Coefficient coding for transform skip mode in video coding

Techniques for coding coefficients in a residual block are described. A video coder (e.g., video encoder or video decoder) may code (e.g., encode or decode), in an interleaving manner, coefficient information on a coefficient-by-coefficient basis for coefficients in a residual block of a current block of the video data in a first pass, wherein the coefficient information for a coefficient includes one or more of a significance flag indicating whether a value of the coefficient is not zero, a parity flag indicating whether the value of the coefficient is odd or even, a sign flag indicating whether the value of the coefficient is positive or negative, and one or more greater than flags indicating whether an absolute value of the coefficient is greater than respective threshold values, and after the first pass, code remainder information for coefficients in the residual block of the current block in a second pass.

TECHNICAL FIELD

BACKGROUND

SUMMARY

In general, this disclosure describes techniques for coefficient coding such as in examples of transform skip mode. In transform skip mode, rather than performing transformation of residual data from one domain to another (e.g., sample domain to transformed domain), the transformation is skipped. The residual data may be the difference between a current block and a prediction block. In such cases, the coefficient values may be the values of the residual data (e.g., values of a residual block), possibly with quantization.

The example techniques described in this disclosure relate to techniques of coding (e.g., encoding or decoding) the coefficient when transformation of the residual data is skipped (e.g., transform skip mode). In some examples, a video coder codes the coefficient information for the coefficients using level values and sign information. Sign information indicates whether the coefficient value is positive or negative. Level values, in combination with parity values, indicate whether the coefficient value is greater than 0, 1, 2, etc., as a few non-limiting examples.

In some examples, in transform skip mode, the level values and the sign information of neighboring coefficients may be more correlated than in examples where transform is applied. This disclosure describes example techniques that may utilize the correlation between neighboring coefficients as a way to increase coding efficiency. In this manner, the example techniques provide a practical application to increase coding efficiency in the video coding technology.

This disclosure also describes examples for interleaved coefficient coding. In the interleaved coefficient coding, level values are coded (e.g., encoded or decoded) on a coefficient-by-coefficient basis in the same pass as other coefficient information such as sign information and parity information. Then, in a second pass, the remainder values are coded coefficient-by-coefficient. With such interleaved coefficient coding, the example techniques may promote better use of coding techniques, as described in more detail. In this way, the example techniques provide a technical solution to technical problems in video coding, such as by providing a practical application for coefficient coding in the video coding process.

In one example, the disclosure describes a method of coding video data, the method comprising coding, in an interleaving manner, coefficient information on a coefficient-by-coefficient basis for coefficients in a residual block of a current block of the video data in a first pass, wherein the coefficient information for a coefficient includes one or more of a significance flag indicating whether a value of the coefficient is not zero, a parity flag indicating whether the value of the coefficient is odd or even, a sign flag indicating whether the value of the coefficient is positive or negative, and one or more greater than flags indicating whether an absolute value of the coefficient is greater than respective threshold values, and after the first pass, coding remainder information for coefficients in the residual block of the current block in a second pass.

In one example, the disclosure describes a device for coding video data, the device comprising a memory configured to store video data and processing circuitry configured to code, in an interleaving manner, coefficient information on a coefficient-by-coefficient basis for coefficients in a residual block of a current block of the video data in a first pass, wherein the coefficient information for a coefficient includes one or more of a significance flag indicating whether a value of the coefficient is not zero, a parity flag indicating whether the value of the coefficient is odd or even, a sign flag indicating whether the value of the coefficient is positive or negative, and one or more greater than flags indicating whether an absolute value of the coefficient is greater than respective threshold values, and after the first pass, code remainder information for coefficients in the residual block of the current block in a second pass.

In one example, the disclosure describes a computer-readable storage medium having stored thereon instructions that, when executed, cause one or more processors to code, in an interleaving manner, coefficient information on a coefficient-by-coefficient basis for coefficients in a residual block of a current block of the video data in a first pass, wherein the coefficient information for a coefficient includes one or more of a significance flag indicating whether a value of the coefficient is not zero, a parity flag indicating whether the value of the coefficient is odd or even, a sign flag indicating whether the value of the coefficient is positive or negative, and one or more greater than flags indicating whether an absolute value of the coefficient is greater than respective threshold values, and after the first pass, code remainder information for coefficients in the residual block of the current block in a second pass.

In one example, the disclosure describes a device for coding video data, the device comprising means for coding, in an interleaving manner, coefficient information on a coefficient-by-coefficient basis for coefficients in a residual block of a current block of the video data in a first pass, wherein the coefficient information for a coefficient includes one or more of a significance flag indicating whether a value of the coefficient is not zero, a parity flag indicating whether the value of the coefficient is odd or even, a sign flag indicating whether the value of the coefficient is positive or negative, and one or more greater than flags indicating whether an absolute value of the coefficient is greater than respective threshold values, and means for coding remainder information for coefficients in the residual block of the current block in a second pass, after the first pass.

DETAILED DESCRIPTION

In video coding, a video encoder determines a prediction block for a current block. In some examples, the prediction block may include samples or interpolated samples from a reference picture, such as in inter-prediction. In some examples, the prediction block may include samples generated from samples in the same picture as the current block, such as in intra-prediction. Intra-block copy may be similar to inter-prediction, but the prediction block is from the same picture as the current block. The video encoder determines a difference between the prediction block and the current block to generate a residual block of the current block.

In some techniques, the video encoder performs a transform on the residual block. For example, the samples in the residual block may be considered to be in the pixel domain or sample domain, and the video encoder transforms the samples in the pixel domain to a frequency or transform domain. The result of the transform is a block of coefficients that may be quantized and then encoded.

In some examples, the video encoder skips the transform step (i.e., in transform-skip mode). In the transform-skip mode, the coefficients may be the same as the samples of the residual block. The coefficients may then be quantized and then encoded.

The example techniques described in this disclosure may relate to ways in which the coefficients are processed for encoding. The term “coefficients” may refer to coefficients for a residual block of the current block where transform is skipped on the residual block or where transform is performed on the residual block. For ease of illustration, the example techniques are described with respect to transform-skip mode. However, the techniques may be applicable to examples where transform is performed.

For the video encoder to signal information for the coefficients, the video encoder may be configured to encode coefficient information that a video decoder receives to determine the value of the coefficient. Examples of the coefficient information include a significance flag (e.g., a flag that indicates whether a value of a coefficient value is not zero), a parity flag (e.g., a flag that indicates whether the value of the coefficient is odd or even), a sign flag (e.g., a flag that indicates whether the value of the coefficient is positive or negative), and one or more greater than flags, that in combination with the parity flag, indicate whether an absolute value of the coefficient is greater than respective threshold values.

The “greater than” flags are referred to as “gt” flags and are each associated with a threshold value. The gt flags by themselves may not be sufficient to indicate whether the value of a coefficient is greater than their respective threshold value. As an example, if the gt2 (e.g., greater than 2) flag is 0, it does not necessarily follow that the value of the coefficient cannot be 3. Rather, the combination of the parity flag and the gt2 flag together may indicate the value.

For example, if the value of the coefficient is 3, then the parity flag is 1, to indicate an odd value, and the gt1 flag is 1 to indicate that the value is greater than the one, but the gt2 flag may be 0. In this example, the video decoder may determine that the value is greater than 1 because the gt1 flag is 1 and determine that the value is odd because the parity flag is 1. Therefore, because the value is greater than 1 and odd, the value cannot be 2. Accordingly, the value can be 3, 5, 7 . . . etc. Also, the gt2 flag is 0, and therefore, based on the combination of gt2 flag being 0 and the coefficient being odd, the video decoder may determine that the value of the coefficient cannot be greater than 3 and is 3. If the coefficient value were 5 instead of 3, then the g2 flag would have been1.

Using the level flags and parity flag, it may be possible to represent values up to 9, assuming that the last greater than flag is the greater than 5 (e.g., gt5) flag. For values greater than 9, the video encoder may signal a remainder value. The remainder value is the remaining absolute value of a coefficient.

