WO2019135630A1 - Hiding sign data of transform coefficient - Google Patents

Abstract

The aim of the present invention is to provide improved image coding and decoding techniques. In particular, provided according to one aspect of the present invention are techniques for efficiently hiding the sign of a non-zero coefficient among quantized transform coefficients. To this end, sign hiding for a non-zero coefficient is implemented by discriminately applying a sign prediction process on the basis of the characteristics between a last non-zero coefficient and a first non-zero coefficient.

Description

Converting the sign data of the transform coefficients

Field of the Invention The present invention relates to encoding and decoding an image, and more particularly, to efficiently signaling a sign of a non-zero coefficient among quantized transform coefficients.

Since video data has a larger amount of data than voice data or still image data, it requires a lot of hardware resources including memory to store or transmit the video data itself without processing for compression. Accordingly, when storing or transmitting moving image data, the moving image data is compressed or stored using an encoder, and the decoder receives the compressed moving image data and decompresses and reproduces the moving image data. There are H.264 / AVC and H.265 (High Efficiency Video Coding: HEVC) standard technology which was established in early 2013 which improved the coding efficiency of about 40% of H.264 / AVC. do.

Table 1 lists the syntaxes defined in the H.265 (HEVC) standard, which are used to signal coefficient blocks that are arrays of quantized coefficients through transform and quantization processes.

last_sig_coeff_x_prefix

last_sig_coeff_y_prefix

last_sig_coeff_x_suffix

last_sig_coeff_y_suffix

coded_sub_block_flag

sig_coeff_flag

coeff_abs_level_greater1_flag

coeff_abs_level_greater2_flag

coeff_sign_flag

coeff_abs_level_remaining

Figure 1 shows the syntax of expressing the exemplary positions of non-zero coefficients in a 16 x 16 TU (transform unit) and coefficients of the corresponding TU. One TU is divided into sub-blocks of 4 × 4 size, and related information is expressed. The syntaxes representing the coefficients in one TU are signaled in the order of Table 1. Among the syntaxes listed in Table 1, coeff_sign_flag is a syntax for representing the sign of a non-zero coefficient. If the coeff_sign_flag indicates a first value (e.g., "0"), the sign of the corresponding non-zero coefficient is positive and the coeff_sign_flag indicates a second value Quot; - "(negative).

In H.265 (HEVC), a new coding tool called sign data hiding or sign bit hiding was introduced to further improve the compression performance of image coding. The code bit concealment improves the coding gain by selectively encoding the sign bit of the first non-zero quantized transform coefficient of the 4x4 sub-block. If there is at least two non-zero coefficients in the 4x4 sub-block and the distance between the scan positions of the first non-zero coefficient and the last non-zero coefficient in the scan order is greater than a predetermined threshold value, The sign of the zero coefficient is hidden (i.e., not signaled) at the encoder side. On the decoder side it is possible to deduce the secret code bit from checking the parity of the sum of the absolute values of the non-zero coefficients. For example, if the sum of the absolute values of the transform coefficients in the 4x4 sub-block is even, the sign of the first non-zero coefficient is inferred as positive, otherwise it is negatively inferred. This can be referred to as sign prediction. If the parity does not indicate the correct sign of the first non-zero coefficient, the encoder selectively compensates for the sign bit that is hidden. This is accomplished on the encoder side by selecting one non-zero coefficient and modifying its value to an adjacent value that is greater than or less than the previous value. For this compensation, a non-zero coefficient having a level close to the boundary of the quantization interval can be selected. Coded bit concealment gives the encoder a freedom to choose which non-zero coefficient to compensate based on the rate-distortion performance.

It is an object of the present invention to provide an improved image encoding and decoding technique and, in particular, to provide techniques for efficiently hiding the sign of non-zero coefficients among the quantized transform coefficients I suggest.

According to an aspect of the present invention, there is provided a method of decoding a bitstream of an encoded image to recover a coefficient block, which is an array of transform coefficients, in an image decoding apparatus, Stage 1; A second step of setting a position of a last non-zero coefficient in a scan order in a current coefficient block to a first position; A third step of determining a second position closest to the scanning order from the first position satisfying the code concealment condition among the non-zero coefficients encountered in the reverse order of the scanning order from the first position; A fourth step of applying a sign prediction process to the non-zero coefficient of the second position; And a fifth step of setting a position of a closest non-zero coefficient that meets in a reverse order of a scan order from a second position to which the sign prediction process is applied to the first position. The third to fifth steps are repeatedly performed until the first non-zero coefficient is reached in the scan order in the current coefficient block.

In some embodiments, the code conceal condition may be such that the first variable defined by the first position and the second position exceeds a predetermined first threshold value. In some embodiments, the code conceal condition is such that a first parameter defined by the first position and the second position exceeds a predetermined first threshold value and is defined by the first position and the second position. The second parameter may exceed the predetermined second threshold value.

According to another aspect of the present invention there is provided a method of decoding a bitstream of an encoded image in order to reconstruct a coefficient block that is an array of transform coefficients in an image decoding apparatus, ; Dividing non-zero coefficients in the current coefficient block, excluding zero coefficients, into a plurality of groups; Identifying, for each group, a last non-zero coefficient and a first non-zero coefficient that are defined by a reverse scan order; And for each group, if the distance in the reverse scan order between the last non-zero coefficient and the first non-zero coefficient is greater than a preset threshold, then a sign prediction process is performed on the last non-zero coefficient The method comprising the steps of:

According to another aspect of the present invention, there is provided a computer program product comprising: one or more processors; And a memory in which instructions are stored. The instructions comprising: a first step of causing the image decoding apparatus, when executed by the one or more processors, to identify positions of significant coefficients in a current coefficient block; A second step of setting a position of a last non-zero coefficient in a scan order in a current coefficient block to a first position; A third step of determining a second position closest to the scanning order from the first position satisfying the code concealment condition among the non-zero coefficients encountered in the reverse direction of the scan order from the first position; A fourth step of applying a sign prediction process to the non-zero coefficient of the second position; And a fifth step of setting a position of a closest non-zero coefficient that meets in a reverse direction of a scan order from a second position to which the sign prediction process is applied to the first position, wherein in the current coefficient block, The third to fifth steps are repeatedly performed until the first non-zero coefficient is reached.

According to another aspect of the present invention, there is provided a computer program product comprising: one or more processors; And a memory in which instructions are stored, wherein the instructions cause the image decoding apparatus, when executed by the one or more processors, to determine, in the current coefficient block, a position of non-zero coefficients ; Dividing non-zero coefficients in the current coefficient block, excluding zero coefficients, into a plurality of groups; Identifying, for each group, a last non-zero coefficient and a first non-zero coefficient that are defined by a reverse scan order; And for each group, if the distance in the reverse scan order between the last non-zero coefficient and the first non-zero coefficient is greater than a preset threshold, then a sign prediction process is performed on the last non-zero coefficient .

