Patent Publication Number: US-2022224885-A1

Title: Method and device for deriving intra-prediction mode

Description:
TECHNICAL FIELD 
     The present disclosure in some embodiments relates to the encoding and decoding of images or a video. More particularly, the present disclosure relates to a method and apparatus for deriving an intra-prediction mode through an intra-prediction mode derivation scheme having an extended application range to provide improved encoding and decoding efficiency. 
     BACKGROUND 
     Since video data has a large data volume compared to audio data or still image data, it requires a lot of hardware resources, including memory, to store or transmit the data in its raw form before undergoing a compression process. 
     Accordingly, storing or transmitting video data typically accompanies compression thereof by using an encoder before a decoder can receive, decompress, and reproduce the compressed video data. Existing video compression technologies include H.264/AVC and High Efficiency Video Coding (HEVC), which improves the encoding efficiency of H.264/AVC by about 40%. 
     However, the constant increase of videos in size, resolution, and frame rate and the resultant increase of data amount to be encoded require a new and superior compression technique with better coding efficiency and higher image quality improvement over existing compression techniques. 
     SUMMARY 
     Technical Problem 
     The present disclosure seeks to provide an improved video encoding and decoding technology to meet these needs, and in particular, at least one aspect of the present disclosure is to provide techniques for improving the efficiency of video encoding and decoding through an intra-prediction mode derivation scheme that is generally applicable to various intra prediction techniques. 
     Technical Solution 
     At least one aspect of the present disclosure provides a method of deriving an intra-prediction mode, including obtaining, based on reference line information indicating any one of a plurality of reference lines, most probable mode (MPM) information indicating whether an intra-prediction mode of a current block is included in an MPM list lacking a planar mode, determining, based on the reference line information and the MPM information, planar information indicating whether the intra-prediction mode of the current block is the planar mode, and deriving the intra-prediction mode of the current block by using the planar information. 
     Another aspect of the present disclosure provides an apparatus for decoding video images, including an acquisition unit, a determination unit, and a derivation unit. The acquisition unit is configured to obtain, based on reference line information indicating any one of a plurality of reference lines, most probable mode (MPM) information indicating whether an intra-prediction mode of a current block is included in an MPM list lacking a planar mode. The determination unit is configured to determine, based on the reference line information and the MPM information, planar information indicating whether the intra-prediction mode of the current block is the planar mode. The derivation unit is configured to derive the intra-prediction mode of the current block by using the planar information. 
     Advantageous Effects 
     As described above, according to at least one embodiment of the present disclosure, an intra-prediction mode derivation scheme is provided with a more extended application range an efficient selection of candidate intra modes for a chroma block and more precise prediction of the chroma block can result in improved compression performance. 
     According to another embodiment of the present disclosure, an MPM list and a non-MPM list can be efficiently set, thereby providing an improved compression performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a video encoding apparatus that can implement the techniques of the present disclosure. 
         FIG. 2  is a diagram for explaining a method of partitioning or splitting a block by using a QTBTTT structure. 
         FIG. 3  is a diagram of multiple intra prediction modes. 
         FIG. 4  is a block diagram illustrating a video decoding apparatus capable of implementing the techniques of the present disclosure. 
         FIG. 5  is a diagram illustrating conventional intra prediction modes. 
         FIG. 6  is a diagram illustrating a multiple reference lines (MRL) function. 
         FIG. 7  is a diagram illustrating an intra sub-partition (ISP) function. 
         FIG. 8  is a block diagram of an example video encoding/decoding apparatus capable of implementing the techniques of the present disclosure. 
         FIG. 9  is a flowchart of an illustrative embodiment of intra-prediction mode derivation. 
         FIG. 10  is a flowchart of an illustrative embodiment for obtaining the most probable mode (MPM) information. 
         FIGS. 11 and 12  are flowcharts illustrating various embodiments for determining planar information. 
         FIG. 13  is a flowchart of an illustrative embodiment of an MPM list configuration. 
         FIG. 14  is a diagram illustrating neighboring blocks used for MPM list configuration. 
         FIGS. 15 and 16  are flowcharts illustrating various embodiments of MPM list configuration. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, some embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In the following description, like reference numerals preferably designate like elements, although the elements are shown in different drawings. Further, in the following description of some embodiments, a detailed description of related known components and functions when considered to obscure the subject of the present disclosure will be omitted for the purpose of clarity and for brevity. 
       FIG. 1  is a block diagram illustrating a video encoding apparatus that can implement the techniques of the present disclosure. Hereinafter, a video encoding apparatus and sub-components of the apparatus will be described with reference to  FIG. 1 . 
     The video encoding apparatus may be configured including a block splitter  110 , a prediction unit  120 , a subtractor  130 , a transformer  140 , a quantizer  145 , an encoder  150 , an inverse quantizer  160 , an inverse transformer  165 , an adder  170 , a filter unit  180 , and a memory  190 . 
     The respective components of the video encoding apparatus may be implemented as hardware or software, or hardware and software combined. Additionally, the function of each component may be implemented by software and the function by software for each component may be implemented to be executed by a microprocessor. 
     A video is composed of a plurality of pictures. The pictures are each split into a plurality of regions, and encoding is performed for each region. For example, one picture is split into one or more tiles or/and slices. Here, one or more tiles may be defined as a tile group. Each tile or/and slice is split into one or more Coding Tree Units (CTUs). And each CTU is split into one or more Coding Units (CUs) by a tree structure. Information applied to the respective CUs are encoded as syntaxes of the CUs, and information commonly applied to CUs included in one CTU is encoded as a syntax of the CTU. Additionally, information commonly applied to all blocks in one slice is encoded as a syntax of a tile or as a syntax of a tile group that is a collection of tiles, and information applied to all blocks constituting one picture is encoded in a Picture Parameter Set (PPS) or a picture header. Furthermore, information commonly referenced by a plurality of pictures is encoded in a Sequence Parameter Set (SPS). Additionally, information commonly referenced by one or more SPSs is encoded in a Video Parameter Set (VPS). 
     The block splitter  110  determines the size of the coding tree unit (CTU). Information on the size of the CTU (CTU size) is encoded as a syntax of the SPS or PPS and transmitted to a video decoding apparatus. 
     The block splitter  110  splits each picture constituting the video into a plurality of coding tree units (CTUs) having a predetermined size and then uses a tree structure to split the CTUs recursively. A leaf node in the tree structure becomes a coding unit (CU), which is a basic unit of encoding. 
     A tree structure for use may be a QuadTree (QT) in which an upper node (or parent node) is split into four equally sized lower nodes (or child nodes), a BinaryTree (BT) in which an upper node is split into two lower nodes, a TernaryTree (TT) in which an upper node is split into three lower nodes in a size ratio of 1:2:1, or a mixture of two or more of the QT structure, BT structure, and TT structure. For example, a QuadTree plus BinaryTree (QTBT) structure may be used, or a QuadTree plus BinaryTree TernaryTree (QTBTTT) structure may be used. Here, BTTT may be collectively referred to as a Multiple-Type Tree (MTT). 
       FIG. 2  shows a QTBTTT split tree structure. As shown in  FIG. 2 , the CTU may be first split into a QT structure. The quadtree splitting may be repeated until the size of a splitting block reaches the minimum block size (MinQTSize) of a leaf node allowed in QT. A first flag (QT_split_flag) indicating whether each node of the QT structure is split into four nodes of a lower layer is encoded by the encoder  150  and signaled to the video decoding apparatus. When 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 any one or more of the BT structure or the TT structure. In the BT structure and/or the TT structure, there may be a plurality of split directions. For example, there may be two directions in which the block of the relevant node is split horizontally and vertically. As shown in  FIG. 2 , when MTT splitting starts, a second flag (mtt_split_flag) indicating whether the nodes are split, and if yes, a further flag indicating split directions (vertical or horizontal) and/or a flag indicating partition or split type (binary or ternary) are encoded by the encoder  150  and signaled to the video decoding apparatus. 
     As another example of the tree structure, when a block is split using the QTBTTT structure, information on a CU split flag (split_cu_flag) indicating that the node was split is first encoded and signaled and then a QT_split_flag (split_qt_flag) information indicating whether the split type is QT split is encoded by the encoder  150  and signaled to the video decoding apparatus. When the CU split flag (split_cu_flag) value indicates that the node was not split, the block of that node becomes a leaf node in the split tree structure and turns into a coding unit (CU), which is a basic unit of coding. When the CU split flag (split_cu_flag) value indicates that the node was split, the video encoding apparatus classifies the split type into QT split or MTT split. When the split type is QT split, there is no additional information. When the split type is MTT split, one or both of flags are additionally encoded and signaled by the encoder  150  and signaled to the video decoding apparatus, which are a flag (mtt_split_cu_vertical_flag) indicating the MTT split direction (vertical or horizontal) and a flag (mtt_split_cu_binary_flag) indicating the MTT split type (binary or ternary). 
     As yet another example of the tree structure, when QTBT is used, there may be two types of partition including a type that horizontally splits the block of the relevant node into two equally sized blocks (i.e., symmetric horizontal splitting) and a type that splits the same vertically (i.e., symmetric vertical splitting). Encoded by the encoder  150  and transmitted to the video decoding apparatus are a split flag (split_flag) indicating whether each node of the BT structure is split into blocks of a lower layer and partition type information indicating its partition type. Meanwhile, there may be a further type in which the block of the relevant node is split into two asymmetrically formed blocks. The asymmetric form may include a form of the block of the relevant node being split into two rectangular blocks having a size ratio of 1:3 or a form of the block of the relevant node being split in a diagonal direction. 
     A CU may have various sizes depending on the QTBT or QTBTTT split of the CTU. Hereinafter, a block corresponding to a CU to be encoded or decoded (i.e., a leaf node of QTBTTT) is referred to as a ‘current block’. 
     The prediction unit  120  predicts the current block to generate a prediction block. The prediction unit  120  includes an intra predictor  122  and an inter predictor  124 . 
     In general, the current blocks in a picture may each be predictively coded. Prediction of the current block may be generally performed using an intra prediction technique or inter prediction technique. The intra prediction technique uses data from a picture containing the current block and the inter prediction technique uses data from the preceding picture coded before the picture containing the current block. Inter prediction includes both unidirectional prediction and bidirectional prediction. 
     The intra predictor  122  predicts pixels in the current block by using the peripheral pixels (reference pixels) located around the current block in the current picture. There are multiple intra prediction modes depending on different prediction directions. For example, as shown in  FIG. 3 , the multiple intra prediction modes may include 2 non-directional modes including a planar mode and a DC mode and 65 directional modes. Neighboring pixels and an equation to be used are defined differently for each prediction mode. 
     The intra predictor  122  may determine an intra prediction mode to be used for encoding the current block. In some examples, the intra predictor  122  may encode the current block by using several intra prediction modes and select an appropriate intra prediction mode to use from tested modes. For example, the intra predictor  122  may calculate rate-distortion values through rate-distortion analysis of several tested intra prediction modes and select an intra prediction mode that has the best rate-distortion characteristics among the tested modes. 
     The intra predictor  122  selects one intra prediction mode from among a plurality of intra prediction modes and predicts the current block by using at least one neighboring pixel (reference pixel) determined according to the selected intra prediction mode and calculation formula. Information on the selected intra prediction mode is encoded by the encoder  150  and transmitted to the video decoding apparatus. 
     The inter predictor  124  generates a prediction block for the current block through a motion compensation process. The inter predictor  124  searches for a block most similar to the current block in the coded and decoded reference picture before the current picture, and generates a prediction block of the current block by using the searched block. Then, the inter predictor  124  generates a motion vector corresponding to the displacement between the current block in the current picture and the prediction block in a reference picture. In general, motion estimation is performed on a luma component, and a motion vector calculated based on the luma component is used for both the luma component and the chroma component. Motion information including information on the reference picture and information on the motion vector used to predict the current block is encoded by the encoder  150  and transmitted to the video decoding apparatus. 
     The subtractor  130  generates a residual block by subtracting, from the current block, the prediction block generated by the intra predictor  122  or the inter predictor  124 . 
