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WO2018124686A1 - Codage et décodage d'image à l'aide d'une prédiction intra - Google Patents

Codage et décodage d'image à l'aide d'une prédiction intra Download PDF

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Publication number
WO2018124686A1
WO2018124686A1 PCT/KR2017/015460 KR2017015460W WO2018124686A1 WO 2018124686 A1 WO2018124686 A1 WO 2018124686A1 KR 2017015460 W KR2017015460 W KR 2017015460W WO 2018124686 A1 WO2018124686 A1 WO 2018124686A1
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Prior art keywords
intra prediction
block
subblocks
mode
candidates
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Korean (ko)
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임정연
이선영
손세훈
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SK Telecom Co Ltd
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SK Telecom Co Ltd
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Priority claimed from KR1020170179413A external-priority patent/KR102488123B1/ko
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/102—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or selection affected or controlled by the adaptive coding
    • H04N19/103—Selection of coding mode or of prediction mode
    • H04N19/11—Selection of coding mode or of prediction mode among a plurality of spatial predictive coding modes
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/169—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
    • H04N19/17—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object
    • H04N19/176—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object the region being a block, e.g. a macroblock
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
    • H04N19/169—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
    • H04N19/186—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being a colour or a chrominance component
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/50—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding
    • H04N19/593—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using predictive coding involving spatial prediction techniques
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
    • H04N19/70—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals characterised by syntax aspects related to video coding, e.g. related to compression standards

Definitions

  • the present invention relates to encoding or decoding of an image using intra prediction.
  • the image encoding apparatus selects the last one mode of the current block, which is the encoding target, from among the plurality of intra prediction modes, and delivers information about the mode to the image decoding apparatus.
  • the most probable mode MPM is used to efficiently represent the selected intra mode.
  • FIG. 1 is a diagram illustrating intra modes that can be used for intra prediction in HEVC.
  • intra modes that can be used for intra prediction in HEVC.
  • three MPMs for the block are used by using the neighboring block and the most frequently used modes of the block to encode the final intra mode. Select.
  • a 1 bit MPM flag indicating whether or not the final mode of the block is the same as the MPM is transmitted. If the final mode is the MPM, an additional MPM index value is transmitted. If the final mode is not the MPM, it will explicitly transmit which of the remaining modes.
  • intra prediction mode candidates composed of some of a total of 35 intra prediction modes are configured and an intra prediction mode of the color difference block is selected among the candidates. And information for indicating a selected candidate among the candidates is signaled.
  • the intra prediction mode candidates are composed of an intra prediction mode (DM) of a luminance block corresponding to a planar mode, a vertical mode, a horizontal mode, a DC mode, and a chrominance block.
  • the present invention seeks to provide an improved method for determining intra prediction modes in this context.
  • An object of the present invention is to provide an improved technique for determining and encoding an intra prediction mode of a current block in intra prediction coding.
  • An aspect of the present invention provides a method of encoding information about an intra prediction mode of a chroma block, comprising: constructing a candidate set including a plurality of candidates for an intra prediction mode of the color difference block; The candidate of at least one intra prediction mode derived from a luma block corresponding to the chrominance block; And encoding color difference intra mode information for indicating an intra prediction mode of the color difference block among the plurality of candidates belonging to the candidate set.
  • the one or more candidates are selected from intra prediction modes of at least some of the subblocks of the plurality of subblocks, and intras of the at least some subblocks within the luminance block are selected.
  • Select the one or more candidates sequentially from intra prediction modes of the at least some subblocks based on at least one of a ratio occupied by each of the prediction modes, or a predefined scan order of scanning the at least some subblocks .
  • Another aspect of the present invention provides a method of determining an intra prediction mode of a chroma block in image decoding, comprising: decoding color difference intra mode information for indicating an intra prediction mode of the color difference block from a bitstream; Constructing a candidate set including a plurality of candidates for an intra prediction mode of the chrominance block, wherein the plurality of candidates include at least one intra prediction mode derived from a luma block corresponding to the chrominance block; ; And setting a candidate indicated by the intra prediction mode of the chrominance block among the plurality of candidates belonging to the candidate set to the intra prediction mode of the chrominance block.
  • the one or more candidates are selected from intra prediction modes of at least some of the subblocks of the plurality of subblocks, and intras of the at least some subblocks within the luminance block are selected.
  • the one or more candidates are sequentially selected from intra prediction modes of the at least some subblocks based on at least one of a ratio occupied by each of the prediction modes or a predefined order of scanning the at least some subblocks.
  • FIG. 1 illustrates intra modes available for intra prediction in HEVC
  • FIG. 2 is a block diagram of an image encoding apparatus according to an embodiment of the present invention.
  • 3 is an exemplary diagram of block division using a QTBT structure
  • FIG. 4 is an exemplary diagram for a plurality of intra prediction modes according to an embodiment of the present invention.
  • FIG. 5 is an exemplary diagram illustrating neighboring blocks of a current block
  • FIG. 6 is an exemplary diagram illustrating a block division structure for a luminance component and a chrominance component of a CTU
  • FIG. 7 is an exemplary diagram illustrating an intra prediction mode of respective luminance blocks in a block division structure for luminance components of a CTU;
  • FIG. 8 is an exemplary diagram illustrating a z-scan order
  • DM direct mode
  • FIG. 10 is another exemplary diagram for inducing DM of the present invention.
  • FIG. 11 is another exemplary diagram for inducing DM of the present invention.
  • Figure 13 is another illustration for inducing DM of the present invention.
  • FIG. 14 is a block diagram of an image decoding apparatus according to an embodiment of the present invention.
