KR20210035025A - Lossless recontruction method and apparatus for using the same - Google Patents

Abstract

Disclosed are a lossless restoration method and an image decoding device. According to an embodiment of the present invention, as a method for lossless restoration of residual samples of a current block, the lossless restoration method comprises the steps of: determining any one of a lossless coding mode and a lossy coding mode in accordance with a first syntax indicating whether a transformation process for residual samples is skipped; and restoring the residual samples depending on lossless coding mode syntaxes decoded from a bitstream when the lossless coding mode is determined.

Description

Lossless restoration method and video decoding device {LOSSLESS RECONTRUCTION METHOD AND APPARATUS FOR USING THE SAME}

The present invention relates to encoding and decoding of an image, and more particularly, to a lossless reconstruction method and an image decoding apparatus in which an image is losslessly restored to improve encoding and decoding efficiency.

Since moving picture data has a larger amount of data than audio data or still image data, it requires a lot of hardware resources including memory in order to store or transmit itself without processing for compression.

Accordingly, when moving or transmitting moving picture data, the moving picture data is compressed and stored or transmitted using an encoder, and the decoder receives the compressed moving picture data, decompresses and plays the compressed moving picture data. As such video compression techniques, there are H.264/AVC and HEVC (High Efficiency Video Coding), which improves coding efficiency by about 40% compared to H.264/AVC.

However, the size, resolution, and frame rate of an image are gradually increasing, and accordingly, the amount of data to be encoded is also increasing. Accordingly, a new compression technique having higher encoding efficiency and higher quality improvement effect than the existing compression technique is required.

In order to meet these needs, the present invention aims to provide an improved video encoding and decoding technology, and in particular, an aspect of the present invention relates to a technology for improving the efficiency of encoding and decoding by restoring an image without loss. .

An aspect of the present invention is a lossless restoration method of residual samples of a current block, in accordance with a first syntax indicating whether a conversion process for the residual samples is skipped, and a lossless coding mode and Determining any one of lossy coding modes; When the lossless coding mode is determined, there is provided a lossless reconstruction method comprising the step of restoring the residual samples depending on lossless coding mode syntaxes decoded from a bitstream.

Another aspect of the present invention is an image decoding apparatus for losslessly reconstructing residual samples of a current block, according to a first syntax indicating whether a conversion process for the residual samples is skipped, A determination unit for determining any one of a lossless coding mode and a lossy coding mode; When the lossless coding mode is determined, there is provided an image decoding apparatus including a reconstruction unit for reconstructing the residual samples depending on lossless coding mode syntaxes decoded from a bitstream.

As described above, according to an embodiment of the present invention, since the current block can be losslessly encoded and decoded, an image having the same quality as the original image can be restored.

1 is an exemplary block diagram of an image encoding apparatus capable of implementing the techniques of the present disclosure.
2 is a diagram for explaining a method of dividing a block using a QTBTTT structure.
3 is a diagram for describing a plurality of intra prediction modes.
4 is an exemplary block diagram of an image decoding apparatus capable of implementing the techniques of the present disclosure.
5 is an exemplary block diagram of an image decoding apparatus capable of implementing a lossless reconstruction method.
6 is a flowchart illustrating an example of a lossless restoration method.
7 is a flowchart illustrating an example of determining whether a current block is losslessly encoded.
8 and 9 are flowcharts illustrating examples of determining a lossless coding mode.
10 is a flowchart illustrating an example of a method of losslessly restoring residual samples.
11 is a diagram for explaining a scanning procedure of lossless restoration.

Hereinafter, some embodiments of the present invention will be described in detail through exemplary drawings. In adding identification symbols to elements of each drawing, it should be noted that the same elements are to have the same symbols as possible even if they are indicated on different drawings. In addition, in describing the present invention, when it is determined that a detailed description of a related known configuration or function may obscure the subject matter of the present invention, a detailed description thereof will be omitted.

1 is an exemplary block diagram of an image encoding apparatus capable of implementing the techniques of the present disclosure. Hereinafter, an image encoding apparatus and sub-elements of the apparatus will be described with reference to FIG. 1.

The image encoding apparatus includes a block division unit 110, a prediction unit 120, a subtractor 130, a transform unit 140, a quantization unit 145, an encoding unit 150, an inverse quantization unit 160, and an inverse transform unit ( 165, an adder 170, a filter unit 180, and a memory 190 may be included.

Each component of the image encoding apparatus may be implemented as hardware or software, or may be implemented as a combination of hardware and software. In addition, functions of each component may be implemented as software, and a microprocessor may be implemented to execute a function of software corresponding to each component.

One image (video) is composed of a plurality of pictures. Each picture is divided into a plurality of regions, and encoding is performed for each region. For example, one picture is divided into one or more tiles or/and slices. Here, one or more tiles may be defined as a tile group. Each tile or/slice is divided into one or more Coding Tree Units (CTUs). And each CTU is divided into one or more CUs (Coding Units) by a tree structure. Information applied to each CU is encoded as the syntax of the CU, and information commonly applied to CUs included in one CTU is encoded as the syntax of the CTU. In addition, information commonly applied to all blocks in one tile is encoded as the syntax of a tile or is encoded as a syntax of a tile group in which a plurality of tiles are collected, and information applied to all blocks constituting one picture is It is encoded in a picture parameter set (PPS) or a picture header. Furthermore, information commonly referred to by a plurality of pictures is encoded in a sequence parameter set (SPS). In addition, information commonly referred to by one or more SPSs is encoded in a video parameter set (VPS).

The block dividing unit 110 determines the size of a coding tree unit (CTU). Information on the size of the CTU (CTU size) is encoded as the syntax of the SPS or PPS and transmitted to the video decoding apparatus.

The block dividing unit 110 divides each picture constituting an image into a plurality of coding tree units (CTUs) having a predetermined size, and then repeatedly divides the CTU using a tree structure. Split (recursively). A leaf node in a tree structure becomes a coding unit (CU), which is a basic unit of coding.

