoder 300 may support a CU size of 2N×2N, 2N×N, or N×2N.

[0393] For other video coding techniques (such as intra-block copy mode coding, affine mode coding, and linear model (LM) mode coding, as a few examples), the mode selection unit 202 generates a prediction block for the current block being encoded through a corresponding unit associated with the coding technique. In some examples, such as palette mode coding, the mode selection unit 202 may not generate a prediction block, but instead generate a syntax element indicating how to reconstruct the block based on the selected palette. In such a mode, the mode selection unit 202 may provide these syntax elements to the entropy coding unit 220 for encoding.

[0394] As described above, the residual generation unit 204 receives video data of a current block and a corresponding prediction block. The residual generation unit 204 then generates a residual block for the current block. To generate the residual block, the residual generation unit 204 calculates the sample-by-sample difference between the prediction block and the current block.

[0395] 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 directional transform, a Karhunen-Loeve 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, for example, a primary transform and a secondary transform, such as a rotation transform. In some examples, the transform processing unit 206 does not apply a transform to the residual block.

[0396] According to the techniques of the present disclosure, the transform processing unit 206 may select a transform scheme (e.g., an MTS scheme) that includes both a primary transform and a secondary transform. The primary transform may be a separable transform including a horizontal transform and a vertical transform, such as one of a plurality of DCTs and / or DSTs. The secondary transform may be an LFNST. The transform processing unit 206 may additionally provide an indication of the selected transform scheme to the entropy coding unit 220 and provide an indication of the selected secondary transform to the entropy coding unit 220 if the selected transform scheme includes a secondary transform. The entropy coding unit 220 may then encode a first codeword representing the selected transform scheme (which may also indicate whether the selected transform scheme includes a secondary transform). If the selected transform scheme includes a secondary transform, such as an LFNST, the entropy coding unit 220 may further encode a second codeword representing the selected secondary transform. If, for example, the primary transform includes a DCT-2 horizontal transform and a DCT-2 vertical transform, the video encoder 200 may determine that the selected transform scheme includes a secondary transform, as discussed above. In addition, the transform processing unit 206 may apply the primary transform to the residual block. If the selected transform scheme includes a secondary transform, transform processing unit 206 may also apply the secondary transform after the primary transform.

[0397] The quantization unit 208 may quantize the transform coefficients in the transform coefficient block to produce a quantized transform coefficient block. The quantization unit 208 may quantize the transform coefficients of the transform coefficient block according to a quantization parameter (QP) value associated with the current block. The video encoder 200 (e.g., via the mode selection unit 202) may adjust the degree of quantization applied to the coefficient block associated with the current block by adjusting the QP value associated with the CU. Quantization may introduce information loss, and therefore, the quantized transform coefficients may have lower precision than the original transform coefficients produced by the transform processing unit 206.

[0398] The inverse quantization unit 210 and the inverse transform processing unit 212 may apply inverse quantization and inverse transform, respectively, to the quantized transform coefficient block to reconstruct a residual block from the transform coefficient block. According to the techniques of the present disclosure, the inverse transform processing unit 212 may apply an inverse secondary transform and then an inverse primary transform to the transform coefficients. The reconstruction unit 214 may generate a reconstructed block corresponding to the current block (although possibly with some degree of distortion) based on the reconstructed residual block and the prediction block generated by the mode selection unit 202. For example, the reconstruction unit 214 may add samples of the reconstructed residual block to corresponding samples from the prediction block generated by the mode selection unit 202 to generate a reconstructed block.

[0399] Filter unit 216 may perform one or more filter operations on the reconstructed block. For example, filter unit 216 may perform a deblocking operation to reduce blocking artifacts along the edges of the CU. In some examples, the operations of filter unit 216 may be skipped.

[0400] The video encoder 200 stores the reconstructed blocks in the DPB 218. For example, in an example where the operation of the filter unit 216 is not required, the reconstruction unit 214 can store the reconstructed blocks to the DPB 218. In an example where the operation of the filter unit 216 is required, the filter unit 216 can store the filtered reconstructed blocks to the DPB 218. The motion estimation unit 222 and the motion compensation unit 224 can extract a reference picture from the DPB 218, which is formed by the reconstructed (and possibly filtered) blocks to perform inter-frame prediction on the blocks of the subsequent encoded pictures. In addition, the intra-frame prediction unit 226 can use the reconstructed blocks of the current picture in the DPB 218 to perform intra-frame prediction on other blocks in the current picture.