     The transformer  140  transforms the residual signal in the residual block having pixel values in the spatial domain into transform coefficients in the frequency domain. The transformer  140  may transform the residual signals in the residual block by using the residual block in full size as a transform unit, or separate the residual block into two subblocks, which are a transform region and a non-transform region, and use only the transform-region subblock as a transform unit to transform the residual signals. Here, the transform-region subblock may be one of two rectangular blocks having a size ratio of 1:1 based on the horizontal axis (or vertical axis). In this case, the flag (cu_sbt_flag) indicating that only the subblock is transformed, directional or vertical/horizontal information (cu_sbt_horizontal_flag), and/or position information (cu_sbt_pos_flag) are encoded by the encoder  150  and signaled to the video decoding apparatus. Additionally, the size of the transform-region subblock may have a size ratio of 1:3 based on the horizontal axis (or vertical axis), wherein the flag (cu_sbt_quad_flag) identifying that partition is additionally encoded by the encoder  150  and signaled to the video decoding apparatus. 
     The quantizer  145  quantizes the transform coefficients outputted from the transformer  140  and outputs the quantized transform coefficients to the encoder  150 . 
     The encoder  150  uses various encoding methods such as Context-based Adaptive Binary Arithmetic Code (CABAC) and the like for encoding the quantized transform coefficients to generate a bitstream. The encoder  150  encodes information on block partition, such as CTU size, CU split flag, QT split flag, MTT split type, and MTT split direction related to the block partition for allowing the video decoding apparatus to split the block in the same way as the video encoding apparatus does. 
     Additionally, the encoder  150  encodes information on a prediction type indicating whether the current block is encoded by intra prediction or inter prediction and encodes, depending on the prediction type, intra prediction information (i.e., information on intra prediction mode) or inter prediction information (i.e., information on reference pictures and motion vectors). 
     The inverse quantizer  160  inverse quantizes the quantized transform coefficients outputted from the quantizer  145  to generate transform coefficients. The inverse transformer  165  transforms the transform coefficients outputted from the inverse quantizer  160  from the frequency domain to the spatial domain to reconstruct the residual block. 
     The adder  170  adds up the reconstructed residual block and the prediction block generated by the prediction unit  120  to reconstruct the current block. Pixels in the reconstructed current block are used as reference pixels when intra-predicting the next block. 
     The filter unit  180  performs filtering on the reconstructed pixels to reduce blocking artifacts, ringing artifacts, blurring artifacts, etc. generated due to block-based prediction and transform/quantization. The filter unit  180  may include a deblocking filter  182  and a sample adaptive offset (SAO) filter  184 . 
     The deblocking filter  180  filters the boundary between the reconstructed blocks to remove a blocking artifact caused by block-by-block encoding/decoding, and the SAO filter  184  performs additional filtering on the deblocking filtered video. The SAO filter  184  is a filter used to compensate for a difference between a reconstructed pixel and an original pixel caused by lossy coding. 
     The reconstructed block is filtered through the deblocking filter  182  and the SAO filter  184  and stored in the memory  190 . When all blocks in one picture are reconstructed, the reconstructed picture may be used as a reference picture for inter-prediction of blocks in the next picture to be encoded. 
       FIG. 4  is a functional block diagram illustrating a video decoding apparatus capable of implementing the techniques of the present disclosure. Hereinafter, the video decoding apparatus and sub-components of the apparatus will be described referring to  FIG. 4 . 
     The video decoding apparatus may be configured including a decoder  410 , an inverse quantizer  420 , an inverse transformer  430 , a prediction unit  440 , an adder  450 , a filter unit  460 , and a memory  470 . 
     As with the video encoding apparatus of  FIG. 1 , the respective components of the video decoding apparatus may be implemented as hardware or software, or hardware and software combined. Additionally, the function of each component may be implemented by software and the function by software for each component may be implemented to be executed by a microprocessor. 
     The decoder  410  decodes the bitstream generated by the video encoding apparatus and extracts information on block partition to determine the current block to be decoded, and extracts prediction information required to reconstruct the current block and information on residual signal, etc. 
     The decoder  410  extracts information on the CTU size from a sequence parameter set (SPS) or a picture parameter set (PPS), determines the size of the CTU, and splits the picture into CTUs of the determined size. Then, the decoder  410  determines the CTU as the highest layer, i.e., the root node of the tree structure, and extracts the split information on the CTU, and thereby splits the CTU by using the tree structure. 
     For example, when splitting the CTU by using the QTBTTT structure, a first flag (QT_split_flag) related to QT splitting is first extracted and each node is split into four nodes of a lower layer. For the node corresponding to the leaf node of QT, the decoder  410  extracts the second flag (MTT_split_flag) related to the partition of MTT and information of the split direction (vertical/horizontal) and/or split type (binary/ternary) to split that leaf node into an MTT structure. This allows the respective nodes below the leaf node of QT to be recursively split into a BT or TT structure. 
     As another example, when splitting the CTU by using the QTBTTT structure, the decoder  410  first extracts a CU split flag (split_cu_flag) indicating whether a CU is split, and upon splitting the relevant block, it extracts QT split flag (split_qt_flag). When the partition type is MTT, not QT, the decoder  410  further extracts the flag (mtt_split_cu_vertical_flag) indicating the MTT split direction (vertical or horizontal) and/or the flag (mtt_split_cu_binary_flag) indicating the MTT split type (binary or ternary). In the splitting process, each node may have zero or more recursive QT splits followed by zero or more recursive MTT splits. For example, the CTU may immediately enter MTT split, or conversely, have multiple QT splits alone. 
     As yet another example, when splitting the CTU by using the QTBT structure, the decoder  410  extracts a first flag (QT_split_flag) related to QT splitting to split each node into four nodes of a lower layer. And, for a node corresponding to a leaf node of QT, the decoder  410  extracts a split flag (split_flag) indicating whether that node is or is not further split into BT and split direction information. 
     Meanwhile, when the decoder  410  determines the current block to be decoded through the tree-structure splitting, it extracts information on a prediction type indicating whether the current block was intra-predicted or inter-predicted. When the prediction type information indicates intra prediction, the decoder  410  extracts a syntax element for intra prediction information (intra prediction mode) of the current block. When the prediction type information indicates inter prediction, the decoder  410  extracts a syntax element for the inter prediction information, that is, information indicating a motion vector and a reference picture referenced by the motion vector. 
     Meanwhile, the decoder  410  extracts information on the quantized transform coefficients of the current block as information on the residual signal. 
     The inverse quantizer  420  inverse quantizes the quantized transform coefficients. The inverse transformer  430  inverse transforms the inverse quantized transform coefficients from the frequency domain to the spatial domain to reconstruct the residual signals and thereby generates a reconstructed residual block of the current block. 
     Additionally, when the inverse transformer  430  inversely transforms only a partial region (subblock) of the transform block, it extracts a flag (cu_sbt_flag) indicating that only the subblock of the transform block has been transformed, the subblock&#39;s directional (vertical/horizontal) information (cu_sbt_horizontal_flag), and/or subblock&#39;s position information (cu_sbt_pos_flag), and inversely transforms the transform coefficients of that subblock from the frequency domain to the spatial domain to reconstruct the residual signals. At the same time, the inverse transformer  430  fills the remaining region of the transform block left untransformed inversely with the “0” value for the residual signals and thereby generates the final residual block for the current block. 
     The prediction unit  440  may include an intra predictor  442  and an inter predictor  444 . The intra predictor  442  is activated when the prediction type of the current block is intra prediction, and the inter predictor  444  is activated when the prediction type of the current block is inter prediction. 
     The intra predictor  442  determines, among a plurality of intra prediction modes, the intra prediction mode of the current block from the syntax element for the intra prediction mode extracted by the decoder  410 , and according to the determined intra prediction mode, it predicts the current block by using neighboring reference pixels of the current block. 
     The inter predictor  444  utilizes the syntax element for the inter prediction information extracted by the decoder  410  to determine a motion vector of the current block and a reference picture referenced by the motion vector, and it predicts the current block by using the motion vector and the reference picture as determined. 
     The adder  450  adds up the residual block outputted from the inverse transformer and the prediction block outputted from the inter predictor or the intra predictor to reconstruct the current block. Pixels in the reconstructed current block are used as reference pixels when intra-predicting a coming block to be decoded. 
     The filter unit  460  may include a deblocking filter  462  and an SAO filter  464 . The deblocking filter  462  deblocks and filters the boundary between reconstructed blocks to remove a blocking artifact caused by block-by-block decoding. The SAO filter  464  performs additional filtering on the reconstructed block after deblocking filtering to compensate for the difference between the reconstructed pixel and the original pixel caused by lossy coding. The reconstructed block is filtered through the deblocking filter  462  and the SAO filter  464  and stored in the memory  470 . When all blocks in one picture are reconstructed, the reconstructed picture is used as a reference picture for inter-prediction of blocks in a picture to be encoded thereafter. 
     The intra prediction modes for intra prediction are composed of directional modes with directionality and non-directional modes without directionality. In the conventional method, as shown in  FIG. 5 , intra prediction is performed using a total of 35 intra prediction modes including 33 angular modes with directionality, i.e., directional modes and two non-angular modes without directionality, i.e., non-directional modes of a planar mode and a direct current (DC) mode. 
     The conventional method determines an intra prediction mode used for intra prediction of the current block (or an intra prediction mode of the current block) by selecting the three most probable modes (MPMs) with a high probability of matching the intra prediction mode of the current block among the  35  modes and configuring a list by including the selected prediction modes. For the list configuration, the conventional method uses the prediction modes of the neighboring blocks located in the vicinity of the current block and the prediction modes that are statistically most frequently used. 
     A list consisting of selected prediction modes may be referred to as an MPM list, and unselected prediction modes, i.e.,  32  prediction modes may be referred to as non-MPM modes or remaining modes. Additionally, a list consisting of non-MPM modes may be referred to as a non-MPM list. 
     The intra prediction mode of the current block may be determined by prediction mode information items signaled from the video encoding apparatus to the video decoding apparatus or video decoding apparatus. Conventionally, a 1-bit flag is signaled first indicating whether the intra prediction mode of the current block is included in the MPM list. If the intra prediction mode of the current block is included in the MPM list, information (MPM index) is signaled to indicate any one of the prediction modes included in the MPM list. If the intra prediction mode of the current block is not included in the MPM list, an index indicating any one of the non-MPM modes is signaled. 
     Meanwhile, intra prediction of the current block may utilize various techniques such as multiple reference lines (MRL), intra sub-partition (ISP), regular intra prediction, etc. 
     As illustrated in  FIG. 6 , the MRL technique or MRL mode refers to a technique for performing intra prediction of a current block (CU) by using multiple reference lines. When the MRL mode is applied, the multiple reference lines are assigned indexes of “0” to “3” according to the degree of proximity to the current block. One reference line is selected from among reference lines of “0,” “1,” and “2,” and samples in the selected reference line are used as reference samples when performing the intra prediction. Information (index) on the selected reference line is signaled from the video encoding apparatus to the video decoding apparatus. The MRL mode is applicable only to the MPM modes and applies only to the directional modes and the DC mode. 
     As illustrated in  FIG. 7 , the ISP technology or ISP mode refers to a technology for dividing a current block into multiple sub-blocks and performing prediction, transform, quantization, etc. for each sub-block. When the ISP mode is applied, the current block is divided according to its size into two sub-blocks ( FIG. 7  at (a)) or into four sub-blocks ( FIG. 7  at (b)). Since the minimum size of the current block to which the ISP mode can be applied is 4×8 or 8×4, when the current block is sized 4×4, the ISP technology is rendered to be off. When the current block is sized 4×8 or 8×4, the current block is bisected, and when the current block exceeds 4×8 or 8×4, the current block is quadrisected. 
     A flag indicating whether the ISP mode is applied or not (on/off) is first signaled from the video encoding apparatus to the video decoding apparatus, and when the ISP mode is on, information is additionally signaled on the direction in which the current block is split. The ISP mode is turned on only when the reference line index in the MRL mode is “0,” and it is applicable to both MPM modes and non-MPM modes and applies to both directional modes and non-directional modes. 
     The regular intra prediction (Regular) is a commonly used intra prediction technique or mode, and it refers to a case in which the reference line index in the MRL mode is “0” and a case in which the ISP mode is off. The Regular mode is applicable to both MPM modes and non-MPM modes and applies to both directional modes and non-directional modes. 
     Table 1 shows syntaxes and structures of the MRL mode, ISP mode, and Regular mode. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
               
