Method and device for video coding using bi-predictive local illumination compensation

Abstract

Disclosed in the present embodiment are a video coding method and device using bi-predictive local illumination compensation. In the present embodiment, an image decoding device acquires reference pictures of the current picture on the basis of a bitstream. The reference pictures include a reference picture in a direction of LX (X=0 or 1) and a reference picture in a direction of LY (Y=1-X). The image decoding device applies local illumination compensation to the current block regardless of whether the distance between the current picture and the reference picture in the LY direction is the same as the distance between the current picture and the reference picture in the LX direction.

Description

Method and device for video coding using bidirectional predictive local illumination compensation

[0001] The present disclosure relates to a video coding method and device using bidirectional predictive local illumination compensation, and a recording medium.

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

[0003] Since video data has a large amount of data compared to voice data or still image data, it requires a lot of hardware resources, including memory, to store or transmit it without processing for compression.

[0004] Therefore, when storing or transmitting video data, the encoder compresses the video data and stores or transmits it, and the decoder receives the compressed video data, decompresses it, and plays it back. These video compression technologies include H.264 / AVC, HEVC (High Efficiency Video Coding), and VVC (Versatile Video Coding), which improves encoding efficiency by about 30% compared to HEVC.

[0005] However, as the size, resolution, and frame rate of images are gradually increasing, and the amount of data that needs to be encoded is also increasing, a new compression technology that has better encoding efficiency and better image quality improvement than existing compression technologies is required.

[0006] In particular, the demand for high-resolution, high-quality images, such as HD (High Definition) images and UHD (Ultra High Definition) images, has been increasing in various fields. As image data becomes higher in resolution and quality, the amount of information or bits transmitted increases relatively compared to existing image data. This increase in the amount of information or bits transmitted leads to an increase in transmission and storage costs. In addition, interest in and demand for immersive media, such as VR (Virtual Reality), AR (Artificial Reality) content, and holograms, has been increasing recently, and the broadcasting of videos / images with different image characteristics from real images, such as game images, is increasing. Accordingly, a highly efficient image compression technology is required to effectively compress, transmit, store, and play information of high-resolution, high-quality images with various characteristics as described above.

[0007] The present disclosure aims to provide a video coding method and device using improved bi-predictive Local Illumination Compensation (LIC).

[0008] An embodiment according to the present disclosure aims to provide a video coding method and device that adaptively uses bidirectional predictive local lighting compensation based on the distance between a current picture and reference pictures.

[0009] One embodiment of the present disclosure provides a method for encoding / decoding an image, performed by an image encoding / decoding device. One embodiment of the present disclosure provides a device for encoding / decoding an image.

[0010] One embodiment of the present disclosure provides a computer-readable digital storage medium storing encoded image information generated by an image encoding method disclosed in at least one of the embodiments of the present disclosure. One embodiment of the present disclosure provides a computer-readable digital storage medium storing encoded information or encoded image information that causes an image decoding device to perform an image decoding method disclosed in at least one of the embodiments of the present disclosure.

[0011] In addition, one embodiment according to the present disclosure provides a method for transmitting image data including a bitstream generated based on an image encoding method disclosed in at least one of the embodiments according to the present disclosure, or a transmission device for transmitting image data.

[0012] According to an embodiment of the present disclosure, a method for restoring a current block, performed by a video decoding device, is provided, comprising: obtaining reference pictures of a current picture based on a bitstream, wherein the reference pictures include a reference picture in an LX (X=0 or 1) direction and a reference picture in an LY (Y=1-X) direction; and applying local illumination compensation to the current block, regardless of whether a first distance and a second distance are the same, wherein the first distance represents a distance between the current picture and the reference picture in the LY direction, and the second distance represents a distance between the current picture and the reference picture in the LX direction.

[0013] According to another embodiment of the present disclosure, a method for encoding a current block, performed by a video encoding apparatus, is provided, comprising: obtaining reference pictures of a current picture based on higher level information, wherein the reference pictures include a reference picture in an LX (X=0 or 1) direction and a reference picture in an LY (Y=1-X) direction; and applying local illumination compensation to the current block regardless of whether a first distance and a second distance are the same, wherein the first distance represents a distance between the current picture and the reference picture in the LY direction, and the second distance represents a distance between the current picture and the reference picture in the LX direction.

[0014] According to another embodiment of the present disclosure, a method for providing video data to a video decoding device is provided, comprising: encoding the video data into a bitstream; and transmitting the bitstream to the video decoding device, wherein the encoding the video data comprises: obtaining reference pictures of a current picture based on higher level information, wherein the reference pictures include a reference picture in an LX (X=0 or 1) direction and a reference picture in an LY (Y=1-X) direction; and applying local illumination compensation to a current block regardless of whether a first distance and a second distance are the same, wherein the first distance represents a distance between the current picture and the reference picture in the LY direction, and the second distance represents a distance between the current picture and the reference picture in the LX direction.

[0015] As described above, according to the present embodiment, by providing a video coding method and device using improved bidirectional predictive local illumination compensation, it is possible to improve video encoding efficiency and enhance video quality.

[0016] In addition, according to the present embodiment, there is provided a video coding method and device that adaptively uses bidirectional predictive local lighting compensation based on the distance between the current picture and reference pictures, thereby making it possible to improve video encoding efficiency and enhance video quality.

[0017] FIG. 1 is an exemplary block diagram of an image encoding device capable of implementing the techniques of the present disclosure.

[0018] Figure 2 is a drawing for explaining a method of dividing a block using the QTBTTT (QuadTree plus BinaryTree TernaryTree) structure.

[0019] FIGS. 3A and 3B are diagrams illustrating multiple intra prediction modes, including wide-angle intra prediction modes.

[0020] Figure 4 is an example diagram of the surrounding blocks of the current block.

[0021] FIG. 5 is an exemplary block diagram of an image decoding device capable of implementing the techniques of the present disclosure.

[0022] Figure 6 is an example diagram showing local lighting compensation.

[0023] FIG. 7 is a flowchart illustrating a method of encoding a current block performed by an image encoding device according to one embodiment of the present disclosure.

[0024] FIG. 8 is a flowchart illustrating a method for restoring a current block performed by an image decoding device according to one embodiment of the present disclosure.

[0025] Hereinafter, embodiments of the present invention will be described in detail with reference to exemplary drawings. When designating components in each drawing, it should be noted that, where possible, identical components are given the same reference numerals, even if they appear in different drawings. Furthermore, in describing the present embodiments, detailed descriptions of related known structures or functions will be omitted if they are deemed to obscure the gist of the present embodiments.

[0026] FIG. 1 is an exemplary block diagram of an image encoding device capable of implementing the techniques of the present disclosure. Hereinafter, the image encoding device and its subcomponents will be described with reference to the illustration in FIG. 1.

[0027] The video encoding device may be configured to include a picture segmentation unit (110), a prediction unit (120), a subtractor (130), a transformation unit (140), a quantization unit (145), a reordering unit (150), an entropy encoding unit (155), an inverse quantization unit (160), an inverse transformation unit (165), an adder (170), a loop filter unit (180), and a memory (190).

[0028] Each component of the video encoding device may be implemented in hardware, software, or a combination of hardware and software. Furthermore, the functions of each component may be implemented in software, with a microprocessor executing the software functions corresponding to each component.

