KR20230013146A - Method for intra prediction and apparatus thereof - Google Patents

Abstract

In the intra prediction method according to the present invention, luma reference information including pixels in a luma reference block and neighboring luma reference pixels located around a luma reference block is obtained based on pixels in a current luma block and neighboring luma pixels located around the current luma block. generating, deriving a prediction parameter for the current chroma block based on a neighboring luma reference pixel and a first neighboring chroma pixel corresponding to the neighboring luma reference pixel among neighboring chroma pixels positioned around the current chroma block, and deriving a luma reference parameter. and deriving a predicted value of a pixel in the current chroma block based on the pixel in the block and the prediction parameter.

Description

Intra prediction method and device thereof {METHOD FOR INTRA PREDICTION AND APPARATUS THEREOF}

The present invention relates to image processing, and more particularly, to an intra prediction method and apparatus.

Recently, as broadcasting services with high definition (HD) resolution are expanded not only domestically but also globally, many users are getting used to high-resolution and high-definition images, and many organizations are accelerating the development of next-generation video equipment. In addition, as interest in UHD (Ultra High Definition) having a resolution four times higher than that of HDTV increases along with HDTV, a compression technology for a higher resolution and higher quality image is required.

For image compression, inter prediction technology predicts pixel values included in the current picture from temporally previous and/or subsequent pictures, and predicts pixel values included in the current picture using pixel information in the current picture. An intra prediction technique, an entropy coding technique in which short codes are assigned to symbols with a high frequency of occurrence and long codes are assigned to symbols with a low frequency of occurrence may be used.

A technical problem of the present invention is to provide an image encoding method and apparatus capable of improving image encoding/decoding efficiency.

Another technical problem of the present invention is to provide an image decoding method and apparatus capable of improving image encoding/decoding efficiency.

Another technical problem of the present invention is to provide an intra prediction method and apparatus capable of improving image encoding/decoding efficiency.

One embodiment of the present invention is an intra prediction method. The method includes generating luma reference information including pixels in a luma reference block and neighboring luma reference pixels located around the luma reference block, based on pixels in a current luma block and neighboring luma pixels located around the current luma block. deriving a prediction parameter for the current chroma block based on a first neighboring chroma pixel corresponding to the neighboring luma reference pixel among neighboring luma reference pixels and neighboring chroma pixels located around the current chroma block, and The method may include deriving a predicted value of a pixel in the current chroma block based on a pixel in the reference block and the prediction parameter. In the generating of the luma reference information, whether or not a neighboring luma pixel is available is determined according to whether a second neighboring chroma pixel corresponding to the neighboring luma pixel is available among neighboring chroma pixels located around the current chroma block. and determine a pixel value of the peripheral luma pixel according to whether the peripheral luma pixel is available.

In the generating of the luma reference information, when the second peripheral chroma pixel is available, it may be determined that the peripheral luma pixel is available.

When the position of the second neighboring chroma pixel is [x, y], the x is -1 and the y has a value of one of 0 to N _U *2-1, based on the second neighboring chroma pixel The location of the neighboring luma pixel for which availability is determined may be [x, 2y] or [x, 2y+1], and N _U may represent the horizontal and vertical lengths of the current chroma block.

When the position of the second neighboring chroma pixel is [x, y], y is -1 and x has a value of one of 0 to N _U *2-1, based on the second neighboring chroma pixel The location of the neighboring luma pixel for which availability is determined may be [2x, y] or [2x+1, y], and N _U may indicate the horizontal and vertical lengths of the current chroma block.

When the position of the second neighboring chroma pixel is [-1, -1], the position of the neighboring luma pixel whose availability is determined based on the second neighboring chroma pixel may be [-1, -1] there is.

In the generating luma reference information, when it is determined that the neighboring luma pixel is unavailable, a pixel value of an available pixel among pixels located around the current luma block may be allocated to the neighboring luma pixel.

In the luma reference information generating step, when it is determined that the peripheral luma pixel is a pixel on a left pixel line located adjacent to the left side of the current luma block and the peripheral luma pixel is available, A pixel value of a luma pixel may be determined as a pixel value of the neighboring luma pixel.

In the luma reference information generating step, if it is determined that the neighboring luma pixel is a pixel on an upper pixel line positioned adjacent to an upper end of the current luma block and the neighboring luma pixel is available, a luma pixel at the same position as the neighboring luma pixel A pixel value of may be determined as a pixel value of the surrounding luma pixel.

Another embodiment of the present invention is a video decoding method. The method includes generating luma reference information including pixels in a luma reference block and neighboring luma reference pixels located around the luma reference block, based on pixels in a current luma block and neighboring luma pixels located around the current luma block. deriving a prediction parameter for the current chroma block based on a first neighboring chroma pixel corresponding to the neighboring luma reference pixel among neighboring luma reference pixels and neighboring chroma pixels located around the current chroma block, generating a prediction block for the current chroma block by deriving a predicted value of a pixel in the current chroma block based on a pixel in the reference block and the prediction parameter; and generating a prediction block for the current chroma block based on the prediction block. It may include generating a restoration block. In the generating of the luma reference information, whether or not a neighboring luma pixel is available is determined according to whether a second neighboring chroma pixel corresponding to the neighboring luma pixel is available among neighboring chroma pixels located around the current chroma block. and determine a pixel value of the peripheral luma pixel according to whether the peripheral luma pixel is available.

In the generating of the luma reference information, when the second peripheral chroma pixel is available, it may be determined that the peripheral luma pixel is available.

When the position of the second neighboring chroma pixel is [x, y], the x is -1 and the y has a value of one of 0 to N _U *2-1, based on the second neighboring chroma pixel The location of the neighboring luma pixel for which availability is determined may be [x, 2y] or [x, 2y+1], and N _U may represent the horizontal and vertical lengths of the current chroma block.

When the position of the second neighboring chroma pixel is [x, y], y is -1 and x has a value of one of 0 to N _U *2-1, based on the second neighboring chroma pixel The location of the neighboring luma pixel for which availability is determined may be [2x, y] or [2x+1, y], and N _U may indicate the horizontal and vertical lengths of the current chroma block.

When the position of the second neighboring chroma pixel is [-1, -1], the position of the neighboring luma pixel whose availability is determined based on the second neighboring chroma pixel may be [-1, -1] there is.

In the generating luma reference information, when it is determined that the neighboring luma pixel is unavailable, a pixel value of an available pixel among pixels located around the current luma block may be allocated to the neighboring luma pixel.

In the luma reference information generating step, when it is determined that the peripheral luma pixel is a pixel on a left pixel line located adjacent to the left side of the current luma block and the peripheral luma pixel is available, A pixel value of a luma pixel may be determined as a pixel value of the neighboring luma pixel.

In the luma reference information generating step, if it is determined that the neighboring luma pixel is a pixel on an upper pixel line positioned adjacent to an upper end of the current luma block and the neighboring luma pixel is available, a luma pixel at the same position as the neighboring luma pixel A pixel value of may be determined as a pixel value of the surrounding luma pixel.

According to the video encoding method according to the present invention, video encoding/decoding efficiency can be improved.

According to the video decoding method according to the present invention, video encoding/decoding efficiency can be improved.

According to the intra prediction method according to the present invention, image encoding/decoding efficiency can be improved.

1 is a block diagram showing a configuration according to an embodiment of an image encoding apparatus to which the present invention is applied.
2 is a block diagram showing a configuration according to an embodiment of a video decoding apparatus to which the present invention is applied.
3 is a conceptual diagram schematically illustrating an embodiment in which one unit is divided into a plurality of sub-units.
4 shows an example of prediction directions of intra prediction modes and mode values assigned to each prediction direction.
5 is a flowchart schematically illustrating an embodiment of an inter-color component prediction method for performing prediction on a chroma component based on a luma component.
6 illustrates an embodiment of a method of deriving luma reference information when a size of a current luma block and a size of a current chroma block are different from each other.
7 is a diagram schematically illustrating an embodiment of a process of generating luma reference information based on an encoding parameter.
8 schematically illustrates an embodiment of a process of generating luma reference information when an unavailable pixel exists among neighboring luma pixels.
9 is a diagram for explaining an embodiment of a process of generating luma reference information after performing a padding process.
10 is a diagram for explaining another embodiment of a process of generating luma reference information after performing a padding process.
11 is a diagram for explaining another embodiment of a process of generating luma reference information after performing a padding process.
12 is a diagram for explaining another embodiment of a process of generating luma reference information after performing a padding process.
13 is a diagram schematically illustrating an embodiment of a process of deriving first color component reference information to be used for prediction of a second color component block when the size of the first color component block is smaller than the size of the second color component block. .
14 is a diagram schematically illustrating an embodiment of a process of deriving first color component reference information to be used for prediction of a second color component block when the size of the first color component block is the same as that of the second color component block. .
15 is a diagram schematically illustrating an example of peripheral luma pixels used to generate luma reference information.
16 is a diagram for explaining an embodiment of a process of deriving prediction parameters and a process of deriving predictive values of pixels in a current chroma block.
17 is a flowchart schematically illustrating an embodiment of a decryption process according to the present invention.
FIG. 18 is a diagram for explaining an embodiment of a process of deriving luma reference information and prediction parameters for a current chroma block when a decoding process is performed as in the embodiment of FIG. 17 .
19A and 19B are diagrams for explaining another embodiment of a decoding process according to the present invention.

EMBODIMENT OF THE INVENTION Hereinafter, embodiment of this invention is described concretely with reference to drawings. In describing the embodiments of the present specification, if it is determined that a detailed description of a related known configuration or function may obscure the gist of the present specification, the detailed description thereof will be omitted.

It is understood that when a component is referred to as “connected” or “connected” to another component, it may be directly connected or connected to the other component, but other components may exist in the middle. It should be. In addition, the description of "including" a specific configuration in the present invention does not exclude configurations other than the corresponding configuration, and means that additional configurations may be included in the practice of the present invention or the scope of the technical spirit of the present invention.

Terms such as first and second may be used to describe various components, but the components should not be limited by the terms. These terms are only used for the purpose of distinguishing one component from another. For example, a first element may be termed a second element, and similarly, a second element may be termed a first element, without departing from the scope of the present invention.

In addition, components appearing in the embodiments of the present invention are shown independently to represent different characteristic functions, and do not mean that each component is made of separate hardware or a single software component unit. That is, each component is listed and included as each component for convenience of explanation, and at least two components of each component can be combined to form a single 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 separated embodiments of components are also included in the scope of the present invention as long as they do not depart from the essence of the present invention.

In addition, some of the components may be optional components for improving performance rather than essential components that perform essential functions in the present invention. The present invention can be implemented by including only components essential to implement the essence of the present invention, excluding 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 invention.

1 is a block diagram showing a configuration according to an embodiment of an image encoding apparatus to which the present invention is applied.

Referring to FIG. 1 , the video encoding apparatus 100 includes a motion estimation unit 111, a motion compensation unit 112, an intra prediction unit 120, a switch 115, a subtractor 125, and a transform unit 130. , a quantization unit 140, an entropy encoding unit 150, an inverse quantization unit 160, an inverse transform unit 170, an adder 175, a filter unit 180, and a reference picture buffer 190.

The image encoding apparatus 100 may encode an input image in an intra mode or an inter mode and output a bitstream. Intra prediction means prediction within a picture, and inter prediction means prediction between pictures. In the case of the intra mode, the switch 115 may be switched to the intra mode, and in the case of the inter mode, the switch 115 may be switched to the inter mode. After generating a prediction block for an input block of an input image, the image encoding apparatus 100 may encode a residual between the input block and the prediction block.

In the case of the intra mode, the intra prediction unit 120 may generate a prediction block by performing spatial prediction using pixel values of previously encoded blocks adjacent to the current block.

In the case of the inter mode, the motion estimation unit 111 may obtain a motion vector by finding a region that best matches the input block in the reference picture stored in the reference picture buffer 190 during the motion prediction process. The motion compensator 112 may generate a prediction block by performing motion compensation using a motion vector. Here, the motion vector is a 2D vector used for inter prediction, and may represent an offset between a current encoding/decoding target image and a reference image.

In the prediction process of the intra prediction unit 120, the motion prediction unit 111, and/or the motion compensation unit 112, the processing unit for which the prediction method or prediction mode is determined and the processing unit in which prediction is performed may be the same, but may be different from each other. may be For example, when a processing unit for which a prediction method is determined is a PU, a processing unit for which prediction is performed may be a TU.

The subtractor 125 may generate a residual block by a difference between the input block and the generated prediction block. The transform unit 130 may output a transform coefficient by performing transform on the residual block. The quantization unit 140 may quantize the input transform coefficient according to a quantization parameter and output a quantized coefficient.

The entropy encoding unit 150 may output a bit stream by performing entropy encoding based on the values calculated by the quantization unit 140 or the encoding parameter values calculated in the encoding process.

When entropy encoding is applied, a small number of bits are allocated to a symbol with a high probability of occurrence and a large number of bits are allocated to a symbol with a low probability of occurrence to represent the symbol, thereby reducing the number of bits for symbols to be coded. The size of the column may be reduced. Therefore, compression performance of image encoding can be improved through entropy encoding. The entropy encoding unit 150 may use an encoding method such as exponential golomb, context-adaptive variable length coding (CAVLC), or context-adaptive binary arithmetic coding (CABAC) for entropy encoding.

Since the video encoding apparatus according to the embodiment of FIG. 1 performs inter-prediction encoding, that is, inter-prediction encoding, a currently encoded video needs to be decoded and stored in order to be used as a reference video. Therefore, the quantized coefficient is inversely quantized in the inverse quantization unit 160 and inversely transformed in the inverse transformation unit 170. The inverse quantized and inverse transformed coefficients are added to the prediction block through an adder 175, and a reconstructed block is generated.

The reconstructed block passes through a filter unit 180, and the filter unit 180 applies at least one of a deblocking filter, a sample adaptive offset (SAO), and an adaptive loop filter (ALF) to the reconstructed block or the reconstructed picture. can do. The filter unit 180 may also be called an adaptive in-loop filter. The deblocking filter may remove block distortion generated at a boundary between blocks. SAO may add an appropriate offset value to a pixel value to compensate for a coding error. ALF may perform filtering based on a value obtained by comparing a reconstructed image with an original image. A reconstructed block that has passed through the filter unit 180 may be stored in the reference picture buffer 190 .

2 is a block diagram showing a configuration according to an embodiment of a video decoding apparatus to which the present invention is applied.

Referring to FIG. 2 , the image decoding apparatus 200 includes an entropy decoding unit 210, an inverse quantization unit 220, an inverse transform unit 230, an intra prediction unit 240, a motion compensation unit 250, an adder ( 255), a filter unit 260, and a reference picture buffer 270.