Figure 1 shows the syntax of expressing the exemplary positions of non-zero coefficients in a 16 x 16 TU (transform unit) and coefficients of the corresponding TU.

2 is an exemplary block diagram of an image encoding apparatus capable of implementing the techniques of this disclosure;

3 is an illustration of block partitioning using a QTBTTT structure.

4 is an exemplary diagram of a plurality of intra prediction modes.

5 is a diagram illustrating exemplary scan schemes used for encoding quantized coefficients of a square coefficient block.

6 is an exemplary block diagram of an image decoding apparatus capable of implementing the techniques of the present disclosure;

7A shows an example of a 32x32 transform block including a total of 64 4x4 subblocks.

FIG. 7B is a plot showing coefficients hidden according to the H.265 (HEVC) standard for the transform block illustrated in FIG. 7A.

Figures 8A-8F show coefficients that are hidden according to embodiments of the present invention for the transform block illustrated in Figure 7A.

9 is a flowchart illustrating a method of applying a code concealment technique to a coefficient block, which is an array of transform coefficients, in an image encoding apparatus according to an aspect of the present invention.

10 is a flowchart illustrating a method of applying a code concealment scheme to a coefficient block, which is an array of transform coefficients, in an image encoding apparatus according to another aspect of the present invention.

11 is a flowchart illustrating a method of decoding a bitstream of an encoded image in order to recover a coefficient block, which is an array of transform coefficients, in an image decoding apparatus according to an aspect of the present invention.

12 is a flowchart illustrating a method of decoding a bitstream of an encoded image to recover a coefficient block, which is an array of transform coefficients, in an image decoding apparatus according to another aspect of the present invention.

Hereinafter, some embodiments of the present invention will be described in detail with reference to exemplary drawings. It should be noted that, in adding the identification codes to the constituent elements of the respective drawings, the same constituent elements are denoted by the same reference symbols as possible even if they are displayed on different drawings. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

The techniques of the present disclosure generally relate to efficiently signaling the sign of non-zero coefficients, also referred to as significant coefficients, for a coefficient block that is an array of quantized coefficients that is the result of transform and quantization .

2 is an exemplary block diagram of an image encoding apparatus capable of implementing the techniques of this disclosure;

The image encoding apparatus includes a block division unit 210, a prediction unit 220, a subtractor 230, a transform unit 240, a quantization unit 245, an encoding unit 250, an inverse quantization unit 260, 265, an adder 270, a filter unit 280, and a memory 290. The image encoding apparatus may be implemented such that each component is implemented as a hardware chip or software, and one or more microprocessors execute the function of the software corresponding to each component.

One video (video) is composed of a plurality of pictures. Each picture is divided into a plurality of areas and coding is performed for each area. For example, one picture is divided into one or more slices or / and tiles, and each slice or tile is divided into one or more CTU (Coding Tree Unit). Each CTU is divided into one or more CUs (Coding Units) by a tree structure. The information applied to each CU is encoded as a syntax of the CU, and the information commonly applied to the 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 a syntax of a tile or is encoded as a syntax of a group of tiles in which a plurality of tiles are collected, and information applied to all blocks constituting one picture is And is encoded into a picture parameter set (PPS) or a picture header. Further, information that is commonly referred to by a plurality of pictures is encoded into a sequence parameter set (SPS). The information that is commonly referred to by one or more SPSs is encoded into a Video Parameter Set (VPS).

The block dividing unit 210 determines the size of the Coding Tree Unit (CTU). The information on the size of the CTU (CTU size) is encoded as the syntax of the SPS or PPS and transmitted to the image decoding apparatus. The block dividing unit 210 divides each picture constituting an image into a plurality of CTUs (Coding Tree Units) having a determined size, and thereafter recursively recursively uses the CTU tree structure. . A leaf node in a tree structure becomes a coding unit (CU) which is a basic unit of coding. In the tree structure, a quad tree (QuadTree, QT) in which an upper node (or a parent node) is divided into four sub nodes (or child nodes) of the same size, or a binary tree in which an upper node is divided into two lower nodes , BT), or a ternary tree (TT) in which an ancestor node is divided into three subnodes at a ratio of 1: 2: 1, or a structure combining one or more of these QT structures, BT structures and TT structures have. For example, a QuadTree plus BinaryTree (QTBT) structure can be used, or a QuadTree plus BinaryTreeTernaryTree (QTBTTT) structure can be used.

3 shows a QTBTTT segmentation tree structure. As shown in FIG. 3, the CTU can be first divided into the QT structure. The quadtree partitioning can be repeated until the size of the splitting block reaches the minimum block size (MinQTSize) of the leaf node allowed in QT. The first flag (QT_split_flag) indicating whether each node of the QT structure is divided into four nodes of the lower layer is encoded by the encoding unit 250 and signaled to the video 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 dividing directions may exist. For example, there may be two directions in which the block of the node is divided horizontally and vertically. As shown in FIG. 3, when the BTTT segmentation is started, a second flag BTTT_split_flag indicating whether the nodes are divided and a flag indicating a division direction (vertical or horizontal) and / or a division type (Binary or Ternary Is encoded by the encoding unit 250 and signaled to the video decoding apparatus.

As another example of a tree structure, when QTBT is used, a type that horizontally divides a block of the node into two blocks of the same size (i.e., symmetric horizontal splitting) and a vertically dividing type (i.e., symmetric vertical splitting) Branches can exist. The split flag (split_flag) indicating whether each node of the BT structure is divided into blocks of the lower layer and the split type information indicating the divided type are encoded by the encoding unit 250 and transmitted to the image decoding apparatus. On the other hand, there may be additional types of dividing the blocks of the node into two asymmetric blocks. In the asymmetric form, the block of the node may be divided into two rectangular blocks having a size ratio of 1: 3, or the block of the corresponding node may be divided into diagonal directions.

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

The prediction unit 220 generates a prediction block by predicting the current block. The prediction unit 220 includes an intra prediction unit 222 and an inter prediction unit 224.

In general, the current blocks in a picture may each be predictively coded. Prediction of the current block is generally 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 prior to a picture containing the current block) . Inter prediction includes both unidirectional prediction and bidirectional prediction.

The intraprediction unit 222 predicts pixels in the current block 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. 4, a plurality of intra prediction modes may include a non-directional mode including a planar mode and a DC mode, and 65 directional modes. The neighboring pixels to be used and the calculation expression are defined differently according to each prediction mode.

The intra prediction unit 222 can determine an intra prediction mode to be used for coding the current block. In some examples, the intra-prediction unit 222 may encode the current block using a plurality of intra-prediction modes and may select an appropriate intra-prediction mode to use from the tested modes. For example, the intra-prediction unit 222 may calculate rate-distortion values using a rate-distortion analysis for various tested intra-prediction modes, and may employ rate- The intra prediction mode may be selected.

