Title of Invention: Video signal processing method and apparatus
technical field
[One]
The present disclosure relates to a video signal processing method and apparatus.
background
[2]
Recently, the demand for high-resolution and high-quality images such as HD (High Definition) images and UHD (Ultra High Definition) images is increasing in various application fields. As the image data becomes higher resolution and higher quality, the amount of data relatively increases compared to the existing image data. The storage cost will increase. High-efficiency image compression techniques can be used to solve these problems that occur as image data becomes high-resolution and high-quality.
[3]
Inter-screen prediction technology that predicts pixel values ​​included in the current picture from pictures before or after the current picture with image compression technology, intra-picture prediction technology that predicts pixel values ​​included in the current picture using pixel information in the current picture, Various techniques exist, such as entropy encoding technology in which a short code is assigned to a value with a high frequency of occurrence and a long code is assigned to a value with a low frequency of occurrence.
[4]
Meanwhile, as the demand for high-resolution images increases, the demand for stereoscopic image content as a new image service is also increasing. A video compression technique for effectively providing high-resolution and ultra-high-resolution stereoscopic image content is being discussed.
DETAILED DESCRIPTION OF THE INVENTION
technical challenge
[5]
An object of the present disclosure is to provide a method and apparatus for intra prediction in encoding/decoding a video signal.
[6]
An object of the present disclosure is to provide a method and apparatus for predicting a chroma component using a luma component reconstruction sample in encoding/decoding a video signal.
[7]
The technical problems to be achieved in the present disclosure are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those of ordinary skill in the art to which the present disclosure belongs from the description below. will be able
means of solving the problem
[8]
A video signal decoding method according to the present disclosure includes determining whether a cross-compoenent linear model (CCLM) mode is applied to a chroma block, and when it is determined that the CCLM mode is determined for the chroma block, adjacent to the chroma block obtaining a filtered neighboring luma sample for a neighboring chroma sample of generating a prediction block for
[9]
The video signal encoding method according to the present disclosure includes the steps of determining whether a cross-compoenent linear model (CCLM) mode is applied to a chroma block, and when it is determined that the CCLM mode is determined for the chroma block, adjacent to the chroma block obtaining a filtered neighboring luma sample for a neighboring chroma sample of generating a prediction block for
[10]
In the video signal decoding method according to the present disclosure, in the filtered neighboring luma sample, a down-sampling filter is applied to a collocated luma sample corresponding to the neighboring chroma sample and neighboring luma samples adjacent to the collocated luma sample. It can be created by applying
[11]
In the video signal decoding method according to the present disclosure, when at least one of the collocated luma sample and the neighboring luma samples is unavailable, a reconstructed sample located at a boundary within a luma block may be padded at an unavailable sample location. have.
[12]
In the video signal decoding method according to the present disclosure, the type of the down-sampling filter may be determined based on the type of the current image.
[13]
In the video signal decoding method according to the present disclosure, the type of the down-sampling filter may be determined based on the position of the neighboring chroma sample.
[14]
In the video signal decoding method according to the present disclosure, the neighboring chroma sample may be extracted by subsampling a plurality of neighboring chroma samples neighboring the chroma block.
[15]
In the video signal decoding method according to the present disclosure, the sub-sampling rate may be determined based on at least one of a size and a shape of the chroma block.
[16]
The features briefly summarized above with respect to the present disclosure are merely exemplary aspects of the detailed description of the present invention that follows, and do not limit the scope of the present disclosure.
Effects of the Invention
[17]
According to the present disclosure, encoding/decoding efficiency can be improved by predicting a chroma sample using a luma reconstruction sample.
[18]
According to the present disclosure, coding/decoding efficiency of the CCLM mode can be improved by determining the down-sampling filter type regardless of the availability of neighboring samples.
[19]
According to the present disclosure, coding/decoding efficiency of the CCLM mode can be improved by subsampling neighboring samples to derive a CCLM parameter.
[20]
Effects obtainable in the present disclosure are not limited to the above-mentioned effects, and other effects not mentioned may be clearly understood by those of ordinary skill in the art to which the present disclosure belongs from the description below. will be.
Brief description of the drawing
[21]
1 is a block diagram illustrating an image encoding apparatus according to an embodiment of the present disclosure.
[22]
2 is a block diagram illustrating an image decoding apparatus according to an embodiment of the present disclosure.
[23]
3 is a flowchart illustrating an intra prediction method according to an embodiment of the present disclosure.
[24]
4 illustrates types of intra prediction modes.
[25]
5 is a diagram for explaining an example of deriving a prediction sample under a planar mode.
[26]
6 illustrates a plurality of reference sample set candidates.
[27]
7 is for explaining a method of deriving a prediction sample under DC mode.
[28]
8 is a flowchart illustrating a method of deriving a prediction sample of a chroma component according to an embodiment of the present disclosure.
[29]
9 illustrates a down-sampling filter type for each chroma sample position when the current image is an HDR image.
[30]
10 illustrates a down-sampling filter type for each chroma sample position when the current image is not an HDR image.
[31]
11 and 12 illustrate examples in which a down-sampling filter type is determined regardless of availability of neighboring samples adjacent to a luma block.
[32]
13 illustrates an example in which ranges of reconstructed pixels used to derive a CCLM parameter are set differently according to a CCLM mode type.
[33]
14 illustrates a down-sampling filter type applied to a collocated luma sample of an upper neighboring sample when the current image is not an HDR image.
[34]
15 illustrates a down-sampling filter type applied to a collocated luma sample of an upper neighboring sample when the current image is an HDR image.
[35]
16 shows an example in which a fixed type of filter is applied according to a position of an upper neighboring sample.
[36]
17 illustrates a down-sampling filter type applied to a collocated luma sample of a left neighboring sample.
Modes for carrying out the invention
[37]
Since the present disclosure can make various changes and can have various embodiments, specific embodiments are illustrated in the drawings and described in detail in the detailed description. However, this is not intended to limit the present disclosure to specific embodiments, and it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present disclosure. In describing each figure, like reference numerals have been used for like elements.
[38]
Terms such as first, second, etc. may be used to describe various elements, but the elements should not be limited by the terms. The above terms are used only for the purpose of distinguishing one component from another. For example, without departing from the scope of the present disclosure, a first component may be referred to as a second component, and similarly, a second component may also be referred to as a first component. and/or includes a combination of a plurality of related listed items or any of a plurality of related listed items.
[39]
When a component is referred to as being “connected” or “connected” to another component, it is understood that the other component may be directly connected or connected to the other component, but other components may exist in between. it should be On the other hand, when it is said that a certain element is "directly connected" or "directly connected" to another element, it should be understood that no other element is present in the middle.
[40]
The terms used in the present application are only used to describe specific embodiments, and are not intended to limit the present disclosure. The singular expression includes the plural expression unless the context clearly dictates otherwise. In the present application, terms such as “comprise” or “have” are intended to designate that a feature, number, step, operation, component, part, or combination thereof described in the specification exists, but one or more other features It is to be understood that this does not preclude the possibility of the presence or addition of numbers, steps, operations, components, parts, or combinations thereof.
[41]
Hereinafter, preferred embodiments of the present disclosure will be described in more detail with reference to the accompanying drawings. Hereinafter, the same reference numerals are used for the same components in the drawings, and repeated descriptions of the same components are omitted.
[42]
[43]
1 is a block diagram illustrating an image encoding apparatus according to an embodiment of the present disclosure.
[44]
Referring to FIG. 1 , the image encoding apparatus 100 includes a picture division unit 110 , prediction units 120 and 125 , a transform unit 130 , a quantization unit 135 , a rearrangement unit 160 , and an entropy encoding unit ( 165 ), an inverse quantization unit 140 , an inverse transform unit 145 , a filter unit 150 , and a memory 155 .
[45]
Each of the constituent units shown in FIG. 1 is independently illustrated to represent different characteristic functions in the image encoding apparatus, and does not mean that each constituent unit is composed of separate hardware or one software constituent unit. That is, each component is listed as each component for convenience of description, and at least two components of each component are combined to form one component, or one component can be divided into a plurality of components to perform a function, and each of these components Integrated embodiments and separate embodiments of components are also included in the scope of the present disclosure without departing from the essence of the present disclosure.
[46]
In addition, some components are not essential components to perform an essential function in the present disclosure, but may be optional components for merely improving performance. The present disclosure may be implemented by including only essential components to implement the essence of the present disclosure, except for components used for performance improvement, and a structure including only essential components excluding optional components used for performance improvement Also included in the scope of the present disclosure.
[47]
The picture divider 110 may divide the input picture into at least one processing unit. In this case, the processing unit may be a prediction unit (PU), a transform unit (TU), or a coding unit (CU). The picture splitter 110 divides one picture into a combination of a plurality of coding units, prediction units, and transformation units, and combines one coding unit, prediction unit, and transformation unit based on a predetermined criterion (eg, a cost function). can be selected to encode the picture.
[48]
For example, one picture may be divided into a plurality of coding units. In order to split a coding unit in a picture, a recursive tree structure such as a quad tree structure may be used. A coding in which one image or a largest coding unit is used as a root and is divided into other coding units. A unit may be divided having as many child nodes as the number of divided coding units. A coding unit that is no longer split according to certain restrictions becomes a leaf node. That is, if it is assumed that only square splitting is possible for one coding unit, one coding unit may be split into up to four different coding units.
[49]
Hereinafter, in an embodiment of the present disclosure, a coding unit may be used as a meaning of a unit performing encoding or may be used as a meaning of a unit performing decoding.
[50]
A prediction unit may be split in the form of at least one square or rectangle of the same size within one coding unit, and one prediction unit among the split prediction units within one coding unit is a prediction of another. It may be divided to have a shape and/or size different from that of the unit.
[51]
When a prediction unit for performing intra prediction based on a coding unit is generated, if it is not the minimum coding unit, intra prediction may be performed without dividing the prediction unit into a plurality of prediction units NxN.
[52]
The prediction units 120 and 125 may include an inter prediction unit 120 performing inter prediction and an intra prediction unit 125 performing intra prediction. Whether to use inter prediction or to perform intra prediction for a prediction unit may be determined, and specific information (eg, intra prediction mode, motion vector, reference picture, etc.) according to each prediction method may be determined. In this case, a processing unit in which prediction is performed and a processing unit in which a prediction method and specific content are determined may be different. For example, a prediction method and a prediction mode may be determined in a prediction unit, and prediction may be performed in a transformation unit. A residual value (residual block) between the generated prediction block and the original block may be input to the transform unit 130 . Also, prediction mode information, motion vector information, etc. used for prediction may be encoded by the entropy encoder 165 together with a residual value and transmitted to a decoding apparatus. When a specific encoding mode is used, it is also possible to encode the original block as it is without generating a prediction block through the predictors 120 and 125 and transmit it to the decoder.
