Video coding using cross-component linear models

Abstract

To provide a method and computing device for video coding using a cross-component linear model (CCLM) mode.SOLUTION: The video coding method includes: deriving a parameter α and a parameter β for a CCLM mode by using a predetermined number of neighboring reconstructed luma samples and chroma samples in a coding unit (CU); and generating a final chroma predictor for the chroma samples of the CU by using the parameter α and the parameter β.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] This application claims the benefit of U.S. Provisional Application No. 62 / 790,459, filed January 9, 2019. The entire disclosure of the aforementioned application is incorporated herein by reference.

[0002] FIELD OF THE DISCLOSURE The present disclosure relates generally to video coding and compression. More specifically, the disclosure relates to a system and method for performing video coding using a cross-component linear model. The method is described in terms of a final chroma predictor for chroma samples of a coding unit in a specific exemplary embodiment. [Background technology]

[0003] This section provides background information related to the present disclosure. Information contained in this section should not necessarily be construed as prior art.

[0004] Various video coding techniques may be used to compress video data. Video coding is performed according to one or more video coding standards. For example, video coding standards include Versatile Video Coding (VVC), Joint Exploration Test Model (JEM), High-Efficiency Video Coding (H.265 / HEVC), Advanced Video Coding (H.264 / AVC), Moving Picture Experts Group (MPEG) coding, etc. Video coding generally uses prediction methods (e.g., inter-prediction, intra-prediction, etc.) that exploit redundancy present in a video image or sequence. One of the key goals of video coding techniques is to compress video data into a format that uses a lower bitrate while avoiding or minimizing degradation to video quality. To enable ever-evolving video services, coding techniques with better compression efficiency are needed. Summary of the Invention

[0005] This section provides an overview of the disclosure and is not a comprehensive disclosure of its entire scope or all of its features.

[0006] According to a first aspect of the present disclosure, a video coding method is executed in a computing device having one or more processors and one or more memories storing a plurality of programs executed by the one or more processors, the method including: deriving a first parameter α and a second parameter β for a cross-component linear model (CCLM) mode by using a predetermined number of neighboring reconstructed luma samples and chroma samples in a coding unit (CU), and generating a final chroma predictor for chroma samples of the CU by using the first parameter α and the second parameter β.

[0007] According to a second aspect of the present disclosure, there is provided a computing device including at least one processor, a non-transitory storage device coupled to the one or more processors, and a plurality of programs stored in the non-transitory storage device that, when executed by the processor, cause the computing device to perform operations including deriving first and second parameters α and β for a cross-component linear model (CCLM) mode by using a predetermined number of neighboring reconstructed luma samples and chroma samples in a coding unit (CU), and generating a final chroma predictor for the chroma sample of the CU by using the first and second parameters α and β. [Brief explanation of the drawings]

[0008] Below, a set of exemplary, non-limiting embodiments of the present disclosure are described. Variations in structure, method, or function may be implemented by those of ordinary skill in the relevant art based on the examples shown herein, and all such variations are included within the scope of the present disclosure. Where no contradiction exists, teachings of different embodiments may, but need not, be combined with each other. [Figure 1] FIG. 1 is a block diagram illustrating an exemplary encoder that can be used with multiple video coding standards. [Figure 2] FIG. 2 is a block diagram illustrating an exemplary decoder that can be used with multiple video coding standards. [Figure 3] FIG. 3 is a flowchart of a video coding method. [Figure 4] Figure 4 is an example of a luma / chroma pixel sampling grid. [Figure 5] FIG. 5 shows the sample locations used for the derivation of α and β. [Figure 6] FIG. 6 shows the linear derivation of α and β using the min-max method. [Figure 7] FIG. 7 shows the LM_A mode. [Figure 8] FIG. 8 shows the LM_A mode. [Figure 9] Figure 9 is the luma / chroma pixel sampling grid for the YUV4:2:2 format. [Figure 10] Figure 10 is the luma / chroma pixel sampling grid for the YUV4:2:4 format. [Figure 11] FIG. 11 shows multiple neighbors of the MMLM. [Figure 12] FIG. 12 shows the locations of three example sample pairs. [Figure 13] FIG. 13 shows another example location of three sample pairs. [Figure 14] FIG. 14 shows the locations of an example of four sample pairs. [Figure 15]FIG. 15 shows another example location of four sample pairs. DETAILED DESCRIPTION OF THE INVENTION