The intra prediction unit 222 selects one intra prediction mode from among a plurality of intra prediction modes, and predicts the current block using neighboring pixels (reference pixels) determined by the selected intra prediction mode and an equation. The information on the selected intra prediction mode is encoded by the encoding unit 250 and transmitted to the image decoding apparatus.

The inter-prediction unit 224 generates a prediction block for the current block through a motion compensation process. A block most similar to the current block is searched in a reference picture coded and decoded earlier than the current picture, and a prediction block for the current block is generated using the searched block. Then, a motion vector corresponding to the 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 motion vectors calculated based on luma components are used for both luma components and chroma components. The motion information including the information on the reference picture used for predicting the current block and the information on the motion vector is encoded by the encoding unit 250 and transmitted to the video decoding apparatus.

The subtracter 230 subtracts the prediction block generated by the intra prediction unit 222 or the inter prediction unit 224 from the current block to generate a residual block.

The transform unit 240 transforms the residual signal in the residual block having pixel values in the spatial domain into transform coefficients in the frequency domain. The transform unit 240 may transform the residual signals in the residual block using the size of the current block as a transform unit or divide the residual block into a plurality of smaller subblocks and transform residual signals into subblock- Conversion. There are various ways of dividing the residual block into smaller sub-blocks. For example, it may be divided into sub blocks of the same size that have been set, or a QT (quadtree) type partition using a residual block as a root node.

The quantization unit 245 quantizes the transform coefficients output from the transform unit 240 and outputs the quantized transform coefficients to the encoding unit 250.

The encoding unit 250 encodes the quantized transform coefficients using a coding scheme such as CABAC to generate a bitstream. This encoding is typically performed on the quantized transform coefficients using one of a plurality of available scan schemes.

5 is a diagram illustrating exemplary scanning schemes used to code quantized coefficients. These scanning methods include an up-right diagonal method (FIG. 5A), a horizontal method (FIG. 5B), and a vertical method (FIG. When the current block is coded using the inter prediction scheme, the coefficients of the corresponding block are scanned in an up-right diagonal manner. If the current block is coded in the intra prediction mode, one of the above three types And scan the coefficients of the corresponding block. Each scan mode has the same scan pattern for the subblocks in the coefficient block and the coefficients in each subblock. For example, in the case of the horizontal scanning method, the scanning order of the sub-blocks is also a horizontal method, and the scanning order of the coefficients in each sub-block is also a horizontal method. However, the order stored in the actual bit stream is stored in the reverse order of the scan order. That is, they are stored in the bit stream in the order of the pixel in the lower right position and the pixel in the upper left position. Once the position of the last significant coefficient (non-zero coefficient) is known, the coefficients may be decoded in the opposite direction of the scan order towards the first coefficient in the scan pattern, as will be described further below.

The encoding unit 250 encodes information such as a CTU size, a QT division flag, a BT division flag, a dividing direction, and a division type associated with the block division so that the video decoding apparatus can divide the block like the video encoding apparatus .

The encoding unit 250 encodes information on a prediction type indicating whether the current block is coded by intraprediction or inter prediction, and outputs the intra prediction information (that is, Information) or inter prediction information (information on reference pictures and motion vectors).

The inverse quantization unit 260 dequantizes the quantized transform coefficients output from the quantization unit 245 to generate transform coefficients. The inverse transform unit 265 transforms the transform coefficients output from the inverse quantization unit 260 from the frequency domain to the spatial domain and restores the residual block.

The adder 270 adds the reconstructed residual block and the prediction block generated by the predictor 220 to reconstruct the current block. The pixels in the reconstructed current block are used as reference pixels when intra prediction of the next-order block is performed.

The filter unit 280 performs filtering on the reconstructed pixels to reduce blocking artifacts, ringing artifacts, blurring artifacts, and the like caused by block-based prediction and transformation / . The filter unit 280 may include a deblocking filter 282 and an SAO filter 284.

The deblocking filter 280 filters boundaries between the restored blocks to remove blocking artifacts caused by coding / decoding on a block basis, and the SAO filter 284 performs additional filtering on the deblocking- Perform filtering. The SAO filter 284 is a filter used to compensate for the difference between the reconstructed pixel and the original pixel due to lossy coding.

The restored blocks filtered through the deblocking filter 282 and the SAO filter 284 are stored in the memory 290. When all the 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 coded later.

6 is an exemplary block diagram of an image decoding apparatus capable of implementing the techniques of the present disclosure;

The image decoding apparatus includes an image reconstructor 600 including a decoding unit 610, an inverse quantization unit 620, an inverse transformation unit 630, a prediction unit 640, an adder 650, And a memory 670. 2, each component may be implemented as a hardware chip, or may be implemented as software, and a microprocessor may be implemented to execute functions of software corresponding to each component.

The decoding unit 610 decodes the bit stream received from the image encoding apparatus to extract information related to the block division to determine a current block to be decoded, and outputs prediction information and residual signal information necessary for restoring the current block .

The decoding unit 610 extracts information on the CTU size from a sequence parameter set (SPS) or a picture parameter set (PPS), determines the size of the CTU, and divides the picture into CTUs of a predetermined size. Then, the CTU is determined as the top 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, when the CTU is divided using the QTBTTT structure, the first flag (QT_split_flag) related to the division of the QT is extracted and each node is divided into four nodes of the lower layer. For the node corresponding to the leaf node of the QT, the second flag BTTT_split_flag, the vertical / horizontal and / or binary / ternary information related to the division of the BTTT are extracted, Structure. In this way, each node below the leaf nodes of the QT is recursively divided into BT or TT structures.

As another example, when the CTU is divided using the QTBT structure, the first flag (QT_split_flag) related to the division of the QT is extracted and each node is divided into four nodes of the lower layer. The node corresponding to the leaf node of the QT is extracted with a split flag (split_flag) and dividing direction information indicating whether the node is further divided by BT.

On the other hand, when the current block to be decoded is determined through division of the tree structure, the decoding unit 610 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 decoding unit 610 extracts a syntax element for intra prediction information (intra prediction mode) of the current block.

When the prediction type information indicates inter prediction, the decoding unit 610 extracts syntax elements for inter prediction information, that is, information indicating a motion vector and a reference picture referred to by the motion vector.

Meanwhile, the decoding unit 610 extracts information on the quantized transform coefficients of the current block as information on the residual signal.

The inverse quantization unit 620 dequantizes the quantized transform coefficients and the inverse transform unit 630 inversely transforms the dequantized transform coefficients from the frequency domain to the spatial domain to generate residual blocks for the current block by restoring the residual signals.

