KR102607887B1 - Method for intra prediction and apparatus thereof - Google Patents

Abstract

The intra prediction method according to the present invention generates luma reference information, including pixels in the current luma block and surrounding luma pixels located around the current luma block, based on pixels in the current luma block and surrounding luma reference pixels located around the luma reference block. Generating a prediction parameter for the current chroma block based on the surrounding luma reference pixel and a first surrounding chroma pixel corresponding to the surrounding luma reference pixel among the surrounding chroma pixels located around the current chroma block, and luma reference. It includes 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 {METHOD FOR INTRA PREDICTION AND APPARATUS THEREOF}

The present invention relates to image processing, and more specifically, to intra prediction methods and devices.

Recently, as broadcasting services with HD (High Definition) resolution have expanded not only domestically but also globally, many users are becoming accustomed to high-resolution, high-definition video, and many organizations are accelerating the development of next-generation video devices. In addition, as interest in UHD (Ultra High Definition), which has a resolution more than four times that of HDTV, increases along with HDTV, compression technology for higher resolution and higher quality images 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. Intra prediction technology, entropy coding technology that assigns short codes to symbols with a high frequency of appearance and long codes to symbols with a low frequency of appearance, etc. may be used.

The technical object of the present invention is to provide an image encoding method and device that can improve image encoding/decoding efficiency.

Another technical object of the present invention is to provide an image decoding method and device that can improve image encoding/decoding efficiency.

Another technical problem of the present invention is to provide an intra prediction method and device that can improve video 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 surrounding luma reference pixels located around the luma reference block, based on pixels in a current luma block and surrounding luma pixels located around the current luma block. , deriving a prediction parameter for the current chroma block based on the peripheral luma reference pixel and a first peripheral chroma pixel corresponding to the peripheral luma reference pixel among the peripheral chroma pixels located around the current chroma block, and the luma It may include deriving a predicted value of a pixel in the current chroma block based on the pixel in the reference block and the prediction parameter. Here, in the luma reference information generation step, whether the neighboring luma pixel is available is determined depending on 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, the pixel value of the surrounding luma pixel may be determined depending on whether the surrounding luma pixel is available.

In the luma reference information generation step, if the second peripheral chroma pixel is available, it may be determined that the peripheral luma pixel is available.

When the location of the second surrounding chroma pixel is [x, y], x is -1 and y has one of 0 to _NU *2-1, based on the second surrounding chroma pixel The location of the surrounding luma pixel, whose 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 location of the second surrounding chroma pixel is [x, y], y is -1 and x has one of the values of 0 to _NU *2-1, based on the second surrounding chroma pixel The location of the surrounding luma pixel, which is determined to be available, may be [2x, y] or [2x+1, y], and N _U may represent the horizontal and vertical lengths of the current chroma block.

When the location of the second peripheral chroma pixel is [-1, -1], the location of the peripheral luma pixel, whether or not it is available based on the second peripheral chroma pixel, is determined to be [-1, -1]. there is.

In the luma reference information generation step, when it is determined that the surrounding luma pixel is not available, the pixel value of an available pixel among pixels located around the current luma block may be assigned to the surrounding luma pixel.

In the luma reference information generation step, if the peripheral luma pixel is a pixel on the left pixel line located adjacent to the left of the current luma block and the peripheral luma pixel is determined to be available, the peripheral luma pixel is located adjacent to the left of the peripheral luma pixel. The pixel value of the luma pixel may be determined by the pixel value of the surrounding luma pixels.

In the luma reference information generation step, if the surrounding luma pixel is a pixel on the top pixel line located adjacent to the top of the current luma block and the surrounding luma pixel is determined to be available, the luma pixel at the same location as the surrounding luma pixel The pixel value of may be determined as the 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 surrounding luma reference pixels located around the luma reference block, based on pixels in a current luma block and surrounding luma pixels located around the current luma block. , deriving a prediction parameter for the current chroma block based on the surrounding luma reference pixel and a first surrounding chroma pixel corresponding to the surrounding luma reference pixel among neighboring chroma pixels located around the current chroma block, the luma generating a prediction block for the current chroma block by deriving a prediction value of a pixel in the current chroma block based on the 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 the step of generating a restoration block. Here, in the luma reference information generation step, whether the neighboring luma pixel is available is determined depending on 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, the pixel value of the surrounding luma pixel may be determined depending on whether the surrounding luma pixel is available.

In the luma reference information generation step, if the second peripheral chroma pixel is available, it may be determined that the peripheral luma pixel is available.

When the location of the second surrounding chroma pixel is [x, y], x is -1 and y has one of 0 to _NU *2-1, based on the second surrounding chroma pixel The location of the surrounding luma pixel, whose 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 location of the second surrounding chroma pixel is [x, y], y is -1 and x has one of the values of 0 to _NU *2-1, based on the second surrounding chroma pixel The location of the surrounding luma pixel, which is determined to be available, may be [2x, y] or [2x+1, y], and N _U may represent the horizontal and vertical lengths of the current chroma block.

When the location of the second peripheral chroma pixel is [-1, -1], the location of the peripheral luma pixel, whether or not it is available based on the second peripheral chroma pixel, is determined to be [-1, -1]. there is.

In the luma reference information generation step, when it is determined that the surrounding luma pixel is not available, the pixel value of an available pixel among pixels located around the current luma block may be assigned to the surrounding luma pixel.

In the luma reference information generation step, if the peripheral luma pixel is a pixel on the left pixel line located adjacent to the left of the current luma block and the peripheral luma pixel is determined to be available, the peripheral luma pixel is located adjacent to the left of the peripheral luma pixel. The pixel value of the luma pixel may be determined by the pixel value of the surrounding luma pixels.

In the luma reference information generation step, if the surrounding luma pixel is a pixel on the top pixel line located adjacent to the top of the current luma block and the surrounding luma pixel is determined to be available, the luma pixel at the same location as the surrounding luma pixel The pixel value of may be determined as the 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 the configuration of a video encoding device according to an embodiment to which the present invention is applied.
Figure 2 is a block diagram showing the configuration of a video decoding device according to an embodiment to which the present invention is applied.
Figure 3 is a conceptual diagram schematically showing an embodiment in which one unit is divided into a plurality of sub-units.
Figure 4 shows an example of a prediction direction of an intra prediction mode and a mode value assigned to each prediction direction.
Figure 5 is a flowchart schematically showing an embodiment of a prediction method between color components that performs prediction of chroma components based on luma components.
Figure 6 shows an embodiment of a method for deriving luma reference information when the size of the current luma block and the size of the current chroma block are different from each other.
Figure 7 is a diagram schematically showing an embodiment of a process for generating luma reference information based on encoding parameters.
FIG. 8 is a diagram schematically showing an embodiment of a process for generating luma reference information when there is an unavailable pixel among surrounding luma pixels.
Figure 9 is a diagram for explaining an embodiment of a process for generating luma reference information after performing a padding process.
FIG. 10 is a diagram for explaining another embodiment of a process for generating luma reference information after performing a padding process.
FIG. 11 is a diagram illustrating another embodiment of a process for generating luma reference information after performing a padding process.
FIG. 12 is a diagram illustrating another embodiment of a process for generating luma reference information after performing a padding process.
FIG. 13 is a diagram schematically showing an embodiment of a process for deriving first color component reference information to be used for prediction of the second color component block when the size of the first color component block is smaller than the size of the second color component block. .
FIG. 14 is a diagram schematically showing an embodiment of a process for deriving first color component reference information to be used for prediction of the second color component block when the size of the first color component block is the same as the size of the second color component block. .
Figure 15 is a diagram schematically showing an embodiment of peripheral luma pixels used to generate luma reference information.
FIG. 16 is a diagram illustrating an embodiment of a process for deriving prediction parameters and a process for deriving predicted values of pixels in the current chroma block.
Figure 17 is a flowchart schematically showing an embodiment of the decoding process according to the present invention.
FIG. 18 is a diagram illustrating an embodiment of a process for deriving luma reference information and prediction parameters for the current chroma block when a decoding process is performed as in the embodiment of FIG. 17.
Figures 19a and 19b are diagrams for explaining another embodiment of the decoding process according to the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the 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 will be omitted.

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

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

Additionally, the components appearing in the embodiments of the present invention are shown independently to show different characteristic functions, and this does not mean that each component is comprised of separate hardware or a single software component. That is, each component is listed and included as a separate component for convenience of explanation, and at least two of each component can be combined to form one component, or one component can be divided into a plurality of components to perform a function, and each of these components can perform a function. Integrated embodiments and separate embodiments of the constituent parts are also included in the scope of the present invention as long as they do not deviate from the essence of the present invention.