  • FIG. 2 is a block diagram of an image encoding apparatus according to an embodiment of the present invention.
  • the image encoding apparatus includes a block divider 210, a predictor 220, a subtractor 230, a transformer 240, a quantizer 245, an encoder 250, an inverse quantizer 260, and an inverse transform unit ( 265, an adder 270, a filter unit 280, and a memory 290.
  • each component may be implemented as a hardware chip, or may be implemented in software and implemented so that the microprocessor executes a function of software corresponding to each component.
  • the block dividing unit 210 After dividing each picture constituting the image into a plurality of coding tree units (CTUs), the block dividing unit 210 recursively divides the CTUs using a tree structure.
  • a leaf node in the tree structure becomes a CU (coding unit) which is a basic unit of coding.
  • CU coding unit
  • QT QuadTree
  • QTBT QuadTree
  • BT binaryTree
  • BinaryTree BinaryTree
  • the CTU is first divided into a QT structure.
  • the leaf nodes of the QT may then be further partitioned by BT.
  • the partition information generated by the block divider 210 by dividing the CTU by the QTBT structure is encoded by the encoder 250 and transmitted to the image decoding apparatus.
  • a first flag (QT split flag, QT_split_flag) indicating whether a block of a corresponding node is split is encoded. If the first flag is 1, the block of the node is divided into four blocks of the same size. If the first flag is 1, the node is no longer divided by QT.
  • a second flag (BT split flag, BT_split_flag) indicating whether a block of the corresponding node is split is encoded.
  • BT there may be a plurality of partition types. For example, there may be two types of partitioning a block of a node horizontally into two blocks of the same size and a type of partitioning vertically. Alternatively, there may further be a type in which blocks of the corresponding node are further divided into two blocks having an asymmetric shape. The asymmetrical form may include dividing a block of a node into two rectangular blocks having a size ratio of 1: 3, or dividing a block of the node in a diagonal direction.
  • partition type information indicating a partition type of the corresponding block is further encoded.
  • FIG. 3 is an exemplary diagram of block division using a QTBT structure.
  • A of FIG. 3 is an example in which a block is divided by a QTBT structure, and (b) shows it in a tree structure.
  • the solid line indicates the division by the QT structure
  • the dotted line indicates the division by the BT structure.
  • the parenthesis indicates a layer of QT
  • the parenthesis indicates a layer of BT.
  • a number represents partition type information. 1 indicates horizontal division and 0 indicates vertical division.
  • a block corresponding to a CU to be encoded or decoded is called a 'current block'.
  • the prediction unit 220 generates a prediction block by predicting the current block.
  • the predictor 220 includes an intra predictor 222 and an inter predictor 224.
  • the intra predictor 222 predicts pixels in the current block by using pixels (reference pixels) positioned around the current block in the current picture including the current block. There are a plurality of intra prediction modes according to the prediction direction, and the peripheral pixels to be used and the equations are defined differently according to each prediction mode. In particular, the intra predictor 222 may determine an intra prediction mode to be used to encode the current block. In some examples, intra prediction unit 222 may encode the current block using several intra prediction modes and select an appropriate intra prediction mode to use from the tested modes. For example, the intra predictor 222 calculates rate distortion values using rate-distortion analysis for several tested intra prediction modes, and has the best rate distortion characteristics among the tested modes. Intra prediction mode may be selected.
  • FIG 4 shows an example of intra prediction modes according to an embodiment of the present invention.
  • the intra prediction modes may include two non-directional modes (planar mode and DC mode) and 65 directional modes.
  • the intra predictor 222 selects one intra prediction mode from among the plurality of intra prediction modes, and predicts the current block by using a neighboring pixel (reference pixel) and an operation formula determined according to the selected intra prediction mode.
  • Information on the selected intra prediction mode is encoded by the encoder 250 and transmitted to the image decoding apparatus.
  • the inter prediction unit 224 searches for the block most similar to the current block in the coded and decoded reference picture before the current picture, and generates a prediction block for the current block using the searched block.
  • a motion vector corresponding to a displacement between the current block in the current picture and the prediction block in the reference picture is generated.
  • Motion information including information about a reference picture and information about a motion vector used to predict the current block is encoded by the encoder 250 and transmitted to the image decoding apparatus.
  • the subtractor 230 subtracts the prediction block generated by the intra predictor 222 or the inter predictor 224 from the current block to generate a residual block.
  • the converter 240 converts the residual signal in the residual block having pixel values of the spatial domain into a transform coefficient of the frequency domain.
  • the transform unit 240 may convert the residual signals in the residual block using the size of the current block as a conversion unit, or divide the residual block into a plurality of smaller subblocks and convert the residual signals in a subblock-sized transform unit. You can also convert. There may be various ways of dividing the residual block into smaller subblocks. For example, it may be divided into sub-blocks of a predetermined same size, or a quadtree (QT) scheme may be used in which the residual block is a root node.
  • QT quadtree
  • the quantization unit 245 quantizes the transform coefficients output from the transform unit 240, and outputs the quantized transform coefficients to the encoder 250.
  • the encoder 250 generates a bitstream by encoding the quantized transform coefficients by using an encoding method such as CABAC.
  • the encoder 250 encodes information such as a CTU size, a QT split flag, a BT split flag, a split type, and the like related to block division, so that the image decoding apparatus may split the block in the same manner as the image encoding apparatus.
  • the encoder 250 encodes information about a prediction type indicating whether the current block is encoded by intra prediction or inter prediction, and uses a syntax element for intra prediction mode or inter prediction information according to the prediction type.
  • the inverse quantizer 260 inversely quantizes the quantized transform coefficients output from the quantizer 245 to generate transform coefficients.