In this disclosure, the sig_coeff_flag refers to the significance flag that indicates whether the value of a coefficient is not zero. The par_level_flag refers to the parity flag that indicates whether the value of a coefficient is odd or even. The coeff_sign_flag refers to the sign flag that indicates whether the value of the coefficient is positive or negative. The abs_level_gtX_flags refer to the flags that indicates whether the value of a coefficient value is greater than a particular threshold. For example, abs_level_gtx_flag[n][j] specifies whether the absolute value of the coefficient level (at scanning position n) is greater than (j<<1)+1. When abs_level_gtx_flag[n][j] is not present, it is inferred to be equal to 0. The gtx (or greater than) flags may be referred to as flags for level values. However, the gtx flag by itself may not specify whether a value is greater than a threshold For example, gt2 value of 0 does not necessarily mean that the value of the coefficient cannot be 3.

The abs_remainder refers to the information signaled for the remainder. For example, abs_remainder[n] is the remaining absolute value of a coefficient level that may be coded with Golomb-Rice code at the scanning position n. When abs_remainder[n] is not present, it is inferred to be equal to 0.

The video decoder receives the various coefficient information (e.g., the sig_coeff_flag, par_level_flag, coeff_sign_flag and all the abs_level_gtX_flags and the abs_remainder) and determines the coefficient value for a coefficient based on the coefficient information. The video decoder may then perform inverse-quantization (if needed) and inverse-transform (if needed) to generate the sample values of the residual block. In one or more examples, the video decoder may skip the inverse-transform since the coefficients may be generated by the video encoder in transform-skip mode.

The video decoder may determine a prediction block using the same techniques as the video encoder. For example, the video encoder may signal information to the video decoder that allows the video decoder to determine the same prediction block as the video encoder. The video decoder then adds the prediction block and residual block together to reconstruct the current block.

In some techniques, the video encoder may signal the coefficient information in a bitplane-by-bitplane basis. For example, the video encoder may encode the sig_coeff_flag (e.g., flag of whether the value of the coefficient is non zero) for each coefficient in the block of coefficients, forming a bitplane of sig_coeff_flag values. Then, the video encoder may encode the par_level_flag (e.g., flag of whether the value of the coefficient is even or odd) for each coefficient in the block of coefficients, forming a bitplane of par_level_flag values, and so forth.

This disclosure describes example techniques in which the video encoder encodes a plurality of the coefficient information on a coefficient-by-coefficient basis in a first pass through the coefficients. Then, in subsequent passes, the video encoder may encode any remaining coefficient information in a coefficient-by-coefficient basis or bitplane-by-bitplane basis.

In the coefficient-by-coefficient basis of encoding coefficient information, the video encoder may interleave different coefficient information. For example, the video encoder may encode the sig_coeff_flag, par_level_flag, coeff_sign_flag and all the abs_level_gtX_flags for the first coefficient. Then, the video encoder may encode the sig_coeff_flag, par_level_flag, coeff_sign_flag and all the abs_level_gtX_flags for the second coefficient, and so forth, in a first pass through the coefficients of the residual block of the current block. In one or more examples, the video encoder may then, in a second pass through the coefficients, encode the abs_remainder values in a coefficient-by-coefficient basis. In the bitplane-by-bitplane basis of encoding coefficient information, the video encoder would have encoded the sig_coeff_flag for the first coefficient, then the sig_coeff_flag for the second coefficient, followed by the par_level_flag for the first coefficient, then the par_level_flag for the second coefficient, and so forth.

Interleaved coefficient coding (e.g., encoding plurality of the coefficient information on a coefficient-by-coefficient basis) rather than encoding the coefficient information on a bitplane-by-bitplane basis may be beneficial such as in transform-skip mode. For example, when coding coefficient information, context-based coding may be preferred. However, there may be a limit to how many bins can be context-based coded (e.g., referred to as a coded bin count limit). One example of the coded bin count limit is 2*block width of the current block*block height of the current block. Another example of the coded bin count limit is 1.75*block width of the current block*block height of the current block. In some cases, the video encoder may need to determine on a coefficient-by-coefficient basis how many bins are going to be used and how many bins are going to be available after coding the coefficient information for a coefficient. How many bins are going to be used and how many bins are going to be available after coding the coefficient information for a coefficient may be needed for determining quantization parameters, such as in the rate distortion optimization quantization (RDOQ) process.

By interleaving the coefficient information that uses context-based coding (e.g., significance information, sign information, parity information, and greater than flags) on a coefficient-by-coefficient basis, the video encoder may be able to better track how many bins are going to be used for a coefficient than in examples where the coefficient information that uses context-based coding is coded in bitplane-by-bitplane basis. For instance, in a bitplane-by-bitplane basis, the video encoder does not know how many bins are needed for one coefficient until the video encoder passes through all coefficients.

In one or more examples described in this disclosure, after the first pass of coefficient information that is context-based coded, the remainder information is coded. The remainder information may not be context-based coded (e.g., bypass coded). Because the remainder information is bypass coded, the remainder information may not impact the number of bins needed for context-based coding, and therefore, can be separated out from the context-based coding of context information such as significance information, sign information, parity information, and level values.

Accordingly, in one or more examples, the example techniques described in this disclosure may improve the coefficient coding process. The one or more techniques described in this disclosure provide for a practical application to coding coefficient information that may improve the overall video coding process for instance by separating out coefficient information that needs context-based coding from coefficient information that can be bypass coded.

In some examples, source device102may output encoded video data to file server114or another intermediate storage device that may store the encoded video generated by source device102. Destination device116may access stored video data from file server114via streaming or download. File server114may be any type of server device capable of storing encoded video data and transmitting that encoded video data to the destination device116. File server114may represent a web server (e.g., for a website), a File Transfer Protocol (FTP) server, a content delivery network device, or a network attached storage (NAS) device. Destination device116may access encoded video data from file server114through any standard data connection, including an Internet connection. This may include a wireless channel (e.g., a Wi-Fi connection), a wired connection (e.g., digital subscriber line (DSL), cable modem, etc.), or a combination of both that is suitable for accessing encoded video data stored on file server114. File server114and input interface122may be configured to operate according to a streaming transmission protocol, a download transmission protocol, or a combination thereof.

Although not shown inFIG. 1, in some examples, video encoder200and video decoder300may each be integrated with an audio encoder and/or audio decoder, and may include appropriate MUX-DEMUX units, or other hardware and/or software, to handle multiplexed streams including both audio and video in a common data stream. If applicable, MUX-DEMUX units may conform to the ITU H.223 multiplexer protocol, or other protocols such as the user datagram protocol (UDP).

Video encoder200and video decoder300may operate according to a video coding standard, such as ITU-T H.265, also referred to as High Efficiency Video Coding (HEVC) or extensions thereto, such as the multi-view and/or scalable video coding extensions. Alternatively, video encoder200and video decoder300may operate according to other proprietary or industry standards, such as ITU-T H.266, also referred to as Versatile Video Coding (VVC). A draft of the VVC standard is described in Bross, et al. “Versatile Video Coding (Draft 5),” Joint Video Experts Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 14 Meeting: Geneva, CH, 19-27 Mar. 2019, JVET-N1001-v7 (hereinafter “VVC Draft 5”). A recent draft of the VVC standard is described in Bross, et al. “Versatile Video Coding (Draft 9),” Joint Video Experts Team (JVET) of ITU-T SG 16 WP 3 and ISO/IEC JTC 1/SC 29/WG 11, 18thMeeting: by teleconference, 15-24 Apr. 2020, JVET-R2001-v8 (hereinafter “VVC Draft 9”). The techniques of this disclosure, however, are not limited to any particular coding standard.

In an MTT partitioning structure, blocks may be partitioned using a quadtree (QT) partition, a binary tree (BT) partition, and one or more types of triple tree (TT) partitions. A triple tree partition is a partition where a block is split into three sub-blocks. In some examples, a triple tree partition divides a block into three sub-blocks without dividing the original block through the center. The partitioning types in MTT (e.g., QT, BT, and TT), may be symmetrical or asymmetrical.

Transforming the residual block to produce transformed data in the transform domain instead of the sample domain is not necessary in all examples. In some examples, transform may be skipped (e.g., in transform-skip mode for the current block). In such examples, video encoder200may perform further operations on the residual values (e.g., residual data) of the residual block. For example, the transform data (e.g., where transformation from sample domain to transform domain occurs) may result in transform coefficients, and video encoder may perform operations on the transform coefficients.