[53]
The inter prediction unit 120 may predict a prediction unit based on information on at least one of a picture before or after a picture of the current picture, and in some cases, prediction based on information of a partial region in the current picture for which encoding has been completed Units can also be predicted. The inter prediction unit 120 may include a reference picture interpolator, a motion prediction unit, and a motion compensator.
[54]
The reference picture interpolator may receive reference picture information from the memory 155 and generate pixel information of integer pixels or less in the reference picture. In the case of luminance pixels, a DCT-based 8-tap interpolation filter in which filter coefficients are different to generate pixel information of integer pixels or less in units of 1/4 pixels may be used. In the case of a color difference signal, a DCT-based 4-tap interpolation filter in which filter coefficients are different to generate pixel information of integer pixels or less in units of 1/8 pixels may be used.
[55]
The motion prediction unit may perform motion prediction based on the reference picture interpolated by the reference picture interpolator. As a method for calculating the motion vector, various methods such as Full search-based Block Matching Algorithm (FBMA), Three Step Search (TSS), and New Three-Step Search Algorithm (NTS) may be used. The motion vector may have a motion vector value of 1/2 or 1/4 pixel unit based on the interpolated pixel. The motion prediction unit may predict the current prediction unit by using a different motion prediction method. Various methods, such as a skip method, a merge method, an AMVP (Advanced Motion Vector Prediction) method, an intra block copy method, etc., may be used as the motion prediction method.
[56]
The intra prediction unit 125 may generate a prediction unit based on reference pixel information around the current block, which is pixel information in the current picture. When a neighboring block of the current prediction unit is a block on which inter prediction is performed, and thus a reference pixel is a pixel on which inter prediction is performed, a reference pixel included in the block on which inter prediction is performed is a reference pixel of the block on which intra prediction is performed. information can be used instead. That is, when the reference pixel is not available, the unavailable reference pixel information may be replaced with at least one reference pixel among the available reference pixels.
[57]
In intra prediction, the prediction mode may have a directional prediction mode in which reference pixel information is used according to a prediction direction and a non-directional mode in which directional information is not used when prediction is performed. A mode for predicting luminance information and a mode for predicting chrominance information may be different, and intra prediction mode information used for predicting luminance information or predicted luminance signal information may be utilized to predict chrominance information.
[58]
When intra prediction is performed, if the size of the prediction unit and the size of the transformation unit are the same, intra prediction for the prediction unit based on the pixel present at the left side, the pixel present at the upper left corner, and the pixel present at the upper side of the prediction unit can be performed. However, when the size of the prediction unit is different from the size of the transformation unit when intra prediction is performed, intra prediction may be performed using a reference pixel based on the transformation unit. In addition, intra prediction using NxN splitting may be used only for the smallest coding unit.
[59]
The intra prediction method may generate a prediction block after applying an adaptive intra smoothing (AIS) filter to a reference pixel according to a prediction mode. The type of AIS filter applied to the reference pixel may be different. In order to perform the intra prediction method, the intra prediction mode of the current prediction unit may be predicted from the intra prediction mode of the prediction unit existing around the current prediction unit. When the prediction mode of the current prediction unit is predicted using mode information predicted from the neighboring prediction unit, if the intra prediction mode of the current prediction unit and the neighboring prediction unit are the same, the current prediction unit and the neighboring prediction unit are used using predetermined flag information It is possible to transmit information indicating that the prediction modes of , and if the prediction modes of the current prediction unit and the neighboring prediction units are different from each other, entropy encoding may be performed to encode prediction mode information of the current block.
[60]
In addition, a residual block including residual information that is a difference value from the original block of the prediction unit and the prediction unit in which prediction is performed based on the prediction unit generated by the prediction units 120 and 125 may be generated. The generated residual block may be input to the transform unit 130 .
[61]
The transform unit 130 converts the original block and the residual block including residual information of the prediction units generated by the prediction units 120 and 125 to DCT (Discrete Cosine Transform), DST (Discrete Sine Transform), KLT and It can be converted using the same conversion method. Whether to apply DCT, DST, or KLT to transform the residual block may be determined based on intra prediction mode information of a prediction unit used to generate the residual block.
[62]
The quantization unit 135 may quantize values ​​transformed in the frequency domain by the transform unit 130 . The quantization coefficient may change according to blocks or the importance of an image. The value calculated by the quantization unit 135 may be provided to the inverse quantization unit 140 and the rearrangement unit 160 .
[63]
The reordering unit 160 may rearrange the coefficient values ​​on the quantized residual values.
[64]
The rearranging unit 160 may change the two-dimensional block form coefficient into a one-dimensional vector form through a coefficient scanning method. For example, the rearranging unit 160 may scan from DC coefficients to coefficients in a high frequency region using a zig-zag scan method and may change it into a one-dimensional vector form. A vertical scan for scanning a two-dimensional block shape coefficient in a column direction and a horizontal scan for scanning a two-dimensional block shape coefficient in a row direction may be used instead of the zig-zag scan according to the size of the transform unit and the intra prediction mode. That is, it may be determined whether any of the zig-zag scan, the vertical scan, and the horizontal scan is used according to the size of the transform unit and the intra prediction mode.
[65]
The entropy encoding unit 165 may perform entropy encoding based on the values ​​calculated by the reordering unit 160 . For entropy encoding, various encoding methods such as Exponential Golomb, Context-Adaptive Variable Length Coding (CAVLC), and Context-Adaptive Binary Arithmetic Coding (CABAC) may be used.
[66]
The entropy encoding unit 165 receives the residual value coefficient information and block type information, prediction mode information, division unit information, prediction unit information and transmission unit information, motion of the coding unit from the reordering unit 160 and the prediction units 120 and 125 . Various information such as vector information, reference frame information, interpolation information of a block, and filtering information may be encoded.
[67]
The entropy encoder 165 may entropy-encode the coefficient values ​​of the coding units input from the reordering unit 160 .
[68]
The inverse quantizer 140 and the inverse transform unit 145 inversely quantize the values ​​quantized by the quantizer 135 and inversely transform the values ​​transformed by the transform unit 130 . The residual values ​​generated by the inverse quantizer 140 and the inverse transform unit 145 are combined with the prediction units predicted through the motion estimation unit, the motion compensator, and the intra prediction unit included in the prediction units 120 and 125 and restored. You can create a Reconstructed Block.
[69]
The filter unit 150 may include at least one of a deblocking filter, an offset correcting unit, and an adaptive loop filter (ALF).
[70]
The deblocking filter may remove block distortion caused by the boundary between blocks in the reconstructed picture. In order to determine whether to perform deblocking, it may be determined whether to apply the deblocking filter to the current block based on pixels included in several columns or rows included in the block. When a deblocking filter is applied to a block, a strong filter or a weak filter can be applied according to the required deblocking filtering strength. In addition, in applying the deblocking filter, horizontal filtering and vertical filtering may be concurrently processed when performing vertical filtering and horizontal filtering.
[71]
The offset correcting unit may correct an offset from the original image in units of pixels with respect to the image on which the deblocking has been performed. In order to perform offset correction on a specific picture, a method of dividing pixels included in an image into a certain number of regions, determining the region to be offset and applying the offset to the region, or taking edge information of each pixel into consideration can be used to apply
[72]
Adaptive loop filtering (ALF) may be performed based on a value obtained by comparing the filtered reconstructed image and the original image. After dividing pixels included in an image into a predetermined group, one filter to be applied to the corresponding group is determined, and filtering can be performed differentially for each group. As for information on whether to apply ALF, the luminance signal may be transmitted for each coding unit (CU), and the shape and filter coefficients of the ALF filter to be applied may vary according to each block. Also, the ALF filter of the same type (fixed type) may be applied regardless of the characteristics of the block to be applied.
[73]
The memory 155 may store the reconstructed block or picture calculated through the filter unit 150 , and the stored reconstructed block or picture may be provided to the predictors 120 and 125 when inter prediction is performed.
[74]
[75]
2 is a block diagram illustrating an image decoding apparatus according to an embodiment of the present disclosure.
[76]
2, the image decoding apparatus 200 includes an entropy decoding unit 210, a reordering unit 215, an inverse quantization unit 220, an inverse transform unit 225, prediction units 230 and 235, and a filter unit ( 240) and a memory 245 may be included.
[77]
When an image bitstream is input from the image encoding apparatus, the input bitstream may be decoded by a procedure reverse to that of the image encoding apparatus.
[78]
The entropy decoding unit 210 may perform entropy decoding in a procedure opposite to that performed by the entropy encoding unit of the image encoding apparatus. For example, various methods such as Exponential Golomb, Context-Adaptive Variable Length Coding (CAVLC), and Context-Adaptive Binary Arithmetic Coding (CABAC) may be applied in correspondence to the method performed by the image encoding apparatus.
[79]
The entropy decoding unit 210 may decode information related to intra prediction and inter prediction performed by the encoding apparatus.
[80]
The reordering unit 215 may perform rearrangement based on a method of rearranging the entropy-decoded bitstream by the entropy decoding unit 210 by the encoder. Coefficients expressed in a one-dimensional vector form may be restored and rearranged in a two-dimensional block form. The reordering unit 215 may receive information related to coefficient scanning performed by the encoder and perform reordering by performing a reverse scanning method based on the scanning order performed by the corresponding encoder.
[81]
The inverse quantizer 220 may perform inverse quantization based on the quantization parameter provided by the encoding apparatus and the reordered coefficient values ​​of the blocks.
[82]
The inverse transform unit 225 may perform inverse transforms, ie, inverse DCT, inverse DST, and inverse KLT, on the transforms performed by the transform unit, ie, DCT, DST, and KLT, on the quantization result performed by the image encoding apparatus. Inverse transform may be performed based on a transmission unit determined by the image encoding apparatus. The inverse transform unit 225 of the image decoding apparatus may selectively perform a transformation technique (eg, DCT, DST, KLT) according to a plurality of pieces of information such as a prediction method, a size of a current block, and a prediction direction.
[83]
The prediction units 230 and 235 may generate a prediction block based on the prediction block generation related information provided from the entropy decoding unit 210 and previously decoded block or picture information provided from the memory 245 .
[84]
As described above, when intra prediction is performed in the same manner as in the operation in the image encoding apparatus, when the size of the prediction unit and the size of the transformation unit are the same, the pixel present at the left side of the prediction unit, the pixel present at the upper left side, and the upper side Intra prediction is performed on the prediction unit based on existing pixels, but when the size of the prediction unit is different from the size of the transformation unit when intra prediction is performed, intra prediction is performed using the reference pixel based on the transformation unit can do. In addition, intra prediction using NxN splitting may be used only for the smallest coding unit.