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

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

Referring to FIG. 1, the image encoding device 100 includes a motion prediction unit 111, a motion compensation unit 112, an intra prediction unit 120, a switch 115, a subtractor 125, and a conversion 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 device 100 may encode an input image in intra mode or inter mode and output a bitstream. Intra prediction refers to prediction within a screen, and inter prediction refers to prediction between screens. In the case of intra mode, the switch 115 may be switched to intra, and in the case of inter mode, the switch 115 may be switched to inter. The image encoding apparatus 100 may generate a prediction block for an input block of an input image and then encode the residual between the input block and the prediction block.

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

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

In the prediction process of the intra prediction unit 120, motion prediction unit 111, and/or motion compensation unit 112, the processing unit in 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. It may be possible. For example, when the processing unit in which the prediction method is determined is PU, the processing unit in which prediction is performed may be TU.

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

The entropy encoding unit 150 may perform entropy encoding based on values calculated by the quantization unit 140 or encoding parameter values calculated during the encoding process, and output a bit stream.

When entropy coding is applied, a small number of bits are allocated to symbols with a high probability of occurrence and a large number of bits are allocated to symbols with a low probability of occurrence to represent symbols, so that the bits for the symbols to be encoded are expressed. The size of the column may be reduced. Therefore, the compression performance of video encoding can be improved through entropy coding. The entropy encoding unit 150 may use encoding methods such as exponential gollomb, context-adaptive variable length coding (CAVLC), and context-adaptive binary arithmetic coding (CABAC) for entropy encoding.

Since the video encoding device according to the embodiment of FIG. 1 performs inter predictive coding, that is, inter-screen predictive coding, the currently encoded video needs to be decoded and stored 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 the adder 175 and a restored block is generated.

The restored block passes through the filter unit 180, and the filter unit 180 applies at least one of a deblocking filter, Sample Adaptive Offset (SAO), and Adaptive Loop Filter (ALF) to the restored block or restored picture. can do. The filter unit 180 may also be called an adaptive in-loop filter. The deblocking filter can remove block distortion at the boundaries between blocks. SAO can add an appropriate offset value to the pixel value to compensate for coding errors. ALF can perform filtering based on the comparison between the restored image and the original image. The restored block that has passed through the filter unit 180 may be stored in the reference picture buffer 190.

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

Referring to FIG. 2, the image decoding device 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, and an adder ( 255), a filter unit 260, and a reference picture buffer 270.

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

The entropy decoding unit 210 may entropy decode the input bitstream according to a probability distribution to generate symbols including symbols in the form of quantized coefficients. 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 symbols with a high probability of occurrence and a large number of bits are allocated to symbols with a low probability of occurrence to represent the symbols, thereby reducing the size of the bit string for each symbol. can be reduced. Therefore, the compression performance of video decoding can 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 transformation unit 230. As a result of the inverse quantization/inverse transformation of the quantized coefficient, a residual block may be generated.

In the case of intra mode, the intra prediction unit 240 may generate a prediction block by performing spatial prediction using pixel values of already encoded blocks surrounding the current block. In 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.

During the prediction process in the intra prediction unit 240 and/or the motion compensation unit 250, the processing unit in which the prediction method or prediction mode is determined and the processing unit in which prediction is performed may be the same or different from each other. For example, when the processing unit in which the prediction method is determined is PU, the processing unit in which prediction is performed may be 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 restored block or a restored picture. The filter unit 260 may output a reconstructed image, that is, a restored image. The reconstructed image can be stored in the reference picture buffer 270 and used for inter prediction.

Hereinafter, a unit may refer to a unit of video encoding and decoding. When encoding or decoding a video, the encoding or decoding unit refers to the divided unit when the video is divided and encoded or decoded, so it is a coding unit (CU: Coding Unit), prediction unit (PU: Prediction Unit), and transformation unit (TU). : Transform Unit), etc. Additionally, in embodiments described later, a unit may also be referred to as a block. One unit may be further divided into smaller sized subunits.

Figure 3 is a conceptual diagram schematically showing 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 extent to which the unit is divided, it may also include information about the size of the sub-unit.

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

Referring to 310 of FIG. 3, the highest node may be called 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 child node with a depth of level 1 may represent a unit in which the original unit was divided once, and a child node with a depth of level 2 may represent a unit in which the original unit was divided twice. For example, unit a corresponding to node a in 320 of FIG. 3 is a unit divided once from the original unit and may have a depth of level 1.

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

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

Meanwhile, an image signal may be composed of color components, which are components of a color space. For example, an image signal may generally include three color components representing the three primary color components of light. The three color components representing the three primary color components of light can be expressed as R (Red), G (Green), and B (Blue). The R, G, and B signals may be converted into one luma component and two chroma components to reduce the frequency band used for image processing. At this time, one video signal may include one luma component and two chroma components. Here, the luma component is a component representing the brightness of the screen and can be represented by Y. Additionally, the two chroma components are components representing the color of the screen and can be expressed as U, V, Cb, or Cr. Therefore, the color space of the image may be expressed as YUV or YCbCr, for example. Hereinafter, in embodiments 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.

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

For example, in a 4:2:0 video format, the number of horizontal pixels of the chroma block corresponding to the luma block is 1/2 of the number of horizontal pixels of the luma block, and the number of vertical pixels of the chroma block corresponding to the luma block is 1/2. The number may correspond to 1/2 of the number of vertical pixels of the luma block. In the 4:2:2 video format, the number of horizontal pixels of the chroma block corresponding to the luma block is 1/2 of the number of horizontal pixels of the luma block, and the number of vertical pixels of the chroma block corresponding to the luma block is 1/2 of the number of horizontal pixels of the luma block. It may be equal to the number of pixels in the vertical direction. In the 4:4:4 video format, the number of horizontal and vertical pixels of the chroma block corresponding to the luma block may be the same as the number of horizontal and vertical pixels 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 mode of adjacent neighboring blocks, the prediction mode of the current block may be encoded based on the prediction mode of neighboring blocks adjacent to the current block. At this time, MPM (Most Probable Mode) for the current block may be used.

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

As described above in the embodiments of FIGS. 1 and 2, the encoder and decoder can 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 can have may be a fixed value, or it 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 can have may be 35 or 36.

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

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

Additionally, the prediction mode with a mode value of 35 in 410 of FIG. 4 may be called LM mode (eg, Intra_FromLuma). LM mode may refer to a prediction mode in which the prediction value of a chroma component (eg, U or V) pixel is determined according to the pixel value of the luma component (eg, Y). Specific examples of intra prediction in LM mode will be described later.

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

Meanwhile, since the luma component and the chroma component in the image are correlated, 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. The mode can be derived. Accordingly, the prediction mode information of the chroma component transmitted from the encoder to the decoder may not be the prediction mode of the chroma component itself, but 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. Table 1 below shows an example of a chroma prediction mode determined according to the prediction mode of the luma component and the transmitted value.

[Table 1]

Referring to Table 1, the value transmitted from the encoder to the decoder may be the value assigned to intra_chroma_pred_mode. IntraPredMode represents the intra prediction mode of the luma component. For example, if the value of intra_chroma_pred_mode 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. Additionally, the values assigned to each prediction mode are one embodiment and may be determined differently depending on implementation and/or need.

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

In addition, the encoder encodes LM flag information indicating whether the LM mode is applied according to the prediction mode of the chroma component within the current sequence, that is, whether the LM mode can be applied to intra prediction of the chroma component within the current sequence and transmits it to the decoder. Can be transmitted. For example, LM flag information may be transmitted to the decoder through a syntax element called chroma_pred_from_luma_enabled_flag. For example, if 1 is assigned to chroma_pred_from_luma_enabled_flag, the flag may indicate that LM mode can be used for intra prediction of the chroma component. Additionally, if 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 the chroma component.

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

[Table 2]

On the other hand, since the plurality of color components that make up the color space are correlated with each other, prediction of another color component (second color component) is performed based on one color component (first color component), thereby Coding efficiency for components can be improved. This prediction process can also be called ‘prediction between color components’. As described above, color components constituting the color space may include R, G, B, luma components (eg, Y), and/or chroma components (eg, U, V, Cb, Cr).

For example, in the LM mode described above, prediction of the chroma component may be performed based on the luma component. At this time, the first color component may correspond to a luma component, and the second color component may correspond to a chroma component. In LM mode, the correlation between the luma component and the chroma component can be expressed as a linear relationship using prediction parameters. A specific example of the process of predicting the chroma component based on the 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 explanation, the first color component is referred to as a luma component, and the second color component is referred to as a chroma component. However, the embodiments described below are not limited to this and may be applied in the same or similar manner to all possible combinations of the first color component and the second color component. At this time, in embodiments described later, the luma component can be viewed as a first color component, and the chroma component can be viewed as a second color component.

