Method for configuring merge candidate list for inter prediction

Abstract

The present embodiment provides a method for configuring a merge candidate list for inter prediction. In the present embodiment, an image decoding device decodes a merge index indicating one of merge candidates and configures a primary merge candidate list of a current block. The image decoding device configures the primary merge candidate list by generating a primary merge candidate list including a plurality of types of merge candidates, resorting the merge candidates on the basis of template matching costs of the merge candidates in the primary merge candidate list, and additionally sorting the merge candidates on the basis of comparison between the resorted merge candidates. The image decoding device selects a merge candidate of the current block from the primary merge candidate list on the basis of the merge index, and predicts the current block on the basis of motion information of the selected merge candidate.

Description

Method for constructing a merge candidate list for inter prediction

[0001] The present disclosure relates to an image encoding / decoding method, an apparatus, and a recording medium for storing a bitstream, and more specifically, to a method for constructing a merge candidate list for inter-prediction.

[0002] The following description merely provides background information related to the present invention and does not constitute prior art.

[0003] Because video data contains a large amount of data compared to audio or still image data, storing or transmitting it as is without compression processing requires significant hardware resources, including memory.

[0004] Therefore, typically when storing or transmitting video data, the encoder compresses the video data for storage or transmission, and the decoder receives the compressed video data, decompresses it, and plays it. Such video compression technologies include H.264 / AVC, HEVC (High Efficiency Video Coding), and VVC (Versatile Video Coding), which improves coding efficiency by more than 30% compared to HEVC.

[0005] However, as video size, resolution, and frame rates are gradually increasing, and the amount of data that needs to be encoded is also growing accordingly, a new compression technology is required that offers better encoding efficiency and higher image quality improvement effects than existing compression technologies.

[0006] The present disclosure aims to provide an image encoding / decoding method and apparatus for constituting a merge candidate list or an AMVP (Advanced Motion Vector Prediction) candidate list in inter-prediction, and a recording medium for storing a bitstream generated by said image encoding method / apparatus.

[0007] According to an embodiment of the present disclosure, a method for restoring a current block, performed by an image decoding device, comprises: decoding a merge index indicating one of the merge candidates; and constructing a primary merge candidate list of the current block, wherein the step of constructing the primary merge candidate list comprises: generating a primary merge candidate list including a plurality of types of merge candidates; rearranging the merge candidates based on the template matching costs of the merge candidates within the primary merge candidate list, wherein the template matching costs represent the costs between the template of the current block and the template of the reference block indicated by each merge candidate; and further rearranging the merge candidates based on a comparison between the rearranged merge candidates.

[0008] According to another embodiment of the present disclosure, a method for encoding a current block performed by an image encoding device comprises: obtaining a merge index indicating one of the merge candidates; constructing a primary merge candidate list of the current block; and encoding the merge index, wherein the step of constructing the primary merge candidate list comprises: generating a primary merge candidate list including a plurality of types of merge candidates; rearranging the merge candidates based on the template matching costs of the merge candidates within the primary merge candidate list, wherein the template matching costs represent the costs between the template of the current block and the template of the reference block indicated by each merge candidate; and further rearranging the merge candidates based on a comparison between the rearranged merge candidates.

[0009] According to another embodiment of the present disclosure, a method for providing video data to an image decoder comprises: encoding the video data into a bitstream; and delivering the bitstream to the image decoder, wherein the step of encoding the video data comprises: obtaining a merge index indicating one of the merge candidates; constructing a primary merge candidate list of a current block; and encoding the merge index, wherein the step of constructing the primary merge candidate list comprises: generating a primary merge candidate list including a plurality of types of merge candidates; rearranging the merge candidates based on the template matching costs of the merge candidates within the primary merge candidate list, wherein the template matching costs represent the costs between the template of the current block and the template of the reference block indicated by each merge candidate; and further rearranging the merge candidates based on a comparison between the rearranged merge candidates.

[0010] As described above, by providing a video encoding / decoding method, an apparatus, and a recording medium storing a bitstream generated by the video encoding method / apparatus according to the present embodiment, it is possible to improve video encoding efficiency and video quality.

[0011] FIG. 1 is an exemplary block diagram of an image encoding device capable of implementing the technologies of the present disclosure.

[0012] Figure 2 is a diagram illustrating a method for dividing blocks using a QTBTTT (QuadTree plus BinaryTree TernaryTree) structure.

[0013] FIGS. 3a and 3b are diagrams showing a plurality of intra prediction modes including wide-angle intra prediction modes.

[0014] Figure 4 is an example diagram of the surrounding blocks of the current block.

[0015] FIG. 5 is an exemplary block diagram of an image decoding device capable of implementing the technologies of the present disclosure.

[0016] FIG. 6 is a flowchart illustrating a method for configuring a merge candidate list according to one embodiment of the present disclosure.

[0017] FIG. 7 is an exemplary diagram showing a candidate reference block and a template of a candidate reference block according to one embodiment of the present disclosure.

[0018] FIG. 8 is a flowchart illustrating a method for predicting a current block according to one embodiment of the present disclosure.

[0019] Hereinafter, embodiments of the present invention will be described in detail with reference to the exemplary drawings. It should be noted that in assigning reference numerals to the components of each drawing, the same components are given the same reference numeral whenever possible, even if they are shown in different drawings. Furthermore, in describing these embodiments, if it is determined that a detailed description of related known components or functions could obscure the essence of these embodiments, such detailed description is omitted.

[0020] FIG. 1 is an exemplary block diagram of an image encoding device capable of implementing the technologies of the present disclosure. Hereinafter, the image encoding device and its sub-components will be described with reference to FIG. 1.

[0021] The video encoding device may be configured to include a picture splitting unit (110), a prediction unit (120), a subtractor (130), a conversion unit (140), a quantization unit (145), a reordering unit (150), an entropy encoding unit (155), an inverse quantization unit (160), an inverse conversion unit (165), an adder (170), a loop filter unit (180), and a memory (190).

[0022] Each component of the video encoding device may be implemented in hardware or software, or as a combination of hardware and software. Additionally, the function of each component may be implemented in software, and a microprocessor may be implemented to execute the software function corresponding to each component.

[0023] A single image (video) consists of one or more sequences containing multiple pictures. Each picture is divided into multiple regions, and encoding is performed for each region. For example, a single picture is divided into one or more tiles and / or slices. Here, one or more tiles can 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 Coding Units (CUs) by a tree structure. Information applicable to each CU is encoded as the syntax of the CU, and information applicable to all CUs included in a single CTU is encoded as the syntax of the CTU. Additionally, information applicable to all blocks within a single slice is encoded as the syntax of the slice header, and information applicable to all blocks constituting one or more pictures is encoded in the Picture Parameter Set (PPS) or the picture header. Furthermore, information commonly referenced by multiple pictures is encoded in a Sequence Parameter Set (SPS). Also, information commonly referenced by one or more SPSs is encoded in a Video Parameter Set (VPS). Additionally, information commonly applicable to a single tile or tile group may be encoded as the syntax of a tile or tile group header. The syntax included in the SPS, PPS, slice header, and tile or tile group header may be referred to as high-level syntax.

[0024] The picture splitting unit (110) determines the size of the CTU. Information regarding the size of the CTU (CTU size) is encoded as a syntax of SPS or PPS and transmitted to an image decoding device.

[0025] The picture division unit (110) divides each picture constituting the image into multiple CTUs having a predetermined size, and then recursively divides the CTUs using a tree structure. The leaf nodes in the tree structure become the CUs, which are the basic units of encoding.

