Intra prediction-based image coding method using MPM list and apparatus therefor

1. A video decoding method performed by a decoding apparatus, the method comprising:

deriving a first candidate intra prediction mode based on a first neighboring block of a current block;

deriving a second candidate intra prediction mode based on a second neighboring block of the current block;

constructing an intra prediction mode candidate list of the current block based on the first candidate intra prediction mode and the second candidate intra prediction mode;

deriving an intra prediction mode for the current block based on the intra prediction mode candidate list; and

generating a prediction sample for the current block based on the intra prediction mode,

wherein the first neighboring block is a left neighboring block located at the lowermost side among neighboring blocks adjacent to a left boundary of the current block,

wherein the second neighboring block is an upper neighboring block located at the rightmost side among neighboring blocks adjacent to an upper boundary of the current block,

wherein, based on a determination that a size of the current block is W×H, and x and y components of a top-left sample position of the current block are xN and yN respectively, the first neighboring block is a block including a sample at the coordinates of (xN−1, yN+H−1), and the second neighboring block is a block including a sample at the coordinates of (xN+W−1, yN−1),

wherein the intra prediction mode candidate list of the current block is derived as a first intra prediction mode candidate list based on a determination that the first candidate intra prediction mode and the second candidate intra prediction mode are non-angular intra modes, and

wherein the first intra prediction mode candidate list comprises a vertical intra prediction mode of which value is 50, a horizontal intra prediction mode of which value is 18, an intra prediction mode having a value of 46 being equal to a value derived by subtracting 4 from the value of the vertical intra prediction mode, and an intra prediction mode having a value of 54 being equal to a value derived by adding 4 to the value of the vertical intra prediction mode.

2. The method of claim 1 , wherein, based on a determination that the first neighboring block is available, and intra prediction is applied to the first neighboring block, the first candidate intra prediction mode is derived as an intra prediction mode of the first neighboring block, and

wherein based on a determination that the first neighboring block is unavailable, or the intra prediction is not applied to the first neighboring block, the first candidate intra prediction mode is derived as a planar intra prediction mode.

3. The method of claim 1 , wherein, based on a determination that the second neighboring block is available, intra prediction is applied to the second neighboring block, and the second neighboring block is included in a current CTU, the second candidate intra prediction mode is derived as an intra prediction mode of the second neighboring block, and

wherein based on a determination that the second neighboring block is unavailable, the intra prediction is not applied to the second neighboring block, or the second neighboring block is not included in the current CTU, the second candidate intra prediction mode is derived as a planar intra prediction mode.

4. The method of claim 1 , wherein the intra prediction mode candidate list of the current block is derived as a second intra prediction mode candidate list based on a determination that the first candidate intra prediction mode is equal to the second candidate intra prediction mode and a mode value of the first candidate intra prediction mode is larger than a mode value of a DC intra prediction mode which is 1, and

wherein the second intra prediction mode candidate list is derived based on the first candidate intra prediction mode and an intra prediction mode having a value obtained by subtracting 1 from the first candidate intra prediction mode.

5. The method of claim 1 , wherein the intra prediction mode candidate list of the current block is derived as a third intra prediction mode candidate list based on a determination that the first candidate intra prediction mode is not equal to the second candidate intra prediction mode, both of the first and the second candidate intra prediction modes are not larger than a mode value of a DC intra prediction mode, and the sum of mode values of the first and second candidate intra prediction modes are larger than or equal to 2, and

wherein the third intra prediction mode candidate list is derived based on an intra prediction mode having a value obtained by subtracting 1 from a larger mode value of mode values of the first and second candidate intra prediction modes.

6. The method of claim 1 , wherein the intra prediction mode candidate list of the current block is derived as a fourth intra prediction mode candidate list based on a determination that the first candidate intra prediction mode is not equal to the second candidate intra prediction mode, both of the first and the second candidate intra prediction modes are larger than a mode value of a DC intra prediction mode, and

wherein the fourth intra prediction mode candidate list is derived based on an intra prediction mode having a value obtained by subtracting 1 from a larger mode value of mode values of the first and second candidate intra prediction modes.

7. The method of claim 4 , wherein, based on a determination that the mode value of the first candidate intra prediction mode is not larger than the mode value of the DC intra prediction mode, the intra prediction mode candidate list of the current block is derived as a second intra prediction mode candidate list.

8. The method of claim 7 , wherein the second intra prediction mode candidate list includes 6 intra prediction mode candidates; and the first intra prediction mode candidate is derived as a planar intra prediction mode, the second intra prediction mode candidate is derived as a DC intra prediction mode, the third intra prediction mode candidate is derived as intra prediction mode 50, the fourth intra prediction mode candidate is derived as intra prediction mode 18, the fifth intra prediction mode candidate is derived as intra prediction mode 46, and the sixth intra prediction mode candidate is derived as intra prediction mode 54.