                   
               
             
            
               
                 if( treeType = = SINGLE_TREE || treeType = = 
                   
               
               
                 DUAL_TREE_LUMA ) { 
               
               
                  if( ( y0 % CtbSizeY ) &gt; 0 ) 
               
               
                   intra_luma_ref_idx[ x0 ][ y0 ] 
                 ae(v) 
               
               
                  if(intra_luma_ref_idx[ x0 ][ y0 ] = = 0 &amp;&amp; 
               
               
                   ( cbWidth &lt;= MaxTbSizeY || cbHeight &lt;= MaxTbSizeY) &amp;&amp; 
               
               
                   ( cbWidth * cbHeight &gt; MinTbSizeY * MinTbSizeY )) 
               
               
                   intra_subpartitions_mode_flag[ x0 ][ y0 ] 
                 ae(v) 
               
               
                  if( intra_subpartitions_mode_flag[ x0 ][ y0 ] = = 1 &amp;&amp; 
               
               
                   cbWidth &lt;= MaxTbSizeY &amp;&amp; cbHeight &lt;= MaxTbSizeY) 
               
               
                   intra_subpartitions_split_flag[ x0 ][ y0 ] 
                 ae(v) 
               
               
                  if( intra_luma_ref_idx[ x0 ][ y0 ] = = 0) 
               
               
                   intra_luma_mpm_flag[ x0 ][ y0 ] 
                 ae(v) 
               
               
                  if( intra_luma_mpm_flag[ x0 ][ y0 ] ) 
               
               
                   intra_luma_mpm_idx[ x0 ][ y0 ] 
                 ae(v) 
               
               
                  else 
               
               
                   intra_luma_mpm_remainder[ x0 ][ y0 ] 
                 ae(v) 
               
               
                 } 
               
               
                 if( treeType = = SINGLE_TREE || treeType = = 
               
               
                 DUAL_TREE_CHROMA ) 
               
               
                  intra_chroma_pred_mode[ x0 ][ y0 ] 
                 ae(v) 
               
               
                   
               
            
           
         
       
     