  • the inverse transformer 265 restores the residual block by converting the transform coefficients output from the inverse quantizer 260 from the frequency domain to the spatial domain.
  • the adder 270 reconstructs the current block by adding the reconstructed residual block and the predicted block generated by the predictor 220.
  • the pixels in the reconstructed current block are used as reference pixels when intra prediction of the next order of blocks.
  • the filter unit 280 deblocks and filters the boundary between the reconstructed blocks in order to remove blocking artifacts that occur due to encoding / decoding of blocks. When all the blocks in a picture are reconstructed, the reconstructed picture is used as a reference picture for inter prediction of a block in a picture to be encoded later.
  • the pixels of the current block may be composed of a luminance component and two color difference components Cb and Cr.
  • the current block includes one luminance block composed of luminance components and two color difference blocks composed of respective color difference components.
  • the luminance block and the chrominance block may be predicted and encoded independently of each other according to the encoding method described above. That is, the luminance block and the chrominance block are predicted independently of each other, and the residual signals of the luminance block and the chrominance block are also independently encoded.
  • the present disclosure relates to determining and encoding an intra prediction mode of each of a luminance block and a chrominance block in encoding of a luminance block and a chrominance block.
  • the method of determining the intra prediction mode of the luminance block and the method of determining the intra prediction mode of the chrominance block may be different.
  • the image encoding apparatus 200 determines an intra prediction mode of the luminance block and classifies a plurality of candidate sets among all intra prediction modes.
  • Luma intra mode information is generated by generating information about which group of the plurality of candidate sets the intra prediction mode of the luminance block belongs and index information for indicating the intra prediction mode of the luminance block within the group.
  • the plurality of candidate sets may be generated by classifying all intra prediction modes of the current block according to a frequency of occurrence.
  • the plurality of candidate sets include the following groups.
  • MPMs are derived from the intra prediction modes of the neighboring block adjacent to the luminance block.
  • the neighboring blocks are the luminance block L adjacent to the left side of the luminance block, the luminance block A adjacent to the upper side, the luminance block BL adjacent to the lower left side, the block AR adjacent to the upper right side, and the luminance block adjacent to the upper left side ( AL).
  • the second group is offset (eg, ⁇ 1) to modes having a mode number of multiples of four when reordering the mode numbers in ascending order except for MPM of all intra prediction modes, or directional modes belonging to the first group. It consists of modes created by adding +1).
  • All intra prediction modes consist of the remaining modes except those belonging to the first group and the second group.
  • the image encoding apparatus 200 configures a candidate set including a plurality of candidates for the intra prediction mode of the chrominance block.
  • the intra prediction mode of the color difference block is determined among a plurality of candidates belonging to the candidate set, and color difference intra mode information for indicating a selected candidate among the plurality of candidates is encoded.
  • the candidate set may consist of the following four types of intra prediction modes.
  • An intra mode that derives a prediction block for a chrominance block by using a reconstructed pixel value in a luminance block. Using the correlation between the luminance block and the chrominance block, the scaling factor ⁇ and the offset ⁇ are obtained. Then, the prediction block for the chrominance block is derived using the reconstructed pixel values in the luminance block and the two values. do.
  • the specific formula is shown in Equation 1.
  • Pred C (i, j) is a predicted chrominance block corresponding to the current block
  • rec L (i, j) is a downsampled reconstructed luminance block corresponding to the current block
  • ⁇ is a scaling factor
  • ⁇ is an offset.
  • Indicates. ⁇ and ⁇ are either block units on which intra prediction is performed, or a block of a top layer corresponding to a root node in a tree structure for a block partition, or a sequence unit that is a slice, a picture, or a group of a plurality of pictures.
  • the bitstream may be included in a unit of and transmitted to the decoding apparatus.
  • ⁇ and ⁇ may be equally calculated by the encoding apparatus and the decoding apparatus using pixel values in the luminance block and the chrominance block that are already reconstructed around the current block. In this case, signaling for ⁇ and ⁇ is not required.
  • the intra prediction mode of the luminance block corresponding to the chrominance block In the tree structure division from the CTU to the CU, the luminance component and the chrominance component can be divided into the same structure. In this case, since there is only one luminance block corresponding to the color difference block, there is one DM. However, the luminance component and the chrominance component may be divided into different structures. Referring to FIG. 6, FIG. 6 (a) shows a block division structure of a CTU composed of luminance components, and FIG. 6 (b) shows a block division structure of a CTU composed of chrominance components. It can be seen that the luminance block located at the upper right of FIG. 6A corresponding to the chrominance block located at the upper right of FIG. 6B is divided into a total of six subblocks. Since each subblock becomes one CU, each subblock may be intra predicted using different intra prediction modes. Accordingly, there may be a plurality of intra prediction modes of the luminance block corresponding to the color difference block.
  • the luminance block corresponding to the chrominance block is divided into a plurality of subblocks, a method of deriving one or more intra prediction modes from the intra prediction modes of the plurality of subblocks is required.
  • Intra prediction modes of neighboring color difference blocks spatially adjacent to the color difference block For example, the position of the peripheral color difference block may be determined as shown in FIG. 5.
  • the priorities for selecting the neighboring chrominance block are left (L), top (A), bottom left (BL), top right (AR), and top left ( AL).
  • Probably frequently used modes may be included in the candidate set in the order of Planar, DC, vertical, horizontal, and diagonal modes. Or, first, the planar and DC modes are included in the candidate set, and then, modes obtained by adding -1 or +1 to the angle modes (directional modes) already included in the candidate set are included in the candidate set. If the number of candidates in the candidate set is still insufficient, the candidate sets are filled in the order of the vertical, horizontal, and diagonal modes.