[0029] A single image (video) is composed 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). Each CTU is then divided into one or more Coding Units (CUs) by a tree structure. Information applied to each CU is encoded as the syntax of the CU, and information commonly applied to CUs included in a CTU is encoded as the syntax of the CTU. In addition, information commonly applied to all blocks within a single slice is encoded as the syntax of the slice header, and information applied 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). And, information commonly referenced by one or more SPS is encoded in a Video Parameter Set (VPS). In addition, information commonly applied to one tile or tile group may be encoded as syntax of a tile or tile group header. Syntaxes included in an SPS, PPS, slice header, tile or tile group header may be referred to as high level syntax.

[0030] The picture segmentation unit (110) determines the size of the CTU. Information about the size of the CTU (CTU size) is encoded as the syntax of SPS or PPS and transmitted to the image decoding device.

[0031] The picture segmentation unit (110) divides each picture constituting an image into a plurality of CTUs having a predetermined size, and then recursively divides the CTUs using a tree structure. A leaf node in the tree structure becomes a CU, which is a basic unit of encoding.

[0032] 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 the same size, a BinaryTree (BT) in which an upper node is divided into two lower nodes, or a TernaryTree (TT) in which an upper node is divided into three lower nodes in a 1:2:1 ratio, or a structure that mixes two or more of the QT structures, BT structures, 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 may be combined and referred to as a MTT (Multiple-Type Tree).

[0033] Figure 2 is a drawing for explaining a method of dividing a block using the QTBTTT structure.

[0034] As illustrated in FIG. 2, a CTU may first be split into a QT structure. The quadtree splitting may be repeated until the size of the splitting block reaches the minimum block size (MinQTSize) of the leaf node allowed in the QT. 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 encoding unit (155) and signaled to the image decoding device. If the leaf node of the QT is not larger than the maximum block size (MaxBTSize) of the root node allowed in the BT, it may be further split into one or more of the BT structure or the TT structure. There may be multiple splitting directions in the BT structure and / or the TT structure. For example, there may be two directions in which the block of the corresponding node is split horizontally and two directions in which the block is split vertically. As illustrated in FIG. 2, when MTT splitting begins, a second flag (mtt_split_flag) indicating whether nodes have been split, and if splitting has occurred, a flag indicating the splitting direction (vertical or horizontal) and / or a flag indicating the splitting type (Binary or Ternary) are encoded by the entropy encoding unit (155) and signaled to the image decoding device.

[0035] Alternatively, before encoding the first flag (QT_split_flag) indicating whether each node is split into four nodes of a lower layer, a CU split flag (split_cu_flag) indicating whether the node is split may be encoded. If the CU split flag (split_cu_flag) value indicates that the node is not split, the block of the corresponding node becomes a leaf node in the split tree structure and becomes a CU (coding unit), which is a basic unit of encoding. If the CU split flag (split_cu_flag) value indicates that the node is split, the video encoding device starts encoding from the first flag in the above-described manner.

[0036] As another example of a tree structure, when QTBT is used, there may be two types: a type that horizontally splits the block of the corresponding node into two blocks of the same size (i.e., symmetric horizontal splitting) and a type that vertically splits it (i.e., symmetric vertical splitting). A split flag (split_flag) indicating whether each node of the BT structure is split into blocks of a lower layer and split type information indicating the type of split are encoded by the entropy encoding unit (155) and transmitted to the image decoding device. Meanwhile, there may additionally be a type that splits the block of the corresponding node into two blocks of an asymmetrical shape. The asymmetric shape may include a shape that splits the block of the corresponding node into two rectangular blocks with a size ratio of 1:3, or a shape that splits the block of the corresponding node in a diagonal direction.

[0037] A CU can have various sizes depending on the QTBT or QTBTTT partitioning from the CTU. Hereinafter, the block corresponding to the CU to be encoded or decoded (i.e., the leaf node of the QTBTTT) is referred to as the "current block." Depending on the QTBTTT partitioning employed, the current block may be rectangular as well as square.

[0038] 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).

[0039] In general, each current block within a picture can be predictively coded. Prediction of the current block can typically be performed using either intra-prediction (using data from the picture containing the current block) or inter-prediction (using data from a picture coded before the picture containing the current block). Inter-prediction encompasses both unidirectional and bidirectional prediction.

[0040] The intra prediction unit (122) predicts pixels within the current block using pixels (reference pixels) located around the current block within the current picture including the current block. There are multiple intra prediction modes depending on the prediction direction. For example, as shown in Fig. 3a, the multiple intra prediction modes may include two non-directional modes including the Planar mode and the DC mode, and 65 directional modes. The surrounding pixels to be used and the calculation formula are defined differently depending on each prediction mode.

[0041] For efficient directional prediction for a rectangular current block, directional modes (intra prediction modes 67 to 80 and -1 to -14) indicated by dotted arrows in Fig. 3b may be additionally used. These may be referred to as "wide-angle intra-prediction modes." In Fig. 3b, the arrows point to 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. At this time, among the wide-angle intra-prediction modes, some wide-angle intra-prediction modes available for the current block may be determined based on the ratio of the width and height of the rectangular current block. For example, wide-angle intra prediction modes (intra prediction modes 67 to 80) having an angle less than 45 degrees are available when the current block is a rectangular shape whose height is smaller than its width, and wide-angle intra prediction modes (intra prediction modes -1 to -14) having an angle greater than -135 degrees are available when the current block is a rectangular shape whose width is larger than its height.

[0042] The intra prediction unit (122) can determine an intra prediction mode to be used to encode the current block. In some examples, the intra prediction unit (122) can encode the current block using multiple intra prediction modes and select an appropriate intra prediction mode to be used from the tested modes. For example, the intra prediction unit (122) can calculate bit-rate distortion values ​​using rate-distortion analysis for multiple tested intra prediction modes and select an intra prediction mode with the best bit-rate distortion characteristics among the tested modes.

[0043] The intra prediction unit (122) selects one intra prediction mode from among multiple intra prediction modes and predicts the current block using surrounding pixels (reference pixels) and an operation formula determined according to the selected intra prediction mode. Information about the selected intra prediction mode is encoded by the entropy encoding unit (155) and transmitted to the image decoding device.

[0044] 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 a block most similar to the current block within reference pictures that were encoded and decoded before the current picture, and generates a prediction block for the current block using the searched block. Then, a motion vector (MV) corresponding to the displacement between the current block within the current picture and the prediction block within the reference picture is generated. Generally, motion estimation is performed on the luma component, and the motion vector calculated based on the luma component is used for both the luma component and the chroma component. The motion information including information on the reference picture used to predict the current block and information on the motion vector is encoded by the entropy encoding unit (155) and transmitted to the image decoding device.

[0045] The inter prediction unit (124) may perform interpolation on a reference picture or a reference block to improve prediction accuracy. That is, subsamples between two consecutive integer samples are interpolated by applying filter coefficients to a plurality of consecutive integer samples including the two integer samples. When a process of searching for a block most similar to the current block is performed on the interpolated reference picture, the motion vector can be expressed up to a precision in decimal units rather than a precision in integer sample units. The precision or resolution of the motion vector can be set differently for each target region to be encoded, such as a slice, tile, CTU, CU, etc. When such adaptive motion vector resolution (AMVR) is applied, information on the motion vector resolution to be applied to each target region must be signaled for each target region. For example, when the target region is a CU, information on the motion vector resolution applied to each CU is signaled. Information on the motion vector resolution may be information indicating the precision of a differential motion vector, which will be described later.