The prediction unit 640 includes an intra prediction unit 642 and an inter prediction unit 644. The intra prediction unit 342 is activated when the intra prediction is the prediction type of the current block, and the inter prediction unit 344 is activated when the intra prediction is the prediction type of the current block.

The intra prediction unit 642 determines the intra prediction mode of the current block among the plurality of intra prediction modes from the syntax element for the intra prediction mode extracted from the decoding unit 610, To predict the current block.

The inter-prediction unit 644 determines a motion vector of the current block and a reference picture referenced by the motion vector using a syntax element for the intra-prediction mode extracted from the decoding unit 610, The block is predicted.

The adder 650 adds the residual block output from the inverse transform unit and the prediction block output from the inter prediction unit or the intra prediction unit to reconstruct the current block. The pixels in the reconstructed current block are utilized as reference pixels for intra prediction of a block to be decoded later.

The image restorer 600 sequentially restores the current blocks corresponding to the CUs, thereby restoring the picture composed of the CTUs and CTUs constituted by the CUs.

The filter unit 660 includes a deblocking filter 662 and an SAO filter 664. The deblocking filter 662 deblocks the boundary between the restored blocks to remove blocking artifacts caused by decoding on a block-by-block basis. The SAO filter 664 performs additional filtering on the reconstructed block after deblocking filtering to compensate for the difference between the reconstructed pixel and the original pixel resulting from lossy coding. The restored block filtered through the deblocking filter 662 and the SAO filter 664 is stored in the memory 670. When all the 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 coded later.

The techniques of this disclosure generally involve efficiently hiding the sign of non-zero coefficients (i.e., significant coefficients) for a coefficient block that is an array of quantized coefficients that is the result of transformations and quantization. Therefore, certain techniques of the present disclosure may be performed by the encoding unit 250 or the decoding unit 610. [ That is, for example, the encoding unit 250 or the decoding unit 610 may perform the techniques of the present disclosure described with reference to Figs. 8A to 12 below. In other instances, one or more other units of the image encoding device or the image decoding device may additionally be involved in performing the techniques of the present disclosure.

The code data concealment technique described in the H.265 (HEVC) standard divides a transform block into 4 × 4 subblocks and applies a code data concealment process to each subblock. That is, if the difference in positions between the first non-zero coefficient and the last non-zero coefficient in the reverse scan order within a 4 × 4 subblock of the transform coefficient block is greater than 3 (ie, lastSignScanPos - firstSigScanPos> 3) And the quantization steps are not bypassed (i.e., skipped) with respect to the current CU (i.e., cu_transquant_bypass_flag = off), the image encoding apparatus returns the sign data of the first non- zero coefficient of the corresponding 4x4 sub- Do not signal.

7A shows an example of a 32x32 transform block including a total of 64 4x4 subblocks. In FIG. 7A, non-zero coefficients are hatched, and the order of the non-zero coefficients recorded in the actual bit stream is listed when the diagonal scanning method illustrated in FIG. 5A is used.

In the transform block illustrated in FIG. 7A, only the first subblock satisfies the sign concealment condition (lastSignScanPos - firstSigScanPos> 3) defined in the H.265 (HEVC) standard, and thus the first ratio of the first subblock - Only the sign of the zero coefficient (coefficient 8) is hidden. Through the code concealment technique defined in the H.265 (HEVC) standard, the coefficients concealed for the transform block illustrated in FIG. 7A are marked in black in FIG. 7B.

Now, several techniques for efficiently concealing the code data of the non-zero coefficients proposed in this disclosure will be described with reference to FIGS. 8A-12.

≪ Embodiment 1 >

In the code concealment scheme adopted in the HEVC, the first non-zero coefficient and the last non-zero coefficient for determining code concealment conditions are defined in one 4 × 4 sub-block. The present embodiment does not limit the positions of the first non-zero coefficient and the last non-zero coefficient for code concealment within one 4x4 sub-block, but the last non-zero coefficient in one transform block 1 coefficient) to the non-zero coefficients in the reverse order of the scan order are set as the first non-zero coefficient and the second non-zero coefficient to measure the distance in the scan order. The procedure for this embodiment is as follows.

Assuming that the threshold value thr is 3, applying the technique of this embodiment to the transform coefficient block illustrated in Fig. 7A is as follows.

First, the image encoding apparatus sets the position of the first coefficient to the first position, designates the position of the second coefficient to the second position, and then calculates the distance between two coefficients in the scan order. Since the distance between the two coefficients exceeds the threshold value (= 3), the sign of the coefficient at the second position (i.e., coefficient 2) is concealed. The sign of the second coefficient can be inferred from the parity check of the sum of the absolute values of the first and second coefficients by the video decoding apparatus. The image encoding apparatus resets the first position and the second position, and then performs the scanning again.

The image coding apparatus sets the position of the third coefficient to the first position, designates the position of the fourth coefficient to the second position, and then calculates the distance between the two coefficients in the scanning order. Since the distance between two coefficients exceeds the threshold value (= 3), the sign of the coefficient at the second position (that is, coefficient 4) is concealed. The sign of the coefficient 4 can be deduced by the image decoding apparatus from the parity check of the sum of the absolute values of the coefficients 3 and 4. The image encoding apparatus resets the first position and the second position, and then performs the scanning again.

The image coding apparatus sets the position of the coefficient 5 to the first position and sets the position of the coefficient 6 to the second position and then calculates the distance between two coefficients in the scanning order. The distance between the two coefficients does not exceed the threshold value (3). After resetting the second position, scanning is resumed again, and the position of the coefficient 7 is designated as the second position. Calculate the distance between two coefficients in the scan order (coefficient 7 and coefficient 5). Since the distance between the two coefficients exceeds the threshold value (= 3), the sign of the coefficient at the second position (i.e., coefficient 7) is concealed. The sign of the coefficient 7 can be deduced from the parity check of the sum of the absolute values of the coefficients 5, 6 and 7 by the image decoding apparatus. The image encoding apparatus resets the first position and the second position, and then performs the scanning again.

The coefficients concealed under the code concealment technique of this embodiment are marked in black in Fig. 8A. Of the eight coefficients, the sign of the three coefficients (the numbers 2, 4, and 7) is concealed. That is, 3 bits are saved through the code hiding technique of the present embodiment.

≪ Embodiment 2 >

The present embodiment counts the non-zero coefficients encountered in the reverse order of the scan order, starting from the last non-zero coefficient (e.g., coefficient 1 in FIG. 7A) in one transform block, When the number of non-zero coefficients exceeds the threshold value thr , the sign of the corresponding non-zero coefficient is concealed and the scanning is performed again. The procedure for this embodiment is as follows.

Assuming that the threshold value thr is 2, applying the technique of this embodiment to the transform coefficient block illustrated in Fig. 7A is as follows.