The image decoding apparatus 200 may receive the bitstream output from the encoder, perform decoding in an intra mode or inter mode, and output a reconstructed image, that is, a reconstructed image. In the intra mode, the switch may be converted to intra, and in the case of the inter mode, the switch may be converted to inter. The image decoding apparatus 200 may generate a reconstructed block, that is, a reconstructed block, by obtaining a residual block from the input bitstream, generating a prediction block, and then adding the residual block and the prediction block.

The entropy decoding unit 210 may generate symbols including symbols in the form of quantized coefficients by entropy decoding the input bitstream according to a probability distribution. The entropy decoding method is similar to the entropy encoding method described above.

When the entropy decoding method is applied, a small number of bits are allocated to a symbol with a high probability of occurrence and a large number of bits are allocated to a symbol with a low probability of occurrence to express the symbol, thereby increasing the size of the bit stream for each symbol. can be reduced Therefore, compression performance of image decoding may be improved through the entropy decoding method.

The quantized coefficient is inversely quantized in the inverse quantization unit 220 and inversely transformed in the inverse transform unit 230, and as a result of inverse quantization/inverse transformation of the quantized coefficient, a residual block may be generated.

In the case of the intra mode, the intra prediction unit 240 may generate a prediction block by performing spatial prediction using pixel values of previously encoded blocks adjacent to the current block. In the case of the inter mode, the motion compensation unit 250 may generate a prediction block by performing motion compensation using a motion vector and a reference image stored in the reference picture buffer 270 .

In the prediction process of the intra prediction unit 240 and/or the motion compensation unit 250, a processing unit in which a prediction method or prediction mode is determined and a processing unit in which prediction is performed may be the same or may be different from each other. For example, when a processing unit for which a prediction method is determined is a PU, a processing unit for which prediction is performed may be a TU.

The residual block and the prediction block are added through the adder 255, and the added block may pass through the filter unit 260. The filter unit 260 may apply at least one of a deblocking filter, SAO, and ALF to a reconstructed block or a reconstructed picture. The filter unit 260 may output a reconstructed image, that is, a restored image. The reconstructed image may be stored in the reference picture buffer 270 and used for inter prediction.

Hereinafter, a unit may mean a unit of image encoding and decoding. When encoding and decoding an image, the encoding or decoding unit refers to the divided unit when encoding or decoding an image by dividing it, such as a coding unit (CU), a prediction unit (PU), and a transform unit (TU). : Transform Unit). Also, in embodiments described below, a unit may also be referred to as a block. One unit can be further divided into sub-units of smaller size.

3 is a conceptual diagram schematically illustrating an embodiment in which one unit is divided into a plurality of sub-units.

One unit may be hierarchically divided with depth information based on a tree structure. Each divided sub-unit may have depth information. Since the depth information indicates the number and/or degree of division of the unit, it may include information about the size of the sub-unit.

310 of FIG. 3 shows an embodiment of a tree structure in which one unit is divided into a plurality of sub-units. Also, 320 in FIG. 3 shows an embodiment of a unit that is divided into a plurality of sub-units. The unit shown in 320 of FIG. 3 may be, for example, a CU or a TU.

Referring to 310 of FIG. 3 , the highest node may be referred to as a root node and may have the smallest depth value. At this time, the highest node may have a depth of level 0 and may represent the first undivided unit.

A lower node having a depth of level 1 may represent a unit in which the first unit is divided once, and a lower node having a depth of level 2 may represent a unit in which the first unit is divided twice. For example, unit a corresponding to node a in 320 of FIG. 3 is a unit divided once from an initial unit and may have a depth of level 1.

A leaf node of level 3 may represent a unit in which the initial unit is divided 3 times. For example, unit d corresponding to node d in 320 of FIG. 3 is a unit divided three times from an initial unit and may have a depth of level 3. Accordingly, a leaf node of level 3, which is the lowest node, may have the deepest depth.

Hereinafter, in embodiments to be described later, an encoding/decoding target block may be referred to as a current block in some cases. Also, when intra prediction is performed on an encoding/decoding object block, the encoding/decoding object block may be referred to as a prediction object block.

Meanwhile, an image signal may be composed of color components that form a color space. For example, an image signal may generally include three color components representing three primary color components of light. Three color components representing the three primary color components of light can be represented by R (Red), G (Green), and B (Blue). The R, G, and B signals may be converted into one luma component and two chroma components in order to reduce a frequency band used for image processing. In this case, one image signal may include one luma component and two chroma components. Here, the luma component is a component representing the brightness of the screen and may be represented by Y. In addition, the two chroma components are components representing the color of the screen and may be represented by U, V, Cb, or Cr. Accordingly, the color space of the image may be expressed as YUV or YCbCr, for example. Hereinafter, in embodiments to be described later, a block having a luma component may be referred to as a luma block, and a block having a chroma component may be referred to as a chroma block.

Human eyes are sensitive to luma component signals and insensitive to chroma component signals. When R, G, and B signals are converted into luma and chroma signals using these characteristics, a frequency band used for image processing can be reduced. Also, due to this characteristic, the number of pixels of a chroma component within one image or block may be smaller than the number of pixels of a luma component.

For example, in the 4:2:0 video format, the number of horizontal pixels of a chroma block corresponding to a luma block is 1/2 of the number of horizontal pixels of a luma block, and the vertical pixels of a chroma block corresponding to a luma block. The number may correspond to 1/2 of the number of pixels in the vertical direction of the luma block. In the 4:2:2 video format, the number of pixels in the horizontal direction of a chroma block corresponding to a luma block is 1/2 of the number of pixels in the horizontal direction of a luma block, and the number of pixels in the vertical direction of a chroma block corresponding to a luma block is 1/2 of the number of pixels in the horizontal direction of a luma block. may be equal to the number of pixels in the vertical direction of In the 4:4:4 video format, the number of pixels in the horizontal and vertical directions of a chroma block corresponding to a luma block may be equal to the number of pixels in the horizontal and vertical directions of the luma block, respectively.

Meanwhile, intra prediction may be performed according to the intra prediction mode of the current block. Since the intra prediction mode of the current block has a high correlation with the prediction modes of neighboring blocks, the prediction mode of the current block may be encoded based on the prediction modes of the neighboring blocks of the current block. At this time, the most probable mode (MPM) for the current block may be used.

4 shows an example of prediction directions of intra prediction modes and mode values assigned to each prediction direction. Hereinafter, in this specification, a pixel may have the same meaning as a sample, and in embodiments to be described later, a pixel may be referred to as a sample in some cases.

As described above in the embodiments of FIGS. 1 and 2 , the encoder and decoder may generate a prediction block by performing intra prediction based on pixel information in the current picture. That is, when performing intra prediction, the encoder and decoder may perform directional prediction and/or non-directional prediction based on at least one reconstructed reference pixel. Here, the prediction block may refer to a block generated as a result of performing intra prediction. The prediction block may correspond to at least one of a coding unit (CU), a prediction unit (PU), and a transform unit (TU).

As described above, intra prediction may be performed according to the intra prediction mode of the current block. The number of intra prediction modes that the current block may have may be a predetermined fixed value or may be a value determined differently depending on the size of the prediction block. For example, the number of intra prediction modes that the current block may have may be 35 or 36.

Referring to 410 of FIG. 4 , for example, in the vertical mode where the mode value is 1, prediction may be performed in the vertical direction based on the pixel value of the reference pixel, and in the case of the horizontal mode where the mode value is 2, the reference pixel Prediction may be performed in a horizontal direction based on a pixel value of . Even in the case of a directional mode other than the above-described mode, the encoder and the decoder may perform intra prediction using a reference pixel according to a corresponding angle.

In 410 of FIG. 4 , a prediction mode having a mode value of 0 may be referred to as a planar mode, and a prediction mode having a mode value of 3 may be referred to as a DC mode. The planar mode (eg, Intra_Planar) and the DC mode (eg, Intra_DC) may correspond to non-directional modes. For example, in the DC mode, a prediction block may be generated by averaging pixel values of a plurality of reference pixels. In the planner mode, the position of a reference pixel to be used for prediction of the target pixel may be determined based on the position of the target pixel to be predicted, and the predicted value of the target pixel may be derived based on the reference pixel existing at the determined position.

Also, a prediction mode having a mode value of 35 in 410 of FIG. 4 may be called an LM mode (eg, Intra_FromLuma). The LM mode may refer to a prediction mode in which a prediction value of a pixel of a chroma component (eg, U or V) is determined according to a pixel value of a luma component (eg, Y). A specific embodiment of intra prediction in LM mode will be described later.

The number of intra prediction modes and/or the mode value assigned to each intra prediction mode is not limited to the above-described embodiment, and may be determined differently according to implementation and/or needs. For example, a prediction direction of an intra prediction mode and a mode value assigned to each prediction mode may be determined differently from 410 of FIG. 4 as in 420 of FIG. 4 . Also, as an example, the LM mode may be applied to the embodiment of 420 of FIG. 4 as in 410 of FIG. 4 . In this case, a mode value of 35 may be assigned to the LM mode. Hereinafter, in embodiments to be described later, for convenience of description, it is assumed that intra prediction is performed based on the intra prediction mode as shown in 410 of FIG. 4 unless otherwise specified.

Meanwhile, since the luma component and the chroma component in the image have a correlation, the prediction mode of the chroma component can be encoded based on the prediction mode of the corresponding luma component, and the decoder predicts the chroma component based on the prediction mode of the luma component. mode can be derived. Accordingly, the prediction mode information of the chroma component transmitted from the encoder to the decoder may be a separate value used to derive the prediction mode of the chroma component in relation to the prediction mode of the luma component, rather than the prediction mode of the chroma component itself. Table 1 below shows an embodiment of a chroma prediction mode determined according to a prediction mode of a luma component and a transmitted value.

[Table 1]

Referring to Table 1, a value transmitted from an encoder to a decoder may be a value assigned to intra_chroma_pred_mode. IntraPredMode represents an intra prediction mode of a luma component. For example, when the intra_chroma_pred_mode value is 2 and the value of IntraPredMode is 26, the intra prediction mode value of the chroma component may be 10. The intra_chroma_pred_mode and IntraPredMode are not limited to their names. In addition, values assigned to each prediction mode may be determined differently according to implementation and/or needs as an example.

In the example of Table 1, when the value of intra_chroma_pred_mode is 4, the prediction mode of chroma components may be called LM mode, and when the value of intra_chroma_pred_mode is 5, the prediction mode of chroma components may be called DM mode. The LM mode is a prediction mode in which a predicted value of a chroma component pixel is determined according to a pixel value of a luma component, and a DM mode is a prediction mode in which a prediction mode of a luma component is used as a prediction mode of a chroma component.

In addition, the encoder encodes LM flag information indicating whether the LM mode is applied according to the prediction mode of chroma components in the current sequence, that is, whether the LM mode can be applied to intra prediction of chroma components in the current sequence, and can transmit The LM flag information may be transmitted to the decoder through, for example, a syntax element called chroma_pred_from_luma_enabled_flag. For example, if 1 is allocated to chroma_pred_from_luma_enabled_flag, the flag may indicate that the LM mode can be used for intra prediction of chroma components. Also, when 0 is assigned to chroma_pred_from_luma_enabled_flag, the flag may indicate that the LM mode is not used for intra prediction of chroma components.

When the LM flag is used, if 1 is allocated to chroma_pred_from_luma_enabled_flag, the chroma prediction mode may be determined according to the embodiment of Table 1. However, if 0 is assigned to chroma_pred_from_luma_enabled_flag, since the LM mode is not used for intra prediction of chroma components, the chroma prediction mode determined according to the luma component prediction mode and transmitted value may be determined differently from the embodiment of Table 1. For example, when the LM mode is not used for intra prediction of a chroma component, the chroma prediction mode may be represented as in the embodiment of Table 2 below.

[Table 2]

Meanwhile, since a plurality of color components constituting the color space have a correlation with each other, by performing prediction on another color component (second color component) based on one color component (first color component), the second color component is predicted. Encoding efficiency for components may be improved. This prediction process may also be called 'inter-color component prediction'. As described above, the color components constituting the color space may include R, G, B, a luma component (eg, Y), and/or a chroma component (eg, U, V, Cb, and Cr).

For example, in the above-described LM mode, prediction of a chroma component may be performed based on a luma component. In this case, the first color component may correspond to a luma component, and the second color component may correspond to a chroma component. In the LM mode, a correlation between a luma component and a chroma component may be expressed as a linear relationship using a prediction parameter. A detailed embodiment of a process of predicting a chroma component based on a luma component will be described later.

The first color component and the second color component may have various combinations, but in this specification, for convenience of description, the first color component is referred to as a luma component and the second color component is referred to as a chroma component. However, embodiments to be described later are not limited thereto and may be applied in the same or similar manner to all possible combinations of the first color component and the second color component. In this case, in embodiments described later, the luma component may be regarded as a first color component, and the chroma component may be regarded as a second color component.

5 is a flowchart schematically illustrating an embodiment of an inter-color component prediction method for performing prediction on a chroma component based on a luma component. A prediction process described later may be performed by intra prediction units of the encoding device and the decoding device.

Hereinafter, in this specification, a chroma block to be encoded/decoded is referred to as a current chroma block. Also, a 'reconstructed' luma block corresponding to the current chroma block is referred to as a current luma block. For example, the current luma block may be a block existing at the same location as the current chroma block. As another example, the current luma block may be a block existing at a predetermined relative position or a predetermined fixed position with respect to the current chroma block.

Referring to FIG. 5 , the encoder and decoder may derive luma reference information to be used for prediction of the current chroma block based on pixels within the current luma block and pixels around the current luma block (S510).

Luma reference information may refer to pixel values of a luma component to be used for prediction of a current chroma block. Therefore, in this specification, luma reference information may also be referred to as a 'luma reference pixel'. In this case, luma reference pixels corresponding to pixels in the current chroma block may constitute a 'luma reference block'. Also, in this specification, pixels located around the current luma block are referred to as 'surrounding luma pixels', pixels located around the current chroma block are referred to as 'surrounding chroma pixels', and luma reference pixels located around the luma reference block are referred to as 'surrounding chroma pixels'. These are called 'peripheral luma reference pixels'. In this case, the luma reference block may correspond to the current chroma block, and the neighboring luma reference pixels may correspond to the neighboring chroma pixels. In this specification, luma reference information may be used as a concept including a pixel in a luma reference block and a neighboring luma reference pixel.

Meanwhile, as described above, the current luma block and the current chroma block may have different sizes according to an image format. In this case, the encoder and decoder perform down-sampling and/or up-sampling on pixels in the current luma block and pixels around the current luma block, so that the luma block has the same size as the current chroma block. A reference block and corresponding neighboring luma reference pixels may be derived.

In this case, the encoder and the decoder may generate luma reference information differently based on the encoding parameter. Here, the encoding parameter is a parameter necessary for encoding and decoding, and may include information that can be inferred in the process of encoding or decoding, as well as information encoded in the encoder and transmitted to the decoder, such as syntax elements, and can be used to encode or decode an image. It can mean the information you need when you do it. Encoding parameters may include, for example, an encoding mode, an intra prediction mode, an inter prediction mode, and a quantization parameter (QP).