[0026] The tree structure may be a QuadTree (QT) in which an upper node (or parent node) is divided into four lower nodes (or child nodes) of equal size, a BinaryTree (BT) in which an upper node is divided into two lower nodes, a TernaryTree (TT) in which an upper node is divided into three lower nodes in a 1:2:1 ratio, or a structure that combines two or more of these QT, BT, and TT structures. For example, a QTBT (QuadTree plus BinaryTree) structure may be used, or a QTBTTT (QuadTree plus BinaryTree TernaryTree) structure may be used. Here, BTTT combined may be referred to as an MTT (Multiple-Type Tree).

[0027] Figure 2 is a diagram illustrating a method for dividing blocks using a QTBTTT structure.

[0028] As illustrated in FIG. 2, the CTU can first be split into a QT structure. Quadtree splitting can be repeated until the size of the splitting block reaches the minimum block size of the leaf node allowed in QT (MinQTSize). A first flag (QT_split_flag) indicating whether each node of the QT structure is split into four nodes of the lower layer is encoded by the entropy encoder (155) and signaled to the image decoder. If the leaf node of the QT is not larger than the maximum block size of the root node allowed in BT (MaxBTSize), it can be further split into one or more of the BT structure or TT structure. In the BT structure and / or TT structure, multiple splitting directions may exist. For example, there may be two directions in which the block of the corresponding node is split horizontally and vertically. As shown in Figure 2, when MTT splitting begins, a second flag (mtt_split_flag) indicating whether the nodes have been split, and if splitting has occurred, a flag indicating the splitting direction (vertical or horizontal) and / or the splitting type (binary or ternary) are encoded by the entropy encoding unit (155) and signaled to the image decoding device.

[0029] Alternatively, prior to encoding the first flag (QT_split_flag) indicating whether each node is split into four nodes of the lower layer, the CU split flag (split_cu_flag) indicating whether the node is split may be encoded. If the value of the CU split flag (split_cu_flag) indicates that it 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 the basic unit of encoding. If the value of the CU split flag (split_cu_flag) indicates that it is split, the video encoding device starts encoding from the first flag in the manner described above.

[0030] When QTBT is used as another example of a tree structure, there may be two types: a type that divides the block of the corresponding node horizontally into two blocks of the same size (i.e., symmetric horizontal splitting) and a type that divides it vertically (i.e., symmetric vertical splitting). A splitting flag (split_flag) indicating whether each node of the BT structure is split into a block of a lower layer and splitting type information indicating the type of splitting are encoded by the entropy encoding unit (155) and transmitted to the image decoding device. Meanwhile, there may also be an additional type that divides the block of the corresponding node into two blocks of an asymmetric shape. The asymmetric shape may include a shape that divides the block of the corresponding node into two rectangular blocks with a size ratio of 1:3, or a shape that divides the block of the corresponding node diagonally.

[0031] A CU can have various sizes depending on the QTBT or QTBTTT partitioning from a CTU. Hereinafter, the block corresponding to the CU to be encoded or decoded (i.e., the leaf node of QTBTTT) is referred to as the 'current block'. Depending on the adoption of QTBTTT partitioning, the shape of the current block may be not only square but also rectangular.

[0032] 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).

[0033] Generally, current blocks within a picture can each be predictively coded. Typically, the prediction of a current block can be performed using an intra-prediction technique (using data from the picture containing the current block) or an inter-prediction technique (using data from a picture coded prior to the picture containing the current block). Inter-prediction includes both unidirectional and bidirectional prediction.

[0034] The intra prediction unit (122) predicts pixels within the current block using pixels (reference pixels) located around the current block within the current picture containing the current block. Multiple intra prediction modes exist depending on the prediction direction. For example, as shown in FIG. 3a, multiple intra prediction modes may include two non-directional modes, including Planar mode and DC mode, and 65 directional modes. The surrounding pixels to be used and the calculation formula are defined differently for each prediction mode.

[0035] For efficient directional prediction for a rectangular current block, directional modes (intra-prediction modes 67 through 80 and -1 through -14) illustrated by dashed arrows in FIG. 3b may be additionally used. These may be referred to as "wide angle intra-prediction modes." In FIG. 3b, the arrows indicate corresponding reference samples used for prediction and do not indicate the prediction direction. The prediction direction is opposite to the direction indicated by the arrows. Wide angle intra-prediction modes are modes that perform prediction in the opposite direction of a specific directional mode without additional bit transmission when the current block is rectangular. Among the wide angle intra-prediction modes, some wide angle intra-prediction modes available for the current block may be determined by the ratio of the width to the height of the rectangular current block. For example, wide-angle intra-prediction modes with an angle less than 45 degrees (intra-prediction modes 67 to 80) are available when the current block is a rectangular shape with a height less than the width, and wide-angle intra-prediction modes with an angle greater than -135 degrees (intra-prediction modes -1 to -14) are available when the current block is a rectangular shape with a width greater than the height.

[0036] The intra prediction unit (122) can determine the intra prediction mode to use for encoding 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) may calculate the rate-distortion values ​​using a rate-distortion analysis of several tested intra prediction modes and select the intra prediction mode having the best rate-distortion features among the tested modes.

[0037] The intra prediction unit (122) selects one intra prediction mode among a plurality of intra prediction modes and predicts the current block using a calculation formula and surrounding pixels (reference pixels) determined according to the selected intra prediction mode. Information regarding the selected intra prediction mode is encoded by the entropy encoding unit (155) and transmitted to the image decoding device.

[0038] The inter prediction unit (124) generates a prediction block for the current block using a motion compensation process. The inter prediction unit (124) searches for the block most similar to the current block within a reference picture that is encoded and decoded before the current picture, and generates a prediction block for the current block using the searched block. Then, it generates a motion vector (MV) corresponding to the displacement between the current block in the current picture and the prediction block in the reference picture. Generally, motion estimation is performed on the lumina component, and the motion vector calculated based on the lumina component is used for both the lumina component and the chroma component. Motion information including information about the reference picture used to predict the current block and information about the motion vector is encoded by the entropy encoding unit (155) and transmitted to the image decoding device.

[0039] The inter prediction unit (124) may perform interpolation on a reference picture or reference block to increase the accuracy of the prediction. That is, subsamples between two consecutive integer samples are interpolated by applying filter coefficients to a plurality of consecutive integer samples including those two integer samples. When the process of searching for the block most similar to the current block is performed for the interpolated reference picture, the motion vector can be expressed with precision in fractional units rather than precision in integer sample units. The precision or resolution of the motion vector can be set differently for each unit of the target area to be encoded, such as slice, tile, CTU, CU, etc. When such Adaptive Motion Vector Resolution (AMVR) is applied, information regarding the motion vector resolution to be applied to each target area must be signaled for each target area. For example, if the target area is a CU, information regarding the motion vector resolution applied to each CU is signaled. The information regarding the motion vector resolution may be information indicating the precision of the difference motion vector described later.

[0040] Meanwhile, the inter prediction unit (124) can perform inter prediction using bi-prediction. In the case of bi-prediction, two reference pictures and two motion vectors representing the block location most similar to the current block within each reference picture are used. The inter prediction unit (124) selects a first reference picture and a second reference picture from the reference picture list 0 (RefPicList0) and reference picture list 1 (RefPicList1), respectively, and generates a first reference block and a second reference block by searching for a block similar to the current block within each reference picture. Then, it generates a prediction block for the current block by averaging or weighting the first reference block and the second reference block. Then, it transmits motion information containing information about the two reference pictures used to predict the current block and information about the two motion vectors to the entropy encoding unit (155). Here, reference picture list 0 consists of restored pictures that are prior to the current picture in the display order, and reference picture list 1 may consist of restored pictures that are prior to the current picture in the display order. However, this is not necessarily limited to this, and restored pictures prior to the current picture in the display order may be additionally included in reference picture list 0, and conversely, restored pictures prior to the current picture may be additionally included in reference picture list 1.

[0041] Various methods can be used to minimize the amount of bits required to encode motion information.

[0042] For example, if the reference picture and motion vector of the current block are identical to the reference picture and motion vector of a neighboring block, the motion information of the current block can be transmitted to an image decoder by encoding information that can identify the neighboring block. This method is called 'merge mode'.