[85]
The prediction units 230 and 235 may include a prediction unit determiner, an inter prediction unit, and an intra prediction unit. The prediction unit determiner receives various information such as prediction unit information input from the entropy decoder 210, prediction mode information of the intra prediction method, and motion prediction related information of the inter prediction method, and divides the prediction unit from the current coding unit, and predicts It may be determined whether the unit performs inter prediction or intra prediction. The inter prediction unit 230 uses information required for inter prediction of the current prediction unit provided from the image encoding apparatus based on information included in at least one picture before or after the current picture including the current prediction unit. Inter prediction may be performed on the prediction unit. Alternatively, inter prediction may be performed based on information on a pre-restored partial region in the current picture including the current prediction unit.
[86]
In order to perform inter prediction, a motion prediction method of a prediction unit included in a corresponding coding unit based on a coding unit is selected from among skip mode, merge mode, AMVP mode, and intra block copy mode. You can decide which way to go.
[87]
The intra prediction unit 235 may generate a prediction block based on pixel information in the current picture. When the prediction unit is a prediction unit on which intra prediction is performed, intra prediction may be performed based on intra prediction mode information of the prediction unit provided by the image encoding apparatus. The intra prediction unit 235 may include an adaptive intra smoothing (AIS) filter, a reference pixel interpolator, and a DC filter. The AIS filter is a part that performs filtering on the reference pixel of the current block, and may be applied by determining whether to apply the filter according to the prediction mode of the current prediction unit. AIS filtering may be performed on the reference pixel of the current block by using the prediction mode and AIS filter information of the prediction unit provided by the image encoding apparatus. When the prediction mode of the current block is a mode in which AIS filtering is not performed, the AIS filter may not be applied.
[88]
When the prediction mode of the prediction unit is a prediction unit that performs intra prediction based on a pixel value obtained by interpolating the reference pixel, the reference pixel interpolator may interpolate the reference pixel to generate a reference pixel of a pixel unit having an integer value or less. When the prediction mode of the current prediction unit is a prediction mode that generates a prediction block without interpolating the reference pixel, the reference pixel may not be interpolated. The DC filter may generate the prediction block through filtering when the prediction mode of the current block is the DC mode.
[89]
The reconstructed block or picture may be provided to the filter unit 240 . The filter unit 240 may include a deblocking filter, an offset correcting unit, and an ALF.
[90]
Information on whether a deblocking filter is applied to a corresponding block or picture and information on whether a strong filter or a weak filter is applied when the deblocking filter is applied may be provided from the image encoding apparatus. The deblocking filter of the image decoding apparatus may receive deblocking filter related information provided from the image encoding apparatus, and the image decoding apparatus may perform deblocking filtering on the corresponding block.
[91]
The offset correction unit may perform offset correction on the reconstructed image based on the type of offset correction applied to the image during encoding and information on the offset value.
[92]
ALF may be applied to a coding unit based on information on whether ALF is applied, ALF coefficient information, etc. provided from the encoding apparatus. Such ALF information may be provided by being included in a specific parameter set.
[93]
The memory 245 may store the reconstructed picture or block to be used as a reference picture or reference block, and may also provide the reconstructed picture to an output unit.
[94]
As described above, hereinafter, in the embodiments of the present disclosure, a coding unit is used as a term for a coding unit for convenience of description, but may also be a unit for performing decoding as well as coding.
[95]
In addition, the current block denotes an encoding/decoding target block, and depending on the encoding/decoding step, a coding tree block (or coding tree unit), a coding block (or a coding unit), a transform block (or a transform unit), or a prediction block (or prediction unit) and the like. In this specification, a 'unit' may indicate a basic unit for performing a specific encoding/decoding process, and a 'block' may indicate a pixel array of a predetermined size. Unless otherwise specified, 'block' and 'unit' may be used interchangeably. For example, in the embodiments to be described later, it may be understood that the coding block (coding block) and the coding unit (coding unit) have the same meaning.
[96]
[97]
3 is a flowchart illustrating an intra prediction method according to an embodiment of the present disclosure.
[98]
Referring to FIG. 3 , the index of the reference sample line of the current block may be determined ( S301 ). The index may specify one of a plurality of reference sample line candidates. The plurality of reference sample line candidates may include an adjacent reference sample line adjacent to the current block and at least one non-adjacent reference sample line not adjacent to the current block.
[99]
As an example, an adjacent reference sample line composed of an adjacent row having a y-axis coordinate smaller by 1 than the top row of the current block and an adjacent column having an x-axis coordinate smaller by 1 than the leftmost column of the current block may be used as a reference sample line candidate. .
[100]
A first non-adjacent reference sample line including a non-adjacent row whose y-axis coordinate is 2 less than the top row of the current block and a non-adjacent column whose x-axis coordinate is 2 less than the leftmost column of the current block is selected as a reference sample line candidate. can be used
[101]
A second non-adjacent reference sample line including a non-adjacent row whose y-axis coordinate is 3 less than the top row of the current block and a non-adjacent column whose x-axis coordinate is 3 less than the leftmost column of the current block is selected as a reference sample line candidate. can be used
[102]
The index may indicate one of an adjacent reference sample line, a first non-adjacent reference sample line, or a second non-adjacent reference sample line. For example, an index of 0 means that the adjacent reference sample line is selected, an index of 1 means that the first non-adjacent reference sample line is selected, and an index of 2 means that the second non-adjacent reference sample line is selected. means selected.
[103]
An index specifying one of the plurality of reference sample line candidates may be signaled through the bitstream.
[104]
Alternatively, the index may be signaled for the luma component block, and the signaling of the index may be omitted for the chroma component block. When the signaling of the index is omitted, the index may be regarded as 0. That is, with respect to the chroma component block, intra prediction may be performed using an adjacent reference sample line.
[105]
Reconstructed samples included in the selected reference sample line may be derived as reference samples.
[106]
Next, an intra prediction mode of the current block may be determined (S302).
[107]
4 illustrates types of intra prediction modes. As in the example shown in FIG. 4 , the intra prediction modes include a non-directional prediction mode (DC and Planar) and a directional prediction mode. In FIG. 4 , it is exemplified that 65 directional prediction modes are defined.
[108]
A flag indicating whether the intra prediction mode of the current block is the same as the Most Probable Mode (MPM) may be signaled through the bitstream. As an example, the value of the MPM flag being 1 indicates that the same MPM as the intra prediction mode of the current block exists. On the other hand, the value of the MPM flag being 0 indicates that the same MPM as the intra prediction mode of the current block does not exist.
[109]
When the value of the MPM flag is 1, a flag indicating whether the intra prediction mode of the current block is the same as the default intra prediction mode may be signaled. The default intra prediction mode may be at least one of DC, a planar, a vertical prediction mode, and a horizontal prediction mode. As an example, a flag intra_not_planar_flag indicating whether the intra prediction mode of the current block is the planar mode may be signaled. A value of the flag intra_not_planar_flag of 0 indicates that the intra prediction mode of the current block is planar. On the other hand, the value of the flag intra_not_planar_flag equal to 1 indicates that the intra prediction mode of the current block is not the planner. When the value of the flag intra_not_planar_flag is 1, an index specifying one of the MPM candidates may be signaled. The intra prediction mode of the current block may be set to be the same as the MPM indicated by the MPM index.
[110]
A prediction sample may be derived based on the reference samples belonging to the reference sample line and the intra prediction mode (S303).
[111]
When the intra prediction mode of the current block is the directional prediction mode, a prediction sample may be derived using a reference sample positioned on a line along the angle of the directional prediction mode.
[112]
When the intra prediction mode of the current block is the planar mode, a prediction sample may be derived using a reference sample positioned in a vertical direction and a reference sample positioned in a horizontal direction of the prediction target sample.
[113]
5 is a diagram for explaining an example of deriving a prediction sample under a planar mode.
[114]
In FIG. 5 , T denotes a reference sample adjacent to the upper right corner of the current block, and L denotes a reference sample adjacent to the lower left corner of the current block.
[115]
Under the planar mode, a horizontal direction prediction sample P1 and a vertical direction prediction sample P2 may be derived with respect to a prediction target sample.
[116]
The horizontal prediction sample P1 may be generated by linearly interpolating the reference sample H and the upper right reference sample T positioned on the same horizontal line as the prediction target sample.
[117]
The vertical direction prediction sample P2 may be generated by linearly interpolating the reference sample V and the lower left reference sample L positioned on the same vertical line as the prediction target sample.
[118]
Thereafter, a prediction sample may be derived based on a weighted sum operation of the horizontal direction prediction sample P1 and the vertical direction prediction sample P2. Equation 1 shows an example in which the prediction sample P is derived by the weighted sum operation of the horizontal direction prediction sample P1 and the vertical direction prediction sample P2.
[119]
[Formula 1]

[120]
In Equation 1, α denotes a weight applied to the horizontal direction prediction sample P1, and β denotes a weight applied to the vertical direction prediction sample P2.
[121]
The weights α and β may be determined based on the size or shape of the current block. Specifically, weights α and β may be determined in consideration of at least one of a width or a height of the current block. For example, when the width and height of the current block are the same, the weights α and β may be set to the same value. When the weights α and β are the same, the prediction sample may be derived as an average value of the horizontal direction prediction sample P1 and the vertical direction prediction sample P2. On the other hand, when the width and height of the current block are different, the weights α and β may be set differently. For example, when the width of the current block is greater than the height, the weight β can be set to a value greater than the weight α, and when the height of the current block is greater than the width, the weight α can be set to a value greater than the weight β. have. Or, conversely, if the width of the current block is greater than the height, set the weight α to a value greater than the weight β, and if the height of the current block is greater than the width, set the weight β to a value greater than the weight α. can be set.
[122]
As another example, weights α and β may be derived from one of a plurality of weight set candidates. For example, when weight candidate sets (1, 1), (3, 1), and (1, 3) representing a combination of weights α and β are predefined, weights α and β are one of the weight candidate sets. can be selected in the same way as
[123]
An index indicating one of the plurality of weight set candidates may be signaled through the bitstream. The index may be signaled at the block level. As an example, the index may be signaled in units of coding blocks or transform blocks.
[124]
Alternatively, the index may be signaled at the coding tree unit, slice, picture, or sequence level. Blocks included in the index transmission unit may determine weights α and β with reference to an index signaled at a higher level. That is, for blocks included in the index transmission unit, the weights α and β may be identically set.
[125]
In the example of FIG. 5 , it is shown that the upper right reference sample T is used to derive the horizontal direction prediction sample P1, and the lower left reference sample L is used to derive the vertical direction prediction sample P2.