[0046] 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 indicating the block position 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 reference picture list 0 (RefPicList0) and reference picture list 1 (RefPicList1), respectively, and searches for a block similar to the current block within each reference picture to generate a first reference block and a second reference block. Then, the first reference block and the second reference block are averaged or weighted averaged to generate a prediction block for the current block. Then, motion information including information on two reference pictures used to predict the current block and information on two motion vectors is transmitted to the entropy encoding unit (155). Here, reference picture list 0 may be composed of pictures that are before the current picture in display order among the restored pictures, and reference picture list 1 may be composed of pictures that are after the current picture in display order among the restored pictures. However, this is not necessarily limited to this, and restored pictures that are after the current picture in display order may be additionally included in reference picture list 0, and conversely, restored pictures that are before the current picture may be additionally included in reference picture list 1.

[0047] Various methods can be used to minimize the number of bits required to encode motion information.

[0048] For example, if the reference picture and motion vector of the current block are identical to those of a neighboring block, the motion information of the current block can be transmitted to the image decoding device by encoding information that can identify the neighboring block. This method is called 'merge mode'.

[0049] 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.

[0050] As the surrounding blocks for deriving merge candidates, all or part of the left block (A0), the lower left block (A1), the upper block (B0), the upper right block (B1), and the upper left block (B2) adjacent to the current block within the current picture may be used, as illustrated in FIG. 4. In addition, a block located within a reference picture (which may or may not be the same as the reference picture used to predict the current block) other than the current picture in which the current block is located may be used as a merge candidate. For example, a block co-located with the current block within the reference picture or blocks adjacent to the block at the co-located block 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.

[0051] The inter prediction unit (124) uses these surrounding blocks to construct a merge list containing a predetermined number of merge candidates. Among the merge candidates included in the merge list, the merge candidate to be used as motion information of the current block is selected and merge index information for identifying the selected candidate is generated. The generated merge index information is encoded by the entropy encoding unit (155) and transmitted to the video decoding device.

[0052] Merge Skip mode is a special case of merge mode. After quantization, when all transform coefficients for entropy encoding are close to zero, only neighboring block selection information is transmitted without transmitting residual signals. By utilizing merge skip mode, relatively high encoding efficiency can be achieved for low-motion images, still images, and screen content images.

[0053] Hereinafter, merge mode and merge skip mode are collectively referred to as merge / skip mode.

[0054] Another method for encoding motion information is Advanced Motion Vector Prediction (AMVP) mode.

[0055] In AMVP mode, the inter prediction unit (124) derives predicted motion vector candidates for the motion vector of the current block using neighboring blocks of the current block. As neighboring blocks used to derive predicted motion vector candidates, all or some of the left block (A0), the lower left block (A1), the upper block (B0), the upper right block (B1), and the upper left block (B2) adjacent to the current block in the current picture as shown in FIG. 4 may be used. In addition, a block located in a reference picture (which may or may not be the same as the reference picture used to predict the current block) other than the current picture in which the current block is located may be used as the neighboring block used to derive predicted motion vector candidates. For example, a block co-located with the current block in the reference picture or blocks adjacent to the block in the co-located block 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.

[0056] The inter prediction unit (124) derives predicted motion vector candidates using the motion vectors of these surrounding blocks, and determines a predicted motion vector for the motion vector of the current block using the predicted motion vector candidates. Then, the predicted motion vector is subtracted from the motion vector of the current block to produce a differential motion vector.

[0057] The predicted motion vector can be obtained by applying a predefined function (e.g., median, mean, etc.) to the predicted motion vector candidates. In this case, the image decoding device also knows the predefined function. In addition, since the surrounding blocks used to derive the predicted motion vector candidates are blocks that have already been encoded and decoded, the image decoding device also already knows the motion vectors of the surrounding blocks. Therefore, the image encoding device does not need to encode information to identify the predicted motion vector candidates. Therefore, in this case, information about the differential motion vector and information about the reference picture used to predict the current block are encoded.

[0058] Alternatively, the predicted motion vector can 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 differential motion vector and the reference picture used to predict the current block.

[0059] The subtractor (130) subtracts the prediction block generated by the intra prediction unit (122) or inter prediction unit (124) from the current block to generate a residual block.

[0060] The transformation unit (140) transforms residual signals within a residual block having pixel values ​​in a spatial domain into transform coefficients in a frequency domain. The transformation unit (140) may transform the residual signals within the residual block using the entire size of the residual block as a transformation unit, or may divide the residual block into a plurality of sub-blocks and use the sub-blocks as transformation units to perform the transformation. Alternatively, the residual signals may be transformed using only the transformation domain sub-block as a transformation unit by dividing the sub-blocks into two sub-blocks, that is, a transformation domain and a non-transform domain. Here, the transformation domain 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 a sub-block has been converted, directionality (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. In addition, the size of the conversion 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.

[0061] Meanwhile, the transformation unit (140) can individually perform transformations on the residual block in the horizontal and vertical directions. For the transformation, various types of transformation functions or transformation matrices can 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 transformation function pair with the best transformation efficiency among the MTS and transform the residual block in the horizontal and vertical directions, respectively. Information (mts_idx) on the transformation function pair selected among the MTS is encoded by the entropy encoding unit (155) and signaled to the image decoding device.

[0062] The quantization unit (145) quantizes the transform coefficients output from the transform unit (140) using quantization parameters and outputs the quantized transform coefficients to the entropy encoding unit (155). The quantization unit (145) may directly quantize a related residual block without transformation for a certain block or frame. The quantization unit (145) may also apply different quantization coefficients (scaling values) according to the positions of the transform coefficients within the transform block. The quantization matrix applied to the quantized transform coefficients arranged in two dimensions may be encoded and signaled to an image decoding device.

[0063] The rearrangement unit (150) can perform rearrangement of coefficient values ​​for quantized residual values.

[0064] The reordering unit (150) can change a two-dimensional coefficient array into a one-dimensional coefficient sequence by using coefficient scanning. For example, the reordering unit (150) can output a one-dimensional coefficient sequence by scanning from the DC coefficient to the coefficients of the high-frequency region by using a zig-zag scan or a diagonal scan. Depending on the size of the transformation unit and the intra prediction mode, a vertical scan that scans the two-dimensional coefficient array in the column direction or a horizontal scan that scans the two-dimensional block-shaped coefficients in the row direction may be used instead of the zig-zag scan. That is, depending on the size of the transformation unit and the intra prediction mode, the scanning method to be used may be determined among the zig-zag scan, the diagonal scan, the vertical scan, and the horizontal scan.

[0065] The entropy encoding unit (155) generates a bitstream by encoding a sequence of one-dimensional quantized transform coefficients output from the rearrangement unit (150) using various encoding methods such as CABAC (Context-based Adaptive Binary Arithmetic Code) and Exponential Golomb.

[0066] In addition, 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 image decoding device can divide the block in the same manner as the image encoding device. In addition, the entropy encoding unit (155) encodes information about the prediction type indicating whether the current block is encoded by intra prediction or inter prediction, and encodes intra prediction information (i.e., information about the intra prediction mode) or inter prediction information (information about the encoding mode of motion information (merge mode or AMVP mode), a merge index in the case of the merge mode, and a reference picture index and a differential motion vector in the case of the AMVP mode) according to the prediction type. In addition, the entropy encoding unit (155) encodes information related to quantization, that is, information about quantization parameters and information about a quantization matrix.

[0067] The inverse quantization unit (160) inversely quantizes the quantized transform coefficients output from the quantization unit (145) to generate transform coefficients. The inverse transform unit (165) transforms the transform coefficients output from the inverse quantization unit (160) from the frequency domain to the spatial domain to restore the residual block.