[0043] In merge mode, the inter prediction unit (124) selects a predetermined number of merge candidate blocks (hereinafter referred to as 'merge candidates') from the surrounding blocks of the current block.

[0044] As for the surrounding blocks for deriving merge candidates, as shown in FIG. 4, all or part of the left block (A0), bottom-left block (A1), top block (B0), top-right block (B1), and top-left block (B2) adjacent to the current block within the current picture may be used. Additionally, a block located within a reference picture (which may be the same as or different from the reference picture used to predict the current block) other than the current picture where the current block is located may be used as a merge candidate. For example, a block located at the same position as the current block within the reference picture (co-located block) or a block adjacent to that same position may be additionally used as a merge candidate. If the number of merge candidates selected by the method described above is less than a preset number, a 0 vector is added to the merge candidates.

[0045] The inter prediction unit (124) constructs a merge list containing a predetermined number of merge candidates using these surrounding blocks. Among the merge candidates included in the merge list, it selects a merge candidate to be used as movement information for the current block and generates merge index information to identify the selected candidate. The generated merge index information is encoded by the entropy encoding unit (155) and transmitted to the image decoding device.

[0046] Merge skip mode is a special case of merge mode; after quantization, when all transform coefficients for entropy coding are close to zero, only neighbor block selection information is transmitted without transmitting residual signals. By utilizing merge skip mode, relatively high coding efficiency can be achieved in images with minimal motion, still images, and screen content images.

[0047] Hereinafter, merge mode and merge skip mode will be collectively referred to as merge / skip mode.

[0048] Another method for encoding motion information is the AMVP (Advanced Motion Vector Prediction) mode.

[0049] In AMVP mode, the inter-prediction unit (124) derives predicted motion vector candidates for the motion vector of the current block using the surrounding blocks of the current block. As surrounding blocks used to derive predicted motion vector candidates, all or part of the left block (A0), bottom-left block (A1), top block (B0), top-right block (B1), and top-left block (B2) adjacent to the current block within the current picture shown in FIG. 4 may be used. Additionally, blocks located within a reference picture (which may be the same as or different from the reference picture used to predict the current block) other than the current picture where the current block is located may be used as surrounding blocks to derive predicted motion vector candidates. For example, blocks located at the same position as the current block within the reference picture (co-located blocks) or blocks adjacent to the blocks at the same position may be used. If the number of motion vector candidates is less than a preset number by the method described above, a 0 vector is added to the motion vector candidates.

[0050] The inter prediction unit (124) derives predicted motion vector candidates using the motion vectors of the surrounding blocks and determines a predicted motion vector for the current block's motion vector using the predicted motion vector candidates. Then, it calculates a difference motion vector by subtracting the predicted motion vector from the current block's motion vector.

[0051] Predicted motion vectors can be obtained by applying a predefined function (e.g., median, mean operation, etc.) to the predicted motion vector candidates. In this case, the image decoder is also aware of the predefined function. Furthermore, since the surrounding blocks used to derive the predicted motion vector candidates have already been encoded and decoded, the image decoder is also aware of the motion vectors of those surrounding blocks. Therefore, the image decoder does not need to encode information to identify the predicted motion vector candidates. Consequently, in this case, information regarding the difference motion vector and the reference picture used to predict the current block is encoded.

[0052] Meanwhile, the predicted motion vector may be determined by selecting one of the predicted motion vector candidates. In this case, information for identifying the selected predicted motion vector candidate is additionally encoded, along with information about the difference motion vector and information about the reference picture used to predict the current block.

[0053] 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.

[0054] The conversion unit (140) converts residual signals within a residual block having pixel values ​​in a spatial domain into conversion coefficients in the frequency domain. The conversion unit (140) can convert the residual signals within the residual block using the entire size of the residual block as the conversion unit, or it can divide the residual block into multiple sub-blocks and use the sub-blocks as the conversion unit to perform the conversion. Alternatively, it can divide the residual signals into two sub-blocks, a conversion area and a non-conversion area, and use only the conversion area sub-block as the conversion unit to convert the residual signals. Here, the conversion area sub-block may be one of two rectangular blocks having a size ratio of 1:1 with respect to the horizontal axis (or vertical axis). In this case, a flag (cu_sbt_flag) indicating that only the sub-block has been converted, direction (vertical / horizontal) information (cu_sbt_horizontal_flag) and / or position information (cu_sbt_pos_flag) are encoded by the entropy encoding unit (155) and signaled to the image decoding device. Additionally, the size of the converted area sub-block may have a size ratio of 1:3 with respect to the horizontal axis (or vertical axis), and in this case, a flag (cu_sbt_quad_flag) distinguishing the corresponding division is additionally encoded by the entropy encoding unit (155) and signaled to the image decoding device.

[0055] Meanwhile, the transformation unit (140) can perform transformations on the residual block individually in the horizontal and vertical directions. For the transformation, various types of transformation functions or transformation matrices may be used. For example, a pair of transformation functions for horizontal transformation and vertical transformation can be defined as a Multiple Transform Set (MTS). The transformation unit (140) can select one pair of transformation functions with the best transformation efficiency among the MTS and transform the residual block in the horizontal and vertical directions, respectively. Information (mts_idx) regarding the selected pair of transformation functions among the MTS is encoded by the entropy encoding unit (155) and signaled to the image decoder.

[0056] The quantization unit (145) quantizes the transformation coefficients output from the transformation unit (140) using quantization parameters and outputs the quantized transformation coefficients to the entropy encoding unit (155). The quantization unit (145) may quantize the associated residual block directly without transformation for any block or frame. The quantization unit (145) may apply different quantization coefficients (scaling values) depending on the position of the transformation coefficients within the transformation block. The quantization matrix applied to the quantized transformation coefficients arranged in two dimensions can be encoded and signaled to an image decoder.

[0057] The reordering unit (150) can perform reordering of coefficient values ​​for quantized residual values.

[0058] The reordering unit (150) can convert a two-dimensional coefficient array into a one-dimensional coefficient sequence using coefficient scanning. For example, the reordering unit (150) can output a one-dimensional coefficient sequence by scanning from DC coefficients to coefficients in the high-frequency range using a zig-zag scan or a diagonal scan. Depending on the size of the conversion unit and the intra-prediction mode, a vertical scan that scans the two-dimensional coefficient array in the column direction and a horizontal scan that scans the two-dimensional block-shaped coefficients in the row direction may be used instead of a zig-zag scan. That is, depending on the size of the conversion unit and the intra-prediction mode, the scanning method to be used among a zig-zag scan, a diagonal scan, a vertical scan, and a horizontal scan may be determined.

[0059] The entropy encoding unit (155) generates a bitstream by encoding a sequence of one-dimensional quantized transformation coefficients output from the reordering unit (150) using various encoding methods such as CABAC (Context-based Adaptive Binary Arithmetic Code) and Exponential Golomb.

[0060] Additionally, the entropy encoding unit (155) encodes information related to block division, such as CTU size, CU division flag, QT division flag, MTT division type, and MTT division direction, so that the video decoder can divide the block in the same way as the video encoding unit. Additionally, the entropy encoding unit (155) encodes information regarding a prediction type indicating whether the current block is encoded by intra prediction or by inter prediction, and encodes intra prediction information (i.e., information regarding the intra prediction mode) or inter prediction information (information regarding the encoding mode of motion information (merge mode or AMVP mode), the merge index in the case of merge mode, and the reference picture index and difference motion vector in the case of AMVP mode) according to the prediction type. Additionally, the entropy encoding unit (155) encodes information related to quantization, i.e., information regarding quantization parameters and information regarding the quantization matrix.