  • a candidate set is constructed by using the intra prediction modes derived from the four types described above.
  • the number m of candidates in the candidate set may have a value in the range of 5-8.
  • the order of filling the intra prediction mode of the color difference block in the candidate set among four types may be variously determined.
  • the candidate set may be filled with intra prediction mode candidates of the color difference block in the order of DM, LM, neighboring mode, and default mode. For example, if the number of candidate sets is defined as six, two DMs and one LM are derived, and three modes are derived from available neighboring blocks L, AR, and AL based on the priority of the neighboring mode. In this case, the default mode is not used because six candidates have already been derived. Therefore, the candidate set is filled in the order of intra prediction modes of two DM, one LM, neighboring blocks L, AR, and AL. If two modes are obtained from one DM and one LM and available neighboring blocks A and AL, two candidates are derived in the order of planar and DC based on the priority in the default mode.
  • the candidate set is filled in order of one DM, one LM, neighboring blocks A, and intra prediction modes of the AL, planar, and DC. If five DMs and one LM are derived, since all six candidates are already derived, the neighboring mode and the default mode are not included in the candidate set.
  • the candidate set may be filled with intra prediction mode candidates of a color difference block in the order of LM, DM, neighboring mode, and default mode.
  • the m intra prediction mode candidates belonging to the candidate set should not overlap.
  • it is checked whether the new intra prediction mode already exists in the candidate set, and if not, adds the new intra prediction mode to the candidate set.
  • the LM may always be included in the candidate set.
  • the number of normal intra prediction modes except for the LM mode is m ⁇ 1.
  • the number of candidate sets is defined as six, up to five general intra prediction modes are derived from DM, neighboring modes, and default modes.
  • the image encoding apparatus 200 should store intra prediction modes of the luminance block.
  • Intra prediction modes of respective luminance blocks may be stored in a memory having an N ⁇ N block size.
  • N may be appropriately set in consideration of the memory capacity and the like.
  • the value of N may have a value of 1 (ie, store an intra prediction mode of a luminance block in units of pixels), and may have a minimum block size among block sizes available for intra prediction.
  • FIG. 7 is an exemplary diagram illustrating an intra prediction mode of respective luminance blocks in the block division structure of FIG.
  • FIG. 7A intra prediction modes A to H of respective luminance blocks are displayed in different patterns.
  • FIG. 7B illustrates an example of storing intra prediction modes of luminance blocks in a memory in units of 4 ⁇ 4 block sizes.
  • the size of the CTU is 32x32, intra prediction modes for a total of 64 regions are stored.
  • the image encoding apparatus 200 may determine from the intra prediction modes of at least some of the subblocks present in the luminance block.
  • One or more candidates for intra prediction mode of the chrominance block i.e., DM, are selected.
  • DM one or more of a ratio occupied by each of the intra prediction modes of the at least some subblocks in the luminance block, or a predefined order of scanning the at least some subblocks may be used.
  • the at least some subblocks may be all of the plurality of subblocks existing in the luminance block or may be subblocks of a predefined position.
  • the subblocks of the predefined positions include pixels (center, CR) located at the center of the luminance block corresponding to the chrominance block, pixels (top left, TL) located at the top left of the luminance block, and pixels located at the top right of the luminance block ( top right, TR), a pixel located at the bottom left of the luminance block (bottom left, BL), and a subblock covering a pixel located at the bottom right of the luminance block (bottom right, BR).
  • CR refers to a pixel (top left center, TLC) positioned at the top left with respect to the center point of the luminance block.
  • TRC top right center
  • BLC bottom left pixel
  • BRC bottom right pixel
  • the ratio may be the area of the block covered by the intra prediction mode of each of the at least some subblocks in the luminance block.
  • at least some subblocks are blocks that cover pixels located in CR, TL, TR, BL, BR.
  • the intra prediction modes of CR, TL, TR, BL, and BR in the luminance block corresponding to the chrominance block located at the upper right of FIG. 6B are CR (G) and TL. (C), TR (C), BL (A), and BR (G) (expressed as "position (mode)").
  • the ratio may be a frequency at which the same intra prediction mode occurs as each of the intra prediction modes of the at least some subblocks in the luminance block.
  • at least some subblocks are blocks that cover pixels located in CR, TL, TR, BL, BR.
  • the intra prediction modes of CR, TL, TR, BL, and BR in the luminance block corresponding to the chrominance block located at the upper right of FIG. 6B are CR (G) and TL. (C), TR (C), BL (A) and BR (G). Therefore, there are three intra prediction modes corresponding to CR, TL, TR, BL, and BR (G, C, A).
  • the frequency of occurrence of the same mode as each intra prediction mode in the luminance block in units of block size stored in the memory (for example, in units of 4 ⁇ 4 blocks as shown in FIG. 7B), these modes are included in the luminance block.
  • the frequency of occurrence is 8 times in mode C, 4 times in mode G, and 1 time in mode A.
  • the candidates are included in the candidate set in the order of high frequency, that is, the modes C, G, and A.
  • the frequency is counted in units of a block size in which the intra prediction mode is stored in the memory, but the present invention is not necessarily limited thereto. For example, it may be counted in units of subblocks divided into tree structures within the luminance block.
  • the ratio may be defined as the number of overlaps between intra prediction modes of at least some subblocks.
  • intra prediction modes corresponding to CR, TL, TR, BL, and BR in the luminance block corresponding to the color difference block located at the upper right of FIG. 6B may include CR (G) and TL. (C), TR (C), BL (A) and BR (G). Therefore, the number of overlaps between the intra prediction modes of at least some subblocks is mode C twice, mode G twice, and mode A once. Therefore, mode C and mode G are included in the candidate set before mode A.
