KR20230134995A - Method and apparatus for encoding/decoding image - Google Patents

Abstract

An image encoding/decoding method according to the present invention includes determining at least one intra prediction mode for a current block; and performing intra prediction on the current block based on the at least one intra prediction mode. At this time, the at least one intra prediction mode may be determined through a histogram derived from the reference area of the current block.

Description

Image encoding/decoding method and device {METHOD AND APPARATUS FOR ENCODING/DECODING IMAGE}

The present invention relates to a video signal processing method and device.

Recently, demand for high-resolution, high-quality images such as HD (High Definition) images and UHD (Ultra High Definition) images is increasing in various application fields. As video data becomes higher resolution and higher quality, the amount of data increases relative to existing video data. Therefore, when video data is transmitted using media such as existing wired or wireless broadband lines or stored using existing storage media, transmission costs and Storage costs increase. High-efficiency video compression technologies can be used to solve these problems that arise as video data becomes higher resolution and higher quality.

Inter-screen prediction technology that predicts the pixel value included in the current picture from pictures before or after the current picture using video compression technology, intra-screen prediction technology that predicts the pixel value included in the current picture using pixel information in the current picture, There are various technologies, such as entropy coding technology, which assigns short codes to values with a high frequency of occurrence and long codes to values with a low frequency of occurrence. Using these video compression technologies, video data can be effectively compressed and transmitted or stored.

Meanwhile, as the demand for high-resolution video increases, the demand for three-dimensional video content as a new video service is also increasing. Discussions are underway regarding video compression technology to effectively provide high-resolution and ultra-high-resolution stereoscopic video content.

The purpose of the present disclosure is to provide an intra prediction method and device for encoding/decoding video signals.

The purpose of the present disclosure is to provide a method and device for determining an intra prediction mode for intra prediction on the decoder side when encoding/decoding a video signal.

The purpose of the present disclosure is to provide a method and device for obtaining a prediction block using a plurality of intra prediction modes when encoding/decoding a video signal.

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 skilled in the art from the description below. You will be able to.

An image decoding method according to the present disclosure includes determining at least one intra prediction mode for a current block; and performing intra prediction on the current block based on the at least one intra prediction mode.

At this time, the at least one intra prediction mode may be determined through a histogram derived from the reference area of the current block.

An image encoding method according to the present disclosure includes determining at least one intra prediction mode for a current block; and performing intra prediction on the current block based on the at least one intra prediction mode.

At this time, the at least one intra prediction mode may be determined through a histogram derived from the reference area of the current block.

In the video decoding/encoding method according to the present disclosure, the reference area includes a top reference area located at the top of the current block and a left reference area located to the left of the current block, and the histogram is the reference area. It can be generated based on the intra prediction mode and amplitude of each reference sample belonging to the region.

In the image decoding/encoding method according to the present disclosure, the intra prediction mode of the reference sample may be obtained based on the ratio between the absolute value of the horizontal slope and the absolute value of the vertical slope of the reference sample.

In the video decoding/encoding method according to the present disclosure, the amplitude of the reference sample may be derived as the sum of the absolute value of the horizontal slope of the reference sample and the absolute value of the vertical descriptor.

In the video decoding/encoding method according to the present disclosure, the horizontal slope and the vertical slope of the reference sample are obtained by applying a filter to the reference sample, and the filter may be a Sobel mask or a Prewitt mask. .

In the image decoding/encoding method according to the present disclosure, the histogram may be obtained by cumulatively summing the amplitudes of reference samples for each intra prediction mode.

In the video decoding/encoding method according to the present disclosure, the at least one intra prediction mode may be determined in descending order of amplitude accumulation values in the histgram.

In the image decoding/encoding method according to the present disclosure, when a plurality of intra prediction modes are selected from the histogram, intra prediction is performed based on each of the plurality of intra prediction modes to obtain a plurality of prediction blocks, The final prediction block of the current block may be obtained through an average operation or a weighted sum operation of a plurality of prediction blocks.

In the video decoding/encoding method according to the present disclosure, intra prediction is performed based on each of the at least one intra prediction mode and the default mode, a plurality of prediction blocks are obtained, and an average calculation or weighting of the plurality of prediction blocks is performed. The final prediction block of the current block can be obtained through a sum operation.

In the video decoding/encoding method according to the present disclosure, the default mode may include at least one of planar mode, DC mode, vertical mode, and horizontal mode.

A computer-readable recording medium that stores a bitstream encoded by the video encoding method according to the present disclosure may be provided.

According to the present disclosure, the coding efficiency of the intra prediction mode of the current block can be improved by deriving the intra prediction mode on the decoder side.

According to the present disclosure, the accuracy of intra prediction can be improved by using a plurality of intra prediction modes.

The effects that can be obtained from the present disclosure are not limited to the effects mentioned above, and other effects not mentioned can be clearly understood by those skilled in the art from the description below. will be.

1 is a block diagram showing a video encoding device according to an embodiment of the present disclosure.
Figure 2 is a block diagram showing a video decoding device according to an embodiment of the present disclosure.
FIG. 3 illustrates an image encoding/decoding method performed by the image encoding/decoding device according to the present disclosure.
Figures 4 and 5 show an example of a plurality of intra prediction modes according to the present disclosure.
Figure 6 illustrates a planar mode-based intra prediction method according to the present disclosure.
Figure 7 shows a DC mode-based intra prediction method according to the present disclosure.
Figure 8 illustrates a directional mode-based intra prediction method according to the present disclosure.
Figure 9 shows a method for deriving samples of fractional positions.
Figures 10 and 11 show that the tangent value for the angle is scaled by 32 times for each intra prediction mode.
12 and 13 are diagrams for explaining an example of setting a reference area.
Figure 14 illustrates filter coefficients for each Sobel mask and Prewitt mask.
Figure 15 shows positions where the vertical slope and horizontal slope within the reference area are obtained.
Figure 16 shows an example of grouping directional modes into a plurality of intra prediction mode groups.
Figure 17 is a diagram for explaining a method of determining the intra prediction mode of a chroma block based on the characteristics of the luma block.
FIG. 18 is a diagram illustrating an example in which an intra prediction mode is determined on a sub-block basis.

Since the present invention can make various changes and have various embodiments, specific embodiments will be illustrated in the drawings and described in detail in the detailed description. However, this is not intended to limit the present invention to specific embodiments, and should be understood to include all changes, equivalents, and substitutes included in the spirit and technical scope of the present invention. While describing each drawing, similar reference numerals are used for similar components.

Terms such as first, second, etc. may be used to describe various components, but the components should not be limited by the terms. The above terms are used only for the purpose of distinguishing one component from another. For example, a first component may be named a second component, and similarly, the second component may also be named a first component without departing from the scope of the present invention. The term and/or includes any of a plurality of related stated items or a combination of a plurality of related stated items.

When a component is said to be "connected" or "connected" to another component, it is understood that it may be directly connected or connected to the other component, but that other components may exist in between. It should be. On the other hand, when it is mentioned that a component is “directly connected” or “directly connected” to another component, it should be understood that there are no other components in between.

The terms used in this application are only used to describe specific embodiments and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this application, terms such as “comprise” or “have” are intended to designate the presence of features, numbers, steps, operations, components, parts, or combinations thereof described in the specification, but are not intended to indicate the presence of one or more other features. It should be understood that this does not exclude in advance the possibility of the existence or addition of elements, numbers, steps, operations, components, parts, or combinations thereof.

Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the attached drawings. Hereinafter, the same reference numerals will be used for the same components in the drawings, and duplicate descriptions of the same components will be omitted.

Figure 1 is a block diagram showing a video encoding device according to an embodiment of the present invention.

Referring to FIG. 1, the image encoding device 100 includes a picture segmentation unit 110, prediction units 120 and 125, a conversion unit 130, a quantization unit 135, a reordering 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.

Each component shown in FIG. 1 is shown independently to represent different characteristic functions in the video encoding device, and does not mean that each component is comprised of separate hardware or a single software component. That is, each component is listed and included as a separate component for convenience of explanation, and at least two of each component can be 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 can be divided into a plurality of components. Integrated embodiments and separate embodiments of the constituent parts are also included in the scope of the present invention as long as they do not deviate from the essence of the present invention.

Additionally, some components may not be essential components that perform essential functions in the present invention, but may simply be optional components to improve performance. The present invention can be implemented by including only essential components for implementing the essence of the present invention excluding components used only to improve performance, and a structure including only essential components excluding optional components used only to improve performance. is also included in the scope of rights of the present invention.

The picture division unit 110 may divide the input picture into at least one processing unit. At this time, the processing unit may be a prediction unit (PU), a transformation unit (TU), or a coding unit (CU). The picture division unit 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 standard (for example, a cost function). You can encode the picture by selecting .

For example, one picture may be divided into a plurality of coding units. To split the coding unit in a picture, a recursive tree structure such as the Quad Tree Structure can be used. Coding is split into other coding units with one image or the largest coding unit as the root. A unit can be divided into child nodes equal to the number of divided coding units. A coding unit that is no longer divided according to certain restrictions becomes a leaf node. That is, assuming that only square division is possible for one coding unit, one coding unit can be divided into up to four different coding units.

Hereinafter, in the embodiments of the present invention, the coding unit may be used to mean a unit that performs encoding, or may be used to mean a unit that performs decoding.

A prediction unit may be divided into at least one square or rectangular shape of the same size within one coding unit, and any one of the prediction units divided within one coding unit may be a prediction unit of another prediction unit. It may be divided to have a different shape and/or size than the unit.

If the prediction unit for which intra prediction is performed based on the coding unit is not the minimum coding unit when generated, intra prediction can be performed without dividing the prediction unit into a plurality of prediction units NxN.

The prediction units 120 and 125 may include an inter prediction unit 120 that performs inter prediction and an intra prediction unit 125 that performs intra prediction. It is possible to determine whether to use inter prediction or intra prediction for a prediction unit, and determine specific information (eg, intra prediction mode, motion vector, reference picture, etc.) according to each prediction method. At this time, the processing unit in which the prediction is performed and the processing unit in which the prediction method and specific contents are determined may be different. For example, the prediction method and prediction mode are determined in prediction units, and prediction may be performed in transformation units. The residual value (residual block) between the generated prediction block and the original block may be input to the conversion unit 130. Additionally, prediction mode information, motion vector information, etc. used for prediction may be encoded in the entropy encoder 165 together with the residual value and transmitted to the decoding device. When using a specific encoding mode, it is possible to encode the original block as is and transmit it to the decoder without generating a prediction block through the prediction units 120 and 125.

The inter prediction unit 120 may predict a prediction unit based on information on at least one picture among the pictures before or after the current picture, and in some cases, prediction based on information on a partially encoded region within the current picture. Units can also be predicted. The inter prediction unit 120 may include a reference picture interpolation unit, a motion prediction unit, and a motion compensation unit.

The reference picture interpolation unit may receive reference picture information from the memory 155 and generate pixel information of an integer number of pixels or less from the reference picture. In the case of luminance pixels, a DCT-based 8-tap interpolation filter with different filter coefficients can be used to generate pixel information of an integer pixel or less in 1/4 pixel units. In the case of color difference signals, a DCT-based 4-tap interpolation filter with different filter coefficients can be used to generate pixel information of an integer pixel or less in 1/8 pixel units.

The motion prediction unit may perform motion prediction based on a reference picture interpolated by the reference picture interpolation unit. Various methods such as FBMA (Full search-based Block Matching Algorithm), TSS (Three Step Search), and NTS (New Three-Step Search Algorithm) can be used to calculate the motion vector. The motion vector may have a motion vector value in units of 1/2 or 1/4 pixels based on the interpolated pixels. The motion prediction unit can predict the current prediction unit by using a different motion prediction method. As a motion prediction method, various methods such as the skip method, the merge method, the Advanced Motion Vector Prediction (AMVP) method, and the intra block copy method can be used.

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. If the neighboring block of the current prediction unit is a block on which inter prediction has been performed and the reference pixel is a pixel on which inter prediction has been performed, the reference pixel included in the block on which inter prediction has been performed is the reference pixel of the block on which intra prediction has been performed. It can be used in place of information. That is, when a reference pixel is not available, the unavailable reference pixel information can be replaced with at least one reference pixel among available reference pixels.

In intra prediction, the prediction mode can include a directional prediction mode that uses reference pixel information according to the prediction direction and a non-directional mode that does not use directional information when performing prediction. The mode for predicting luminance information and the mode for predicting chrominance information may be different, and intra prediction mode information used to predict luminance information or predicted luminance signal information may be used to predict chrominance information.

When performing intra prediction, if the size of the prediction unit and the size of the transformation unit are the same, intra prediction for the prediction unit is made based on the pixel on the left, the pixel on the top left, and the pixel on the top of the prediction unit. can be performed. However, when performing intra prediction, if the size of the prediction unit and the size of the transformation unit are different, intra prediction can be performed using a reference pixel based on the transformation unit. Additionally, intra prediction using NxN partitioning can be used only for the minimum coding unit.

The intra prediction method can generate a prediction block after applying an Adaptive Intra Smoothing (AIS) filter to the reference pixel according to the prediction mode. The type of AIS filter applied to the reference pixel may be different. To perform the intra prediction method, the intra prediction mode of the current prediction unit can be predicted from the intra prediction mode of prediction units existing around the current prediction unit. When predicting the prediction mode of the current prediction unit using predicted mode information from neighboring prediction units, if the intra prediction mode of the current prediction unit and neighboring prediction units are the same, predetermined flag information is used to predict the current prediction unit and neighboring prediction units. Information that the prediction modes of are the same can be transmitted, and if the prediction modes of the current prediction unit and neighboring prediction units are different, entropy encoding can be performed to encode the prediction mode information of the current block.

Additionally, based on the prediction units generated by the prediction units 120 and 125, a residual block may be generated that includes residual information that is the difference between the prediction unit on which prediction was performed and the original block of the prediction unit. The generated residual block may be input to the conversion unit 130.

The transform unit 130 transforms the residual block, including the original block and the residual value information of the prediction unit generated through the prediction units 120 and 125, into 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 can be determined based on intra prediction mode information of the prediction unit used to generate the residual block.

The quantization unit 135 may quantize the values converted to the frequency domain by the conversion unit 130. The quantization coefficient may change depending on the block or the importance of the image. The value calculated by the quantization unit 135 may be provided to the inverse quantization unit 140 and the realignment unit 160.