When transform is skipped, video encoder200may perform operations on the residual values. For example, in examples where transform is skipped, the coefficients may correspond to the residual data (e.g., difference between samples of current block and prediction block). For example, where transform is skipped a value for a first coefficient may be the first residual value in the residual block, a value for a second coefficient may be the second residual value in the residual block, and so on. In the following description, where transform coefficients are described, rather than using transform coefficients, the techniques may utilize coefficient values where transform is skipped. In other words, the example techniques described for transform coefficients may also be applied to coefficient values where transform is skipped.

As noted above, following any transforms to produce transform coefficients or where transform is skipped, video encoder200may perform quantization of the coefficients. Quantization generally refers to a process in which coefficients are quantized to possibly reduce the amount of data used to represent the coefficients, providing further compression. By performing the quantization process, video encoder200may reduce the bit depth associated with some or all of the coefficients. For example, video encoder200may round an n-bit value down to an m-bit value during quantization, where n is greater than m. In some examples, to perform quantization, video encoder200may perform a bitwise right-shift of the value to be quantized.

In one or more examples, video encoder200may skip quantization. For instance, in some examples, where transform skip is enabled, it may be possible for video encoder200to skip quantization. In the below description, although quantization is described as occurring, it should be understood that in some examples, quantization may be also be skipped.

Following quantization, video encoder200may scan the coefficients, producing a one-dimensional vector from the two-dimensional matrix including the quantized coefficients. The scan may be designed to place higher energy (and therefore lower frequency) coefficients at the front of the vector and to place lower energy (and therefore higher frequency) coefficients at the back of the vector. However, in examples where transform is skipped, the scan may not place higher energy coefficients at the front of the vector and place lower energy coefficients at the back of the vector.

In some examples, video encoder200may utilize a predefined scan order to scan the coefficients to produce a serialized vector, and then encode the coefficients of the vector. In other examples, video encoder200may perform an adaptive scan. After scanning the coefficients to form the one-dimensional vector, video encoder200may encode the one-dimensional vector, e.g., according to context-adaptive binary arithmetic coding (CABAC) (e.g., context-based coding) and/or bypass coding (e.g., not context-based coding). Video encoder200may also entropy encode values for syntax elements describing metadata associated with the encoded video data for use by video decoder300in decoding the video data.

The residual information may be represented by, for example, quantized transform coefficients (or quantized coefficients where transform is skipped or coefficients where transform and quantization is skipped). For ease of description, in this disclosure, the term coefficient may include examples where quantization is skipped, transform is skipped, quantization and transform are skipped, quantization is skipped but transform is performed, or quantization is performed but transform is skipped. In one or more examples, in the techniques described in this disclosure, the examples may be performed with coefficients where transform is skipped but quantization may or may not be skipped.

Video decoder300may inverse quantize (if needed) and inverse transform (if needed) the quantized transform coefficients of a block to reproduce a residual block for the block. Video decoder300uses a signaled prediction mode (intra- or inter-prediction) and related prediction information (e.g., motion information for inter-prediction) to form a prediction block for the block. Video decoder300may then combine (e.g., add) the prediction block and the residual block (on a sample-by-sample basis) to reproduce (e.g., reconstruct) the original block. Video decoder300may perform additional processing, such as performing a deblocking process to reduce visual artifacts along boundaries of the block.

In accordance with the techniques of this disclosure, a video coder (e.g., video encoder200or video decoder300) may be configured to perform operations in examples where transform skip is enabled and code (e.g., encode or decode) coefficient values. For example, a video coder may determine that transform skip is enabled and code a coefficient value for a coefficient in a residual block based on one or more coefficient values of one or more neighboring coefficients. In some examples, the video coder may code, in an interleaving manner, one or more syntax elements on a coefficient-by-coefficient basis for coefficients in a residual block in a first pass and after the first pass, coding a syntax element on the coefficient-by-coefficient basis for coefficients in the residual block in a second pass.

In one or more examples, the video coder may code, in an interleaving manner, coefficient information on a coefficient-by-coefficient basis for coefficients in a residual block of a current block in a first pass. The coefficient information for a coefficient includes a significance flag indicating whether a value of the coefficient is not zero, a parity flag indicating whether the value of the coefficient is odd or even, a sign flag indicating whether the value of the coefficient is positive or negative, and one or more greater than flags indicating whether an absolute value of the coefficient is greater than respective threshold values.

Table 1 below illustrates an example of coefficient values and the respective flags that are signaled for each of the coefficients. The boxes labeled as NA are for the flags that are not signaled for corresponding coefficient values. In table 1, Rem stands for the remainder, and is not signaled as a flag, but as a value. Also, in table 1, the values are positive, which is why the sign value is 0.

To code, in an interleaving manner, the coefficient information (e.g., a significance flag indicating whether a value of the coefficient is not zero, a parity flag indicating whether the value of the coefficient is odd or even, a sign flag indicating whether the value of the coefficient is positive or negative, and one or more greater than flags), video encoder200may signal such coefficient information for a first coefficient. Then, video encoder200may signal such coefficient information for a second coefficient, and so forth. In contrast, in a bitplane-by-bitplane (e.g., not interleaved manner), video encoder200may signal the significance flag for each coefficient in the residual block of the current block, then signal the parity flag for each coefficient in the residual block of the current block, and so forth.

However, in one or more examples described in this disclosure, video encoder200may signal, in an interleaving manner, the coefficient information, in a first pass. After the first pass, the video coder may code remainder information for coefficients in the residual block of the current block in a second pass (e.g., on the coefficient-by-coefficient basis, but not limited to coefficient-by-coefficient basis).

To code, in the interleaving manner, video decoder300may parse the coefficient information on a coefficient-by-coefficient basis for coefficients in the residual block of the current block in the first pass. For example, video decoder300may parse, in the first pass, a significance flag indicating whether a value of the coefficient is not zero, a parity flag indicating whether the value of the coefficient is odd or even, a sign flag indicating whether the value of the coefficient is positive or negative, and one or more greater than flags. Video decoder300may parse this example coefficient information for one coefficient, then parse this example coefficient information for the next coefficient, and so forth for parsing, in an interleaving manner, the coefficient information on the coefficient-by-coefficient basis for coefficients in a residual block for a current in a first pass. After the first pass, to code remainder information, video decoder300may parse remainder information for coefficients in the residual block of the current block in the second pass.

Not all of the example coefficient information may be signaled and parsed in all examples. For example, if the coefficient value is 0, then video encoder200may signal and video decoder300may parse the significance flag as 0 (e.g., indicating that value of the coefficient is zero). In this example, video encoder200may not signal and video decoder300may not receive parity or sign information or any of the greater than flags. As another example, if the coefficient value is 1.8, then video encoder200may signal and video decoder300may parse the significance flag, the parity flag, the sign flag, and the greater than 1 flag, but video encoder200may not signal and video decoder300may not parse any of the other greater than flags.

There may be benefits in coding, in an interleaving manner, the coefficient information in the first pass and coding remainder information in a second pass. In some cases, the coefficient information that is coded in interleaving manner may be coded using context-based coding and the remainder information may be coded using bypass coding. Context-based coding refers to examples where contexts are used to determine a probability that a bin will be a 0 or 1, and using the probabilities to encode or decode values. Example of context-based coding includes CABAC. Bypass coding refers to examples where contexts are not used to determine a probability (or the probability is assumed to be 0.5).

In context-based coding, there may be a coded bin count limit that sets a maximum number of bins in a residual block that can be context-based coded. One example of the coded bin count limit is 2*block width of the current block*block height of the current block or 1.75*block width of the current block*block height of the current block. However, there may be other examples of the coded bin count limit.

For rate distortion optimization quantization (RDOQ), video encoder200may need to determine how many bins for a current coefficient are to be context-based coded and how many bins are going to be left after the current coefficient (e.g., a running total of bins used for context-based coding minus the coded bin count limit). For RDOQ, video encoder200may determine, on a coefficient-by-coefficient basis, how many bins for a current coefficient are to be context-based coded and how many bins are going to be left after the current coefficient. With coding, in the interleaving manner, video encoder200may be able to determine for a current coefficient how many bins the current coefficient will use for context-based coding before video encoder200begins to determine how many bins the next coefficient will use for context-based coding. With bitplane-by-bitplane processing, video encoder200may not be able to determine how many bins a current coefficient will use until video encoder200processes all coefficients, which can negatively impact how quickly video encoder200can encode and signal information needed to reconstruct the current block.