[0009] The terms used in this disclosure are intended to describe particular examples rather than to limit the disclosure. As used in this disclosure and the appended claims, the singular forms "a," "an," and "the" also refer to the plural forms unless the context clearly dictates otherwise. As used herein, the term "and / or" should be understood to refer to any and all possible combinations of one or more of the associated listed items.

[0010] Terms such as "first," "second," and "third" may be used herein to refer to various pieces of information, and it should be understood that the information is not limited by these terms. These terms are used merely to distinguish one category of information from another. For example, first information may be referred to as second information, and similarly, second information may be referred to as first information, without departing from the scope of this disclosure. The term "if," as used herein, may be understood to mean "when," "when," or "depending on," depending on the context.

[0011] Throughout this specification, references to "one embodiment," "an embodiment," "another embodiment," or the like in the singular or plural form mean that one or more of the specific features, structures, or properties described in connection with an embodiment are included in at least one embodiment of the present disclosure. Thus, the appearances of the phrases "in one embodiment," "in an embodiment," "another embodiment," or the like in the singular or plural form in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, specific features, structures, or properties in one or more embodiments may be combined in any suitable manner.

[0012] Conceptually, many video coding standards, including those mentioned above in the "Background" section, are similar: for example, nearly all video coding standards use block-based processing and share similar video coding block diagrams to achieve video compression.

[0013] 1 shows a block diagram of an exemplary encoder 100 that can be used with multiple video coding standards. In encoder 100, a video frame is divided into multiple video blocks for processing. For each given video block, a prediction is made based on either an inter-prediction approach or an intra-prediction approach. In inter-prediction, one or more predictors are formed based on pixels from a previously reconstructed frame through motion estimation and motion compensation. In intra-prediction, predictors are formed based on reconstructed pixels in a current frame. Through a mode decision, the best predictor is selected to predict the current block.

[0014] A prediction residual, which represents the difference between the current video block and its predictor, is sent to a transform circuit 102, which then sends transform coefficients to a quantization circuit 104 for entropy reduction. The quantized coefficients are then provided to an entropy coding circuit 106 to generate a compressed video bitstream. As shown in FIG. 1, prediction-related information 110, such as video block partition information, motion vectors, reference picture indexes, intra-prediction modes, etc., from an inter-prediction circuit and / or an intra-prediction circuit 112 are also provided through the entropy coding circuit 106 and stored in the compressed video bitstream 114.

[0015] Decoder-related circuitry is also required in encoder 100 to reconstruct pixels for prediction purposes. First, a prediction residual is reconstructed through inverse quantization 116 and inverse transform circuitry 118. This reconstructed prediction residual is combined with block predictor 120 to generate unfiltered reconstructed pixels for the current video block.

[0016] In-loop filters are commonly used to improve coding efficiency and image quality. For example, a deblocking filter is available for AVC, HEVC, as well as the current version of VVC. HEVC defines an additional in-loop filter called sample adaptive offset (SAO) to further improve coding efficiency. In the current version of the VVC standard, another in-loop filter called adaptive loop filter (ALF) is being actively researched and has a good chance of being included in the final standard.

[0017] The operation of these in-loop filters is optional: performing these operations helps improve coding efficiency and image quality, and they may be turned off as a decision made by encoder 100 to save computational effort.

[0018] It should be noted that if these filter options are turned on by the encoder 100, inter prediction is based on filtered reconstructed pixels, while intra prediction is usually based on unfiltered reconstructed pixels.