The rearrangement unit 160 may rearrange coefficient values for the quantized residual values.

The rearrangement unit 160 can change the coefficients in a two-dimensional block form into a one-dimensional vector form through a coefficient scanning method. For example, the realignment unit 160 can scan from DC coefficients to coefficients in the high frequency region using a zig-zag scan method and change it into a one-dimensional vector form. Depending on the size of the transformation unit and the intra prediction mode, a vertical scan that scans the two-dimensional block-type coefficients in the column direction or a horizontal scan that scans the two-dimensional block-type coefficients in the row direction may be used instead of the zig-zag scan. That is, depending on the size of the transformation unit and the intra prediction mode, it can be determined which scan method among zig-zag scan, vertical scan, and horizontal scan will be used.

The entropy encoding unit 165 may perform entropy encoding based on the values calculated by the reordering unit 160. Entropy coding can use various coding methods, such as Exponential Golomb, Context-Adaptive Variable Length Coding (CAVLC), and Context-Adaptive Binary Arithmetic Coding (CABAC).

The entropy encoding unit 165 receives the residual value coefficient information and block type information of the coding unit, prediction mode information, division unit information, prediction unit information and transmission unit information, and motion information from the reordering unit 160 and the prediction units 120 and 125. Various information such as vector information, reference frame information, block interpolation information, and filtering information can be encoded.

The entropy encoding unit 165 may entropy encode the coefficient value of the coding unit input from the reordering unit 160.

The inverse quantization unit 140 and the inverse transformation unit 145 inversely quantize the values quantized in the quantization unit 135 and inversely transform the values transformed in the transformation unit 130. The residual value generated in the inverse quantization unit 140 and the inverse transform unit 145 is restored by combining the prediction units predicted through the motion estimation unit, motion compensation unit, and intra prediction unit included in the prediction units 120 and 125. You can create a block (Reconstructed Block).

The filter unit 150 may include at least one of a deblocking filter, an offset correction unit, and an adaptive loop filter (ALF).

The deblocking filter can remove block distortion caused by boundaries between blocks in the restored picture. To determine whether to perform deblocking, it is possible to determine whether to apply a deblocking filter to the current block based on the pixels included in several columns or rows included in the block. When applying a deblocking filter to a block, a strong filter or a weak filter can be applied depending on the required deblocking filtering strength. Additionally, when applying a deblocking filter, horizontal filtering and vertical filtering can be processed in parallel when vertical filtering and horizontal filtering are performed.

The offset correction unit may correct the offset of the deblocked image from the original image in pixel units. In order to perform offset correction for a specific picture, the pixels included in the image are divided into a certain number of areas, then the area to perform offset is determined and the offset is applied to that area, or the offset is performed by considering the edge information of each pixel. You can use the method of applying .

Adaptive Loop Filtering (ALF) can be performed based on a comparison between the filtered restored image and the original image. After dividing the pixels included in the image into predetermined groups, filtering can be performed differentially for each group by determining one filter to be applied to that group. Information related to whether to apply ALF may be transmitted for each coding unit (CU), and the shape and filter coefficients of the ALF filter to be applied may vary for each block. Additionally, an ALF filter of the same type (fixed type) may be applied regardless of the characteristics of the block to which it is applied.

The memory 155 may store a reconstructed block or picture calculated through the filter unit 150, and the stored reconstructed block or picture may be provided to the prediction units 120 and 125 when inter prediction is performed.

Figure 2 is a block diagram showing an image decoding device according to an embodiment of the present invention.

Referring to FIG. 2, the image decoding device 200 includes an entropy decoding unit 210, a reordering unit 215, an inverse quantization unit 220, an inverse transform unit 225, a prediction unit 230, 235, and a filter unit ( 240) and memory 245 may be included.

When a video bitstream is input from a video encoding device, the input bitstream can be decoded in a procedure opposite to that of the video encoding device.

The entropy decoding unit 210 may perform entropy decoding in a procedure opposite to the procedure in which entropy encoding is performed in the entropy encoding unit of the video encoding device. For example, various methods such as Exponential Golomb, CAVLC (Context-Adaptive Variable Length Coding), and CABAC (Context-Adaptive Binary Arithmetic Coding) may be applied in response to the method performed in the image encoding device.

The entropy decoder 210 can decode information related to intra prediction and inter prediction performed by the encoding device.

The reordering unit 215 may rearrange the bitstream entropy-decoded by the entropy decoding unit 210 based on the method in which the encoder rearranges the bitstream. Coefficients expressed in the form of a one-dimensional vector can be restored and rearranged as coefficients in the form of a two-dimensional block. The reordering unit 215 may receive information related to coefficient scanning performed by the encoder and perform reordering by reverse scanning based on the scanning order performed by the encoder.

The inverse quantization unit 220 may perform inverse quantization based on the quantization parameters provided by the encoding device and the coefficient values of the rearranged blocks.

The inverse transform unit 225 may perform inverse transform, that is, inverse DCT, inverse DST, and inverse KLT, on the transform performed by the transformer, that is, DCT, DST, and KLT, on the quantization result performed by the image encoding device. Inverse transformation may be performed based on the transmission unit determined by the video encoding device. The inverse transform unit 225 of the video decoding device may selectively perform a transformation technique (eg, DCT, DST, KLT) according to a plurality of information such as a prediction method, the size of the current block, and the prediction direction.

The prediction units 230 and 235 may generate a prediction block based on prediction block generation-related information provided by the entropy decoder 210 and previously decoded block or picture information provided by the memory 245.

As described above, when performing intra prediction in the same manner as the operation in the video encoding device, when the size of the prediction unit and the size of the transformation unit are the same, the pixel existing on the left of the prediction unit, the pixel existing on the upper left, and the size of the transformation unit are the same. Intra prediction of the prediction unit is performed based on existing pixels, but when performing intra prediction, if the size of the prediction unit and the size of the transformation unit are different, intra prediction is performed using a reference pixel based on the transformation unit. can do. Additionally, intra prediction using NxN partitioning only for the minimum coding unit can be used.

The prediction units 230 and 235 may include a prediction unit determination unit, an inter prediction unit, and an intra prediction unit. The prediction unit discriminator 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, distinguishes the prediction unit from the current coding unit, and makes predictions. It is possible to determine whether a unit performs inter-prediction or intra-prediction. The inter prediction unit 230 uses the information required for inter prediction of the current prediction unit provided by the video encoding device to determine the current prediction unit based on the information included in at least one of the pictures before or after the current picture including the current prediction unit. Inter prediction can be performed on prediction units. Alternatively, inter prediction may be performed based on information on a pre-restored partial region within the current picture including the current prediction unit.

To perform inter prediction, based on the coding unit, the motion prediction method of the prediction unit included in the coding unit is Skip Mode, Merge Mode, AMVP Mode, and Intra Block Copy Mode. You can judge whether it is a certain method or not.

The intra prediction unit 235 may generate a prediction block based on pixel information in the current picture. If the prediction unit is a prediction unit that has performed intra prediction, intra prediction may be performed based on intra prediction mode information of the prediction unit provided by the video encoding device. The intra prediction unit 235 may include an Adaptive Intra Smoothing (AIS) filter, a reference pixel interpolation unit, and a DC filter. The AIS filter is a part that performs filtering on the reference pixels of the current block, and can be applied by determining whether or not to apply the filter according to the prediction mode of the current prediction unit. AIS filtering can be performed on the reference pixel of the current block using the prediction mode and AIS filter information of the prediction unit provided by the video encoding device. If the prediction mode of the current block is a mode that does not perform AIS filtering, the AIS filter may not be applied.