[126]
A horizontal direction prediction sample P1 may be derived using a reference sample other than the upper right reference sample, or a vertical direction prediction sample P2 may be derived using a reference sample other than the lower left reference sample. As an example, reference sample set candidates for the first reference sample used to derive the horizontal direction prediction sample P1 and the second reference sample used to derive the vertical direction prediction sample P2 are configured, and selected from among the plurality of reference sample set candidates Using one, the horizontal direction prediction sample P1 and the vertical direction prediction sample P2 can be derived.
[127]
An index identifying one of the plurality of reference sample set candidates may be signaled through the bitstream. The index may be signaled in units of blocks, sub-blocks, or samples.
[128]
Alternatively, a reference sample set candidate may be selected based on the location of the prediction target sample.
[129]
6 illustrates a plurality of reference sample set candidates.
[130]
In the example shown in FIG. 6 , the notation of (y, x) indicates a combination of the y coordinate and the x coordinate of each sample. For example, (2, 1) represents a sample having a y-coordinate of 2 and an x-coordinate of 1.
[131]
In the example of FIG. 6 , the first reference sample set candidate may include a reference sample T1 adjacent to an upper-right corner of the current block and a reference sample L1 adjacent to an upper-left corner of the current block. When the position of the upper left sample of the current block is (0, 0), T1 indicates a reference sample of (-1, W) coordinates, and L1 indicates a reference sample of (H, -1) coordinates. Here, W and H represent the width and height of the current block, respectively.
[132]
The second reference sample set candidate may include a reference sample T2 adjacent to the top of T1 and a reference sample L2 adjacent to the left of L1. For example, T2 denotes a reference sample of (-2, W) coordinates, and L2 denotes a reference sample of (H, -2) coordinates.
[133]
The third reference sample set candidate may include a reference sample T3 adjacent to the top of T2 and a reference sample L3 adjacent to the left of L2. For example, T3 denotes a reference sample of (-3, W) coordinates, and L3 denotes a reference sample of (H, -3) coordinates.
[134]
The fourth reference sample set candidate may be composed of a reference sample T4 adjacent to the top of T3 and a reference sample L4 adjacent to the left of L3. For example, T4 denotes a reference sample of (-4, W) coordinates, and L4 denotes a reference sample of (H, -4) coordinates.
[135]
Reference sample set candidates are not limited to the illustrated examples. As an example, in the example of FIG. 6 , a combination of a reference sample having an x-axis coordinate of W and a reference sample having a y-axis coordinate of H of the current block is set as a reference sample set candidate. However, unlike the illustrated example, a reference sample having an x-axis coordinate of W/2 or (W/2)-1 or a reference sample having a y-axis coordinate of H/2 or (H/2)-1 is a reference sample set Candidates may be formed.
[136]
The reference sample set may be adaptively selected according to at least one of the position or size of the current block. For example, when the current block is a square having the same width and height, a reference sample set including reference samples (-1, W) and reference samples (H, -1) may be used. When the current block is a non-square having a width greater than a height, a reference sample set composed of a reference sample (-1, W/2) and a reference sample (H, -1) may be used. If the current block has a non-square shape with a height greater than a width, a reference sample set composed of a reference sample (-1, W) and a reference sample (H/2, -1) may be used.
[137]
When the intra prediction mode of the current block is DC, a prediction sample may be derived based on an average value of reference samples.
[138]
7 is for explaining a method of deriving a prediction sample under DC mode.
[139]
An average value of reference samples adjacent to the current block may be calculated, and an average value calculated for all samples in the current block may be set as a prediction value.
[140]
The average value may be derived based on top reference samples adjacent to the top of the current block and left reference samples adjacent to the left side of the current block.
[141]
Alternatively, according to the shape of the current block, an average value may be derived using only the upper reference samples or only the left reference samples. For example, when the current block is a square block, an average value may be derived using upper reference samples and left reference samples. When the width of the current block is greater than the height or the ratio of the width to the height is greater than (or less than) a predefined value, the average value may be derived using only the upper reference samples. When the height of the current block is greater than the width or when the ratio of the width to the height is greater than (or less than) a predefined value, an average value can be derived using only the left reference samples.
[142]
In derivation of the mean value, it is possible to exclude unusual reference samples. For example, when the average of the reconstructed pixels is m and the standard deviation is σ, a reference sample that does not deviate from the range of k times the standard deviation from the average value is used for the average value calculation, and the reference samples that do not are excluded from the average value calculation have. Here, k is a natural number and may have a value of 1, 2, 3, 4, or the like. The value of k may be predefined in the encoder and decoder. Alternatively, the value of k may be determined based on at least one of the size and shape of the block. Alternatively, information indicating the value of k may be signaled through a bitstream.
[143]
Instead of the standard deviation σ, an arbitrary threshold may be set to determine whether to use a reference sample. For example, a reference sample having an absolute value of a difference from the average value equal to or less than a threshold value may be set to be usable when the average value is derived. On the other hand, a reference sample in which the absolute value of the difference from the average value is greater than the threshold value may be set to be unavailable when the average value is derived. The threshold value may be predefined in the encoder and decoder. Alternatively, the threshold value may be determined based on at least one of the size and shape of the block. Alternatively, information indicating a threshold may be signaled through a bitstream.
[144]
In order to reduce the complexity of calculating the average value, reference samples may be subsampled and the average value may be calculated using the subsampled reference samples. For example, when sub-sampling is performed at an interval of two samples, upper reference samples located at (-1, 2m) coordinates among the upper reference samples are used to derive an average value, or (-1, 2m+1) coordinates The upper reference samples located in ? can be used to derive the average value. Also, among the left reference samples, left reference samples located at (2n, -1) coordinates are used to derive an average value, or left reference samples located at (2n+1, -1) coordinates are used to derive an average value. can Here, m is a natural number from 0 to (W/2)-1, and n is a natural number from 0 to (H/2)-1. Based on the sub-sampling rate, ranges of m and n may be determined. The sub-sampling rate may be adaptively determined according to the size or shape of the current block.
[145]
In subsampling, reference samples may be selected at fixed intervals. In this case, the value indicating the interval between the reference samples may be predefined in the encoder and the decoder. Alternatively, an interval between reference samples may be adaptively determined based on at least one of a size or a shape of the current block. Alternatively, an interval between reference samples may be determined based on index information specifying one of a plurality of candidates.
[146]
As another example, after configuring a plurality of set candidates with respect to reference samples, an average value may be derived based on a selected one of the plurality of set candidates.
[147]
As an example, the first set candidate may include all upper reference samples touching the upper boundary of the current block and all left reference samples touching the left boundary of the current block.
[148]
The second set candidate may include upper reference samples at a (-1, 2m) position among upper reference samples of the current block and left reference samples at a (2n, -1) position among left reference samples of the current block. have.
[149]
The third set candidate is the top reference samples at the (-1, 2m+1) position among the top reference samples of the current block and the left reference sample at the (2n+1, -1) position among the left reference samples of the current block. may include
[150]
The number and types of set candidates are not limited to the above-described example. It is also possible to define a larger number or a smaller number of set candidates than in the above-described example.
[151]
The encoder may generate a prediction block for each set candidate, measure a cost for each prediction block, and determine an optimal set candidate. And, an index specifying an optimal set candidate may be encoded and signaled through a bitstream.
[152]
Alternatively, an optimal set candidate may be determined based on at least one of a size or a shape of the current block.
[153]
As another example, one of a set candidate composed of reference samples before subsampling is performed (eg, at least one of left reference samples and upper reference samples) and a set candidate composed of reference samples on which subsampling is performed is optimally selected may be selected as a set candidate of
[154]
[155]
The intra prediction mode of the chroma component may be determined based on the intra prediction mode of the luma component. Specifically, the intra prediction mode of the chroma component is determined with reference to the intra prediction mode of the luma component, but a method of determining the intra prediction mode of the chroma component may be different depending on the chroma mode.
[156]
The chroma mode includes at least one of a DC mode, a planar mode, a VER mode, a HOR mode, and a DM mode. Table 1 shows a method of inducing an intra prediction mode of a chroma component according to a chroma mode.
[157]
[Table 1]
color difference mode index Luminance In-Screen Prediction Mode
0 50 18 One X ( 0 <= X <= 66 )
0 (Planar mode) 66 0 0 0 0
1 (VER mode) 50 66 50 50 50
2 (HOR mode) 18 18 66 18 18
3 (DC mode) One One One 66 One
4 (DM mode) 0 50 18 One X
[158]
Index information for specifying the chroma mode may be signaled through a bitstream. For example, a chroma mode index indicating one of DC mode, planar mode, VER mode, HOR mode, and DM mode may be signaled through the bitstream.
[159]
Referring to Table 1, when the chroma mode is the planar mode (eg, when the color difference mode index is 0), the intra prediction mode of the chroma component is the planar mode, except when the intra prediction mode of the luma component is 0 (planar). mode can be set.
[160]
When the chroma mode is the VER mode (eg, when the color difference mode index is 1), the intra prediction mode of the chroma component can be set in the vertical direction, except when the intra prediction mode of the luma component is 50 (vertical direction). have.
[161]
When the chroma mode is the HOR mode (eg, when the color difference mode index is 2), the intra prediction mode of the chroma component can be set in the horizontal direction, except when the intra prediction mode of the luma component is 18 (horizontal direction). have.
[162]
When the chroma mode is the DC mode (eg, when the color difference mode index is 3), the intra prediction mode of the chroma component may be set to DC, except when the intra prediction mode of the luma component is 1 (DC).
[163]
When the chroma mode is the DM mode (eg, when the color difference mode index is 4), the intra prediction mode of the chroma component may be set to be the same as the intra prediction mode of the luma component.
[164]
In the example of Table 1, a cross-component linear mode (CCLM) mode may be additionally defined as a chroma mode. Table 2 shows an example in which the CCLM mode is added as a new chroma mode.
[165]
[Table 2]
color difference mode index Luminance In-Screen Prediction Mode
0 50 18 One X ( 0 <= X <= 66 )
0 (Planar mode) 66 0 0 0 0
1 (VER mode) 50 66 50 50 50
2 (HOR mode) 18 18 66 18 18
3 (DC mode) One One One 66 One
4 81 81 81 81 81
5 82 82 82 82 82
6 83 83 83 83 83
7 (DM mode) 0 50 18 One X
[166]
In Table 2, it is exemplified that the index assigned to the DM mode is changed from No. 4 to No. 7, and CCLM modes are added to No. 4 to No. 6.
[167]
A plurality of CCLM modes may be defined. For example, in Table 2, indices 4 to 6 are the first CCLM mode (LM mode), the second CCLM mode (LM-A (Above) mode), and the third CCLM mode (LM-L (Left) mode) points to
[168]
Based on the chroma mode index, it may be determined whether the chroma mode is the CCLM mode.