In addition, there may be unavailable pixels among pixels positioned around the current luma block, luma reference block, and/or current chroma block. In this case, the encoder and decoder may perform a padding process on unavailable pixels. Here, padding may refer to a process of filling an unavailable pixel with a new pixel value based on an available pixel.

Specific embodiments of the process of deriving luma reference information will be described later.

Referring back to FIG. 5 , the encoder and decoder may derive a prediction parameter for the current chroma block based on luma reference information and chroma component information (S520).

For example, the encoder and decoder may derive prediction parameters based on pixel values of neighboring luma reference pixels and pixel values of neighboring chroma pixels. Here, the prediction parameters may be alpha (α) and beta (β) based on the least squares method.

Specific embodiments of the prediction parameter derivation process will be described later.

Referring back to FIG. 5 , the encoder and decoder may derive predicted values of pixels in the current chroma block based on the prediction parameter and the luma reference pixel value in the luma reference block (S530). That is, the encoder and decoder may perform intra prediction on the current chroma block based on the prediction parameter and the luma reference pixel value in the luma reference block. A detailed embodiment of a process of deriving predicted values of pixels in the current chroma block will be described later.

6 illustrates an embodiment of a method of deriving luma reference information when a size of a current luma block and a size of a current chroma block are different from each other.

As described above, the current luma block and the current chroma block may have different sizes depending on the image format. For example, when the color space of an image is YUV 4:2:0 or YCbCr 4:2:0, the size of the current luma block may be 4 times larger than the size of the current chroma block. In this case, the encoder and decoder may derive pixels in the luma reference block having the same size as the current chroma block based on the current luma block, and may derive neighboring luma reference pixels located around the luma reference block. That is, in this case, the encoder and decoder may generate luma reference information to correspond to the size of the current chroma block.

610 of FIG. 6 illustrates an embodiment of the current luma block 612 , surrounding luma pixels 614 , current chroma block 616 , and surrounding chroma pixels 618 . Referring to 610 of FIG. 6 , when the size of the current chroma block 616 is 4x4, the size of the corresponding current luma block 612 may be 8x8. In this case, the encoder and decoder may generate luma reference pixels corresponding to a 4x4 luma reference block.

620 of FIG. 6 illustrates an embodiment of a luma reference information derivation method.

In the embodiment 620 of FIG. 6 , as in 610 of FIG. 6 , a method of deriving luma reference information 626 and 628 when the size of the current luma block 612 is 8x8 and the size of the current chroma block 616 is 4x4. this is shown

Referring to 620 of FIG. 6 , an encoder and a decoder determine a luma reference block 626 and its surroundings to be used for prediction of the current chroma block 616 based on the pixels in the current luma block 612 and the surrounding luma pixels 624 . Luma reference pixels 628 may be created. Here, the neighboring luma pixels 624 used for deriving the luma reference information are pixels included in two pixel lines located closest to the left side of the current luma block 612 and pixels located on top of the current luma block 612. Pixels included in one pixel line located closest to each other may be included.

Meanwhile, in 620 of FIG. 6 , since the size of the current luma block 612 and the current chroma block 616 are different from each other, the encoder and the decoder use a luma reference block 626 having the same size (4x4) as the current chroma block 616. and peripheral luma reference pixels 628 corresponding thereto. At this time, since the size of the current chroma block 616 is smaller than the size of the current luma block 612, the encoder and decoder down-samples pixels in the current luma block 612 and neighboring luma pixels 624 ( down sampling) may generate luma reference information.

Hereinafter, coordinates (eg, (x, y)) in this specification may be coordinates based on the position of the upper leftmost pixel in the block. For example, in this specification, the coordinates of the top leftmost pixel in the block may be (0,0).

In one embodiment, the encoder and decoder may use a 2-tap filter, a 3-tap filter and/or a 4-tap filter for the pixels in the current luma block 612 and surrounding luma pixels 624. Downsampling may be performed by applying a (4-tap) filter or the like. In this case, the downsampling process may be represented by Equation 1 below as an example.

[Equation 1]

p'[x,y] = (p[2x,2y] + p[2x,2y+1]) >> 1

p'[x,y] = (p[2x+1,2y] + p[2x+1,2y+1]) >> 1

p'[x,y] = (p[2x-1,2y+1] + 2*p[2x,2y+1] + p[2x+1,2y+1] + 2) >> 2

Here, p[x,y] may indicate pixel values of reconstructed luma component pixels, that is, pixels in the current luma block 612 and neighboring luma pixels 624 . Also, p'[x,y] may represent pixel values of the luma reference pixels 626 and 628 generated according to the size of the current chroma block 616 .

In another embodiment, the encoder and decoder may perform downsampling by selecting pixels to be used as luma reference pixels from among pixels in the current luma block 612 and neighboring luma pixels 624 according to a predetermined operation. In this case, for example, the encoder and decoder may select one pixel from a pixel group having a predetermined size and use it as a luma reference pixel. This down-sampling process may be performed by at least one of the down-sampling methods represented by Equations 2, 3, and 4 below, for example.

[Equation 2]

p'[x,y] = p[2x,2y]

[Equation 3]

p'[x,y] = p[2x,2y+1]

[Equation 4]

p'[x,y] = p[2x+1,2y]

In addition, different downsampling schemes may be applied to pixels in the current luma block 612, neighboring luma pixels positioned on top of the current luma block 612, and neighboring luma pixels positioned on the left side of the current luma block 612. there is. For example, when the downsampling method according to Equation 1 is applied to pixels within the current luma block 612, the downsampling method according to Equation 2 is applied to neighboring luma pixels located on top of the current luma block 612. Alternatively, the downsampling method according to Equation 3 may be applied to the neighboring luma pixels located to the left of the current luma block 612 . Here, although combinations of possible embodiments to which the downsampling scheme is applied are not all described, those skilled in the art will recognize that other combinations of the above-described downsampling schemes may be used.

630 of FIG. 6 illustrates another embodiment of a luma reference information derivation method.

In the embodiment 630 of FIG. 6 , as in 610 of FIG. 6 , a method of deriving luma reference information 636 and 638 when the size of the current luma block 612 is 8x8 and the size of the current chroma block 616 is 4x4. this is shown

Referring to 630 of FIG. 6 , the encoder and the decoder determine a luma reference block 636 and its surroundings to be used for prediction of the current chroma block 616 based on the pixel in the current luma block 612 and the surrounding luma pixels 634 . Luma reference pixels 638 may be created.

In 630 of FIG. 6 , unlike 620 of FIG. 6 , pixels included in “one” pixel line located closest to the left side of the current luma block 612 and closest to the top of the current luma block 612 Luma reference information 636 and 638 may be derived by performing down-sampling based on pixels included in one located pixel line. That is, in the embodiment 630 of FIG. 6 , only pixels 634 included in one pixel line at the left and top of the current luma block 612 may be used to derive the luma reference information 636 and 638 .

Since the method of performing downsampling in 630 of FIG. 6 is the same as that of 620 of FIG. 6 , a detailed description of the method of performing downsampling will be omitted.

Meanwhile, the encoder and decoder may generate luma reference information differently based on a block including neighboring luma pixels and/or an encoding parameter of the current luma block. As described above, the encoding parameter may include an encoding mode, an intra prediction mode, an inter prediction mode, a quantization parameter (QP), and the like. For example, the encoder and decoder may generate luma reference information using only pixel values of pixels having small distortion.

As an embodiment, an encoder and a decoder may generate luma reference information differently based on an encoding mode. For example, the encoder and decoder may use only pixels within a block encoded in an intra mode among pixels within a current luma block and neighboring luma pixels to derive luma reference information.

An additional embodiment of a method of generating luma reference information based on an encoding parameter will be described later with reference to FIG. 7 .

7 is a diagram schematically illustrating an embodiment of a process of generating luma reference information based on an encoding parameter.

710 of FIG. 7 represents an embodiment of a method of generating luma reference information 716 and 718 based on the intra prediction mode of the current luma block 712 . In 710 of FIG. 7 , it is assumed that the size of the current luma block 712 is 8x8 and the size of the current chroma block is 4x4.

In 710 of FIG. 7 , a luma reference block 716 and neighboring luma reference pixels 718 to be used for prediction of the current chroma block are generated based on the pixels in the current luma block 712 and the neighboring luma pixels 714. It can be. Since the size of the current chroma block is 4x4, the size of the generated luma reference block 716 may be 4x4.

At this time, since the intra prediction mode of the current luma block 712 is the vertical mode 722, the encoder and the decoder select only pixels located adjacent to the top of the current luma block 712 from among the surrounding luma pixels 714. may be used to generate luma reference information (716, 718). Accordingly, the peripheral luma reference pixels 718 derived from 710 of FIG. 7 may be pixels positioned adjacent to an upper end of the luma reference block 716 .

Unlike 710 of FIG. 7 , if the intra prediction mode of the current luma block is a horizontal mode, the encoder and decoder may use only pixels adjacent to the left side of the current luma block among neighboring luma pixels to generate luma reference information. there is. In this case, the derived peripheral luma reference pixels may be pixels positioned adjacent to the left side of the luma reference block. When the intra prediction mode of the current luma block is the DC mode or the planner mode, the encoder and decoder can use both pixels located on the left side of the current luma block and pixels located on the top side of the current luma block to generate luma reference information. In this case, the derived peripheral luma reference pixels may include both pixels located on the left side of the luma reference block and pixels located on top of the luma reference block.

730 of FIG. 7 shows an embodiment of a method of generating luma reference information 752 and 754 based on quantization parameters of neighboring luma blocks 734 , 736 , and 738 located around the current luma block 732 . In 730 of FIG. 7 , it is assumed that the size of the current luma block 732 is 8x8 and the size of the current chroma block is 4x4.

Referring to 730 in FIG. 7 , the quantization parameter value of the luma block 734 adjacent to the upper end adjacent to the upper end of the current luma block 732 is 22, and the upper left corner located at the upper left corner outside the current luma block 732. The value of the quantization parameter of the peripheral luma block 736 is 26. In addition, the quantization parameter value of the left peripheral luma block 738 positioned adjacent to the left side of the current luma block 732 is 28. In this case, the encoder and decoder may use neighboring luma pixels belonging to a neighboring luma block having the smallest quantization parameter value among the neighboring luma pixels 744 , 746 , and 748 to generate luma reference information.

In 730 of FIG. 7 , among the top peripheral luma block 734, the upper left peripheral luma block 736, and the left peripheral luma block 738, the upper peripheral luma block 734 has the smallest quantization parameter value (eg, 22). Therefore, the peripheral luma pixels 744 in the upper peripheral luma block 734 may be used to generate luma reference information. That is, in 730 of FIG. 7 , the encoder and decoder determine that the pixels in the current luma block 732 and the peripheral luma pixels 744 in the upper peripheral luma block 734 are the luma reference block 752 and the peripheral luma reference pixels ( 754) can be used.

In 730 of FIG. 7 , since the size of the current chroma block is 4x4, the size of the generated luma reference block 752 may be 4x4. Also, the peripheral luma reference pixels 754 derived from 730 of FIG. 7 may be pixels positioned adjacent to an upper end of the luma reference block 752 .

In the embodiment of FIG. 7 , a downsampling process performed to generate luma reference information is similar to that in the embodiment of FIG. 6 , so a detailed description thereof will be omitted here.

8 schematically illustrates an embodiment of a process of generating luma reference information when an unavailable pixel exists among neighboring luma pixels.

As described above, there may be unavailable pixels among the neighboring luma pixels located around the current luma block. Cases in which the neighboring luma pixels are not available are when the current luma block is located adjacent to the boundary of the current picture, when the current luma block is located adjacent to the boundary of the current slice, and CIP (Constrained Intra Prediction) may be applied.

810 of FIG. 8 illustrates an embodiment of a method of deriving luma reference information when a current luma block is positioned adjacent to a boundary of a current picture and/or a current slice. Referring to 810 of FIG. 8 , the current luma block 812 may be a block located adjacent to the left boundary 818 of the current picture and/or current slice. In this case, since the neighboring luma pixels 816 positioned to the left of the current luma block 812 are positioned outside the current picture and/or current slice, they may correspond to unavailable pixels.

At this time, according to an embodiment, the encoder and decoder may perform a padding process on unavailable pixels prior to performing the downsampling process. As described above, padding may refer to a process of filling in unavailable pixel positions with new pixel values.

As shown in 810 of FIG. 8 , when the surrounding luma pixels 816 located on the left side of the current luma block 812 are not available, the encoder and decoder use the available surrounding luma pixels located on the top side of the current luma block 812. A pixel value of at least one pixel among the pixels 814 may be filled in with positions of neighboring luma pixels positioned to the left of the current luma block 812 . For example, a process of filling pixel values into a first pixel line located closest to the left of the current luma block 812 may be expressed by Equation 5 below.

[Equation 5]

p[-1,y] = p[0,-1], (y=0~7)

Here, p[x,y] may represent a reconstructed pixel value of a luma component.

A process of filling a pixel value into a pixel line located second closest to the left side of the current luma block 812, that is, a second pixel line located adjacent to the left side of the first pixel line, is represented by Equation 6 below as an example. can

[Equation 6]

p[-2,y] = p[0,-1], (y=0~7)

When the padding process is performed, the encoder and decoder may use newly filled pixel values to generate luma reference information. That is, the encoder and decoder generate luma reference information based on the pixel values of available neighboring luma pixels 814, the pixel values of pixels 816 filled by the padding process, and the pixel values of pixels in the current luma block 812. can create

In another embodiment, the encoder and decoder may not use unavailable neighboring luma pixels to generate luma reference information. That is, the encoder and decoder may generate luma reference information based on pixels in the current luma block and available neighboring luma pixels. For example, in 810 of FIG. 8 , pixels in the current luma block 812 and available peripheral luma pixels 814 located on top of the current luma block 812 may be used to generate luma reference information.

In the luma reference information generation process in 810 of FIG. 8 , at least one of the luma reference information generation methods described above in the embodiments of FIGS. 6 and 7 may be applied together with the above-described embodiments.

820 of FIG. 8 illustrates an embodiment of a method of deriving luma reference information when Constrained Intra Prediction (CIP) is applied.

CIP may refer to an intra prediction method in which only pixels within a block to be encoded/decoded by intra prediction among blocks located around a block to be predicted are used for prediction. When CIP is applied, pixels in a block encoded/decoded by inter prediction may not be used for intra prediction.

For example, the encoder may transmit flag information on whether CIP is applied to the decoder. At this time, the decoder may determine whether to apply CIP based on the flag information. Flag information on whether CIP is applied may be represented as 'constrained_intra_pred_flag', for example.