If the prediction mode of the prediction unit is a prediction unit that performs intra prediction based on pixel values obtained by interpolating the reference pixel, the reference pixel interpolator may interpolate the reference pixel to generate a reference pixel in pixel units of an integer value or less. If 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 can generate a prediction block through filtering when the prediction mode of the current block is DC mode.

The restored block or picture may be provided to the filter unit 240. The filter unit 240 may include a deblocking filter, an offset correction unit, and an ALF.

Information on whether a deblocking filter has been applied to the corresponding block or picture can be provided from the video encoding device, and when a deblocking filter has been applied, information on whether a strong filter or a weak filter has been applied. The deblocking filter of the video decoding device receives information related to the deblocking filter provided by the video encoding device, and the video decoding device can perform deblocking filtering on the corresponding block.

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 offset value information.

ALF can be applied to the coding unit based on ALF application availability information, ALF coefficient information, etc. provided from the coding device. This ALF information may be included and provided in a specific parameter set.

The memory 245 can store the restored picture or block so that it can be used as a reference picture or reference block, and can also provide the restored picture to an output unit.

As described above, hereinafter, in the embodiments of the present invention, the term coding unit is used as a coding unit for convenience of explanation, but it may also be a unit that performs not only encoding but also decoding.

In addition, the current block represents an encoding/decoding target block and, depending on the encoding/decoding stage, is a coding tree block (or coding tree unit), a coding block (or coding unit), a transform block (or transform unit), or a prediction block. (or prediction unit), etc. In this specification, 'unit' may represent a basic unit for performing a specific encoding/decoding process, and 'block' may represent a pixel array of a predetermined size. Unless otherwise specified, ‘block’ and ‘unit’ can be used with the same meaning. For example, in embodiments described later, a coding block (coding block) and a coding unit (coding unit) may be understood to have equivalent meanings.

FIG. 3 illustrates an image encoding/decoding method performed by the image encoding/decoding device according to the present disclosure.

Referring to FIG. 3, a reference line for intra prediction of the current block can be determined (S300).

The current block may use one or more of a plurality of reference line candidates pre-defined in the video encoding/decoding device as a reference line for intra prediction. Here, the plurality of pre-defined reference line candidates may include a neighboring reference line adjacent to the current block to be decoded and N non-neighboring reference lines that are 1-sample to N-sample away from the boundary of the current block. N may be an integer of 1, 2, 3, or more. Hereinafter, for convenience of explanation, it is assumed that the plurality of reference line candidates available for the current block consists of a neighboring reference line candidate and three non-neighboring reference line candidates, but is not limited thereto. That is, of course, the plurality of reference line candidates available for the current block may include four or more non-neighboring reference line candidates.

The video encoding device can determine an optimal reference line candidate among a plurality of reference line candidates and encode an index for specifying it. The video decoding device can determine the reference line of the current block based on the index signaled through the bitstream. The index may specify one of a plurality of reference line candidates. The reference line candidate specified by the index can be used as the reference line of the current block.

The number of indices signaled to determine the reference line of the current block may be 1, 2, or more. For example, when the number of signaled indices is 1, the current block may perform intra prediction using only a single reference line candidate specified by the signaled index among a plurality of reference line candidates. Alternatively, when the number of signaled indices is two or more, the current block may perform intra prediction using a plurality of reference line candidates specified by a plurality of indices among a plurality of reference line candidates.

Referring to FIG. 3, the intra prediction mode of the current block can be determined (S310).

The intra prediction mode of the current block may be determined from a plurality of intra prediction modes predefined in the video encoding/decoding device. The plurality of pre-defined intra prediction modes will be examined with reference to FIGS. 4 and 5.

Figure 4 shows an example of a plurality of intra prediction modes according to the present disclosure.

Referring to FIG. 4, a plurality of intra prediction modes pre-defined in the video encoding/decoding device may be comprised of a non-directional mode and a directional mode. The non-directional mode may include at least one of planar mode or DC mode. The directional mode may include directional modes numbered 2 to 66.

The directional mode may be expanded further than shown in FIG. 4. Figure 5 shows an example in which the directional mode is expanded.

In Figure 5, modes -1 to -14 and modes 67 to 80 are illustrated as being added. These directional modes may be referred to as wide angle intra prediction modes. Whether to use the wide angle intra prediction mode can be determined depending on the type of the current block. For example, if the current block is a non-square block with a width greater than the height, some directional modes (eg, 2 to 15) may be converted to wide angle intra prediction modes 67 to 80. On the other hand, if the current block is a non-square block with a height greater than the width, some directional modes (e.g., numbers 53 to 66) may be converted to wide angle intra prediction modes between -1 and -14. there is.

The range of available wide-angle intra prediction modes can be adaptively determined depending on the width-to-height ratio of the current block. Table 1 shows the range of available wide-angle intra prediction modes according to the width and height ratio of the current block.

width/height Wide angle intra prediction mode range available W/H = 16 67~80 W/H = 8 67~78 W/H = 4 67~76 W/H = 2 67~74 W/H = 1 doesn't exist W/H = 1/2 -1~-8 W/H = 1/4 -1~-10 W/H = 1/8 -1~-12 W/H = 1/16 -1~-14

Among the plurality of intra prediction modes, K candidate modes (most probable mode, MPM) can be selected. A candidate list including the selected candidate mode may be created. An index indicating one of the candidate modes belonging to the candidate list may be signaled. The intra prediction mode of the current block may be determined based on the candidate mode indicated by the index. As an example, the candidate mode indicated by the index may be set as the intra prediction mode of the current block. Alternatively, the intra prediction mode of the current block may be determined based on the value of the candidate mode indicated by the index and a predetermined difference value. The difference value may be defined as the difference between the value of the intra prediction mode of the current block and the value of the candidate mode indicated by the index. The difference value may be signaled through a bitstream. Alternatively, the difference value may be a value pre-defined in the video encoding/decoding device. Alternatively, the intra prediction mode of the current block may be a flag indicating whether a mode identical to the intra prediction mode of the current block exists in the candidate list. It can be decided based on . For example, when the flag is the first value, the intra prediction mode of the current block may be determined from the candidate list. In this case, an index indicating one of a plurality of candidate modes belonging to the candidate list may be signaled. The candidate mode indicated by the index may be set as the intra prediction mode of the current block. On the other hand, when the flag is the second value, one of the remaining intra prediction modes may be set as the intra prediction mode of the current block. The remaining intra prediction modes may refer to modes excluding candidate modes belonging to the candidate list among a plurality of pre-defined intra prediction modes. When the flag is the second value, an index indicating one of the remaining intra prediction modes may be signaled. The intra prediction mode indicated by the signaled index may be set as the intra prediction mode of the current block.