In a tree structure, a quadtree (QuadTree, QT) in which an upper node (or parent node) is divided into four lower nodes (or child nodes) of the same size, or a binary tree (BinaryTree) in which an upper node is divided into two lower nodes. , BT), or a ternary tree (TT) in which an upper node is divided into three lower nodes in a 1:2:1 ratio, or a structure in which two or more of these QT structures, BT structures, and TT structures are mixed. have. For example, a QTBT (QuadTree plus BinaryTree) structure may be used, or a QTBTTT (QuadTree plus BinaryTree TernaryTree) structure may be used. Here, BTTT may be collectively referred to as MTT (Multiple-Type Tree).

2 shows a QTBTTT split tree structure. As shown in FIG. 2, the CTU may be first divided into a QT structure. The quadtree division may be repeated until the size of a splitting block reaches the minimum block size (MinQTSize) of a leaf node allowed in QT. A first flag (QT_split_flag) indicating whether each node of the QT structure is divided into four nodes of a lower layer is encoded by the encoder 150 and signaled to the image decoding apparatus. If the leaf node of the QT is not larger than the maximum block size (MaxBTSize) of the root node allowed in BT, it may be further divided into one or more of a BT structure or a TT structure. In the BT structure and/or the TT structure, a plurality of division directions may exist. For example, there may be two directions in which a block of a corresponding node is divided horizontally and a direction vertically divided. As shown in FIG. 2, when MTT splitting starts, a second flag (mtt_split_flag) indicating whether nodes are split, and if split, a flag indicating a split direction (vertical or horizontal) and/or a split type (Binary or Ternary). A flag indicating) is encoded by the encoder 150 and signaled to the image decoding apparatus.

As another example of the tree structure, when a block is divided using a QTBTTT structure, information about a CU split flag (split_cu_flag) indicating that the block has been split first and a QT split flag (split_qt_flag) indicating whether the split type is QT splitting is information from the encoding unit 150 ) And signaled to the video decoding apparatus. When it is indicated that the value of the CU split flag (split_cu_flag) is not split, the block of the corresponding node becomes a leaf node in the split tree structure and becomes a coding unit (CU), which is a basic unit of encoding. When indicating that the value of the CU split flag (split_cu_flag) is split, whether the split type is QT or MTT is distinguished through the value of the QT split flag (split_qt_flag). If the split type is QT, there is no additional information, and if the split type is MTT, a flag indicating the MTT split direction (vertical or horizontal) (mtt_split_cu_vertical_flag) and/or a flag indicating the MTT split type (Binary or Ternary) (mtt_split_cu_binary_flag) is encoded by the encoder 150 and signaled to the video decoding apparatus.

When QTBT is used as another example of the tree structure, there are two types of horizontally splitting a block of the node into two blocks of the same size (i.e., symmetric horizontal splitting) and a type splitting vertically (i.e., symmetric vertical splitting). Branches can exist. A split flag indicating whether each node of the BT structure is divided into blocks of a lower layer and split type information indicating a type to be divided are encoded by the encoder 150 and transmitted to the image decoding apparatus. Meanwhile, there may be additional types of dividing the block of the corresponding node into two blocks of an asymmetric form. The asymmetric form may include a form of dividing a block of a corresponding node into two rectangular blocks having a size ratio of 1:3, or a form of dividing a block of a corresponding node in a diagonal direction.

The CU may have various sizes according to the QTBT or QTBTTT split from the CTU. Hereinafter, a block corresponding to a CU to be encoded or decoded (ie, a leaf node of QTBTTT) is referred to as a'current block'.

The prediction unit 120 predicts the current block and generates a prediction block. The prediction unit 120 includes an intra prediction unit 122 and an inter prediction unit 124.

In general, each of the current blocks in a picture can be predictively coded. In general, prediction of the current block is performed using an intra prediction technique (using data from a picture containing the current block) or an inter prediction technique (using data from a picture coded before a picture containing the current block). Can be done. Inter prediction includes both one-way prediction and two-way prediction.

The intra prediction unit 122 predicts pixels in the current block by using pixels (reference pixels) located around the current block in the current picture including the current block. There are a plurality of intra prediction modes according to the prediction direction. For example, as shown in FIG. 3, the plurality of intra prediction modes may include a non-directional mode including a planar mode and a DC mode, and 65 directional modes. Depending on each prediction mode, the surrounding pixels to be used and the equation are defined differently.

The intra prediction unit 122 may determine an intra prediction mode to be used to encode the current block. In some examples, the intra prediction unit 122 may encode the current block using several intra prediction modes, and select an appropriate intra prediction mode to use from the tested modes. For example, the intra prediction unit 122 calculates rate distortion values using rate-distortion analysis for several tested intra prediction modes, and has the best rate distortion characteristics among the tested modes. It is also possible to select an intra prediction mode.

The intra prediction unit 122 selects one intra prediction mode from among a plurality of intra prediction modes, and predicts the current block by using a neighboring pixel (reference pixel) and an equation determined according to the selected intra prediction mode. Information on the selected intra prediction mode is encoded by the encoder 150 and transmitted to the image decoding apparatus.

The inter prediction unit 124 generates a prediction block for the current block through a motion compensation process. A block that is most similar to a current block is searched for in a reference picture that is coded and decoded before the current picture, and a prediction block for the current block is generated using the searched block. Then, a motion vector corresponding to a displacement between the current block in the current picture and the prediction block in the reference picture is generated. In general, motion estimation is performed on a luma component, and a motion vector calculated based on the luma component is used for both the luma component and the chroma component. Motion information including information on a reference picture used to predict a current block and information on a motion vector is encoded by the encoder 150 and transmitted to an image decoding apparatus.

The subtractor 130 generates a residual block by subtracting the prediction block generated by the intra prediction unit 122 or the inter prediction unit 124 from the current block.

The transform unit 140 converts the residual signal in the residual block having pixel values in the spatial domain into transform coefficients in the frequency domain. The transform unit 140 may transform residual signals in the residual block using the total size of the residual block as a transform unit, or divide the residual block into two sub-blocks, which are transform regions and non-transform regions, Residual signals can be transformed using only a block as a transform unit. Here, the transform region subblock may be one of two rectangular blocks having a size ratio of 1:1 based on the horizontal axis (or vertical axis). In this case, a flag indicating that only the subblock has been transformed (cu_sbt_flag), directional (vertical/horizontal) information (cu_sbt_horizontal_flag), and/or location information (cu_sbt_pos_flag) are encoded by the encoder 150 and signaled to the video decoding apparatus. . In addition, the size of the transform region subblock may have a size ratio of 1:3 based on the horizontal axis (or vertical axis), and in this case, a flag (cu_sbt_quad_flag) that separates the division is additionally encoded by the encoder 150 to decode the image. Signaled to the device.