Figure 5 is a flowchart schematically showing an embodiment of a prediction method between color components that performs prediction of chroma components based on luma components. The prediction process described later may be performed in the intra prediction unit of the encoding device and the decoding device.

Hereinafter, in this specification, the chroma block subject to encoding/decoding is referred to as the current chroma block. Additionally, the 'restored' luma block corresponding to the current chroma block is referred to as the current luma block. For example, the current luma block may be a block that exists in the same location as the current chroma block. As another example, the current luma block may be a block that exists in 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 surrounding the current luma block (S510).

Luma reference information may refer to pixel values of luma components to be used for prediction of the current chroma block. Accordingly, in this specification, luma reference information may be referred to as ‘luma reference pixel.’ At this time, luma reference pixels corresponding to pixels in the current chroma block may constitute a ‘luma reference block’. Additionally, 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 luma pixels'. It is called ‘peripheral luma reference pixel’. At this time, the luma reference block may correspond to the current chroma block, and the surrounding luma reference pixel may correspond to the surrounding chroma pixel. In this specification, luma reference information may be used as a concept including pixels within a luma reference block and surrounding luma reference pixels.

Meanwhile, as described above, the current luma block and the current chroma block may have different sizes depending on the video format. In this case, the encoder and decoder perform down sampling and/or up sampling on pixels within the current luma block and pixels around the current luma block, thereby producing a luma of the same size as the current chroma block. A reference block and surrounding luma reference pixels corresponding to it can be derived.

At this time, the encoder and decoder may generate luma reference information differently based on encoding parameters. Here, the encoding parameters are parameters necessary for encoding and decoding, and may include information that is encoded by the encoder and transmitted to the decoder, such as syntax elements, as well as information that can be inferred during the encoding or decoding process, and may include information that can be inferred during the encoding or decoding process. It can refer to information needed when doing something. Encoding parameters may include, for example, an encoding mode, an intra prediction mode, an inter prediction mode, and a quantization parameter (QP).

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

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

Referring again to FIG. 5, the encoder and decoder may derive prediction parameters 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 surrounding luma reference pixels and pixel values of surrounding chroma pixels. Here, the prediction parameters may be alpha (α) and beta (β) based on least squares.

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

Referring again to FIG. 5, the encoder and decoder may derive prediction 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 can 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 specific example of the process of deriving predicted values of pixels in the current chroma block will be described later.

Figure 6 shows an embodiment of a method for deriving luma reference information when the size of the current luma block and the size of the 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 video format. For example, if the color space of the image is YUV 4:2:0 or YCbCr 4:2:0, the size of the current luma block may be four times larger than the size of the current chroma block. In this case, the encoder and decoder can derive pixels in the luma reference block having the same size as the current chroma block based on the current luma block, and derive surrounding luma reference pixels located around the luma reference block. That is, in this case, the encoder and decoder can generate luma reference information to correspond to the size of the current chroma block.

610 in 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 can generate luma reference pixels corresponding to a 4x4 luma reference block.

620 in FIG. 6 illustrates an embodiment of a method for deriving luma reference information.

In the embodiment of 620 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 as in 610 of FIG. 6 This is shown.

Referring to 620 in FIG. 6, the encoder and decoder use the luma reference block 626 and surrounding luma blocks 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 624. Luma reference pixels 628 may be generated. Here, the surrounding luma pixels 624 used to derive luma reference information include pixels included in the two pixel lines located closest to the left of the current luma block 612, and pixels located at the top of the current luma block 612. It may include pixels included in one pixel line located closest to each other.

Meanwhile, in 620 of FIG. 6, the sizes of the current luma block 612 and the current chroma block 616 are different from each other, so the encoder and decoder use a luma reference block 626 of 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 perform down-sampling ( Luma reference information can be generated by performing down sampling.

Hereinafter, coordinates (eg, (x,y)) in this specification may be coordinates based on the position of the pixel that exists at the top left of the block. For example, in this specification, the coordinates of the pixel located at the top left of the block may be (0,0).

In one embodiment, the encoder and decoder apply a 2-tap filter, a 3-tap filter, and/or a 4-tap filter for pixels within the current luma block 612 and surrounding luma pixels 624. Down sampling can be performed by applying a (4-tap) filter, etc. At this time, the down sampling process can be expressed as an example by Equation 1 below.

[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 represent pixel values of the restored luma component pixels, that is, pixels within the current luma block 612 and surrounding luma pixels 624. Additionally, 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 down-sampling by selecting pixels to be used as luma reference pixels from pixels in the current luma block 612 and surrounding luma pixels 624 according to a predetermined operation. At this time, for example, the encoder and decoder can select one pixel from a pixel group with a predetermined size and use it as a luma reference pixel. For example, this down-sampling process may be performed by at least one of the down-sampling methods represented by Equation 2, Equation 3, and Equation 4 below.

[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]

Additionally, different down-sampling methods may be applied to pixels in the current luma block 612, surrounding luma pixels located on top of the current luma block 612, and surrounding luma pixels located to the left of the current luma block 612. there is. For example, when the down-sampling method according to Equation 1 is applied to the pixels in the current luma block 612, the down-sampling method according to Equation 2 is applied to the surrounding luma pixels located at the top of the current luma block 612. Alternatively, the down-sampling method according to Equation 3 may be applied to surrounding luma pixels located to the left of the current luma block 612. Here, not all combinations of possible embodiments in which the down-sampling method is applied are described, but those skilled in the art will recognize that other combinations of the above-described down-sampling methods can be used.

630 in FIG. 6 shows another embodiment of a method for deriving luma reference information.

In the embodiment of 630 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 as in 610 of FIG. 6 This is shown.

Referring to 630 in FIG. 6, the encoder and decoder use the luma reference block 636 and the surrounding luma block 616 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 generated.

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

Since the down-sampling method at 630 of FIG. 6 is the same as that of 620 of FIG. 6, detailed description of the down-sampling method will be omitted.

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

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

Additional embodiments of a method for generating luma reference information based on encoding parameters are described later with reference to FIG. 7 .

Figure 7 is a diagram schematically showing an embodiment of a process for generating luma reference information based on encoding parameters.

710 of FIG. 7 represents an embodiment of a method for generating luma reference information 716 and 718 based on the intra prediction mode of the current luma block 712. At 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 surrounding luma reference pixels 718 to be used for prediction of the current chroma block are generated based on pixels in the current luma block 712 and surrounding 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 decoder only use pixels located adjacent to the top of the current luma block 712 among the surrounding luma pixels 714. Can 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 located adjacent to the top of the luma reference block 716.

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

Reference numeral 730 of FIG. 7 represents an embodiment of a method for 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. At 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 top surrounding luma block 734 located adjacent to the top of the current luma block 732 is 22, and the top left corner located at the top left corner outside the current luma block 732. The quantization parameter value of the surrounding luma block 736 is 26. Additionally, the quantization parameter value of the left peripheral luma block 738 located adjacent to the left of the current luma block 732 is 28. At this time, the encoder and decoder may use peripheral luma pixels belonging to the peripheral luma block with the smallest quantization parameter value among the peripheral luma pixels 744, 746, and 748 to generate luma reference information.

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

Since the size of the current chroma block at 730 of FIG. 7 is 4x4, the size of the generated luma reference block 752 may be 4x4. Additionally, the peripheral luma reference pixels 754 derived from 730 of FIG. 7 may be pixels located adjacent to the top of the luma reference block 752.

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

FIG. 8 is a diagram schematically showing an embodiment of a process for generating luma reference information when there is an unavailable pixel among surrounding luma pixels.

As described above, there may be unavailable pixels among surrounding luma pixels located around the current luma block. Cases in which surrounding luma pixels are not available include 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 when CIP (Constrained Intra) is used for the current luma block. There may be cases where prediction) is applied.

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

At this time, in one embodiment, the encoder and decoder may perform a padding process on unavailable pixels before performing the down-sampling process. As described above, padding may refer to the process of filling the position of an unavailable pixel with a new pixel value.

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

[Equation 5]

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

Here, p[x,y] may represent the restored pixel value of the luma component.

The process of filling in pixel values in the pixel line located second closest to the left of the current luma block 812, that is, the second pixel line located adjacent to the left of the first pixel line, is represented by the following equation 6, for example: You can.

[Equation 6]

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

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

In another embodiment, the encoder and decoder may not use unavailable surrounding luma pixels to generate luma reference information. That is, the encoder and decoder can generate luma reference information based on pixels in the current luma block and available surrounding luma pixels. For example, in 810 of FIG. 8, pixels within the current luma block 812 and available surrounding 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 at 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 along with the above-described embodiments.