[0068] An adder (170) adds the restored residual block and the predicted block generated by the prediction unit (120) to restore the current block. The pixels within the restored current block are used as reference pixels when intra-predicting the next block.

[0069] The loop filter unit (180) performs filtering on restored pixels to reduce blocking artifacts, ringing artifacts, blurring artifacts, etc. that occur due to block-based prediction and transformation / quantization. The loop filter unit (180) may include all or part of a deblocking filter (182), a sample adaptive offset (SAO) filter (184), and an adaptive loop filter (ALF, 186) as an in-loop filter.

[0070] The deblocking filter (182) filters the boundaries between restored blocks to remove blocking artifacts caused by block-based encoding / decoding, and the SAO filter (184) and the ALF (186) perform additional filtering on the deblocking-filtered image. The SAO filter (184) and the ALF (186) are filters used to compensate for the differences 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 units of CTUs. In contrast, the ALF (186) performs block-based filtering, and compensates for distortion by applying different filters by distinguishing the edges and degrees of variation of the corresponding block. Information on filter coefficients to be used in the ALF can be encoded and signaled to an image decoding device.

[0071] The restored blocks filtered through the deblocking filter (182), SAO filter (184), and ALF (186) are stored in the memory (190). When 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.

[0072] The video encoding device can store the bitstream of encoded video data on a non-transitory storage medium or transmit it to the video decoding device using a communication network.

[0073] FIG. 5 is an exemplary block diagram of an image decoding device capable of implementing the techniques of the present disclosure. Hereinafter, the image decoding device and its subcomponents will be described with reference to FIG. 5.

[0074] The video decoding device may be configured to include an entropy decoding unit (510), a rearrangement 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).

[0075] Similar to the video encoding device of FIG. 1, each component of the video decoding device may be implemented in hardware, software, or a combination of hardware and software. Furthermore, the functions of each component may be implemented in software, with a microprocessor executing the software functions corresponding to each component.

[0076] The entropy decoding unit (510) decodes the bitstream generated by the image encoding device to extract information related to block division, thereby determining the current block to be decoded, and extracts prediction information, information on residual signals, etc. required to restore the current block.

[0077] The entropy decoding unit (510) extracts information about the CTU size from the Sequence Parameter Set (SPS) or the Picture Parameter Set (PPS), determines 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 the CTU is divided using the tree structure by extracting division information about the CTU.

[0078] For example, when splitting a CTU using the QTBTTT structure, first, the first flag (QT_split_flag) related to the splitting of QT is extracted, and each node is split into four nodes of the lower layer. Then, for the nodes corresponding to the leaf nodes of QT, the second flag (mtt_split_flag) related to the splitting of MTT and the split direction (vertical / horizontal) and / or split type (binary / ternary) information are extracted, and the corresponding leaf nodes are split into the MTT structure. Accordingly, each node below the leaf nodes of QT are split recursively into the BT or TT structure.

[0079] As another example, when splitting a CTU using the QTBTTT structure, the CU split flag (split_cu_flag) indicating whether the CU is split is first extracted, and if the block is split, the first flag (QT_split_flag) may be extracted. During the splitting process, each node may undergo zero or more repeated QT splits followed by zero or more repeated MTT splits. For example, a CTU may undergo an MTT split right away, or conversely, may undergo only multiple QT splits.

[0080] As another example, when splitting a CTU using the QTBT structure, the first flag (QT_split_flag) related to the splitting of QT is extracted, and each node is split into four nodes of the lower layer. Furthermore, for nodes corresponding to leaf nodes of QT, a split flag (split_flag) indicating whether to further split into BTs and splitting direction information are extracted.

[0081] Meanwhile, when the entropy decoding unit (510) determines the current block to be decoded by using the division of the tree structure, it extracts information on the prediction type indicating whether the current block is intra-predicted or inter-predicted. If the prediction type information indicates intra-prediction, the entropy decoding unit (510) extracts syntax elements for intra-prediction information (intra-prediction mode) of the current block. If the prediction type information indicates inter-prediction, the entropy decoding unit (510) extracts syntax elements for inter-prediction information, i.e., information indicating a motion vector and a reference picture referenced by the motion vector.

[0082] Additionally, the entropy decoding unit (510) extracts information about the quantized transform coefficients of the current block as information related to quantization and information about residual signals.

[0083] The rearrangement unit (515) can change the sequence of one-dimensional quantized transform 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 performed by the image encoding device.

[0084] The inverse quantization unit (520) inversely quantizes the quantized transform coefficients and inversely quantizes the quantized transform coefficients using the quantization parameters. The inverse quantization unit (520) may also apply different quantization coefficients (scaling values) to the quantized transform coefficients arranged in two dimensions. The inverse quantization unit (520) may perform inverse quantization by applying a matrix of quantized coefficients (scaling values) from an image encoding device to a two-dimensional array of quantized transform coefficients.

[0085] The inverse transform unit (530) inversely transforms the inverse quantized transform coefficients from the frequency domain to the spatial domain to restore residual signals, thereby generating a residual block for the current block.

[0086] In addition, when the inverse transform unit (530) inversely transforms only a portion of a transform block (sub-block), it extracts a flag (cu_sbt_flag) indicating that only a sub-block of the transform block has been transformed, directionality (vertical / horizontal) information (cu_sbt_horizontal_flag) of the sub-block, and / or position information (cu_sbt_pos_flag) of the sub-block, and inversely transforms the transform coefficients of the corresponding sub-block from the frequency domain to the spatial domain to restore residual signals, and fills “0” values ​​with residual signals for areas that have not been inversely transformed, thereby generating a final residual block for the current block.

[0087] In addition, when MTS is applied, the inverse transform unit (530) determines a transform function or a transform matrix to be applied in the horizontal and vertical directions using MTS information (mts_idx) signaled from the image encoding device, and performs inverse transform on the transform coefficients within the transform block in the horizontal and vertical directions using the determined transform function.

[0088] 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 intra prediction, and the inter prediction unit (544) is activated when the prediction type of the current block is inter prediction.

[0089] 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.

[0090] The inter prediction unit (544) uses the syntax elements for the inter prediction mode extracted from the entropy decoding unit (510) to determine the motion vector of the current block and the reference picture referenced by the motion vector, and predicts the current block using the motion vector and the reference picture.

[0091] An adder (550) adds the residual block output from the inverse transform unit (530) and the predicted block output from the inter prediction unit (544) or the intra prediction unit (542) to restore the current block. The pixels within the restored current block are used as reference pixels when intra-predicting a block to be decoded later.

[0092] The loop filter unit (560) may include a deblocking filter (562), an SAO filter (564), and an ALF (566) as in-loop filters. The deblocking filter (562) deblocks the boundaries between restored blocks to remove blocking artifacts caused by block-by-block decoding. The SAO filter (564) and the ALF (566) perform additional filtering on restored blocks after deblocking filtering to compensate for differences between restored pixels and original pixels caused by lossy coding. The filter coefficients of the ALF are determined using information about filter coefficients decoded from the non-stream.

[0093] The restored blocks filtered through the deblocking filter (562), SAO filter (564), and ALF (566) are stored in the memory (570). When all blocks within a picture are restored, the restored picture is used as a reference picture for inter-predicting blocks within a picture to be encoded later.

[0094] The present embodiment relates to encoding and decoding of images (video) as described above. More specifically, a video coding method and device using improved bidirectional predictive local illumination compensation are provided. Furthermore, a video coding method and device adaptively using bidirectional predictive local illumination compensation based on the distance between a current picture and reference pictures are provided.