The quantization unit 145 quantizes the transform coefficients output from the transform unit 140 and outputs the quantized transform coefficients to the encoding unit 150.

The encoding unit 150 generates a bitstream by encoding the quantized transform coefficients using an encoding method such as Context-based Adaptive Binary Arithmetic Code (CABAC). The encoder 150 encodes information such as a CTU size related to block division, a CU division flag, a QT division flag, an MTT division direction, and an MTT division type, so that the video decoding apparatus can divide the block in the same manner as the video encoding apparatus. To be there.

In addition, the encoder 150 encodes information on a prediction type indicating whether the current block is encoded by intra prediction or inter prediction, and intra prediction information (ie, intra prediction mode) according to the prediction type. Information) or inter prediction information (information on a reference picture and a motion vector) is encoded.

The inverse quantization unit 160 inverse quantizes the quantized transform coefficients output from the quantization unit 145 to generate transform coefficients. The inverse transform unit 165 converts transform coefficients output from the inverse quantization unit 160 from the frequency domain to the spatial domain to restore the residual block.

The addition unit 170 restores the current block by adding the reconstructed residual block and the prediction block generated by the prediction unit 120. The pixels in the reconstructed current block are used as reference pixels when intra-predicting the next block.

The filter unit 180 filters reconstructed pixels to reduce blocking artifacts, ringing artifacts, blurring artifacts, etc. that occur due to block-based prediction and transformation/quantization. Perform. The filter unit 180 may include a deblocking filter 182 and a sample adaptive offset (SAO) filter 184.

The deblocking filter 180 filters the boundary between reconstructed blocks to remove blocking artifacts caused by block-based encoding/decoding, and the SAO filter 184 is additionally applied to the deblocking filtered image. Perform filtering. The SAO filter 184 is a filter used to compensate for a difference between a reconstructed pixel and an original pixel caused by lossy coding.

The reconstructed block filtered through the deblocking filter 182 and the SAO filter 184 is stored in the memory 190. When all blocks in one picture are reconstructed, the reconstructed picture is used as a reference picture for inter prediction of a block in a picture to be encoded later.

4 is an exemplary block diagram of an image decoding apparatus capable of implementing the techniques of the present disclosure. Hereinafter, an image decoding apparatus and sub-elements of the apparatus will be described with reference to FIG. 4.

The image decoding apparatus may include a decoding unit 410, an inverse quantization unit 420, an inverse transform unit 430, a prediction unit 440, an adder 450, a filter unit 460, and a memory 470. have.

Like the image encoding apparatus of FIG. 1, each component of the image decoding apparatus may be implemented as hardware or software, or may be implemented as a combination of hardware and software. In addition, functions of each component may be implemented as software, and a microprocessor may be implemented to execute a function of software corresponding to each component.

The decoding unit 410 determines the current block to be decoded by decoding the bitstream received from the video encoding apparatus and extracting information related to block division, and information on prediction information and residual signals necessary to restore the current block, etc. Extract.

The decoder 410 determines the size of the CTU by extracting information on the CTU size from a sequence parameter set (SPS) or a picture parameter set (PPS), and divides the picture into CTUs of the determined size. Then, the CTU is determined as the highest layer of the tree structure, that is, the root node, and the CTU is divided using the tree structure by extracting the partition information for the CTU.

For example, in the case of splitting the CTU using the QTBTTT structure, a first flag (QT_split_flag) related to the splitting of the QT is first extracted, and each node is split into four nodes of a lower layer. And, for the node corresponding to the leaf node of QT, the second flag (MTT_split_flag) related to the splitting of the MTT and the splitting direction (vertical / horizontal) and/or split type (binary / ternary) information are extracted and the corresponding leaf node is MTT. Divide into structure Through this, each node below the leaf node of the QT is recursively divided into a BT or TT structure.

As another example, when a CTU is split using a QTBTTT structure, first, a CU split flag indicating whether to split a CU (split_cu_flag) is extracted, and when the corresponding block is split, a QT split flag (split_qt_flag) is extracted. . If the split type is not QT but MTT, a flag indicating the MTT split direction (vertical or horizontal) (mtt_split_cu_vertical_flag) and/or a flag indicating the MTT split type (Binary or Ternary) (mtt_split_cu_binary_flag) is additionally extracted. In the partitioning process, each node may have zero or more repetitive MTT partitions after zero or more repetitive QT partitions. For example, in the CTU, MTT division may occur immediately, or, conversely, only multiple QT divisions may occur.

As another example, when the CTU is divided using the QTBT structure, each node is divided into four nodes of a lower layer by extracting the first flag (QT_split_flag) related to the division of the QT. In addition, a split flag indicating whether or not the node corresponding to the leaf node of the QT is further split into BT and split direction information are extracted.

On the other hand, when determining the current block to be decoded through division of the tree structure, the decoder 410 extracts information on a prediction type indicating whether the current block is intra predicted or inter predicted. When the prediction type information indicates intra prediction, the decoder 410 extracts a syntax element for intra prediction information (intra prediction mode) of the current block. When the prediction type information indicates inter prediction, the decoder 410 extracts a syntax element for the inter prediction information, that is, information indicating a motion vector and a reference picture referenced by the motion vector.

Meanwhile, the decoding unit 410 extracts information on quantized transform coefficients of the current block as information on the residual signal.

The inverse quantization unit 420 inverse quantizes the quantized transform coefficients, and the inverse transform unit 430 inversely transforms the inverse quantized transform coefficients from the frequency domain to the spatial domain to restore residual signals to generate a residual block for the current block. .

In addition, when the inverse transform unit 430 inverse transforms only a partial region (subblock) of the transform block, a flag indicating that only the subblock of the transform block has been transformed (cu_sbt_flag), and the direction (vertical/horizontal) information of the subblock (cu_sbt_horizontal_flag) ) And/or sub-block location information (cu_sbt_pos_flag), and inverse transform coefficients of the sub-block from the frequency domain to the spatial domain to restore residual signals. By filling in, the final residual block for the current block is created.