The intra prediction mode of a chroma block may be selected from among intra prediction mode candidates of a plurality of chroma blocks. To this end, index information indicating one of the intra prediction mode candidates of the chroma block can be explicitly encoded and signaled through a bitstream. Table 2 illustrates intra prediction mode candidates for chroma block.

index Intra prediction mode candidates for chroma blocks 0 0 (Planar) One 50 (Vertical) 2 18 (Horizontal) 3 1 (DC) 4 DM

In the example of Table 2, DM (Direct Mode) means setting the intra prediction mode of the luma block existing at the same location as the chroma block to the intra prediction mode of the chroma block. As a result, intra prediction of the chroma block The mode can also be set to one of the intra prediction modes shown in FIG. 4 or FIG. 5. The intra prediction mode of the current block may be used to determine the reference line of the current block, in which case step S310 may be performed before step S300.

Referring to FIG. 3, intra prediction may be performed on the current block based on the reference line and intra prediction mode of the current block (S320).

Hereinafter, we will look at the intra prediction method for each intra prediction mode in detail with reference to FIGS. 6 to 8. However, for convenience of explanation, it is assumed that a single reference line is used for intra prediction of the current block. However, even when multiple reference lines are used, the intra prediction method described later may be applied in the same/similar manner.

Figure 6 illustrates a planar mode-based intra prediction method according to the present disclosure.

Referring to FIG. 6, T represents a reference sample located at the upper right corner of the current block, and L represents a reference sample located at the lower left corner of the current block. P1 can be generated through horizontal interpolation. As an example, P1 can be generated by interpolating T with a reference sample located on the same horizontal line as P1. P2 can be generated through interpolation in the vertical direction. As an example, P2 can be generated by interpolating L with a reference sample located on the same vertical line as P2. The current sample in the current block can be predicted through the weighted sum of P1 and P2, as shown in Equation 1 below.

In Equation 1, the weights α and β can be determined considering the width and height of the current block. Depending on the width and height of the current block, weights α and β may have the same value or different values. If the width and height of the current block are the same, the weights α and β can be set to be the same, and the prediction sample of the current sample can be set to the average value of P1 and P2. If the width and height of the current block are not the same, the weights α and β may have different values. For example, if the width is greater than the height, a smaller value can be set to the weight corresponding to the width of the current block, and a larger value can be set to the weight corresponding to the height of the current block. Conversely, if the width is greater than the height, a larger value can be set to the weight corresponding to the width of the current block, and a smaller value can be set to the weight corresponding to the height of the current block. Here, the weight corresponding to the width of the current block may mean β, and the weight corresponding to the height of the current block may mean α.

Figure 7 shows a DC mode-based intra prediction method according to the present disclosure.

Referring to FIG. 7, the average value of neighboring samples adjacent to the current block can be calculated, and the calculated average value can be set as the predicted value of all samples in the current block. Here, the surrounding samples may include the top reference sample and the left reference sample of the current block. However, depending on the type of the current block, the average value may be calculated using only the top reference sample or the left reference sample. For example, if the width of the current block is greater than the height, the average value can be calculated using only the top reference sample of the current block. Alternatively, if the ratio of the width and height of the current block is greater than or equal to a predetermined threshold, the average value can be calculated using only the top reference sample of the current block. Alternatively, if the ratio of the width and height of the current block is less than or equal to a predetermined threshold, the average value can be calculated using only the upper reference sample of the current block. On the other hand, if the width of the current block is smaller than the height, the average value can be calculated using only the left reference sample of the current block. Alternatively, if the ratio of the width and height of the current block is less than or equal to a predetermined threshold, the average value can be calculated using only the left reference sample of the current block. Alternatively, if the ratio of the width and height of the current block is greater than or equal to a predetermined threshold, the average value can be calculated using only the left reference sample of the current block.

Figure 8 illustrates a directional mode-based intra prediction method according to the present disclosure.

If the intra prediction mode of the current block is a directional mode, projection can be performed to a reference line according to the angle of the directional mode. If a reference sample exists at the projected position, the reference sample can be set as the prediction sample of the current sample. If a reference sample does not exist at the projected position, a sample corresponding to the projected position may be generated using one or more surrounding samples adjacent to the projected position. As an example, interpolation may be performed based on two or more neighboring samples in both directions based on the projected position, thereby generating a sample corresponding to the projected position. Alternatively, one surrounding sample adjacent to the projected position can be set as the sample corresponding to the projected position. At this time, among a plurality of neighboring samples adjacent to the projected position, the neighboring sample closest to the projected position may be used. The sample corresponding to the projected position can be set as the predicted sample of the current sample.

Referring to FIG. 8, in the case of the current sample B, when projection is performed from that position to the reference line according to the angle of the intra prediction mode, a reference sample exists at the projected position (i.e., a reference sample at an integer position, R3 ). In this case, the reference sample of the projected position can be set as the predicted sample of the current sample B. In the case of the current sample A, when projection is performed from that position to the reference line according to the angle of the intra prediction mode, there is no reference sample (i.e., reference sample at the integer position) at the projected position. In this case, interpolation may be performed based on surrounding samples (e.g., R2 and R3) neighboring the projected position to generate a sample (r) of the fractional position. The sample (r) at the generated fractional position can be set as the predicted sample of the current sample A.

Figure 9 shows a method for deriving samples of fractional positions.

In the example of Figure 9, the variable h means the vertical distance (i.e., vertical distance) from the position of the predicted sample A to the reference sample line, and the variable w means the horizontal distance from the position of the predicted sample A to the fractional position sample. (i.e., horizontal distance). Additionally, the variable θ refers to an angle predefined according to the directionality of the intra prediction mode, and the variable x refers to the fractional position.

The variable w can be derived as in Equation 2 below.

Then, by removing the integer positions from the variable w, finally, the fractional positions can be derived.

Fractional position samples can be generated by interpolating adjacent integer position reference samples. As an example, the integer position reference sample R2 and the integer position reference sample R3 may be interpolated to generate a fractional position reference sample at the x position.

In deriving fractional position samples, a scaling factor can be used to avoid real numbers. For example, when the scaling factor f is set to 32, the distance between neighboring integer reference samples may be set to 32 instead of 1, as in the example shown in (b) of FIG. 8.

Additionally, the tangent value for the angle θ determined according to the directionality of the intra prediction mode can also be scaled up using the same scaling factor (eg, 32).

Figures 10 and 11 show that the tangent value for the angle is scaled by 32 times for each intra prediction mode.

FIG. 10 shows the scaled results of tangent values for the non-wide angle intra prediction mode, and FIG. 11 shows the scaled results of the tangent values for the wide angle intra prediction mode.

The intra prediction mode of the current block may be derived using reference samples surrounding the current block. Specifically, after calculating the gradient in the horizontal and vertical directions for reference samples, the intra prediction mode of the current block can be derived using the calculated gradient.

Figure 12 is a diagram for explaining an example of setting a reference area.

For convenience of explanation, it is assumed that the current block size is 4x4.

A reference area for deriving the intra prediction mode of the current block may be set. As an example, in FIG. 12, it is assumed that w0 columns adjacent to the left of the current block and h0 rows adjacent to the top of the current block are set as the reference area.

The number of columns (w0) and/or the number of rows (h0) constituting the reference area may be fixed in the encoder and decoder. Alternatively, based on at least one of the size/shape of the current block, whether Intra Sub-Partitioning (ISP) is applied to the current block, or whether the current block is adjacent to a CTU boundary, the number of columns (w0) and/or the number of rows The number (h0) may be determined.