Referring to 820 of FIG. 8 , an upper peripheral luma block 824 located on top of the current luma block 822 is a block encoded/decoded in an intra mode, and is located on the left side of the current luma block 822. The peripheral luma block 826 may be a block encoded/decoded in an inter mode. In this case, the peripheral luma pixels 836 located in the left peripheral luma block 826 may correspond to unavailable pixels.

At this time, according to an embodiment, the encoder and decoder may perform a padding process on unavailable pixels prior to performing the downsampling process. As described above, padding may refer to a process of filling in unavailable pixel positions with new pixel values. In this case, the encoder and decoder may fill unavailable peripheral luma pixels with pixel values of available peripheral luma pixels.

For example, as shown in 820 of FIG. 8 , when the peripheral luma pixels 836 located within the left peripheral luma block 826 are not available, the encoder and decoder use the available peripherals located within the upper peripheral luma block 824. A pixel value of at least one pixel from among the luma pixels 834 may be filled in at locations of unavailable neighboring luma pixels 836 . As an example, a process of performing padding based on pixels in the upper peripheral luma block 824 may be expressed by Equation 7 below.

[Equation 7]

p[-2,y] = p[-2,-1] (y=0~7) or

p[-2,y] = p[-1,-1] (y=0~7) or

p[-1,y] = p[-1,-1] (y=0~7)

As another example, the pixels 838 positioned adjacent to the lower end of the left peripheral luma block 826 may be pixels in a block encoded/decoded in an intra mode. At this time, the encoder and the decoder use a pixel located on top of the left peripheral luma block (eg, a pixel located at a position of (-2,-1) or a pixel located at a position of (-1,-1)) and a left periphery. Neighboring luma pixels 836 for which the pixel average value of the pixel located at the lower end of the luma block (eg, the pixel located at the position of (-2,8) or the pixel located at the position of (-1,8)) is not available ) can be filled in. This padding process may be represented by Equation 8 below as an example.

[Equation 8]

p[-2,y] = (p[-2,-1] + p[-2,8] + 1)>>1, (y=0~7) or

p[-2,y] = (p[-1,-1] + p[-1,8] + 1)>>1, (y=0~7)

When the *163 padding process is performed, the encoder and decoder may use newly filled pixel values to generate luma reference information. That is, the encoder and decoder generate luma reference information based on the pixel values of available neighboring luma pixels 834, the pixel values of pixels 836 filled by the padding process, and the pixel values of pixels in the current luma block 822. can create

As another embodiment, the encoder and decoder in 820 of FIG. 8 may not use unavailable neighboring luma pixels to generate luma reference information. That is, the encoder and decoder may generate luma reference information based on pixels in the current luma block and available neighboring luma pixels. For example, in 820 of FIG. 8 , pixels in the current luma block 822 and available peripheral luma pixels 834 located in the upper peripheral luma block 824 may be used to generate luma reference information.

In the luma reference information generation process in 820 of FIG. 8 , at least one of the luma reference information generation methods described above in the embodiments of FIGS. 6 and 7 may be applied together with the above-described embodiments.

9 is a diagram for explaining an embodiment of a process of generating luma reference information after performing a padding process.

In the embodiment of FIG. 9 , luma reference information is generated by neighboring luma pixels in the first left pixel line closest to the left side of the current luma block 910 and the first upper pixel line closest to the top of the current luma block 910 . It shows an embodiment when used in . In the embodiment of FIG. 9 , a pixel line positioned second closest to the left side of the current luma block 910, that is, a pixel line positioned adjacent to the left side of the first left pixel line will be referred to as a second left pixel line. Also, a pixel line positioned second closest to the top of the current luma block 910, that is, a pixel line positioned adjacent to the top of the first top pixel line will be referred to as a second top pixel line.

In the embodiment of FIG. 9 , it is assumed that the intra prediction mode of the current chroma block to be predicted is the LM mode. As an example, the LM mode may be represented as Intra_FromLuma. Also, in FIG. 9 , it is assumed that the size of the current luma block 910 is 8x8 and the size of the current chroma block is 4x4. As described above, the size of the luma reference block to be used for prediction of the current chroma block may be the same as the size of the current chroma block. Accordingly, the size of the luma reference block 930 derived in the embodiment of FIG. 9 may be 4x4.

In addition, among the peripheral luma pixels 922, 924, 926, and 928 shown in the embodiment of FIG. 9, the peripheral luma pixels corresponding to 922 and 926 are available pixels, It is assumed that peripheral luma pixels corresponding to 924 and 928 are unavailable pixels. In this case, the encoder and decoder may generate luma reference information after performing a padding process on unavailable neighboring luma pixels.

In the embodiment of FIG. 9 , the encoder and decoder may perform a padding process after copying peripheral luma pixel values on the second left pixel line to the first left pixel line. That is, the encoder and decoder replaces the peripheral luma pixel values of the first left pixel line with the peripheral luma pixel values of the second left pixel line, and then outputs values to the unavailable peripheral luma pixels on the first left pixel line and the first upper pixel line. A padding process can be performed for A process of copying the peripheral luma pixel values on the second left pixel line to the first left pixel line may be expressed by Equation 9 below, for example.

[Equation 9]

P _Y [x,y] = P _Y [x-1,y], (x=-1, y=0~2*nS-1)

*173 Here, P _Y may represent a pixel value of a neighboring luma pixel. Also, nS may represent the horizontal and vertical lengths of the current luma block 910 .

After the neighboring luma pixel values on the second, that is, pixel line are copied to the first left pixel line, the encoder and decoder perform a padding process on the neighboring luma pixels to derive pixel values of the neighboring luma pixels to be used for generating luma reference information. can do. Pixel values of neighboring luma pixels to be used for generating luma reference information may be expressed by Equation 10 below.

[Equation 10]

P _Y [x,y], (x=-1, y=-1~nS*2-1), (x=0~nS*2-1, y=-1)

Unavailable pixels among the surrounding luma pixels may constitute a pixel group as shown in 924 and 928 of FIG. 9 . In the embodiment of FIG. 9 , for convenience of description, a group of unavailable pixels on the first left pixel line (eg, four pixels located on the left side in 928 of FIG. 9 ) is referred to as a left pixel group, and the first upper pixel line The group of unavailable pixels 924 on the top is referred to as the top pixel group. In the embodiment of FIG. 9 , pixels belonging to the upper pixel group may be represented by P _Y [x,y], (x=nS˜nS*2-1, y=-1), and pixels belonging to the left pixel group can be represented by P _Y [x,y], (x=-1, y=nS~nS+(nS>>1)-1).

To perform padding, the encoder and decoder may determine whether available pixels located adjacent to the pixel group exist at both ends of each pixel group (eg, the left pixel group and the top pixel group).

In one example, available pixels located adjacent to a pixel group may exist at only one end of the pixel group. Referring to FIG. 9 , no available pixels may exist on the right side of the upper pixel group 924, and there may be available pixels c on the left side of the upper pixel group 924. Here, pixel c may be represented by P _Y [nS-1,-1].

In this case, only one available pixel positioned adjacent to the upper pixel group 924 within the first upper pixel line may exist. Accordingly, the encoder and decoder may fill positions of unavailable pixels in the upper pixel group 924 with the pixel value of the available pixel c located adjacent to the upper pixel group 924 . This padding process may be represented by Equation 11 below as an example.

[Equation 11]

P _Y [x,y] = P _Y [nS-1,-1], (x=nS~nS*2-1, y=-1)

As another example, available pixels located adjacent to a pixel group may exist at both ends of the pixel group. Referring to FIG. 9 , an available pixel b may be adjacently positioned at the top of the left pixel group, and an available pixel a may be adjacently positioned at the bottom of the left pixel group. Here, pixel a may be represented by P _Y [-1, nS+(nS>>1)], and pixel b may be represented by P _Y [-1, nS-1].

In this case, two available pixels positioned adjacent to the left pixel group in the first left pixel line may exist. Accordingly, the encoder and decoder may fill the positions of the unavailable pixels in the left pixel group with the average pixel value of the available pixels (a, b) located adjacent to the left pixel group. This padding process may be represented by Equation 12 below as an example.

[Equation 12]

P _Y [x,y] = (P _Y [-1,nS-1] + P _Y [-1,nS+(nS>>1)] + 1) >> 1,

*187(x=-1, y=nS~nS+(nS>>1)-1)

Pixel values of the peripheral luma pixels on the first left pixel line and the peripheral luma pixels on the first upper pixel line may be determined through the above-described padding process. At this time, the encoder and the decoder use luma reference information (luma reference information) based on pixel values of neighboring luma pixels on the first left pixel line, neighboring luma pixels on the first upper pixel line, and pixels in the current luma block 910. Block 930 and surrounding luma reference pixels 940 may be created. However, a neighboring luma pixel existing at a position of [-1,-1] may not be used to generate luma reference information. In the embodiment of FIG. 9 , the process of generating luma reference information may be represented by Equation 13 below as an example.

[Equation 13]

P _Y '[x,y] = (P _Y [2x-1,y] + 2*P _Y [2x,y] + P _Y [2x+1,y] + 2) >> 2, (x=0 ~nS-1, y=-1)

P _Y '[x,y] = (P _Y [x,2y] + P _Y [x,2y+1]) >> 1, (x=-1, y=0~nS-1)

P _Y '[x,y] = (P _RecY [2x,2y] + P _RecY [2x,2y+1]) >> 1, (x=0~nS-1, y=0~nS-1)

Here, P _RecY may represent a pixel value of a pixel in the current luma block. Also, P _Y ′ may indicate a pixel value corresponding to luma reference information (eg, a pixel value of a pixel in a luma reference block and/or a neighboring luma reference pixel).

10 is a diagram for explaining another embodiment of a process of generating luma reference information after performing a padding process.

In the embodiment of FIG. 10 , luma reference information is generated by neighboring luma pixels in the first left pixel line closest to the left side of the current luma block 1010 and the first upper pixel line closest to the top of the current luma block 1010 . It shows an embodiment when used in . In the embodiment of FIG. 10 , a pixel line positioned second closest to the left side of the current luma block 1010, that is, a pixel line positioned adjacent to the left side of the first left pixel line will be referred to as a second left pixel line. Also, a pixel line positioned second closest to the top of the current luma block 1010, that is, a pixel line positioned adjacent to the top of the first top pixel line will be referred to as a second top pixel line.

In the embodiment of FIG. 10 , it is assumed that the intra prediction mode of the current chroma block to be predicted is the LM mode. As an example, the LM mode may be represented as Intra_FromLuma. Also, in FIG. 10 , it is assumed that the size of the current luma block 1010 is 8x8 and the size of the current chroma block is 4x4. As described above, the size of the luma reference block to be used for prediction of the current chroma block may be the same as the size of the current chroma block. Accordingly, the size of the luma reference block 1030 derived in the embodiment of FIG. 10 may be 4x4.

In addition, among the peripheral luma pixels 1022, 1024, 1026, and 1028 shown in the embodiment of FIG. 10 , peripheral luma pixels corresponding to 1022 and 1026 are available pixels, It is assumed that peripheral luma pixels corresponding to 1024 and 1028 are unavailable pixels. In this case, the encoder and decoder may generate luma reference information after performing a padding process on unavailable neighboring luma pixels.

In the embodiment of FIG. 10 , the encoder and decoder perform a padding process on unavailable peripheral luma pixels on the first left pixel line and the first upper pixel line, thereby obtaining pixel values P _Y [x,y] of the peripheral luma pixels. , (x = -1, y = -1 ~ nS * 2-1), (x = 0 ~ nS * 2-1, y = -1) can be derived. Here, P _Y may represent a pixel value of a neighboring luma pixel. Also, nS may represent the horizontal and vertical lengths of the current luma block 1010 .

Unavailable pixels among the surrounding luma pixels may constitute a pixel group as shown in 1024 and 1028 of FIG. 10 . In the embodiment of FIG. 10 , for convenience of description, a group of unavailable pixels on the first left pixel line (eg, four pixels located on the left side in 1028 of FIG. 10 ) is referred to as a left pixel group, and the first upper pixel line The group of unavailable pixels 1024 on the top is referred to as the top pixel group. 10 , pixels belonging to the upper pixel group may be represented by P _Y [x,y], (x=nS˜nS*2-1, y=-1), and pixels belonging to the left pixel group can be represented by P _Y [x,y], (x=-1, y=nS~nS+(nS>>1)-1).

To perform padding, the encoder and decoder may determine whether available pixels located adjacent to the pixel group exist at both ends of each pixel group (eg, the left pixel group and the top pixel group).

In one example, available pixels located adjacent to a pixel group may exist at only one end of the pixel group. Referring to FIG. 10 , no available pixels may exist on the right side of the upper pixel group 1024, and there may be available pixels c on the left side of the upper pixel group 1024. Here, pixel c may be represented by P _Y [nS-1,-1].

In this case, only one available pixel positioned adjacent to the upper pixel group 1024 within the first upper pixel line may exist. Accordingly, the encoder and decoder may fill positions of unavailable pixels in the upper pixel group 1024 with the pixel value of the available pixel c located adjacent to the upper pixel group. This padding process may be represented by Equation 14 below as an example.

[Equation 14]

P _Y [x,y] = P _Y [nS-1,-1], (x=nS~nS*2-1, y=-1)

As another example, available pixels located adjacent to a pixel group may exist at both ends of the pixel group. Referring to FIG. 10 , an available pixel b may be adjacently positioned at the top of the left pixel group, and an available pixel a may be adjacently positioned at the bottom of the left pixel group. Here, pixel a may be represented by P _Y [-1, nS+(nS>>1)], and pixel b may be represented by P _Y [-1, nS-1].

In this case, two available pixels positioned adjacent to the left pixel group in the first left pixel line may exist. Accordingly, the encoder and decoder may fill the positions of the unavailable pixels in the left pixel group with the average pixel value of the available pixels (a, b) located adjacent to the left pixel group. This padding process may be represented by Equation 15 below as an example.

[Equation 15]

P _Y [x,y] = (P _Y [-1,nS-1] + P _Y [-1,nS+(nS>>1)] + 1) >> 1,

*209(x=-1, y=nS~nS+(nS>>1)-1)

Pixel values of the peripheral luma pixels on the first left pixel line and the peripheral luma pixels on the first upper pixel line may be determined through the above-described padding process. The encoder and the decoder use luma reference information (the luma reference block 1030) based on pixel values of neighboring luma pixels on the first left pixel line, neighboring luma pixels on the first upper pixel line, and pixels in the current luma block 1010. ) and surrounding luma reference pixels 1040).

In this case, available pixels may exist among pixels on the second left pixel line adjacent to the left side of the first left pixel line. If a pixel located adjacent to the left of a pixel on the first left pixel line (a pixel on a second left pixel line) is available, the encoder and decoder use a pixel located adjacent to the second left pixel line instead of a pixel on the first left pixel line. Pixels of the image may be used to generate luma reference information. However, if a pixel located adjacent to the left side of a pixel on the first left pixel line (a pixel on the second left pixel line) is not available, the encoder and decoder refer to the pixel on the first left pixel line on which the padding process has been performed as luma. Can be used to generate information.