[0095] The following embodiments may be performed by an inter prediction unit (124) within a video encoding apparatus. In addition, the following embodiments may be performed by an inter prediction unit (544) within a video decoding apparatus.

[0096] The video encoding device can generate signaling information related to the present embodiment in terms of rate distortion optimization in encoding 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 decoding device. The video decoding device can decode the signaling information related to the decoding of the current block from the bitstream using the entropy decoding unit (510).

[0097] In the following description, the term "target block" may be used interchangeably with the current block or coding unit (CU). Alternatively, the term "target block" may also refer to a portion of a coding unit.

[0098] A flag having a value of true indicates that the flag is set to 1. A flag having a value of false indicates that the flag is set to 0.

[0099] The decoder side includes all or part of the inverse quantization unit (160), the inverse transform unit (165), the prediction unit (120), the adder (170), the loop filter unit (180), and the memory (190) in the video encoding device illustrated in FIG. 1. The decoder side includes all or part of the inverse quantization unit (520), the inverse transform unit (530), the prediction unit (540), the adder (550), the loop filter unit (560), and the memory (570) in the video decoding device illustrated in FIG. 5. With respect to a series of decoding processes, the decoder side of the video encoding device and the decoder side of the video decoding device perform the same operation. The video encoding device determines information related to the operation of the decoder side and signals the determined information to the video decoding device. The video decoding device can decode the signaled information and operate the decoder side based on the decoded information.

[0100] I. Local Illumination Compensation (LIC)

[0101] When generating prediction signals for the current block using motion compensation based on inter-prediction, local or global illumination changes between the current frame and the reference frame may occur. LIC (Local Illumination Compensation) is a technology that improves prediction efficiency for the current block by performing illumination compensation according to changes in the image acquisition environment.

[0102] Figure 6 is an example diagram showing local lighting compensation.

[0103] To effectively perform local illumination compensation, an image decoding device derives parameters representing a linear relationship between a surrounding L-shaped template (A_refp) of a current block and a surrounding L-shaped template (B_refp) of a reference block (i.e., a prediction block). For example, the parameters of the linear relationship can be derived based on a linear least square method. The surrounding L-shaped template of the current block exists in the original restoration area of ​​the current block. The image decoding device uses the parameters of the derived linear relationship to perform local illumination compensation of the current block based on the reference block, as shown in Equation 1.

[0104]

[0105] In Equation 1, α and β are parameters of the linear relationship, representing the slope and offset, respectively. P(x,y) represents a pixel within the reference block (i.e., a predicted pixel), and P'(x,y) represents a predicted pixel within the current block to which local illumination compensation has been applied.

[0106] As described above, local lighting compensation is applied to the current block, and a linear relationship is derived based on the surrounding reference pixels adjacent to the reference block and the surrounding reference pixels adjacent to the current block. If there are no available pixels among the surrounding reference pixels of the reference block, local lighting compensation may not be performed on the current block. If available pixels are not located on the left or top of the reference block, but are located in a portion of the top or left, local lighting compensation may be performed using some of the available pixels.

[0107] α and β, the parameters of the linear relationship used in local illumination compensation, are calculated identically in both the image encoding device and the image decoding device. Therefore, the image encoding device may not transmit the parameters of the linear relationship to the image decoding device.

[0108] Meanwhile, BOOF is a technology that additionally compensates for the movement of predicted samples based on optical flow (OF) using bidirectional motion prediction, based on the assumption that the samples or objects that make up the image move at a constant speed and there is little change in the sample values.

[0109] DMVR is a technology that improves motion vectors on the decoder side by finely adjusting the motion vectors of bilateral prediction using bilateral matching (BM) technology.

[0110] Multi-pass Decoder-side Motion Vector Refinement (DMVR) is a technology that refines motion vectors in multiple passes at the decoder side using BM technology. In the first pass of the multiple passes, the video decoder searches for motion vectors on a CU-by-CU basis, and in the second pass, it searches for motion vectors for each 16×16 sub-block within the CU. In the third pass, the video decoder applies Bi-directional Optical Flow (BDOF) to search for motion vectors for each 8×8 sub-block.

[0111] Adaptive DMVR is an extension of multipass DMVR that constructs two new merge modes to improve motion vectors in one of the bidirectional prediction directions for merge candidates that satisfy the DMVR conditions.

[0112] Affine DMVR (DMVR) is a technique that applies DMVR to the current block predicted according to the affine mode. The video decoding device applies the BM to the current block subblocks to improve the subblock motion vectors.

[0113] AMVP (Advanced Motion Vector Prediction)-Merge mode is a technology that uses merge mode in one direction and AMVP mode in the remaining direction in bidirectional prediction.

[0114] Recently, lighting compensation technology has been expanded to cover not only unidirectional prediction blocks but also bidirectional prediction blocks. The lighting compensation technology applied to bidirectional prediction blocks is called bidirectional predictive local illumination compensation (BILC).

[0115] With respect to a bidirectional prediction block in which any one-way prediction block is in merge mode, DMVR and / or BDOF may be performed. For example, in AMVP-Merge mode, AMVP can express relatively accurate motion information. However, since Merge mode directly uses the motion vector information of a merge candidate selected from the merge candidate list without using additional motion vector information, it has limitations in expressing optimal motion information. For example, an image decoding device can correct the motion information derived in the merge mode of the AMVP-Merge mode based on DMVR and / or BDOF.

[0116] In the next generation VVC (beyond VVC), a technology is being applied to activate bidirectional prediction LIC in blocks to which DMVR (e.g., multi-pass DMVR, adaptive DMVR, affine DMVR, etc.) and BDOF are applied.

[0117] For example, a video decoding device checks the DMVR and BDOF application conditions for the current block. If all application conditions are satisfied, the video decoding device applies bidirectional prediction LIC to the current block. While the existing DMVR and BDOF used the same application conditions, the DMVR application conditions have recently changed. Therefore, there are differences between the DMVR and BDOF application conditions. The revised DMVR application conditions are shown in Table 1.

[0118]

[0119] As shown in Table 1, among the existing DMVR application conditions, the requirement that the distances between the two reference pictures and the current picture be equal has been removed. In other words, DMVR can be performed if all remaining conditions are satisfied, regardless of the distance between the two reference pictures and the current picture.

[0120] However, the application condition of BDOF still includes the condition that the distance between the two reference pictures and the current picture must be equal. Since bidirectional prediction LIC is activated only when both the DMVR application condition and the BDOF application condition are satisfied, bidirectional prediction LIC is performed only when the distance between the two reference pictures and the current picture is equal. The velocity of an arbitrary object can change every time unit, and the reference block for bidirectional prediction of the current block may not always exist at the same distance due to various factors. Nevertheless, since the existing bidirectional prediction LIC is performed when the distance between the two reference pictures and the current picture is equal, the opportunity for lighting compensation may be limited with respect to the bidirectional prediction block.

[0121] The following embodiments are described with a focus on a video decoding device, but can be implemented in the same or similar manner in a video encoding device. Alternatively, the following embodiments are described with a focus on the decoder side of a video decoding device, but can also be implemented in the same or similar manner in the decoder side of a video encoding device.