The image encoding apparatus first initializes a variable " count " and counts the non-zero coefficients that start in the reverse order of the scan order, starting from the first coefficient. The count value (= 3) exceeds the threshold value (= 2) by reaching the third coefficient, and the image coding apparatus applies the code concealment process to the third coefficient. The sign of the third coefficient can be inferred from the parity check of the sum of the absolute values of the first, second and third coefficients by the video decoding apparatus.

The image encoding apparatus resets the variable " count ", and then counts non-zero coefficients from the non-zero coefficient applied with the code concealment process. That is, starting from the fourth coefficient, the non-zero coefficients encountered in the reverse order of the scan order are counted. The count value (= 3) exceeds the threshold value (= 2) by reaching the 6th coefficient, and the image coding apparatus applies the code concealment process to the 6th coefficient. The sign of the sixth coefficient can be deduced from the parity check of the sum of the absolute values of the fourth, fifth and sixth coefficients by the video decoding apparatus.

The image encoding apparatus resets the variable " count ", and then counts non-zero coefficients from the non-zero coefficient applied with the code concealment process. That is, starting from coefficient 7, the non-zero coefficients encountered in the reverse order of the scan order are counted. Since the count value (= 2) does not exceed the threshold value (= 2) even after reaching the coefficient of 8, which is the first non-zero coefficient in the scan order, there is no nonzero coefficient to which the code concealment process is applied.

The coefficients concealed under the code concealment technique of this embodiment are marked in black in FIG. 8B. The sign of the two total coefficients (the numbers 3 and 6) of the eight coefficients is concealed. That is, the code hiding technique of this embodiment saves 2 bits.

≪ Third Embodiment >

This embodiment divides consecutive non-zero coefficients into a predefined number of groups from the last non-zero coefficient in the transform block (e.g., coefficient 1 in FIG. 7A) to the reverse order of the scan order, The technique of hiding the sign of the first non-zero coefficient in the scan order is used. Since the technique of the present embodiment must be able to know the number of all non-zero coefficients in the corresponding transform block before generating the sign data, the sign data at the end of the syntax for all coefficient information, As shown in Fig.

As an example, assuming that the number of predefined groups is 2, applying the technique of the present embodiment to the transform coefficient block illustrated in FIG. 7A is as follows.

The video encoding apparatus first signals the syntax information for all eight non-zero coefficients. And divides the total of eight non-zero coefficients into two groups. That is, consecutive {1, 2, 3, 4} coefficients in the reverse order of the scan order are grouped into a first group, and consecutive {5, 6, 7, 8} . In each group, the sign for the first non-zero coefficient (4, 8) in the scan order is concealed. The secret code can be deduced by the video decoding apparatus from the parity check on the sum of the absolute values of the intra-group coefficients. The coefficients concealed under the code concealment technique of this embodiment are marked in black in FIG. 8B. The sign of the total of two coefficients (the coefficients of No. 4 and No. 8) among the 8 coefficients is concealed. That is, the code hiding technique of this embodiment saves 2 bits.

As another example, assuming that the number of predefined groups is 3, applying the technique of this embodiment to the transform coefficient block illustrated in FIG. 7A is as follows.

Divide all eight nonzero coefficients into three groups. As in the example of FIG. 7A, if the total number of non-zero coefficients in the transform block and the number of groups are not in a multiple relation, the non-zero coefficients are grouped according to a predetermined rule. Here, for convenience, {1, 2, 3} coefficients are assigned to the first group, {4, 5, 6} coefficients to the second group, and {7, } Coefficients are assigned to the third group. In each group, the sign for the first non-zero coefficient (i.e., number 3, number 6, and number 8) in the scan order is concealed. Note that, unlike the second embodiment, the sign of the coefficient 8 is further hidden. The coefficients concealed under the sign cloaking technique of this embodiment are marked in black in Fig. 8d. Of the eight coefficients, the sign of the total of three coefficients ( numbers 3, 6, and 8) was concealed. That is, 3 bits are saved through the code hiding technique of the present embodiment.

<Fourth Embodiment>

In this embodiment, the non-zero coefficients in one transform block are divided into a predetermined number of groups, and the coefficients of the low-frequency component and the high-frequency component are mixed with each other on the frequency domain basis (that is, Unlike the third embodiment, continuous non-zero coefficients in the scan sequence are not grouped). Then, the sign of the coefficient of the low frequency (or high frequency) component is concealed in each group. By mixing the low-frequency coefficients and the high-frequency coefficients in each group, the operation of adjusting the coefficient values to conform to the code prediction rule can be achieved without the operation of low-frequency components. That is, code hiding can be performed by modifying the high frequency component rather than modifying the relatively low frequency component. Since the technique of the present embodiment must be able to know the number of all non-zero coefficients in the corresponding transform block before generating the sign data, the sign data at the end of the syntax for all coefficient information, As shown in Fig.

Example # 1

Assuming that the number of predefined groups is 2, applying the technique of this embodiment to the transform coefficient block illustrated in FIG. 7A is as follows.

The image encoding apparatus first signals the syntax information for all eight non-zero coefficients. And divides the total eight non-zero coefficients into two groups. In this example, the low frequency components and the high frequency components are mixed so that {1, 2, 5, 6} coefficients are assigned to the first group, and {3, 4, 7, 8} Lt; / RTI &gt; The codes for the first non-zero coefficients (coefficients 6 and 8) are hidden in the scan sequence in the above two groups. The coefficients concealed under the code concealment technique of this embodiment are marked in black in FIG. 8E. Alternatively, when the sign of the coefficient of the high frequency component is concealed, the sign of the coefficients of the first and third coefficients is concealed. Of the eight coefficients, the sign of two total coefficients (coefficients 6 and 8) was concealed. That is, the code hiding technique of this embodiment saves 2 bits.

Example # 2

As another example, when the number of predefined groups is 2, and a non-zero coefficient is placed as shown in FIG. 7A, the fourth embodiment first signals the syntax information for all eight non-zero coefficients. Then, all eight non-zero coefficients are divided into two groups based on the threshold value (= 2). In this example, the low frequency component and the high frequency component are mixed so that {1, 2, 7, 8} coefficients are assigned to the first group, and {3, 4, 5, 6} Lt; / RTI &gt; The codes for the first non-zero coefficients (coefficients 8 and 6) on the above two intra-group scan orders are hidden. The coefficients concealed under the code concealment technique of this embodiment are marked in black in Fig. 8f. Of the eight coefficients, the sign of the two total coefficients (coefficients 8 and 6) was concealed. Alternatively, when the sign of the coefficient of the high frequency component is concealed, the sign of the coefficients of the first and third coefficients is concealed. That is, the code hiding technique of this embodiment saves 2 bits.