The prediction unit 440 may include an intra prediction unit 442 and an inter prediction unit 444. The intra prediction unit 442 is activated when the prediction type of the current block is intra prediction, and the inter prediction unit 444 is activated when the prediction type of the current block is inter prediction.

The intra prediction unit 442 determines an intra prediction mode of the current block among a plurality of intra prediction modes from a syntax element for the intra prediction mode extracted from the decoder 410, and a reference pixel around the current block according to the intra prediction mode. The current block is predicted by using them.

The inter prediction unit 444 determines a motion vector of the current block and a reference picture referenced by the motion vector using the syntax element for the intra prediction mode extracted from the decoding unit 410, and uses the motion vector and the reference picture. To predict the current block.

The adder 450 restores the current block by adding the residual block output from the inverse transform unit and the prediction block output from the inter prediction unit or the intra prediction unit. The pixels in the reconstructed current block are used as reference pixels for intra prediction of a block to be decoded later.

The filter unit 460 may include a deblocking filter 462 and an SAO filter 464. The deblocking filter 462 performs deblocking filtering on the boundary between reconstructed blocks in order to remove blocking artifacts occurring due to block-by-block decoding. The SAO filter 464 performs additional filtering on the reconstructed block after deblocking filtering in order to compensate for the difference between the reconstructed pixel and the original pixel caused by lossy coding. The reconstructed block filtered through the deblocking filter 462 and the SAO filter 464 is stored in the memory 470. When all blocks in one picture are reconstructed, the reconstructed picture is used as a reference picture for inter prediction of a block in a picture to be encoded later.

High Efficiency Video Coding (HEVC) supports CU-level syntax as shown in Table 1 for lossless encoding/decoding of CU.

cu_transquant_bypass_flag is a syntax indicating whether lossless encoding/decoding is bypassed in the corresponding CU.If cu_transquant_bypass_flag=1, a transform process, quantization process, and in-loop filtering (DBF, SAO) process are applied to the corresponding CU. It doesn't work. That is, if cu_transquant_bypass_flag=1, the encoded data becomes a difference value, not coefficients.

In HEVC, since a CU is divided into a plurality of TUs, and one TU is composed of three components (Y, Cb, Cr), if cu_transquant_bypass_flag = 1, the transformation process, quantization process and in- The loop filtering process is omitted, and the difference value after intra or inter prediction is encoded.

When cu_transquant_bypass_flag=1, the residual encoding method is the same as the general coefficient encoding method of HEVC. However, under certain conditions, a general coefficient encoding method is applied after a process of rotating residual values is performed prior to encoding a coefficient (residual). Here, the specific condition is'if transform_skip_rotation_enabled_flag defined in HLS is “on”, the block is predicted to be intra, and the size of the TU is 4x4.'

A method of rotating the residual values is shown in Equation 1.

In Equation 1, r denotes a residual value before rotation, r'denotes a residual value after rotation, and nTbS denotes the size of a TB having a square shape.

Syntaxes for general coefficient coding are as follows.

1.last_sig_coeff_x_prefix,

2. last_sig_coeff_y_prefix,

3. last_sig_coeff_x_suffix,

4. last_sig_coeff_y_suffix,

5. coded_sub_block_flag,

6. sig_coeff_flag,

7. coeff_abs_level_greater1_flag,

8. coeff_abs_level_greater2_flag,

9. coeff_sign_flag,

10. coeff_abs_level_remaining.

The preceding four syntax elements (last_sig_coeff_x_prefix, last_sig_coeff_y_prefix, last_sig_coeff_x_suffix, and last_sig_coeff_y_suffix) are non-zero coefficients (last significant coefficients) positioned last in the scan order in the coefficient block. For the last effective coefficient, the x component (last_sig_coeff_x_prefix, last_sig_coeff_x_suffix) and the y component (last_sig_coeff_y_prefix, last_sig_coeff_y_suffix) for the position are separately displayed, and each component is divided into a prefix and a suffix.

The fifth syntax element (coded_sub_block_flag) is a flag indicating whether subblocks of 4x4 size located in the coefficient block include one or more non-zero coefficients. The coded_sub_block_flag is signaled only for subblocks preceding the last significant sub-block in which the last significant coefficient exists in the scan order. Here, coded_sub_block_flag=0 indicates that all coefficients in the subblock are “0”, and coded_sub_block_flag=1 indicates that at least one non-zero coefficient exists in the subblock.

The sixth syntax element sig_coeff_flag is a flag indicating whether the corresponding coefficient is non-zero or zero for each of the coefficients in the subblock. sig_coeff_flag=0 indicates that the coefficient is zero, and sig_coeff_flag=1 indicates that the coefficient is non-zero. sig_coeff_flag is signaled for each of all coefficients in a corresponding subblock when coded_sub_block_flag=1.

The seventh syntax element (coeff_abs_level_greater1_flag) is a flag indicating whether the absolute value of the coefficient level is greater than 1, and the eighth syntax element (coeff_abs_level_greater2_flag) is a flag indicating whether the absolute value of the coefficient level is greater than 2. The ninth syntax element (coeff_sign_flag) is a flag indicating the sign (positive or negative) of the coefficient level, and the tenth syntax element (coeff_abs_level_remaining) is the residual value for the absolute value of the coefficient level greater than 2 (remainder, that is, the coefficient level). Absolute value-3).

Using these ten syntax elements, residual values may be losslessly encoded/decoded.

In this specification, methods for lossless encoding/decoding of a current block (CU) are proposed. General lossy processes occurring during the process of encoding/decoding a video are omitted. In addition, in this specification, a syntax structure suitable for lossless encoding/decoding and methods of signaling the same are proposed.

As shown in FIG. 5, an image decoding apparatus capable of implementing the lossless restoration method may include a decoding unit 410, a determination unit 510, and a restoration unit 520.

The image encoding apparatus may determine whether the current block is losslessly encoded, and may signal the determination result by encoding the determination result with the first syntax. The decoder 410 may decode the first syntax from the bitstream (S610).