In the embodiment of FIG. 10 , the process of generating luma reference information may be represented by Equation 16 below as an example.

[Equation 16]

P _Y '[x,y] = (P _Y [2x-1,y] + 2*P _Y [2x,y] + P _Y [2x+1,y] + 2) >> 2, (x=0 ~nS-1, y=-1)

P _Y '[x,y] = (P _RecY [2x,2y] + P _RecY [2x,2y+1]) >> 1, (x=0~nS-1, y=0~nS-1)

If P _Y [2x,2y], (x=-1, y=0~nS-1) is available,

P _Y '[x,y] = (P _Y [2x,2y] + P _Y [2x,2y+1]) >> 1, (x=-1, y=0~nS-1)

If P _Y [2x,2y], (x=-1, y=0~nS-1) is not available,

P _Y '[x,y] = (P _Y [x,2y] + P _Y [x,2y+1]) >> 1, (x=-1, y=0~nS-1)

Here, P _RecY may represent a pixel value of a pixel in the current luma block. Also, P _Y ′ may indicate a pixel value corresponding to luma reference information (eg, a pixel value of a pixel in a luma reference block and/or a neighboring luma reference pixel).

11 is a diagram for explaining another embodiment of a process of generating luma reference information after performing a padding process.

In the embodiment of FIG. 11 , luma reference information is generated by neighboring luma pixels in the first left pixel line closest to the left side of the current luma block 1110 and the first upper pixel line closest to the top of the current luma block 1110 . It shows an embodiment when used in . In the embodiment of FIG. 11 , a pixel line positioned second closest to the left side of the current luma block 1110, that is, a pixel line positioned adjacent to the left side of the first left pixel line will be referred to as a second left pixel line. Also, a pixel line positioned second closest to the top of the current luma block 1110, that is, a pixel line positioned adjacent to the top of the first top pixel line will be referred to as a second top pixel line.

In the embodiment of FIG. 11 , it is assumed that the intra prediction mode of the current chroma block to be predicted is the LM mode. As an example, the LM mode may be represented as Intra_FromLuma. 11, it is assumed that the size of the current luma block 1110 is 8x8 and the size of the current chroma block is 4x4. As described above, the size of the luma reference block to be used for prediction of the current chroma block may be the same as the size of the current chroma block. Accordingly, the size of the luma reference block 1130 derived in the embodiment of FIG. 11 may be 4x4.

In addition, among the peripheral luma pixels 1122, 1124, 1126, and 1128 shown in the embodiment of FIG. 11, the peripheral luma pixels corresponding to 1122 and 1126 are available pixels, It is assumed that peripheral luma pixels corresponding to 1124 and 1128 are unavailable pixels. In this case, the encoder and decoder may generate luma reference information after performing a padding process on unavailable neighboring luma pixels.

In the embodiment of FIG. 11 , the encoder and decoder may perform a padding process on unavailable peripheral luma pixels on the first left pixel line, the second left pixel line, and the first upper pixel line. Also, the encoder and decoder may copy peripheral luma pixel values on the second left pixel line to the first left pixel line. Pixel values of neighboring luma pixels generated by the above-described process and used for generating reference information may be expressed by Equation 17 below.

[Equation 17]

P _Y [x,y], (x=-1, y=-1~nS*2-1), (x=0~nS*2-1, y=-1)

Here, P _Y may represent pixel values of peripheral luma pixels belonging to the first left pixel line and/or the first upper pixel line. Also, nS may represent the horizontal and vertical lengths of the current luma block 1110 .

Unavailable pixels among the surrounding luma pixels may constitute a pixel group as shown in 1124 and 1128 of FIG. 11 . In the embodiment of FIG. 11, for convenience of explanation, a group of unavailable pixels on the first left pixel line (eg, four pixels located on the left side in 1128 of FIG. 11) is referred to as a first left pixel group, and a second left pixel group The group of unavailable pixels on the pixel line (e.g., the four pixels located on the right in 1128 of FIG. The group is referred to as the top pixel group. 11 , pixels belonging to the upper pixel group may be represented by P _Y [x,y], (x=nS˜nS*2-1, y=-1), and in the first left pixel group The belonging pixels may be represented by P _Y [x, y], (x = -1, y = nS ~ nS + (nS>> 1) -1), and the pixels belonging to the second left pixel group are P _Y2 [x ,y], (x=-2, y=nS~nS+(nS>>1)-1). Here, P _Y2 may indicate a pixel value of a peripheral luma pixel belonging to the second left pixel line.

The encoder and the decoder determine whether there are available pixels located adjacent to the pixel group at both ends of each pixel group (e.g., the first left pixel group, the second left pixel group, and the top pixel group) to perform padding. can judge

In one embodiment, usable pixels located adjacent to a pixel group may exist at only one end of the pixel group. Referring to FIG. 11 , no usable pixels may exist on the right side of the upper pixel group 1124, and there may be an available pixel c on the left side of the upper pixel group 1124. Here, pixel c may be represented by P _Y [nS-1,-1].

In this case, only one available pixel positioned adjacent to the upper pixel group 1124 within the first upper pixel line may exist. Accordingly, the encoder and decoder may fill positions of unavailable pixels in the upper pixel group 1124 with the pixel value of the available pixel c located adjacent to the upper pixel group. This padding process may be represented by Equation 18 below as an example.

[Equation 18]

P _Y [x,y] = P _Y [nS-1,-1], (x=nS~nS*2-1, y=-1)

In another embodiment, available pixels located adjacent to a pixel group may exist at both ends of the pixel group.

For example, in the embodiment of FIG. 11 , an available pixel b may be adjacently located at the upper end of the first left pixel group, and an available pixel a may be adjacently located at the lower end of the first left pixel group. Here, pixel a may be represented by P _Y [-1, nS+(nS>>1)], and pixel b may be represented by P _Y [-1, nS-1].

In this case, two available pixels positioned adjacent to the left pixel group in the first left pixel line may exist. Accordingly, the encoder and decoder may fill the positions of the unavailable pixels in the first left pixel group with the average pixel values of the available pixels a and b located adjacent to the first left pixel group. This padding process may be represented by Equation 19 below as an example.

[Equation 19]

P _Y [x,y] = (P _Y [-1,nS-1] + P _Y [-1,nS+(nS>>1)] + 1) >> 1,

(x=-1, y=nS~nS+(nS>>1)-1)

As another example, in the embodiment of FIG. 11 , an available pixel e may be adjacently located at the upper end of the second left pixel group, and an available pixel d may be adjacently located at the lower end of the second left pixel group. Here, pixel d may be represented by P _Y2 [-2, nS+(nS>>1)], and pixel e may be represented by P _Y2 [-2, nS-1].

In this case, two available pixels positioned adjacent to the left pixel group in the second left pixel line may exist. Accordingly, the encoder and decoder may fill the positions of the unavailable pixels in the second left pixel group with the average pixel values of the available pixels e and d located adjacent to the second left pixel group. This padding process may be represented by Equation 20 below as an example.

[Equation 20]

P _Y2 [x,y] = (P _Y2 [-2,nS-1] + P _Y2 [-2,nS+(nS>>1)] + 1) >> 1,

(x=-2, y=nS~nS+(nS>>1)-1)

Pixel values on the second left pixel line determined after the padding process is performed may be expressed as, for example, P _Y2 [x,y], (x=-2, y=-1 to 2*nS-1).

Pixel values of peripheral luma pixels on the first left pixel line, the second left pixel line, and the first upper pixel line may be determined by the padding process described above. In this case, the encoder and the decoder may copy peripheral luma pixel values on the second left pixel line to the first left pixel line to derive pixel values on the first left pixel line to be used for generating luma reference information. That is, the encoder and decoder may replace the luma pixel values of the first left pixel line with the luma pixel values of the second left pixel line. A process of copying the peripheral luma pixel values on the second left pixel line to the first left pixel line may be expressed by Equation 21 below, for example.

[Equation 21]

P _Y [x,y] = P _Y2 [x-1,y], (x=-1, y=0~2*nS-1)

After the ambient luma pixel values on the second, i.e., pixel line are copied to the first left pixel line, the encoder and decoder calculate the ambient luma pixels on the first left pixel line, the ambient luma pixels on the first top pixel line, and the current luma block. Based on the pixel values of pixels in 1110 , luma reference information (a luma reference block 1130 and neighboring luma reference pixels 1140 ) may be generated. In the embodiment of FIG. 11 , the process of generating luma reference information may be represented by Equation 22 below as an example.

[Equation 22]

P _Y '[x,y] = (P _Y [2x-1,y] + 2*P _Y [2x,y] + P _Y [2x+1,y] + 2) >> 2, (x=0 ~nS-1, y=-1)

P _Y '[x,y] = (P _Y [x,2y] + P _Y [x,2y+1]) >> 1, (x=-1, y=0~nS-1)

P _Y '[x,y] = (P _RecY [2x,2y] + P _RecY [2x,2y+1]) >> 1, (x=0~nS-1, y=0~nS-1)

Here, P _RecY may represent a pixel value of a pixel in the current luma block. Also, P _Y ′ may indicate a pixel value corresponding to luma reference information (eg, a pixel value of a pixel in a luma reference block and/or a neighboring luma reference pixel).

12 is a diagram for explaining another embodiment of a process of generating luma reference information after performing a padding process.

In the embodiment of FIG. 12 , the surrounding luma pixels in the first left pixel line closest to the left side of the current luma block 1210 and the first upper pixel line closest to the top of the current luma block 1210 generate luma reference information. It shows an example of a case in which it is used. In the embodiment of FIG. 12 , a pixel line positioned second closest to the left side of the current luma block 1210, that is, a pixel line positioned adjacent to the left side of the first left pixel line will be referred to as a second left pixel line. Also, a pixel line positioned second closest to the top of the current luma block 1210, that is, a pixel line positioned adjacent to the top of the first top pixel line will be referred to as a second top pixel line.

In the embodiment of FIG. 12 , it is assumed that the intra prediction mode of the current chroma block to be predicted is the LM mode. As an example, the LM mode may be represented as Intra_FromLuma. 12, it is assumed that the size of the current luma block 1210 is 8x8 and the size of the current chroma block is 4x4. As described above, the size of the luma reference block to be used for prediction of the current chroma block may be the same as the size of the current chroma block. Accordingly, the size of the luma reference block 1230 derived in the embodiment of FIG. 12 may be 4x4.

In addition, among the peripheral luma pixels 1222, 1224, 1226, and 1228 shown in the embodiment of FIG. 12, the peripheral luma pixels corresponding to 1222 and 1226 are available pixels, It is assumed that peripheral luma pixels corresponding to 1224 and 1228 are unavailable pixels. In this case, the encoder and decoder may generate luma reference information after performing a padding process on unavailable neighboring luma pixels.

In the embodiment of FIG. 12 , the encoder and decoder perform a padding process on unavailable peripheral luma pixels on the first left pixel line and the first upper pixel line, thereby obtaining pixel values of the peripheral luma pixels P _Y [x,y] , (x = -1, y = -1 ~ nS * 2-1), (x = 0 ~ nS * 2-1, y = -1) can be derived. Here, P _Y may represent a pixel value of a neighboring luma pixel. Also, nS may represent the horizontal and vertical lengths of the current luma block 1210 .

Unavailable pixels among the surrounding luma pixels may constitute a pixel group as shown in 1224 and 1228 of FIG. 12 . In the embodiment of FIG. 12, for convenience of description, a group of unavailable pixels on the first left pixel line (eg, four pixels located on the left side in 1228 of FIG. 12) is referred to as a left pixel group, and the first upper pixel line The group of unavailable pixels 1224 on the top is referred to as the top pixel group. 12 , pixels belonging to the upper pixel group may be represented by P _Y [x,y], (x=nS˜nS*2-1, y=-1), and pixels belonging to the left pixel group can be represented by P _Y [x,y], (x=-1, y=nS~nS+(nS>>1)-1).

To perform padding, the encoder and decoder may determine whether available pixels located adjacent to the pixel group exist at both ends of each pixel group (eg, the left pixel group and the top pixel group).

In one example, available pixels located adjacent to a pixel group may exist at only one end of the pixel group. Referring to FIG. 12 , there may be no available pixels on the right side of the upper pixel group 1224, and an available pixel c on the left side of the upper pixel group 1224. Here, pixel c may be represented by P _Y [nS-1,-1].

In this case, only one available pixel positioned adjacent to the upper pixel group 1224 within the first upper pixel line may exist. Accordingly, the encoder and decoder may fill positions of unavailable pixels in the upper pixel group 1224 with the pixel value of the available pixel c located adjacent to the upper pixel group. This padding process may be represented by Equation 23 below as an example.

[Equation 23]

P _Y [x,y] = P _Y [nS-1,-1], (x=nS~nS*2-1, y=-1)

As another example, available pixels located adjacent to a pixel group may exist at both ends of the pixel group. Referring to FIG. 12 , an available pixel b may be adjacently positioned at the top of the left pixel group, and an available pixel a may be adjacently positioned at the bottom of the left pixel group. Here, pixel a may be represented by P _Y [-1, nS+(nS>>1)], and pixel b may be represented by P _Y [-1, nS-1].

In this case, two available pixels positioned adjacent to the left pixel group in the first left pixel line may exist. Accordingly, the encoder and decoder may fill the positions of the unavailable pixels in the left pixel group with the average pixel value of the available pixels (a, b) located adjacent to the left pixel group. This padding process may be represented by Equation 24 below as an example.

[Equation 24]

P _Y [x,y] = (P _Y [-1,nS-1] + P _Y [-1,nS+(nS>>1)] + 1) >> 1,

*271(x=-1, y=nS~nS+(nS>>1)-1)

Pixel values of the peripheral luma pixels on the first left pixel line and the peripheral luma pixels on the first upper pixel line may be determined through the above-described padding process. The encoder and the decoder use luma reference information (the luma reference block 1230 based on pixel values of neighboring luma pixels on the first left pixel line, neighboring luma pixels on the first upper pixel line, and pixels in the current luma block 1210). ) and surrounding luma reference pixels 1240). In the embodiment of FIG. 12 , the process of generating luma reference information may be represented by Equation 25 below as an example.

[Equation 25]

P _Y '[x,y] = (P _Y [2x-1,y] + 2*P _Y [2x,y] + P _Y [2x+1,y] + 2) >> 2, (x=0 ~nS-1, y=-1)

P _Y '[x,y] = (P _RecY [2x,2y] + P _RecY [2x,2y+1]) >> 1, (x=0~nS-1, y=0~nS-1)

P _Y '[x,y] = (P _Y [x,2y] + P _Y [x,2y+1]) >> 1, (x=-1, y=0~nS-1)

Here, P _RecY may represent a pixel value of a pixel in the current luma block. Also, P _Y ′ may indicate a pixel value corresponding to luma reference information (eg, a pixel value of a pixel in a luma reference block and/or a neighboring luma reference pixel).