The difference between Examples # 1 and # 2 is a grouping criterion of low frequency and high frequency components. FIGS. 8G and 8H are diagrammatic representations of grouping differences of Example # 1 and Example # 2, in which the non-zero coefficients in the transform coefficient block of FIG. 7A are listed in reverse order of scan order. In Example # 1, {1, 2, 5, 6} coefficients are grouped into the same group as in FIG. 8G, and the {3, 4, 7, 8} coefficients are grouped into the same group. As shown in FIG. 8H, the coefficients {1, 2, 7, 8} are grouped into the same group, and the coefficients {3, 4, 5, 6} are grouped into the same group. If the number of total non-zero coefficients and the number of groups are not in multiple relation, the non-zero coefficients should be grouped according to the predetermined rules.

The gist of this embodiment is to mix low frequency and high frequency position coefficients into each group for code concealment and code prediction, and such grouping type is not limited to the above two types. For example, odd-order non-zero coefficients may be assigned to the first group and even-numbered non-zero coefficients may be assigned to the second group in the scan order. Similarly, when the non-zero coefficients are divided into three groups, the (first, fourth, seventh, ...) } Non-zero coefficients into the first group, {second, fifth, eighth, ... } The coefficients belong to the second group, {3rd, 6th, 9th, ... } Non-zero coefficients may be assigned to the third group. This grouping can be accomplished by performing a modulo-N operation based on the number of predefined groups (N) for the order of the non-zero coefficients on the scan order.

<Fifth Embodiment>

In this embodiment, the absolute values of the non-zero coefficients meeting in the reverse order of the scan order from the last non-zero coefficient (for example, coefficient 1 in Fig. 7A) in one transform block are accumulated and added, , The code of the corresponding non-zero coefficient is concealed, and scanning is performed again. The procedure for this embodiment is as follows.

Assuming that the threshold value thr is 5, the technique of this embodiment is applied to the transform coefficient block illustrated in Fig. 7A as follows.

For illustrative purposes, the level (i.e., the absolute value) of the non-zero coefficients in the transform coefficient block illustrated in FIG.

Coefficient number		number 1	No.2	number 3	No. 4	5 times	No. 6	No. 7	8 times
level	One	3	2	3	2	2	3	One

The image encoding apparatus first initializes the variable &quot; sum &quot; and accumulates the absolute values of the non-zero coefficients that are encountered in the reverse order of the scan order, starting from the first coefficient. (I.e., the sum of the absolute values of the non-zero coefficients = 6) reaches the threshold value (= 5), and therefore the image encoding apparatus performs the sign concealment process Is applied. The sign of the coefficient 3 can be inferred from the parity of the sum of the absolute values of the coefficients 1, 2 and 3 by the image decoding apparatus. The image coding apparatus resets the variable &quot; sum &quot; and accumulates the absolute value again after the non-zero coefficient applied with the sign concealment process. That is, starting from the fourth coefficient, the absolute values of the non-zero coefficients encountered in the reverse order of the scan order are accumulated. The accumulated absolute value (= 7) reaches the threshold value (= 5) when reaching the coefficient of 6, so that the image coding apparatus applies the code concealment process to the coefficient of 6. The sign of the sixth coefficient can be inferred from the parity of the sum of the absolute values of the coefficients of the fourth, fifth and sixth coefficients by the image decoding apparatus.

The image coding apparatus resets the variable &quot; sum &quot; and accumulates the absolute value again after the non-zero coefficient applied with the sign concealment process. That is, starting from coefficient 7, the absolute values of the non-zero coefficients encountered in the reverse order of the scan order are accumulated. Since the accumulated absolute value (= 4) does not exceed the threshold value (= 5) even after reaching the first non-zero coefficient of scan order, there is no non-zero coefficient to which the code concealment process is applied . As a result, the sign of the total of the two coefficients (the numbers 3 and 6) among the eight coefficients was concealed. That is, the code hiding technique of this embodiment saves 2 bits.

<Sixth Embodiment>

This embodiment is a combination of the methods of the first and fifth embodiments described above. That is, the sum of the absolute values of the non-zero coefficients and the distance of non-zero coefficients encountered in the reverse order of the scan order from the last non-zero coefficient (e.g., coefficient 1 in FIG. 7A) When the distance is equal to or larger than the first threshold value thr1 and the sum of absolute values is equal to or larger than the second threshold value thr2 , the sign of the non-zero coefficient is concealed and the scanning is performed again. The procedure for this embodiment is as follows.

<Seventh Embodiment>

The present embodiment is a combination of the methods of the above-described second and fifth embodiments. That is, the "number of non-zero coefficients" and the "sum of absolute values" that meet in the reverse order of the scan order from the last non-zero coefficient (coefficient 1 in FIG. 7A) in one conversion block are obtained, If the sum of the absolute values is equal to or larger than the first threshold value thr1 and is greater than or equal to the second threshold value thr2 , the sign of the non-zero coefficient is concealed and scanning is performed again. The procedure for this embodiment is as follows.

&Lt; Eighth Embodiment >

This embodiment relates to a technique for differentiating the non-zero coefficient used to check whether the code concealment condition is satisfied and the non-zero coefficient whose actual code is concealed and predicted. In the previous embodiments, this embodiment distinguishes if the coefficients whose coefficients are concealed (and thus predicted by the image decoding apparatus) were one non-zero coefficient used to identify code concealment conditions.

For example, assume that there are n non-zero coefficients in one transform block (or one sub-block). According to this embodiment, the sum of the absolute values of m (< n) non-zero coefficients among the n non-zero coefficients is used to calculate one or more non- The sign of the zero coefficient is hidden by the encoder and is predicted (inferred) by the decoder. In this case, the value of m is the value promised between the encoder and the decoder, and the non-zero coefficient whose actual code is predicted is the position or order of the coefficient by the promise of the encoder and decoder among the remaining (n - m) non- And the like. In general, the m non-zero coefficients are located in the front in the reverse order of the scan order, and the coefficient whose sign is predicted is located after this.

In each of the above embodiments, the prediction of the hidden code is based on a parity check of the sum of the absolute values of all associated non-zero coefficients. For example, if the sum of the absolute values is an even number, the hidden code is positive, and if the sum of the absolute values is an odd number, the hidden code is predicted to be negative. This code prediction process can be simplified to the AND operation of least significant bits of all associated non-zero coefficients. For example, when there are a total of three nonzero coefficients, an AND operation between the least significant bits of the representations of three coefficients (A, B, C) (C &amp; 0x01) "). That is, if the result of the AND operation is "0", the sign is determined as "+", and if the AND operation result is "1", the sign is determined as "1". The opposite is also possible.

9 is a flowchart illustrating a method of applying a code concealment technique to a coefficient block, which is an array of transform coefficients, in an image encoding apparatus according to an aspect of the present invention.