The first syntax indicates only whether the transform process is skipped or bypassed (hereinafter referred to as'skip') for residual samples of the current block, or both the transform process and the quantization process are skipped or It can also indicate whether or not it has been bypassed. When the first syntax only indicates whether to skip the transformation process, the first syntax may be implemented as transform_skip_flag. When the first syntax indicates whether both the transformation process and the quantization process are skipped, the first syntax may be implemented as cu_transquant_bypass_flag.

transform_skip_flag=1 indicates that a transform process for residual samples is skipped, and transform_skip_flag=0 may indicate that a transform process for residual samples is not skipped. cu_transquant_bypass_flag=1 indicates that both the transform process and the quantization process for the residual samples are skipped, and cu_transquant_bypass_flag=0 may indicate that the conversion process and the quantization process for the residual samples are not skipped.

The determiner 510 may determine one of a lossless coding mode and a lossy coding mode according to the decoded first syntax (S620, S630, and S660).

When the first syntax indicates that the conversion process is skipped (S620), a lossless coding mode may be determined (S630). In contrast, when the first syntax indicates that the transformation process is not skipped (S620), a lossy coding mode (general coefficient coding) may be determined (S640). Here, the lossless coding mode refers to a mode in which residual samples are losslessly decoded, and the lossy coding mode may refer to a mode in which residual samples are decoded in a general manner.

Depending on the embodiment, the second syntax (tu_cbf) may be further used in the process of determining the lossless coding mode. The second syntax is information indicating whether one or more non-zero coefficients are included in the transform block of the current block, and may be signaled from the image encoding apparatus to the image decoding apparatus. In this case, when indicating that the second syntax is included and that the first syntax is skipped, the video decoding apparatus may determine the lossless coding mode.

When the first syntax indicates that the transformation process has been skipped, the image encoding apparatus may encode and signal the lossless coding mode syntaxes. In contrast, when the first syntax indicates that the transformation process has not been skipped, the video encoding apparatus may encode and signal the lossy coding mode syntaxes.

Lossless coding mode syntaxes mean syntaxes for lossless decoding of residual samples, and lossy coding mode syntaxes may mean syntaxes for decoding residual samples in a general manner.

When the lossless coding mode is determined (S630), the decoding unit 410 decodes the lossless coding mode syntaxes (S640), and the reconstruction unit 520 can losslessly restore the residual samples depending on the lossless coding mode syntaxes. Yes (S650). In contrast, when the lossy coding mode is determined (S660), the decoding unit 410 may decode the lossy coding mode syntaxes (S670), and the recovery unit 520 may calculate residual samples depending on the lossy coding mode syntaxes. It can be restored (S650).

Example 1

Embodiment 1 is a method of directly signaling whether a current block is losslessly encoded.

First, an enable flag (eg, transquant_bypass_enabled_flag) for supporting lossless encoding/decoding of the current block may be signaled from the image encoding device to the image decoding device.

transquant_bypass_enabled_flag is a high level syntax (HLS), and may be defined at one or more of a sequence parameter set (SPS), a picture parameter set (PPS), a video parameter set (VPS), and a decoder parameter set (DPS).

An example of transquant_bypass_enabled_flag defined in PPS is shown in Table 2.

When transquant_bypass_enabled_flag is “on”, a first syntax (eg, cu_transquant_bypass_flag) indicating whether the corresponding current block is losslessly encoded for each current block may be signaled from the image encoding device to the image decoding device.

cu_transquant_bypass_flag may be signaled when transquant_bypass_enabled_flag is “on” (Table 3), or when cu_cbf=1 and transquant_bypass_enabled_flag is “on” (Table 4).

cu_cbf is a syntax indicating whether one or more coefficients or residuals having a non-zero level are included in the block for the component (Y, Cb, Cr) of the corresponding current block. cu_cbf=0 indicates that coefficients or residual values in the block for all components of the current block are all-zero, and cu_cbf=1 indicates that non-zero coefficients or residuals are included in one or more blocks among blocks for components of the current block. Show. When cu_cbf=0, since all residual values are all-zero, the conversion and quantization process may be omitted, and signaling of cu_transquant_bypass_flag may not be required.

Information indicating whether in-loop filtering (DBF, SAO, ALF) is performed may also be signaled from the image encoding device to the image decoding device. Information indicating whether the in-loop filter is performed may be signaled when transquant_bypass_enabled_flag is “on” as shown in Table 3, or when cu_cbf=1 and transquant_bypass_enabled_flag is “on” as shown in Table 4. In the latter case, when cu_cbf=0, signaling of information indicating whether the in-loop filter is performed may be omitted.

The video decoding apparatus may determine whether lossless decoding is possible by extracting transquant_bypass_enabled_flag from the bitstream (S710). In addition, when transquant_bypass_enabled_flag=1 (S720), the image decoding apparatus may extract cu_transquant_bypass_flag from the bitstream (S730) to determine whether the current block is losslessly encoded.

When cu_transquant_bypass_flag is “on”, the transform process, quantization process, and in-loop filtering process are all omitted, so that lossless decoding of the current block may be performed.

When the current block is losslessly encoded/decoded (cu_transquant_bypass_flag=1), all CU-level syntaxes for the transformation process and quantization process may not be signaled. The following is a list of syntaxes for performing the CU-level transformation process and the quantization process, and when cu_transquant_bypass_flag=1, the corresponding syntaxes can be derived or inferred as “off” values without signaling.

1. Syntaxes related to the conversion process

1) sub-block transform: cu_sbt_flag, cu_sbt_quad_flag, cu_sbt_horizontal_flag, cu_sbt_pos_flag

2) primary transform: tu_mts_idx

3) secondary transform: lfnst_idx

4) transform skip: transform_skip_flag

2. Syntaxes related to the quantization process

1) delta qp: cu_qp_delta_abs, cu_qp_delta_sign_flag

Since transform_skip_flag is a syntax indicating whether the transformation process is skipped (skipped) for the current block, when the current block is losslessly encoded/decoded, transform_skip_flag may not be signaled and may be derived with a value of “1”.

When the current block is losslessly encoded/decoded, syntax for coding tools having a lossy effect may not be signaled. The following is a list of syntaxes for performing coding tools having a lossy effect, and when cu_transquant_bypass_flag=1, the corresponding syntaxes can be derived or inferred as “off” values without signaling.

3. Syntax related to coding tools with loss effect

1) joint cb and cr coding: tu_joint_cbcr_residual

When the current block is losslessly coded/decoded, coding tools without a separate syntax may not be performed among coding tools having a lossy effect. That is, coding tools that have a loss effect but do not have a separate syntax may be omitted during the encoding/decoding process. Below is a list of coding tools with lossy effects but no separate syntax.