[0401] In general, the entropy coding unit 220 may entropy encode syntax elements received from other functional components of the video encoder 200. For example, the entropy coding unit 220 may entropy encode a block of quantized transform coefficients from the quantization unit 208. As another example, the entropy coding unit 220 may entropy encode prediction syntax elements (e.g., motion information for inter-frame prediction or intra-frame mode information for intra-frame prediction) from the mode selection unit 202. The entropy coding unit 220 may perform one or more entropy coding operations on the syntax elements (which are another example of video data) to generate entropy-encoded data. For example, the entropy coding unit 220 may perform a context-adaptive variable length coding (CAVLC) operation, a CABAC operation, a variable-to-variable (V2V) length coding operation, a syntax-based context-adaptive binary arithmetic coding (SBAC) operation, a probability interval partitioning entropy (PIPE) coding operation, an exponential Golomb coding operation, or another entropy coding operation on the data. In some examples, the entropy coding unit 220 may operate in a bypass mode where the syntax elements are not entropy-encoded.

[0402] The video encoder 200 may output a bitstream that includes entropy-encoded syntax elements required to reconstruct blocks of a slice or picture. Specifically, the entropy encoding unit 220 may output a bitstream.

[0403] The above operations are described for blocks. Such description should be understood as operations for luma coding blocks and / or chroma coding blocks. As described above, in some examples, the luma coding blocks and chroma coding blocks are the luma and chroma components of a CU. In some examples, the luma coding blocks and chroma coding blocks are the luma and chroma components of a PU.

[0404] In some examples, operations performed for luma coding blocks need not be repeated for chroma coding blocks. As one example, operations for identifying motion vectors (MVs) and reference pictures for luma coding blocks need not be repeated in order to identify MVs and reference pictures for chroma blocks. Instead, the MVs for luma coding blocks may be scaled to determine the MVs for chroma blocks, and the reference pictures may be the same. As another example, the intra prediction process may be the same for luma coding blocks and chroma coding blocks.

[0405] The video encoder 200 represents an example of a device configured to encode video data, including: a memory configured to store video data; and one or more processing units implemented in circuits and configured to: decode a first codeword representing a selected transform scheme from a transform candidate set of a multiple transform selection (MTS) scheme for a current block of video data, the selected transform scheme being: a secondary transform from an available secondary transform set to be applied in addition to a primary transform; decode a second codeword representing a secondary transform from the available secondary transform set; and apply the primary transform and the secondary transform during decoding of residual data for the current block.

[0406] Fig.13 is a block diagram illustrating an exemplary video decoder 300 that can perform the techniques of this disclosure. Fig.13 This is for illustrative purposes only and is not intended to limit the techniques extensively illustrated and described in this disclosure. For illustrative purposes, this disclosure describes the video decoder 300 according to the techniques of VVC and HEVC. However, the techniques of this disclosure may be performed by video decoding devices configured for other video decoding standards.

[0407] exist Fig.13 In the example of , the video decoder 300 includes a coded picture buffer (CPB) memory 320, an entropy decoding unit 302, a prediction processing unit 304, an inverse quantization unit 306, an inverse transform processing unit 308, a reconstruction unit 310, a filter unit 312 and a decoded picture buffer (DPB) 314. Fig.13 A transform library may also be included, and the inverse transform processing unit 308 selects a transform from the transform library according to the technology of the present disclosure, as described above in Figure 4 As shown in . Similarly, Figure 4 Inversely to the techniques shown in , entropy decoding unit 302 may decode data indicating which of a plurality of transforms of the MTS scheme is selected for a current block of video data according to the techniques of this disclosure, and provide an indication of the transform to inverse transform processing unit 308.

[0408] 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 to perform predictions according to other prediction modes. As an example, the prediction processing unit 304 may include a palette unit, an intra-block copy unit (which may form part of the motion compensation unit 316), an affine unit, a linear model (LM) unit, etc. In other examples, the video decoder 300 may include more, fewer, or different functional components.