[169]
The maximum length of the chroma mode index may be variably determined depending on whether the CCLM mode is enabled. For example, when the CCLM mode is not allowed, the chroma mode index may indicate one of 0 to 4 as illustrated in Table 1. On the other hand, when the CCLM mode is allowed, the chroma mode index may indicate one of 0 to 7, as illustrated in Table 2.
[170]
As another example, a flag indicating whether the chroma mode is the CCLM mode may be signaled. For example, when the flag cclm_mode_flag is 1, the chroma mode indicates the CCLM mode. On the other hand, when the flag cclm_mode_flag is 0, it indicates that the chroma mode is not the CCLM mode.
[171]
When the flag cclm_mode_flag is 0, a chroma mode index specifying one of the remaining chroma modes may be signaled through the bitstream. Whether the CCLM mode is applied is determined by a separate flag, and the maximum length of the chroma mode index may have a fixed value regardless of whether the CCLM mode is allowed.
[172]
When the flag cclm_mode_flag is 1, an index for specifying one of a plurality of CCLM modes may be additionally signaled. For example, according to the index cclm_mode_idx, any one of the LM mode, the LM-A mode, and the LM-L mode may be determined as the chroma mode.
[173]
As another example, based on at least one of the chroma subsampling format, the size and/or shape of the current block, or whether CCLM is applied to a neighboring block, whether to use a flag indicating whether CCLM is applied or not, a plurality of CCLM modes It may be determined whether to use an index specifying one of them, or whether to encode/decode a flag and an index indicating whether CCLM mode is applied before the chroma mode index.
[174]
Under the CCLM mode, a prediction sample of a chroma component may be derived based on a reconstructed luma component sample. Accordingly, by using the CCLM mode, it is possible to remove the redundancy between the luma component sample and the color difference component sample. Equation 2 shows an example of deriving a prediction sample of a chroma component under the CCLM mode.
[175]
[Formula 2]

[176]
In Equation 2, Pred C denotes a prediction sample of a chroma component. Pred L ' represents the reconstructed luma component sample. Also, α and β represent CCLM parameters. Specifically, α denotes a weight and β denotes an offset.
[177]
Hereinafter, a method of deriving a prediction sample of a chroma component under the CCLM mode will be described in detail.
[178]
8 is a flowchart illustrating a method of deriving a prediction sample of a chroma component according to an embodiment of the present disclosure.
[179]
First, it is checked whether the sizes of the luma image and the chroma image are the same (S801). For example, when the chroma sub-sampling format is 4:4:4, the size of the luma image and the chroma image may be determined to be the same. On the other hand, when the chroma sub-sampling format is 4:2:2 or 4:2:0, it may be determined that the sizes of the luma image and the chroma image are different.
[180]
When the sizes of the luma image and the chroma image are different, reconstructed samples included in the luma image may be downsampled (S802). By applying the down-sampling filter to the luma block corresponding to the current chroma block, a filtered luma block having the same size as the current chroma block may be obtained.
[181]
When the size of the luma image and the chroma image are the same, applying the downsampling filter to the luma block may be omitted.
[182]
The down-sampling filter type may be determined based on at least one of a current image type, a CCLM mode type, and a sample position. Different types of filters may differ in at least one of a shape of the filter, a number of taps, or a coefficient. The type of the current image indicates whether the current picture is a high dynamic range (HDR) image. The CCLM mode type indicates one of an LM mode, an LM-A mode, or an LM-L mode.
[183]
Information for determining the current image type may be signaled through a bitstream. For example, a flag indicating whether the position of the chroma component sample is moved relative to the position of the co-located luma sample may be signaled through the bitstream. When the flag is 1, it indicates that the position of the chroma component sample and the position of the collocated luma sample are the same. This indicates that the current image is an HDR image. On the other hand, when the flag is 0, it indicates that the position of the chroma component sample is moved downward by 0.5 pixels relative to the position of the collocated luma sample. This indicates that the current image is not an HDR image.
[184]
A position of a collocated luma sample corresponding to the chroma sample may be determined according to the size of the luma image and the size of the chroma image. As an example, the position of the collocated luma sample corresponding to the chroma sample at the (y, x) position may be determined as (y*subHeightC, x*subWidthC). Here, the variables subWidthC and subHeightC may be determined based on the chroma subsampling format. For example, when the chroma subsampling format is 4:4:4, the variable subWidthC and the variable subHeightC may be set to 1. When the chroma subsampling format is 4:2:2, the variable subWidthC may be set to 2, and the variable subHeightC may be set to 1. When the chroma subsampling format is 4:2:0, the variable subWidthC and the variable subHeightC may be set to 2.
[185]
9 illustrates a down-sampling filter type for each chroma sample position when the current image is an HDR image.
[186]
In the illustrated example, A indicates a sample located at the upper left of the current chroma block. B represents the remaining samples except for the upper left sample A among samples included in the top row of the current chroma block. C represents the remaining samples except for the upper left sample A among samples included in the leftmost column of the current chroma block. D denotes residual samples excluding samples included in the top row and samples included in the leftmost column of the current chroma block.
[187]
The variable AvailL indicates whether the left neighboring samples of the luma block are available. The variable AvailT indicates whether the top neighboring samples of the luma block are available. The variables AvailL and AvailT are based on at least one of a CCLM mode type, whether a neighboring block is coded with intra prediction, whether a luma block and a neighboring block are included in the same coding tree unit, or whether a neighboring block is out of a picture boundary can be determined by
[188]
A cross-shaped down-sampling filter may be applied to the luma sample corresponding to the chroma sample D. Specifically, the down-sampling filter may be applied to the collocated luma sample of the chroma sample D, horizontally neighboring luma samples of the collocated luma sample, and vertically neighboring luma samples of the collocated luma sample. have. In this case, a ratio of the filter coefficient applied to the collocated luma sample to the filter coefficient applied to the neighboring luma sample may be 4:1.
[189]
The down-sampling filter type for the luma sample corresponding to the chroma sample C included in the leftmost column in the chroma block may be determined based on the variable AvailL. For example, when left neighboring samples neighboring the luma block are available, a cross-shaped filter may be applied. When the cross-shaped filter is applied, down-sampling of the collocated luma sample of chroma sample C, horizontally neighboring luma samples of the collocated luma sample, and vertically neighboring luma samples of the collocated luma sample Filters can be applied. In this case, a ratio of the filter coefficient applied to the collocated luma sample to the filter coefficient applied to the neighboring luma sample may be 4:1.
[190]
On the other hand, when left neighboring samples neighboring the luma block are unavailable, a vertical filter may be applied. When the vertical filter is applied, the down-sampling filter may be applied to the collocated luma sample of the chroma sample C and to luma samples adjacent in the vertical direction of the collocated luma sample. In this case, a ratio of the filter coefficient applied to the collocated luma sample and the filter coefficient applied to the neighboring luma sample may be 2:1.
[191]
The down-sampling filter type for the luma sample corresponding to the chroma sample B included in the uppermost row in the chroma block may be determined based on the variable AvailT. For example, when upper neighboring samples neighboring the luma block are available, a cross-shaped filter may be applied. When the cross filter is applied, down-sampling of the collocated luma sample of the chroma sample B, horizontally neighboring luma samples of the collocated luma sample, and vertically neighboring luma samples of the collocated luma sample Filters can be applied. In this case, a ratio of the filter coefficient applied to the collocated luma sample to the filter coefficient applied to the neighboring luma sample may be 4:1.
[192]
On the other hand, when upper neighboring samples neighboring the luma block are unavailable, a horizontal filter may be applied. When the horizontal filter is applied, the down-sampling filter may be applied to the collocated luma sample of the chroma sample C and luma samples adjacent to the collocated luma sample in the horizontal direction. In this case, a ratio of the filter coefficient applied to the collocated luma sample and the filter coefficient applied to the neighboring luma sample may be 2:1.
[193]
The down-sampling filter type for the luma sample corresponding to the upper left chroma sample A in the chroma block may be determined based on the variable AvailL and the variable AvailT. As an example, when both left neighboring samples and upper neighboring samples neighboring the luma block are available, a cross-shaped filter may be applied. When the cross-shaped filter is applied, down-sampling of the collocated luma sample of chroma sample A, horizontally neighboring luma samples of the collocated luma sample, and vertically neighboring luma samples of the collocated luma sample Filters can be applied. In this case, a ratio of the filter coefficient applied to the collocated luma sample to the filter coefficient applied to the neighboring luma sample may be 4:1.
[194]
When the left neighboring samples neighboring the luma block are available while the upper neighboring samples are unavailable, a vertical filter may be applied. When the vertical filter is applied, the down-sampling filter may be applied to the collocated luma sample of chroma sample A and luma samples vertically adjacent to the collocated luma sample. In this case, a ratio of the filter coefficient applied to the collocated luma sample and the filter coefficient applied to the neighboring luma sample may be 2:1.
[195]
When the top neighbor samples neighboring the luma block are available while the left neighbor samples are unavailable, a horizontal filter may be applied. When the horizontal filter is applied, the down-sampling filter may be applied to the collocated luma sample of chroma sample A and luma samples horizontally adjacent to the collocated luma sample. In this case, a ratio of the filter coefficient applied to the collocated luma sample and the filter coefficient applied to the neighboring luma sample may be 2:1.
[196]
When both the top neighbor samples and the left neighbor samples neighboring the luma block are unavailable, the down-sampling filter may not be applied to the collocated luma sample corresponding to the chroma sample A.
[197]
10 illustrates a down-sampling filter type for each chroma sample position when the current image is not an HDR image.
[198]
A 6-tap down-sampling filter may be applied to collocated luma samples corresponding to samples (eg, B and D) included in a residual column other than the leftmost column of the chroma block. Specifically, with the collocated luma sample and the lower neighboring sample located at the bottom of the collocated luma sample as the center, the collocated luma sample and the lower neighboring sample are horizontally adjacent to each other in the horizontal direction. A sampling filter can be applied. In this case, a ratio of the filter coefficient applied to the collocated luma sample and the lower neighboring sample to the filter coefficient applied to the horizontal neighboring samples may be 2:1.
[199]
With respect to the chroma component samples (eg, A and C) included in the leftmost column of the luma block, a down-sampling filter having a different shape may be applied according to the variable AvailL. As an example, when the left neighboring samples of the luma block are available, a 6-tap down-sampling filter may be applied to the collocated luma sample corresponding to the chroma component sample A or C. On the other hand, when the left neighboring samples of the luma block are unavailable, a 2-tap down-sampling filter may be applied to the collocated luma sample corresponding to the chroma component sample A or C. The 2-tap down-sampling filter may be applied to the collocated luma sample and a lower neighboring sample positioned at the bottom of the collocated luma sample. In this case, a ratio of filter coefficients applied to the collocated luma sample and the lower neighboring sample may be 1:1.