[0019] FIG. 2 is a block diagram illustrating an exemplary decoder 200 that can be used with multiple video coding standards. This decoder 200 is similar to the reconstruction-related section present in the encoder 100 of FIG. 1. In the decoder 200 (FIG. 2), an input video bitstream 201 is first decoded through entropy decoding 202 to derive quantized coefficient levels and prediction-related information. The quantized coefficient levels are then processed through inverse quantization 204 and inverse transform 206 to obtain reconstructed prediction residuals. A block predictor mechanism, implemented in an intra / inter mode selector 212, is configured to perform intra prediction 208 or motion compensation 210 based on the decoded prediction information. A set of unfiltered reconstructed pixels is obtained by summing the reconstructed prediction residuals from the inverse transform 206 and the prediction output generated by the block predictor mechanism using a summer 214. In the situation where an in-loop filter is turned on, a filtering operation is performed on these reconstructed pixels to derive the final reconstructed video for output.

[0020] To reduce cross-component redundancy, a cross-component linear model (CCLM) prediction mode is used in VVC. YUV format 4:2:0 is used in common test conditions during the development of VVC, and the sampling grid of luma samples and chroma samples for YUV format 4:2:0 is shown in FIG. 4. The coordinates of the luma samples and chroma samples (downsampled luma samples are also shown in FIG. 4) are shown. RecL′[x,y] represents the downsampled upper and left adjacent reconstructed luma samples, RecC′[x,y] represents the upper and left adjacent reconstructed chroma samples, and x and y represent pixel indices as shown in the figure. This disclosure proposes several methods to reduce the complexity of deriving CCLM parameters.

[0021] FIELD OF THE DISCLOSURE This disclosure generally relates to coding (e.g., encoding and decoding) video data. More specifically, this disclosure relates to a video coding method and a computing device for reducing cross-component redundancy in the video coding method. A CCLM predictor mode is used to predict chroma samples based on reconstructed luma samples of the same CU. The computing device includes at least one processor, a non-transitory storage device coupled to the one or more processors, and a plurality of programs stored in the non-transitory storage device that, when executed by the processor, causes the computing device to perform operations of the video coding method.

[0022] As shown in FIG. 3, the video coding method includes at least the following steps:

[0023] Step 10: Derive the first parameter α and the second parameter β for the CCLM mode by using a predetermined number of adjacent reconstructed luma and chroma samples in the CU.

[0024] Step 20: Generate a final chroma predictor for the chroma samples of the CU by using the first parameter α and the second parameter β.

[0025] Step 20 generates the final chroma predictor for the chroma samples of the CU by using the following equation:

number

[0026] pred C (x,y) is the final chroma predictor for the chroma samples of the CU, and rec L '(x,y) is the downsampled reconstructed luma sample of the CU, where x denotes the row index and y denotes the column index.

[0027] In FIG. 5, the coordinates of the luma samples and chroma samples (downsampled luma samples) are shown.

[0028] The parameters α and β are derived by the following equation (referred to as the min-max method in the next section):

number

[0029] Each chroma sample and its corresponding luma sample is called a sample pair. B is the chroma sample value of the largest sample pair, and y A is the chroma sample value of the smallest sample pair, and x B is the luma sample value of the largest sample pair, and x A is the luma sample value of the smallest sample pair.

[0030] As depicted in Figure 6, two points (luma and chroma combinations) (A, B) are the minimum and maximum values ​​among a set of adjacent luma samples. Figure 6 illustrates a line between the minimum and maximum values ​​of luma when parameters α and β of the linear model are obtained according to equation (2).

[0031] In Figure 5, Rec L '[x,y] represents the downsampled top and left adjacent reconstructed luma samples, and Rec C[x,y] represents the top and left neighboring reconstructed chroma samples, x indicates the row index, and y indicates the column index. Note that the square blocks in Figure 5 are reconstructed luma samples corresponding to the luma sample positions depicted in Figure 4, and the circles in Figure 5 correspond to the chroma sample or downsampled luma sample positions depicted in Figure 4. For rectangular coding blocks, the min-max method is applied directly. For non-rectangular coding blocks, the neighboring samples on the longer boundary are first subsampled to have the same number as the samples on the shorter boundary. Figure 5 shows the locations of the left and top samples and the samples of the current block involved in CCLM mode.