Referring to the foregoing, in the embodiment of FIG. 12 , pixels on the second left pixel line are not used to generate luma reference information, and peripheral luma pixels on the first left pixel line and peripheral luma pixels on the first upper pixel line are not used to generate luma reference information. and only the pixels within the current luma block 1210 may be used for generating luma reference information (the luma reference block 1230 and surrounding luma reference pixels 1240).

Meanwhile, in FIGS. 5 to 12 described above, embodiments in which the first color component is a luma component and the second color component is a chroma component are described, but the present invention is not limited thereto. As described above, the present invention can be applied in the same or similar manner to all possible combinations of the first color component and the second color component.

In addition, in FIGS. 6 to 12 described above, embodiments in which the size of the first color component block (eg, luma block) is greater than the size of the second color component block (eg, chroma block) have been described, but the first color component block (eg, chroma block) has been described. The component block may have a smaller size than or the same size as the second color component block.

Even in this case, the encoder and decoder may derive first color component reference information to be used for prediction of the second color component block based on pixels in the first color component block and pixels around the first color component block.

Here, the first color component reference information may mean pixel values of the first color component to be used for prediction of the second color component block. Accordingly, in this specification, the first color component information may be referred to as a 'first color component pixel'. In this case, first color component pixels corresponding to pixels in the second color component block may constitute a 'first color component reference block'. Further, in this specification, pixels located around the first color component block are referred to as 'surrounding first color component pixels', and pixels located around the second color component block are referred to as 'surrounding second color component pixels'. Pixels positioned around the 1 color component reference block are referred to as 'surrounding first color component reference pixels'. In this case, the first color component reference block may correspond to the second color component block, and the neighboring first color component reference pixels may correspond to the neighboring second color component pixels. In this specification, the first color component reference information may be used as a concept including pixels in the first color component reference block and neighboring first color component reference pixels.

Deriving first color component reference information to be used for prediction of a second color component block based on the first color component block when the first color component block has a size smaller than or equal to that of the second color component block may be different from those in the above-described embodiments of FIGS. 6 to 12 .

13 is a diagram schematically illustrating an embodiment of a process of deriving first color component reference information to be used for prediction of a second color component block when the size of the first color component block is smaller than the size of the second color component block. .

1310 of FIG. 13 illustrates an embodiment of the first color component block 1312, the surrounding first color component pixels 1314, the second color component block 1316, and the surrounding second color component pixels 1318. . Referring to 1310 of FIG. 13 , when the size of the second color component block 1316 is 8x8, the size of the corresponding first color component block 1312 may be 4x4. In this case, the encoder and decoder may generate first color component reference pixels corresponding to the 8x8 first color component reference block.

1320 of FIG. 13 illustrates an embodiment of a method of deriving first color component reference information.

In the example 1320 of FIG. 13 , as in 1310 of FIG. 13 , the first color component reference information when the size of the first color component block 1312 is 4x4 and the size of the second color component block 1316 is 8x8. The derivation method is shown.

Referring to 1320 of FIG. 13 , the encoder and the decoder are to be used for prediction of the second color component block 1316 based on the pixel in the first color component block 1312 and the surrounding first color component pixels 1314 . A 1 color component reference block 1326 and surrounding first color component reference pixels 1328 may be generated. Here, the neighboring first color component pixels 1314 used to derive the first color component reference information are, for example, pixels included in one pixel line located closest to the left side of the first color component block 1312, and pixels included in one pixel line located closest to the top of the first color component block 1312 .

Meanwhile, in 1320 of FIG. 13 , since the sizes of the first color component block 1312 and the second color component block 1316 are different, the encoder and the decoder have the same size (8x8) as the second color component block 1316. The first color component reference block 1326 and neighboring first color component reference pixels 1328 corresponding to the first color component reference block 1326 may be generated. At this time, since the size of the first color component block 1312 is smaller than the size of the second color component block 1316, the encoder and decoder use pixels in the first color component block 1312 and neighboring first color component pixels. First color component reference information may be generated by up-sampling the s 1314 .

When there are unavailable pixels among the neighboring first color component pixels 1314, the encoder and decoder may perform upsampling after performing a padding process on the unavailable pixels. Since the padding process is similar to that in the above-described embodiments, a detailed description thereof will be omitted here.

A process of performing up-sampling on pixels in the first color component block 1312 and neighboring first color component pixels 1314 may be expressed by Equation 26 below, for example.

[Equation 26]

*293p'[2x,2y] = p[x,y]

p'[2x+1,2y] = (p[x,y] + p[x+1,y])>>1

p'[2x,2y+1] = (p[x,y] + p[x,y+1])>>1

p'[2x+1,2y+1] = (p[x+1,y] + p[x,y+1])>>1

Here, p[x,y] may represent pixel values of restored first color component pixels, that is, pixels in the first color component block 1312 and neighboring first color component pixels 1314 . Also, p'[x,y] may represent pixel values of the first color component reference pixels 1326 and 1328 generated according to the size of the second color component block 1316 .

14 is a diagram schematically illustrating an embodiment of a process of deriving first color component reference information to be used for prediction of a second color component block when the size of the first color component block is the same as that of the second color component block. .

As described above, when the 4:4:4 video format is used, that is, when the color space of the video is RGB 4:4:4 or YUV 4:4:4, the size of the luma block and the chroma block may be the same. there is. Accordingly, in this case, if the first color component block is a luma block and the second color component block is a chroma block, the size of the first color component block and the size of the second color component block may be the same. Also, when the first color component block is a chroma block corresponding to U and the second color component block is a chroma block corresponding to V, the size of the first color component block and the size of the second color component block may be the same. . In the embodiment of FIG. 14 , it is assumed that the size of the first color component block 1412 is 8x8 and the size of the second color component block 1422 is 8x8.

Referring to FIG. 14 , an encoder and a decoder are configured to use a first color component reference block to be used for prediction of a second color component block based on a pixel in a first color component block 1412 and neighboring first color component pixels 1414 . 1422 and neighboring first color component reference pixels 1424 may be generated. The generated first color component reference block 1422 may exist at a position corresponding to the second color component block. Here, the neighboring first color component pixels 1414 used to derive the first color component reference information are, for example, pixels included in one pixel line located closest to the left side of the first color component block 1412, and pixels included in one pixel line located closest to the top of the first color component block 1412 .

At this time, since the size of the first color component block 1412 is the same as the size of the second color component block, the encoder and decoder perform down sampling and sub sampling when generating the first color component information. and up sampling may not be performed. Accordingly, pixel values of pixels in the first color component block 1412 and pixel values of neighboring first color component pixels 1414 may be used as first color component reference information.

Also, in the intra mode, a lossless encoding/decoding method may be applied instead of a lossy encoding/decoding method using transform/quantization. In the lossless encoding/decoding method, a process such as transform quantization may not be applied to an encoding target block. When the size of the first color component block 1412 is equal to the size of the second color component block, even when the lossless encoding/decoding method is applied, the first color component reference information can be derived as in the above-described embodiment. can In this case, the encoder and decoder may not perform downsampling, subsampling, and upsampling when generating the first color component information. The generated first color component reference block 1422 may exist at a position corresponding to the second color component block.

Meanwhile, in the above-described embodiments, both upper peripheral luma pixels positioned on top of the current luma block and left peripheral luma pixels positioned on the left side of the current luma block may be used to generate luma reference information. However, the present invention is not limited thereto, and neighboring luma pixels used to generate luma reference information may be determined in various ways. For example, only upper peripheral luma pixels may be used to generate luma reference information, and only left peripheral luma pixels may be used to generate luma reference information. Also, neighboring luma pixels used to generate luma reference information may be determined according to an intra prediction mode of a current luma block.

15 is a diagram schematically illustrating an example of peripheral luma pixels used to generate luma reference information.

In the embodiment of *305 FIG. 15, it is assumed that the size of the current luma block 1512 is 8x8 and the size of the corresponding current chroma block is 4x4. As an example, FIG. 15 may correspond to an embodiment in which a video format of YUV 4:2:0 is used. In addition, it is assumed that the peripheral luma pixels A, B, C, D, E, and F shown in FIG. 15 are already filled with pixel values through the above-described padding process. And, in FIG. 15, it is assumed that the mode value of the intra prediction mode as in 410 of FIG. 4 is used.

In the embodiment of FIG. 15 , a luma reference block (a luma reference block (a 1522, 1532 or 1542) and peripheral luma reference pixels 1524, 1534 or 1544 may be generated. In this case, since the size of the current chroma block is 4x4, the size of the generated luma reference block 1522, 1532 or 1542 may be 4x4.

In the embodiment of FIG. 15 , peripheral luma pixels to be used for generating luma reference information may be obtained by, for example, at least one of the combinations shown in Table 3 below.

[Table 3]

In the example of Table 3, P may mean that left peripheral luma pixels located on the left side of the current luma block 1512 and upper peripheral luma pixels located on the top side of the current luma block 1512 are used to generate luma reference information. . Also, Q may mean that upper peripheral luma pixels located on the upper side of the current luma block 1512 are used to generate luma reference information. Also, R may mean that left peripheral luma pixels located on the left side of the current luma block 1512 are used to generate luma reference information.

In addition, in the example of Table 3, L indicates that when left peripheral luma pixels are used to generate luma reference information, pixels on the first left pixel line located closest to the left side of the current luma block 1512 are used to generate luma reference information. can mean Further, M may mean that, when left peripheral luma pixels are used to generate luma reference information, pixels on a second left pixel line located second closest to the left side of the current luma block 1512 are used to generate luma reference information. there is. In this case, pixels on the first left pixel line may be used together for subsampling.

Also, N may mean that neighboring luma pixels to be used for generating luma reference information are determined based on the intra prediction mode of the current luma block 1512 . In this case, luma reference information may be adaptively generated based on the intra prediction mode of the current luma block 1512 . The left peripheral luma pixels used when luma reference information is generated based on the intra prediction mode of the current luma block 1512 may be, for example, pixels on the first left pixel line or, for example, pixels on the second left pixel line. may be

For example, in an embodiment in which a combination of P and L is applied, upper peripheral luma pixels positioned at an upper portion of the current luma block 1512 and left peripheral luma pixels on the first left pixel line may be used to generate luma reference information. . Accordingly, in this case, neighboring luma pixels of [A, D] or [A, B, D, F] shown in 1510 of FIG. 15 may be used to generate luma reference information.

When luma reference information is generated based on [A, D], the generated luma reference information may be shown as 1520 of FIG. 15 as an example. In 1520 of FIG. 15 , the peripheral luma reference pixels 1524 may include pixels adjacent to an upper end of the luma reference block 1522 and pixels adjacent to a left side of the luma reference block 1522 .

As another example, in an embodiment in which a combination of Q and L is applied, upper peripheral luma pixels located on an upper portion of the current luma block 1512 may be used to generate luma reference information. Accordingly, in this case, neighboring luma pixels of [A, B] shown in 1510 of FIG. 15 may be used to generate luma reference information.

When luma reference information is generated based on [A, B], the generated luma reference information may be shown as 1530 of FIG. 15 as an example. In 1530 of FIG. 15 , the peripheral luma reference pixels 1534 may be pixels located on top of the luma reference block 1532 .

As another example, in an embodiment in which a combination of R and L is applied, left peripheral luma pixels on the first left pixel line may be used to generate luma reference information. Accordingly, in this case, neighboring luma pixels of [D, F] shown in 1510 of FIG. 15 may be used to generate luma reference information.

When luma reference information is generated based on [D, F], the generated luma reference information may be shown as 1540 of FIG. 15 as an example. In 1540 of FIG. 15 , the peripheral luma reference pixels 1544 may be pixels positioned to the left of the luma reference block 1542 .

As another example, in an embodiment in which a combination of P and M is applied, upper peripheral luma pixels positioned at an upper portion of the current luma block 1512 and left peripheral luma pixels on the second left pixel line may be used to generate luma reference information. there is. Accordingly, in this case, neighboring luma pixels of [A, C] or [A, B, C, E] shown in 1510 of FIG. 15 may be used to generate luma reference information. Luma reference information generated when a combination of P and M is applied may be shown as 1520 of FIG. 15 as an example.

As another example, in an embodiment in which a combination of Q and M is applied, upper peripheral luma pixels positioned at an upper portion of the current luma block 1512 may be used to generate luma reference information. Accordingly, in this case, neighboring luma pixels of [A, B] shown in 1510 of FIG. 15 may be used to generate luma reference information. Luma reference information generated when a combination of Q and M is applied may be shown as 1530 of FIG. 15 as an example.

As another example, in an embodiment in which a combination of R and M is applied, left peripheral luma pixels on the second left pixel line may be used to generate luma reference information. Accordingly, in this case, neighboring luma pixels of [C, E] shown in 1510 of FIG. 15 may be used to generate luma reference information. Luma reference information generated when a combination of R and M is applied may be shown as 1540 in FIG. 15 as an example.

As another example, in an embodiment in which a combination of P and N is applied, based on the intra prediction mode of the current luma block 1512, upper peripheral luma pixels located on top of the current luma block 1512 and the current luma block 1512 ) may be determined as peripheral luma pixels to be used for generating luma reference information.

In this case, the intra prediction mode of the current luma block 1512 may be a first intra prediction mode using all pixels located on the top and left sides of the current luma block 1512 . For example, the first intra prediction mode has a DC mode, a planner mode, or a mode value of 4, 19, 11, 20, 5, 21, 12, 22, 27, 15, 28, 8, 29, 16, or 30. can be a mod

Luma reference information generated when a combination of P and N is applied may be shown as 1520 of FIG. 15 as an example.

As another example, in an embodiment in which a combination of Q and N is applied, luma pixels located on top of the current luma block 1512 are used to generate luma reference information based on the intra prediction mode of the current luma block 1512 . It can be determined as a peripheral luma pixel to be.

In this case, the intra prediction mode of the current luma block 1512 may be a second intra prediction mode mainly using pixels located on top of the current luma block 1512 . For example, the second intra prediction mode may be a mode having a mode value of 1, 23, 13, 24, 6, 25, 14, 26, or 7.

Luma reference information generated when a combination of Q and N is applied may be shown as 1530 in FIG. 15 as an example.

As another example, in an embodiment in which a combination of R and N is applied, left peripheral luma pixels located on the left side of the current luma block 1512 are used to generate luma reference information based on the intra prediction mode of the current luma block 1512. It can be determined as a peripheral luma pixel to be.

In this case, the intra prediction mode of the current luma block 1512 may be a third intra prediction mode mainly using pixels located on the left side of the current luma block 1512 . For example, the third intra prediction mode may be a mode having a mode value of 2, 31, 17, 32, 9, 33, 18, 34, or 10.