     Syntaxes expressed in Table 1 are described below. 
     intra_luma_ref_idx: A syntax that indicates an index of a reference line. intra_luma_ref_idx≠0 means a reference line for MRL mode, and intra_luma_ref_idx=0 means a reference line for ISP mode or Regular mode. MRL mode is not executed in the first row of CTU. 
     intra_subpartitions_mode_flag: A syntax that indicates whether ISP mode is applied or not (on/off). 
     intra_subpartitions_split_flag: Information indicating a direction in which the current block is split when the ISP mode is on. 
     intra_luma_mpm_flag: Information indicating whether the intra prediction mode of the current block is included in the MPM list. When the MRL mode is on, the condition that intra_luma_mpm_flag=1 may be set or derived. Here, the occasion when the MRL mode is on means that intra_luma_ref_idx≠0. 
     intra_luma_mpm_idx: Information indicating one of the prediction modes included in the MPM list, which is the same prediction mode as the intra prediction mode of the current block. intra_luma_mpm_idx is signaled when intra_luma_mpm_flag=1. 
     intra_luma_mpm_remainder: Information indicating any one of non-MPM modes (the same prediction mode as the intra prediction mode of the current block). When intra_luma_mpm_flag=0, intra_luma_mpm_remainder is signaled. 
     A comparison of the three techniques described above is shown in Table 2. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Ref idx 
                 MPM 
                 Mode 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 MRL 
                 ! = 0 
                 MPM 
                 Angular + DC 
               
               
                 ISP 
                 =0 
                 MPM + non-MPM 
                 Angular + planar + DC 
               
               
                 Regular 
                 =0 
                 MPM + non-MPM 
                 Angular + planar + DC 
               
               
                   
               
            
           
         
       
     
     As can be seen from Table 2, in the MRL mode, the index of the reference line is non-zero, only MPM modes are supported, and only directional modes and DC mode are supported. In the ISP mode, the index of the reference line is zero, both MPM modes and non-MPM modes are supported, and both directional modes and non-directional modes are supported. In the Regular mode, the index of the reference line is zero, both MPM modes and non-MPM modes are supported, and both directional modes and non-directional modes are supported. 
     The present disclosure intends to provide a method of configuring an MPM list that can be applied to all of the MRL mode, ISP mode, and Regular mode. Specifically, the present disclosure provides a method of configuring the MPM list by excluding the planar mode and/or DC mode (one or more of the non-directional modes) and signaling separate information for the non-directional modes. 
     As shown in  FIG. 8 , a video encoding/decoding apparatus may include an acquisition unit  810 , an determination unit  820 , a derivation unit  830 , a candidate deriving unit  840 , and a list configuration unit  850 . The determination unit  820  may include a first determination unit  822  and a second determination unit  824 . 
     The acquisition unit  810  may obtain MPM information based on reference line information (in Step S 910 ). Here, the reference line information is one indicating a reference line used for intra prediction of the current block among a plurality of reference lines. The reference line information may be implemented as intra_luma_ref_idx. When no reference line information is present, a reference line information may be set or derived as indicating a reference line having an index of 0 (intra_luma_ref_idx=0). 
     The MPM information is one indicating whether the current block&#39;s intra prediction mode (current intra mode) is included in the MPM list. The MPM information may be implemented as intra_luma_mpm_flag. When no MPM information is present (when multiple reference lines or MRL is on), an MPM information may be set or derived as indicating that the current block&#39;s intra prediction mode is included in the MPM list (intra_luma_mpm_flag=1). 
     The determination unit  820  may determine planar information based on the reference line information and the MPM information (S 920 ). The planar information is one indicating whether the current block&#39;s intra prediction mode is the planar mode, and it may be implemented as intra_luma_planar_mode_flag or intra_luma_not_planar_flag. When planar information is absent, the planar information may be inferred as indicating that the current block&#39;s intra prediction mode is not the planar mode (intra_luma_planar_mode_flag=0, intra_luma_not_planar_flag=1). 
     When the planar information is implemented as intra_luma_planar_mode_flag, the condition that intra_luma_planar_mode_flag=1 may indicate that the current block&#39;s intra prediction mode is the planar mode, and the condition that intra_luma_planar_mode_flag=0 may indicate that the current block&#39;s intra prediction mode is not the planar mode. When planar information is implemented as intra_luma_not_planar_flag, the condition that intra_luma_not_planar_flag=0 may indicate that the current block&#39;s intra prediction mode is the planar mode, and the condition that intra_luma_not_planar_flag=1 may indicate that the current block&#39;s intra prediction mode is not the planar mode. 
     The derivation unit  830  may derive the intra prediction mode of the current block by using the planar information (S 930 ). For example, when the planar information indicates the planar mode, the planar mode may be set as the intra prediction mode of the current block. When the planar information does not indicate the planar mode, the intra prediction mode of the current block may be determined by what is indicated by the MPM information. Where the MPM information indicates that the intra prediction mode of the current block is included in the MPM list, the candidate intra prediction mode, which is indicated by the MPM index among the intra prediction modes (i.e., candidate intra prediction modes included in the MPM list), may be derived as the intra prediction mode of the current block. On the contrary, when the MPM information indicates that the intra prediction mode of the current block is not included in the MPM list, the non-MPM mode indicated by the non-MPM information (intra_luma_mpm_remainder) may be derived for the intra prediction mode of the current block. 
     Hereinafter, the respective steps described above will be described separately for each embodiment. 
     Embodiment 1 
     Embodiment 1 is for the process of obtaining MPM information. 
     First, the video encoding apparatus may signal reference line information (intra_luma_ref_idx) by including the same in a bitstream to the video decoding apparatus. The video decoding apparatus may decode the reference line information from the bitstream (S 1020 ). 
     MRL Mode 
     When the reference line information indicates a reference line having an index other than “0” (intra_luma_ref_idx≠0), the MRL mode may be applied to intra prediction of the current block. Since the MRL mode supports the MPM modes exclusively, the video encoding apparatus may not signal the MPM information (intra_luma_mpm_flag), and the MPM information may be derived or set to “1” (intra_luma_mpm_flag=1). 
     When the decoded reference line information indicates a reference line having an index other than “0” (intra_luma_ref_idx≠0) in Step S 1040 , since the MPM information is not signaled, the video decoding apparatus may obtain the MPM information by inducing or setting the same to “1” (intra_luma_mpm_flag=1) in Step S 1080 . 
     ISP Mode 
     When the reference line information indicates a reference line having an index of “0” (intra_luma_ref_idx=0), the ISP mode or the Regular mode may be applied to intra prediction of the current block. To distinguish between the ISP mode and the Regular mode, the video encoding apparatus may signal, to the video decoding apparatus, information (intra_subpartitions_mode_flag) indicating whether to apply the ISP mode by including the same in the bitstream. 
     Additionally, when the ISP mode is applied (intra_subpartitions_mode_flag=1), the video encoding apparatus may signal information (intra_subpartitions_split_flag) indicating the splitting direction of the current block by further including the same in the bitstream. Furthermore, since the ISP mode supports both MPM modes and non-MPM modes, the video encoding apparatus may signal MPM information. 
     When the decoded reference line information indicates a reference line having an index of “0” (intra_luma_ref_idx=0) in Step S 1040 , the video decoding apparatus may distinguish the ISP mode from the Regular mode by further decoding information indicating whether the ISP mode is applied (S 1050 ). When this information indicates that the ISP mode is applied (intra_subpartitions_mode_flag=1) in Step S 1060 , the video decoding apparatus may further decode information indicating the splitting direction (intra subpartition split_flag) in Step S 1070 . Furthermore, the video decoding apparatus may obtain the MPM information by decoding the same from the bitstream (S 1090 ). 
     Regular Mode 
     When the ISP mode is not applied (intra_subpartitions_mode_flag=0), the Regular mode may be used for intra prediction of the current block. Since the Regular mode supports both MPM modes and non-MPM modes, the video encoding apparatus may signal MPM information to the video decoding apparatus. 
     When the information indicating whether or not the ISP mode is applied indicates that the ISP mode is not applied (intra_subpartitions_mode_flag=0) in Step S 1060 , the Regular mode may be used for intra prediction of the current block. In this case, the video decoding apparatus may decode and obtain the MPM information from the bitstream (S 1090 ). 
     Table 3 shows the syntax structure for Embodiment 1 described above. 
     