[0032] The min-max calculation is performed as part of the decoding process, not just as a search operation in the encoder. Therefore, no syntax is used to communicate the values ​​of parameters α and β to the decoder. Currently, equation / filter (3) is used as the luma downsampling filter to generate the downsampled luma samples. However, as shown in equations (3) through (19), a different equation / filter can be selected to generate the downsampled luma samples. Note that equations (5) through (10) can be considered to directly obtain the samples without the downsampling process.

number

[0033] The top and left templates can both be used to calculate the coefficients of the linear model, and alternatively, two other LM modes, LM_A and LM_B. L As shown in Figure 7, in LM_A mode, the above template is used to calculate the coefficients of the linear model. To obtain more samples, the above template is extended to (W+H). As shown in Figure 8, in LM_A mode, LIn this mode, only the left template is used to calculate the coefficients of the linear model. To get more samples, the left template is dilated to (H+W). For non-rectangular blocks, the top template is dilated to W+W and the left template is dilated to H+H. If the top / left template is not available, LM_A / LM L The mode is not checked or signaled. If there are not enough samples available, the template is padded by copying the rightmost (for the top template) or bottommost (for the left template) sample to the nearest log2 number. In addition to the 4:2:0 YUV format, the codec also supports the 4:2:2 format (Figure 9) and the 4:4:4 format (Figure 10).

[0034] At the JVET meeting, several methods for improving the LM mode were proposed:

[0035] MMLM mode: MMLM corresponds to the multi-model LM mode, where two linear models are used to derive the predictions of the chroma samples. The reconstructed luma values ​​are divided into two categories, with one model for each category. The derivation of the α and β parameters for each model is performed as in CCLM mode, but the reconstructed luma (downsampled) used to derive the parameters is also divided according to each model.

[0036] MFLM mode: MFLM corresponds to the multi-filter LM mode, where different filters are used to downsample the reconstructed luma samples used in the prediction model. Four such filters are used, and the particular filter used is indicated / signaled in the bitstream.

[0037] LM Angle Prediction: In this mode, the MMLM mode and the non-LM mode are combined by averaging the prediction samples obtained by the two modes.

[0038] The Multiple Neighbor-based Linear Model (MNLM) uses multiple neighbor sets to derive the MMLM, covering various linear relationships between the luma and chroma samples of a CU. As illustrated in Figure 11, three MNLMs with different neighbor sets are proposed in the MMLM.

[0039] MMLM: A, B, C, D (including top and left neighbors)

[0040] Above MMLM: C, D, F, H (including only the above neighbors)

[0041] Left MMLM: A, B, E, G (includes only left neighbors)

[0042] As shown in Figure 11, A is the second neighbor on the left. B is the first neighbor on the left. C is the first neighbor above. D is the second neighbor above. E is the third neighbor on the left. F is the third neighbor above. G is the fourth neighbor on the left. H is the fourth neighbor above.

[0043] The different CCLM prediction modes for MNLM are listed in the table below. [Table 1]

[0044] Mode 0, Mode 1, Mode 2, and Mode 3 use the same downsampling filter but different neighbor sets for LM and MMLM derivation.

[0045] To reduce the complexity of deriving the CCLM parameters, in the first embodiment, three sample pairs are used to derive the parameters α and β. As shown in Figure 12, the sample pairs are the topmost left adjacent sample (Rec' L [-1,0],Rec C[-1,0]) and the bottom-most adjacent sample to the left (Rec' L [-1,H-1],Rec C [-1,H-1]) and the rightmost sample of the adjacent sample above (Rec' L [W-1,-1],Rec C [W-1,-1]), where W and H indicate the width and height of the block of chroma.