[200]
In FIGS. 9 and 10 , the downsampling filter type is determined based on at least one of a variable AvailL indicating whether left neighboring samples of the luma block are available or a variable AvailT indicating whether upper neighboring blocks of the luma block are available was exemplified as
[201]
As another example, the downsampling filter may be determined independently of the variables AvailL and AvailT. Specifically, regardless of the variables AvailL and AvailT, a fixed downsampling filter may be applied.
[202]
11 and 12 illustrate examples in which a down-sampling filter type is determined regardless of availability of neighboring samples adjacent to a luma block. 11 illustrates an application aspect of the down-sampling filter when the current image is an HDR image, and FIG. 12 illustrates an application aspect of the down-sampling filter when the current image is not an HDR image.
[203]
For example, in the example illustrated in FIG. 11 , a down-sampling filter corresponding to each chroma sample may be determined without considering availability of neighboring samples adjacent to the luma block.
[204]
To this end, the down-sampling filter type using neighboring samples adjacent to the luma block may be set to not available. That is, in the examples shown in FIGS. 9 and 10 , the down-sampling filter selected when the variables AvailL and AvailT are false may be fixedly applied.
[205]
For example, when the luma block is an HDR image, a cross-shaped filter may be applied to a collocated luma sample corresponding to a chroma sample D in the chroma block.
[206]
A vertical filter may be applied to the collocated luma sample corresponding to the chroma sample C in the chroma block.
[207]
A horizontal filter may be applied to the collocated luma sample corresponding to the chroma sample B in the chroma block.
[208]
It may be set not to apply the down sampling filter to the collocated luma sample corresponding to the chroma sample A in the chroma block.
[209]
When the luma block is not an HDR image, a 6-tap down-sampling filter may be applied to collocated luma samples corresponding to chroma samples D or B in the chroma block.
[210]
A 2-tap down-sampling filter may be applied to collocated luma samples corresponding to chroma samples A or C in the chroma block.
[211]
Contrary to the examples shown in FIGS. 11 and 12 , a down-sampling filter type using neighboring samples adjacent to the luma block may be fixedly applied to the luma block. That is, in the examples shown in FIGS. 9 and 10 , the down-sampling filter selected when the variables AvailL and AvailT are true may be fixedly applied.
[212]
In this case, when the neighboring sample adjacent to the luma block is unavailable, pixels positioned at the boundary within the luma block may be padded at the unavailable neighboring sample location. That is, the unavailable neighboring samples may be replaced with pixels located at the boundary within the luma block. For example, when left neighboring samples adjacent to the left side of the luma block are unavailable, reconstructed samples included in the leftmost column in the luma block may be padded to the left. Alternatively, when upper neighboring samples adjacent to the upper end of the luma block are unavailable, reconstructed samples included in the uppermost row in the luma block may be padded to the upper end.
[213]
9 to 12 , after classifying each of the chroma samples into one of A to D, a different down-sampling filter type for each category is determined. In this case, it is exemplified that the classification of the chroma samples is performed based on at least one of whether the chroma samples are included in the uppermost row or whether the chroma samples are included in the leftmost column.
[214]
Depending on the block size, a classification criterion for chroma samples may be set differently. For example, when the size of the chroma block is 4x4, the classification criteria shown in FIGS. 9 to 12 may be followed.
[215]
On the other hand, when the size of the chroma block is 16x16 or more, the chroma samples may be classified based on whether they are included in the top two rows of the chroma block or whether they are included in the leftmost two columns of the chroma block. As an example, when the chroma block is 16x16, chroma samples included in the upper left 2x2 region in the chroma block may be classified as A. Also, among the chroma samples included in the top two rows in the chroma block, the remaining chroma samples excluding the chroma samples classified as A may be classified as B. Among the chroma samples included in the leftmost two columns of the chroma block, residual chroma samples excluding the chroma samples classified as A may be classified as C. Residual chroma samples in the chroma block may be classified as D.
[216]
CCLM parameters α and β may be derived based on reconstructed pixels around the chroma block and reconstructed pixels around the luma block ( S803 ). When the sizes of the luma image and the chroma image are different, reconstructed pixels around the luma block may be down-sampled.
[217]
CCLM parameters may be derived based on at least one of top neighbor samples adjacent to the top of the chroma block and the luma block or left neighbor samples adjacent to the left of the chroma block and the luma block.
[218]
According to the CCLM mode, when the CCLM parameters are derived, whether to use the top neighbor samples and the left neighbor samples may be determined. As an example, under the LM mode, CCLM parameters may be derived based on top neighbor samples and left neighbor samples. Under LM-A mode, CCLM parameters can be derived based only on top neighbor samples. Under LM-L mode, CCLM parameters can be derived based only on left neighboring samples.
[219]
Alternatively, the number or range of neighboring reconstructed pixels may be determined based on at least one of the size, shape, current image type, CCLM mode type, and chroma subsampling format of the current block.
[220]
13 illustrates an example in which ranges of reconstructed pixels used to derive a CCLM parameter are set differently according to a CCLM mode type.
[221]
For convenience of description, it is assumed that both the width W and the height H of the chroma block are 4.
[222]
When the LM mode is applied to the chroma block, the CCLM parameter may be derived using W upper neighboring samples touching the upper boundary of the chroma block and H left neighboring samples touching the left boundary of the chroma block.
[223]
When the LM-A mode is applied to the chroma block, the CCLM parameter may be derived using 2W upper neighboring samples adjacent to the upper end of the chroma block.
[224]
When the LM-L mode is applied to the chroma block, CCLM parameters may be derived using 2H left neighboring samples adjacent to the left of the chroma block.
[225]
Alternatively, after sub-sampling neighboring samples adjacent to the chroma block, CCLM parameters may be derived using only the sub-sampled neighboring samples.
[226]
For example, when the LM mode is applied to the chroma block, W upper neighbor samples touching the upper boundary of the chroma block are subsampled at two intervals, and H neighboring samples touching the left boundary of the chroma block are subsampled at two intervals. can be subsampled. Equation 3 shows the position combination of sub-sampled neighboring samples used to derive the CCLM parameter under the LM mode.
[227]
[Formula 3]

[228]
In Equation 3, the first value in parentheses indicates the y-coordinate of the neighboring sample, and the second value indicates the x-coordinate of the neighboring sample.
[229]
When the LM-A mode is applied to the chroma block, 2W upper neighboring samples adjacent to the upper end of the chroma block may be subsampled. Equation 4 shows the position combination of sub-sampled neighboring samples used to derive the CCLM parameter under the LM-A mode.
[230]
[Formula 4]

[231]
When the LM-L mode is applied to the chroma block, 2H left neighboring samples adjacent to the left of the chroma block may be subsampled. Equation 5 shows the position combination of sub-sampled neighboring samples used to derive the CCLM parameter under the LM-L mode.
[232]
[Formula 5]

[233]
A collocated luma sample corresponding to a neighboring chroma sample may be extracted from the luma image. In this case, when the sizes of the luma image and the chroma image are different, a luma sample to which a down-sampling filter is applied may be derived.
[234]
The down-sampling filter may be applied to the collocated luma pixel and neighboring pixels adjacent to the collocated luma pixel.
[235]
The type of the down-sampling filter may be determined based on the type of the current image, the position of the collocated luma pixel, the variable AvailL, the variable AvailT, or whether the current block is in contact with the boundary of the coding tree unit. Different types of filters may differ in at least one of a shape of the filter, a number of taps, or a coefficient.
[236]
Hereinafter, an example of deriving a filtered luma sample of an upper neighboring block adjacent to the top of a chroma block and an example of deriving a filtered luma sample of a left neighboring block adjacent to the left of the chroma block will be described in detail.
[237]
In order to derive the filtered luma sample for the top neighbor sample neighboring the top of the chroma block, reconstructed samples included in up to N lines from the top boundary of the luma block may be used. In this case, the number of lines may be determined based on at least one of a chroma subsampling format, a CCLM mode type, an image type, a shape or size of a current block, whether the current block is in contact with a coding tree unit boundary, or a downsampling filter type. have. In the present disclosure, it is assumed that reconstructed samples included in two rows around the upper boundary of the luma block are used when the down-sampling filter is applied to the collocated luma sample at the upper end of the luma block.
[238]
14 illustrates a down-sampling filter type applied to a collocated luma sample of an upper neighboring sample when the current image is not an HDR image.
[239]
15 illustrates a down-sampling filter type applied to a collocated luma sample of an upper neighboring sample when the current image is an HDR image.
[240]
14 and 15 , the variable AvailTL may be derived by applying an AND operator between the variable AvailL and the variable AvailT. That is, when both the left neighbor samples and the top neighbor samples are available, the value of the variable AvailTL is set to 1, and when at least one of the left neighbor samples and the top neighbor samples is unavailable, the variable AvailTL is set to 0. can
[241]
The down-sampling filter type applied to the collocated luma sample may be determined based on at least one of a variable AvailL, a variable AvailT, a position of an upper neighboring sample, and whether the current block is in contact with an upper boundary of a coding tree unit.
[242]
First, when the current image is not an HDR image, an application aspect of the down-sampling filter will be described.
[243]
For a chroma component top neighbor sample whose x-axis coordinate is greater than 0, if both top neighbor samples and left neighbor samples are available and the current block does not touch the top boundary of the coding tree unit, the collocated luma sample has A 6-tap down-sampling filter may be applied. Specifically, the collocated luma sample and the lower neighboring sample located at the lower end of the collocated sample are used as the center, and the collocated sample and the lower neighboring sample are down-sampling filters in the horizontal direction neighboring samples in the horizontal direction respectively. can be applied. In this case, the ratio of the filter coefficients applied to the collocated sample and the lower neighboring sample to the filter coefficients applied to the horizontal direction neighboring samples may be 2:1.
[244]
On the other hand, when at least one of the upper neighboring samples and the left neighboring samples is unavailable or the current block is in contact with the upper boundary of the coding tree unit, a horizontal filter may be applied to the collocated luma sample. Specifically, the down-sampling filter may be applied to the collocated luma sample and to luma samples adjacent to the collocated luma sample in the horizontal direction. In this case, a ratio of the filter coefficient applied to the collocated luma sample and the filter coefficient applied to the neighboring luma sample may be 2:1.
[245]
For a chroma component top neighbor sample whose x-axis coordinate is 0 or less, if both top neighbor samples and left neighbor samples are available and the current block does not touch the top boundary of the coding tree unit, the collocated luma sample has 6 A tap-down sampling filter may be applied.
[246]
Both the top neighbor samples and the left neighbor samples are available, but when the current block touches the top boundary of the coding tree unit, a horizontal down-sampling filter may be applied.