[0409] The CPB memory 320 may store video data, such as an encoded video bitstream, to be decoded by components of the video decoder 300. The video data stored in the CPB memory 320 may be, for example, received from the computer-readable medium 110 ( Figure 1 ). The CPB memory 320 may include a CPB that stores encoded video data (e.g., syntax elements) from an encoded video bitstream. Moreover, the CPB memory 320 may store video data other than syntax elements of a decoded picture, such as temporary data representing outputs from various units of the video decoder 300. The DPB 314 typically stores decoded pictures, which the video decoder 300 may output and / or use 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 by any of a variety of storage devices, such as a dynamic random access memory (DRAM), including synchronous DRAM (SDRAM), magnetoresistive RAM (MRAM), resistive RAM (RRAM), or other types of storage devices. The CPB memory 320 and the DPB 314 may be provided by the same storage device or a separate storage device. In various examples, the CPB memory 320 may be on-chip with other components of the video decoder 300, or off-chip relative to those components.

[0410] Additionally or alternatively, in some examples, video decoder 300 may retrieve the video from memory 120 ( Figure 1 ) to extract the decoded video data. That is, the memory 120 may store data together with the CPB memory 320 as described above. Similarly, when some or all of the functions of the video decoder 300 are implemented in software to be performed by the processing circuits of the video decoder 300, the memory 120 may store instructions to be executed by the video decoder 300.

[0411] Show Fig.13 The various units shown in FIG. 1 and FIG. 2 are provided to help understand the operations performed by the video decoder 300. These units may be implemented as fixed function circuits, programmable circuits, or a combination thereof. Fig.12 , fixed-function circuits refer to circuits that provide specific functions and pre-set operations that can be performed. Programmable circuits refer to circuits that can be programmed to perform various tasks and provide flexible functions in the operations that can be performed. For example, a programmable circuit can execute software or firmware that causes the programmable circuit to operate in a manner defined by the instructions of the software or firmware. Fixed-function circuits can execute software instructions (for example, to receive parameters or output parameters), but the type of operation performed by the fixed-function circuit is generally immutable. In some examples, one or more units may be different circuit blocks (fixed function or programmable), and in some examples, one or more units may be integrated circuits.

[0412] The video decoder 300 may include an ALU, an EFU, a digital circuit, an analog circuit, and / or a programmable core formed by a programmable circuit. In an example where the operation of the video decoder 300 is performed by software executed in a programmable circuit, an on-chip or off-chip memory may store instructions (e.g., object code) of the software received and executed by the video decoder 300.

[0413] Entropy decoding unit 302 may receive encoded video data from the CPB and entropy decode the video data to reproduce syntax elements. Prediction processing unit 304, inverse quantization unit 306, inverse transform processing unit 308, reconstruction unit 310, and filter unit 312 may generate decoded video data codes based on syntax elements extracted from the bitstream.

[0414] According to the techniques of the present disclosure, entropy decoding unit 302 may decode a first codeword representing a transform scheme to be applied to decoded transform coefficients of a current block of video data. Entropy decoding unit 302 may further determine whether the selected transform scheme includes a secondary transform (e.g., LFNST) to be applied in addition to the primary transform. For example, if the primary transform includes a DCT-2 horizontal transform and a DCT-2 vertical transform, entropy decoding unit 302 may determine that a secondary transform is also to be applied. In addition, in response to determining that a secondary transform is to be applied, entropy decoding unit 302 may also decode a second codeword representing a secondary transform in a set of available secondary transforms.

[0415] Typically, the video decoder 300 reconstructs a picture on a block-by-block basis. The video decoder 300 may perform a reconstruction operation on each block individually (where the block currently being reconstructed (ie, decoded) may be referred to as a "current block").