In one or more examples, examples of coefficient information that is context-based coded include a significance flag indicating whether a value of the coefficient is not zero, a parity flag indicating whether the value of the coefficient is odd or even, a sign flag indicating whether the value of the coefficient is positive or negative, and one or more greater than flags indicating whether an absolute value of the coefficient is greater than respective threshold values (e.g., abs_level_gtx_flag[n][j] specifies whether the absolute value of a coefficient level is greater than (J<<1)+1). The remainder information may be coded in bypass mode. Therefore, the remainder information can be coded in a second pass after the first pass that includes the coefficient information that is context-based coded because the coding of the remainder information does not impact whether the coded bin count limit is reached or not (e.g., because reminder information is bypass coded).

As described above, there may be a context bin count limit. Accordingly, in some examples, to code, in the interleaving manner, the video coder may context-based code, in the interleaving manner, coefficient information on the coefficient-by-coefficient basis until the coded bin count limit is reached, and bypass code, in the interleaving manner, coefficient information on the coefficient-by-coefficient basis after the coded bin count limit is reached. For example, if during coding of a coefficient, video encoder200and video decoder300reaches the coded bin count limit after coding the significance flag, then video encoder200and video decoder300may bypass code the remaining coefficient information for the coefficient and for subsequent coefficients.

Also, to code remainder information, the video coder may be configured to coding information indicative of a remaining absolute value of a coefficient. However, in some examples, if the video coder reaches the coded bin count limit during the coding of the particular coefficient, the video coder may code respective values of coefficients following the particular coefficient, as part of the second pass. For example, rather than coding greater than flags for coefficients following the particular flag, the video coder may code remainder information for the coefficients following the particular flag, where the remainder information is the actual value of the coefficient or the absolute value of the coefficient minus 1.

FIGS. 2A and 2Bare conceptual diagram illustrating an example quadtree binary tree (QTBT) structure130, and a corresponding coding tree unit (CTU)132. The solid lines represent quadtree splitting, and dotted lines indicate binary tree splitting. In each split (i.e., non-leaf) node of the binary tree, one flag is signaled to indicate which splitting type (i.e., horizontal or vertical) is used, where 0 indicates horizontal splitting and 1 indicates vertical splitting in this example. For the quadtree splitting, there is no need to indicate the splitting type, since quadtree nodes split a block horizontally and vertically into 4 sub-blocks with equal size. Accordingly, video encoder200may encode, and video decoder300may decode, syntax elements (such as splitting information) for a region tree level of QTBT structure130(i.e., the solid lines) and syntax elements (such as splitting information) for a prediction tree level of QTBT structure130(i.e., the dashed lines). Video encoder200may encode, and video decoder300may decode, video data, such as prediction and transform data, for CUs represented by terminal leaf nodes of QTBT structure130.

In general, CTU132ofFIG. 2Bmay be associated with parameters defining sizes of blocks corresponding to nodes of QTBT structure130at the first and second levels. These parameters may include a CTU size (representing a size of CTU132in samples), a minimum quadtree size (MinQTSize, representing a minimum allowed quadtree leaf node size), a maximum binary tree size (MaxBTSize, representing a maximum allowed binary tree root node size), a maximum binary tree depth (MaxBTDepth, representing a maximum allowed binary tree depth), and a minimum binary tree size (MinBTSize, representing the minimum allowed binary tree leaf node size).

The root node of a QTBT structure corresponding to a CTU may have four child nodes at the first level of the QTBT structure, each of which may be partitioned according to quadtree partitioning. That is, nodes of the first level are either leaf nodes (having no child nodes) or have four child nodes. The example of QTBT structure130represents such nodes as including the parent node and child nodes having solid lines for branches. If nodes of the first level are not larger than the maximum allowed binary tree root node size (MaxBTSize), then the nodes can be further partitioned by respective binary trees. The binary tree splitting of one node can be iterated until the nodes resulting from the split reach the minimum allowed binary tree leaf node size (MinBTSize) or the maximum allowed binary tree depth (MaxBTDepth). The example of QTBT structure130represents such nodes as having dashed lines for branches. The binary tree leaf node is referred to as a coding unit (CU), which is used for prediction (e.g., intra-picture or inter-picture prediction) and transform, without any further partitioning. As discussed above, CUs may also be referred to as “video blocks” or “blocks.”

In one example of the QTBT partitioning structure, the CTU size is set as 128×128 (luma samples and two corresponding 64×64 chroma samples), the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64, the MinBTSize (for both width and height) is set as 4, and the MaxBTDepth is set as 4. The quadtree partitioning is applied to the CTU first to generate quad-tree leaf nodes. The quadtree leaf nodes may have a size from 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size). If the leaf quadtree node is 128×128, it will not be further split by the binary tree, since the size exceeds the MaxBTSize (i.e., 64×64, in this example). Otherwise, the leaf quadtree node will be further partitioned by the binary tree. Therefore, the quadtree leaf node is also the root node for the binary tree and has the binary tree depth as 0. When the binary tree depth reaches MaxBTDepth (4, in this example), no further splitting is permitted. When the binary tree node has width equal to MinBTSize (4, in this example), it implies no further horizontal splitting is permitted. Similarly, a binary tree node having a height equal to MinBTSize implies no further vertical splitting is permitted for that binary tree node. As noted above, leaf nodes of the binary tree are referred to as CUs, and are further processed according to prediction and transform without further partitioning.

As described above, the example techniques described in this disclosure relate to coefficient coding for transform skip mode. For example, this disclosure describes examples of coefficient coding methods targeting at coding of transform skip mode. For instance, this disclosure is related to an entropy decoding process that converts a binary representation to a series of non-binary valued quantized coefficients. The corresponding entropy encoding process, which is the reverse process of entropy decoding, is part of this disclosure as well. For instance, the entropy encoding process may be performed as the reverse of the decoding process. The techniques described in this disclosure may be applied to any of the existing video codecs, such as High Efficiency Video Coding (HEVC), or be a coding tool in standards currently being developed, such as Versatile Video Coding (VVC), and applied to other future video coding standards.

The following describes correlation between transform skip (TS) coefficients. In transform-skip mode, transform process is skipped for residual signals before quantization step on the encoder side (e.g., video encoder200) and inverse transform step after the dequantization step on the decoder side (e.g., video decoder300). The characteristics of a not transformed residual signal are quite different than the characteristics of transformed signals.

The coefficients are more correlated with their neighboring coefficients in the transform skip case. As a result, the level values of neighboring coefficients (e.g., actual value of the coefficient) as well as the sign information of neighboring coefficients are more correlated. Coding of the levels (e.g., greater than flags) and the sign information in the transform skip (TS) coefficient coding was proposed in B. Bross, T. Nguyen, P. Keydel, H. Schwarz, D. Marpe, T. Wiegand, “Non-CE8: Unified Transform Type Signalling and Residual Coding for Transform Skip”, JVET document JVET-M0464, Marrackech, MA, January 2019 (herein JVET-M0464). This disclosure describes examples of exploiting the signal characteristics for more efficient coding.

The following describes coding of transform skip (TS) coefficients. In some techniques, transform skip residual coding, sig_coeff_flag, coeff_sign_flag, abs_level_gt1_flag, par_level_flag syntax elements are coded interleaved coefficient by coefficient in the first pass. Starting from second pass, abs_level_gtX_flags, (currently up to 5, and corresponding passes), syntax elements are coded bitplane-by-bitplane fashion. The following is one example of definitions of the syntax elements.

sig_coeff_flag[xC][yC] specifies for the transform coefficient location (xC, yC) within the current transform block whether the corresponding transform coefficient level at the location (xC, yC) is non-zero.

abs_level_gtX_flag[n] specifies whether the absolute value of the transform coefficient level (at scanning position n) is greater than X. Examples of abs_level_gtX_flag includes abs_level_gt1_flag, abs_level_gt2_flag, and so on. Another example of abs_level_gtX_flag is: abs_level_tx_flag[n][j] specifies whether the absolute value of the transform coefficient level (at scanning position n) is greater than (j<<1)+1. When abs_level_gtx_flag[n][j] is not present, it is inferred to be equal to 0.

par_level_flag[n] specifies the parity of the transform coefficient level (e.g., odd or even) at scanning position n.

coeff_sign_flag[n] specifies the sign of a transform coefficient level for the scanning position n.