Luma reference information generated when a combination of R and N is applied may be shown as 1540 in FIG. 15 as an example.

When the pixel values of the neighboring luma reference pixels are derived based on the neighboring luma pixels determined according to the above-described embodiment, a prediction parameter may be derived using the neighboring chroma pixels located at the same location as each neighboring luma reference pixel. . A specific embodiment of the process of deriving the prediction parameters will be described later.

16 is a diagram for explaining an embodiment of a process of deriving prediction parameters and a process of deriving predictive values of pixels in a current chroma block.

16 shows a luma reference block 1610 , surrounding luma reference pixels 1620 , a current chroma block 1630 and reconstructed surrounding chroma pixels 1640 . In the embodiment of FIG. 16 , the size of the luma reference block 1610 and the size of the current chroma block 1630 may each be 8x8.

As an embodiment, the encoder and decoder may derive a prediction parameter for the current chroma block 1630 based on luma reference information and chroma component information. For example, the encoder and decoder may derive a prediction parameter based on pixel values of neighboring luma reference pixels 1620 and pixel values of neighboring chroma pixels 1640 . Here, the prediction parameters may be alpha (α) and beta (β) based on the least squares method.

A process of deriving a prediction parameter based on the pixel values of the neighboring luma reference pixels 1620 and the neighboring chroma pixels 1640 may be expressed by Equation 27 below, for example.

[Equation 27]

In Equation 27, p1 may represent a pixel value of the neighboring luma reference pixels 1620, and p2 may represent a pixel value of the neighboring chroma pixels 1640. Also, nS may represent the horizontal and vertical lengths of the luma reference block 1610 and the current chroma block 1630, and 'Bitdepth' may represent the bit depth of a chroma component pixel. And, abs(x) may represent the absolute value of x. 'lmDiv' is a variable determined according to the value of 'a2s', and may be determined by Table 4 below, for example.

[Table 4]

In Equation 27, a may represent alpha (α) and b may represent beta (β).

In the process of deriving the prediction parameters described above, an unavailable pixel may exist among the neighboring luma reference pixels 1620 and/or the neighboring chroma pixels 1640 . In this case, for example, the encoder and decoder may derive a prediction parameter based on only available pixels among the neighboring luma reference pixels 1620 and/or the neighboring chroma pixels 1640 .

In another embodiment, the encoder and decoder do not derive a prediction parameter based on the neighboring luma reference pixels 1620 and the neighboring chroma pixels 1640, but use a default prediction parameter as the current chroma block 1630. can also be used for prediction of For example, alpha (α) used as a basic prediction parameter may be 1, and beta (β) used as a basic prediction parameter may be 0.

When prediction parameters are derived according to the above-described embodiments, the encoder and decoder may derive prediction values of pixels in the current chroma block 1630 based on the prediction parameters and pixel values of pixels in the luma reference block 1610. there is. For example, the encoder and the decoder are based on the prediction parameters 'a', 'k', and 'b' derived by Equation 27 and pixel values of pixels in the luma reference block 1610 shown in FIG. Prediction values of pixels in 1630 may be derived. In this case, a process of deriving predicted values of pixels within the current chroma block 1630 may be expressed by Equation 28 below, for example.

[Equation 28]

predSamples[x,y] = Clip1(((p1[x,y]*a)>>k)+b), (x,y=0~(nS-1))

Clip1(i) = Clip3(0,(1<<BitDepth)-1,i)

Here, 'predSamples[x,y]' may indicate a predicted value of a pixel in the current chroma block 1630. Also, clip(x,y,z) may be x if z is less than x, y if z is greater than y, and z otherwise.

17 is a flowchart schematically illustrating an embodiment of a decryption process according to the present invention.

In the embodiment of FIG. 17 , P _Y may represent pixel values of neighboring luma pixels used for prediction of a current luma block. Also, N _Y may indicate the horizontal and vertical lengths of the current luma block, and P _LM may indicate pixel values of neighboring luma pixels used to generate luma reference information in the LM mode. Here, P _Y [x,y] and P _LM [x,y] may have the same value.

The embodiment of FIG. 17 is described based on the luma component (Y). However, the present invention is not limited thereto, and the embodiment of FIG. 17 may be applied to the chroma components (U and V) other than the luma component (Y) in the same or similar manner as the luma component. Also, in this case, P _C may be used instead of variable P _Y , N _C may be used instead of variable N _Y , and a value assigned to cIdx may be 1 or 2 other than 0. Here, cIdx may be an index indicating which component of Y, U, and V corresponds to the color component of the current block. For example, if the color component of the current block is Y, 0 may be assigned to cIdx, if the color component of the current block is U, 1 may be assigned to cIdx, and if the color component of the current block is V, cIdx may be assigned 2 can be assigned. The relationship between variables applied to the embodiment of FIG. 17 according to color components may be represented by Table 5 below, for example.

[Table 5]

Here, P _C may represent pixel values of neighboring chroma pixels located around the current chroma block. Also, N _C may represent the horizontal and vertical lengths of the current chroma block.

As an example, the embodiment of FIG. 17 may be applied in order of a Y component, a U component, and a V component to each color component constituting the color space.

Referring to FIG. 17, the encoder and decoder are P _Y [x,y], (x=-1, y=-1~N _Y *2-1 and x=0~N _Y *2-1, y=- 1) may be marked by checking whether or not is available (S1710). Here, marking may mean a process of storing or displaying information on availability. Since the embodiments in which the peripheral luma pixels are not available have been described above in the embodiment of FIG. 8 , a detailed description thereof will be omitted here.

Referring back to FIG. 17 , the encoder and decoder may determine whether conditions A and B are satisfied for the current luma block (S1720).

Here, condition A is a condition indicating that the LM mode can be applied to intra prediction of the current chroma block corresponding to the current luma block. For example, condition A may be satisfied when 1 is allocated to the LM flag (chroma_pred_from_luma_enabled_flag). As described above, chroma_pred_from_luma_enabled_flag may indicate whether the LM mode can be supported for intra prediction of the current chroma block. Also, condition B is a condition indicating that the current block is the luma component block (Y). As an example, condition B can be satisfied when 0 is assigned to cIdx.

When conditions A and B are satisfied, the encoder and decoder can generate P _LM .

Referring back to FIG. 17, if conditions A and B are satisfied, the encoder and decoder P _LM [x,y], (x=-1, y=-1~N _Y *2-1 and x=0~ N _Y *2-1, y = -1) may be checked to mark whether or not is available (S1730).

At this time, since P _Y [x,y] and P _LM [x,y] can have the same value, if P _Y [x,y] is available, P _LM [x,y] is also marked as available. and if P _Y [x,y] is not available, P _LM [x,y] may also be marked as not available.

In addition, for example, after the marking process for P _LM is performed, the encoder and decoder are available among pixels corresponding to P _LM [x,y], (x = -1, y = 0 to N _Y * 2-1) The pixel values of the pixels determined to be correct may be replaced with the luma pixel values at the [x-1,y] position.

Referring back to FIG. 17 , the encoder and decoder may determine whether condition C is satisfied with respect to neighboring luma pixels to be used for prediction of the current luma block (S1740). Here, condition C is at least one of P _Y [x,y], (x=-1, y=-1~N _Y *2-1 and x=0~N _Y *2-1, y=-1) A condition indicating that a pixel in is not available.

If condition C is satisfied, that is, if at least one pixel among the neighboring luma pixels to be used for prediction of the current luma block is not available, the encoder and decoder P _Y [x,y], (x=-1, y= A padding process may be performed on unavailable pixels among -1 to N _Y *2-1 and x=0 to N _Y *2-1, y = -1 (S1750). Since the embodiments of the padding process have been described above, a detailed description thereof will be omitted here.

Referring back to FIG. 17 , the encoder and decoder may determine whether conditions A, B, and D are satisfied with respect to neighboring luma pixels to be used for generating luma reference information (S1760). Here, condition D is at least one of P _LM [x,y], (x=-1, y=-1~N _Y *2-1 and x=0~N _Y *2-1, y=-1) A condition indicating that a pixel in is not available.

When conditions A, B, and D are satisfied, that is, when at least one pixel among the surrounding luma pixels to be used for luma reference information generation is unavailable, the encoder and decoder P _LM [x,y], (x=-1 , y = -1 to N _Y *2-1 and x = 0 to N _Y *2-1, y = -1), a padding process may be performed on unavailable pixels (S1770). Since the embodiments of the padding process have been described above, a detailed description thereof will be omitted here.

After the above-described padding process is performed, the encoder and decoder may generate a reconstructed block for the current luma block by performing a process of predicting and decoding the current luma block based on neighboring luma pixels (S1780). When the embodiment of FIG. 17 is applied to a chroma component (U or V), the encoder and decoder generate a prediction block for the current chroma block based on neighboring luma pixels, pixels in the reconstructed current luma block, and neighboring chroma pixels. and a reconstruction block for the current chroma block may be generated based on the generated prediction block.

FIG. 18 is a diagram for explaining an embodiment of a process of deriving luma reference information and prediction parameters for a current chroma block when a decoding process is performed as in the embodiment of FIG. 17 .

18 shows a luma component block 1810 to be predicted, neighboring luma pixels 1820 (P _Y ) used for prediction of the luma component block, a reconstructed current luma block (1830, P _RecY ), and luma reference information generation It shows the surrounding luma pixels 1840 (P _LM ) used, the current chroma block 1850 and the surrounding chroma pixels 1860 (P _C ). In the embodiment of FIG. 18 , the size of the luma component block 1810 to be predicted and the reconstructed current luma block 1830 (P _RecY ) may be 8x8, and the size of the current chroma block 1850 may be 4x4.

As described above, the embodiment of FIG. 17 may be applied to chroma components (U and V) other than the luma component (Y). When the prediction mode of the current chroma block 1850 is the LM mode, the encoder and decoder may derive luma reference information and prediction parameters for the current chroma block 1850 after the padding process is performed.

The encoder and decoder may derive predicted values of pixels within the luma component block 1810 to be predicted based on the neighboring luma pixels 1820 (P _Y ). In this case, the encoder and decoder may generate a reconstructed block of a luma component based on the derived prediction value, which may correspond to the reconstructed current luma block 1830 (P _RecY ). In addition, the encoder and decoder may determine neighboring luma pixels 1840 (P _LM ) to be used for generating luma reference information based on the neighboring luma pixels 1820 (P _Y ) used in prediction of the luma component block 1810. there is.

The encoder and decoder may generate luma reference information to be used for prediction of the current chroma block 1850 based on the neighboring luma pixels 1840 (P _LM ) and the reconstructed current luma block 1830 (P _RecY ). In FIG. 18, since the sizes of the reconstructed current luma block 1830 and the current chroma block 1850 are different, the encoder and the decoder use a luma reference block of the same size (4x4) as the current chroma block 1850 and its surroundings. Luma reference pixels can be created. At this time, since the size of the current chroma block 1850 is smaller than the size of the reconstructed current luma block 1830, the encoder and the decoder use pixels in the reconstructed current luma block 1830 and neighboring luma pixels 1840. Luma reference information may be generated by performing down-sampling for .

A down-sampling process may be performed, for example, by Equation 29 below on the neighboring luma pixels positioned above the reconstructed current luma block 1830 .

[Equation 29]

P _Y '[x,-1] = (P _LM [2x-1,-1] + 2*P _LM [2x,-1] + P _LM [2x+1,-1] + 2) >> 2,

(x = 0 to N _C -1)

Here, P _Y ' may indicate a pixel value of a pixel corresponding to luma reference information (eg, a pixel in a luma reference block or a neighboring luma reference pixel). Also, P _LM may indicate a pixel value of a neighboring luma pixel 1840 to be used for generating luma reference information. Also, N _C may represent the horizontal length and vertical length of the current chroma block 1850 .

A down-sampling process may be performed, for example, by Equation 30 below for peripheral luma pixels positioned to the left of the reconstructed current luma block 1830 .

[Equation 30]

P _Y '[-1,y] = (P _LM [-1,2y] + P _LM [-1,2y+1]) >> 1, (y = 0~N _C -1)

For the pixels in the reconstructed current luma block 1830, a downsampling process may be performed, for example, by Equation 31 below. A luma reference block corresponding to the current chroma block 1850 may be generated by Equation 31 below.

[Equation 31]

P _Y '[x,y] = (P _RecY [2x,2y] + P _RecY [2x,2y+1]) >> 1, (x,y = 0~N _C -1)

Here, P _RecY may indicate a pixel value of a pixel in the current luma block 1830 .

When luma reference information is generated by the above-described process, the encoder and decoder may derive a prediction parameter for the current chroma block 1850 based on the luma reference information and chroma component information. For example, the encoder and decoder may derive a prediction parameter based on pixel values of the neighboring luma reference pixels 1840 and pixel values of the neighboring chroma pixels 1860 . Equation 32 below may represent a part of a process of deriving a prediction parameter.

[Equation 32]

Here, P _C may indicate a pixel value of a neighboring chroma pixel 1860 to be used for generating luma reference information, and 'Bitdepth' may indicate a bit depth of a chroma component pixel.

Implementation of the process of deriving k2, a1, a2, k1, a1s, a2s, a3, α (alpha), k and β (beta) based on k3, L, C, LL, LC derived by Equation 32 Since the example is similar to the embodiment of Equation 27, a detailed description thereof will be omitted here. However, in this case, in the embodiment of Equation 27, nS may be replaced with N _C , p1 may be replaced with P _Y ', and p2 may be replaced with P _C .

After the prediction parameters are derived, the encoder and decoder may derive prediction values of pixels in the current chroma block 1850 based on the prediction parameters and pixel values of pixels in the luma reference block 1830 . Since an embodiment of a process of deriving predicted values of pixels in the current chroma block 1850 is similar to the embodiment of Equation 28, a detailed description thereof will be omitted here. However, in this case, in the embodiment of Equation 28, nS may be replaced with N _C , and p1 may be replaced with P _RecY .

19A and 19B are diagrams for explaining another embodiment of a decoding process according to the present invention.

19a shows an embodiment of a decryption process according to the present invention. 19B shows that one CU 1990 is divided into a plurality of TUs based on a quad-tree structure.

19A and 19B (hereinafter referred to as FIG. 19 ), P _k may be P _Y , P _U or P _V , and N _k may be N _Y , N _U or N _V . Here, P _Y may represent pixel values of neighboring luma pixels used for prediction of the current luma block. Also, P _U and P _V may represent pixel values of neighboring chroma pixels used for prediction of the current chroma block. Here, P _U and P _V may be represented by P _C . Also, N _Y may represent the horizontal and vertical lengths of the current luma block, and N _U and N _V may represent the horizontal and vertical lengths of the current chroma block. Here, N _U and N _V may be represented by N _C . In the example of FIG. 19 , P _LM may represent pixel values of neighboring luma pixels used to generate luma reference information in LM mode. Here, P _Y [x,y] and P _LM [x,y] may have the same value.