[0046] In another embodiment, as shown in FIG. 13, the sample pair is divided into two adjacent samples, one of which is the leftmost sample (Rec' L [0,-1],Rec C [0,-1]) and the bottom-most adjacent sample to the left (Rec' L [-1,H-1],Rec C [-1,H-1]) and the rightmost sample of the adjacent sample above (Rec' L [W-1,-1],Rec C [W-1,-1]).

[0047] It should be noted that the selection of sample pairs is not limited to the above-described embodiment, and the three sample pairs can be any three sample pairs selected from the reconstructed adjacent samples above or to the left, and the adjacent samples are not limited to only one line above or one line to the left.

[0048] In one embodiment, sample pairs having the maximum, median, and minimum luma sample values, respectively, are identified through luma sample comparison. The weighted average of the luma sample values ​​of the maximum and median sample pairs is x B (shown in equation (21)), and the weighted average of the chroma sample values ​​of the maximum and middle sample pairs is y B (23)). The weighted average of the luma sample values ​​of the middle and minimum sample pairs is x A (shown in equation (20)), and the weighted average of the chroma sample values ​​of the middle and minimum sample pairs is y A(shown in equation (22)). Then, the parameters α and β are calculated using equation (2).

number

number

number

number

[0049] x max is the luma sample value of the largest sample pair, and x mid is the luma sample value of the middle sample pair, and x min is the luma sample value of the smallest sample pair, and y max is the chroma sample value of the largest sample pair, and y mid is the chroma sample value of the middle sample pair, and y min is the chroma sample value of the smallest sample pair, and w1+w2=(1< <N1)、offset1=1<<(N1-1)であり、w3+w4=(1<<N2)、offset2=1<<(N2-1)である。

[0050] where w1 is the first weighting coefficient, w2 is the second weighting coefficient, w3 is the third weighting coefficient, and w4 is the fourth weighting coefficient. N1 is the first average value, N2 is the second average value, offset1 is the first offset coefficient, and offset2 is the second offset coefficient.

[0051] An example where equal weighting is applied is where w1=1, w2=1, w3=1, w4=1, N1=1, N2=1, and offset1=1, offset2=1.

[0052] In yet another example, w1=3, w2=1, w3=1, w4=3, N1=2, N2=2, and offset1=2, offset2=2.

[0053] In yet another example, w1=3, w2=1, w3=1, w4=3, N1=2, N2=2, and offset1=2, offset2=2.

[0054] In yet another embodiment, i, j, k are used as indices for the three sample pairs, and luma i and luma j and luma i and luma k Only two comparisons are made: and . These two comparisons allow the three sample pairs to be completely sorted by luma value, or to be divided into two groups, one containing the two larger values ​​and the other containing the smaller values, or vice versa. If the values ​​are completely sorted, the method described in the previous section can be used. If the sample pairs are divided into two groups, the luma and chroma samples in the same group are each weighted averaged (a single sample pair in a group effectively does not require weighted averaging). For example, if there are two sample pairs in a group, the two luma values ​​of the group are equally weighted and averaged, while the two chroma values ​​are also equally weighted and averaged. Here, the weighted averaged values ​​are expressed as x to derive the CCLM parameters using (2). A , x B , y A and y B Used as.

[0055] In yet another embodiment, the sample pair with the largest luma sample value and the sample pair with the smallest luma sample value are identified through a comparison of the luma samples. B and the chroma sample value of the largest sample pair is denoted by y Band the luma sample value of the smallest sample pair is denoted by x A and the chroma sample value of the smallest sample pair is denoted by y A Then, the parameters α and β are calculated using equation (2).

[0056] It should be noted that the method for deriving the CCLM parameters is not limited to the above-described embodiment, and the selected three sample pairs can be used in any manner to derive the CCLM parameters.