In all passes, the syntax elements are coded as regular coded bins if the number of regular coded bin count limit is not reached. If during the encoding of the passes, the regular coded bin count is reached, the rest of the syntax element are bypass coded. In the last pass, abs_remainder parts of coefficients are coded using Rice codes. Rice codes are one example of bypass coding (e.g., where context-based coding is not used). One example definition of abs_remainder is abs_remainder[n] is the remaining absolute value of a transform coefficient level that is coded with Golomb-Rice code at the scanning position n. There is an upper limit for the number of regular coded bins that can be used in a TS block (e.g., a current block coded in transform-skip mode).

In one example, the disclosure describes level-mapping based coefficient coding. In transform skip residual coding of JVET-M0464, coefficient absolute levels absCoeffLevel are coded using sig_coeff_flag, abs_level_gtX_flags, par_level_flag, and abs_remainder value to form the final absolute transform coefficient value, where X can be 1, . . . ,5 (or some other cut off value C). In this example, the absCoeffLevel value may be constructed by: absCoeffLevel=1+abs_level_gt1_flag+par_level_flag+2*(abs_level_gt2_flag+abs_level_gt3_flag+ . . . +abs_level_gtC_flag)+2*abs_remainder.

In one or more examples described in this disclosure, instead of or in addition to representing the absCoeffLevel directly as in JVET-M0464, the absCoeffLevel is mapped to a modified level to be coded as described below on the encoder side (e.g., video encoder200) and inverse mapped on the decoder side (e.g., video decoder300) as described below.

The absCoeffLevel information of left and above coefficients are used to perform the mapping. In this case, let X0denote the absolute coefficient level to the left of the current coefficient, and let X1denote the absolute coefficient level of above coefficient. For representing a coefficient with absolute coefficient level absCoeff, a mapped absCoeffMod is coded which is derived as follows:

For Video Encoder200:

In the above pseudo-code, if min(X0,X1)==0, then max(X0, X1) is used as the pred; otherwise min(X0,X1) is used as the pred. If the absolute value of coefficient to be coded is equal to the predictor pred, then the modified level absCoeffMod is set to 1; otherwise if the absolute value of coefficient is less than the predictor, then the value to be coded is incremented by 1; otherwise the absCoeff value is not modified.

For Video Decoder300:

The following describes examples of interleaved coefficient coding. In some examples, video encoder200and/or video decoder300may convert all the coding of syntax elements up to coding of abs_remainder in an interleaved fashion instead of separating them into several bitplanes. The TS residual coding is changed such that the sig_coeff_flag, par_level_flag, coeff_sign_flag and all the abs_level_gtX_flags are coded in an interleaved way coefficient by coefficient in the first pass. After the first pass, abs_remainder is coded coefficient by coefficient. In some examples, when the regular coded bin count limit is reached, the rest of the syntax elements are coded in bypass mode.

For example, a video coder (e.g., video encoder200and video decoder300) may code, in an interleaving manner, coefficient information on a coefficient-by-coefficient basis for coefficients in a residual block of a current block in a first pass. The coefficient information for a coefficient includes a significance flag indicating whether a value of the coefficient is not zero (e.g., sig_coeff_flag), a parity flag indicating whether the value of the coefficient is odd or even (e.g., par_level_flag), a sign flag indicating whether the value of the coefficient is positive or negative (e.g., coeff_sign_flag), and one or more greater than flags (e.g., the abs_level_gtX_flags). The video coder may, after the first pass, code remainder information (e.g., abs_reminader) for coefficients in the residual block of the current block in a second pass.

In some examples, to code, in the interleaving manner, the coefficient information, the video coder may context-based code, in the interleaving manner, coefficient information on the coefficient-by-coefficient basis until a coded bin count limit is reached. The video coder may bypass code, in the interleaving manner, coefficient information on the coefficient-by-coefficient basis after the coded bin count limit is reached.

FIG. 3is a conceptual diagram illustrating an example of coding coefficient information for coefficients.FIG. 3illustrates coefficient 0 to coefficient N−1. In this example, in a first pass, a video coder may code the significance flag, sign flag, gt1 flag, parity flag, and then gt2-gt5 flags for coefficient 0 to coefficient N−1, in an interleaving manner. The gt1-gt5 flags are short for abs_level_gtX_flags, where X equals 1, 2, 3, 4, or 5. The gt1-gt5 or the abs_level_gtX_flags may be referred to as “greater than” flags as well.

In the first pass through the coefficients, the video coder may code the significance flag, sign flag, gt1 flag, parity flag, and gt2-gt5 flags for coefficient 0, then, still in the first pass through the coefficients, the video coder may code the significance flag, sign flag, gt1 flag, parity flag, and gt2-gt5 flags for coefficient 1, and so forth. For example, video encoder200may signal and video decoder300may parse, in a first pass through the coefficients, the significance flag, sign flag, gt1 flag, parity flag, and gt2-gt5 flags for coefficient 0, followed by the significance flag, sign flag, gt1 flag, parity flag, and gt2-gt5 flags for coefficient 1, as part of the first pass through the coefficients, through to coefficient N−1.

After the first pass, the video coder may code the remainder information for coefficient 0 to coefficient N−1. For example, video encoder200may signal and video decoder300may parse the remainder information for coefficient 0 to coefficient N−1 in the second pass. In the example illustrated inFIG. 3, video encoder200and video decoder300may not have reached the coded bin count limit when context-based coding the coefficient information in the first pass.

FIG. 4is a conceptual diagram illustrating another example of coding coefficient information for coefficients. In the example ofFIG. 4, the video coder may code all components of a coefficient (including the remainder) in an interleaved manner. For example, rather than coding coefficient information like a significance flag indicating whether a value of the coefficient is not zero (e.g., sig_coeff_flag), a parity flag indicating whether the value of the coefficient is odd or even (e.g., par_level_flag), a sign flag indicating whether the value of the coefficient is positive or negative (e.g., coeff_sign_flag), and one or more greater than flags (e.g., the abs_level_gtX_flags) in a first pass, and remainder information in a second pass, the video coder may code the coefficient information and the remainder information in one pass, i.e., in the same pass.

For example, as shown inFIG. 4, each coefficient may be split into sig_coeff_flag, coeff_sign_flag, abs_level_gt1_flag, par_level_flag, abs_level_gtX_flags(X=2, 3, 4, 5) and abs_remainder. All the syntax elements for one coefficient are coded before coding the next coefficient.

FIG. 5is a conceptual diagram illustrating another example of coding coefficient information for coefficients. In the example ofFIG. 5, the coding of sig_coeff_flag, coeff_sign_flag, abs_level_gt1_flag, par_level_flag, abs_level_gtX_flags is split into two passes. In the first pass, sig_coeff_flag, coeff_sign_flag, abs_level_gt1_flag are coded. In the second pass, abs_level__gtX_flags and par_level_flag are coded. The par_level_flag may be after all the abs_level_gtX_flags, and part of the second pass. In the third pass, the video coder may code the remainder information, as shown inFIG. 5.

FIG. 6is a conceptual diagram illustrating another example of coding coefficient information for coefficients. In the example ofFIG. 6, the syntax sig_coeff_flag, coeff_sign_flag, abs_level_gt1_flag, and abs_level_gt2_flag for all the coefficients in the coefficient group (e.g., residual block) are coded interleaved in the first pass. In the second pass, coefficient information: abs_level_gtX_flags (X=3, 4, 5) and par_level_flag are coded. In the second pass, the video coder may code par_level_flag after all the abs_level_gtX_flags. In the last pass (e.g., third pass), the video coder may code remainder information (e.g., abs_remainder) for all of the coefficients in the coefficient group.