First, the image encoding apparatus determines the position of non-zero coefficients in the current coefficient block (S910). The image encoding apparatus can signal the positions and levels of non-zero coefficients in the current coefficient block using at least some of the syntax elements illustrated in Table 1. [

Next, the image encoding apparatus divides the non-zero coefficients in the current coefficient block, excluding the zero coefficients, into a plurality of groups (S920). In some examples, the image encoding apparatus may group the non-zero coefficients in the current coefficient block, in a reverse scan order, by a predetermined number of non-zero coefficients. In some other instances, the image encoding apparatus may divide non-zero coefficients into a plurality of groups by performing a modulo-N operation based on the number of groups (N) for each non-zero coefficient in the current coefficient block have.

Next, the image encoding apparatus identifies the last non-zero coefficient and the first non-zero coefficient defined by the reverse order of the scanning order for each group (S930).

For each group, if the distance in the scan order between the last non-zero coefficient and the first non-zero coefficient is greater than a predetermined threshold value, the image encoding apparatus performs a code data concealment process on the last non-zero coefficient (S940). That is, the image encoding apparatus does not signal sign data (i.e., sign bit) for the last non-zero coefficient. The code data concealment process may include a parity check on the sum of the absolute values of the non-zero coefficients included in the group. If the parity does not point to the correct sign of the first non-zero coefficient, the image encoding apparatus selectively compensates for the sign bit to be hidden. This is accomplished in a video encoding device by selecting one non-zero coefficient in the group and modifying the value to an adjacent value that is greater or smaller than the previous value. For this compensation, a non-zero coefficient having a level close to the boundary of the quantization interval can be selected. Alternatively, a non-zero coefficient corresponding to a relatively high frequency component in the group may be selected. The sign data (i.e., sign bit) for the remaining non-zero coefficients of each group, except for the last non-zero coefficient, is explicitly signaled.

10 is a flowchart illustrating a method of applying a code concealment scheme to a coefficient block, which is an array of transform coefficients, in an image encoding apparatus according to another aspect of the present invention.

First, the image encoding apparatus determines the position of non-zero coefficients in the current coefficient block (S1010). The image encoding apparatus can signal the positions and levels of non-zero coefficients in the current coefficient block using at least some of the syntax elements illustrated in Table 1. [

Next, the image encoding apparatus sets the position of the last non-zero coefficient in the scan order in the current coefficient block to the first position (S1020).

Next, the image encoding apparatus determines a second position closest to the scanning order from the first position, which satisfies the code concealment condition, among the non-zero coefficients encountered in the reverse order of the scanning order from the first position (S1030) . Here, the code concealment condition is such that the first variable defined by the first position and the second position exceeds a predetermined first threshold value. In some embodiments, the first variable may be the sum of the absolute values of the non-zero coefficients encountered in the reverse order of the scan order from the first position to the second position within the coefficient block. In another embodiment, the first variable may be a number of non-zero coefficients located between the first and second positions in the scan order. In another embodiment, the first variable may be a distance between the first position and the second position in the scan order.

The image encoding apparatus applies a sign concealment process to the non-zero coefficient of the second position determined in step S1030 (S1040). That is, the image encoding apparatus does not signal sign data (i.e., sign bit) for the non-zero coefficient of the second position. The code data concealment process may include a parity check on the sum of the absolute values of all non-zero coefficients encountered from the first position to the second position in the scan order. If the parity does not indicate the correct sign of the non-zero coefficient in the second position, the image encoding apparatus selectively compensates for the sign bit to be hidden. This is accomplished in a video encoding apparatus by selecting one non-zero coefficient from among all non-zero coefficients encountered from the first position to the second position and modifying the value to an adjacent value that is greater or smaller than the previous value. For this compensation, a non-zero coefficient having a level close to the boundary of the quantization interval can be selected. Alternatively, a non-zero coefficient corresponding to a relatively high frequency component may be selected from among all non-zero coefficients encountered from the first position to the second position. The sign data (i.e., sign bit) for the other non-zero coefficients to which the sign data hiding process is not applied is explicitly signaled.

The image encoding apparatus sets the position of the nearest non-zero coefficient that is in the reverse direction of the scan order from the second position to which the sign prediction process is applied to the new first position (S1050).

The image encoding apparatus repeatedly performs S1030 to S1050 until it reaches the first non-zero coefficient in the scan order in the current coefficient block. That is, if the position of the first non-zero coefficient is selected as the first position or the second position, or exists between the first position and the second position (Yes in S1060), the above procedure is terminated.

11 is a flowchart illustrating a method of decoding a bitstream of an encoded image in order to recover a coefficient block, which is an array of transform coefficients, in an image decoding apparatus according to an aspect of the present invention.

First, the image decoding apparatus determines the position of non-zero coefficients in the current coefficient block (S1110). To this end, the image decoding apparatus can parse at least a part of the syntax elements illustrated in Table 1 from the bitstream.

Next, the image decoding apparatus divides the non-zero coefficients in the current coefficient block, excluding the zero coefficients, into a plurality of groups (S1120). In some examples, the image decoding apparatus may group the non-zero coefficients in the current coefficient block by a predetermined number of non-zero coefficients according to the reverse order of the scan order. In some other instances, the image decoding apparatus may divide non-zero coefficients into a plurality of groups by performing a modulo-N operation based on the number of groups (N) for each non-zero coefficient in the current coefficient block have.

Next, the video decoding apparatus identifies the last non-zero coefficient and the first non-zero coefficient defined by the reverse order of the scanning order for each group (S1130).

For each group, the image decoding apparatus performs a code data prediction process on the last non-zero coefficient if the distance on the scan order between the last non-zero coefficient and the first non-zero coefficient is greater than a preset threshold value (S1140). The sign data prediction process may include a parity check on the sum of the absolute values of the non-zero coefficients included in the group. If the sum of the absolute values of the non-zero coefficients contained in the group is even, then the sign of the last non-zero coefficient is inferred as positive, otherwise it is negatively inferred. The sign data prediction process may include an AND operation of least significant bits of non-zero coefficients included within the group. If the result of the AND operation is "0 ", the sign is determined as" + " The opposite is also possible.

12 is a flowchart illustrating a method of decoding a bitstream of an encoded image to recover a coefficient block, which is an array of transform coefficients, in an image decoding apparatus according to another aspect of the present invention.

First, the image decoding apparatus determines the position of non-zero coefficients in the current coefficient block (S1210). To this end, the image decoding apparatus can parse at least a part of the syntax elements illustrated in Table 1 from the bitstream.

Next, the image decoding apparatus sets the position of the last non-zero coefficient in the scan order in the current coefficient block to the first position (S1220).