[0122] II. Embodiments according to the present disclosure

[0123] In the present disclosure, to solve the above-described problem, the video decoding device performs bidirectional prediction LIC regardless of the condition that the distances between two reference pictures and the current picture must be the same. In the present disclosure, the video decoding device does not consider the condition regarding the distance between two reference pictures and the current picture when determining whether to apply the bidirectional prediction LIC. That is, the video decoding device can perform lighting compensation on bidirectional prediction blocks in which the distances between the current picture and the two reference pictures are different. Therefore, by relaxing the application condition of the bidirectional prediction LIC, lighting compensation can be performed on more bidirectional prediction blocks.

[0124] As an example, a bidirectional LIC works as follows:

[0125] The video decoding device derives different linear models for the two prediction blocks of the current block. Hereinafter, L0 is defined as a list of reference pictures that exist before the current picture in the playback order, and L1 is defined as a list of reference pictures that exist after the current picture in the playback order. The video decoding device derives a linear model between the current block and the prediction block included in L0 as in Equation 2, and a linear model between the current block and the prediction block included in L1 as in Equation 3.

[0126]

[0127]

[0128] In Equations 2 and 3, α0, β0, α1, and β1 are lighting compensation parameters. α0 and α1 represent L0-direction and L1-direction scale parameters, respectively, and β0 and β1 represent L0-direction and L1-direction offset parameters. P0'[x,y] is a lighting-compensated prediction block in the L0 direction, and P1'[x,y] is a lighting-compensated prediction block in the L1 direction.

[0129] The image decoding device can calculate the lighting compensation parameters as follows. The template of the current block is represented as T[x, y], the template of the L0-direction prediction block is represented as T0[x, y], and the template of the L1-direction prediction block is represented as T1[x, y]. The template is composed of already decoded pixels around the current block and can have various sizes or shapes (e.g., L-shape). The image decoding device uses the linear model of Equation 4 to minimize the difference between T0[x, y] and T[x, y] by α. 0,tmp and β 0,tmp It induces.

[0130]

[0131] Induced α 0,tmp and β 0,tmp T[x,y] is updated using , and the updated template is represented as T'[x,y]. The image decoding device derives α1 and β1 that minimize the difference between T'[x,y] and T1[x,y] using the linear model of Equation 5.

[0132]

[0133] Using the derived α1 and β1, T'[x,y] is updated, and the updated template is represented as T"[x,y]. The image decoding device derives α0 and β0 that minimize the difference between T"[x,y] and T0[x,y] using the linear model of Equation 6.

[0134]

[0135] As described above, the calculated α0, β0, α1, and β1 are the final parameters of the bidirectional prediction LIC.

[0136] In the above description, the template of the L0 direction prediction block is considered first, but in deriving the lighting compensation parameters, the template of the L1 direction prediction block may be considered first. In addition to the method of using all pixels of the template area, as another example, using a method such as sampling, the image decoding device can derive α0, β0, α1, and β1 based on some pixels of the template area.

[0137] The video decoding device can generate the illumination-compensated prediction blocks P0'[x,y] and P1'[x,y] by applying α0, β0, α1, and β1 to Equations 2 and 3. The video decoding device can generate the final bidirectional prediction block P'[x,y] by performing a weighted sum on P0'[x,y] and P1'[x,y] as in Equation 7.

[0138]

[0139] In mathematical expression 7, the weight w used in the weighted sum can be determined by one of the following methods.

[0140] As an example, a video decoding device can calculate a weight using BCW_idx signaled at the CU level. Here, BCW (Bi-prediction with CU-level Weight) represents a weight for adding bidirectional prediction blocks.

[0141] As another example, the image decoding device can calculate weights using distortion values ​​(e.g., SAD (Sum of Absolute Differences), SSE (Sum of Square Error), SATD (Sum of Absolute Transformed Differences), MR-SAD (Mean Removed Sum of Absolute Differences), etc.) between reference templates and the current template obtained in the process of performing illumination compensation.

[0142] As another example, preset weights may be used according to an agreement between an image encoding device and an image decoding device.

[0143] As another example, there may be a list of weights according to an agreement between an image encoding device and an image decoding device, and one of the weights from the above-described list may be selected based on a preset method.

[0144] As another example, the video decoding device may calculate weights using the aspect ratio, width, and / or height of the current block.

[0145] As another example, the image decoding device can derive weights based on surrounding information of the current block.

[0146] As another example, the video decoding device can calculate the weight based on the distance ratio between the current picture and two reference pictures.

[0147] In the present disclosure, the video decoding device adaptively applies bidirectional prediction LIC to a bidirectional prediction block. If the distances between two reference pictures and the current picture are different, and one of the two reference pictures is far from the current picture, applying bidirectional prediction LIC may be meaningless. Rather, it may degrade coding efficiency due to inaccurate compensation.

[0148] In this embodiment, to solve the above-mentioned problem, the image decoding device adaptively performs bidirectional prediction LIC by considering the distance between the picture and the reference pictures. Hereinafter, the two reference pictures of the bidirectional prediction are referred to as the LX direction (X = 0 or 1) reference picture and the LY direction reference picture (Y = 1 - X). Hereinafter, DiffPOC LY Go DiffPOC LX Assume it is larger than that.

[0149] As an example, DiffPOC is the distance between the current picture and the LY direction reference picture. LY DiffPOC is the distance between the current picture and the LX direction reference picture. LX If the error is greater than 100 times (i.e. DiffPOC LY > c×DiffPOC LX . Here, c is a real number greater than 1), the image decoding device can perform unidirectional illumination compensation using only the LX reference picture.

[0150] As another example, DiffPOC LY If the LX reference picture is greater than a preset threshold, the image decoding device can perform unidirectional illumination compensation using only the LX reference picture.

[0151] Hereinafter, using the cities of FIGS. 7 and 8, a method of applying bidirectional prediction LIC to the current block is described, regardless of the condition that the distance between two reference pictures and the current picture must be equal.

[0152] FIG. 7 is a flowchart illustrating a method of encoding a current block performed by an image encoding device according to one embodiment of the present disclosure.

[0153] The video encoding device obtains reference pictures of the current picture based on upper level information (S700). Here, the reference pictures include reference pictures in the LX (X=0 or 1) direction and reference pictures in the LY (Y=1-X) direction. For example, the video encoding device may obtain indices indicating reference pictures from the upper level based on the merge mode or AMVP mode, and use the reference pictures indicated by the indices.

[0154] The video encoding device verifies whether the first distance and the second distance satisfy the conditions for bidirectional local lighting compensation (S702).

[0155] Here, the first distance represents the distance between the current picture and the reference picture in the LY direction, and the second distance represents the distance between the current picture and the reference picture in the LX direction. It is assumed that the first distance is greater than the second distance.

[0156] The conditions for bidirectional local lighting compensation are that the first distance is less than or equal to a real number (where the real number is greater than 1) times the second distance, and that the first distance and the second distance are less than or equal to a preset threshold.

[0157] As another example, regardless of whether the first and second distances are equal, the image encoding device can perform bidirectional local illumination compensation. That is, the image encoding device can use process S702 to determine the conditions for applying unidirectional local illumination compensation.

[0158] If the conditions for bidirectional local illumination compensation are satisfied (S702 Yes), the video encoding device can perform bidirectional local illumination compensation as follows. Alternatively, regardless of whether the first distance and the second distance are the same, the video encoding device can perform bidirectional local illumination compensation as follows.

[0159] The video encoding device generates reference blocks from reference pictures (S704). Here, the reference blocks include a reference block in the LX direction and a reference block in the LY direction.

[0160] The image encoding device obtains bidirectional local illumination compensation parameters (S706). Here, the local illumination compensation parameters define linear relationships between reference blocks and the current block to which local illumination compensation has been applied.