[0416] The entropy decoding unit 302 may entropy decode the following: syntax elements defining the quantized transform coefficients of the quantized transform coefficient block and transform information such as a quantization parameter (QP) and / or a transform mode indication. The inverse quantization unit 306 may use the QP associated with the quantized transform coefficient block to determine a quantization degree, and likewise, determine an inverse quantization degree for the inverse quantization unit 306 to apply. The inverse quantization unit 306 may, for example, perform a bitwise left shift operation to inverse quantize the quantized transform coefficients. The inverse quantization unit 306 may thereby form a transform coefficient block including the transform coefficients.

[0417] After the inverse quantization unit 306 forms the transform coefficient block, the inverse transform processing unit 308 may apply one or more inverse transforms to the transform coefficient block to generate a residual block associated with the current block. For example, the inverse transform processing unit 308 may apply an inverse DCT, an inverse integer transform, an inverse Karhunen-Loeve transform (KLT), an inverse rotational transform, an inverse directional transform, or another inverse transform to the coefficient block. If the transform scheme includes a secondary transform, the inverse quantization unit 306 may apply the secondary transform before applying the primary transform.

[0418] In addition, prediction processing unit 304 generates a prediction block based on the prediction information syntax element entropy decoded by entropy decoding unit 302. For example, if the prediction information syntax element indicates that the current block is inter-predicted, motion compensation unit 316 can generate the prediction block. In this case, the prediction information syntax element can indicate a reference picture in DPB 314 from which the reference block is extracted, and a motion vector that identifies the position of the reference block in the reference picture relative to the position of the current block in the current picture. Motion compensation unit 316 can generally generate a prediction block in the same manner as described with respect to motion compensation unit 224 ( Fig.12 ) is performed in a manner substantially similar to that described herein.

[0419] As another example, if the prediction information syntax element indicates that the current block is intra-predicted, the intra-prediction unit 318 can generate a prediction block according to the intra-prediction mode indicated by the prediction information syntax element. Similarly, the intra-prediction unit 318 can generally generate a prediction block in the same manner as that for the intra-prediction unit 226 ( Fig.12 ) The intra prediction unit 318 may extract data of neighboring samples of the current block from the DPB 314.

[0420] The reconstruction unit 310 may reconstruct the current block using the prediction block and the residual block. For example, the reconstruction unit 310 may add samples of the residual block to corresponding samples of the prediction block to reconstruct the current block.

[0421] The filter unit 312 may perform one or more filter operations on the reconstructed block. For example, the filter unit 312 may perform a deblocking operation to reduce blocking artifacts along the edges of the reconstructed block. The operations of the filter unit 312 may not necessarily be performed in all examples.

[0422] The video decoder 300 may store the reconstructed blocks in the DPB 314. As described above, the DPB 314 may provide reference information to the prediction processing unit 304, such as samples of the current picture for intra-frame prediction and previously decoded pictures for subsequent motion compensation. In addition, the video decoder 300 may output a decoded picture from the DPB for subsequent display on a display device (e.g., Figure 1The image is presented on a display device 118).

[0423] The video decoder 300 represents an example of a video decoding device, including: a memory configured to store video data; and one or more processing units implemented in circuits and configured to: decode a first codeword, the first codeword representing a selected transform scheme from a transform candidate set of a multiple transform selection (MTS) scheme for a current block of video data, the selected transform scheme being: a secondary transform from an available secondary transform set to be applied in addition to a primary transform; decode a second codeword representing a secondary transform from the available secondary transform set; and apply the primary transform and the secondary transform during decoding of residual data for the current block.

[0424] Fig.14 2 is a flowchart illustrating an example method for encoding a current block according to the techniques of the present disclosure. The current block may include a current CU. Although with respect to the video encoder 200 ( Figure 1 and 12 ), but it should be understood that other devices can be configured to perform the same Fig.14 Similar approach.

[0425] In this example, the video encoder 200 initially predicts a current block (350). For example, the video encoder 200 may form a prediction block for the current block. The video encoder 200 may then calculate a residual block for the current block (352). To calculate the residual block, the video encoder 200 may calculate the difference between the original uncoded block and the prediction block for the current block. The video encoder 200 may then select a transform and use the selected transform and quantize the coefficients of the residual block (354). The selected transform may include a primary transform and / or a secondary transform (such as, LFNST. The video encoder 200 may apply either or both of the primary transform and / or the secondary transform according to the selected transform.