Next, the video decoding apparatus determines a second position closest to the scanning order from the first position, which satisfies the code concealment condition, among the non-zero coefficients meeting in the reverse order of the scanning order from the first position (S1230) . Here, the code concealment condition is such that the first variable defined by the first position and the second position exceeds a predetermined first threshold value. In some embodiments, the first variable may be the sum of the absolute values of the non-zero coefficients encountered in the reverse order of the scan order from the first position to the second position within the coefficient block. In another embodiment, the first variable may be a number of non-zero coefficients located between the first and second positions in the scan order. In another embodiment, the first variable may be a distance between the first position and the second position in the scan order.

The image decoding apparatus applies the sign prediction process to the non-zero coefficient of the second position determined in step S1230 (S1240). The sign data prediction process may include a parity check on the sum of the absolute values of all non-zero coefficients encountered from the first position to the second position in the scan order. If the sum of the absolute values is even, the sign of the non-zero coefficient of the last second position is deduced as positive, otherwise it is negatively deduced. The sign data prediction process may comprise an AND operation of least significant bits of all non-zero coefficients encountered from the first position to the second position. If the result of the AND operation is "0 ", the sign is determined as" + " The opposite is also possible.

The image decoding apparatus sets the position of the closest non-zero coefficient to a new first position in a reverse order of the scan order from the second position to which the sign prediction process is applied (S1250).

The image decoding apparatus repeatedly performs S1230 to S1250 until it reaches the first non-zero coefficient in the scan order in the current coefficient block. That is, when the position of the first non-zero coefficient is selected as the first position or the second position, or exists between the first position and the second position (Yes in S1260) And is terminated.

The foregoing description is merely illustrative of the technical idea of the present embodiment, and various modifications and changes may be made to those skilled in the art without departing from the essential characteristics of the embodiments. Therefore, the present embodiments are to be construed as illustrative rather than restrictive, and the scope of the technical idea of the present embodiment is not limited by these embodiments. The scope of protection of the present embodiment should be construed according to the following claims, and all technical ideas within the scope of equivalents thereof should be construed as being included in the scope of the present invention.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is related to Korean Patent Application No. 10-2018-0001710 filed on Jan. 5, 2018, which is incorporated herein by reference in its entirety, and Korean Patent Application Priority is claimed on Application No. 10-2018-0070469.

Claims (13)

1. A method for decoding a bitstream of an encoded image to recover a coefficient block, which is an array of transform coefficients, in an image decoding apparatus,

   A first step of identifying positions of effective coefficients in a current coefficient block;

   A second step of setting a position of a last non-zero coefficient in a scan order in a current coefficient block to a first position;

   A third step of determining a second position closest to the scanning order from the first position satisfying the code concealment condition among the non-zero coefficients encountered in the reverse order of the scanning order from the first position;

   A fourth step of applying a sign prediction process to the non-zero coefficient of the second position; And

   A fifth step of setting a position of a closest non-zero coefficient that meets in a reverse order of a scan order from a second position to which the sign prediction process is applied to the first position

   Lt; / RTI &gt;

   Repeatedly performing the third to fifth steps until the first non-zero coefficient on the scan order is reached in the current coefficient block.
2. The method according to claim 1,

   Wherein the code conceal condition is such that a first parameter defined by the first position and the second position exceeds a predetermined first threshold.
3. 3. The method of claim 2,

   Wherein the first variable comprises:

   Wherein in the coefficient block, the absolute value of the non-zero coefficients encountered in the reverse order of the scan order from the first position to the second position.
4. 3. The method of claim 2,

   Wherein the first variable comprises:

   Zero coefficients located between the first position and the second position in the scan order.
5. 3. The method of claim 2,

   Wherein the first variable comprises:

   Wherein the first position is a distance between the first position and the second position in the scan order.
6. The method according to claim 1,

   Wherein the sign conceal condition is such that a first variable defined by the first position and the second position exceeds a predetermined first threshold value and a second variable defined by the first position and the second position Wherein the second threshold value is greater than a predetermined second threshold value.
7. The method according to claim 6,

   Wherein the first variable is a distance between the first position and the second position in the scan order, and the second variable is a ratio in which the convergence occurs in the reverse order of the scan order from the first position to the second position in the coefficient block - the sum of the absolute values of the zero coefficients.
8. The method according to claim 6,

   Wherein the first variable is a number of non-zero coefficients located between a first position and a second position in the scan order and the second variable is the number of non-zero coefficients in the scan block from the first position to the second position, Zero coefficients encountered in the reverse order of the order.
9. A method for decoding a bitstream of an encoded image to recover a coefficient block, which is an array of transform coefficients, in an image decoding apparatus,

   Determining, at the current coefficient block, a position of non-zero coefficients;

   Dividing non-zero coefficients in the current coefficient block, excluding zero coefficients, into a plurality of groups;

   Identifying, for each group, a last non-zero coefficient and a first non-zero coefficient that are defined by a reverse scan order; And

   For each group, if the distance in the reverse scan order between the last non-zero coefficient and the first non-zero coefficient is greater than a preset threshold, then a sign prediction process is performed on the last non-zero coefficient step

   / RTI &gt;
10. 10. The method of claim 9,

    Dividing the non-zero coefficients into a plurality of groups,

    Grouping the non-zero coefficients in the current coefficient block by a predetermined number according to a reverse order of the scan order.
11. 10. The method of claim 9,

    Dividing the non-zero coefficients into a plurality of groups,

    Performing a modulo-N operation based on the number of groups (N) for an order on a scan order of each non-zero coefficients.
12. A video decoding apparatus comprising:

    One or more processors; And

    The instructions comprising instructions for causing the image decoding apparatus to, when executed by the one or more processors,

    A first step of identifying positions of effective coefficients in a current coefficient block;

    A second step of setting a position of a last non-zero coefficient in a scan order in a current coefficient block to a first position;

    A third step of determining a second position closest to the scanning order from the first position satisfying the code concealment condition among the non-zero coefficients encountered in the reverse direction of the scan order from the first position;

    A fourth step of applying a sign prediction process to the non-zero coefficient of the second position; And

    A fifth step of setting a position of a closest non-zero coefficient which meets in a reverse direction of a scan order from a second position to which the sign prediction process is applied to the first position

    , And performs the third to fifth steps repeatedly until the first non-zero coefficient reaches the first non-zero coefficient in the scan order in the current coefficient block. .
13. A video decoding apparatus comprising:

    One or more processors; And

    The instructions comprising instructions for causing the image decoding apparatus to, when executed by the one or more processors,

    Determining, at the current coefficient block, a position of non-zero coefficients;

    Dividing non-zero coefficients in the current coefficient block, excluding zero coefficients, into a plurality of groups;

    Identifying, for each group, a last non-zero coefficient and a first non-zero coefficient that are defined by a reverse scan order; And

    For each group, if the distance in the reverse scan order between the last non-zero coefficient and the first non-zero coefficient is greater than a preset threshold, then a sign prediction process is performed on the last non-zero coefficient step

    To the video decoding apparatus.

Images