[0161] The video encoding device can use an iterative method, as in Equations 4 to 6, based on the template of the current block, the template of the reference block in the LX direction, and the template of the reference block in the LY direction. The video encoding device can calculate first local lighting compensation parameters indicating a linear relationship between the template of the current block and the template of the reference block in the LX direction, and second local lighting compensation parameters indicating a linear relationship between the template of the current block and the template of the reference block in the LY direction.

[0162] The image encoding device generates a final prediction block of the current block by applying local lighting compensation to reference blocks based on local lighting compensation parameters (S708).

[0163] The video encoding device can generate a first prediction block of the current block, as in Equation 2, by applying local lighting compensation to a reference block in the LX direction based on the first local lighting compensation parameters. The video encoding device can generate a second prediction block of the current block, as in Equation 3, by applying local lighting compensation to a reference block in the LY direction based on the second local lighting compensation parameters. The video encoding device can generate a final prediction block by performing a weighted sum of the first prediction block and the second prediction block, as in Equation 7.

[0164] As an example, a video encoding device can calculate a weight using BCW_idx signaled at the CU level. Here, BCW represents a weight for adding bidirectional prediction blocks.

[0165] As another example, the image encoding device can use the distortion values ​​between the reference templates and the template of the current block, which are obtained in the process of calculating local illumination compensation parameters, as weights for performing a weighted sum. Here, the reference templates include the template of the reference block in the LX direction and the template of the reference block in the LY direction.

[0166] The video encoding device encodes information of reference pictures (S710).

[0167] If the condition for two-way local illumination compensation is not satisfied (No of 702), i.e., if the first distance is greater than a real number (wherein the real number is greater than 1) times the second distance, or if the first distance is greater than a preset threshold, the image encoding device can perform one-way local illumination compensation as follows.

[0168] The video encoding device generates a reference block in the LX direction from a reference picture in the LX direction (S720).

[0169] The image encoding device obtains unidirectional local illumination compensation parameters (S722). Here, the local illumination compensation parameters define a linear relationship between a reference block in the LX direction and a current block to which local illumination compensation is applied.

[0170] The image encoding device generates a final prediction block of the current block by applying local lighting compensation to a reference block in the LX direction based on local lighting compensation parameters (S724).

[0171] After generating the final prediction block, the video encoding device can generate a residual block by subtracting the final prediction block from the current block. The video encoding device can transform / quantize the residual block to generate quantized transform coefficients and encode the quantized transform coefficients.

[0172] FIG. 8 is a flowchart illustrating a method for restoring a current block performed by an image decoding device according to one embodiment of the present disclosure.

[0173] The video decoding device obtains reference pictures of the current picture based on the bitstream (S800).

[0174] Here, the reference pictures include reference pictures in the LX (X=0 or 1) direction and reference pictures in the LY (Y=1-X) direction. For example, the image decoding device can obtain indices indicating reference pictures from a bitstream based on a merge mode or an AMVP mode, and use the reference pictures indicated by the indices.

[0175] The image decoding device verifies whether the first distance and the second distance satisfy the conditions for bidirectional local lighting compensation (S802).

[0176] Here, the first distance represents the distance between the current picture and the reference picture in the LY direction, and the second distance represents the distance between the current picture and the reference picture in the LX direction. It is assumed that the first distance is greater than the second distance.

[0177] The conditions for bidirectional local lighting compensation are that the first distance is less than or equal to a real number (where the real number is greater than 1) times the second distance, and that the first distance and the second distance are less than or equal to a preset threshold.

[0178] As another example, regardless of whether the first and second distances are equal, the image decoding device can perform bidirectional local illumination compensation. That is, the image decoding device can use the S802 process to determine the conditions for applying unidirectional local compensation.

[0179] If the conditions for bidirectional local illumination compensation are satisfied (S802 Yes), the image decoding device can perform bidirectional local illumination compensation as follows. Alternatively, regardless of whether the first distance and the second distance are the same, the image decoding device can perform bidirectional local illumination compensation as follows.

[0180] The video decoding device generates reference blocks from reference pictures (S804). Here, the reference blocks include a reference block in the LX direction and a reference block in the LY direction.

[0181] The image decoding device obtains bidirectional local illumination compensation parameters (S806). Here, the local illumination compensation parameters define linear relationships between reference blocks and the current block to which local illumination compensation has been applied.

[0182] The image decoding device can use an iterative method, as in Equations 4 to 6, based on the template of the current block, the template of the reference block in the LX direction, and the template of the reference block in the LY direction. The image decoding device can calculate first local lighting compensation parameters indicating a linear relationship between the template of the current block and the template of the reference block in the LX direction, and second local lighting compensation parameters indicating a linear relationship between the template of the current block and the template of the reference block in the LY direction.

[0183] The image decoding device generates a final prediction block of the current block by applying local lighting compensation to reference blocks based on local lighting compensation parameters (S808).

[0184] The image decoding device can generate a first prediction block of the current block, as in Equation 2, by applying local lighting compensation to a reference block in the LX direction based on the first local lighting compensation parameters. The image decoding device can generate a second prediction block of the current block, as in Equation 3, by applying local lighting compensation to a reference block in the LY direction based on the second local lighting compensation parameters. The image decoding device can generate a final prediction block by performing a weighted sum of the first prediction block and the second prediction block, as in Equation 7.

[0185] As an example, a video decoding device can calculate weights using BCW_idx signaled at the CU level, where BCW represents a weight for adding bidirectional prediction blocks.

[0186] As another example, the image decoding device can use the distortion values ​​between the reference templates and the template of the current block, which are obtained in the process of calculating local illumination compensation parameters, as weights for performing a weighted sum. Here, the reference templates include the template of the reference block in the LX direction and the template of the reference block in the LY direction.

[0187] If the condition for bidirectional local illumination compensation is not satisfied (No of 802), i.e., if the first distance is greater than a real number (wherein the real number is greater than 1) times the second distance, or if the first distance is greater than a preset threshold, the image decoding device can perform unidirectional local illumination compensation as follows.

[0188] The video decoding device generates a reference block in the LX direction from a reference picture in the LX direction (S820).

[0189] The image decoding device obtains unidirectional local illumination compensation parameters (S822). Here, the local illumination compensation parameters define a linear relationship between a reference block in the LX direction and a current block to which local illumination compensation is applied.

[0190] The image decoding device generates a final prediction block of the current block by applying local lighting compensation to a reference block in the LX direction based on local lighting compensation parameters (S824).

[0191] An image decoding device can decode quantized transform coefficients from a bitstream and generate a residual block by inversely quantizing / inversely transforming the quantized transform coefficients. Thereafter, the image decoding device can add the final prediction block and the residual block to generate a reconstruction block of the current block.

[0192] Although the flowchart / timing diagram of this specification describes each process as being executed sequentially, this is merely an illustrative description of the technical idea of ​​one embodiment of the present disclosure. In other words, a person of ordinary skill in the art to which one embodiment of the present disclosure belongs may modify and apply various modifications and variations by changing the order described in the flowchart / timing diagram without departing from the essential characteristics of one embodiment of the present disclosure, or by executing one or more of the processes in parallel. Therefore, the flowchart / timing diagram is not limited to a chronological order.

[0193] It should be understood that the exemplary embodiments described above can be implemented in many different ways. The functions or methods described in one or more examples can be implemented in hardware, software, firmware, or any combination thereof. It should be understood that the functional components described herein are labeled as "units" to further emphasize their implementation independence.