[0061] The inverse quantization unit (160) inversely quantizes the quantized transformation coefficients output from the quantization unit (145) to generate transformation coefficients. The inverse transformation unit (165) converts the transformation coefficients output from the inverse quantization unit (160) from the frequency domain to the spatial domain to restore the residual block.

[0062] The adder (170) restores the current block by adding the restored residual block and the prediction block generated by the prediction unit (120). The pixels within the restored current block are used as reference pixels when intra-predicting the next block in sequence.

[0063] The loop filter section (180) performs filtering on the restored pixels to reduce blocking artifacts, ringing artifacts, blurring artifacts, etc. caused by block-based prediction and transformation / quantization. The loop filter section (180) may include all or part of a deblocking filter (182), a SAO (Sample Adaptive Offset) filter (184), and an ALF (Adaptive Loop Filter, 186) as an in-loop filter.

[0064] The deblocking filter (182) filters the boundaries between restored blocks to remove blocking artifacts caused by block-unit encoding / decoding, and the SAO filter (184) and ALF (186) perform additional filtering on the deblocking filtered image. The SAO filter (184) and ALF (186) are filters used to compensate for the difference between restored pixels and original pixels caused by lossy coding. The SAO filter (184) improves not only subjective image quality but also encoding efficiency by applying an offset in CTU units. In contrast, the ALF (186) performs block-unit filtering, and compensates for distortion by applying different filters by distinguishing the degree of edge and change of the corresponding block. Information regarding the filter coefficients to be used in the ALF can be encoded and signaled to an image decoder.

[0065] The restored blocks filtered through the deblocking filter (182), SAO filter (184), and ALF (186) are stored in memory (190). Once all blocks within a picture are restored, the restored picture can be used as a reference picture for inter-predicting blocks within a picture to be encoded later.

[0066] The video encoding device can store the bitstream of encoded video data on a non-transient recording medium or transmit it to a video decoding device using a communication network.

[0067] FIG. 5 is an exemplary block diagram of an image decoding device capable of implementing the technologies of the present disclosure. Hereinafter, the image decoding device and its sub-components will be described with reference to FIG. 5.

[0068] The image decoding device may be configured to include an entropy decoding unit (510), a reordering unit (515), an inverse quantization unit (520), an inverse transformation unit (530), a prediction unit (540), an adder (550), a loop filter unit (560), and a memory (570).

[0069] Similar to the image encoding device of FIG. 1, each component of the image decoding device may be implemented in hardware or software, or in combination of hardware and software. Additionally, the function of each component may be implemented in software, and a microprocessor may be implemented to execute the function of the software corresponding to each component.

[0070] The entropy decoding unit (510) determines the current block to be decoded by decoding the bitstream generated by the video encoding device and extracting information related to block division, and extracts prediction information, information on residual signals, etc., necessary to restore the current block.

[0071] The entropy decoding unit (510) extracts information about the CTU size from the SPS (Sequence Parameter Set) or PPS (Picture Parameter Set) to determine the size of the CTU and divides the picture into CTUs of the determined size. Then, the CTU is determined as the top layer of the tree structure, i.e., the root node, and divides the CTU using the tree structure by extracting division information for the CTU.

[0072] For example, when splitting a CTU using a QTBTTT structure, first, a first flag (QT_split_flag) related to QT splitting is extracted to split each node into four nodes of the lower layer. Then, for the nodes corresponding to the leaf nodes of QT, a second flag (mtt_split_flag) related to MTT splitting and splitting direction (vertical / horizontal) and / or splitting type (binary / ternary) information are extracted to split the corresponding leaf nodes into an MTT structure. Accordingly, each node below the leaf nodes of QT is recursively split into a BT or TT structure.

[0073] As another example, when splitting a CTU using the QTBTTT structure, a CU splitting flag (split_cu_flag) indicating whether to split the CU is first extracted, and if the block is split, a first flag (QT_split_flag) is extracted. During the splitting process, each node may undergo zero or more iterative MTT splittings after zero or more iterative QT splittings. For example, the CTU may undergo MTT splitting immediately, or conversely, only multiple QT splittings may occur.

[0074] As another example, when splitting a CTU using a QTBT structure, a first flag (QT_split_flag) related to the splitting of QT is extracted to split each node into four nodes of the lower layer. Then, for the nodes corresponding to the leaf nodes of QT, a split flag (split_flag) indicating whether to further split into BTs and split direction information are extracted.

[0075] Meanwhile, when the entropy decoding unit (510) determines the current block to be decoded using the division of the tree structure, it extracts information regarding the prediction type indicating whether the current block is intra-predicted or inter-predicted. If the prediction type information indicates intra-predicted, the entropy decoding unit (510) extracts syntax elements for the intra-predicted information (intra-predicted mode) of the current block. If the prediction type information indicates inter-predicted, the entropy decoding unit (510) extracts syntax elements for the inter-predicted information, namely information indicating the motion vector and the reference picture that the motion vector refers to.

[0076] Additionally, the entropy decoding unit (510) extracts information regarding quantization-related information and information regarding residual signals, as well as information regarding the quantized transformation coefficients of the current block.

[0077] The reordering unit (515) can change the sequence of one-dimensional quantized transformation coefficients entropy-decoded in the entropy decoding unit (510) back into a two-dimensional coefficient array (i.e., block) in the reverse order of the coefficient scanning order performed by the image encoding device.

[0078] The inverse quantization unit (520) inversely quantizes the quantized transformation coefficients and inversely quantizes the quantized transformation coefficients using quantization parameters. The inverse quantization unit (520) may apply different quantization coefficients (scaling values) to the quantized transformation coefficients arranged in two dimensions. The inverse quantization unit (520) may perform inverse quantization by applying a matrix of quantization coefficients (scaling values) from an image encoding device to a two-dimensional array of quantized transformation coefficients.

[0079] The inverse transformation unit (530) generates a residual block for the current block by inversely transforming the inversely quantized transformation coefficients from the frequency domain to the spatial domain and restoring the residual signals.

[0080] Additionally, when the inverse transformation unit (530) inversely transforms only a part of the transformation block (sub-block), it extracts a flag (cu_sbt_flag) indicating that only the sub-block of the transformation block has been transformed, information on the directionality (vertical / horizontal) of the sub-block (cu_sbt_horizontal_flag) and / or information on the position of the sub-block (cu_sbt_pos_flag), restores residual signals by inversely transforming the transformation coefficients of the corresponding sub-block from the frequency domain to the spatial domain, and creates a final residual block for the current block by filling the areas that have not been inversely transformed with "0" values ​​of residual signals.

[0081] Additionally, when MTS is applied, the inverse transformation unit (530) determines a transformation function or transformation matrix to be applied in the horizontal and vertical directions, respectively, using MTS information (mts_idx) signaled from the video encoding device, and performs an inverse transformation on the transformation coefficients within the transformation block in the horizontal and vertical directions using the determined transformation function.

[0082] The prediction unit (540) may include an intra prediction unit (542) and an inter prediction unit (544). The intra prediction unit (542) is activated when the prediction type of the current block is an intra prediction, and the inter prediction unit (544) is activated when the prediction type of the current block is an inter prediction.

[0083] The intra prediction unit (542) determines the intra prediction mode of the current block among a plurality of intra prediction modes from the syntax elements for the intra prediction mode extracted from the entropy decoding unit (510), and predicts the current block using reference pixels around the current block according to the intra prediction mode.

[0084] The inter prediction unit (544) determines the motion vector of the current block and the reference picture that the motion vector refers to using the syntax elements for the inter prediction mode extracted from the entropy decoding unit (510), and predicts the current block using the motion vector and the reference picture.

[0085] The adder (550) restores the current block by adding the residual block output from the inverse transformation unit (530) and the prediction block output from the inter prediction unit (544) or the intra prediction unit (542). The pixels within the restored current block are used as reference pixels when intra-predicting the block to be decoded later.