[0057] In the second embodiment, to reduce the complexity of the CCLM derivation, the parameters α and β are derived using four sample pairs. As shown in FIG. 14, the sample pairs are the topmost sample of the left adjacent sample (Rec' L [-1,0],Rec C [-1,0]) and the leftmost adjacent sample above (Rec' L [0,-1],Rec C [0,-1]) and the bottom-most adjacent sample to the left (Rec' L [-1,H-1],Rec C [-1,H-1]) and the rightmost sample of the adjacent sample above (Rec' L [W-1,-1],Rec C [W-1,-1]).

[0058] In another embodiment, as shown in FIG. 15, the sample pair is split into a quarter width portion of the leftmost adjacent sample above (Rec' L [W / 4,-1],Rec C [W / 4,-1]) and one-quarter of the width of the topmost adjacent sample to the left (Rec' L [-1,H / 4],Rec C [-1,H / 4]) and the bottom-most adjacent sample to the left (Rec' L [-1,H-1],Rec C[-1,H-1]) and the rightmost sample of the adjacent sample above (Rec' L [W-1,-1],Rec C [W-1,-1]).

[0059] It should be noted that the selection of sample pairs is not limited to the above-described embodiment. The four sample pairs may be any four sample pairs selected from the reconstructed adjacent samples above or to the left, and the adjacent samples are not limited to only one line above or one line to the left. For example, one set of sample pairs may include one-quarter of the width of the leftmost sample among the adjacent samples above, one-quarter of the width of the topmost sample among the adjacent samples to the left, three-quarters of the width of the leftmost sample among the adjacent samples above, and three-quarters of the width of the topmost sample among the adjacent samples to the left.

[0060] Alternatively, another set of sample pairs includes one-eighths of the width of the leftmost adjacent sample above, three-eighths of the width of the leftmost adjacent sample above, five-eighths of the width of the leftmost adjacent sample above, and seven-eighths of the width of the leftmost adjacent sample above.

[0061] Alternatively, another set of sample pairs includes one-eighth of the height of the topmost one of the left adjacent samples, three-eighths of the height of the topmost one of the left adjacent samples, five-eighths of the height of the topmost one of the left adjacent samples, and seven-eighths of the height of the topmost one of the left adjacent samples.

[0062] In one embodiment, a sample pair having the largest two luma sample values ​​and the smallest two luma sample values, respectively, is identified through a comparison of the luma samples. B0 , x B1and are shown, and the chroma sample values of the two larger maximum sample pairs are y B0 , y B1 and are shown. The luma sample values of the two smaller sample pairs are x A0 , x A1 and are shown, and the chroma sample values of the two smaller minimum sample pairs are y A0 , y A1 and are shown. And, as shown by the following equations, equations (24)-(27), x[[ID=1​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​An example where equal weighting is applied is where w1=1, w2=1, w3=1, w4=1, N1=1, N2=1, and offset1=1, offset2=1.

[0065] In yet another example, w1=3, w2=1, w3=1, w4=3, N1=2, N2=2, and offset1=2, offset2=2.

[0066] In another embodiment, a sample pair having the maximum and minimum luma sample values, respectively, is identified through a comparison of the luma samples. The luma sample value of the maximum sample pair is denoted by x B and the chroma sample value of the largest sample pair is denoted by y B Also, let the luma sample value of the smallest sample pair be x A and the chroma sample value of the smallest sample pair is denoted by y A Then, the parameters α and β are calculated using equation (2).

[0067] It should be noted that the method for deriving the CCLM parameters is not limited to the above-described embodiment, and the selected four sample pairs can be used in any manner to derive the CCLM parameters.

[0068] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention in accordance with its general principles, including departures from the present disclosure as come within known or customary practice in the art. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

[0069] It will be appreciated that the present invention is not limited to the specific examples described above and illustrated in the accompanying drawings, and that various modifications and changes can be made without departing from the scope thereof, which is intended to be limited only by the appended claims.