[0194] Meanwhile, the various functions or methods described in this embodiment may also be implemented as instructions stored on a non-transitory storage medium that can be read and executed by one or more processors. Non-transitory storage media include, for example, all types of storage devices that store data in a form readable by a computer system. For example, non-transitory storage media include storage media such as erasable programmable read-only memory (EPROM), flash drives, optical drives, magnetic hard drives, and solid-state drives (SSDs).

[0195] The above description is merely an example of the technical idea of ​​the present embodiment, and those skilled in the art will appreciate that various modifications and variations can be made without departing from the essential characteristics of the present embodiment. Therefore, the present embodiments are not intended to limit the technical idea of ​​the present embodiment, but rather to explain it, 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 interpreted by the claims below, and all technical ideas within a scope equivalent thereto should be interpreted as being included in the scope of rights of the present embodiment.

[0196]

[0197]

[0198] CROSS-REFERENCE TO RELATED APPLICATION

[0199] This patent application claims priority to Korean patent application No. 10-2024-0046084, filed on April 4, 2024, the entire contents of which are incorporated herein by reference.

Claims

1. In a method for restoring a current block performed by an image decryption device, A step of obtaining reference pictures of a current picture based on a bitstream, wherein the reference pictures include a reference picture in the LX (X=0 or 1) direction and a reference picture in the LY (Y=1-X) direction; and A step of applying local illumination compensation to the current block, regardless of whether the first distance and the second distance are the same. Including, but not limited to, The steps for applying the above local lighting compensation are: A step of generating reference blocks from the above reference pictures, wherein the reference blocks include a reference block in the LX direction and a reference block in the LY direction; A step of obtaining bidirectional local lighting compensation parameters, wherein the local lighting compensation parameters define linear relationships between the reference blocks and the current block to which local lighting compensation is applied; and A step of generating a final prediction block of the current block by applying local lighting compensation to the reference blocks based on the local lighting compensation parameters. Including, but not limited to, A method wherein the first distance represents a distance between the current picture and a reference picture in the LY direction, and the second distance represents a distance between the current picture and a reference picture in the LX direction.

2. In paragraph 1, The step of obtaining the above local lighting compensation parameters is: A method for calculating first local lighting compensation parameters indicating a linear relationship between the template of the current block and the template of the reference block in the LX direction, and second local lighting compensation parameters indicating a linear relationship between the template of the current block and the template of the reference block in the LY direction, using an iterative method based on the template of the current block, the template of the reference block in the LX direction, and the template of the reference block in the LY direction.

3. In paragraph 2, The step of generating the final prediction block is as follows: A step of generating a first prediction block of the current block by applying local lighting compensation to the reference block in the LX direction based on the first local lighting compensation parameters; A step of generating a second prediction block of the current block by applying local lighting compensation to the reference block in the LY direction based on the second local lighting compensation parameters; and A step of generating the final prediction block by performing a weighted sum of the first prediction block and the second prediction block. A method comprising:

4. In paragraph 3, The step of generating the final prediction block is as follows: A method in which the distortion values ​​between the reference templates and the template of the current block, obtained in the process of calculating the local lighting compensation parameters, are used as weights for performing the weighted sum, wherein the reference templates include a template of the reference block in the LX direction and a template of the reference block in the LY direction.

5. In paragraph 1, If the first distance is greater than a real number (wherein the real number is greater than 1) times the second distance, or if the first distance is greater than the preset threshold, The steps for applying the above local lighting compensation are: A step of generating a reference block in the LX direction from a reference picture in the LX direction; A step of obtaining unidirectional local lighting compensation parameters, wherein the local lighting compensation parameters define a linear relationship between a reference block in the LX direction and a current block to which local lighting compensation is applied; and A step of generating a final prediction block of the current block by applying local lighting compensation to the reference block in the LX direction based on the local lighting compensation parameters. A method comprising:

6. In a method for encoding a current block performed by a video encoding device, A step of obtaining reference pictures of a current picture based on upper level information, wherein the reference pictures include a reference picture in the LX (X=0 or 1) direction and a reference picture in the LY (Y=1-X) direction; and A step of applying local illumination compensation to the current block, regardless of whether the first distance and the second distance are the same. Including, but not limited to, The steps for applying the above local lighting compensation are: A step of generating reference blocks from the above reference pictures, wherein the reference blocks include a reference block in the LX direction and a reference block in the LY direction; A step of obtaining bidirectional local lighting compensation parameters, wherein the local lighting compensation parameters define linear relationships between the reference blocks and the current block to which local lighting compensation is applied; and A step of generating a final prediction block of the current block by applying local lighting compensation to the reference blocks based on the local lighting compensation parameters. Including, but not limited to, A method wherein the first distance represents a distance between the current picture and a reference picture in the LY direction, and the second distance represents a distance between the current picture and a reference picture in the LX direction.

7. In paragraph 6, A method further comprising a step of encoding information of the above reference pictures.

8. In paragraph 7, The step of obtaining the above local lighting compensation parameters is: A method for calculating first local lighting compensation parameters indicating a linear relationship between the template of the current block and the template of the reference block in the LX direction, and second local lighting compensation parameters indicating a linear relationship between the template of the current block and the template of the reference block in the LY direction, using an iterative method based on the template of the current block, the template of the reference block in the LX direction, and the template of the reference block in the LY direction.

9. In paragraph 8, The step of generating the final prediction block is as follows: A step of generating a first prediction block of the current block by applying local lighting compensation to the reference block in the LX direction based on the first local lighting compensation parameters; A step of generating a second prediction block of the current block by applying local lighting compensation to the reference block in the LY direction based on the second local lighting compensation parameters; and A step of generating the final prediction block by performing a weighted sum of the first prediction block and the second prediction block. A method comprising:

10. In paragraph 6, If the first distance is greater than a real number (wherein the real number is greater than 1) times the second distance, or if the first distance is greater than the preset threshold, The steps for applying the above local lighting compensation are: A step of generating a reference block in the LX direction from a reference picture in the LX direction; A step of obtaining unidirectional local lighting compensation parameters, wherein the local lighting compensation parameters define a linear relationship between a reference block in the LX direction and a current block to which local lighting compensation is applied; and A step of generating a final prediction block of the current block by applying local lighting compensation to the reference block in the LX direction based on the local lighting compensation parameters. A method comprising:

11. In a method for providing video data to a video decoding device, A step of encoding the above video data into a bitstream; and A step of transmitting the above bitstream to the image decoding device Including, but not limited to, The step of encoding the above video data is: A step of obtaining reference pictures of a current picture based on upper level information, wherein the reference pictures include a reference picture in the LX (X=0 or 1) direction and a reference picture in the LY (Y=1-X) direction; and A step of applying local illumination compensation to the current block, regardless of whether the first and second distances are the same. Including, but not limited to, The steps for applying the above local lighting compensation are: A step of generating reference blocks from the above reference pictures, wherein the reference blocks include a reference block in the LX direction and a reference block in the LY direction; A step of obtaining bidirectional local lighting compensation parameters, wherein the local lighting compensation parameters define linear relationships between the reference blocks and the current block to which local lighting compensation is applied; and A step of generating a final prediction block of the current block by applying local lighting compensation to the reference blocks based on the local lighting compensation parameters. Including, but not limited to, A method wherein the first distance represents a distance between the current picture and a reference picture in the LY direction, and the second distance represents a distance between the current picture and a reference picture in the LX direction.

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