In the embodiment of FIG. 19 , since k may be Y, U, or V, the embodiment of FIG. 19 may be applied to both the luma component Y and chroma components U and V. The relationship between variables applied to the embodiment of FIG. 19 according to color components may be represented by Table 6 below, for example.

[Table 6]

Here, cIdx may be an index indicating which color component of the current block corresponds to Y, U(Cb), and V(Cr). For example, if the color component of the current block is Y, 0 may be assigned to cIdx, if the color component of the current block is U(Cb), 1 may be assigned to cIdx, and if the color component of the current block is V( Cr), 2 may be assigned to cIdx.

In addition, the embodiment of FIG. 19 may be applied in order of a Y component, a U(Cb) component, and a V(Cr) component to each color component constituting a color space in one CU, for example.

Referring to FIG. 19B, one CU 1990 may be divided into a plurality of TUs (1, 2, 3, 4, 5, 6, 7) based on a quad tree structure. At this time, the encoder and decoder may first perform the decoding process according to the embodiment of FIG. 19A on the Y component blocks in the CU 1990. Also, the encoder and decoder may sequentially perform the decoding process according to the embodiment of FIG. 19A on U component blocks and V component blocks in the CU 1990. As an example, the decoding process according to the embodiment of FIG. 19 is [Y1->Y2->...->Y7->U1->U2->...->U7->V1->V2...- >V7]. As an example, Y1 may mean that decoding is performed on the luma component of block 1 in FIG. 19B.

Referring to FIG. 19A, the encoder and decoder P _k [x,y], (x = -1, y = -1 to N _k *2-1 and x = 0 to N _k *2-1, y = - 1) can be marked by checking whether it is available (S1910). As described above, marking may mean a process of storing or displaying information about availability. Since the embodiments in which the peripheral luma pixels are not available have been described above in the embodiment of FIG. 8 , a detailed description thereof will be omitted here.

Referring back to FIG. 19A, the encoder and decoder may determine whether conditions A, B, and C are satisfied (S1920).

Here, condition A is a condition indicating that the LM mode can be applied to intra prediction of the current chroma block. For example, condition A may be satisfied when 1 is allocated to the LM flag (chroma_pred_from_luma_enabled_flag). As described above, chroma_pred_from_luma_enabled_flag may indicate whether the LM mode can be supported for intra prediction of the current chroma block. Also, condition B is a condition indicating that the intra prediction mode of the current block is the LM mode. Here, as an example, the LM mode may be represented as Intra_FromLuma. Also, condition C is a condition indicating that the current block is a chroma component block (U(Cb) or V(Cr)). As an example, condition C can be satisfied when 1 or 2 is assigned to cIdx.

When condition A, condition B, and condition C are satisfied, the encoder and decoder can generate P _LM .

Referring back to FIG. 19A, if condition A, condition B, and condition C are satisfied, the encoder and decoder P _LM [x,y], (x=-1, y=-1 to N _Y *2-1 and x = 0 to N _Y * 2-1, y = -1) may be checked and marked (S1930).

At this time, for example, after the marking process for P _LM is performed, the encoder and decoder pixels corresponding to P _LM [x,y], (x=-1, y=0 to N _Y *2-1) The pixel values of the pixels determined to be usable may be replaced with the pixel value at the position [x-1,y], that is, the pixel value of P _LM [x-1,y].

Also at this time, as an embodiment, the encoder and decoder may check whether P _LM is available based on the availability of neighboring chroma pixels corresponding to neighboring luma pixels, that is, P _U or P _V . At this time, whether P _U or P _V is available may be already marked in step S1910. For example, the encoder and protector may determine whether the P _LM is available based on whether a pixel corresponding to P _U is available.

For example, if the surrounding chroma pixel P _U [x,y] is marked as unavailable for x=-1, y=0~N _U *2-1, then the encoder and decoder can use the surrounding luma pixel P _LM [x ,2y] and P _LM [x,2y+1] can also be determined to be unavailable. On the other hand, if the surrounding chroma pixel P _U [x,y] is marked as available for x=-1, y=0~N _U *2-1, then the encoder and decoder use the surrounding luma pixel P _LM [x,2y] and P _LM [x, 2y + 1] may also be determined to be available.

At this time, the encoder and decoder assign luma pixel values located at [x-1,2y] and [x-1,2y+1] to P _LM [x,2y] and P _LM [x,2y+1], respectively. can do. Accordingly, if it is determined that a peripheral luma pixel is a pixel on the left pixel line located adjacent to the left side of the current luma block and the peripheral luma pixel is available, the pixel value of the luma pixel located adjacent to the left side of the peripheral luma pixel is the pixel value of the adjacent luma pixel. It may be determined as a pixel value of a luma pixel.

Also, if the surrounding chroma pixel P _U [x,y] is marked as unavailable for x=0~N _U *2-1, y=-1, then the encoder and decoder use the surrounding luma pixel P _LM [2x,y ] and P _LM [2x+1,y] can also be determined to be unavailable. On the other hand, if the surrounding chroma pixel P _U [x,y] is marked as available for x=0~N _U *2-1, y=-1, then the encoder and decoder use the surrounding luma pixel P _LM [2x,y] and P _LM [2x+1,y] may also be determined to be available.

In this case, the encoder and decoder may allocate luma pixel values located at [2x,y] and [2x+1,y] to P _LM [2x,y] and P _LM [2x+1,y], respectively. Accordingly, if it is determined that the peripheral luma pixel is a pixel on the top pixel line located adjacent to the top of the current luma block and the peripheral luma pixel is available, the pixel value of the luma pixel at the same location as the peripheral luma pixel is the peripheral luma pixel It can be determined as a pixel value of

Also, if the surrounding chroma pixel P _U [x,y] is marked as unavailable for x=-1, y=-1, then the encoder and decoder determine that the surrounding luma pixel P _LM [x,y] is also unavailable. can judge On the other hand, if the surrounding chroma pixel P _U [x,y] is marked as available for x = -1 and y = -1, the encoder and decoder will determine that the surrounding luma pixel P _LM [x,y] is also available. can

At this time, the encoder and decoder may allocate the luma pixel value located at [x,y] to P _LM [x,y]. Therefore, if the position of the neighboring luma pixel is [-1,-1] and it is determined that the neighboring luma pixel is available, the pixel value of the luma pixel at the same position as the neighboring luma pixel is determined as the pixel value of the neighboring luma pixel. can

In the above-described embodiment, a process of checking whether P _LM is available based on P _U is described, but the present invention is not limited thereto. For example, whether P _LM is available may be checked based on P _V .

Referring back to FIG. 19A, the encoder and decoder may determine whether condition D is satisfied (S1940). Here, condition D is at least one of P _k [x,y], (x=-1, y=-1~N _k *2-1 and x=0~N _k *2-1, y=-1) A condition indicating that a pixel in is not available.

If condition D is satisfied, the encoder and decoder P _k [x,y], (x=-1, y=-1~N _k *2-1 and x=0~N _k *2-1, y= A padding process may be performed on unavailable pixels among -1) (S1950). Since the embodiments of the padding process have been described above, a detailed description thereof will be omitted here.

Referring back to FIG. 19A , the encoder and decoder may determine whether conditions A, B, C, and E are satisfied (S1960). Here, condition E is at least one of P _LM [x,y], (x=-1, y=-1~N _Y *2-1 and x=0~N _Y *2-1, y=-1) A condition indicating that a pixel in is not available.

If condition A, condition B, condition C and condition E are satisfied, the encoder and decoder P _LM [x,y], (x=-1, y=-1~N _Y *2-1 and x=0~ A padding process may be performed on unavailable pixels among N _Y *2-1, y = -1 (S1970). Since specific embodiments of the padding process have been described above, a detailed description thereof will be omitted here.

After the above-described padding process is performed, the encoder and the decoder perform a process of predicting and decoding the current luma block and/or the current chroma block to obtain a reconstruction block for the current luma block and/or a reconstruction block for the current chroma block. It can be created (S1980).

In this case, LM mode prediction may be applied to the current chroma block. Since the prediction process in the LM mode is similar to that in the embodiment of FIG. 18, a detailed description thereof will be omitted here.

In the foregoing embodiments, the methods are described on the basis of a flowchart as a series of steps or blocks, but the present invention is not limited to the order of steps, and some steps may occur in a different order or concurrently with other steps as described above. can In addition, those skilled in the art will understand that the steps shown in the flow chart are not exclusive, that other steps may be included, or that one or more steps of the flow chart may be deleted without affecting the scope of the present invention. You will understand.

The foregoing embodiment includes examples of various aspects. It is not possible to describe all possible combinations to represent the various aspects, but those skilled in the art will recognize that other combinations are possible. Accordingly, it is intended that the present invention cover all other substitutions, modifications and variations falling within the scope of the following claims.

Claims (9)

determining whether an LM mode is applied to the current chroma block according to intra prediction mode information indicating the intra prediction mode of the current chroma block;
When the LM mode is applied to the current chroma block, whether left neighboring luma pixels adjacent to the left side of the luma reference block among the neighboring luma pixels of the luma reference block corresponding to the current chroma block are available and of the luma reference block determining whether top perimeter luma pixels adjacent to the top are available;
determining peripheral luma reference pixels according to whether the left peripheral luma pixels are available and whether the top peripheral luma pixels are available;
determining neighboring chroma reference pixels corresponding to the neighboring luma reference pixels from among neighboring chroma pixels located around the current chroma block;
deriving a prediction parameter for the current chroma block based on the neighboring luma reference pixels and the neighboring chroma reference pixels; and
Deriving a predicted value of a pixel in the current chroma block based on a pixel in the luma reference block and the prediction parameter;
the left peripheral luma pixels include a first left peripheral luma pixel immediately adjacent to the left of the luma reference block and a second left peripheral luma pixel immediately adjacent to the left of the first left peripheral luma pixel;
If the left peripheral luma pixels are available, a downsampled peripheral luma reference pixel determined based on the first left peripheral luma pixel and the second left peripheral luma pixel and a peripheral chroma corresponding to the downsampled peripheral luma reference pixel Based on a reference pixel, the prediction parameter for the current chroma block is derived.
According to claim 1,
Determining the peripheral luma reference pixels,
if the left peripheral luma pixels are not available, the left peripheral luma pixels are not included in peripheral luma reference pixels;
When the upper peripheral luma pixels are not available, the upper peripheral luma pixels are not included in peripheral luma reference pixels.
According to claim 1,
In the step of deriving the prediction parameter,
The peripheral luma pixels are included in pixel groups having a predetermined size;
For each of the pixel groups, a peripheral luma sample at a predetermined position among the peripheral luma pixels is selected;
Based on the selected peripheral luma sample of the predetermined location, the peripheral luma pixels are downsampled;
Wherein the prediction parameter is derived based on the downsampled neighboring luma pixels.
According to claim 1,
The image decoding method of claim 1 , wherein the upper peripheral luma pixels further include a second upper peripheral luma pixel immediately adjacent to an upper end of the first upper peripheral luma pixel.
According to claim 1,
Determining the peripheral luma reference pixels,
and determining the peripheral luma reference pixels according to the left peripheral luma pixels and the lower-left peripheral luma pixels when the left peripheral luma pixels are available.
According to claim 1,
Determining the peripheral luma reference pixels,
and determining the peripheral luma reference pixels according to the top peripheral luma pixels and the top right peripheral luma pixel when the upper peripheral luma pixels are available.
According to claim 1,
The video decoding method,
obtaining LM flag information indicating whether an LM mode can be applied to intra prediction of a chroma block in a current sequence including the current chroma block; and
When the LM flag information indicates that the LM mode is applied to intra prediction of a chroma block in the current sequence, whether the LM mode is applied to the current chroma block according to intra prediction mode information indicating the intra prediction mode of the current chroma block An image decoding method further comprising the step of determining whether or not.
Determine whether left neighboring luma pixels adjacent to the left side of the luma reference block among the neighboring luma pixels of the luma reference block corresponding to the current chroma block are available and whether upper neighboring luma pixels adjacent to the top of the luma reference block are available doing;
determining peripheral luma reference pixels according to whether the left peripheral luma pixels are available and whether the top peripheral luma pixels are available;
determining neighboring chroma reference pixels corresponding to the neighboring luma reference pixels from among neighboring chroma pixels located around the current chroma block;
deriving a prediction parameter for the current chroma block based on the neighboring luma reference pixels and the neighboring chroma reference pixels;
deriving a predicted value of a pixel in the current chroma block based on a pixel in the luma reference block and the prediction parameter; and
Determining whether the LM mode is applied to the current chroma block according to a prediction result of the current chroma block, and encoding intra prediction mode information indicating an intra prediction mode of the current chroma block according to the determination result do,
the left peripheral luma pixels include a first left peripheral luma pixel immediately adjacent to the left of the luma reference block and a second left peripheral luma pixel immediately adjacent to the left of the first left peripheral luma pixel;
If the left peripheral luma pixels are available, a downsampled peripheral luma reference pixel determined based on the first left peripheral luma pixel and the second left peripheral luma pixel and a peripheral chroma corresponding to the downsampled peripheral luma reference pixel Based on a reference pixel, the prediction parameter for the current chroma block is derived.
A computer-readable recording medium storing a bitstream,
The bitstream includes image data decoded by an image decoding method,
The image data includes intra prediction mode information indicating an intra prediction mode of a current chroma block;
The video decoding method,
determining whether an LM mode is applied to the current chroma block according to the intra prediction mode information;
When the LM mode is applied to the current chroma block, whether left neighboring luma pixels adjacent to the left side of the luma reference block among the neighboring luma pixels of the luma reference block corresponding to the current chroma block are available and of the luma reference block determining whether top perimeter luma pixels adjacent to the top are available;
determining peripheral luma reference pixels according to whether the left peripheral luma pixels are available and whether the top peripheral luma pixels are available;
determining neighboring chroma reference pixels corresponding to the neighboring luma reference pixels from among neighboring chroma pixels located around the current chroma block;
deriving a prediction parameter for the current chroma block based on the neighboring luma reference pixels and the neighboring chroma reference pixels; and
Deriving a predicted value of a pixel in the current chroma block based on a pixel in the luma reference block and the prediction parameter;
the left peripheral luma pixels include a first left peripheral luma pixel immediately adjacent to the left of the luma reference block and a second left peripheral luma pixel immediately adjacent to the left of the first left peripheral luma pixel;
If the left peripheral luma pixels are available, a downsampled peripheral luma reference pixel determined based on the first left peripheral luma pixel and the second left peripheral luma pixel and a peripheral chroma corresponding to the downsampled peripheral luma reference pixel Based on a reference pixel, the prediction parameter for the current chroma block is derived.

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