       
         
           
               
               
             
               
                 TABLE 3 
               
               
                   
               
             
            
               
                 if( treeType = = SINGLE_TREE || treeType = = 
                   
               
               
                 DUAL_TREE_LUMA ) { 
               
               
                  if( ( y0 % CtbSizeY ) &gt; 0 ) 
               
               
                   intra_luma_ref_idx[ x0 ][ y0 ] 
                 ae(v) 
               
               
                  if (intra_luma_ref_idx[ x0 ][ y0 ] = = 0 &amp;&amp; 
               
               
                   ( cbWidth &lt;= MaxTbSizeY || cbHeight &lt;= MaxTbSizeY) &amp;&amp; 
               
               
                   ( cbWidth * cbHeight &gt; MinTbSizeY * MinTbSizeY )) 
               
               
                   intra_subpartitions_mode_flag[ x0 ][ y0 ] 
                 ae(v) 
               
               
                  if( intra_subpartitions_mode_flag[ x0 ][ y0 ] = = 1 &amp;&amp; 
               
               
                   cbWidth &lt;= MaxTbSizeY &amp;&amp; cbHeight &lt;= MaxTbSizeY) 
               
               
                   intra_subpartitions_split_flag[ x0 ][ y0 ] 
                 ae(v) 
               
               
                  if( intra_luma_ref_idx[ x0 ][ y0 ] = = 0) 
               
               
                   intra_luma_mpm_flag[ x0 ][ y0 ] 
                 ae(v) 
               
               
                   
               
            
           
         
       
     
     Some embodiments implement the step of obtaining MPM information by using information indicating the activation/deactivation (enablement/disenablement, i.e., on/off) of the MRL mode (e.g., mrl_enabled_flag) or information indicating the activation/deactivation (on/off) of the ISP mode (e.g., isp_enabled_flag). 
     The information indicating the activation/deactivation of the MRL mode and the information indicating the activation/deactivation of the ISP mode may be defined in one or more positions of a sequence parameter set, a picture parameter set, a slice header, and a tile group header. 
     The video encoding apparatus may signal the reference line information when the MRL mode is enabled (mrl_enabled_flag=1), and it may not signal the reference line information when the MRL mode is not enabled (mrl_enabled_flag=0) but set or derive the reference line information as indicating a reference line having an index of “0” (intra_luma_ref_idx=0). The video decoding apparatus in Step S 1020  may decode the reference line information when the MRL mode is enabled (mrl_enabled_flag=1 in Step S 1010 ), and it may not decode the reference line information when the MRL mode is not enabled (mrl_enabled_flag=0 in Step S 1010 ), but it may infer the reference line information as indicating a reference line having an index of “0” (intra_luma_ref_idx=0). 
     The video encoding apparatus may signal intra_subpartitions_mode_flag and intra_subpartitions_split_flag when the ISP mode is enabled (isp_enabled_flag=1), and may not signal intra_subpartitions_mode_flag and intra_subpartitions_split_flag when the ISP mode is disabled (isp_enabled_flag=0). In response, when the ISP mode is enabled (isp_enabled_flag=1) while the reference line information indicates a reference line having an index of “0” (intra_luma_ref_idx=0) in Step S 1040 , the video decoding apparatus may decode intra_subpartitions_mode_flag and intra_subpartitions_split_flag in Steps S 1050  and S 1070 . When the ISP mode is disabled (isp_enabled_flag=0) or the reference line information indicates a reference line having an index other than “0” (intra_luma_ref_idx≠0) in Step S 1040 , the video decoding apparatus may not decode intra_subpartitions_mode_flag and intra_subpartitions_split_flag (S 1050 , S 1070 ). 
     Embodiment 2 
     Embodiment 2 concerns the step of determining the planar information and the step of deriving the current block&#39;s intra prediction mode based on the planar information. 
     In Embodiment 2 and Embodiment 3 to be described below, it is assumed that a total of 67 intra prediction modes are used to derive the intra prediction mode of the current block. The total of 67 intra prediction modes may include two non-directional modes (planar mode and DC mode) and 65 directional modes. 
     Embodiment 2 may be divided into two embodiments as follows according to the type of the non-directional mode excluded from the MPM list among the non-directional modes (planar mode and DC mode). 
     Embodiment 2-1 
     In embodiment 2-1, the planar mode alone may be excluded from the MPM list. In other words, in Embodiment 2-1, the MPM list may be configured by using the DC mode and the directional modes. 
     MRL Mode 
     MRL mode may support MPM modes, support directional modes and DC mode, and support reference lines having an index other than “0”. Accordingly, with the MRL mode applied, the MPM information may indicate the current intra mode (the intra prediction mode of the current block) as included in the MPM list (intra_luma_mpm_flag=1), and the reference line information may indicate a reference line having an index other than “0” (intra_luma_ref_idx≠0). 
     When MPM information indicates the current intra mode as included in the MPM list (intra_luma_mpm_flag=1) while the reference line information indicates a reference line having an index other than “0” (intra_luma_ref_idx≠0), the planar information may be infer as a value indicating that the current intra mode is not the planar mode (intra_luma_planar_mode_flag=0 or intra_luma_not_planar_mode_flag=1). 
     Additionally, to indicate any one of the candidate intra prediction modes included in the MPM list, the MPM index (intra_luma_mpm_idx) may be signaled to the video decoding apparatus. Here, the MPM index may be information indicating any one of the candidate intra prediction modes (MPM modes) included in the MPM list. 
     The video decoding apparatus may determine MPM information and reference line information (S 1110 , S 1120 ). With the MRL mode applied, the MPM information may indicate the current intra mode as included in the MPM list (intra_luma_mpm_flag=1), and the reference line information may indicate a reference line having an index other than “0” (intra_luma_ref_idx≠0). 
     Additionally, the planar information may not be signaled and may be inferred as a value not indicating the planar mode (intra_luma_planar_mode_flag=0) in Step S 1140 . 
     Since the planar information does not indicate the planar mode (intra_luma_planar_mode_flag=0 or intra_luma_not_planar_mode_flag=1) in Step S 1150 , the MPM index may be decoded from the bitstream (S 1160 ). Additionally, the candidate intra prediction mode in the MPM list, as indicated by the MPM index may be derived as the current block&#39;s intra prediction mode. 
     ISP Mode 
     The ISP mode may support both MPM modes and non-MPM modes, support both directional modes and non-directional modes, and support a reference line having an index of “0.” Accordingly, with the ISP mode applied, the MPM information may indicate that the current intra mode is included in the MPM list (intra_luma_mpm_flag=1) or indicate the current intra mode is not included in the MPM list (intra_luma_mpm_flag=0). The reference line information may indicate a reference line having an index of “0” (intra_luma_ref_idx=0). 
     When MPM information indicates that the current intra mode is included in the MPM list (intra_luma_mpm_flag=1) while the reference line information indicates a reference line having an index of “0” (intra_luma_ref_idx=0), the planar information may be signaled to the video decoding apparatus. 
     When the planar information indicates that the current intra mode is not the planar mode (intra_luma_planar_mode_flag=0 or intra_luma_not_planar_mode_flag=1), the MPM index may be signaled to the video decoding apparatus. On the contrary, when the planar information indicates that the current intra mode is the planar mode (intra_luma_planar_mode_flag #0 or intra_luma_not_planar_mode_flag=0), the MPM index may not be signaled to the video decoding apparatus. 
     The video decoding apparatus may determine MPM information and reference line information (S 1110 , S 1120 ). With the ISP mode applied, the MPM information may indicate that the current intra mode is included in the MPM list (intra_luma_mpm_flag=1) or indicate that the current intra mode is not included in the MPM list (intra_luma_mpm_flag=0). The reference line information may indicate a reference line having an index of “0” (intra_luma_ref_idx=0). 
     When intra_luma_mpm_flag=0 (S 1110 ), non-MPM information may be decoded from the bitstream (S 1170 ). When intra_luma_mpm_flag=1 (S 1110 ), since intra_luma_ref_idx=0 (S 1120 ), the planar information may be decoded from the bitstream and thereby the value thereof may be determined (S 1130 ). 
     When the planar information does not indicate the planar mode (intra_luma_planar_mode_flag=0 or intra_luma_not_planar_mode_flag=1, S 1150 ), the MPM index may be decoded from the bitstream (S 1160 ). 
     Additionally, the candidate intra prediction mode, as indicated by the MPM index, in the MPM list may be set as the intra prediction mode of the current block. On the contrary, when the planar information indicates the planar mode (intra_luma_planar_mode_flag=1 or intra_luma_not_planar_mode_flag=0, S 1150 ), the planar mode may be set as the intra prediction mode of the current block. 
     Regular Mode 
     The Regular mode may support MPM modes and non-MPM modes, support directional modes and non-directional modes, and support a reference line having an index of “0.” Therefore, with the Regular mode applied, the MPM information may indicate the current intra mode is included in the MPM list (intra_luma_mpm_flag=1) or indicate that the current intra mode is not included in the MPM list (intra_luma_mpm_flag≠1), and the reference line information may indicate a reference line having an index of “0” (intra_luma_ref_idx=0). 
     When the MPM information indicates that the current intra mode is not included in the MPM list (intra_luma_mpm_flag=0), non-MPM information (intra_luma_mpm_remainder) may be signaled to the video decoding apparatus. Here, the non-MPM information may be information indicating one of the candidate intra prediction modes (non-MPM modes) included in the non-MPM list. 
     On the contrary, when MPM information indicates that the current intra mode is included in the MPM list (intra_luma_mpm_flag=1) and the reference line information indicates a reference line having an index of “0” (intra_luma_ref_idx=0), the planar information may be signaled to the video decoding apparatus. 
     When the planar information indicates that the current intra mode is not the planar mode (intra_luma_planar_mode_flag=0 or intra_luma_not_planar_mode_flag=1), the MPM index may be signaled to the video decoding apparatus. On the contrary, when the planar information indicates that the current intra mode is the planar mode (intra_luma_planar_mode_flag≠0 or intra_luma_not_planar_mode_flag=0), the MPM index may not be signaled. 
     The video decoding apparatus may determine MPM information and reference line information (S 1110 , S 1120 ). When the MPM information indicates that the current intra mode is not included in the MPM list (intra_luma_mpm_flag=0), non-MPM information may be decoded from the bitstream (S 1170 ), and the candidate intra prediction mode in the non-MPM list, as indicated by the non-MPM information may be selected as the current block&#39;s intra prediction mode. 
     On the contrary, when MPM information indicates that the current intra mode is included in the MPM list (intra_luma_mpm_flag=1), reference line information may be determined (S 1120 ). When the reference line information indicates a reference line having an index of 0″ (intra_luma_ref_idx=0), planar information may be decoded from the bitstream (S 1130 ). 
     When the planar information does not indicate the planar mode (intra_luma_planar_mode_flag=0 or intra_luma_not_planar_mode_flag=1) in Step S 1150 , the MPM index may be decoded from the bitstream (S 1160 ), and the candidate intra prediction mode in the MPM list, as indicated by the MPM index may be derived for the intra prediction mode of the current block. On the contrary, when the planar information indicates the planar mode (intra_luma_planar_mode_flag=1 or intra_luma_not_planar_mode_flag=0) in Step S 1150 , the planar mode may be derived for the intra prediction mode of the current block. 
     Table 4 shows the syntax structure of Embodiment 2-1 described above. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
            