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

CIP may refer to an intra prediction method that uses only pixels within a block encoded/decoded by intra prediction among blocks located around the prediction target block for prediction. When CIP is applied, pixels within a block encoded/decoded by inter prediction may not be used for intra prediction.

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

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

At this time, in one embodiment, the encoder and decoder may perform a padding process on unavailable pixels before performing the down-sampling process. As described above, padding may refer to the process of filling the position of an unavailable pixel with a new pixel value. At this time, the encoder and decoder can fill unavailable peripheral luma pixels with pixel values of available peripheral luma pixels.

For example, when the surrounding luma pixels 836 located within the left peripheral luma block 826 are not available as shown in 820 of FIG. 8, the encoder and decoder use the available peripheral luma pixels 836 located within the top peripheral luma block 824. The pixel value of at least one pixel among the luma pixels 834 may be filled into the positions of unavailable neighboring luma pixels 836. For example, the process of performing padding based on pixels in the upper peripheral luma block 824 can 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, pixels 838 located adjacent to the bottom of the left peripheral luma block 826 may be pixels within a block encoded/decoded in intra mode. At this time, the encoder and decoder use pixels located at the top of the left peripheral luma block (e.g., pixels present at the (-2,-1) position or pixels present at the (-1,-1) position) and the left peripheral Surrounding luma pixels 836 for which the pixel average value of the pixel located at the bottom of the luma block (e.g., the pixel at the (-2,8) position or the pixel at the (-1,8) position) is not available. ) can be filled in. For example, this padding process can be expressed by Equation 8 below.

[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)

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

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

In the luma reference information generation process at 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 along with the above-described embodiments.

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

9, the first left pixel line located closest to the left of the current luma block 910 and the surrounding luma pixels within the first top pixel line located closest to the top of the current luma block 910 generate luma reference information. An example of when used is shown. In the embodiment of FIG. 9 , the pixel line located second closest to the left of the current luma block 910, that is, the pixel line located adjacent to the left of the first left pixel line, will be referred to as the second left pixel line. Additionally, the pixel line located second closest to the top of the current luma block 910, that is, the pixel line located adjacent to the top of the first top pixel line, will be referred to as the 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, LM mode may be indicated as Intra_FromLuma. Additionally, 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 the peripheral luma pixels corresponding to 926 are available pixels, It is assumed that the surrounding luma pixels corresponding to 924 and the surrounding luma pixels corresponding to 928 are unavailable pixels. In this case, the encoder and decoder may generate luma reference information after performing a padding process on unavailable surrounding luma pixels.

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

[Equation 9]

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

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

After the surrounding luma pixel values on the second pixel line are copied to the first left pixel line, the encoder and decoder perform a padding process on the surrounding luma pixels to derive pixel values of the surrounding luma pixels to be used for generating luma reference information. can do. Pixel values of surrounding luma pixels to be used to generate luma reference information can 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 form pixel groups as indicated by numbers 924 and 928 in FIG. 9 . In the embodiment of FIG. 9, for convenience of explanation, a group of unavailable pixels on the first left pixel line (e.g., four pixels located on the left at 928 in FIG. 9) is referred to as a left pixel group, and the first upper pixel line is referred to as a left pixel group. The group of unavailable pixels 924 on the top is called the top pixel group. In the embodiment of FIG. 9, pixels belonging to the top pixel group can be expressed as P _Y [x,y], (x=nS~nS*2-1, y=-1), and pixels belonging to the left pixel group can be expressed as P _Y [x,y], (x=-1, y=nS~nS+(nS>>1)-1).

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

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

In this case, there may be only one available pixel located adjacent to the top pixel group 924 within the first top pixel line. Accordingly, the encoder and decoder can fill the positions of unavailable pixels in the top pixel group 924 with the pixel value of the available pixel c located adjacent to the top pixel group 924. For example, this padding process can be expressed by Equation 11 below.

[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, available pixel b may be located adjacent to the top of the left pixel group, and available pixel a may be located adjacent to the bottom of the left pixel group. Here, pixel a can be represented as P _Y [-1, nS+(nS>>1)], and pixel b can be represented as P _Y [-1, nS-1].

In this case, there may be two available pixels located adjacent to the left pixel group within the first left pixel line. Accordingly, the encoder and decoder can fill the pixel average values of the available pixels (a, b) located adjacent to the left pixel group into the positions of unavailable pixels in the left pixel group. For example, this padding process can be expressed by Equation 12 below.

[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)

The pixel values of the surrounding luma pixels on the first left pixel line and the surrounding luma pixels on the first top 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) based on the pixel values of the surrounding luma pixels on the first left pixel line, the surrounding luma pixels on the first top pixel line, and the pixels in the current luma block 910. A block 930 and peripheral luma reference pixels 940) may be generated. However, the surrounding luma pixels present at the position [-1,-1] may not be used to generate luma reference information. In the embodiment of FIG. 9, the luma reference information generation process can be expressed by the following equation (13), for 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 the pixel value of a pixel in the current luma block. Additionally, P _Y 'may represent a pixel value corresponding to luma reference information (eg, a pixel value of a pixel in a luma reference block and/or a surrounding luma reference pixel).

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

10 , the first left pixel line located closest to the left of the current luma block 1010 and the surrounding luma pixels within the first top pixel line located closest to the top of the current luma block 1010 generate luma reference information. An example of when used is shown. In the embodiment of FIG. 10, the pixel line located second closest to the left of the current luma block 1010, that is, the pixel line located adjacent to the left of the first left pixel line, will be referred to as the second left pixel line. Additionally, the pixel line located second closest to the top of the current luma block 1010, that is, the pixel line located adjacent to the top of the first top pixel line, will be referred to as the 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, LM mode may be indicated as Intra_FromLuma. Additionally, 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, the peripheral luma pixels corresponding to 1022 and the peripheral luma pixels corresponding to 1026 are available pixels, It is assumed that the surrounding luma pixels corresponding to 1024 and the surrounding luma pixels corresponding to 1028 are unavailable pixels. In this case, the encoder and decoder may generate luma reference information after performing a padding process on unavailable surrounding luma pixels.

In the embodiment of FIG. 10, the encoder and decoder perform a padding process on the unavailable surrounding luma pixels on the first left pixel line and the first top pixel line, so that the pixel values of the surrounding 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 the pixel value of the surrounding luma pixel. Additionally, nS may represent the horizontal and vertical lengths of the current luma block 1010.

Unavailable pixels among the surrounding luma pixels may form pixel groups as shown in numbers 1024 and 1028 of FIG. 10 . In the embodiment of FIG. 10, for convenience of explanation, a group of unavailable pixels on the first left pixel line (e.g., four pixels located on the left at 1028 in FIG. 10) is referred to as a left pixel group, and the first upper pixel line is referred to as a left pixel group. The group of unavailable pixels 1024 on the top is called the top pixel group. In the embodiment of FIG. 10, pixels belonging to the top pixel group can be expressed as P _Y [x,y], (x=nS~nS*2-1, y=-1), and pixels belonging to the left pixel group can be expressed as P _Y [x,y], (x=-1, y=nS~nS+(nS>>1)-1).

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

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

In this case, there may be only one available pixel located adjacent to the top pixel group 1024 within the first top pixel line. Accordingly, the encoder and decoder can fill the positions of unavailable pixels in the top pixel group 1024 with the pixel value of the available pixel c located adjacent to the top pixel group 1024. For example, this padding process can be expressed by Equation 14 below.

[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, available pixel b may be located adjacent to the top of the left pixel group, and available pixel a may be located adjacent to the bottom of the left pixel group. Here, pixel a can be represented as P _Y [-1, nS+(nS>>1)], and pixel b can be represented as P _Y [-1, nS-1].

In this case, there may be two available pixels located adjacent to the left pixel group within the first left pixel line. Accordingly, the encoder and decoder can fill the pixel average values of the available pixels (a, b) located adjacent to the left pixel group into the positions of unavailable pixels in the left pixel group. For example, this padding process can be expressed by Equation 15 below.

[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)

The pixel values of the surrounding luma pixels on the first left pixel line and the surrounding luma pixels on the first top pixel line may be determined through the above-described padding process. The encoder and the decoder generate luma reference information (luma reference block 1030 ) and surrounding luma reference pixels 1040) can be generated.