  • the z-scan order may be used as the predefined scan order.
  • the z-scan order means the order of the top left, top right, bottom left and bottom right.
  • the processing order between sub-blocks is also a z-scan order, and the processing order within the sub-blocks is also a z-scan order.
  • the predefined scan order is not limited to the z-scan order.
  • the predefined location may be scanned using a scan order different from the z-scan order.
  • a scan order of CR, TL, TR, BL, BR, or TL, CR, TR, BL, BR May be used.
  • This embodiment selects one or more DMs from all intra prediction modes of all subblocks present in the luminance block based on priority in z-scan order.
  • FIG. 9 is an exemplary diagram for inducing DM according to the first embodiment.
  • Capital letters A to H shown in FIG. 9A indicate the intra prediction mode, and the numerals shown in FIG. 9B indicate the number of DMs in each color difference block.
  • two subblocks having different intra prediction modes are present in the luminance block corresponding to the color difference block located at the upper left of FIG. 9B.
  • a total of two DMs are selected in the order of intra prediction mode A and intra prediction mode B according to the z-scan order.
  • the remaining three candidates are selected from the neighboring mode and the default mode.
  • a total of six subblocks exist in the luminance block corresponding to the color difference block located at the upper right of FIG. 9B, and the six subblocks have different intra prediction modes.
  • a total of five DMs are selected in the order of intra prediction modes C, D, E, A and F. Since the maximum number of DMs is five, intra prediction mode G is not selected as DM.
  • This embodiment selects one or more DMs sequentially in accordance with the ratio and z-scan order from all intra prediction modes of all subblocks present in the luminance block.
  • DM is selected in order of proportion, and in the case of equal proportions, in accordance with z-scan order.
  • any one of the above-described block area, frequency, and number of overlaps may be used as a ratio, but it will be described based on the block area.
  • FIG. 10 is an exemplary diagram for inducing DM according to the second embodiment.
  • Capital letters A through H shown in FIG. 10A indicate an intra prediction mode, and the number described in FIG. 10B indicates the number of DMs of each color difference block.
  • two subblocks having different intra prediction modes are present in the luminance block corresponding to the color difference block located at the upper left of FIG. 10B.
  • the area of the blocks covered by the two intra prediction modes in the luminance block is the same. Therefore, a total of two DMs are selected in the order of intra prediction mode A and intra prediction mode B according to the z-scan order. The remaining three candidates are selected from the neighboring mode and the default mode.
  • each intra prediction mode in the luminance block is large in the order of intra prediction modes C and G.
  • the block areas covered by the intra prediction modes D, E, A, and F are the same.
  • two DMs are first selected in the intra prediction modes C and G according to the block area, and intra prediction modes D according to the z-scan order among the intra prediction modes D, E, A and F covering the same area.
  • the three DMs are selected in the order of, E, and A. Since the maximum number of DMs is five, intra prediction mode F is not selected as DM. Therefore, the present embodiment selects a total of five DM in the order of intra prediction modes C, G, D, E, and A.
  • This embodiment selects one or more DMs based on a ratio and a predefined scan order from subblocks of a predefined position among all subblocks present in the luminance block.
  • the subblocks of the predefined position include subblocks covering pixels located in CR, TL, TR, BL, and BR in the luminance block.
  • the DMs are selected in order of proportion, and when the proportions are the same, they are selected in accordance with a predefined scan order.
  • the present embodiment can use any one of block area, frequency, or number of overlaps as a ratio, and Z-scan order, or scan order of CR, TL, TR, BL, BR, or TL as a predefined scan order. , Scan order of CR, TR, BL, BR, etc. may be used.
  • the Z-scan order is used as the predefined scan order and the block area or the number of overlaps is used as the ratio.
  • FIG. 11 is an exemplary diagram for inducing DM according to the third embodiment.
  • Capital letters A to H shown in FIG. 11A indicate an intra prediction mode, and a number described in FIG. 11B indicates the number of DMs of each color difference block.
  • each of CR, TL, TR, BL, and BR in the luminance block corresponding to the chrominance block located at the upper left of FIG. There are two intra prediction modes corresponding to the position. Since the block areas covered by the two intra prediction modes in the luminance block are the same, this embodiment selects two DMs in the order of intra prediction modes A and B according to the Z-scan order. The remaining three candidates are selected from the neighboring mode and the default mode. On the other hand, in the luminance block corresponding to the chrominance block located at the upper right of FIG.
  • Intra prediction modes are the same as CR (B), TL (A), TR (B), BL (A) and BR (B).
  • the number of overlaps between the intra prediction modes in CR, TL, TR, BL, and BR is two times in mode A and three times in mode B. Therefore, the present embodiment selects two DMs in the order of intra prediction modes B and A. FIG. The remaining three candidates are selected from the neighboring mode and the default mode.
  • intra prediction modes corresponding to positions of CR, TL, TR, BL, and BR in the luminance block corresponding to the chrominance block located at the upper right of FIG. 11 (b) include CR (G), TL (C), Same as TR (C), BL (A), BR (G).
  • the number of overlaps between intra prediction modes in CR, TL, TR, BL, and BR is mode A once, mode C twice, and mode G twice. Therefore, modes C and G are selected as DMs before mode A.
  • the intra prediction mode C is first selected as the DM according to the Z-scan order. Therefore, three DMs are selected in the order of C, G, and A. The remaining two candidates are selected from the neighboring mode and the default mode.