4. Coding tools with lossy effect but no syntax

1) sign hiding

2) RDOQ (rate-distortion optimization quantization)

Example 2

Embodiment 2 is a method of determining a residual coding mode for lossless coding/decoding.

In the case of general lossy coding (loss mode), spatial domain values are changed to frequency domain values while undergoing a transformation process. The spatial domain is divided into a low-frequency component and a high-frequency component, and the low-frequency component is located at the upper left of the block, and the high-frequency component is located at the lower right of the block. By the conversion process, most of the high frequency components take zero values, and some of the low frequency components take non-zero values. Accordingly, coefficient coding for the lossy mode uses a method for efficiently coding a non-zero component located at the upper left of a block.

In contrast, in the case of lossless encoding (lossless mode), since a transformation process and a quantization process are omitted, difference values after prediction (intra prediction or inter prediction) may be scattered in the block. Therefore, the coefficient coding scheme for the lossy mode may be inappropriate.

In Embodiment 2, a method of selecting or determining a lossless coding mode when a current block is coded in a lossless mode is proposed. Embodiment 2 can be divided into an embodiment for a luma block and an embodiment for a chroma block.

Example 2-1: Luma block

The image encoding apparatus may encode the first syntax (transform_skip_flag or cu_transquant_bypass_flag) and signal the first syntax to the image decoding apparatus. The image decoding apparatus may decode the first syntax and determine whether the luma block of the current block is encoded in the lossless mode by using the decoded first syntax (S830).

When the first syntax is “on”, since the luma block is encoded in a lossless mode, a lossless coding mode (residual_ts_coding) may be determined (S840). In contrast, when the first syntax is “off”, since the luma block is encoded in the lossy mode, a lossy coding mode (residual_coding) may be determined (S850). Here, residual_ts_coding refers to a residual coding method for a spatial domain (where a transformation process is skipped), and residual_coding refers to a coefficient coding method for a general frequency domain (to which a transformation process has been applied).

Depending on the embodiment, the second syntax (tu_cbf_luma) may be further used in the process of determining the lossless coding mode of the luma block.

The image encoding apparatus encodes the second syntax and signals it to the image decoding apparatus, and the image decoding apparatus may decode the second syntax from the bitstream (S810).

When tu_cbf_luma=0 (S820), since only all-zero coefficients are included in the luma block, coefficient coding is not performed, and the corresponding luma block may be filled with coefficients having a value of “0”. In contrast, when tu_cbf_luma=1 (S820) and the first syntax is “on” (S830), a lossless coding mode (residual_ts_coding) is determined (S840), and tu_cbf_luma=1 (S820) and the first syntax is “off” In the case of (S830), a lossy coding mode (residual_coding) may be determined (S850).

The syntax structure for Example 2-1 is shown in Table 5.

Meanwhile, as described above, when the lossless mode is applied, transform_skip_flag may be derived to a value of “1”. The syntax structure for this example is shown in Table 6.

Example 2-2: Chroma block

The image encoding apparatus may encode the first syntax (transform_skip_flag or cu_transquant_bypass_flag) and signal the first syntax to the image decoding apparatus. The image decoding apparatus may determine whether the chroma block of the current block is encoded in the lossless mode by using the first syntax (S930).

When the first syntax is “on”, since the chroma block is encoded in a lossless mode, a lossless coding mode (residual_ts_coding) may be determined (S940). In contrast, when the first syntax is “off”, since the chroma block is encoded in a lossy mode, a lossy coding mode (residual_coding) may be determined (S950).

Depending on the embodiment, the second syntax (tu_cbf_cb or tu_cbf_cr) may be further used in the process of determining the lossless coding mode of the chroma block.

The image encoding apparatus encodes the second syntax and signals the signal to the image decoding apparatus, and the image decoding apparatus may decode the second syntax from the bitstream (S910).

When tu_cbf_cb=0 (S920), since only all-zero coefficients are included in the chroma block, coefficient coding is not performed and the corresponding chroma block may be filled with coefficients having a value of “0”. In contrast, when tu_cbf_cb=1 (S920) and the first syntax is “on” (S930), a lossless coding mode (residual_ts_coding) is determined (S940), and tu_cbf_cb=1 (S920) and the first syntax is “off” In the case of (S930), a lossy coding mode (residual_coding) may be determined (S950).

The syntax structure for Example 2-2 is shown in Table 7.

Example 3

Embodiment 3 is a method of restoring residual samples in a lossless coding mode (residual_ts_coding).

The biggest difference between the lossless coding mode and the lossy coding mode can be summarized as (1) whether the position information of the last non-zero coefficient is signaled, and (2) the scanning order.

(1) Whether to signal the location information of the last non-zero coefficient

In the lossy coding mode, the position information of the last non-zero (last non-zero) coefficient located in the transform block in the scan order is signaled with the highest priority, and the values of the coefficients located after the last non-zero coefficient are all-zero. Under the assumption, the rest of the other syntaxes are signaled. This is because non-zero coefficients are located in the upper left of the block and zero coefficients are located in the lower right of the block.

However, when the current block is coded in the lossless mode, there is a high probability that non-zero coefficients are evenly distributed within the block, so it is necessary to take another method. That is, in the lossless coding mode, the position information of the last non-zero coefficient is not signaled.

(2) Scanning procedure

In the lossy coding mode, coded_sub_block_flag and sig_coeff_flag to be described below are signaled in the reverse order of the diagonal-up scanning order. In contrast, in the lossless coding mode, coded_sub_block_flag and sig_coeff_flag are signaled in the diagonal-up scanning order (in the correct order of the diagonal-up scanning order).

An example of the diagonal-up scanning sequence is shown in FIG. 11. Blocks separated by dashed-dotted lines indicate subblocks included in the current block, and unit blocks separated by solid lines indicate samples (transformation coefficients or residual samples) included in each of the subblocks.

In the lossless coding mode, syntaxes used in the lossless coding mode may be signaled in the order of numbers of FIG. 11. That is, coded_sub_block_flag is signaled in the order of numbers (1 to 16) connected by arrows, and sig_coeff_flag is signaled in the order of numbers (0 to 255) indicated in residual samples.

Syntaxes (lossless coding mode syntaxes) used in the lossless coding mode are as follows.