As another example, the size of the reference area may be determined depending on the type of filter applied to the reference area. Specifically, the horizontal and vertical lengths of the filter can be set to the number of columns w0 and the number of rows h0, respectively. As an example, assuming that the 3x3 mask shown in FIG. 14, which will be described later, is used, the number of columns w0 and the number of rows h0 may each be set to 3.

The reference area may extend beyond the right border and/or bottom border of the current block. For example, in the example shown in FIG. 12, the reference area is illustrated as extending by w1 from the right border of the current block and by h1 from the bottom border of the current block.

The right extension distance w1 and/or the bottom extension distance h1 may be set equal to the width and/or height of the current block. For example, if the size of the current block is 4x4, the right extension distance w1 may be set to 4, equal to the width of the current block, and the bottom extension distance h1 may be set to 4, equal to the height of the current block.

As another example, as in the example shown in FIG. 13, a reference area may be set.

Specifically, as in the example shown in (a) of FIG. 13, the right extension distance w1 and/or the bottom extension distance h1 may be set to 0. Furthermore, as in the example shown in (b) of FIG. 13, the upper reference area is composed only of reference samples with x-axis coordinates from 0 to (w-1), and y-axis coordinates from 0 to (h-1). ) The left reference area can also be constructed using only the reference samples. Here, w represents the width of the current block, and h represents the height of the current block.

As another example, reference line candidates for intra prediction of the current block or at least one of the reference line candidates may be set as a reference area.

As another example, depending on whether the current block is adjacent to a CTU boundary, the reference area may be configured only from the top reference area or only from the left reference area.

Filtering (i.e., convolution) using a mask within the reference area can be performed. At this time, the filter used may be at least one of a Sobel mask or a Prewitt mask through which a slope value is output.

Figure 14 illustrates filter coefficients for each Sobel mask and Prewitt mask.

A different type of filter from that shown in FIG. 14 may be applied to the reference area. For example, instead of a square filter with a size of 3x3, a 1D filter with a size of 1x3 or 3x1, a rectangular filter with a size of 2x3 or 2x3, a cross-shaped filter, or a diamond-shaped filter can be applied to the reference area. Alternatively, a filter of a different size from that shown in FIG. 14 (eg, 2x2, 4x4, or 5x5, etc.) may be applied to the reference area.

The type of filter applied to the reference area may be predefined in the encoder and decoder. Alternatively, after pre-defining a plurality of filter candidates, index information indicating one of the plurality of filter candidates may be encoded and explicitly signaled through a bitstream.

As another example, based on at least one of the size/shape of the current block, whether an ISP is applied to the current block, the size of the reference region, the intra prediction mode of a neighboring block, or whether the current block borders a CTU boundary, a plurality of filters At least one of the candidates may be adaptively selected. Here, the neighboring block may include at least one of the top neighboring block or the left neighboring block of the current block.

The type of filter applied to the upper reference area may be different from the type of filter applied to the left reference area.

By applying a vertical mask to a specific reference sample in the reference area, the vertical gradient Dy with respect to the reference sample can be obtained. Additionally, by applying a horizontal mask to a specific reference sample in the reference area, the horizontal slope Dx for the reference sample can be obtained.

Figure 15 shows positions where the vertical slope and horizontal slope within the reference area are obtained.

As in the example shown in FIG. 14, assuming that a 3x3 sized mask is applied, the vertical slope Dy and the horizontal slope Dx can be obtained for each of the reference samples that are not adjacent to the border of the reference area. For example, when w0 and h0 are 3 and w1 and h1 are 4, as in the example shown in FIG. 14, for each of the 17 reference samples, there are 17 vertical slope Dy and 17 horizontal slope Dx. can be obtained.

When a filter of a different size or shape from that shown in FIG. 14 is applied, the vertical slope Dy and the horizontal slope Dx may be obtained for more/fewer reference samples than shown.

Based on the vertical slope Dy and the horizontal slope Dx of each of the reference samples, an intra prediction mode for each of the reference samples may be determined.

A method for determining the intra prediction mode of a reference sample using the vertical slope Dy and the horizontal slope Dx will be described. horizontal direction

For example, when either the vertical slope Dy or the horizontal slope Dx is 0, the directionality mode of the reference sample may be determined as the horizontal mode (No. 18) or the vertical direction mode (No. 50). Specifically, when the horizontal slope Dx is 0 and the vertical slope Dy is non-0, the intra prediction mode of the corresponding reference sample may be determined to be the vertical mode (No. 50). Conversely, if the vertical slope Dy is 0 and the horizontal slope Dx is not 0, the intra prediction mode of the corresponding reference sample may be determined to be the horizontal mode (No. 18).

If both the vertical slope Dy and the horizontal slope Dx are not 0, one of the remaining directional modes, excluding the horizontal direction mode and the vertical direction mode, may be determined as the intra prediction mode of the reference sample.

Here, the intra prediction mode group to which the intra prediction mode of the reference sample belongs can be determined by comparing the absolute values of the vertical slope Dy and the horizontal slope Dx. Here, the intra prediction mode group may be composed of a plurality of directional modes with similar directionality.

Figure 16 shows an example of grouping directional modes into a plurality of intra prediction mode groups.

In Figure 16, the directional modes are classified into four intra prediction mode groups (a to d) based on the horizontal mode (No. 18), the diagonal mode (No. 34), and the vertical mode (No. 50). It has been exemplified.

In the illustrated example, group a and group b have a symmetrical structure with respect to the horizontal mode (no. 18), and group c and group d have a symmetrical structure with respect to the vertical mode (no. 50).

Additionally, the angle of directional modes 36 to 66 is the same as the transposed angle of modes 2 to 34.

If the absolute value of the horizontal slope Dx of the reference sample is greater than the absolute value of the vertical slope Dy, the directional mode of the reference sample may belong to group a or group b.

On the contrary, if the absolute value of the vertical slope Dy of the reference sample is greater than the slope of the horizontal slope Dx, the directional mode of the reference sample may belong to group c or group d.

Table 3 shows the intra prediction mode group to which the intra prediction mode of the reference sample belongs according to the magnitude of the horizontal slope Dx and the vertical slope Dy.

if (|Dx| > |Dy|) Else Dx >= 0 Dy >= 0 b Dx >= 0 Dy >= 0 c Dx < 0 Dy >= 0 a Dx < 0 Dy >= 0 d Dx >= 0 Dy < 0 a Dx >= 0 Dy < 0 d Dx < 0 Dy < 0 b Dx < 0 Dy < 0 c

Using the horizontal slope Dx and the vertical slope Dy of the reference sample, the slope of the directional mode to be assigned to the reference sample can be derived. To this end, a variable R representing the ratio between the horizontal slope and the vertical slope can be derived, as shown in Equation 3 below.

As illustrated in Equation 3, the variable R can be derived by using the larger absolute value of the horizontal slope Dx and the vertical slope Dy as the denominator.

Thereafter, the directional mode of the reference sample can be determined by comparing the variable R and the tangent value (tanθ) for each angle of the directional modes. Specifically, the directional mode with the same tangent value or the most similar tangent value as the variable R may be assigned to the reference sample.