  • both the ratio and the predefined scan order are considered, but the present embodiment may use only the predefined scan order without considering the block area.
  • the DM of the color difference block located at the upper left of FIG. 11B is selected in the order of the intra prediction modes G, C, and A.
  • This embodiment is the same as embodiment # 3 in that one or more DMs are selected based on a ratio and a predefined scan order from subblocks of a predetermined position among all subblocks existing in the luminance block.
  • the present embodiment uses subblocks that cover pixels located in TLC, TRC, BLC, and BRC as well as TL, TR, BL, and BR in the luminance block as subblocks of a predefined position.
  • the z-scan order is used as the predefined scan order and the block area or the number of overlaps is used as the ratio.
  • FIG. 12 is an exemplary diagram for inducing DM according to the fourth embodiment.
  • Capital letters A through H shown in FIG. 9A indicate an intra prediction mode, and the number described in FIG. 12B indicates the number of DMs of each color difference block.
  • intra prediction modes there are four intra prediction modes corresponding to the positions of TL, TR, BL, BR, TLC, TRC, BLC, and BRC in the luminance block corresponding to the color difference block located at the upper right of FIG.
  • Two DMs are selected first in the order of intra prediction modes C and G according to the block area covered by each intra prediction mode. Since the intra prediction modes corresponding to the BL and BLC positions cover the same block area, the intra prediction modes E and A are sequentially selected as DM in the order of BLC and BL according to the Z-scan order. That is, in this embodiment, a total of four DMs are selected in the order of intra prediction modes C, G, E, and A. The other candidate is selected from neighboring mode and / or default mode.
  • TL, TR, BL, BR, TLC, TRC, BLC, and BRC in the luminance block corresponding to the color difference block located in the upper left of FIG.
  • Corresponding intra prediction modes are TL (A), TR (B), BL (A), BR (B), TLC (A), TRC (B), BLC (A), BRC (B). Since the number of overlaps between two intra prediction modes corresponding to the positions of TL, TR, BL, BR, TLC, TRC, BLC, and BRC is the same as four times, DM is selected in the order of intra prediction modes A and B in the Z-scan order. Choose. The remaining three candidates are selected from neighboring mode and / or default mode.
  • intra prediction modes corresponding to positions of TL, TR, BL, BR, TLC, TRC, BLC, and BRC in the luminance block corresponding to the color difference block located at the upper right of FIG. 12B are TL (C). , TR (C), BL (A), BR (G), TLC (C), TRC (C), BLC (E), BRC (G).
  • the number of overlaps between the intra prediction modes corresponding to each position is four times C, one A, two G, and one E. Two DMs are first selected in the order of intra prediction modes C and G according to the number of overlaps.
  • intra prediction modes E and A are sequentially selected as DMs in the order of BLC and BL according to the Z-scan order. That is, in this embodiment, a total of four DMs are selected in the order of intra prediction modes C, G, E, and A.
  • the other candidate is selected from neighboring mode and / or default mode.
  • This embodiment is the same as embodiment # 3 in that one or more DMs are selected based on a ratio and a predefined scan order from subblocks of a predetermined position among all subblocks existing in the luminance block.
  • the present embodiment uses subblocks that cover pixels located in TLC, TRC, BLC, and BRC in the luminance block as subblocks of a predefined position.
  • the z-scan order is used as the predefined scan order and the block area or the number of overlaps is used as the ratio.
  • FIG. 13 is an exemplary diagram for inducing DM according to the fifth embodiment.
  • Capital letters A through H shown in FIG. 9A indicate an intra prediction mode, and a number described in FIG. 13B indicates the number of DMs of each color difference block.
  • FIG. 13A there are two intra prediction modes corresponding to positions of TLC, TRC, BLC, and BRC in the luminance block corresponding to the color difference block located at the upper left of FIG. 13B. . Since the block area covered by the two intra prediction modes in the luminance block corresponding to the chrominance block located at the upper left is the same, DM is selected in the order of intra prediction modes A and B in the Z-scan order. The remaining three modes are selected from neighboring mode and / or default mode.
  • each intra prediction mode corresponding to positions of TLC, TRC, BLC, and BRC are included in the luminance block corresponding to the color difference block located at the upper right of FIG. 13B.
  • the block area covered by each intra prediction mode is different. Therefore, three DMs are selected in the order of intra prediction modes C, G, and E according to the block area. The remaining two candidates are selected from neighboring mode and / or default mode.
  • the number of overlaps is used as the ratio
  • two intra prediction modes corresponding to the positions of TLC, TRC, BLC, and BRC in the luminance block corresponding to the color difference block located in the upper left of FIG. exist.
  • the intra prediction mode of the subblocks of each position it is the same as TLC (A), TRC (B), BLC (A), and BRC (B).
  • DM is selected in the order of intra prediction modes A and B according to the Z-scan order. The remaining three modes are selected from neighboring mode and / or default mode.
  • intra luminance modes corresponding to positions of TLC, TRC, BLC, and BRC are TLC (C), TRC (C), and BLC (in the luminance block corresponding to the color difference block located at the upper right of FIG. 13B).
  • E same as BRC (G).
  • the number of overlaps between the intra prediction modes at each position is C twice, E and G once. Therefore, the intra prediction mode C is first selected as the DM according to the number of overlaps. Since the number of overlaps of the intra prediction modes corresponding to the BLC and BRC positions is the same, the intra prediction modes E and G are sequentially selected as DM in the order of BLC and BRL according to the Z-scan order. That is, in this embodiment, a total of three DMs are selected in the order of intra prediction modes C, E, and G. The remaining two candidates are selected from neighboring mode and / or default mode.