At this time, available pixels may exist among pixels on the second left pixel line located adjacent to the left of the first left pixel line. When a pixel located adjacent to the left of a pixel on the first left pixel line (pixel on the second left pixel line) is available, the encoder and decoder use a second left pixel line located adjacent thereto instead of the pixel on the first left pixel line. The pixels on the image can be used to generate luma reference information. However, if a pixel located adjacent to the left of the pixel on the first left pixel line (pixel on the second left pixel line) is not available, the encoder and decoder refer to the luma of the pixel on the first left pixel line on which the padding process was performed. It can be used to generate information.

In the embodiment of FIG. 10, the luma reference information generation process can be expressed by the following equation (16), for 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 the pixel value of a pixel in the current luma block. Additionally, P _Y 'may represent a pixel value corresponding to luma reference information (eg, a pixel value of a pixel in a luma reference block and/or a surrounding luma reference pixel).

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

11 , the first left pixel line located closest to the left of the current luma block 1110 and the surrounding luma pixels within the first top pixel line located closest to the top of the current luma block 1110 generate luma reference information. An example of when used is shown. In the embodiment of FIG. 11 , the pixel line located second closest to the left of the current luma block 1110, that is, the pixel line located adjacent to the left of the first left pixel line, will be referred to as the second left pixel line. Additionally, the pixel line located second closest to the top of the current luma block 1110, that is, the pixel line located adjacent to the top of the first top pixel line, will be referred to as the 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, LM mode may be indicated as Intra_FromLuma. Additionally, in FIG. 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 the peripheral luma pixels corresponding to 1126 are available pixels, It is assumed that the surrounding luma pixels corresponding to 1124 and the surrounding luma pixels corresponding to 1128 are unavailable pixels. In this case, the encoder and decoder may generate luma reference information after performing a padding process on unavailable surrounding 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 top pixel line. Additionally, the encoder and decoder may copy surrounding luma pixel values on the second left pixel line to the first left pixel line. The pixel values of the surrounding luma pixels generated through the above-described process to be used for generating reference information can 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 the pixel value of the surrounding luma pixel belonging to the first left pixel line and/or the first top pixel line. Additionally, nS may represent the horizontal and vertical lengths of the current luma block 1110.

Unavailable pixels among the surrounding luma pixels may form pixel groups as shown in numbers 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 (e.g., four pixels located on the left at 1128 in FIG. 11) is referred to as a first left pixel group, and the group of unavailable pixels on the first left pixel line is referred to as a first left pixel group, and the group of unavailable pixels on the first left pixel line (e.g., four pixels located on the left at 1128 in FIG. The group of unavailable pixels on the pixel line (e.g., the four pixels located on the right at 1128 in FIG. 11) is called the second left pixel group, and the group of unavailable pixels 1124 on the first top pixel line is called the second left pixel group. The group is called the top pixel group. In the embodiment of FIG. 11, pixels belonging to the top pixel group can be represented as P _Y [x,y], (x=nS~nS*2-1, y=-1), and are in the first left pixel group. The pixels belonging to the second left pixel group can be expressed as 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], can be expressed as (x=-2, y=nS~nS+(nS>>1)-1). Here, P _Y2 may represent the pixel value of the surrounding luma pixel belonging to the second left pixel line.

In order to perform padding, the encoder and decoder determine whether available pixels located adjacent to the pixel group exist at both ends of each pixel group (e.g., the first left pixel group, the second left pixel group, and the top pixel group). can be judged.

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

In this case, there may be only one available pixel located adjacent to the top pixel group 1124 within the first top pixel line. Accordingly, the encoder and decoder can fill the positions of unavailable pixels in the top pixel group 1124 with the pixel value of the available pixel c located adjacent to the top pixel group 1124. For example, this padding process can be expressed by Equation 18 below.

[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, available pixel b may be located adjacent to the top of the first left pixel group, and available pixel a may be located adjacent to the bottom of the first left pixel group. Here, pixel a can be represented as P _Y [-1, nS+(nS>>1)], and pixel b can be represented as P _Y [-1, nS-1].

In this case, there may be two available pixels located adjacent to the left pixel group within the first left pixel line. Accordingly, the encoder and decoder can fill the positions of unavailable pixels in the first left pixel group with the pixel average value of the available pixels (a, b) located adjacent to the first left pixel group. For example, this padding process can be expressed by Equation 19 below.

[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, available pixel e may be located adjacent to the top of the second left pixel group, and available pixel d may be located adjacent to the bottom of the second left pixel group. Here, pixel d can be represented as P _Y2 [-2, nS+(nS>>1)], and pixel e can be represented as P _Y2 [-2, nS-1].

In this case, there may be two available pixels located adjacent to the left pixel group within the second left pixel line. Accordingly, the encoder and decoder can fill the positions of unavailable pixels in the second left pixel group with the pixel average values of the available pixels (e, d) located adjacent to the second left pixel group. For example, this padding process can be expressed by Equation 20 below.

[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=-1~2*nS-1).

By the above-described padding process, pixel values of surrounding luma pixels on the first left pixel line, the second left pixel line, and the first top pixel line may be determined. At this time, the encoder and decoder can derive pixel values on the first left pixel line to be used for generating luma reference information by copying surrounding luma pixel values on the second left pixel line to the first left pixel line. That is, the encoder and decoder can replace the surrounding luma pixel values of the first left pixel line with the surrounding luma pixel values of the second left pixel line. The process of copying the surrounding luma pixel values on the second left pixel line to the first left pixel line can be expressed, for example, by Equation 21 below.

[Equation 21]

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

After the peripheral luma pixel values on the second pixel line are copied to the first left pixel line, the encoder and decoder output the peripheral luma pixels on the first left pixel line, the peripheral luma pixels on the first top pixel line, and the current luma block. Based on the pixel values of pixels within 1110, luma reference information (luma reference block 1130 and surrounding luma reference pixels 1140) may be generated. In the embodiment of FIG. 11, the luma reference information generation process can be expressed by the following equation 22, for 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 the pixel value of a pixel in the current luma block. Additionally, P _Y 'may represent a pixel value corresponding to luma reference information (eg, a pixel value of a pixel in a luma reference block and/or a surrounding luma reference pixel).

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

12 , the first left pixel line located closest to the left of the current luma block 1210 and the surrounding luma pixels within the first top pixel line located closest to the top of the current luma block 1210 are used to generate luma reference information. An example of use is shown. In the embodiment of FIG. 12 , the pixel line located second closest to the left of the current luma block 1210, that is, the pixel line located adjacent to the left of the first left pixel line, will be referred to as the second left pixel line. Additionally, the pixel line located second closest to the top of the current luma block 1210, that is, the pixel line located adjacent to the top of the first top pixel line, will be referred to as the 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, LM mode may be indicated as Intra_FromLuma. Additionally, in FIG. 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 the peripheral luma pixels corresponding to 1226 are available pixels, It is assumed that the surrounding luma pixels corresponding to 1224 and the surrounding luma pixels corresponding to 1228 are unavailable pixels. In this case, the encoder and decoder may generate luma reference information after performing a padding process on unavailable surrounding luma pixels.

In the embodiment of FIG. 12, the encoder and decoder perform a padding process on the unavailable surrounding luma pixels on the first left pixel line and the first top pixel line, so that the pixel values of the surrounding 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 the pixel value of the surrounding luma pixel. Additionally, nS may represent the horizontal and vertical lengths of the current luma block 1210.

Unavailable pixels among the surrounding luma pixels may form pixel groups as shown in numbers 1224 and 1228 of FIG. 12 . In the embodiment of FIG. 12, for convenience of explanation, a group of unavailable pixels on the first left pixel line (e.g., four pixels located on the left at 1228 in 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 called the top pixel group. In the embodiment of FIG. 12, pixels belonging to the top pixel group can be expressed as P _Y [x,y], (x=nS~nS*2-1, y=-1), and pixels belonging to the left pixel group can be expressed as P _Y [x,y], (x=-1, y=nS~nS+(nS>>1)-1).

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

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

In this case, there may be only one available pixel located adjacent to the top pixel group 1224 within the first top pixel line. Accordingly, the encoder and decoder can fill the positions of unavailable pixels in the top pixel group 1224 with the pixel value of the available pixel c located adjacent to the top pixel group 1224. For example, this padding process can be expressed by Equation 23 below.

[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, available pixel b may be located adjacent to the top of the left pixel group, and available pixel a may be located adjacent to the bottom of the left pixel group. Here, pixel a can be represented as P _Y [-1, nS+(nS>>1)], and pixel b can be represented as P _Y [-1, nS-1].

In this case, there may be two available pixels located adjacent to the left pixel group within the first left pixel line. Accordingly, the encoder and decoder can fill the pixel average values of the available pixels (a, b) located adjacent to the left pixel group into the positions of unavailable pixels in the left pixel group. For example, this padding process can be expressed by Equation 24 below.