At this time, as in the example shown in FIG. 10 or FIG. 11, if the tangent value for each angle of the directional modes is stored in the encoder and decoder in a scaled state, the variable R is scaled using the same scaling factor, The directional mode of the reference sample can be determined.

Next, the amplitude of each reference sample can be derived. The amplitude can be derived as the sum of the absolute value of the horizontal slope Dx and the absolute value of the vertical slope Dy, as shown in Equation 4 below.

Next, for each of the intra prediction modes, the amplitude value of each reference sample to which the same intra prediction mode is assigned may be accumulated.

In Equation 5, intra_mode represents the intra prediction mode. As an example, the amplitude accumulation value for the directional mode with mode number N is derived by summing the amplitude values of reference samples to which mode N is assigned in the reference area, and the amplitude accumulation value for the directional mode with mode number M is, Mode M in the reference area can be derived by summing the amplitude values of the assigned reference samples.

The buffer storing the accumulated amplitude value may be initialized in block units. For example, when designating a reference area around the current block, the amplitude accumulation value for each intra prediction mode may be initialized to 0.

If a histogram recording the amplitude accumulation value for each of the intra prediction modes is derived through the above process, at least one intra prediction mode can be selected in the order of the amplitude accumulation value in the histogram. The number of selected intra prediction modes may be M, and M may be a natural number of 1 or more. The value of M may be predefined in the encoder and decoder. Alternatively, the value of M may be adaptively determined by considering at least one of the size/shape of the current block and whether an ISP is applied to the current block. That is, M intra prediction modes can be selected in descending order of amplitude accumulation value.

At least one intra prediction mode selected from the histogram may be set as the intra prediction mode of the current block, and the prediction block of the current block may be obtained based on the intra prediction mode of the current block. For example, when one intra prediction mode is selected from the histogram, the prediction block obtained based on the selected intra prediction mode can be used as the final prediction block of the current block.

When a plurality of intra prediction modes are selected from the histogram, intra prediction may be performed based on each of the plurality of intra prediction modes. Accordingly, when a plurality of prediction blocks are generated, the final prediction block of the current block can be obtained through an average operation or a weighted sum operation of the plurality of prediction blocks.

At this time, for the weighted sum calculation, the weight applied to each prediction block may be determined based on the amplitude of the intra prediction mode. That is, among the plurality of intra prediction modes, the largest weight is assigned to the prediction block derived based on the intra prediction mode with the largest amplitude, and the smallest weight is assigned to the prediction block derived based on the intra prediction mode with the smallest amplitude. Can be assigned.

At this time, the weight assigned to each prediction block may be determined based on the ratio between amplitudes. Alternatively, the weight value for each amplitude rank may be previously stored, and then the weight mapped to the amplitude rank of the corresponding intra prediction mode may be applied to the prediction block.

A prediction block of the current block may be obtained by further considering at least one default mode along with at least one intra prediction mode selected from the histogram. As an example, intra prediction may be performed based on each of the intra prediction mode and default mode selected from the histogram to obtain a plurality of prediction blocks for the current block. Afterwards, the final prediction block of the current block can be obtained through an average operation or a weighted sum operation of a plurality of prediction blocks.

The number N of default modes may be 0 or an integer of 1 or more. When M intra prediction modes are selected from the histogram, (M+N) prediction blocks can be obtained by performing intra prediction based on each of the M intra prediction modes and N default modes. Afterwards, the final prediction block of the current block can be obtained through an average operation or a weighted sum operation of (M+N) prediction blocks.

The number N of default modes may be predefined in the encoder and decoder. Alternatively, based on at least one of the size/shape of the current block, whether an ISP is applied to the current block, or whether at least one intra prediction mode selected from the histogram includes a default mode, the number N of default modes is adaptively can be decided.

The default mode may include at least one of planar mode, DC mode, or predefined directional mode.

The encoder and decoder may be set to use a predefined mode (eg, planar mode) among the modes listed above as the default mode.

Alternatively, the type of default mode may be adaptively determined according to the type of directional mode selected through the histogram. For example, when at least one orientation mode selected through the histogram is a vertical mode or a horizontal mode, the planar mode or DC mode may be set as the default mode. On the other hand, if the vertical direction and/or the horizontal direction mode is not selected through the histogram, the vertical direction mode or the horizontal direction mode can be set as the default mode.

Depending on the type of the current block, whether or not the wide angle intra prediction mode can be used may be determined. For example, if the current block has a square shape with the same width and height, the directional modes selected from the histogram may consist of non-wide angle intra prediction modes. On the other hand, if the current block has a non-square shape with different widths and heights, some of the directional modes selected from the histogram can be converted to wide-angle intra prediction mode.

When performing intra prediction based on the intra prediction mode derived through a histogram, a predefined reference line can be used. Here, the predefined reference line may be an adjacent reference line adjacent to the current block (eg, index 0) or a non-adjacent reference line (eg, index 1).

Information indicating whether to apply the method of performing intra prediction by selecting an intra prediction mode through the histogram described above may be encoded and signaled through a bitstream. The information may be a 1-bit flag.

Alternatively, the intra prediction mode via the histogram, based on at least one of the following: size/shape of the current block, whether an ISP is applied to the current block, whether the current block borders a CTU boundary, or whether a neighboring block is encoded with intra prediction. It can be decided whether to select .

For example, when at least one of the top neighboring block or the left neighboring block of the current block is not encoded by intra prediction, a method of selecting an intra prediction mode through a histogram may be applied to the current block.

The method of selecting an intra prediction mode through a histogram can be applied to both luma components and chroma components. Alternatively, the above-described method may be applied only to the luma component. Alternatively, whether to select the intra prediction mode may be determined independently for each of the luma component and the chroma component through the histogram.

When encoding/decoding a chroma block, the intra prediction mode of the chroma block can be determined based on the characteristics of the luma block. Here, the characteristics of the luma block may represent the slope of samples belonging to the luma block (specifically, the horizontal slope Dx and the vertical slope Dy). Additionally, the sample used to derive the above gradient may refer to a reconstructed sample within the luma block.

Figure 17 is a diagram for explaining a method of determining the intra prediction mode of a chroma block based on the characteristics of the luma block.

The entire luma block or a partial area of the luma block can be set as the reference area. Then, through filtering, the slope for the sample in the reference region can be obtained. As an example, when the 3x3 sized mask shown in FIG. 14 is applied, as in the example shown in FIG. 17, for each of the remaining samples excluding samples adjacent to the border of the luma block, the horizontal slope Dx and the vertical direction The slope Dy can be obtained.

As another example, at least a partial area of the luma block and a previously restored area around the luma block may be set as a reference area. For example, at least one of at least one restored sample line adjacent to the top of the luma block or at least one restored sample line adjacent to the left of the luma block may be further included in the reference area.

After determining the intra prediction mode for each sample using the horizontal slope and vertical slope of each sample, a histogram can be derived based on the amplitude accumulation value for each intra prediction mode. Since the method for deriving the histogram has been explained in detail through Equations 3 to 5, detailed description thereof will be omitted.

The intra prediction mode with the largest accumulated amplitude value in the histogram can be set as the intra prediction mode of the chroma block at the same location as the luma block. That is, by performing intra prediction based on the intra prediction mode with the largest accumulated amplitude value in the histogram, the prediction block of the chroma block can be obtained.