9. The method of claim 4 , wherein the constructing the intra prediction mode candidate list of the current block further includes:

based on a determination that the first candidate intra prediction mode is not the same as the second candidate intra prediction mode, deriving a first intra prediction mode candidate of the intra prediction mode candidate list as the first candidate intra prediction mode and a second intra prediction mode candidate of the intra prediction mode candidate list as the second candidate intra prediction mode; and

deriving the remaining intra prediction mode candidates of the intra prediction mode candidate list based on whether the mode values of the first candidate intra prediction mode and the second candidate intra prediction mode are larger than that of the DC intra prediction mode.

10. The method of claim 9 , wherein, based on a determination that the mode values of the first candidate intra prediction mode and the second candidate intra prediction mode are larger than that of the DC intra prediction mode, the third intra prediction mode candidate is derived as the planar intra prediction mode, the fourth intra prediction mode candidate is derived as the DC intra prediction mode, and the fifth intra prediction mode candidate and the sixth intra prediction mode candidate are derived based on a difference between the mode values of the largest candidate intra prediction mode and the smallest candidate intra prediction mode.

11. The method of claim 10 , wherein, based on a determination that a difference between the mode values of the largest candidate intra prediction mode and the smallest candidate intra prediction mode is not 1 nor 64, the fifth intra prediction mode candidate is derived as an intra prediction mode represented by a value obtained by subtracting 1 from the mode value of the first candidate intra prediction mode according to modular arithmetic, and the sixth intra prediction mode candidate is derived as an intra prediction mode represented by a value obtained by adding 1 to the mode value of the first candidate intra prediction mode according to the modular arithmetic.

12. The method of claim 11 , wherein the candidate intra prediction with the larger mode value between the first candidate intra prediction mode and the second intra prediction mode is derived as the largest candidate intra prediction mode, and the candidate intra prediction mode with the smaller mode value between the two is derived as the smallest candidate intra prediction mode.

13. A video encoding method performed by an encoding apparatus, the method comprising:

deriving a first candidate intra prediction mode based on a first neighboring block of a current block;

deriving a second candidate intra prediction mode based on a second neighboring block of the current block;

constructing an intra prediction mode candidate list of the current block based on the first candidate intra prediction mode and the second candidate intra prediction mode;

determining an intra prediction mode for the current block based on the intra 7 prediction mode candidate list;

generating a prediction sample for the current block based on the intra prediction mode; and

encoding video information comprising intra prediction mode information for the current block,

wherein the first neighboring block is a left neighboring block located at the lowermost side among neighboring blocks adjacent to a left boundary of the current block,

wherein the second neighboring block is an upper neighboring block located at the rightmost side among neighboring blocks adjacent to an upper boundary of the current block,

wherein, based on a determination that a size of the current block is W x H, and x and y components of a top-left sample position of the current block are xN and yN respectively, the first neighboring block is a block including a sample at the coordinates of (xN−1, yN+H−1), and the second neighboring block is a block including a sample at the coordinates of (xN+W−1, yN−1),

wherein the intra prediction mode candidate list of the current block is derived as a first intra prediction mode candidate list based on a determination that the first candidate intra prediction mode and the second candidate intra prediction mode are non-angular intra modes, and

wherein the first intra prediction mode candidate list comprises a vertical intra prediction mode of which value is 50, a horizontal intra prediction mode of which value is 18, an intra prediction mode having a value of 46 being equal to a value derived by subtracting 4 from the value of the vertical intra prediction mode, and an intra prediction mode having a value of 54 being equal to a value derived by adding 4 to the value of the vertical intra prediction mode.

14. A non-transitory computer-readable storage medium storing a bitstream, the bitstream causing a decoding apparatus to perform a video decoding method including the following steps:

deriving a first candidate intra prediction mode based on a first neighboring block of a current block;

deriving a second candidate intra prediction mode based on a second neighboring block of the current block;

constructing an intra prediction mode candidate list of the current block based on the first candidate intra prediction mode and the second candidate intra prediction mode;

deriving an intra prediction mode for the current block based on the intra prediction mode candidate list; and

generating a prediction sample for the current block based on the intra prediction mode,

wherein the first neighboring block is a left neighboring block located at the lowermost side among neighboring blocks adjacent to a left boundary of the current block,

wherein the second neighboring block is an upper neighboring block located at the rightmost side among neighboring blocks adjacent to an upper boundary of the current block,

wherein, based on a determination that a size of the current block is W x H, and x and y components of a top-left sample position of the current block are xN and yN respectively, the first neighboring block is a block including a sample at the coordinates of (xN−1, yN+H−1), and the second neighboring block is a block including a sample at the coordinates of (xN+W−1, yN−1),

wherein the intra prediction mode candidate list of the current block is derived as a first intra prediction mode candidate list based on a determination that the first candidate intra prediction mode and the second candidate intra prediction mode are non-angular intra modes, and

wherein the first intra prediction mode candidate list comprises a vertical intra prediction mode of which value is 50, a horizontal intra prediction mode of which value is 18, an intra prediction mode having a value of 46 being equal to a value derived by subtracting 4 from the value of the vertical intra prediction mode, and an intra prediction mode having a value of 54 being equal to a value derived by adding 4 to the value of the vertical intra prediction mode.