[0086] The loop filter section (560) may include a deblocking filter (562), an SAO filter (564), and an ALF (566) as an in-loop filter. The deblocking filter (562) deblocks the boundaries between restored blocks to remove blocking artifacts caused by block-unit decoding. The SAO filter (564) and the ALF (566) perform additional filtering on the restored blocks after deblocking filtering to compensate for the difference between the restored pixels and the original pixels caused by lossy coding. The filter coefficients of the ALF are determined using information about the filter coefficients decoded from the bitstream.

[0087] The restored blocks filtered through the deblocking filter (562), SAO filter (564), and ALF (566) are stored in memory (570). When all blocks within a picture are restored, the restored picture is used as a reference picture to inter-predict blocks within the picture to be encoded later.

[0088] The present embodiment relates to the encoding and decoding of an image (video) as described above. More specifically, the present invention provides an image encoding / decoding method and apparatus for constituting a merge candidate list or an AMVP candidate list in inter-prediction, and a recording medium for storing a bitstream generated by the image encoding method / apparatus.

[0089] The following embodiments may be performed by an inter prediction unit (124) within a video encoding apparatus. Additionally, the following embodiments may be performed by an inter prediction unit (544) within a video decoding apparatus.

[0090] The video encoding device can generate signaling information related to the present embodiment in terms of rate distortion optimization during the encoding of the current block. The video encoding device can encode the signaling information using the entropy encoding unit (155) and then transmit it to the video decoder. The video decoder can decode the signaling information related to the decoding of the current block from the bitstream using the entropy decoder (510).

[0091] In the following description, the term 'target block' may be used interchangeably with 'current block' or 'Coding Unit (CU).' Alternatively, 'target block' may refer to a specific area of ​​a Coding Unit.

[0092] Also, a value of one flag being true indicates that the flag is set to 1. Also, a value of one flag being false indicates that the flag is set to 0.

[0093] The decoder-side includes all or part of an inverse quantizer (160), an inverse transform (165), a prediction unit (120), an adder (170), a loop filter (180), and a memory (190) in the image encoding device illustrated in FIG. 1. Alternatively, the decoder-side includes all or part of an inverse quantizer (520), an inverse transform (530), a prediction unit (540), an adder (550), a loop filter (560), and a memory (570) in the image decoding device illustrated in FIG. 5. In relation to a series of decoding processes, the decoder-side of the image encoding device and the decoder-side of the image decoding device perform the same operation. The image encoding device determines information related to the operation of the decoder-side and signals the determined information to the image decoding device. The image decoding device can decode the signaled information and operate the decoder-side based on the decoded information.

[0094] I. Adaptive Reordering of Merge Candidates (ARMC)

[0095] In conventional inter-prediction, the image encoding device constructs a merge candidate list using multiple merge candidates. The merge candidate list may include various types of merge candidates. That is, a merge candidate may be one of various types, such as adjacent and non-adjacent spatial merge candidates, temporal merge candidates, constructed merge candidates, bi-directional prediction merge candidates, history-based merge candidates, pairwise average-based merge candidates, pairwise weighted-summation-based merge candidates, zero merge candidates, etc. In this case, if multiple merge candidates exist within the same reference picture, a constructed merge candidate may be generated by averaging the aforementioned multiple merge candidates.

[0096] In the process of constructing the merge candidate list, a technique (ARMC: Adaptive Reordering of Merge Candidates or ARMC-TM: Adaptive Reordering of Merge Candidates with Template Matching) that rearranges the order of merge candidates based on the matching cost between the current template and the reference template may be applied. In this case, the current template represents the surrounding area of ​​the current block, and the reference template represents the surrounding area of ​​the candidate reference block of the current block. The candidate reference block exists within the reference picture indicated by the merge candidate. Based on the template matching cost, the ARMC technique sorts candidates determined to have a high probability of selection at the front of the list. When selecting a merge candidate and transmitting a merge index indicating the selected merge candidate, the ARMC technique can provide the effect of reducing the transmission bits. The video encoding device may signal to the video decoder the merge index indicating one or more merge candidates from the final merge candidate list.

[0097] The video encoding device may, based on the types of merge candidates, separately perform a reordering of merge candidates by type, select multiple candidates by type, and add the selected candidates to the initial merge candidate list. The initial merge candidate list represents a list to which the reordering of merge candidates by type has been applied. Additionally, to ensure diversity of merge candidates, the template matching costs of two consecutive merge candidates may be compared during the ARMC process. If the absolute value of the difference between the costs is smaller than a predefined threshold, the subsequent candidate may be considered a redundant candidate and moved to a lower priority in the list. The aforementioned techniques may or may not be applied depending on the types of merge candidates.

[0098] Below, the merge candidate list, candidate list, merge list, or list may be used interchangeably. Additionally, the merge candidate or candidate may be used interchangeably.

[0099] The ARMC technology described above can also be utilized for rearranging candidates within the AMVP candidate list.

[0100] The following embodiments are described with reference to an image decoding device, but may be implemented identically or similarly in an image encoding device. Alternatively, the following embodiments are described with reference to the decoder side of the image decoding device, but may be implemented identically or similarly in the decoder side of an image encoding device.

[0101] II. Embodiments according to the present disclosure

[0102] FIG. 6 is a flowchart illustrating a method for configuring a merge candidate list according to one embodiment of the present disclosure.

[0103] As described above, the image decoding device rearranges the merge candidates by type (S600) and constructs an initial merge candidate list using the rearranged merge candidates by type (S602). When constructing the initial merge candidate list, a process of selecting all or some of the rearranged merge candidates of each type may be performed.

[0104] The video decoder improves the motion vector of each merge candidate in the initial merge candidate list (S604). For example, the motion vector of each merge candidate can be improved using Decoder-side Motion Vector Refinement (DMVR) based on Bilateral Matching (BM). Alternatively, the motion vector of each merge candidate can be improved using Decoder-side Motion Vector Derivation based on Template Matching (TM).

[0105] The image decoding device rearranges the improved merge candidates within the initial merge candidate list (S606) and rearranges the rearranged initial merge candidate list using comparisons between the merge candidates (S608). A final merge candidate list can be generated from the initial merge candidate list according to the series of alignment processes described above.

[0106] A merge candidate is selected from the final merge candidate list (S610).

[0107] For example, the video encoding device may select one or more merge candidates from the final merge candidate list in terms of rate distortion optimization. The composition of the merge candidate list as described above may be performed identically by the video encoding device. The video encoding device may signal a merge index indicating the selected merge candidate to the video decoder. The video decoder may parse the merge index from the bitstream and derive one or more merge candidates from the final merge candidate list based on the parsed merge index.

[0108] The following describes in detail the rearrangement based on comparison between merge candidates, i.e., Step S608. Rearrangement based on comparison between merge candidates can be used compatiblely with additional rearrangement.

[0109] Additional sorting determines redundant candidates by comparing candidates within the reordered candidate list and moves those candidates to a lower rank within the candidate list. The number of candidates moved can be N (where N is an integer greater than or equal to 1).

[0110] In comparisons between candidates, one or more of the differences, such as the absolute difference between the candidates' template matching costs, the squared difference between the candidates' template matching costs, and the distance between the candidates' motion vectors, may be used.

[0111] As an example, if the comparison value between two adjacent candidates within a reordered list is the smallest, that candidate may be determined to be a duplicate candidate.

[0112] If the absolute value of the difference between the template matching costs of candidate A and candidate A-1 within the reordered merge candidate list is the smallest, candidate A or candidate A-1 may be determined to be a duplicate candidate. The image decoder may move candidate A or candidate A-1 to a lower rank in the list.

[0113] In a reordered list of merge candidates, if the squared difference between the template matching costs of the A-th candidate and the A-1-th candidate is the smallest, the A-th candidate or the A-1-th candidate may be determined to be a duplicate candidate. The image decoder may move the A-th candidate or the A-1-th candidate to a lower priority in the list.