[0426] Next, the video encoder 200 may scan the quantized transform coefficients of the residual block (356). During or after the scan, the video encoder 200 may entropy encode the coefficients and the data representing the selected transform (358). For example, the video encoder 200 may entropy encode the data representing the transform using any of the various techniques of the present disclosure as discussed above. The video encoder 200 may encode the coefficients using CAVLC or CABAC. Specifically, the video encoder 200 may select a transform scheme according to the techniques of the present disclosure and entropy encode a codeword representing the selected transform according to any of the techniques of the present disclosure. If the selected transform scheme includes a secondary transform, the video encoder 200 may further encode a second codeword representing the secondary transform from the set of available secondary transforms, for example, as discussed above with respect to Figure 8 Video encoder 200 may then output entropy encoded data representing the transform and coefficients of the block (360).

[0427] In this way, Fig.14 The method of encoding video data represents an example of a method of encoding video data, the method comprising: decoding a first codeword, the first codeword representing a selected transform scheme in a transform candidate set of a multiple transform selection (MTS) scheme for a current block of video data, the selected transform scheme being: a secondary transform in an available secondary transform set to be applied in addition to a primary transform; decoding a second codeword representing a secondary transform from the available secondary transform set; and applying the primary transform and the secondary transform during decoding of residual data of the current block.

[0428] Fig.15 1 is a flowchart illustrating an example method for decoding a current block of video data according to the techniques of the present disclosure. The current block may include a current CU. Although with respect to the video decoder 300 ( Figure 1 and 13 ), but it should be understood that other devices can be configured to perform the same Fig.15 Similar approach.

[0429] The video decoder 300 may receive entropy coded data for a current block, such as entropy coded prediction information and entropy coded data of coefficients of a residual block corresponding to the current block (370). The video decoder 300 may entropy decode the entropy coded data to determine prediction information for the current block, a transform of the current block, and reproduce coefficients of the residual block (372). The video decoder 300 may entropy decode the transform information according to any of the various techniques of the present disclosure. The video decoder 300 may predict the current block (374), for example, using an intra-frame or inter-frame prediction mode as indicated by the prediction information of the current block to calculate the prediction block of the current block. The video decoder 300 may then inverse scan the reproduced coefficients (376) to produce a block of quantized transform coefficients. The video decoder 300 may then inverse quantize and inverse transform the coefficients using the indicated transform to produce a residual block (378). For example, the video decoder 300 may decode a codeword representing the transform to be applied according to any of the techniques of the present disclosure. The video decoder 300 may finally decode the current block by combining the prediction block with the residual block ( 380 ).

[0430] In this way, Fig.15The method of the present invention represents an example of a method of decoding video data, the method comprising: decoding a first codeword, the first codeword representing a selected transform scheme in a transform candidate set of a multiple transform selection (MTS) scheme for a current block of the video data, the selected transform scheme being: a secondary transform in an available secondary transform set to be applied in addition to a primary transform; decoding a second codeword representing a secondary transform from the available secondary transform set; and applying the primary transform and the secondary transform during decoding of residual data of the current block.

[0431] Fig.16 is a flowchart illustrating an example video encoding method according to the techniques of the present disclosure. Figure 1 and 12 Video Encoder 200 Explanation Fig.16 method, but it should be understood that other video encoders may be configured to perform this method or similar methods.

[0432] Initially, video encoder 200 may select a transform scheme including a primary and a secondary transform (400). Mode selection unit 202 may also select a secondary transform from a set of available secondary transforms (402). For example, mode selection unit 202 may cause various components of video encoder 200 to perform various encoding passes, including testing of various transform schemes. Mode selection unit 202 may calculate a rate-distortion metric and determine that the selected transform scheme including the primary transform and the selected secondary transform produces the best tested rate-distortion characteristic.

[0433] The video encoder 200 may then encode the first codeword representing the selected transform scheme (404). In addition, the video encoder 200 may encode the second codeword representing the selected secondary transform scheme (406). Specifically, the entropy encoding unit 220 may entropy encode the first codeword and the second codeword.