[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)

The pixel values of the surrounding luma pixels on the first left pixel line and the surrounding luma pixels on the first top pixel line may be determined through the above-described padding process. The encoder and decoder generate luma reference information (luma reference block 1230 ) and peripheral luma reference pixels 1240) can be generated. In the embodiment of FIG. 12, the luma reference information generation process can be expressed by the following equation 25, for 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 the pixel value of a pixel in the current luma block. Additionally, P _Y 'may represent a pixel value corresponding to luma reference information (eg, a pixel value of a pixel in a luma reference block and/or a surrounding luma reference pixel).

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

Meanwhile, in FIGS. 5 to 12 described above, embodiments where the first color component is a luma component and the second color component is a chroma component are described, but the application of 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 where the size of the first color component block (e.g., luma block) is larger than the size of the second color component block (e.g., chroma block) are described, but the first color The component block may have a smaller size or the same size as the second color component block.

Even in this case, the encoder and decoder can derive first color component reference information to be used for prediction of the second color component block based on pixels within 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.’ At this time, first color component pixels corresponding to pixels in the second color component block may constitute a ‘first color component reference block’. Additionally, in this specification, pixels located around the first color component block are referred to as ‘surrounding first color component pixels’, pixels located around the second color component block are referred to as ‘surrounding second color component pixels’, and A pixel located around a color component reference block is called a ‘surrounding first color component reference pixel’. At this time, the first color component reference block may correspond to the second color component block, and the surrounding first color component reference pixel may correspond to the surrounding second color component pixel. In this specification, first color component reference information may be used as a concept including pixels within a first color component reference block and surrounding first color component reference pixels.

When the first color component block has a smaller size or the same size as the second color component block, a process of deriving first color component reference information to be used for prediction of the second color component block based on the first color component block may be different from the embodiments of FIGS. 6 to 12 described above.

FIG. 13 is a diagram schematically showing an embodiment of a process for deriving first color component reference information to be used for prediction of the 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 in 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 in 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 an 8x8 first color component reference block.

1320 in FIG. 13 shows an example of a method for deriving first color component reference information.

In the embodiment of 1320 of FIG. 13, 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 as in 1310 of FIG. 13. The derivation method is shown.

Referring to 1320 of FIG. 13, the encoder and decoder generate a first color component 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 first color component reference block 1326 and surrounding first color component reference pixels 1328 may be generated. Here, the surrounding 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 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, the sizes of the first color component block 1312 and the second color component block 1316 are different from each other, so the encoder and decoder use a block of the same size (8x8) as the second color component block 1316. A first color component reference block 1326 and surrounding first color component reference pixels 1328 corresponding thereto 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 the pixels in the first color component block 1312 and the surrounding first color component pixels. First color component reference information can be generated by performing up sampling on the fields 1314.

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

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

[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 the restored first color component pixels, that is, pixels within the first color component block 1312 and surrounding first color component pixels 1314. Additionally, 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.

FIG. 14 is a diagram schematically showing an embodiment of a process for deriving first color component reference information to be used for prediction of the second color component block when the size of the first color component block is the same as the size 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 sizes of the luma block and chroma block may be the same. there is. Therefore, 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. Additionally, 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, the encoder and decoder select a first color component reference block to be used for prediction of the second color component block based on the pixel in the first color component block 1412 and the surrounding first color component pixels 1414. 1422 and surrounding 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 surrounding 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 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, the pixel values of the pixels in the first color component block 1412 and the pixel values of the surrounding first color component pixels 1414 can be used as the first color component reference information.

Additionally, in intra mode, a lossless coding/decoding method may be applied instead of a lossy coding/decoding method that uses transform/quantization, etc. In the lossless encoding/decoding method, processes such as transform quantization may not be applied to the encoding target block. When the size of the first color component block 1412 is the same as the size of the second color component block, even when the lossless encoding/decoding method is applied, the first color component reference information may be derived as in the above-described embodiment. You can. In this case, the encoder and decoder may not perform down-sampling, sub-sampling, or up-sampling 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 the top peripheral luma pixels located on top of the current luma block and the left peripheral luma pixels located on the left of the current luma block can be used to generate luma reference information. However, the present invention is not limited to this, and surrounding luma pixels used to generate luma reference information may be determined in various ways. For example, only the top peripheral luma pixels may be used to generate luma reference information, and only the left peripheral luma pixels may be used to generate luma reference information. Additionally, surrounding luma pixels used to generate luma reference information may be determined according to the intra prediction mode of the current luma block.

Figure 15 is a diagram schematically showing an embodiment of peripheral luma pixels used to generate luma reference information.

In the embodiment of *305 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, Figure 15 may correspond to an embodiment when the video format of YUV 4:2:0 is used. Additionally, it is assumed that the surrounding luma pixels (A, B, C, D, E, F) shown in FIG. 15 are already filled with pixel values through the padding process described above. And, in FIG. 15, it is assumed that the mode value of the intra prediction mode as in 410 of FIG. 4 described above is used.

In the embodiment of FIG. 15, based on at least one of the surrounding luma pixels (A, B, C, D, E, F) and pixels in the current luma block 1512, a luma reference block ( 1522, 1532 or 1542) and surrounding luma reference pixels 1524, 1534 or 1544 may be generated. At this time, 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, surrounding luma pixels to be used for generating luma reference information may be obtained, for example, by at least one of the combinations shown in Table 3 below.

[Table 3]

In the embodiment of Table 3, P may mean that the left peripheral luma pixels located on the left of the current luma block 1512 and the top peripheral luma pixels located on the top of the current luma block 1512 are used to generate luma reference information. . Additionally, Q may mean that luma pixels around the top of the current luma block 1512 are used to generate luma reference information. And, R may mean that the left surrounding luma pixels located to the left of the current luma block 1512 are used to generate luma reference information.

Additionally, in the embodiment 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 of the current luma block 1512 are used to generate luma reference information. It can mean. In addition, M may mean that when left surrounding luma pixels are used to generate luma reference information, pixels on the second left pixel line located second closest to the left 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.

Additionally, N may mean that surrounding 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. When luma reference information is generated based on the intra prediction mode of the current luma block 1512, the left surrounding luma pixels used may be pixels on the first left pixel line, for example, or pixels on the second left pixel line, as another example. It may be possible.

For example, in an embodiment in which a combination of P and L is applied, the upper peripheral luma pixels located at the top of the current luma block 1512 and the left peripheral luma pixels on the first left pixel line may be used to generate luma reference information. . Therefore, in this case, the surrounding luma pixels of [A, D] or [A, B, D, F] shown at 1510 in FIG. 15 can 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, for example, at 1520 in FIG. 15. At 1520 of FIG. 15 , the surrounding luma reference pixels 1524 may include pixels adjacent to the top of the luma reference block 1522 and pixels adjacent to the left 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 at the top of the current luma block 1512 may be used to generate luma reference information. Therefore, in this case, the surrounding luma pixels of [A, B] shown at 1510 in FIG. 15 can 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, for example, at 1530 in FIG. 15. At 1530 of FIG. 15 , the peripheral luma reference pixels 1534 may be pixels located at the 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. Therefore, in this case, the surrounding luma pixels of [D, F] shown at 1510 in FIG. 15 can 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, for example, at 1540 in FIG. 15. At 1540 of FIG. 15 , the peripheral luma reference pixels 1544 may be pixels located 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, the upper peripheral luma pixels located at the top of the current luma block 1512 and the left peripheral luma pixels on the second left pixel line may be used to generate luma reference information. there is. Therefore, in this case, the surrounding luma pixels of [A, C] or [A, B, C, E] shown at 1510 in FIG. 15 can be used to generate luma reference information. Luma reference information generated when the combination of P and M is applied may be shown, for example, at 1520 in FIG. 15 .

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

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. Therefore, in this case, the surrounding luma pixels of [C, E] shown at 1510 in FIG. 15 can be used to generate luma reference information. Luma reference information generated when the combination of R and M is applied may be shown, for example, at 1540 in FIG. 15 .

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, the 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 to generate luma reference information.

At this time, the intra prediction mode of the current luma block 1512 may be a first intra prediction mode that uses all pixels located on the top and left of the current luma block 1512. For example, the first intra prediction mode is DC mode, planar mode, or has a mode value of 4, 19, 11, 20, 5, 21, 12, 22, 27, 15, 28, 8, 29, 16, or 30. It may be a mode.

Luma reference information generated when a combination of P and N is applied may be shown, for example, at 1520 in FIG. 15 .