1.coded_sub_block_flag (third syntax): This is a syntax indicating whether one or more non-zero coefficients are included in a plurality of subblocks included in the transform block (whether coefficients included in the subblocks are all-zero). . coded_sub_block_flag=0 indicates all-zero, and coded_sub_block_flag=1 indicates that one or more non-zero coefficients are included. If coded_sub_block_flag does not exist, it can be inferred as a value of “1”.

2. sig_coeff_flag (fourth syntax): This is a syntax indicating whether the level (coefficient level) of coefficients included in the subblock (target subblock indicated by coded_sub_block_flag=1) is zero. sig_coeff_flag=0 indicates that the coefficient level is zero, and sig_coeff_flag=1 indicates that the coefficient level is non-zero.

3. coeff_sign_flag: This is a syntax indicating the sign of the coefficient level. coeff_sign_flag=0 represents a positive number, and coeff_sign_flag=1 represents a negative number.

4. abs_level_gtx_flag(x=1): This is a syntax indicating whether the absolute value of the coefficient level is greater than x(=1).

5. par_level_flag: This is a syntax indicating whether the absolute value of the coefficient level is even or odd.

6. abs_level_gtx_flag(x=3, 5, 7, 9): This is a syntax indicating whether the absolute value of the coefficient level is greater than x (= 3, 5, 7, 9).

7. abs_remainder: This syntax expresses the remaining value of the absolute value of the coefficient level greater than 9.

In this way, the syntaxes indicating the position of the last non-zero coefficient (last_sig_coeff_x_prefix, last_sig_coeff_y_prefix, last_sig_coeff_x_suffix, and last_sig_coeff_y_suffix) are not included in the lossless coding mode syntax. That is, when the lossless coding mode is applied, the position information of the last non-zero coefficient is not signaled.

The video encoding apparatus may determine whether a non-zero coefficient is included in the i-th subblock, and signal the result as coded_sub_block_flag[i]. The video decoding apparatus may decode coded_sub_block_flag[i] from the bitstream (S1010).

coded_sub_block_flag[i] may be signaled for all subblocks. If, in the scan order, coded_sub_block_flag[i] for all subblocks from the last subblock to the second subblock are all “0”, coded_sub_block_flag[i] for the first subblock is not signaled and “1” It can also be derived by value.

When coded_sub_block_flag[i]=0, the video decoding apparatus determines whether determination of all subblocks has been completed (S1094). If the determination on the subblocks is not completed, the process S1010 is again performed for the next (i++) subblock, and when the determination on all subblocks is completed, the lossless coding mode may be terminated.

The video encoding apparatus may determine whether the j-th coefficient or residual sample in the target subblock with coded_sub_block_flag[i]=1 is non-zero, and signal the result as sig_coeff_flag[j]. When coded_sub_block_flag[i]=1 (S1020), the video decoding apparatus may decode sig_coeff_flag[j] from the bitstream (S1030).

sig_coeff_flag[j] may be signaled for all coefficients in the target subblock. If sig_coeff_flag[j] for all coefficients from the last coefficient to the second coefficient in the scan order are all “0”, sig_coeff_flag[j] for the first coefficient is not signaled and is derived as a value of “1”. It could be.

When sig_coeff_flag[j]=1, the video encoding apparatus determines whether the absolute value of the coefficient level is greater than x(=1) and the sign of the coefficient, and determines the result of coeff_sign_flag[j] and abs_level_gtx_flag[j]( It can be signaled with x=1). The video decoding apparatus may decode coeff_sign_flag[j] and abs_level_gtx_flag[j] (x=1) from the bitstream when sig_coeff_flag[j]=1 (S1050).

coeff_sign_flag may be encoded using context-adaptive binary arithmetic coding (CABAC). BAC is an arithmetic coding method based on binary (0 or 1) values. It has a probability value for "0" and a probability value for "1", and the syntax is changed to a probability value for "0" or a probability value for "1". It is a method of arithmetic coding with probability values. In this case, a method of determining one probability model from among one or more BACs using the context of the corresponding syntax is CABAC.

For example, in CABAC, there are n probability models with a probability value of “0” and a probability value of “1”, and any one of n probability models is selected using the surrounding information of the syntax. It is done.

coeff_sign_flag has 6 probability models for each of I-slices (0 to 5), B-slices (6 to 11) and P-slices (12 to 17). Here, numbers 0 to 17 denote index values of each of the probability models. Of a total of 18 probability models, three probability models for each of I-slice, B-slice and P-slice are used for intra bdpcm mode, and 3 for each of I-slice, B-slice and P-slice. The two probabilistic models are used for modes other than the intra bdpcm mode.

For example, 3rd to 5th probability models may be used for I-slices for intra bdpcm mode, and 0th to 2nd probability models may be used for I-slices for purposes other than intra bdpcm mode. The criterion for selecting one probability model from among three probability models (0th to 2nd) for the I-slice for purposes other than the intra bdpcm mode is the surrounding information of the corresponding syntax.

In the case of coeff_sign_flag, the sum of the coeff_sign_flag value of the residual adjacent to the right and the coeff_sign_flag value of the residual adjacent to the lower side is used as the context value. For example, if the coeff_sign_flag value of the residual adjacent to the right of the corresponding residual is “0” and the coeff_sign_flag value of the residual adjacent to the lower side is “1”, the sum becomes “1”, and the first probability model based on the value of “1” This residual is selected as a probabilistic model for coeff_sign_flag. As another example, if the coeff_sign_flag value of the residual adjacent to the right of the corresponding residual is "1" and the coeff_sign_flag value of the residual adjacent to the lower side is "1", the sum is "2", and the second probability based on the value of "2" The model is selected as a probabilistic model for the coeff_sign_flag of the corresponding residual. In sum, since the sum of the coeff_sign_flag value of the residual adjacent to the right side of the residual and the coeff_sign_flag value of the residual adjacent to the lower side is within the range of 0 to 2, any one of the 0th, 1st and 2nd probability models corresponds to It is chosen as a probabilistic model of the residuals. As a result, the syntax is encoded as a probability value of the selected probability model.