[0434] Video encoder 200 may then apply the primary transform to the residual block (408). Specifically, transform processing unit 206 may apply the primary transform to the residual block, thereby generating a transform block of transform coefficients. Video encoder 200 (specifically, transform processing unit 206) may also apply a secondary transform to the transform block (410).

[0435] In this way, Fig.16The method of encoding video data represents an example of a method of encoding video data, the method comprising: decoding a first codeword, the first codeword representing a selected transform scheme in a transform candidate set of a multiple transform selection (MTS) scheme for a current block of video data, the selected transform scheme being: a secondary transform in an available secondary transform set to be applied in addition to a primary transform; decoding a second codeword representing a secondary transform from the available secondary transform set; and applying the primary transform and the secondary transform during decoding of residual data of the current block.

[0436] Fig.17 is a flowchart illustrating an example video decoding method according to the techniques of the present disclosure. Figure 1 and 13 The video decoder 300 explains Fig.17 method, but it should be understood that other video encoders may be configured to perform this method or similar methods.

[0437] The video decoder 300 may initially decode a first codeword representing a transform scheme including a primary transform and a secondary transform (420). Specifically, the entropy decoding unit 302 may entropy decode the first codeword. The entropy decoding unit 302 of the video decoder 300 may also entropy decode a second codeword representing a secondary transform in a set of available secondary transforms (422). For example, the second codeword may serve as an index into the set of available secondary transforms.

[0438] Video decoder 300 may then apply the secondary transform to the decoded transform coefficients of the transform block (424) to produce an intermediate transform block. Video decoder 300 may also apply the primary transform to the intermediate transform block to reproduce a residual block (426). In particular, inverse transform processing unit 308 may apply the secondary transform and the primary transform.

[0439] In this way, Fig.17 The method of the present invention represents an example of a method of decoding video data, the method comprising: decoding a first codeword, the first codeword representing a selected transform scheme in a transform candidate set of a multiple transform selection (MTS) scheme for a current block of the video data, the selected transform scheme being: a secondary transform in an available secondary transform set to be applied in addition to a primary transform; decoding a second codeword representing a secondary transform from the available secondary transform set; and applying the primary transform and the secondary transform during decoding of residual data of the current block.

[0440] It should be appreciated that, according to examples, certain operations or events of any of the techniques described herein may be performed in a different order, may be added, combined, or omitted entirely (e.g., not all described operations or events are required to implement the technique). In addition, in some examples, operations or events may be performed concurrently rather than sequentially, such as through multithreading, interrupt handling, or multiple processors.

[0441] In one or more exemplary designs, the described functions may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored or sent as one or more instructions or codes on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media include: computer storage media, which corresponds to tangible media such as data storage media, or communication media, including, for example, any media that facilitates the transfer of a computer program from one place to another according to a communication protocol. In this manner, computer-readable media may generally correspond to (1) a non-temporary tangible computer-readable storage medium, or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to extract instructions, codes, and / or data structures to implement the techniques described in the present disclosure. A computer program product may include a computer-readable medium.

[0442] For example, but not limiting, such computer-readable media may include RAM, ROM, EEPROM, CD-ROM or other optical disk storage devices, magnetic disk storage devices or other magnetic storage devices, flash memory or any other medium that can be used to store the required program code in the form of instructions or data structures and can be accessed by a computer. In addition, any connection can be appropriately referred to as a computer-readable medium. For example, if the software is sent from a website, server or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL) or wireless technologies such as infrared, wireless and microwaves, the coaxial cable, fiber optic cable, twisted pair, DSL or wireless technologies such as infrared, wireless and microwaves are also included in the definition of the medium. However, it should be understood that computer-readable storage media and data storage media do not include connections, carrier waves, signals or other temporary media, but are directed to non-temporary tangible storage media. The disks and optical disks used herein include compact disks (CDs), laser optical disks, optical disks, digital versatile disks (DVDs), floppy disks and blue-ray disks, wherein the disks usually reproduce data magnetically, and the optical disks usually reproduce data optically using lasers. The above combination is also included in the scope of computer-readable media.