This disclosure also describes examples of handling the bypass coding after the context bin limit (e.g., coded bin count limit) is reached. As one example, the coded bin count limit is 2*block width*block height. As another example, the coded bin count limit is 1.75*block width*block height. In some techniques, if the coded bin count limit is reached, video encoder200may separate the absolute value of a coefficient into abs_level__gt1_flag, par_level_flag, abs_level_gtX_flags(X=2, 3, 4, 5) and abs_remainder, and bypass code each of these coefficient information. However, in accordance with one or more examples described in this disclosure, rather than bypass code each one of abs_level_gt1_flag, par_level_flag, abs_level_gtX_flags(X=2, 3, 4, 5) and abs_remainder, video encoder200may encode respective values of the coefficients following a particular coefficient, where during or at completion of coding the particular coefficient, the coded bin count limit is reached. As one example, video encoder200may bypass encode the absolute value abs(coefficient value)−1 directly, rather than bypass code each one of abs_level_gt1_flag, par_level_flag, abs_level_gtX_flags(X=2, 3, 4, 5) and abs_remainder. Video decoder300may bypass decode the coefficient value (e.g., based on the bypass encoding of the coefficient value or the abs (coefficient value)−1)) for a coefficient following a particular coefficient for which the coded bin count limit is reached. For example, video encoder200and video decoder300may utilize Rice-Golomb coding of the abs_remainder to encode and decode the absolute value of coefficients after the coded bin count limit is reached, and after sig_coeff_flag and coeff_sign_flag are bypass coded.

Rice-Golomb coding is a scheme of binarization to convert a value to a series bins of 1 or 0. In some examples, it may be possible to use context based coding when Rice-Golomb coding is used. However, for the remainder, Rice-Golomb coding is used and the bins are bypass coded.

If the regular bin count (e.g., coded bin count limit) is reached before coding of gt1 flag, all remaining significance and sign flags are bypass coded and for significant coefficients after reaching the coded bin count limit, the remainder is coded as an absolute value of the coefficient or the absolute value of the coefficient−1 without splitting the coefficient into gt1, par, gt2, . . . , gt5 and corresponding remainder. Accordingly, in this example, for a coefficient at and after reaching the coded bin count limit (e.g., coefficients following a particular coefficient, where the coded bin count limit is reached during the coding of the particular coefficient), video encoder200and video decoder300may bypass code the significance and sign flags, and bypass code the remainder portion without utilizing gtX and parity flags. Example techniques of bypass coding include Rice-Golomb coding.

If the coded bin count limit is reached after coding of gt1 flag for a coefficient, then video encoder200and video decoder300may bypass code the remaining gtX and parity flags for that coefficient before switching to the coding of significance and sign flags as bypass and the remainder as Rice-Golomb code. The example techniques may be applied to various schemes changing the coding order of significance, sign, gt1, par, gtX flags where once the regular bin count (e.g., coded bin count limit) is reached, for the rest (and including the current) of the coefficients, only significance and sign flags are bypass coded and the remaining portion of the coefficient is represented by one Rice-Golomb coded value. Rice-Golomb coding is one example, and other types of binarization with bypass coding techniques may be used as well, such as unary coding. In this disclosure, the remainder information may refer to a remaining value after coding of the greater than flags.

In some examples, the parameter of Rice-Golomb coding may be redesigned because when encoding or decoding the whole abs(coeff)−1 instead of the remainder of the abs(coeff), larger values are expected to be coded. One example of changes to Rice-Golomb design may be as follows:

Let posX, posY be the position of the current coefficient, and LeftCoeff be the left neighbor of the current coefficient, and AboveCoeff be the above neighbor of the current coefficient.

The example techniques described in this disclosure may be combined together. For example,FIG. 7is a conceptual diagram illustrating another example of coding coefficient information for coefficients. In the example ofFIG. 7, video encoder200and video decoder300may perform techniques similar to the combination of the example ofFIG. 4and the example where the coefficient value or coefficient value−1 is directly bypass coded.

For instance, in the example illustrated inFIG. 7, all the components of each coefficient are coded in an interleaved manner, and if regular coded bins are used up (e.g., coded bin count limit is reached) when coding one of the coefficients (e.g., Coeff 2 inFIG. 7), then starting from the next coefficient, after encoding or decoding the sig flag (e.g., significance flag) and the sign flag, the remainder information (e.g., abs(coeff)−1) may not be split before being coded. In other words, for coefficients following Coeff2 (e.g., Coeff3 to CoeffN−1), video encoder200and video decoder300may, in an interleaving manner, bypass encode or decode the significance flag (e.g., sig_coeff_flag) and sign flag (e.g., coeff_sign_flag) for each coefficient, and for each coefficient, video encoder200and video decoder300may bypass code the remainder information.

FIG. 8is a conceptual diagram illustrating another example of coding coefficient information for coefficients. In the example ofFIG. 8, video encoder200and video decoder300may perform techniques similar to the combination of the example ofFIG. 3and the example where the coefficient value or coefficient value−1 is directly bypass coded.

For instance, similar to the example ofFIG. 3, video encoder200and video decoder300may encode or decode the coefficients in a two-pass manner. For example, before the coded bin count limit is reached, video encoder200and video decoder300may encode and decode, in an interleaving manner, coefficient information on a coefficient-by-coefficient basis for coefficients in a residual block for a current block in a first pass, where the coefficient information for a coefficient includes a significance flag (e.g., sig_coeff_flag) indicating whether a value of the coefficient is not zero, a parity flag (e.g., par_level_flag) indicating whether the value of the coefficient is odd or even, a sign flag (e.g., coeff_sign_flag) indicating whether the value of the coefficient is positive or negative, and one or more greater than flags (e.g., abs_level_gtX_flags). After the first pass, video encoder200and video decoder300may encode and decode remainder information (e.g., abs_remainder) for coefficients in the residual block in a second pass.

After a context bin limit is reached, for a particular coefficient, if none of the abs_level__gt1_flag, abs_level_gtX_flags, par_level_flag can be coded for the particular coefficient using context (e.g., due to the coded count bin limit being reached), then video encoder200and video decoder300may not split the absolute value of that coefficient into abs_level_gt1_flag, abs_level_gtX_flags, par_level_flag and abs_remainder. Instead, video encoder200and video decoder300may encode or decode respective values of coefficients following the particular coefficient (e.g., encode or decode abs(coefficient value)−1)) as part of the second pass (e.g., as Rice-Golomb coding).

For instance, as illustrated inFIG. 8, when coding the Coeff 0 of the current coding group, video encoder200and video decoder300may reach the coded bin count limit, such as when coding GT2 (the first abs_level_gtX_flag (X=2, 3, 4, 5)). Because the coded bin count limit is reached, video encoder200and video decoder300may bypass encode the rest of the greater than flags (e.g., greater than 3, greater than 4, and greater than 5), as illustrated inFIG. 8with (BP), which stands for bypass, next to the GT3 (greater than 3) flag, GT4 (greater than 4) and GT5 (greater than 5) flags. In this example, starting from Coeff 1, video encoder200and video decoder300may no longer split the absolute values of each coefficient into abs_level_gt1_flag, abs_level_gtX_flags, par_level_flag and abs_remainder. Instead, video encoder200and video decoder300may bypass code the abs(coefficient)−1 value.

In the example ofFIG. 8, to code, in the interleaving manner, coefficient information, video encoder200and video decoder300may context-based encode and decode, in the interleaving manner, coefficient information on the coefficient-by-coefficient basis until a coded bin count limit is reached. For example, assuming the coded bin count limit is not reached, video encoder200and video decoder300may have context-based encoded and decoded, on a coefficient-by-coefficient basis a significance flag (e.g., sig_coeff_flag) indicating whether a value of the coefficient is not zero, a parity flag (e.g., par_level_flag) indicating whether the value of the coefficient is odd or even, a sign flag (e.g., coeff_sign_flag) indicating whether the value of the coefficient is positive or negative, and one or more greater than flags (e.g., abs_level_gtX_flags) indicating whether an absolute value of the coefficient is greater than respective threshold values.