               
                   
                 if( intra_luma_mpm_flag[ x0 ][ y0 ] ) { 
                   
               
               
                   
                  if( intra_luma_ref_idx[ x0 ][ y0 ] = = 0 ) 
               
               
                   
                   intra_luma_planar_mode_flag[ x0 ][ y0 ] 
               
               
                   
                  if( intra_luma_planar_mode_flag[ x0 ][ y0 ] = = 0 ) 
               
               
                   
                   intra_luma_mpm_idx[ x0 ][ y0 ] 
                 ae(v) 
               
               
                   
                 } 
               
               
                   
                 else 
               
               
                   
                  intra_luma_mpm_remainder[ x0 ][ y0 ] 
                 ae(v) 
               
               
                   
                   
               
            
           
         
       
     
     Embodiment 2-2 
     In Embodiment 2-2, non-directional modes, i.e., the planar mode and DC mode may be excluded from the MPM list. In other words, Embodiment 2-2 may configure the MPM list by using only the directional modes. 
     Compared with Embodiment 2-1, Embodiment 2-2 may distinguish between the planar mode and the DC mode by further using non-directional information (e.g., intra_luma_non-angular_mode_flag) indicating whether the current block&#39;s intra prediction mode corresponds to the non-directional mode. The condition that Intra_luma_non-angular_mode_flag=1 may indicate that the current block&#39;s intra prediction mode corresponds to the non-directional mode (planar mode or DC mode), and the condition that intra_luma_non-angular_mode_flag=0 may indicate that the current block&#39;s intra prediction mode corresponds to the directional mode. When the non-directional mode is absent, the current block&#39;s intra prediction mode may be set or derived into a value indicating that the current block&#39;s intra prediction mode corresponds to the directional mode (intra_luma_non-angular_mode_flag=0). 
     The step of determining the non-directional information may be performed by the first determination unit  822 , and the step of determining the planar information may be performed by the second determination unit  824 . 
     MRL Mode 
     When the MPM information indicates the current intra mode is included in the MPM list (intra_luma_mpm_flag=1) while the reference line information indicates a reference line having an index other than “0” (intra_luma_ref_idx≠0), non-directional information (intra_luma_non-angular_mode_flag) may be set or derived into a value indicating that the current intra mode does not correspond to the non-directional mode (intra_luma_non-angular_mode_flag=0). 
     With the non-directional information set to indicate that the current intra mode does not correspond to the non-directional mode (intra_luma_non-angular_mode_flag=0), the MPM index may be signaled to the video decoding apparatus to indicate one of the candidate intra prediction modes included in the MPM list. 
     The video decoding apparatus may determine MPM information and reference line information (S 1210 , S 1220 ). When the MRL mode is applied, the MPM information may indicate the current intra mode is included in the MPM list (intra_luma_mpm_flag=1) in Step S 1210 , and the reference line information may indicate a reference line having an index other than “0” (intra_luma_ref_idx≠0) in Step S 1220 . Accordingly, the non-directional information may be inferred as a value indicating that the current intra mode does not correspond to the non-directional mode (intra_luma_non-angular_mode_flag=0) in Step S 1240 . 
     Additionally, with the non-directional information indicating that the current intra mode does not correspond to the non-directional mode (intra_luma_non-angular_mode_flag=0, S 1280 ), the MPM index may be decoded from the bitstream (S 1290 ). Furthermore, the candidate intra prediction mode in the MPM list as indicated by the MPM index may be derived as the intra prediction mode of the current block. 
     ISP Mode 
     When MPM information indicates that the current intra mode is included in the MPM list (intra_luma_mpm_flag=1) while the reference line information indicates a reference line having an index of “0” (intra_luma_ref_idx=0), non-directional information (intra_luma_non-angular_mode_flag) may be signaled to the video decoding apparatus. 
     With the ISP mode applied (intra_subpartitions_mode_flag=1), the planar information may be inferred as a value that indicates the current block&#39;s intra-prediction mode is the planar mode (intra_luma_planar_mode_flag=1 or intra_luma_not_planar_mode_flag=0). Additionally, with the planar information indicating that the current intra mode is the planar mode (intra_luma_planar_mode_flag=1 or intra_luma_not_planar_mode_flag=0), the MPM index may not be signaled to the video decoding apparatus. 
     The video decoding apparatus may determine MPM information and reference line information (S 1210 , S 1220 ). When the ISP mode is applied, the MPM information may indicate the current intra mode as included in the MPM list (intra_luma_mpm_flag=1) in Step S 1210 , and the reference line information may indicate a reference line having an index of “0” (intra_luma_ref_idx=0) in Step S 1220 . Accordingly, non-directional information may be decoded from the bitstream (S 1230 ). 
     Since the ISP mode is applied and intra_subpartitions_mode_flag=1 (S 1250 ), the planar information may be inferred as a value indicating that the current block&#39;s intra-prediction mode is the planar mode (intra_luma_planar_mode_flag=1 or intra_luma_not_planar_mode_flag=0, S 1270 ). 
     When the non-directional information indicates that the current intra mode corresponds to the non-directional mode (intra_luma_non-angular_mode_flag=0, S 1250 ), the planar mode may be derived as the intra prediction mode of the current block. On the other hand, when the non-directional information indicates that the current intra mode does not correspond to the non-directional mode (intra_luma_non-angular_mode_flag=1) in Step S 1280 , the MPM index is decoded from the bitstream (S 1290 ), and a candidate intra prediction mode in the MPM list as indicated by the MPM index may be derived as the intra prediction mode of the current block. 
     Regular Mode 
     When MPM information indicates that the current intra mode is not included in the MPM list (intra_luma_mpm_flag=0), non-MPM information (intra_luma_mpm_remainder) may be signaled to the video decoding apparatus. On the contrary, when MPM information indicates that the current intra mode is included in the MPM list (intra_luma_mpm_flag=1) while the reference line information indicates a reference line having an index of “0” (intra_luma_ref_idx=0), non-directional information (intra_luma_non-angular_mode_flag) may be signaled to the video decoding apparatus. 
     Additionally, when the ISP mode is not applied (intra_subpartitions_mode_flag=0) and the non-directional information indicates that the current intra mode corresponds to the non-directional mode (intra_luma_non-angular_mode_flag=1), planar information may be signaled to the video decoding apparatus. On the contrary, when the ISP mode is applied (intra_subpartitions_mode_flag=1) or the non-directional information indicates that the current intra mode does not correspond to the non-directional mode (intra_luma_non-angular_mode_flag=0), the planar information may be inferred as a value indicating that the current intra mode is the planar mode (intra_luma_planar_mode_flag=1 or intra_luma_not_planar_mode_flag=0). Since the Regular mode is applied, this step may assume that intra_subpartitions_mode_flag=0. 
     When the non-directional information indicates that the current intra mode corresponds to the non-directional mode (intra_luma_non-angular_mode_flag=1), the MPM index (intra_luma_mpm_idx) may be signaled to the video decoding apparatus. On the contrary, when the non-directional information indicates that the current intra mode does not correspond to the non-directional mode (intra_luma_non-angular_mode_flag=0), the MPM index (intra_luma_mpm_idx) may not be signaled. 
     The video decoding apparatus may determine MPM information and reference line information (S 1210 , S 1220 ). When the MPM information indicates that the current intra mode is not included in the MPM list (intra_luma_mpm_flag=0), non-MPM information (intra_luma_mpm_remainder) may be decoded from the bitstream (S 1295 ), and a candidate intra prediction mode in the non-MPM list as indicated by the syntax of intra_luma_mpm_remainder may be derived as the intra prediction mode of the current block. 
     On the contrary, when MPM information indicates that the current intra mode is included in the MPM list (intra_luma_mpm_flag=1, S 1210 ), reference line information may be determined (S 1220 ). When the reference line information indicates a reference line having an index of “0” (intra_luma_ref_idx=0, S 1220 ), the non-directional information may be decoded from the bitstream (S 1230 ) and thereby the value thereof may be determined. 
     When the ISP mode is not applied (intra_subpartitions_mode_flag=0) and the non-directional information indicates that the current intra mode corresponds to the non-directional mode (intra_luma_non-angular_mode_flag=1) (S 1250 ), planar information may be decoded from the bitstream. On the contrary, when the ISP mode is applied (intra_subpartitions_mode_flag=1) or when the non-directional information indicates that the current intra mode does not corresponds to the non-directional mode (intra_luma_non-angular_mode_flag=0) (S 1250 ), the planar information may be inferred as a value indicating that the current intra mode is the planar mode (intra_luma_planar_mode_flag=1 or intra_luma_not_planar_mode_flag=0) in Step S 1270 . Since the Regular mode is applied, intra_subpartitions_mode_flag=0 may be assumed at S 1270 . 
     When the non-directional information indicates that the current intra mode does not correspond to the non-directional mode (intra_luma_non-angular_mode_flag=0) in Step S 1280 , the MPM index (intra_luma_mpm_idx) may be decoded from the bitstream (S 1290 ). In this case, the candidate intra prediction mode in the MPM list, as indicated by the decoded MPM index may be derived as the intra prediction mode of the current block. 
     On the contrary, when the non-directional information indicates that the current intra mode corresponds to the non-directional mode (intra_luma_non-angular_mode_flag=1) in Step S 1280 , the current block&#39;s intra prediction mode may be derived by what is indicated by the planar information. Specifically, when the planar information indicates that the current block&#39;s intra-prediction mode is the planar mode (intra_luma_planar_mode_flag=1 or intra_luma_not_planar_mode_flag=0), the planar mode is derived as the intra prediction mode of the current block, and when the planar information indicates that the current block&#39;s intra-prediction mode is not the planar mode (intra_luma_planar_mode_flag)=0 or intra_luma_not_planar_mode_flag=1), the DC mode may be derived as the intra prediction mode of the current block. 
     Table 5 shows the syntax structure for the operation of the video encoding/decoding apparatus described above. 
     