[0443] Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general-purpose microprocessors, application-specific integrated circuits (ASICs), field-programmable gate arrays (FPGAs), or other equivalent integrated or discrete logic circuits. Accordingly, the term "processor" as used herein may refer to any of the foregoing structures or any other structure suitable for implementing the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and / or software modules configured for encoding and decoding, or incorporated in a combined codec. Similarly, the techniques may be fully implemented in one or more circuits or logic elements.

[0444] The techniques of the present disclosure may be implemented in a variety of devices or apparatuses including wireless handsets, integrated circuits (ICs), or a set of ICs (e.g., chipsets). Various components, modules, or units are described in the present disclosure to emphasize the functional aspects of devices configured to perform the disclosed techniques, but do not necessarily need to be implemented by different hardware units. Instead, as described above, the various units may be combined in a codec hardware unit, or provided by a collection of interoperable hardware units, including one or more processors as described above in combination with appropriate software and / or firmware.

[0445] Various examples have been described. These and other examples are within the scope of the following claims.

Claims

1. A method for transforming a video data block, the method comprising: coding a value of a low frequency non-separable transform (LFNST) index (LFNST_idx) syntax element for a current block of video data, the value of the LFNST_idx syntax element being non-zero and indicating a secondary transform for the current block of video data; determining a multiple transform select (MTS) scheme for a current block of video data indicating a discrete cosine transform 2 (DCT-2) primary transform for the current block of video data; as well as The DCT-2 primary transform and the secondary transform are applied during coding of residual data for a current block of video data.

2. The method according to claim 1, wherein: Determining that the MTS scheme for the current block of video data indicates a DCT-2 primary transform for the current block of video data includes determining that an MTS_idx syntax element has a value of zero.

3. The method according to claim 2, wherein: Coding the value of the LFNST_idx syntax element includes: performing context adaptive binary arithmetic coding (CABAC) coding on the value of the LFNST_idx syntax element, including: determining a context for CABAC coding the value of the LFNST_idx syntax element without using the value of the MTS_idx syntax element.

4. The method according to claim 1, in, Decoding the value of the LFNST_idx syntax element includes: decoding the value of the LFNST_idx syntax element; and Wherein, applying the DCT-2 primary transform and the secondary transform comprises: applying the secondary transform to decoded transform coefficients to produce intermediate transform coefficients; and The DCT-2 primary transform is applied to the intermediate transform coefficients to produce a residual block for the current block of video data.

5. The method according to claim 1, in, Decoding the value of the LFNST_idx syntax element includes: encoding the value of the LFNST_idx syntax element; and Wherein, applying the DCT-2 primary transform and the secondary transform comprises: applying the DCT-2 primary transform to a residual block of a current block of video data to generate intermediate transform coefficients; and The secondary transform is applied to the intermediate transform coefficients.

6. A device for transforming a video data block, the device comprising: a memory configured to store video data; as well as One or more processors implemented in circuitry and configured to: coding a value of a low frequency non-separable transform (LFNST) index (LFNST_idx) syntax element for a current block of video data, the value of the LFNST_idx syntax element being non-zero and indicating a secondary transform for the current block of video data; determining a multiple transform select (MTS) scheme for a current block of video data indicating a discrete cosine transform 2 (DCT-2) primary transform for the current block of video data; as well as The DCT-2 primary transform and the secondary transform are applied during coding of residual data for a current block of video data.

7. The device according to claim 6, wherein: To determine that the MTS scheme for the current block of video data indicates a DCT-2 primary transform for the current block of video data, the one or more processors are configured to determine that the MTS_idx syntax element has a value of zero.

8. The device according to claim 7, wherein: To code the value of the LFNST_idx syntax element, the one or more processors are configured to perform context-adaptive binary arithmetic coding (CABAC) coding on the value of the LFNST_idx syntax element, and wherein the one or more processors are configured to determine a context for CABAC coding the value of the LFNST_idx syntax element without using the value of the MTS_idx syntax element.

9. The device according to claim 6, in, To decode the value of the LFNST_idx syntax element, the one or more processors are configured to decode the value of the LFNST_idx syntax element; and Wherein, in order to apply the DCT-2 primary transform and the secondary transform, the one or more processors are configured to: applying the secondary transform to decoded transform coefficients to produce intermediate transform coefficients; and The DCT-2 primary transform is applied to the intermediate transform coefficients to produce a residual block for the current block of video data.