  • the image encoding apparatus 200 assigns a mode index to a predetermined number of candidates belonging to the candidate set and encodes an index corresponding to the candidate selected as the intra prediction mode of the color difference block, as index intra mode information.
  • the mode index is given in the order described above in the order of LM, DM, neighboring mode, default mode, or DM, LM, neighboring mode, default mode.
  • the mode index may be binarized such that the index of the candidate selected first in the candidate set has a smaller number of bits than the index of the candidate selected later.
  • the indexes for the two candidates last selected from the candidate set may have the same number of bits and may be binarized so that the remaining bits except the least significant bit (LSB) are the same.
  • Table 3 shows a method of binarizing a mode index when the number of candidates belonging to the candidate set is six.
  • Codeword # 1 is the result of binarization using the TU (truncated unary) method. For example, when an index is assigned in the order of LM, DM, neighboring mode, and default mode, the first bin of codeword # 1 indicates whether the intra prediction mode of the color difference block is LM. If the first bin has a first value (0 in Table 1) it is an LM, and if it has a second value (1 in Table 1) it is not an LM. Therefore, in the case of LM, the mode index is represented by zero.
  • the first bin is represented by 1.
  • Normal modes derived in the order of DM, neighboring mode, and default mode are identified by second and subsequent bins.
  • the mode index for the intra prediction mode included in the candidate set first among the normal modes is binarized with a total of 2 bits, the second bin is represented by 0, and the second bins of the mode indices of candidates having a higher priority than the first candidate are all 1 It is expressed as
  • the mode index for the intra prediction mode included in the candidate set for the second time among the normal modes is binarized to a total of 3 bits by increasing the number of bits by 1 than the first candidate, and the third bin is expressed as 0.
  • the third bins of all the mode indices of candidates having a lower rank than the second candidate are all represented by one.
  • the mode index for the intra prediction mode included in the candidate set is the third of the normal modes, and the number of bits is increased by one, and the fourth bin is represented by zero.
  • the mode indexes for the general modes included in the candidate set are binarized.
  • the two candidates included in the candidate set among the normal modes, that is, the fourth candidate and the fifth candidate are binarized with the same number of bits and binarized so that the remaining bits except for the lowest bin are the same.
  • the fourth and fifth candidates of the general modes are represented by the first four bins equal to 1, and only the fifth bin, which is the lowest bin, is represented by 0 and 1, respectively.
  • LM low-density diode
  • DM low-density diode
  • neighboring mode low-density diode
  • default mode a predetermined number of candidates (eg, 6), and which one of the candidates is used as the intra prediction mode of the color difference block. It was described as encoding a mode index indicating. However, the present invention is not necessarily limited thereto.
  • the first bin indicates whether LM is used as an intra prediction mode of a color difference block. Therefore, it is also possible to indicate whether the LM is used as the intra prediction mode of the chrominance block through a separate flag instead of the first bin, which is extremely obvious to those skilled in the art. In this case, if the flag indicates that the LM is not used as an intra prediction mode of the chrominance block, a predetermined number of normal modes are selected in the order of DM, neighboring mode, and default mode, and among the normal modes.
  • a mode index indicating which candidate is used as the intra prediction mode of the color difference block may be binarized using the above-described binarization scheme.
  • the image encoding apparatus 200 may encode the chrominance intra prediction mode.
  • a predetermined number of DMs may be selected and a first index indicating which candidate from among the predetermined number of DMs is used as the intra prediction mode of the color difference block may be binarized using the above-described binarization scheme.
  • the second index indicating which candidate among the predetermined number of modes derived from the LM, neighboring mode, and default mode is used as the intra prediction mode of the chrominance block using the above-described binarization scheme. It can be binarized.
  • FIG 8 illustrates an image decoding apparatus according to an embodiment of the present invention.
  • the image decoding apparatus includes a decoder 1410, an inverse quantizer 1420, an inverse transformer 1430, a predictor 1440, an adder 1450, a filter 1460, and a memory 1470.
  • the image decoding apparatus may be implemented by each component as a hardware chip, or may be implemented by software and a microprocessor to execute a function of software corresponding to each component.
  • the decoder 1410 decodes a bitstream received from the image encoding apparatus, extracts information related to block division, determines a current block to be decoded, and includes prediction information and information on a residual signal necessary for reconstructing the current block. Extract
  • the decoder 1410 extracts information on the CTU size from a Sequence Parameter Set (SPS) or a Picture Parameter Set (PPS) to determine the size of the CTU, and divides the picture into a CTU of the determined size.
  • the CTU is determined as the highest layer of the tree structure, that is, the root node, and the CTU is partitioned using the tree structure by extracting partition information about the CTU. For example, when splitting a CTU using a QTBT structure, first, a first flag (QT_split_flag) related to splitting of QT is extracted, and each node is divided into four nodes of a lower layer. For the node corresponding to the leaf node of the QT, the second flag BT_split_flag and the split type information related to the splitting of the BT are extracted to split the corresponding leaf node into the BT structure.
  • QT_split_flag a first flag related to splitting of QT
  • the decoder 1410 determines the current block to be decoded by splitting the tree structure, the decoder 1410 extracts information about a prediction type indicating whether the current block is intra predicted or inter predicted.
  • the decoder 1410 decodes a syntax element for intra prediction information (intra prediction mode) of the current block and transmits the syntax element to the intra prediction unit 1444.
  • the decoder 1410 extracts information on the quantized transform coefficients of the current block as information on the residual signal.
  • the inverse quantization unit 1420 inversely quantizes the quantized transform coefficients, and the inverse transform unit 1430 inversely transforms the inverse quantized transform coefficients from the frequency domain to the spatial domain to generate a residual block for the current block.