However, in the example ofFIG. 8, video encoder200and video decoder300may have reached the coded bin count limit after the greater than 2 flag of Coeff0. In this example, video encoder200and video decoder300may then bypass encode and decode the remaining flags of Coeff0, and bypass encode and decode the significance flag and the sign flag for coefficients following Coeff0 in the first pass. For example, to code, in the interleaving manner, coefficient information, video encoder200and video decoder300may context-based code (e.g., encode or decode), in the interleaving manner, coefficient information on the coefficient-by-coefficient basis until a coded bin count limit is reached, and bypass code (e.g., encode or decode), in the interleaving manner, coefficient information on the coefficient-by-coefficient basis after the coded bin count limit is reached.

In the example ofFIG. 8, after the first pass, video encoder200and video decoder300may code remainder information for coefficients in the residual block of the current block in a second pass. For example, to code the remainder information, video encoder200and video decoder300may code information indicative of a difference between a value of a particular coefficient and a largest threshold value associated with the greater than flags (e.g., such as the remainder information of Coeff0). The coded bin count limit may be reached during the coding of the particular coefficient (e.g., where the particular coefficient is Coeff0 inFIG. 8). Video encoder200and video decoder300may code respective values of coefficients following the particular coefficient. For example, video encoder200and video decoder300may code information indicative of a difference between absolute values of respective values of the coefficients following the particular coefficient and 1 (e.g., code information indicative of the value of abs(Coeff1)−1, code information indicative of the value of abs(Coeff2)−1, and so forth until abs(CoeffN−1)−1). In one or more examples, video encoder200and video decoder300may bypass code the remainder information.

FIG. 9is a block diagram illustrating an example video encoder200that may perform the techniques of this disclosure.FIG. 9is provided for purposes of explanation and should not be considered limiting of the techniques as broadly exemplified and described in this disclosure. For purposes of explanation, this disclosure describes video encoder200in the context of video coding standards such as the H.265 (HEVC) video coding standard and the H.266 (VVC) video coding standard in development. However, the techniques of this disclosure are not limited to these video coding standards, and are applicable generally to video encoding and decoding.

In the example ofFIG. 9, video encoder200includes video data memory230, mode selection unit202, residual generation unit204, transform processing unit206, quantization unit208, inverse quantization unit210, inverse transform processing unit212, reconstruction unit214, filter unit216, decoded picture buffer (DPB)218, and entropy encoding unit220. Any or all of video data memory230, mode selection unit202, residual generation unit204, transform processing unit206, quantization unit208, inverse quantization unit210, inverse transform processing unit212, reconstruction unit214, filter unit216, DPB218, and entropy encoding unit220may be implemented in one or more processors or in processing circuitry. Moreover, video encoder200may include additional or alternative processors or processing circuitry to perform these and other functions.

In some examples, transform processing unit206does not apply transforms to a residual block. For example, in examples where mode selection unit202determines that transform skip mode is enabled, the operations of transform processing unit206may be skipped. In such examples, the values of the coefficients may be for coefficients of the residual block (e.g., first position in the residual block is for a first coefficient and the residual value in the first position is the value for the first coefficient, the second position in the residual block is for a second coefficient and the residual value in the second position is the value for the second coefficient, and so forth).

Quantization unit208may quantize the coefficients in a coefficient block (e.g., which may be a transform coefficient block or a residual block), to produce a quantized coefficient block. In some examples, the operations of quantization unit208may be skipped. Quantization unit208may quantize coefficients of a coefficient block according to a quantization parameter (QP) value associated with the current block. Video encoder200(e.g., via mode selection unit202) may adjust the degree of quantization applied to the coefficient blocks associated with the current block by adjusting the QP value associated with the CU. Quantization may introduce loss of information, and thus, quantized transform coefficients may have lower precision than the original transform coefficients produced by transform processing unit206.

Inverse quantization unit210and inverse transform processing unit212may apply inverse quantization and inverse transforms (if needed) to a quantized coefficient block, respectively, to reconstruct a residual block from the coefficient block. Reconstruction unit214may produce a reconstructed block corresponding to the current block (albeit potentially with some degree of distortion) based on the reconstructed residual block and a prediction block generated by mode selection unit202. For example, reconstruction unit214may add samples of the reconstructed residual block to corresponding samples from the prediction block generated by mode selection unit202to produce the reconstructed block.

Video encoder200stores reconstructed blocks in DPB218. For instance, in examples where operations of filter unit216are not needed, reconstruction unit214may store reconstructed blocks to DPB218. In examples where operations of filter unit216are needed, filter unit216may store the filtered reconstructed blocks to DPB218. Motion estimation unit222and motion compensation unit224may retrieve a reference picture from DPB218, formed from the reconstructed (and potentially filtered) blocks, to inter-predict blocks of subsequently encoded pictures. In addition, intra-prediction unit226may use reconstructed blocks in DPB218of a current picture to intra-predict other blocks in the current picture.

Video encoder200represents 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 circuitry and configured to determine that transform skip is enabled and, based on transform skip being enabled, encode a coefficient value for a coefficient in a residual block based on one or more coefficient values of one or more neighboring coefficients. Video encoder200may also be configured to, based on transform skip being enabled, encode in an interleaving manner one or more syntax elements on a coefficient-by-coefficient basis for coefficients in a residual block in a first pass and after the first pass, and encode a syntax element on the coefficient-by-coefficient basis for coefficients in the residual block in a second pass.

As an example, mode selection unit202may determine that the current block is coded in transform-skip mode, meaning that the operation of transform processing unit206may be skipped. In this example, residual generation unit204generates the coefficients that are encoded by entropy encoding unit220.

Entropy encoding unit220may encode, in an interleaving manner (e.g., when transform skip is enabled), coefficient information on a coefficient-by-coefficient basis for coefficients in a residual block of a current block in a first pass. For example, entropy encoding unit220may determine the coefficient information for each coefficient in the residual block. The coefficient information for a coefficient includes one or more of a significance flag indicating whether a value of the coefficient is not zero, a parity flag indicating whether the value of the coefficient is odd or even, a sign flag indicating whether the value of the coefficient is positive or negative, and one or more greater than flags indicating whether an absolute value of the coefficient is greater than respective threshold values.

In this example, to code, in the interleaving manner, the coefficient information, entropy encoding unit220may context-based encode (e.g., such as CABAC), in the interleaving manner, coefficient information on the coefficient-by-coefficient basis until a coded bin count limit is reached. For example, entropy encoding unit220may determine the coded bin count limit (e.g., 2*block width*block height or 1.75*block width*block height), and track how many of bins of the coded bin count limit are used. When the number of bins reaches (e.g., is greater than or equal to) the coded bin count limit (e.g., after the coded bin count limit is reached), entropy encoding unit220may bypass encode, in the interleaving manner, coefficient information on the coefficient-by-coefficient basis.

After the first pass, entropy encoding unit220may encode remainder information for coefficients in the residual block of the current block in a second pass. To encode the remainder information, entropy encoding unit220may bypass encode the remainder information.

In one or more examples, entropy encoding unit220may signal, in the interleaving manner, coefficient information on the coefficient-by-coefficient basis for coefficients in the residual block of the current block in the first pass. Entropy encoding unit220may signal remainder information for coefficients in the residual block of the current block in the second pass.

In the example ofFIG. 10, video decoder300includes coded picture buffer (CPB) memory320, entropy decoding unit302, prediction processing unit304, inverse quantization unit306, inverse transform processing unit308, reconstruction unit310, filter unit312, and decoded picture buffer (DPB)314. Any or all of CPB memory320, entropy decoding unit302, prediction processing unit304, inverse quantization unit306, inverse transform processing unit308, reconstruction unit310, filter unit312, and DPB314may be implemented in one or more processors or in processing circuitry. Moreover, video decoder300may include additional or alternative processors or processing circuitry to perform these and other functions.

Entropy decoding unit302may entropy decode syntax elements defining quantized transform coefficients of a quantized transform coefficient block, as well as transform information, such as a quantization parameter (QP) and/or transform mode indication(s). Inverse quantization unit306may use the QP associated with the quantized transform coefficient block to determine a degree of quantization and, likewise, a degree of inverse quantization for inverse quantization unit306to apply. Inverse quantization unit306may, for example, perform a bitwise left-shift operation to inverse quantize the quantized transform coefficients. Inverse quantization unit306may thereby form a coefficient block including coefficients (e.g., coefficient block where transforms are used or coefficient block where transform is skipped). In some examples, the operations of inverse quantization unit306may be skipped. When transform is skipped, the coefficient block may be residual values of a residual block, i.e., such that each coefficient for a position in the residual block is a corresponding residual value for the position.