10. The device according to claim 6, further comprising: A display configured to display the decoded video data.

11. The device according to claim 6, wherein: The device includes one or more of a camera, a computer, a mobile device, a broadcast receiver, or a set-top box.

12. A non-transitory computer-readable storage medium having stored thereon instructions that, when executed, cause a processor to: coding a value of a low frequency non-separable transform (LFNST) index (LFNST_idx) syntax element for a current block of video data, the value of the LFNST_idx syntax element being non-zero and indicating a secondary transform for the current block of video data; determining a multiple transform select (MTS) scheme for a current block of video data indicating a discrete cosine transform 2 (DCT-2) primary transform for the current block of video data; as well as The DCT-2 primary transform and the secondary transform are applied during coding of residual data for a current block of video data.

13. The non-transitory computer-readable storage medium of claim 12, wherein: The instructions that cause the processor to determine that the MTS scheme for the current block of video data indicates a DCT-2 primary transform for the current block of video data include instructions that cause the processor to determine that an MTS_idx syntax element has a value of zero.

14. The non-transitory computer-readable storage medium of claim 13, wherein: The instructions that cause the processor to decode the value of the LFNST_idx syntax element include instructions that cause the processor to perform the following operations: context-adaptive binary arithmetic coding (CABAC) decoding of the value of the LFNST_idx syntax element, including instructions that cause the processor to perform the following operations: determine a context for CABAC decoding of the value of the LFNST_idx syntax element without using the value of the MTS_idx syntax element.

15. The non-transitory computer-readable storage medium of claim 12, in, The instructions for causing the processor to decode the value of the LFNST_idx syntax element include instructions for causing the processor to: decode the value of the LFNST_idx syntax element; and in, The instructions for causing the processor to apply the DCT-2 primary transform and the secondary transform include instructions for causing the processor to: applying the secondary transform to decoded transform coefficients to produce intermediate transform coefficients; and The DCT-2 primary transform is applied to the intermediate transform coefficients to produce a residual block for the current block of video data.

16. The non-transitory computer-readable storage medium of claim 12, in, The instructions for causing the processor to decode the value of the LFNST_idx syntax element include instructions for causing the processor to: encode the value of the LFNST_idx syntax element; and The instructions for causing the processor to apply the DCT-2 primary transform and the secondary transform include instructions for causing the processor to perform the following operations: applying the DCT-2 primary transform to a residual block of a current block of video data to generate intermediate transform coefficients; and The secondary transform is applied to the intermediate transform coefficients.

17. A device for transforming a video data block, the device comprising: means for coding a value of a low frequency non-separable transform (LFNST) index (LFNST_idx) syntax element for a current block of video data, the value of the LFNST_idx syntax element being non-zero and indicating a secondary transform for the current block of video data; a multiple transform selection (MTS) scheme for determining a unit of a discrete cosine transform 2 (DCT-2) primary transform for a current block of video data; as well as Means for applying the DCT-2 primary transform and the secondary transform during coding of residual data for a current block of video data.

18. The apparatus according to claim 17, wherein: Means for determining an MTS scheme for a current block of video data indicates a DCT-2 primary transform for the current block of video data includes means for determining that an MTS_idx syntax element has a value of zero.

19. The device according to claim 17, in, The unit for decoding the value of the LFNST_idx syntax element includes: a unit for decoding the value of the LFNST_idx syntax element; and Wherein, the unit for applying the DCT-2 primary transform and the secondary transform comprises: means for applying the secondary transform to decoded transform coefficients to produce intermediate transform coefficients; and Means for applying the DCT-2 primary transform to the intermediate transform coefficients to generate a residual block for a current block of video data.

20. The apparatus according to claim 17, in, The unit for decoding the value of the LFNST_idx syntax element includes: a unit for encoding the value of the LFNST_idx syntax element; and Wherein, the unit for applying the DCT-2 primary transform and the secondary transform comprises: means for applying said DCT-2 primary transform to a residual block of a current block of video data to generate intermediate transform coefficients; and Means for applying the secondary transform to the intermediate transform coefficients.

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