  • the predictor 1440 includes an intra predictor 642 and an inter predictor 644.
  • the intra predictor 1442 is activated when the intra prediction is the prediction type of the current block
  • the inter predictor 1444 is activated when the intra prediction is the prediction type of the current block.
  • the intra prediction unit 1442 determines the intra prediction mode of the current block by using the syntax element for the intra prediction mode extracted from the decoder 1410, and uses the reference pixels around the current block according to the intra prediction mode. Predict the block.
  • the inter prediction unit 1444 determines motion information of the current block by using a syntax element of the inter prediction information extracted from the decoder 1410, and predicts the current block by using the determined motion information.
  • the adder 1450 reconstructs the current block by adding the residual block output from the inverse transformer and the prediction block output from the inter predictor or the intra predictor.
  • the pixels in the reconstructed current block are utilized as reference pixels when intra prediction of a block to be decoded later.
  • the filter unit 1460 deblocks and filters the boundary between the reconstructed blocks in order to remove blocking artifacts that occur due to block-by-block decoding and stores them in the memory 1470.
  • the reconstructed picture is used as a reference picture for inter prediction of a block in a picture to be decoded later.
  • the syntax element for the intra prediction information (intra prediction mode) decoded by the decoder 1410 may predict the luminance intra mode information for predicting the luminance block composed of the luminance component of the current block and the color difference block composed of the chrominance component. Color difference intra mode information is included.
  • the intra predictor 1442 predicts the luminance block by using the luminance intra mode information.
  • the luminance intra mode information includes information indicating a group to which an intra prediction mode of the luminance block belongs and an index indicating an intra prediction mode of the luminance block among intra prediction modes in the group.
  • the intra prediction unit 1442 derives the intra prediction mode candidates constituting the group in the same manner as the image encoding apparatus according to the information indicating the group to which the intra prediction mode of the luminance block belongs. If the information indicating a group indicates an MPM group, an MPM group is created. If a group indicates a selected mode group, a selected mode group is created. The candidate identified by the index in the corresponding group is set to the intra prediction mode of the luminance block, and the luminance block is predicted using the set intra prediction mode. The predicted luminance block is added with the residual block for the luminance component to restore the luminance component of the current block.
  • the intra predictor 1442 predicts a color difference block using color difference intra mode information.
  • the intra predictor 1442 configures a candidate set for the intra prediction mode of the chrominance block in the same manner as the image encoding apparatus 200. That is, the candidate set is configured by using at least one of DM, LM, neighboring mode, and default mode. Since the method of deriving each type of mode has already been described in the image encoding apparatus, the detailed description thereof will be omitted in order to avoid duplication.
  • the intra prediction unit 1442 selects one candidate among the candidates in the candidate set using the color difference intra mode information as the intra prediction mode of the color difference block, and predicts the color difference block using the selected intra prediction mode.
  • the color difference intra mode information decoded from the decoder 1410 is total m.
  • Information indicating the intra prediction mode of the color difference block among the candidates is included.
  • the intra prediction unit 1442 configures a candidate set by sequentially selecting m candidates in the order of DM, LM, neighboring mode, default mode, or LM, DM, neighboring mode, and default mode.
  • the candidate indicated by the chrominance intra mode information among the m candidates in the candidate set is determined as the intra prediction mode of the chrominance block.
  • the chrominance intra mode information may include information indicating whether the intra prediction mode of the chrominance block is LM, and, if not, the predetermined number of normal modes derived from DM, neighboring mode, and default mode. ) May include information indicating the intra prediction mode of the chrominance block.
  • the intra prediction unit 1442 selects the LM as the intra prediction mode of the color difference block.
  • the intra prediction unit 1442 configures the candidate set by selecting a predetermined number of normal modes sequentially from the DM, neighboring mode, and default mode.
  • the intra prediction mode of the color difference block is selected from candidates in the candidate set according to the information indicating the intra prediction mode of the color difference block.
  • the chrominance intra mode information includes information (first index) indicating the intra prediction mode of the chrominance block among the predetermined number of DMs. It may include information (second index) indicating an intra prediction mode of the chrominance block among a predetermined number of modes derived from the default mode.
  • first index indicating the intra prediction mode of the chrominance block among the predetermined number of DMs.
  • second index indicating an intra prediction mode of the chrominance block among a predetermined number of modes derived from the default mode.
  • the intra prediction unit 1442 configures the candidate set by selecting a predetermined number of normal modes sequentially from the LM, neighboring mode, and default mode.
  • the intra prediction mode of the color difference block is selected from the candidates in the candidate set by the second index.

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Abstract

La présente invention concerne un procédé de codage d'informations pour un mode de prédiction intra d'un bloc chroma, comprenant les étapes consistant à : configurer un ensemble de candidats comprenant une pluralité de candidats pour le mode de prédiction intra du bloc chroma, la pluralité de candidats comprenant au moins un mode de prédiction intra induit à partir d'un bloc luma correspondant au bloc chroma ; et coder des informations de mode intra de chroma pour indiquer le mode de prédiction intra du bloc chroma parmi la pluralité de candidats appartenant à l'ensemble de candidats. L'invention concerne également une technique de sélection d'un ou de plusieurs candidats à partir de modes de prédiction intra d'au moins certains sous-blocs parmi une pluralité de sous-blocs lorsque le bloc luma est divisé en multiples sous-blocs.
PCT/KR2017/015460 2016-12-26 2017-12-26 Codage et décodage d'image à l'aide d'une prédiction intra Ceased WO2018124686A1 (fr)

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