       
         
           
               
               
             
               
                 TABLE 5 
               
               
                   
               
             
            
               
                 if( intra_luma_mpm_flag[ x0 ][ y0 ] ) { 
                   
               
               
                  if( intra_luma_ref_idx[ x0 ][ y0 ] = = 0 ) 
               
               
                   intra_luma_non-angular_mode_flag[ x0 ][ y0 ] 
               
               
                  if( intra_luma_ref_idx[ x0 ][ y0 ] = = 0 &amp;&amp; 
               
               
                   intra_subpartitions_mode_flag[ x0 ][ y0 ] == 0 &amp;&amp; 
               
               
                   intra_luma_non-angular_mode_flag[ x0 ][ y0 ] == 1 ) 
               
               
                   intra_luma_planar_mode_flag[ x0 ][ y0 ] 
               
               
                  if( intra_luma_non-angular_mode_flag[ x0 ][ y0 ] = = 0 ) 
               
               
                   intra_luma_mpm_idx[ x0 ][ y0 ] 
                 ae(v) 
               
               
                 } 
               
               
                 else 
               
               
                  intra_luma_mpm_remainder[ x0 ][ y0 ] 
                 ae(v) 
               
               
                   
               
            
           
         
       
     
     Embodiment 3 
     Embodiment 3 provides various embodiments for configuring the MPM list. 
     Neighboring blocks of the current block may be used as blocks for configuring the MPM list. As shown in  FIG. 14 , the third embodiment may configure an MPM list by using a left block L located leftward of the current block CU and an upper block A located upward of the current block CU. Hereinafter, the prediction mode of left block L is referred to as a left prediction mode (Left), and the prediction mode of upper block A is referred to as an upper prediction mode (Above). 
     The video encoding/decoding apparatus (candidate deriving unit) may obtain a left prediction mode (Left) from left block L and obtain an upper prediction mode (Above) from upper block A (S 1310 ). In this process, a determination may be made on the availability of the left prediction mode (Left) and the upper prediction mode (Above), and as a result, a step may be performed for setting or modifying an unavailable prediction mode to the planar mode. 
     The video encoding/decoding apparatus (candidate deriving unit) may apply various criteria to the left prediction mode (Left) and the upper prediction mode (Above) to derive candidate intra prediction modes (MPM modes) to be included in the MPM list (S 1320 ). 
     The various criteria may include whether at least one of the left prediction mode (Left) and the upper prediction mode (Above) corresponds to the directional mode, and whether the left prediction mode (Left) and the upper prediction mode (Above) are identical to each other. The various criteria may further include determining whether the left prediction mode (Left) and the upper prediction mode (Above) are different directional modes, and if so, determining whether the difference between the left prediction mode (Left) and the upper prediction mode (Above) is within a preset range. 
     The video encoding/decoding apparatus (list configuration unit) may configure the MPM list by including the candidate intra prediction modes that are derived (S 1330 ). The MPM list may include one or more directional modes. The MPM list may further include a DC mode that is a non-directional mode. For example, the MPM list may consist of only 4 or 5 directional modes. Alternatively, the MPM list may be composed of a total of 4 or 5 intra prediction modes inclusive of a DC mode and directional modes. 
     When the MPM list is configured by including the DC mode, the DC mode may be positioned at the end of the MPM list. In other words, among the prediction modes included in the MPM list, the largest index may be allocated to the DC mode (located at the end of the MPM list). Conversely, the smallest index may be allocated to the DC mode (at the beginning of the MPM list). 
     For example, when the MPM list is configured as {3, DC, 12, 4, 45}, to represent the mode No. “12” for the intra prediction mode of the current block, index “2” corresponding to the mode No. “12” may be expressed as “001” (truncated unary method). On the other hand, when the MPM list is configured as {3, 12, 4, 45, DC}, to represent the mode No. “12” for the intra prediction mode of the current block, index “1” corresponding to the mode No. “12” may be expressed as “01.” 
     Embodiment 3 may be divided into the following specific embodiments according to criteria applied to the left prediction mode (Left) and the upper prediction mode (Above). 
     Embodiment 3-1 
     When the left prediction mode (Left) and the upper prediction mode (Above) are obtained (S 1510 ), it can be determined whether both the left prediction mode (Left) and the upper prediction mode (Above) correspond to the non-directional mode (S 1520 ). 
     When both the left prediction mode (Left) and the upper prediction mode (Above) are non-directional modes, the MPM list may be composed of a total of four directional modes such as {V, H, V−4, V+4}, or it may be composed of four directional modes and a DC mode such as {V, H, V−4, V+4, DC} or {DC, V, H, V−4, V+4} (S 1530 ). Here, V denotes a vertical mode (angular_50, directional mode No. 50), H a horizontal mode (angular_18, directional mode No. 18), V−4 a directional mode No. 46, and V+4 a directional mode No. 54. 
     When an addition or subtraction operator exists in a value of a prediction mode to be added to the MPM list, the MPM list may be provided with a result value obtained by applying the remainder operation as in Equation 1 below. 
     
       
         
           
             
               
                 
                   
                     
                       A 
                       ′ 
                     
                     = 
                     
                       
                         
                           A 
                           - 
                           1 
                         
                         → 
                         
                           A 
                           ′ 
                         
                       
                       = 
                       
                         2 
                         + 
                         
                           ( 
                           
                             
                               ( 
                               
                                 A 
                                 + 
                                 61 
                               
                               ) 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             % 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             64 
                           
                           ) 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       A 
                       ′ 
                     
                     = 
                     
                       
                         
                           A 
                           + 
                           1 
                         
                         → 
                         
                           A 
                           ′ 
                         
                       
                       = 
                       
                         2 
                         + 
                         
                           ( 
                           
                             
                               ( 
                               
                                 A 
                                 - 
                                 1 
                               
                               ) 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             % 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             64 
                           
                           ) 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       A 
                       ′ 
                     
                     = 
                     
                       
                         
                           A 
                           - 
                           2 
                         
                         → 
                         
                           A 
                           ′ 
                         
                       
                       = 
                       
                         2 
                         + 
                         
                           ( 
                           
                             
                               ( 
                               
                                 A 
                                 + 
                                 60 
                               
                               ) 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             % 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             64 
                           
                           ) 
                         
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       A 
                       ′ 
                     
                     = 
                     
                       
                         
                           A 
                           + 
                           2 
                         
                         → 
                         
                           A 
                           ′ 
                         
                       
                       = 
                       
                         2 
                         + 
                         
                           ( 
                           
                             
                               ( 
                               A 
                               ) 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             % 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             64 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   1 
                 
               
             
           
         
       
     
     An input value used for calculation may be designated by A, and A′ is a result value included in the MPM list. 
     Additionally, when the value of the prediction mode to be added to the MPM list is out of a predetermined range (e.g., 2 or more to 66 or less), the result of applying the clipping function as in Equation 2 below may be included in the MPM list. 
     