As another example, in an embodiment where the combination of Q and N is applied, based on the intra prediction mode of the current luma block 1512, the upper peripheral luma pixels located on top of the current luma block 1512 are used to generate luma reference information. It can be determined by the surrounding luma pixels that will be.

At this time, the intra prediction mode of the current luma block 1512 may be a second intra prediction mode that mainly uses pixels located at the top of the current luma block 1512. For example, the second intra prediction mode may be a mode with a mode value of 1, 23, 13, 24, 6, 25, 14, 26, or 7.

Luma reference information generated when the combination of Q and N is applied may be shown, for example, at 1530 in FIG. 15 .

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

At this time, the intra prediction mode of the current luma block 1512 may be a third intra prediction mode that mainly uses pixels located to the left of the current luma block 1512. For example, the third intra prediction mode may be a mode with a mode value of 2, 31, 17, 32, 9, 33, 18, 34, or 10.

Luma reference information generated when the combination of R and N is applied may be shown, for example, at 1540 in FIG. 15 .

When the pixel values of the peripheral luma reference pixels are derived based on the peripheral luma pixels determined according to the above-described embodiment, prediction parameters can be derived using the surrounding chroma pixels located at the same location as each peripheral luma reference pixel. . Specific examples of the prediction parameter derivation process will be described later.

FIG. 16 is a diagram illustrating an embodiment of a process for deriving prediction parameters and a process for deriving predicted values of pixels in the current chroma block.

FIG. 16 shows a luma reference block 1610, surrounding luma reference pixels 1620, current chroma block 1630, and restored 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.

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

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

[Equation 27]

In Equation 27, p1 may represent the pixel value of the peripheral luma reference pixels 1620, and p2 may represent the pixel value of the peripheral chroma pixels 1640. Additionally, nS represents 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 the chroma component pixel. And abs(x) can represent the absolute value of x. 'lmDiv' is a variable determined according to the value of 'a2s', and can be determined according to the following Table 4, for example.

[Table 4]

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

In the above-described prediction parameter derivation process, unavailable pixels may exist among the peripheral luma reference pixels 1620 and/or the peripheral chroma pixels 1640. In this case, for example, the encoder and decoder may derive prediction parameters based only on available pixels among the surrounding luma reference pixels 1620 and/or the surrounding chroma pixels 1640.

In another embodiment, the encoder and decoder do not derive a prediction parameter based on the surrounding luma reference pixels 1620 and the surrounding chroma pixels 1640, but set a default prediction parameter to the current chroma block 1630. It can also be used for prediction. 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 can 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 decoder determine the current chroma block based on the prediction parameters 'a', 'k', and 'b' derived by Equation 27 and the pixel values of pixels in the luma reference block 1610 shown in FIG. 16. (1630) Predicted values of my pixels can be derived. At this time, the process of deriving the predicted values of pixels within the current chroma block 1630 can be expressed, for example, by Equation 28 below.

[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 represent the predicted value of a pixel in the current chroma block 1630. Additionally, clip(x,y,z) can be x if z is less than x, y if z is greater than y, and z otherwise.

Figure 17 is a flowchart schematically showing an embodiment of the decoding process according to the present invention.

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

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

[Table 5]

Here, P _C may represent the pixel value of a surrounding chroma pixel located around the current chroma block. Additionally, N _C may represent the horizontal and vertical lengths of the current chroma block.

For example, the embodiment of FIG. 17 may be applied in the order of Y component, U component, and 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) can be marked by checking whether it is available (S1710). Here, marking may refer to the process of storing or displaying information about availability. Embodiments in which peripheral luma pixels are not available have been described above in the embodiment of FIG. 8, so detailed description thereof will be omitted here.

Referring again to FIG. 17, the encoder and decoder can determine whether condition A and condition 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 assigned to the LM flag (chroma_pred_from_luma_enabled_flag). As described above, chroma_pred_from_luma_enabled_flag may indicate whether LM mode can be supported for intra prediction of the current chroma block. Additionally, condition B is a condition indicating that the current block is a luma component block (Y). As an example, condition B may be satisfied when 0 is assigned to cIdx.

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

Referring again to FIG. 17, when condition A and condition B are satisfied, the encoder and decoder are P _LM [x,y], (x=-1, y=-1~N _Y *2-1 and x=0~ N _Y *2-1, y=-1) can be checked and marked (S1730).

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

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

Referring again to FIG. 17, the encoder and decoder may determine whether condition C is satisfied for the surrounding 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) This condition indicates that the pixels in are not available.

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

Referring again to FIG. 17, the encoder and decoder may determine whether conditions A, B, and D are satisfied for the surrounding 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) This condition indicates that the pixels in are not available.

If conditions A, B, and D are satisfied, that is, if at least one pixel among the surrounding luma pixels to be used for generating luma reference information is not available, the encoder and decoder use P _LM [x,y], (x=-1 , y=-1~N _Y *2-1 and x=0~N _Y *2-1, y=-1), a padding process can be performed on unavailable pixels (S1770). Since embodiments of the padding process have been described above, detailed description thereof will be omitted here.

After the above-described padding process is performed, the encoder and decoder can generate a restored block for the current luma block by performing prediction and decoding processes for the current luma block based on surrounding luma pixels (S1780). When the embodiment of FIG. 17 is applied to the chroma component (U or V), the encoder and decoder generate a prediction block for the current chroma block based on surrounding luma pixels, pixels in the reconstructed current luma block, and surrounding chroma pixels. You can do this, and you can create a restored block for the current chroma block based on the generated prediction block.

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

Figure 18 shows the luma component block 1810 to be predicted, the surrounding luma pixels 1820 (P _Y ) used for prediction of the luma component block, the restored current luma block 1830 (P _RecY ), and the luma reference information generation. It shows the surrounding luma pixels (1840, P _LM ) being 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 can also be applied to the chroma components (U, V) rather than the luma component (Y). When the prediction mode of the current chroma block 1850 is the LM mode, the encoder and decoder can derive luma reference information and prediction parameters for the current chroma block 1850 after the padding process is performed.

The encoder and decoder can derive predicted values of pixels in the luma component block 1810 that are the prediction target based on the surrounding luma pixels 1820 (P _Y ). At this time, the encoder and decoder may generate a restored block of the luma component based on the derived prediction value, which may correspond to the restored current luma block (1830, P _RecY ). In addition, the encoder and decoder can determine the surrounding luma pixels (1840, P _LM ) to be used for generating luma reference information based on the surrounding luma pixels (1820, P _Y ) used for 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 surrounding luma pixels 1840 (P _LM ) and the restored current luma block 1830 (P _RecY ). In FIG. 18, since the restored current luma block 1830 and the current chroma block 1850 have different sizes, the encoder and decoder use a luma reference block of the same size (4x4) as the current chroma block 1850 and its corresponding surroundings. Luma reference pixels can be generated. At this time, since the size of the current chroma block 1850 is smaller than the size of the restored current luma block 1830, the encoder and decoder use pixels in the restored current luma block 1830 and surrounding luma pixels 1840. Luma reference information can be generated by performing down-sampling.

For example, a down-sampling process may be performed on surrounding luma pixels located at the top of the restored current luma block 1830 according to Equation 29 below.

[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~N _C -1)

Here, P _Y 'may represent the pixel value of a pixel corresponding to luma reference information (eg, a pixel within a luma reference block or a surrounding luma reference pixel). Additionally, P _LM may represent the pixel value of the peripheral luma pixel 1840 to be used to generate luma reference information. And, N _C may represent the horizontal and vertical lengths of the current chroma block 1850.

For example, a down-sampling process may be performed on surrounding luma pixels located to the left of the restored current luma block 1830 according to Equation 30 below.

[Equation 30]

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

For example, a down-sampling process may be performed on pixels in the restored current luma block 1830 according to Equation 31 below. A luma reference block corresponding to the current chroma block 1850 can be generated using 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 represent the pixel value of a pixel in the current luma block 1830.

When luma reference information is generated through the above-described process, the encoder and decoder can derive prediction parameters for the current chroma block 1850 based on the luma reference information and chroma component information. For example, the encoder and decoder may derive prediction parameters based on the pixel values of the surrounding luma reference pixels 1840 and the pixel values of the surrounding chroma pixels 1860. The following equation 32 may represent part of the process of deriving prediction parameters.

[Equation 32]

Here, P _C may represent the pixel value of the surrounding chroma pixel 1860 to be used for generating luma reference information, and 'Bitdepth' may represent the bit depth of the 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, and LC derived by Equation 32 Since the example is similar to the embodiment of Equation 27, detailed description thereof will be omitted here. However, in this case, in the embodiment of Equation 27, nS can be replaced with N _C , p1 can be replaced with P _Y ', and p2 can be replaced with P _C.