[0070] In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over a computer-readable medium as one or more instructions or code and executed by a hardware-based processing unit. Computer-readable media includes computer-readable storage media, which correspond to tangible media such as data storage media, and communication media, including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. As such, computer-readable media may generally correspond to (1) non-transitory, tangible computer-readable storage media or (2) communication media, such as signals, carrier waves, etc. Data storage media may be any available medium that can be accessed by one or more computers or one or more processors to retrieve instructions, code, and / or data structures for implementations described herein. A computer program product may include computer-readable media.

[0071] Furthermore, the above methods may be implemented using an apparatus including one or more circuits, including application specific integrated circuits (ASICs), digital signal processors (DSPs), digital signal processing devices (DSPDs), programmable logic devices (PLDs), field programmable gate arrays (FPGAs), controllers, microcontrollers, microprocessors, or other electronic components. The apparatus may use the circuits in combination with other hardware or software components to perform the above methods. Each of the above modules, sub-modules, units, or sub-units may be implemented, at least in part, using one or more circuits.

[0072] Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of the invention in accordance with its general principles, including departures from the present disclosure that come within known or customary practice in the art. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

[0073] It will be appreciated that the present invention is not limited to the specific examples described above and illustrated in the accompanying drawings, and that various modifications and changes can be made without departing from the scope thereof, which is intended to be limited only by the appended claims.

Claims

Claim 1 Receiving a bitstream, determining respective prediction modes from the bitstream, deriving a first parameter α and a second parameter β for a cross-component linear model (CCLM) mode by using a plurality of adjacent reconstructed luma samples and adjacent chroma samples in a coding unit (CU); generating predicted chroma samples in the CU by using the first parameter α and the second parameter β; Deriving the first parameter α and the second parameter β includes deriving the first parameter α and the second parameter β by using four adjacent downsampled luma samples and four corresponding chroma samples, and each of the downsampled luma samples and the corresponding chroma sample has the same row index and the same column index. According to each of the prediction modes, the four adjacent downsampled luma samples and the four corresponding chroma samples include any one of the following sets: A sample with a width of one-fourth from the leftmost sample among the upper adjacent samples, a sample with a height of one-fourth from the uppermost sample among the left adjacent samples, the lowermost sample among the left adjacent samples, and the rightmost sample among the upper adjacent samples; A sample with a width of one-fourth from the leftmost sample among the upper adjacent samples, a sample with a height of one-fourth from the uppermost sample among the left adjacent samples, a sample with a width of three-fourths from the leftmost sample among the upper adjacent samples, and a sample with a width of three-fourths from the uppermost sample among the left adjacent samples; A sample with a width of one-eighth from the leftmost sample among the upper adjacent samples, a sample with a width of three-eighths from the leftmost sample among the upper adjacent samples, a sample with a width of five-eighths from the leftmost sample among the upper adjacent samples, a sample with a width of seven-eighths from the leftmost sample among the upper adjacent samples, and The sample at a height of 1 / 8 from the uppermost sample among the left adjacent samples, the sample at a height of 3 / 8 from the uppermost sample among the left adjacent samples, the sample at a height of 5 / 8 from the uppermost sample among the left adjacent samples, the sample at a height of 7 / 8 from the uppermost sample among the left adjacent samples A method for video decoding comprising the above. **Claim 2** Generating the predicted chroma sample comprises Obtaining the predicted chroma sample using the following formula 【Number 1】 comprising predC(x, y) is the predicted chroma sample of the CU, recL'(x, y) is the downsampled reconstructed luma sample of the CU, x indicates the row index, and y indicates the column index The method according to claim 1. **Claim 3** Deriving the first parameter α and the second parameter β using adjacent chroma samples and their corresponding downsampled luma samples comprises Obtaining the first parameter α and the second parameter β using the following formula 【Number 2】 comprising xA, xB, yA and yB are derived based on the weighted average of the adjacent chroma samples and their corresponding downsampled luma samples The method according to claim 1. **Claim 4** Deriving the first parameter α and the second parameter β by using four adjacent downsampled luma samples and four corresponding chroma samples comprises Obtaining the first parameter α and the second parameter β by using the weighted average luma value xA of the two smaller luma samples xA0, xA1 from the four adjacent downsampled luma samples, the weighted average luma value xB of the two larger luma samples xB0, xB1 from the four adjacent downsampled luma samples, the weighted average chroma value yA of the two chroma samples yA0, yA1 corresponding to the two smaller luma samples, and the weighted average chroma value yB of the two chroma samples yB0, yB1 corresponding to the two larger luma samples, comprising xA, xB, yA and yB are obtained by using the following formula 【Number 3】 w1 + w2 = (1 << N1), offset1 = 1 << (N1 - 1), w3 + w4 = (1 << N2), offset2 = 1 << (N2 - 1), where w1 is the first weighting coefficient, w2 is the second weighting coefficient, w3 is the third weighting coefficient, w4 is the fourth weighting coefficient, N1 is the first average value, N2 is the second average value, offset1 is the first offset coefficient, and offset2 is the second offset coefficient, The method according to claim 1.