The video encoding apparatus may determine whether the absolute value of the j-th coefficient level is odd or even, and signal the result as par_level_flag[j]. The image decoding apparatus determines whether the absolute value of the j-th coefficient level is greater than 1 (S1060), and when the absolute value of the level is greater than 1, par_level_flag[j] is decoded from the bitstream (S1070), and the corresponding coefficient level It can be determined whether the absolute value of is odd or even.

The video decoding apparatus determines whether the determination of all coefficients in the i-th subblock has been completed (S1080), and when the determination of the coefficients is not completed, the process S1030 to the next (j++) coefficient. Perform the process S1070. This process may be repeatedly performed until determination of all coefficients in the i-th subblock is completed.

The video encoding apparatus determines whether the absolute value of the coefficient level is greater than x (=1, 3, 5, 7), and signals the result as abs_level_gtx_flag[j] (x=3, 5, 7, 9). can do. When the absolute value of the coefficient level is greater than x (=1, 3, 5, 7) (S1082), the video decoding apparatus decodes abs_level_gtx_flag[j] (x=3, 5, 7, 9) from the bitstream. Can be (S1084).

Specifically, the image decoding apparatus decodes abs_level_gtx_flag[j] (x=3) when the absolute value of the coefficient level is greater than 1, and abs_level_gtx_flag[j](x =5), and when the absolute value of the coefficient level is greater than 5, abs_level_gtx_flag[j](x=7) is decoded, and when the absolute value of the coefficient level is greater than 7, abs_level_gtx_flag[j](x =9) can be decrypted.

The image decoding apparatus determines whether absolute value determination for all coefficients in the i-th subblock has been completed (S1086), and when the determination of the coefficients is not completed, the next coefficient (j++) is S1082. The process and S1084 process are performed. This process may be repeatedly performed until determination of all coefficients in the i-th subblock is completed.

The video encoding apparatus determines whether the absolute value of the j-th coefficient level is greater than x (= 9), and when the absolute value of the j-th coefficient level is greater than 9, 9 is determined from the absolute value of the j-th coefficient level. Signaling can be performed by subtracting the remaining value as abs_remainder[j].

When the absolute value of the j-th coefficient level is greater than 9 (S1088), the image decoding apparatus may determine the absolute value of the j-th coefficient level by decoding abs_remainder[j] from the bitstream (S1088).

The image decoding apparatus determines whether absolute value determination for all coefficients in the i-th subblock has been completed (S1092), and when the determination of the coefficients is not completed, the next coefficient (j++) is S1088. The process and S1090 process are performed. This process may be repeatedly performed until determination of all coefficients in the i-th subblock is completed.

When the determination of all the coefficients is completed, the image decoding apparatus determines whether the determination of all the subblocks has been completed (S1094). If the determination on the subblocks is not completed, the processes described above are performed again for the next subblock (i++), and if the determination on all subblocks is completed, the lossless coding mode may be terminated. have.

The above description is merely illustrative of the technical idea of the present embodiment, and those of ordinary skill in the technical field to which the present embodiment belongs will be able to make various modifications and variations without departing from the essential characteristics of the present embodiment. Accordingly, the present embodiments are not intended to limit the technical idea of the present embodiment, but to explain the technical idea, and the scope of the technical idea of the present embodiment is not limited by these embodiments. The scope of protection of this embodiment should be interpreted by the following claims, and all technical ideas within the scope equivalent thereto should be construed as being included in the scope of the present embodiment.

120, 440: prediction unit 130: subtractor
170, 450: adder 180, 460: filter unit

Claims (14)

As a lossless restoration method for the residual samples of the current block,
Determining any one of a lossless coding mode and a lossy coding mode according to a first syntax indicating whether a conversion process for the residual samples has been skipped;
And restoring the residual samples depending on lossless coding mode syntaxes decoded from a bitstream when the lossless coding mode is determined. The method of claim 1,
The determining step,
When the second syntax decoded from the bitstream indicates that at least one non-zero coefficient is included in the transform block of the current block and indicates that the first syntax is skipped, the lossless coding mode To determine the lossless restoration method. The method of claim 1,
The lossless coding mode syntaxes are:
In the scan order, the non-zero coefficient position syntax that is last located in the transform block of the current block is not included. The method of claim 1,
The lossless coding mode syntaxes are:
A third syntax indicating whether one or more non-zero coefficients are included in subblocks other than the first subblock in scan order among subblocks included in the transform block of the current block; And
And a fourth syntax indicating whether coefficients other than the first coefficient in the scan order are non-zero among coefficients in the target subblock indicating that the third syntax is included. The method of claim 4,
The third syntax for the first subblock is,
A lossless recovery method that is not decoded and is derived with a value indicating that it is included. The method of claim 4,
The fourth syntax for the first coefficient is,
A lossless recovery method that is not decoded and is derived with a value indicating non-zero. The method of claim 4,
The third syntax and the fourth syntax,
A lossless recovery method that is decoded according to the diagonal-up scanning order. As an image decoding apparatus that restores the residual samples of the current block losslessly,
A determination unit determining one of a lossless coding mode and a lossy coding mode according to a first syntax indicating whether a conversion process for the residual samples is skipped;
When the lossless coding mode is determined, the video decoding apparatus comprising a reconstruction unit for reconstructing the residual samples depending on the lossless coding mode syntax decoded from the bitstream. The method of claim 8,
The determination unit,
When the second syntax decoded from the bitstream indicates that at least one non-zero coefficient is included in the transform block of the current block and indicates that the first syntax is skipped, the lossless coding mode The video decoding device to determine. The method of claim 8,
The lossless coding mode syntaxes are:
An image decoding apparatus that does not include position syntaxes of non-zero coefficients last located in a transform block of the current block in a scan order. The method of claim 8,
The lossless coding mode syntaxes are:
A third syntax indicating whether one or more non-zero coefficients are included in subblocks other than the first subblock in scan order among subblocks included in the transform block of the current block; And
And a fourth syntax indicating whether coefficients other than the first coefficient in the scan order are non-zero among coefficients in the target subblock indicating that the third syntax is included. The method of claim 11,
The third syntax for the first subblock is,
A video decoding apparatus that is not decoded and is derived to a value indicating that it is included. The method of claim 11,
The fourth syntax for the first coefficient is,
The video decoding apparatus, which is not decoded and is derived to a value indicating non-zero. The method of claim 11,
The third syntax and the fourth syntax,
An image decoding apparatus that is decoded according to a diagonal-up scanning order.

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