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

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

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

In the embodiments of FIGS. 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 _NY , _NU , or N _V. Here, P _Y may represent the pixel value of the surrounding luma pixel used for prediction of the current luma block. Additionally, P _U and P _V may represent pixel values of surrounding chroma pixels used for prediction of the current chroma block. Here, P _U and P _V may be expressed as P _C . Additionally, 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 also be expressed as N _C. In the embodiment of FIG. 19, P _LM may represent a pixel value of a surrounding luma pixel 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, k may be Y, U, or V, so the embodiment of FIG. 19 can be applied to both the luma component (Y) and the chroma component (U, V). The relationship between variables applied to the embodiment of FIG. 19 according to color components can be shown, for example, in Table 6 below.

[Table 6]

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

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

Referring to FIG. 19b, one CU (1990) can 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 a decoding process according to the embodiment of FIG. 19A on the Y component blocks within the CU (1990). Additionally, the encoder and decoder may sequentially perform the decoding process on the U component blocks and V component blocks within the CU 1990 according to the embodiment of FIG. 19A. For example, the decoding process according to the embodiment of Figure 19 is [Y1->Y2->...->Y7->U1->U2->...->U7->V1->V2...- >V7]. Here, as an example, Y1 may mean that decoding is performed on the luma component of block 1 in FIG. 19B.

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

Referring again to FIG. 19A, the encoder and decoder can determine whether condition A, condition B, and condition C are satisfied (S1920).

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

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

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

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

Also, in this case, in one embodiment, the encoder and decoder may check whether P _LM is available based on the availability of neighboring chroma pixels, that is, P _U or P _V , corresponding to neighboring luma pixels. At this time, whether P _U or P _V is available may already be marked in step S1910. For example, the encoder and protector can determine whether P _LM is available based on whether the 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, the encoder and decoder 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, the encoder and decoder select the surrounding luma pixel P _LM [x,2y] and P _LM [x,2y+1] can 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. Therefore, if the surrounding luma pixel is a pixel on the left pixel line located adjacent to the left of the current luma block, and it is determined that the surrounding luma pixel is available, the pixel value of the luma pixel located adjacent to the left of the surrounding luma pixel is the surrounding luma pixel. It can be determined by the pixel value of the luma pixel.

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

At this time, the encoder and decoder may assign luma pixel values located at [2x,y] and [2x+1,y] to P _LM [2x,y] and P _LM [2x+1,y], respectively. Therefore, if the surrounding luma pixel is a pixel on the top pixel line located adjacent to the top of the current luma block, and the surrounding luma pixel is determined to be available, the pixel value of the luma pixel at the same location as the surrounding luma pixel is the surrounding luma pixel. It can be determined by the pixel value of .

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

At this time, the encoder and decoder can assign the luma pixel value located at [x,y] to P _LM [x,y]. Therefore, when the position of the surrounding luma pixel is [-1,-1] and the surrounding luma pixel is determined to be available, the pixel value of the luma pixel at the same location as the surrounding luma pixel will be determined as the pixel value of the surrounding luma pixel. You 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 to this. For example, whether P _LM is available may be checked based on P _V.

Referring again to FIG. 19A, the encoder and decoder can 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) This condition indicates that the pixels in are not available.

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

Referring again to FIG. 19A, the encoder and decoder can determine whether condition A, condition B, condition C, and condition 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) This condition indicates that the pixels in are not available.

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

After the above-described padding process is performed, the encoder and decoder perform prediction and decoding processes on the current luma block and/or the current chroma block, thereby generating a restoration block for the current luma block and/or a restoration block for the current chroma block. Can be created (S1980).

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

In the above-described embodiments, the methods are described based on flowcharts 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 simultaneously with other steps as described above. You can. Additionally, a person of ordinary skill in the art will recognize that the steps shown in the flowchart are not exclusive and that other steps may be included or one or more steps in the flowchart may be deleted without affecting the scope of the present invention. You will understand.

The above-described embodiments include examples of various aspects. Although it is not possible to describe all possible combinations for representing the various aspects, those skilled in the art will recognize that other combinations are possible. Accordingly, the present invention is intended to include all other substitutions, modifications and changes falling within the scope of the following claims.

Claims (9)

determining whether 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;
As the LM mode is applied to the current chroma block, whether left peripheral luma pixels adjacent to the left side of the luma reference block among the peripheral luma pixels of the luma reference block corresponding to the current chroma block are available, and whether the left peripheral luma pixels of the luma reference block corresponding to the current chroma block are available. determining whether top peripheral luma pixels adjacent to the top are available;
determining peripheral luma reference pixels depending on whether the left peripheral luma pixels are available and whether the top peripheral luma pixels are available;
determining peripheral chroma reference pixels corresponding to the peripheral luma reference pixels from among peripheral chroma pixels located around the current chroma block;
Deriving a prediction parameter for the current chroma block based on the surrounding luma reference pixels and the surrounding chroma reference pixels; and
Deriving a prediction value of a pixel in the current chroma block based on the 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,
When 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. An image decoding method, wherein the prediction parameter for the current chroma block is derived based on a reference pixel.
According to clause 1,
Determining the peripheral luma reference pixels includes:
If the left peripheral luma pixels are not available, the left peripheral luma pixels are not included in the peripheral luma reference pixels,
When the top peripheral luma pixels are not available, the top peripheral luma pixels are not included in the peripheral luma reference pixels.
According to clause 1,
In the step of deriving the prediction parameters,
The surrounding luma pixels are included in pixel groups having a predetermined size,
For each of the pixel groups, a peripheral luma sample at a predetermined location among the peripheral luma pixels is selected,
Based on the surrounding luma sample of the selected predetermined position, the surrounding luma pixels are downsampled,
An image decoding method, wherein the prediction parameter is derived based on the downsampled surrounding luma pixels.
According to clause 1,
The top peripheral luma pixels further include a second top peripheral luma pixel immediately adjacent to the top of the first top peripheral luma pixel.
According to clause 1,
Determining the peripheral luma reference pixels includes:
When the left peripheral luma pixels are available, determining the peripheral luma reference pixels according to the left peripheral luma pixels and the lower left peripheral luma pixels.
According to clause 1,
Determining the peripheral luma reference pixels includes:
When the top peripheral luma pixels are available, determining the peripheral luma reference pixels according to the top peripheral luma pixels and the upper right peripheral luma pixel.
According to paragraph 1,
The video decoding method is,
Obtaining LM flag information indicating whether LM mode can be applied to intra prediction of a chroma block in the current sequence including the current chroma block; and
When the LM flag information indicates that the LM mode is applied to intra prediction of the 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. A video decoding method further comprising the step of determining whether or not.
Among the surrounding luma pixels of the luma reference block corresponding to the current chroma block, determine whether the left surrounding luma pixels adjacent to the left side of the luma reference block are available and whether the top surrounding luma pixels adjacent to the top of the luma reference block are available. steps;
determining peripheral luma reference pixels depending on whether the left peripheral luma pixels are available and whether the top peripheral luma pixels are available;
determining peripheral chroma reference pixels corresponding to the peripheral luma reference pixels from among peripheral chroma pixels located around the current chroma block;
Deriving a prediction parameter for the current chroma block based on the surrounding luma reference pixels and the surrounding chroma reference pixels;
Deriving a prediction value of a pixel in the current chroma block based on the pixel in the luma reference block and the prediction parameter; and
According to the prediction result of the current chroma block, determining whether the LM mode is applied to the current chroma block, and encoding intra prediction mode information indicating the 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,
When 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. An image encoding method wherein the prediction parameter for the current chroma block is derived based on a reference pixel.
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 the intra prediction mode of the current chroma block,
The video decoding method is,
determining whether LM mode is applied to the current chroma block according to the intra prediction mode information;
As the LM mode is applied to the current chroma block, whether left peripheral luma pixels adjacent to the left side of the luma reference block among the peripheral luma pixels of the luma reference block corresponding to the current chroma block are available, and whether the left peripheral luma pixels of the luma reference block corresponding to the current chroma block are available. determining whether top peripheral luma pixels adjacent to the top are available;
determining peripheral luma reference pixels depending on whether the left peripheral luma pixels are available and whether the top peripheral luma pixels are available;
determining peripheral chroma reference pixels corresponding to the peripheral luma reference pixels from among peripheral chroma pixels located around the current chroma block;
Deriving a prediction parameter for the current chroma block based on the surrounding luma reference pixels and the surrounding chroma reference pixels; and
Deriving a prediction value of a pixel in the current chroma block based on the 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,
When 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 the reference pixel, the prediction parameter for the current chroma block is derived.