5. The method according to claim 1, wherein the chroma format of the CU is 4:2:

2.

6. The method according to claim 1, wherein the chroma format of the CU is 4:4:

4.

7. A computing device, comprising at least one processor, a non - temporary storage device connected to one or more of the processors, a plurality of programs stored in the non - temporary storage device and, when executed by the processor, causing the computing device to execute the method according to any one of claims 1 to 6, A computing device comprising.

8. At least one program that, when executed by at least one processor of a computing device, causes the computing device to execute the method according to any one of claims 1 to 6, A computer - readable storage medium storing it.

9. At least one instruction that, when executed by at least one processor, causes the at least one processor to execute the method according to any one of claims 1 to 4, A computer program comprising.

10. The method according to claim 9, wherein the chroma format of the CU is 4:2:

2.

11. The method according to claim 9, wherein the chroma format of the CU is 4:4:

4.

12. Generating a bitstream by executing an encoding method, Transmitting the bitstream, comprising, The encoding method is, Deriving a first parameter α and a second parameter β for a cross - component linear model (CCLM) mode by using a plurality of adjacent reconstructed luma samples and adjacent chroma samples in a coding unit (CU), Generating a predicted chroma sample within the CU by using the first parameter α and the second parameter β; Deriving the first parameter α and the second parameter β includes deriving the first parameter α and the second parameter β by using four adjacent downsampled luma samples and four corresponding chroma samples, and each downsampled luma sample and corresponding chroma sample have the same row index and the same column index; Depending on each prediction mode, the four adjacent downsampled luma samples and four corresponding chroma samples include any one of the following sets: A sample with a width of one-fourth from the leftmost sample among the upper adjacent samples, a sample with a height of one-fourth from the uppermost sample among the left adjacent samples, the lowermost sample among the left adjacent samples, the rightmost sample among the upper adjacent samples; A sample with a width of one-fourth from the leftmost sample among the upper adjacent samples, a sample with a height of one-fourth from the uppermost sample among the left adjacent samples, a sample with a width of three-fourths from the leftmost sample among the upper adjacent samples, a sample with a width of three-fourths from the uppermost sample among the left adjacent samples; A sample with a width of one-eighth from the leftmost sample among the upper adjacent samples, a sample with a width of three-eighths from the leftmost sample among the upper adjacent samples, a sample with a width of five-eighths from the leftmost sample among the upper adjacent samples, a sample with a width of seven-eighths from the leftmost sample among the upper adjacent samples and A sample with a height of one-eighth from the uppermost sample among the left adjacent samples, a sample with a height of three-eighths from the uppermost sample among the left adjacent samples, a sample with a height of five-eighths from the uppermost sample among the left adjacent samples, a sample with a height of seven-eighths from the uppermost sample among the left adjacent samples; A method for transmitting a bitstream including.

13. The method according to claim 12, wherein the chroma format of the CU is 4:2:

2.

14. The method according to claim 12, wherein the chroma format of the CU is 4:4:4.

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