[247]
On the other hand, if the current coding block does not touch the upper boundary of the coding tree unit, but at least one of the upper neighboring samples and the left neighboring samples is unavailable, a 2-tap vertical filter may be applied to the collocated luma sample. . The 2-tap vertical filter may be applied to a collocated luma sample and a lower neighbor sample positioned below the collocated luma sample. In this case, a ratio of filter coefficients applied to the collocated luma sample and the lower neighboring sample may be 1:1.
[248]
On the other hand, when at least one of the upper neighboring samples and the left neighboring samples is unavailable and the current block is in contact with the upper boundary of the coding tree unit, it may be set not to apply the down sampling filter to the collocated luma sample.
[249]
Next, when the current image is an HDR image, an application aspect of the down-sampling filter will be described.
[250]
For a chroma component top neighbor sample whose x-axis coordinate is greater than 0, if both top neighbor samples and left neighbor samples are available and the current block does not touch the top boundary of the coding tree unit, the collocated luma sample has A cross-shaped filter may be applied. Specifically, a down-sampling filter may be applied to a collocated luma sample, horizontally neighboring luma samples of the collocated sample, and vertically neighboring luma samples of the collocated luma sample. In this case, a ratio of the filter coefficient applied to the collocated luma sample to the filter coefficient applied to the neighboring luma sample may be 4:1.
[251]
On the other hand, when at least one of the upper neighboring samples and the left neighboring samples is unavailable or the current block is in contact with the upper boundary of the coding tree unit, a horizontal filter may be applied to the collocated luma sample. Specifically, the down-sampling filter may be applied to the collocated luma sample and to luma samples adjacent to the collocated luma sample in the horizontal direction. In this case, a ratio of the filter coefficient applied to the collocated luma sample and the filter coefficient applied to the neighboring luma sample may be 2:1.
[252]
For a chroma component top neighbor sample whose x-axis coordinate is 0 or less, if both top neighbor samples and left neighbor samples are available and the current block does not touch the top boundary of the coding tree unit, the collocated luma sample has a cross A shape filter may be applied.
[253]
Both the top neighbor samples and the left neighbor samples are available, but when the current block touches the top boundary of the coding tree unit, a horizontal down-sampling filter may be applied.
[254]
On the other hand, when the current coding block does not touch the upper boundary of the coding tree unit, but at least one of the upper neighboring samples and the left neighboring samples is unavailable, it is possible to set not to apply the downsampling filter to the collocated luma sample. .
[255]
Even when at least one of the top neighbor samples and the left neighbor samples is unavailable and the current block touches the top boundary of the coding tree unit, it may be set not to apply the down sampling filter to the collocated luma sample.
[256]
14 and 15 , the down-sampling filter type applied to the collocated luma sample is determined based on the image type, the variable AvailTL, and whether the current block touches the upper boundary of the coding tree unit. has been exemplified
[257]
As another example, the downsampling filter type may be determined irrespective of at least one of the image type, the variable AvailTL, or whether the current block is in contact with the upper boundary of the coding tree unit.
[258]
16 shows an example in which a fixed type of filter is applied according to a position of an upper neighboring sample.
[259]
Regardless of whether the current image is an HDR image, when the x-coordinate of the upper neighboring sample of the chroma component is greater than 0, a horizontal filter may be applied to the collocated luma sample.
[260]
On the other hand, when the x-axis coordinate of the upper neighboring sample of the chroma component is less than or equal to 0, the down-sampling filter may not be applied to the collocated luma sample.
[261]
As another example, in consideration of whether the current image is an HDR image, when the current image is an HDR image, the first type of down-sampling filter may be applied to all upper neighboring samples. On the other hand, when the current image is not an HDR image, the second type of down-sampling filter may be applied to all upper neighboring samples.
[262]
Here, the first type and the second type may be different from each other in at least one of a filter shape, number of taps, and coefficients. As an example, the first type down-sampling filter may represent a 3-tap horizontal direction filter, and the second-type down-sampling filter may represent a 6-tap filter.
[263]
In this case, when a neighboring sample adjacent to the luma block is unavailable, available reconstructed samples may be padded at an unavailable neighboring sample position. As an example, when left neighboring samples adjacent to the left of the luma block are unavailable, reconstructed samples included in the leftmost column in the luma block may be padded to the left. Alternatively, when upper neighboring samples adjacent to the upper end of the luma block are unavailable, reconstructed samples included in the uppermost row in the luma block may be padded to the upper end.
[264]
By padding the unavailable sample location, the down-sampling filter type can be determined without considering the availability of neighboring samples.
[265]
Next, a method of deriving a filtered luma sample with respect to a left neighboring sample will be described.
[266]
When the filtered luma sample is derived, reconstructed samples included in up to M lines from the left boundary of the luma block may be used. In this case, the number of lines may be determined based on at least one of a chroma subsampling format, a CCLM mode type, an image type, a shape or size of a current block, whether the current block is in contact with a coding tree unit boundary, or a downsampling filter type. have. In the present disclosure, it is assumed that reconstructed samples included in three columns around the left boundary of the luma block are used when the down-sampling filter is applied to the collocated luma sample on the left side of the luma block.
[267]
The down-sampling filter may be applied to the collocated luma pixel and neighboring pixels adjacent to the collocated luma pixel.
[268]
17 illustrates a down-sampling filter type applied to a collocated luma sample of a left neighboring sample.
[269]
The down-sampling filter type applied to the collocated luma sample may be determined based on at least one of an image type, a variable AvailL, a variable AvailT, and a position of a left neighboring sample.
[270]
First, when the current image is an HDR image, an application aspect of the down-sampling filter will be described.
[271]
When both upper neighboring samples and left neighboring samples are available, a cross-shaped filter may be applied to the collocated luma sample. Specifically, a down-sampling filter may be applied to a collocated luma sample, horizontally neighboring luma samples of the collocated sample, and vertically neighboring luma samples of the collocated luma sample. In this case, a ratio of the filter coefficient applied to the collocated luma sample to the filter coefficient applied to the neighboring luma sample may be 4:1.
[272]
When the y-axis coordinate of the chroma component upper neighboring sample is greater than 0, the cross-shaped filter may be applied to the collocated luma sample regardless of availability of the left neighboring reference samples and the upper neighboring reference samples.
[273]
When the y-axis coordinate of the chroma component upper neighboring sample is less than or equal to 0, and at least one of the left neighboring reference samples and the upper neighboring reference samples is unavailable, a horizontal downsampling filter may be applied. Specifically, the down-sampling filter may be applied to the collocated luma sample and to luma samples adjacent to the collocated sample in the horizontal direction. In this case, a ratio of the filter coefficient applied to the collocated luma sample to the filter coefficient applied to the neighboring samples in the horizontal direction may be 2:1.
[274]
When the current image is not an HDR image, a 6-tap down-sampling filter may be applied to the collocated luma sample. Specifically, the collocated luma sample and the lower neighboring sample located at the lower end of the collocated sample are used as the center, and the collocated sample and the lower neighboring sample are down-sampling filters in the horizontal direction neighboring samples in the horizontal direction respectively. can be applied. In this case, a ratio of the filter coefficients applied to the collocated sample and the lower neighboring sample to the filter coefficients applied to the horizontal neighboring samples may be 2:1.
[275]
In the example shown in FIG. 16 , the down-sampling filter type applied to the collocated luma sample is exemplified as being determined based on the image type and the variable AvailTL.
[276]
As another example, the down-sampling filter type may be determined regardless of at least one of the image type and the variable AvailTL.
[277]
17 illustrates an example in which a fixed type of filter is applied according to the type of the current image.
[278]
When the current image is an HDR image, a horizontal direction filter may be applied to the collocated luma sample.
[279]
On the other hand, when the current image is not an HDR image, a 6-tap down-sampling filter may be applied to the collocated luma sample.
[280]
In this case, when a neighboring sample adjacent to the luma block is unavailable, available reconstructed samples may be padded at an unavailable neighboring sample position. As an example, when left neighboring samples adjacent to the left of the luma block are unavailable, reconstructed samples included in the leftmost column in the luma block may be padded to the left. Alternatively, when upper neighboring samples adjacent to the upper end of the luma block are unavailable, reconstructed samples included in the uppermost row in the luma block may be padded to the upper end.
[281]
By padding the unavailable sample location, the down-sampling filter type can be determined without considering the availability of neighboring samples.
[282]
Information on a method of determining a down-sampling filter may be signaled through a bitstream. As an example, the first method of determining the down-sampling filter type in consideration of at least one of the image type, the variable AvailTL, or whether the current block is in contact with the upper boundary of the coding tree unit, or the down-sampling regardless of at least one of the conditions Information specifying one of the second methods of determining the filter type may be signaled through the bitstream.
[283]
As another example, at least one of the type of the current image, the size and/or shape of the current block, whether the current block is adjacent to the upper boundary of the coding tree unit, the chroma subsampling format, the CCLM mode type, the variable AvailL, the variable AvailT, or the variable AvailTL Based on the one, at least one of the first method or the second method may be selected.
[284]
Alternatively, as described above, the filter type may be determined by considering only whether the current image is an HDR image. For example, when the current image is an HDR image, only the first type of down-sampling filter is fixedly applied to the upper neighboring sample or the left neighboring sample, and when the current image is not an HDR image, the upper neighboring sample or the left neighboring sample Only two types of downsampling filters can be fixedly applied. Here, the first type and the second type may be different from each other in at least one of a filter shape, number of taps, and coefficients.
[285]
In this case, padding may be performed in advance on N rows around the upper boundary of the luma block and/or M rows around the left boundary of the luma block. As padding is performed, all neighboring samples around the luma block may be set to be available, and accordingly, the filter type may be determined regardless of whether the neighboring samples are available.
[286]
In the example shown in FIG. 13 , through subsampling, it has been described that neighboring samples used to derive CCLM parameters are selected. In this case, the positions of the subsampled neighboring samples may be set in a combination different from that of the described example. As an example, instead of Equation 3, sampled neighboring samples may be derived according to Equation 6 below.
[287]
[Formula 6]

[288]
In Equation 6, W and H represent the width and height of the current chroma block, respectively. In Equation 6, a combination of variables a, b, and c may be determined based on one of Table 3 below.
[289]
[Table 3]
(a, b) c
(3, 5) 8
(5, 11) 16
(11, 21) 32
(21, 43) 64
[290]
Previously, rather than subsampling neighboring samples using Equation 3, subsampling neighboring samples using a combination of Equation 6 and Table 3 maintains an appropriate interval between subsampled neighboring samples. can be effective for As an example, when subsampling is performed using Equation 3 above, as the width or height of the current block is greater than the other one, the interval between the two subsampled samples may change non-uniformly. On the other hand, when one of the variables defined in Equation 6 and Table 3 is used, compared to the case of using Equation 3, the interval between the two subsampled samples can be more evenly maintained.