After inverse quantization unit306forms the coefficient block, inverse transform processing unit308may apply one or more inverse transforms to the transform coefficient block to generate a residual block associated with the current block. When transform skip mode is enabled, the operations of inverse transform processing unit308may be skipped. Inverse transform processing unit308, if needed, 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.

Video decoder300may store the reconstructed blocks in DPB314. As discussed above, DPB314may provide reference information, such as samples of a current picture for intra-prediction and previously decoded pictures for subsequent motion compensation, to prediction processing unit304. Moreover, video decoder300may output decoded pictures from DPB314for subsequent presentation on a display device, such as display device118ofFIG. 1.

In this manner, video decoder300represents an example of a video decoding device including a memory configured to store video data, and one or more processing units implemented in circuitry and configured to determine that transform skip is enabled and, based on transform skip being enabled, decode a coefficient value for a coefficient in a residual block based on one or more coefficient values of one or more neighboring coefficients. Video decoder300may also be configured to decode in an interleaving manner (e.g., when transform skip is enabled) one or more syntax elements on a coefficient-by-coefficient basis for coefficients in a residual block in a first pass and after the first pass, decode a syntax element on the coefficient-by-coefficient basis for coefficients in the residual block in a second pass.

As an example, prediction processing unit304may determine that the current block is coded in transform-skip mode (e.g., based on information signaled by video encoder200), meaning that the operation of inverse transform processing unit308may be skipped. In this example, reconstruction unit310receives coefficients (e.g., residual values since inverse transform is skipped) that are decoded by entropy decoding unit302.

Entropy decoding unit302may decode, in an interleaving manner (e.g., based on transform skip being enabled), coefficient information on a coefficient-by-coefficient basis for coefficients in a residual block of a current block in a first pass. For example, entropy decoding unit302may determine the coefficient information for each coefficient in the residual block. The coefficient information for a coefficient includes one or more of a significance flag indicating whether a value of the coefficient is not zero, a parity flag indicating whether the value of the coefficient is odd or even, a sign flag indicating whether the value of the coefficient is positive or negative, and one or more greater than flags indicating whether an absolute value of the coefficient is greater than respective threshold values.

In this example, to decode, in the interleaving manner (e.g., based on transform skip being enabled), the coefficient information, entropy decoding unit302may context-based decode (e.g., such as CABAC), in the interleaving manner, coefficient information on the coefficient-by-coefficient basis until a coded bin count limit is reached. For example, entropy decoding unit302may determine the coded bin count limit (e.g., 2*block width*block height or may be signaled by video encoder200), and track how many of bins of the coded bin count limit are used. When the number of bins reaches (e.g., is greater than or equal to) the coded bin count limit (e.g., after the coded bin count limit is reached), entropy decoding unit302may bypass decode, in the interleaving manner, coefficient information on the coefficient-by-coefficient basis.

After the first pass, entropy decoding unit302may decode remainder information for coefficients in the residual block of the current block in a second pass. To decode the remainder information, entropy decoding unit302may bypass decode the remainder information.

In one or more examples, entropy decoding unit302may parse, in the interleaving manner, coefficient information on the coefficient-by-coefficient basis for coefficients in the residual block of the current block in the first pass. Entropy decoding unit302may parse remainder information for coefficients in the residual block of the current block in the second pass.

FIG. 11is a flowchart illustrating an example method for coding video data. For example, processing circuitry of a video coder (e.g., video encoder200or video decoder300) may determine that a current block of the video data is coded (e.g., encoded or decoded) in transform-skip mode. Based on the current block being coded in transform-skip mode, the processing circuitry of the video coder may be configured to perform the example techniques.

The processing circuitry of the video coder (e.g., such as with entropy encoding unit220or entropy decoding unit302) may code, in an interleaving manner, coefficient information on a coefficient-by-coefficient basis for coefficients in a residual block of a current block of the video data in a first pass (400). The coefficient information for a coefficient includes one or more of a significance flag indicating whether a value of the coefficient is not zero, a parity flag indicating whether the value of the coefficient is odd or even, a sign flag indicating whether the value of the coefficient is positive or negative, and one or more greater than flags indicating whether an absolute value of the coefficient is greater than respective threshold values.

As one example, to code in the interleaving manner, the coefficient information, the processing circuitry of the video coder (e.g., such as with entropy encoding unit220or entropy decoding unit302) may context-based code (e.g., CABAC), in the interleaving manner, coefficient information on the coefficient-by-coefficient basis until a coded bin count limit is reached. In this example, the processing circuitry of the video coder (e.g., such as with entropy encoding unit220or entropy decoding unit302) may bypass code, in the interleaving manner, coefficient information on the coefficient-by-coefficient basis after the coded bin count limit is reached.

After the first pass, the processing circuitry of the video coder (e.g., such as with entropy encoding unit220or entropy decoding unit302) may code remainder information for coefficients in the residual block of the current block in a second pass (402). For example, to code the remainder information, the processing circuitry may bypass code the remainder information.

As one example, to code remainder information, the processing circuitry of the video coder may be configured to code remaining information. In this example, the coded bin count limit is reached during the coding of the particular coefficient. The processing circuitry of the video coder may be configured to code respective values of coefficients following the particular coefficient. As one example, the processing circuitry of the video coder may code information indicative of a difference between absolute values of respective values of the coefficients following the particular coefficient and 1.

In one or more examples, the processing circuitry of the video coder, such as in examples where the video coder is video encoder200, may signal, in the interleaving manner, the coefficient information on the coefficient-by-coefficient basis for coefficients in the residual block of the current block in the first pass, and signal remainder information for coefficients in the residual block of the current block in the second pass. In one or more examples, the processing circuitry of the video coder, such as in examples where the video coder is video decoder300, may parse, in the interleaving manner, the coefficient information on the coefficient-by-coefficient basis for coefficients in the residual block of the current block in the first pass, and parse remainder information for coefficients in the residual block of the current block in the second pass.

The following are some example techniques that may be used alone or in combination.

A method of coding video data, the method comprising determining that transform skip is enabled and coding a coefficient value for a coefficient in a residual block based on one or more coefficient values of one or more neighboring coefficients.

The method of example 1, wherein the one or more neighboring coefficients comprise a left coefficient and an above coefficient.

The method of any of examples 1 and 2, wherein coding the coefficient value comprises one of for encoding the coefficient value, mapping a coefficient absolute level of the coefficient to a modified value based on the one or more coefficient values of the one or neighboring coefficients or for decoding the coefficient value, inverse mapping the modified value to the coefficient absolute level of the coefficient based on the one or more coefficient values of the one or more neighboring coefficients.

A method of coding video data, the method comprising coding in an interleaving manner one or more syntax elements on a coefficient-by-coefficient basis for coefficients in a residual block in a first pass and after the first pass, coding a syntax element on the coefficient-by-coefficient basis for coefficients in the residual block in a second pass.

The method of example 4, wherein the one or more syntax elements coded in the first pass comprise one or more of the sig_coeff_flag, par_level_flag, coeff_sign_flag, and all abs_level_gtX_flags, examples of which are described in the disclosure.

The method any of examples 4 and 5, wherein the syntax element coded in the second pass comprises the abs_remainder, examples of which are described in the disclosure.

The method of any one or combination of examples 1-6.

The method of any one or combination of examples 1-6, wherein coding comprises decoding.

The method of any one or combination of examples 1-6, wherein coding comprises encoding.

A device for coding video data, the device comprising a memory configured to store video data and a video coder comprising fixed-function or programmable circuitry, wherein the video coder is configured to perform the method of any one or combination of examples 1-6.

The device of example 10, wherein the video coder comprises a video decoder.

The device of example 10, wherein the video coder comprises a video encoder.

The device of any of examples 10-12, further comprising a display configured to display decoded video data.

The device of any of examples 10-13, wherein the device comprises one or more of a camera, a computer, a mobile device, a broadcast receiver device, or a set-top box.

A computer-readable storage medium having stored thereon instructions that, when executed, cause one or more processors to perform the method of any one or combination of examples 1-6.

A device for coding video data, the device comprising means for performing the method of any one or combination of examples 1-6.