[0114] Alternatively, if the distance between the motion vector of candidate A and the motion vector of candidate A-1 within the reordered merge candidate list is the smallest, candidate A or candidate A-1 may be determined to be a duplicate candidate. The image decoder may move candidate A or candidate A-1 to a lower rank in the list. In this case, one or more of distances, such as Euclidean distance, Manhattan distance, or cosine similarity, may be used as the distance.

[0115] As another example, if the comparison value between two adjacent candidates within a reordered candidate list is smaller than a threshold, the candidate may be determined to be a duplicate candidate. In this case, there may be one or more duplicate candidates.

[0116] If the absolute value of the difference between the template matching costs of the A-th candidate and the A-1-th candidate within the reordered merge candidate list is smaller than a threshold, the A-th candidate or the A-1-th candidate may be determined to be a duplicate candidate. The image decoder may move the A-th candidate or the A-1-th candidate to a lower priority in the list.

[0117] If the squared difference between the template matching costs of candidate A and candidate A-1 within the reordered merge candidate list is smaller than a threshold, candidate A or candidate A-1 may be determined to be a duplicate candidate. The image decoder may move candidate A or candidate A-1 to a lower rank in the list.

[0118] Alternatively, if the distance between the motion vector of candidate A and the motion vector of candidate A-1 within the reordered merge candidate list is smaller than a threshold, candidate A or candidate A-1 may be determined to be a duplicate candidate. The image decoder may move candidate A or candidate A-1 to a lower rank in the list. In this case, one or more of distances, such as Euclidean distance, Manhattan distance, or cosine similarity, may be used as the distance. For two points (x1, y1) and (x2, y2), the Manhattan distance is calculated as |x1-x2|+|y1-y2|. For two vectors X and Y, the cosine similarity is calculated as (X·Y) / (|X||Y|). Here, X·Y is represented as the inner product of the two vectors X and Y.

[0119] The aforementioned threshold can be derived or set as follows.

[0120] The threshold can be derived based on the number of pixels within the template. For example, if the number of pixels within the template is less than or equal to K, the threshold can be a natural number X, and if the number of pixels within the template is greater than K, the threshold can be set to Y (where Y > X). Here, the values ​​of K, X, and Y can each be pre-set values ​​or signaled values.

[0121] The threshold may be a preset value. The threshold may be a value transmitted while included in one of the levels of sequence, frame, slice, tile, or CU. Alternatively, the threshold may be a multiple of the template matching cost of a candidate with the highest index in the candidate list. Here, the highest merge index may indicate the first merge candidate in the candidate list.

[0122] Meanwhile, the image decoding device selects two candidates whose comparison value between merge candidates is the smallest or smaller than a threshold, and can select one duplicate candidate from among the two candidates as follows. A duplicate candidate may be selected to increase the expectation that the optimal candidate among the two candidates will have a higher index within the candidate list.

[0123] As an example, the Picture Order Count (POC) values ​​of the reference frames associated with the two candidates can be utilized. For instance, the candidate closest to the current frame may be determined as a duplicate candidate. Alternatively, the candidate furthest from the current frame may be determined as a duplicate candidate.

[0124] As another example, in the reordering process of merge candidates, a method different from the one used to calculate the cost of merge candidates may be utilized to calculate the cost for determining duplicate candidates. That is, the image decoder may determine duplicate candidates based on a different type of template matching cost. In this case, the template matching cost may be calculated based on one or a combination of two or more cost functions such as SAD (Sum of absolute differences), MR-SAD (Mean-removed SAD), SATD (Sum of absolute transformed differences), and MR-SATD (Mean-removed SATD). For example, if the SATD value was not used in the reordering process of merge candidates, the image decoder may calculate the SATD values ​​of the two candidates and determine the candidate with the larger SATD value as the duplicate candidate.

[0125] As another example, duplicate candidates can be determined by comparing them with the merge candidate having the highest index within the candidate list.

[0126] For example, an image decoding device may calculate the cost between a reference block indicated by a merge candidate having the highest index and reference blocks indicated by the remaining candidates, and determine the candidate with the largest cost among the remaining candidates as a duplicate candidate. The cost may be calculated according to one or a combination of two or more cost functions such as SAD, MR-SAD, SATD, MR-SATD, etc.

[0127] For example, an image decoding device may calculate the template matching cost between the template of a reference block indicated by a merge candidate having the highest index and the templates of reference blocks indicated by the remaining candidates, and determine the candidate with the largest template matching cost among the remaining candidates as a duplicate candidate. The template matching cost may be calculated according to one or a combination of two or more cost functions such as SAD, MR-SAD, SATD, MR-SATD, etc.

[0128] As another example, as shown in the example of FIG. 7, the image decoding device can calculate the template matching cost between samples within the template of the reference block and samples adjacent to the template of the reference block within the reference block, and determine duplicate candidates based on the calculated cost. Here, the reference block may be indicated by motion information of each merge candidate in the candidate list. In the example of FIG. 7, the candidate reference block represents the reference block as described above, and the candidate motion vector may represent motion information.

[0129] For example, the candidate with the largest template matching cost may be determined as a duplicate candidate. Alternatively, the candidate with the smallest template matching cost may be determined as a duplicate candidate.

[0130] For example, candidates with a template matching cost smaller than a threshold may be determined as duplicate candidates. Alternatively, candidates with a template matching cost larger than a threshold may be determined as duplicate candidates. In this case, the threshold may be a pre-set value or a signaled value. Alternatively, the threshold may be derived by utilizing information about neighboring blocks, the current block, or the reference block. The block information described above may include one or more items among items such as the block size, the block area, the existence of neighboring inter-blocks, whether the reference block of the merge candidate with the highest index in the candidate list is an inter-predicted block, a multiple of the template matching cost associated with the merge candidate with the highest index in the candidate list, and the predicted slice of the current block. Here, the predicted slice may be an I-slice (Intra-predicted slice), a P-slice (Predicted slice), or a B-slice (Bi-predicted slice).

[0131] The image decoding device may select one of preset thresholds based on a combination of items included in the aforementioned block information. Alternatively, the image decoding device may derive a threshold using the items of the block information.

[0132] If multiple candidates are determined to be duplicate candidates according to the aforementioned conditions, the image decoder may move the duplicate candidates to a lower priority within the candidate list in order of increasing template matching cost. For example, if duplicate candidate A and duplicate candidate B exist, and the template matching cost of duplicate candidate A is smaller than that of duplicate candidate B, duplicate candidate A may be moved to a lower priority while maintaining a higher index than duplicate candidate B.

[0133] If multiple candidates are determined to be duplicate candidates according to the aforementioned conditions, the image decoder may move the duplicate candidates to a lower priority within the candidate list in order of decreasing template matching cost. For example, if duplicate candidate A and duplicate candidate B exist, and the template matching cost of duplicate candidate A is smaller than that of duplicate candidate B, duplicate candidate A may be moved to a lower priority while having a lower index than duplicate candidate B.

[0134] Meanwhile, the image decoding device can construct N (where N is an integer greater than or equal to 2) merge candidate lists. Hereinafter, to distinguish between the two merge candidate lists, the first list is referred to as the primary merge candidate list and the second list as the secondary merge candidate list.

[0135] The video decoder obtains the size of the secondary merge candidate list.

[0136] The size of the secondary merge candidate list can be S (where S is a preset integer greater than or equal to 1). The size of the secondary merge candidate list can be transmitted at the sequence, frame, slice, tile, CU (Coding Unit), and PU (Prediction Unit) levels.

[0137] The video decoder forms two merge candidate lists.

[0138] The main merge candidate list includes adjacent and non-adjacent spatial candidates, temporal candidates, combination merge candidates, bidirectional prediction merge candidates, history-based merge candidates, merge candidates based on the average of two candidates, merge candidates based on the weighted sum of two candidates, and zero merge candidates, and may include P (where P is a preset integer greater than or equal to 1) merge candidates.