As another example, N intra prediction modes with large amplitude accumulation values may be selected from the histogram, then intra prediction may be performed based on each of the N intra prediction modes to obtain N prediction blocks for the chroma block. Here, N may be a natural number of 1 or more. N may be determined based on at least one of the size/shape of the current block, the color format, or the number of intra prediction modes selected from the histogram during intra prediction of the luma block. Afterwards, the final prediction block of the chroma block can be obtained through an average operation or a weighted sum operation of the N prediction blocks.

At this time, for the weighted sum calculation, the weight applied to each prediction block may be determined based on the amplitude of the intra prediction mode. That is, among the plurality of intra prediction modes, the largest weight is assigned to the prediction block derived based on the intra prediction mode with the largest amplitude, and the smallest weight is assigned to the prediction block derived based on the intra prediction mode with the smallest amplitude. Can be assigned.

At this time, the weight assigned to each prediction block may be determined based on the ratio between amplitudes. Alternatively, the weight value for each amplitude rank may be previously stored, and then the weight mapped to the amplitude rank of the corresponding intra prediction mode may be applied to the prediction block.

As another example, at least one intra prediction mode with a large amplitude that does not overlap with the intra prediction mode of the previously derived chroma block may be selected from the histogram. Here, the intra prediction mode of the previously derived chroma block may be one of the intra prediction mode candidates determined through index information (see Table 2).

In this case, intra prediction may be performed based on each of the previously derived intra prediction mode of the chroma block and at least one intra prediction mode selected from the histogram, thereby obtaining a plurality of prediction blocks for the chroma block. Afterwards, the final prediction block of the chroma block can be obtained through an average operation or a weighted sum operation of multiple prediction blocks.

As another example, intra prediction may be performed based on at least one intra prediction mode and at least one default mode derived from the histogram, respectively, to obtain a plurality of prediction blocks for the chroma block. Here, the default mode may include at least one of a planar mode, a DC mode, or a predefined directional mode. Afterwards, the final prediction block of the chroma block can be obtained through an average operation or a weighted sum operation of multiple prediction blocks.

The intra prediction mode can also be determined on a sub-block basis within the chroma block.

Figure 18 is a diagram for explaining an example in which an intra prediction mode is determined on a sub-block basis.

For convenience of explanation, the color format is assumed to be 4:2:0.

As shown in the example, the chroma block may be divided into sub-blocks of a predetermined size. In the illustrated example, the chroma block is divided into sub-blocks of 2x2 size.

Additionally, a reference area can be set centered on the sub-block of the luma block corresponding to the sub-block of the chroma block. In the example shown in FIG. 18, the reference area of each 2x2 subblock in the chroma block is designated as a 4x4 area in the luma block corresponding to the 2x2 subblock. Unlike the example shown, the reference area for each sub-block may be designated in a size larger than 4x4.

Afterwards, a histogram can be derived for each sub-block. Based on the histogram of each subblock, at least one intra prediction mode can be obtained for each subblock. Thereafter, at least one sub-prediction block may be obtained for each sub-block based on at least one intra prediction mode. When a plurality of sub-prediction blocks are obtained, the final prediction block of each sub-block can be obtained based on an average operation or a weighted sum operation of the sub-prediction blocks.

The names of syntaxes used in the above-described embodiments are merely named for convenience of explanation.

Applying the embodiments described with a focus on the decoding process or encoding process to the encoding process or decoding process is included in the scope of the present invention. Modification of the embodiments described in the given order to an order different from that described is also within the scope of the present invention.

Although the above-described embodiments are explained based on a series of steps or flowcharts, this does not limit the chronological order of the invention, and may be performed simultaneously or in a different order as needed. Additionally, in the above-described embodiment, each of the components (e.g., units, modules, etc.) constituting the block diagram may be implemented as a hardware device or software, and a plurality of components may be combined to form a single hardware device or software. It may be implemented. The above-described embodiments may be implemented in the form of program instructions that can be executed through various computer components and recorded on a computer-readable recording medium. The computer-readable recording medium may include program instructions, data files, data structures, etc., singly or in combination. Examples of computer-readable recording media include magnetic media such as hard disks, floppy disks and magnetic tapes, optical recording media such as CD-ROMs and DVDs, and magneto-optical media such as floptical disks. media), and hardware devices specifically configured to store and perform program instructions, such as ROM, RAM, flash memory, etc. The hardware device may be configured to operate as one or more software modules to perform processing according to the invention and vice versa.

Claims (15)

determining at least one intra prediction mode for the current block; and
Comprising: performing intra prediction on the current block based on the at least one intra prediction mode,
An image decoding method, wherein the at least one intra prediction mode is determined through a histogram derived from a reference area of the current block. According to claim 1,
The reference area includes a top reference area located at the top of the current block and a left reference area located to the left of the current block,
The histogram is generated based on the intra prediction mode and amplitude of each reference sample belonging to the reference region. According to clause 2,
The intra prediction mode of the reference sample is obtained based on the ratio between the absolute value of the horizontal slope and the absolute value of the vertical slope of the reference sample. According to clause 3,
The amplitude of the reference sample is derived as the sum of the absolute value of the horizontal slope of the reference sample and the absolute value of the vertical descriptor. According to clause 3,
The horizontal slope and the vertical slope of the reference sample are obtained by applying a filter to the reference sample,
A video decoding method, wherein the filter is a Sobel mask or a Prewitt mask. According to clause 2,
The histogram is obtained by cumulatively summing the amplitudes of reference samples for each intra prediction mode. According to clause 6,
The at least one intra prediction mode is determined in descending order of amplitude accumulation values in the histgram. According to clause 7,
When multiple intra prediction modes are selected from the histogram,
Perform intra prediction based on each of the plurality of intra prediction modes to obtain a plurality of prediction blocks,
An image decoding method, characterized in that the final prediction block of the current block is obtained through an average operation or a weighted sum operation of the plurality of prediction blocks. According to claim 1,
By performing intra prediction based on each of the at least one intra prediction mode and the default mode, a plurality of prediction blocks are obtained,
An image decoding method, characterized in that the final prediction block of the current block is obtained through an average operation or a weighted sum operation of the plurality of prediction blocks. According to clause 9,
The default mode includes at least one of planar mode, DC mode, vertical mode, and horizontal mode. determining at least one intra prediction mode for the current block; and
Comprising: performing intra prediction on the current block based on the at least one intra prediction mode,
An image encoding method, wherein the at least one intra prediction mode is determined through a histogram derived from a reference region of the current block. According to claim 11,
The reference area includes a top reference area located at the top of the current block and a left reference area located to the left of the current block,
The histogram is generated based on the intra prediction mode and amplitude of each reference sample belonging to the reference region. According to claim 12,
The intra prediction mode of the reference sample is obtained based on the ratio between the absolute value of the horizontal slope and the absolute value of the vertical slope of the reference sample. According to claim 13,
The amplitude of the reference sample is derived as the sum of the absolute value of the horizontal slope of the reference sample and the absolute value of the vertical descriptor. determining at least one intra prediction mode for the current block; and
Comprising: performing intra prediction on the current block based on the at least one intra prediction mode,
A computer-readable recording medium storing a bitstream generated by an image encoding method, wherein the at least one intra prediction mode is determined through a histogram derived from a reference area of the current block.

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