[291]
One of the variable combinations defined in Table 3 may be selected based on at least one of the size and shape of the current block, the CCLM mode type, the type of the current image, and the chroma subsampling format. As an example, at least one of a width or a height of the current block may be set as a variable c, and a combination of (a, b) corresponding to the determined variable c may be called. For example, when at least one of the width or height of the current block is 4, the combination of (a, b) is set to (3, 5), and when at least one of the width or height of the current block is 8, Set the combination of (a, b) to (5, 11), and if at least one of the width or height of the current block is 16, set the combination of (a, b) to (11, 21), When at least one of the width or the height of the block is 32 or more, the combination of (a, b) may be set to (21, 43).
[292]
When the current block is a non-square shape, a combination of variables for a horizontal direction and a combination of variables for a vertical direction may be set differently. As an example, subsampling of neighboring samples above the current block may be performed by setting the width W of the current block as a variable c, and then calling a combination of (a, b) corresponding to the set variable c. On the other hand, sub-sampling of samples left of the current block may be performed by setting the width H of the current block as a variable c, and then calling a combination of (a, b) corresponding to the set variable c.
[293]
As another example, information for specifying at least one of variables a, b, and c may be signaled through a bitstream. As an example, information for specifying each of the variables a, b, and c may be signaled through a sequence parameter set or a picture parameter set.
[294]
Alternatively, index information specifying one of combinations of variables a, b and c may be signaled through the bitstream.
[295]
Alternatively, the values ​​of variables a, b, and c may be fixed in the encoder and the decoder.
[296]
The number of sub-sampled chrominance samples may be increased or decreased compared to that described above through Equations 3 to 5. In this case, the number of sub-sampled chrominance samples is the size and shape of the current block, the type of the current image, whether the current block is in contact with the boundary of the coding tree unit, the CCLM mode type, the chroma subsampling format, the variable AvailT, the variable AvailL Alternatively, it may be determined based on at least one of the variable AvailTL.
[297]
As an example, in the example shown in FIG. 13 , when the chroma block has a size of 4x4, it has been described that a total of four neighboring samples are selected through subsampling.
[298]
When the size of the chroma block is greater than 4x4, a larger number of neighboring samples may be selected. For example, when a total of 8 neighboring samples are to be selected, Equations 3 to 5 may be changed to Equations 7 to 9 below.
[299]
[Formula 7]

[300]
[Formula 8]

[301]
[Formula 9]

[302]
Equations 7 to 9 represent combinations of subsampled neighboring samples under the LM mode, the LM-A mode, and the LM-L mode, respectively.
[303]
Based on the size of the current block, the number of subsampled neighboring samples may be adjusted. For example, when the current block is smaller than the threshold WxH, 4 neighboring samples may be selected. On the other hand, when the current block is equal to or greater than the threshold WxH, 8 neighboring samples may be selected.
[304]
In the above-described embodiments, it has been exemplified that a down-sampling filter is applied to a collocated luma sample corresponding to a chroma sample in order to derive a CCLM parameter. As another example, the CCLM parameter may be derived without applying a down-sampling filter to the collocated luma samples. Here, not applying the down-sampling filter indicates that the value of the collocated luma sample is used as it is in deriving the CCLM parameter.
[305]
As another example, whether to apply the down sampling filter may be determined according to the number of neighboring samples. For example, when four neighboring samples are selected, a down-sampling filter may be applied to the collocated luma sample. On the other hand, when 8 neighboring samples are selected, the down-sampling filter may not be applied to the collocated luma sample.
[306]
As another example, the down-sampling filter type may be adaptively determined according to the number of neighboring samples. For example, when four neighboring samples are selected and when eight neighboring samples are selected, at least one of a shape, number of taps, and coefficients of the down-sampling filter may be different.
[307]
By applying a low-pass filter to the chroma image or the luma image, singularity of the reconstructed pixel may be removed or mitigated. For example, before selecting neighboring samples around the chroma block, a low pass filter may be applied. When the low-pass filter is applied, the block characteristics may be better reflected when the CCLM parameter is derived.
[308]
It can be set to apply the low-pass filter only to the chroma image. This is because the specificity of the restored pixel in the luma image may be removed or mitigated through a down-sampling filter.
[309]
As another example, the low-pass filter may be applied to both the chroma image and the luma image.
[310]
Information indicating whether to apply the low-pass filter may be signaled through a bitstream. The information may be signaled for each of a luma component and a chroma component.
[311]
CCLM parameters α and β may be derived using neighboring samples neighboring the chroma block and neighboring samples neighboring the luma block.
[312]
Upon derivation of the CCLM parameter, luma component neighboring samples may be classified into two groups. The classification may be based on values ​​of neighboring samples. For example, when four neighboring samples are selected, two of the four neighboring samples may be classified as a first group, and two of the four neighboring samples may be classified as a second group.
[313]
Chroma component neighboring samples can also be classified into two groups. The classification may be based on a classification result of the luma component. That is, if the neighboring luma samples are classified into the N-th group, the corresponding neighboring chroma samples may also be classified into the N-th group.
[314]
Thereafter, for each of the luma component and the chroma component, an average value of samples for each group may be derived. As an example, for the luma component, an average value Xb may be derived by averaging neighboring luma samples belonging to the first group, and an average value Xa may be derived by averaging the neighboring luma samples belonging to the second group. Also, with respect to the chroma component, an average value Yb may be derived by averaging neighboring chroma samples belonging to the first group, and an average value Ya may be derived by averaging the neighboring chroma samples belonging to the second group.
[315]
Based on the derived average values, the CCLM parameter can be derived. As an example, the weight α and the offset β may be derived based on Equations 10 and 11 below.
[316]
[Formula 10]

[317]
[Formula 11]

[318]
When the CCLM parameter is derived, a chroma sample may be predicted using the down-sampled luma sample (S804). Specifically, the chroma prediction sample may be derived by adding the offset β to the product of the down-sampled luma sample and the weight α.
[319]
The luma component neighboring samples and the chroma component neighboring samples may be classified into three or more groups. As an example, n of the N luma neighboring samples may be classified as a first group, m may be classified as a second group, and Nnm may be classified as a third group. The classification may be based on values ​​of neighboring luma samples.
[320]
The chroma component neighboring samples may also be classified into three or more groups with reference to the classification results of the luma component neighboring samples.
[321]
When the neighboring samples are classified into three or more groups, the CCLM parameter may be derived using only two of the three or more groups. As an example, the CCLM parameter may be derived using a group including a neighbor sample with the largest value and a group including a neighbor sample with the smallest value.
[322]
In the above-described example, it has been described that chroma neighboring samples are classified with reference to the classification results of luma neighboring samples. Contrary to the described example, after classifying the chroma neighbor samples, the luma neighbor samples may be classified by referring to the classification result of the crowbar neighbor samples.
[323]
As another example, the neighboring luma samples and the neighboring chroma samples may be independently classified into a plurality of groups.
[324]
The names of syntaxes used in the above-described embodiments are merely named for convenience of description.
[325]
Applying the decoding process or the embodiments described based on the encoding process to the encoding process or the decoding process is included in the scope of the present disclosure. It is also within the scope of the present disclosure to change the embodiments described in a certain order in an order different from that described.
[326]
Although the above-described embodiment has been described based on a series of steps or a flowchart, this does not limit the time-series order of the invention, and may be performed simultaneously or in a different order, if necessary. In addition, each of the components (eg, unit, module, etc.) constituting the block diagram in the above-described embodiment may be implemented as a hardware device or software, or a plurality of components may be combined to form one hardware device or software. may be implemented. The above-described embodiment may be implemented in the form of program instructions that can be executed through various computer components and recorded in a computer-readable recording medium. The computer-readable recording medium may include program instructions, data files, data structures, etc. alone or in combination. Examples of the computer-readable recording medium include a hard disk, a magnetic medium such as a floppy disk and a magnetic tape, an optical recording medium such as a CD-ROM and DVD, and a magneto-optical medium such as a floppy disk. media), and hardware devices specially configured to store and execute program instructions, such as ROM, RAM, flash memory, and the like. The hardware device may be configured to operate as one or more software modules to perform processing according to the present disclosure, and vice versa.
Industrial Applicability
[327]
The present invention can be applied to an electronic device capable of encoding/decoding an image.
Claims
[Claim 1]
determining whether a Cross-compoenent Linear Model (CCLM) mode is applied to the chroma block; when it is determined that the CCLM mode is determined for the chroma block, obtaining filtered neighboring luma samples for neighboring chroma samples adjacent to the chroma block; deriving a CCLM parameter by using the neighboring chroma sample and the filtered neighboring luma sample; and generating a prediction block for the chroma block by using the CCLM parameter.
[Claim 2]
The method of claim 1, wherein the filtered neighboring luma sample is generated by applying a down-sampling filter to a collocated luma sample corresponding to the neighboring chroma sample and neighboring luma samples adjacent to the collocated luma sample. Characterized in, the video decoding method.
[Claim 3]
The image of claim 2, wherein when at least one of the collocated luma sample and the neighboring luma samples is unavailable, a reconstructed sample located at a boundary within the luma block is padded at an unavailable sample location. Decryption method.
[Claim 4]
The method of claim 2 , wherein the type of the down-sampling filter is determined based on a type of a current image.
[Claim 5]
The method of claim 2 , wherein the type of the down-sampling filter is determined based on positions of the neighboring chroma samples.
[Claim 6]
The method of claim 1 , wherein the neighboring chroma samples are extracted by subsampling a plurality of neighboring chroma samples neighboring the chroma block.
[Claim 7]
The method of claim 6 , wherein the sub-sampling rate is determined based on at least one of a size and a shape of the chroma block.
[Claim 8]
determining whether a Cross-compoenent Linear Model (CCLM) mode is applied to the chroma block; when it is determined that the CCLM mode is determined for the chroma block, obtaining filtered neighboring luma samples for neighboring chroma samples adjacent to the chroma block; deriving a CCLM parameter by using the neighboring chroma sample and the filtered neighboring luma sample; and generating a prediction block for the chroma block by using the CCLM parameter.
[Claim 9]
A computer-readable recording medium storing a bitstream encoded by a video encoding method, the video encoding method comprising: determining whether a cross-component linear model (CCLM) mode is applied to a chroma block; when it is determined that the CCLM mode is determined for the chroma block, obtaining filtered neighboring luma samples for neighboring chroma samples adjacent to the chroma block; deriving a CCLM parameter by using the neighboring chroma sample and the filtered neighboring luma sample; and generating a prediction block for the chroma block by using the CCLM parameter.