[0139] The main merge candidate list can be reordered based on the template matching cost. For example, an image decoder can construct a list containing L candidates using adjacent and non-adjacent spatial candidates, temporal candidates, and history-based candidates, and then apply reordering to construct a list containing P candidates. In this case, L ≥ P.

[0140] As candidates corresponding to each type, the top N (where N is an integer greater than or equal to 1) candidates that are reordered based on template costs may be selected. For example, an image decoder may construct M spatial candidates and select N candidates by reordering the candidates. Here, M > N may be possible. An image decoder may construct a plurality of temporal candidates and select N candidates by reordering the candidates.

[0141] The image decoding device can construct a primary merge candidate list and a secondary merge candidate list based on the aforementioned information. The secondary merge candidate list may not include candidates included in the primary merge candidate list. The secondary merge candidate list may be constructed based on the size of the primary merge candidate list that is pre-configured. For example, when the size of the primary merge candidate list is N, the secondary merge candidate list may consist of M candidates. Here, M ≤ N.

[0142] The video decoder constructs a secondary merge candidate list.

[0143] The secondary merge candidate list may include the following types of candidates. The secondary merge candidate list may include candidates excluded due to reordering during the construction process of the primary merge candidate list. For example, when constructing temporal candidates, candidates not included in the primary merge candidate list due to template matching-based reordering may be included in the secondary merge candidate list. When constructing non-adjacent candidates, candidates not included in the primary merge candidate list due to template matching-based reordering may be included in the secondary merge candidate list. When constructing the primary merge candidate list, candidates not included in the primary merge candidate list due to template matching-based reordering may be included in the secondary merge candidate list. The order of construction of the secondary merge candidate list may be determined by a fixed order or the order of construction of the primary merge candidate list.

[0144] The video decoder can improve candidates within the secondary merge candidate list.

[0145] As described above, the secondary merge candidate list may include S candidates (where S is a preset integer of 1 or more). In this case, S may be smaller than or equal to the size P of the primary merge candidate list. The image decoding device may rearrange the candidates within the secondary merge candidate list configured according to the method described above based on the template cost.

[0146] The image decoding device can determine duplicate candidates within the secondary merge candidate list as described above, and move the determined duplicate candidates to a lower priority in the list.

[0147] The video decoder determines whether to use the secondary merge candidate list.

[0148] A flag indicating whether to use a secondary merge candidate list may be used. The aforementioned flag may be signaled at the sequence, frame, slice, tile, CU (Coding Unit), and PU (Prediction Unit) levels.

[0149] Alternatively, the flag value can be derived using information from surrounding blocks without explicit signaling. For example, the image decoder can derive the flag value to indicate the use of the secondary merge candidate list based on the template matching costs of candidates within the primary merge candidate list.

[0150] If the template matching cost of the candidate with the highest index within the primary merge candidate list is smaller than a preset threshold, a secondary merge candidate list may be used. Alternatively, if the template matching cost of the candidate with the highest index is larger than a preset threshold, a secondary merge candidate list may be used.

[0151] If the template matching cost of the candidate with the lowest index within the primary merge candidate list is smaller than a preset threshold, a secondary merge candidate list may be used. Alternatively, if the template matching cost of the candidate with the lowest index is larger than a preset threshold, a secondary merge candidate list may be used.

[0152] If the sum of the template matching costs of candidates with specific indices within the primary merge candidate list is less than a preset threshold, a secondary merge candidate list may be used. Additionally, if the sum of the template matching costs of candidates with specific indices within the primary merge candidate list is greater than a preset threshold, a secondary merge candidate list may be used. In this case, the specific indices may be the top-level and bottom-level indices.

[0153] The image decoding device parses the merge index and can derive a merge candidate for the current block from the primary merge candidate list or the secondary merge candidate list based on the parsed merge index. A single merge index may be used for two candidate lists. As an example, information distinguishing the two candidate lists may be used together with the single merge index. As another example, a single merge index may be used even without information distinguishing the two candidate lists. That is, if the value of the merge index is smaller than the size P of the primary merge candidate list, the merge index may indicate a candidate within the primary merge candidate list. On the other hand, if the value of the merge index is greater than or equal to the size P of the primary merge candidate list, the merge index may indicate a candidate within the secondary merge candidate list.

[0154] Embodiments related to the configuration of the merge candidate list according to the present disclosure can be similarly utilized in AMVP mode as well.

[0155] Hereinafter, a method for predicting the current block using a merge candidate list is described using the illustration of FIG. 8. The illustration of FIG. 8 can be performed by an image encoding device and an image decoding device. FIG. 8 is described based on the image encoding device, and if necessary, the operation by the image decoding device is additionally described.

[0156] FIG. 8 is a flowchart illustrating a method for predicting a current block according to one embodiment of the present disclosure.

[0157] The video encoding device obtains a merge index indicating one of the merge candidates (S800).

[0158] The video encoding device can, for example, obtain a merge index from a high level. As another example, the video encoding device can determine the merge index in terms of rate distortion optimization. The video encoding device can encode the merge index. The video decoder can decode the merge index from the bitstream.

[0159] The video encoding device constructs a primary merge candidate list (S802).

[0160] As illustrated in FIG. 6, the video encoding device can configure a main merge candidate list including multiple types of merge candidates.

[0161] The video encoding device may rearrange each type of merge candidate based on the template matching cost of each type of merge candidate, select all or some of the rearranged merge candidates of each type, and include the selected merge candidates of each type in the main merge candidate list.

[0162] Here, the template matching cost represents the cost between the template of the current block and the template of the reference block indicated by each merge candidate.

[0163] The video encoding device can improve merge candidates within the main merge candidate list. The video encoding device can reorder merge candidates within the main merge candidate list based on the template matching costs of the merge candidates. The video encoding device can further sort merge candidates based on comparisons between the reordered merge candidates.

[0164] The video encoding device checks whether to use a secondary merge candidate list (S804).

[0165] For example, the video encoding device can obtain information from an upper level indicating whether to use a secondary merge candidate list. As another example, the video encoding device can determine the aforementioned information in terms of rate distortion optimization. The video encoding device can encode the information indicating whether to use a secondary merge candidate list. The video decoder can decode the aforementioned information from a bitstream.

[0166] As another example, without explicit signaling, information indicating whether to use a secondary merge candidate list based on information from surrounding blocks can be derived.

[0167] If it is determined that the secondary merge candidate list is not being used (No of S804), the video encoding device selects a merge candidate for the current block from the main merge candidate list based on the merge index (806).

[0168] On the other hand, if it is determined that a secondary merge candidate list is used (Yes in S804), the video encoding device may perform the following steps.

[0169] The video encoding device constructs a secondary merge candidate list (S820). The secondary merge candidate list can be constructed based on the merge candidates excluded during the process of rearranging each type of merge candidate.

[0170] The video encoding device selects a merge candidate for the current block from the main merge candidate list or the secondary merge candidate list based on the merge index (S822).

[0171] Finally, the video encoding device generates a prediction block of the current block based on the motion information of the selected merge candidate (S808).

[0172] Subsequently, the video encoding device can generate a residual block by subtracting the prediction block from the current block. The video encoding device can generate transformation coefficients by applying transformation / quantization to the residual block and encode the generated transformation coefficients.

[0173] The image decoder can decode the quantized transform coefficients of the current block from the bitstream and generate a residual block by applying inverse quantization / inverse transform to the quantized transform coefficients. The image decoder can restore the current block by adding the prediction block and the residual block.