       
         
           
             
               
                 
                   
                     A 
                     ′ 
                   
                   = 
                   
                     
                       
                         A 
                         - 
                         1 
                       
                       → 
                       
                         A 
                         ′ 
                       
                     
                     = 
                     
                       Clip 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         ( 
                         
                           
                             min 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               : 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             2 
                           
                           , 
                           
                             max 
                             ⁢ 
                             
                               : 
                             
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             66 
                           
                           , 
                           
                             ( 
                             
                               A 
                               - 
                               1 
                             
                             ) 
                           
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     In Step S 1520 , when at least one of the left prediction mode (Left) and the upper prediction mode (Above) is a directional mode, a determination may be mode on whether any one of the left prediction mode (Left) and the upper prediction mode (Above) is a non-directional mode, or whether both the left prediction mode (Left) and the upper prediction mode (Above) are directional modes while the left prediction mode (Left) and the upper prediction mode (Above) are identical to each other (S 1550 ). To determine this, a step may be performed first for determining a maximum value and a minimum value between the left prediction mode (Left) and the upper prediction mode (Above) (S 1520 ). Min=min(Left, Above) is the process of determining the minimum value between the left prediction mode (Left) and the upper prediction mode (Above), and Max=max(Left, Above) is the process of determining the maximum value between the left prediction mode (Left) and the upper prediction mode (Above). 
     When any one of the two modes is a non-directional mode (Min&lt;=1), or when both modes are directional modes and the same (Min=Max), the MPM list may be configured as {Max, Max−1, Max+1, Max−2} including a total of four directional modes or as {Max, Max−1, Max+1, Max−2, DC} including four directional modes and a DC mode (S 1560 ). 
     On the other hand, when the two modes are directional modes and different from each other, the MPM list may be configured as {Left, Above, Min+1, Max−1} including a total of four directional modes or as {Left, Above, Min+1, Max−1, DC} including four directional modes and a DC mode (S 1570 ). 
     Embodiment 3-2 
     When the left prediction mode (Left) and the upper prediction mode (Above) are obtained (S 1610 ), it can be determined whether both the left prediction mode (Left) and the upper prediction mode (Above) correspond to the non-directional mode (S 1620 ). 
     When both the left prediction mode (Left) and the upper prediction mode (Above) are non-directional modes, the MPM list may be composed of a total of four directional modes such as {V, H, V−4, V+4} or composed of four directional modes and a DC mode such as {V, H, V−4, V+4, DC} (S 1630 ). 
     When at least one of the left prediction mode (Left) and the upper prediction mode (Above) is a directional mode, it may be determined whether any one of the left prediction mode (Left) and the upper prediction mode (Above) is a non-directional mode (S 1640 ). To determine this, a step may be performed first for determining the minimum value (Min) between the left prediction mode (Left) and the upper prediction mode (Above) (S 1630 ). 
     When any one of the left prediction mode (Left) and the upper prediction mode (Above) is a non-directional mode (Min&lt;=1), the MPM list may be composed of a total of four directional modes such as {Max, Max−1, Max+1, Max−2}, or it may be composed of four directional modes and a DC mode such as {Max, Max−1, Max+1, Max−2, DC} (S 1650 ). 
     In Step S 1640 , when both the left prediction mode (Left) and the upper prediction mode (Above) are directional modes, it may be determined whether the two directional modes are identical to each other (S 1660 ). When the left prediction mode (Left) and the upper prediction mode (Above) are directional modes and identical to each other (Min=Max), the MPM list may be composed of a total of four directional modes such as {Left, Left−1, Left+1, Left−2}, or it may be composed of four directional modes and a DC mode such as {Left, Left−1, Left+1, Left−2, DC} (S 1670 ). 
     In Step S 1660 , when the left prediction mode (Left) and the upper prediction mode (Above) are different from each other, it may be determined whether a difference between the two modes is within a preset range (S 1680 ). Here, the preset range may be 2 or more to 66 or less. when the difference between the left prediction mode (Left) and the upper prediction mode (Above) is 2 or more and 66 or less, the MPM list may be composed of a total of four directional modes such as {Left, Above, Max−1, Max+1}, or it may be composed of four directional modes and a DC mode such as {Left, Above, Max−1, Max+1, DC} (S 1690 ). When the difference between the left prediction mode (Left) and the upper prediction mode (Above) is less than 2 or more than 66, the MPM list may be composed of a total of four directional modes such as {Left, Above, Max−2, Max+2}, or it may be composed of four directional modes and a DC mode such as {Left, Above, Max−2, Max+2, DC} (S 1695 ). 
     Embodiment 4 
     Embodiment 4 is an example method for binarizing indexes of MPM modes (candidate intra prediction modes included in the MPM list) and indexes of non-MPM modes (candidate intra prediction modes included in the non-MPM list. 
     In the present disclosure, the indexes of MPM modes are encoded using a truncated rice method, and the indexes of non-MPM modes are encoded by using a truncated binary method. 
     When the MPM list is configured by including four MPM modes, the parameter values for the truncated rice method may be k=0, Max=3. When the MPM list is configured by including five MPM modes, the parameter values for the truncated rice method may be k=0, Max=4. However, in an embodiment that configures the MPM list by including five MPM modes, parameter values for the truncated rice method may also be k=0, Max=3. Here, the parameter value k may be referred to as cRiceParam, and Max may be referred to as cMax. 
     Examples of the above two binarization methods are shown in Table 6. 
     
       
         
           
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                   
                 (MPM: 4) 
                 (MPM: 5) 
               
               
                 index 
                 (k = 0, Max = 3) 
                 (k = 0, Max = 4) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 0 
                 0 
                 0 
               
               
                 1 
                 10 
                 10 
               
               
                 2 
                 110 
                 110 
               
               
                 3 
                 111 
                 1110 
               
               
                 4 
                 — 
                 1111 
               
               
                   
               
            
           
         
       
     
     As represented in Table 6, with the truncated rice method, the indexes of MPM modes may be expressed with at most 3 bits (configuring the MPM list with 4 MPM modes) or at most 4 bits (configuring the MPM list with 5 MPM modes). 
     The non-MPM list is composed of a total of 61 non-MPM modes, except for the planar mode and 5 MPM modes among the total of 67 intra prediction modes. When the non-MPM list is composed of a total of 61 non-MPM modes, the parameter value for the truncated binary method may be Max=60. Table 7 shows an example in which the indexes of a total of 61 non-MPM modes are expressed in a truncated binary method. In Table 7, Max may be cMax. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                   
                 Max = 60, n = 59 
               
               
                   
                 index 
                 (32 &lt; n &lt; 64, k = 5) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                   
                 0 
                 00000 
               
               
                   
                 1 
                 00001 
               
               
                   
                 2 
                 00010 
               
               
                   
                 3 
                 00011 
               
               
                   
                 4 
                 00100 
               
               
                   
                 5 
                 001010 
               
               
                   
                 6 
                 001011 
               
               
                   
                 7 
                 001100 
               
               
                   
                 8 
                 001101 
               
               
                   
                 . . . 
                 . . . 
               
               
                   
                 60  
                 111111 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 7, indexes of non-MPM modes may be expressed with 5 bits or 6 bits by using the truncated binary method. The index values of the non-MPM modes shown in Table 7 are reconstructed index values of 61 non-MPM modes excluding the planar mode and 5 MPM modes among the total of 67 intra prediction modes. In other words, index values of 61 non-MPM modes are reconstructed through a shifting operation. For example, when 5 MPM modes are configured as {1, 3, 10, 15, 23} with the planar mode and MPM modes being shifted in all intra prediction modes, the non-MPM list is configured as {2, 4, 5, 6, 7, 8, 9, 11, 12, 13, 14, 16, . . . , 22, 24, 25, . . . , 65, 66}. In this case, prediction mode No. 2 is set to index 0, prediction mode No. 4 is set to index 1, and prediction mode No. 5 is set to index 2. 
     Although exemplary embodiments of the present disclosure have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions, and substitutions are possible, without departing from the idea and scope of the claimed invention. Therefore, exemplary embodiments of the present disclosure have been described for the sake of brevity and clarity. The scope of the technical idea of the present embodiments is not limited by the illustrations. Accordingly, one of ordinary skill would understand the scope of the claimed invention is not to be limited by the above explicitly described embodiments but by the claims and equivalents thereof. 
     CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Application No. 10-2019-0061737 filed on May 27, 2019, and Korean Patent Application No. 10-2019-0157997 filed on Dec. 2, 2019, the disclosures of which are incorporated by reference herein in their entireties.