[0174] Although the flowcharts and timing diagrams in this specification describe each process as being executed sequentially, this is merely an illustrative explanation of the technical concept of one embodiment of the present disclosure. In other words, a person skilled in the art to which one embodiment of the present disclosure belongs may modify and adapt the flowcharts and timing diagrams in various ways, such as changing the order described in the flowcharts and timing diagrams or executing one or more of the processes in parallel, without departing from the essential characteristics of one embodiment of the present disclosure; therefore, the flowcharts and timing diagrams are not limited to a chronological order.

[0175] It should be understood that the exemplary embodiments described above may be implemented in many different ways. The functions or methods described in one or more examples may be implemented in hardware, software, firmware, or any combination thereof. It should be understood that the functional components described herein are labeled as "...unit" to particularly emphasize their implementation independence.

[0176] Meanwhile, the various functions or methods described in the present embodiment may be implemented as instructions stored in a non-transient recording medium that can be read and executed by one or more processors. A non-transient recording medium includes, for example, any type of recording device in which data is stored in a form readable by a computer system. For example, a non-transient recording medium includes storage media such as an EPROM (erasable programmable read-only memory), a flash drive, an optical drive, a magnetic hard drive, and a solid-state drive (SSD).

[0177] The above description is merely an illustrative explanation of the technical concept of the present embodiment, and a person skilled in the art to which the present embodiment belongs would be able to make various modifications and variations within the scope of the essential characteristics of the present embodiment. Accordingly, the present embodiments are intended to explain, not limit, the technical concept of the present embodiment, and the scope of the technical concept of the present embodiment is not limited by these embodiments. The scope of protection of the present embodiment shall be interpreted by the claims below, and all technical concepts within an equivalent scope shall be interpreted as being included within the scope of rights of the present embodiment.

[0178]

[0179] CROSS-REFERENCE TO RELATED APPLICATION

[0180] This patent application claims priority to Korean patent application No. 10-2024-0173280 filed on November 28, 2024, the entire contents of which are incorporated into this patent application by reference.

Claims

1. A method for restoring a current block performed by an image decoding device, The step of configuring a primary merge candidate list of the above-mentioned current block; and The step of decrypting the merge index indicating one of the merge candidates Includes, The step of constructing the above-mentioned main merge candidate list is, A step of generating a main merge candidate list including multiple types of merge candidates; A step of rearranging the merge candidates based on the template matching costs of the merge candidates within the above-mentioned main merge candidate list, wherein the template matching cost represents the cost between the template of the current block and the template of the reference block indicated by each merge candidate; and Step of further aligning the merge candidates based on a comparison between the reordered merge candidates A method including 2. In Paragraph 1, A step of selecting a merge candidate of the current block from the main merge candidate list based on the merge index; and A step of predicting the current block based on movement information of the selected merge candidate A method that further includes.

3. In Paragraph 1, The step of further aligning the above merge candidates is, A step of determining duplicate candidates based on a comparison among the above merge candidates; and Step of moving the above duplicate candidates to a lower priority within the above main merge candidate list A method including 4. In Paragraph 3, The step of determining the above duplicate candidates is, A step of selecting two adjacent merge candidates when the comparison value between the two adjacent merge candidates is minimized; and The step of selecting one of the two adjacent merge candidates as the above duplicate candidate Includes, A method in which the comparison value is one of the absolute value of the difference between the template matching costs of the two adjacent merge candidates, the squared value of the difference between the template matching costs of the two adjacent merge candidates, or the distance between the motion vectors of the two adjacent merge candidates, or a combination of two or more.

5. In Paragraph 4, The step of selecting one of the two adjacent merge candidates above is, A method for selecting one of two adjacent merge candidates based on the Picture Order Count (POC) values ​​of reference frames associated with the two adjacent merge candidates.

6. In Paragraph 4, The step of selecting one of the two adjacent merge candidates above is, A step of calculating a different type of template matching cost for two adjacent merge candidates using a method different from the method used to calculate the template matching cost in the process of rearranging the merge candidates; and A step of selecting one of the two adjacent merge candidates based on the above different types of template matching costs A method including 7. In Paragraph 3, The step of determining the above duplicate candidates is, A step of calculating the cost between a reference block indicated by a merge candidate having the highest index within the above-mentioned main candidate list and reference blocks indicated by the remaining merge candidates; and Step of determining the duplicate candidate among the remaining merge candidates based on the above cost A method including 8. In Paragraph 3, The step of determining the above duplicate candidates is, A step of calculating the matching cost between the template of a reference block indicated by a merge candidate having the highest index within the above-mentioned main candidate list and the templates of reference blocks indicated by the remaining merge candidates; and A step of determining the duplicate candidate among the remaining merge candidates based on the above matching cost A method including 9. In Paragraph 3, The step of determining the above duplicate candidates is, A step of calculating the matching cost between samples within the template of the reference block and samples existing within the reference block and adjacent to the template of the reference block; and Step of determining the duplicate candidates based on the above matching cost A method including 10. In Paragraph 1, The step of generating the above-mentioned main merge candidate list is, A step of rearranging each type of merge candidate based on the template matching cost of each type of merge candidate; A step of selecting some or all of the reordered merge candidates of each type; and Step of including each selected type of merge candidate in the above main merge candidate list A method including 11. In Paragraph 10, If it is decided to use the secondary merge candidate list of the above current block, A step of constructing the secondary merge candidate list based on the merge candidates excluded during the process of rearranging each of the above types of merge candidates; A step of selecting a merge candidate of the current block from the main merge candidate list or the secondary merge candidate list based on the merge index; and A step of predicting the current block based on movement information of the selected merge candidate A method that further includes.

12. A method for encoding a current block performed by an image encoding device, A step of obtaining a merge index that indicates one of the merge candidates; The step of configuring a primary merge candidate list of the above-mentioned current block; and Step of encoding the above merge index Includes, The step of constructing the above-mentioned main merge candidate list is, A step of generating a main merge candidate list including multiple types of merge candidates; A step of rearranging the merge candidates based on the template matching costs of the merge candidates within the above-mentioned main merge candidate list, wherein the template matching cost represents the cost between the template of the current block and the template of the reference block indicated by each merge candidate; and Step of further aligning the merge candidates based on a comparison between the reordered merge candidates A method including 13. In Paragraph 12, A step of selecting a merge candidate of the current block from the main merge candidate list based on the merge index; and A step of predicting the current block based on movement information of the selected merge candidate A method that further includes.

14. In Paragraph 12, The step of further aligning the above merge candidates is, A step of determining duplicate candidates based on a comparison among the above merge candidates; and Step of moving the above duplicate candidates to a lower priority within the above main merge candidate list A method including 15. In Paragraph 12, The step of generating the above-mentioned main merge candidate list is, A step of rearranging each type of merge candidate based on the template matching cost of each type of merge candidate; A step of selecting some or all of the reordered merge candidates of each type; and Step of including each selected type of merge candidate in the above main merge candidate list A method including 16. In Paragraph 15, If it is decided to use the secondary merge candidate list of the above current block, A step of constructing the secondary merge candidate list based on the merge candidates excluded during the process of rearranging each of the above types of merge candidates; A step of selecting a merge candidate of the current block from the main merge candidate list or the secondary merge candidate list based on the merge index; and A step of predicting the current block based on movement information of the selected merge candidate A method that further includes.

17. A method for providing video data to a video decoder, A step of encoding the above video data into a bitstream; and Step of transmitting the above bitstream to the above video decoder Includes, The step of encoding the above video data is, A step of obtaining a merge index that indicates one of the merge candidates; A step of constructing a primary merge candidate list of the current block; and Step of encoding the above merge index Includes, The step of constructing the above-mentioned main merge candidate list is, A step of generating a main merge candidate list including multiple types of merge candidates; A step of rearranging the merge candidates based on the template matching costs of the merge candidates within the above-mentioned main merge candidate list, wherein the template matching cost represents the cost between the template of the current block and the template of the reference block indicated by each merge candidate; and Step of further aligning the merge candidates based on a comparison between the reordered merge candidates A method including

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