Method and device for decoding image according to intra prediction in image coding system
According to the present disclosure, an image decoding method performed by a decoding device comprises the steps of: deriving a first intra prediction mode of a current block; deriving neighboring samples including left neighboring samples and upper neighboring samples of the current block; deriving a second intra prediction mode of a target sample on the basis of a location of the target sample and the first intra prediction mode of the current block; and performing prediction for the target sample on the basis of the second intra prediction mode of the target sample.
1. A video decoding method performed by a decoding apparatus, the method comprising:
deriving a first intra-prediction mode for a current block;
deriving neighboring samples including left neighboring samples and upper neighboring samples of the current block;
deriving a second intra-prediction mode for a target sample based on a position of the target sample and the first intra-prediction mode; and
performing prediction of the target sample based on the second intra-prediction mode for the target sample,
wherein a specific value is derived based a size of the current block, and
when the size of the current block is N×M and the first intra-prediction mode is one of vertical directional intra-prediction modes, the specific value is derived as N,
when the size of the current block is N×M and the first intra-prediction mode is one of horizontal directional intra-prediction modes, the specific value is derived as N, and
wherein the horizontal directional intra-prediction modes include #2 intra-prediction mode to #33 intra-prediction mode, and the vertical directional intra-prediction modes include #34 intra-prediction mode to #66 intra-prediction mode;
wherein the step of deriving the second intra-prediction mode for the target sample based on the position of the target sample and the first intra-prediction mode includes:
deriving a first reference sample located in a prediction direction of the first intra-prediction mode based on the target sample among the neighboring samples; and
when the first intra-prediction mode is #m intra-prediction mode, and a distance between the target sample and the first reference sample is greater than the specific value, deriving another intra-prediction mode as the second intra-prediction mode for the target sample.
2. The video decoding method of claim 1 , wherein
when a x component of a top left sample of the current block is 0 and a y component of the top left sample of the current block is 0, a x component of the target sample is a and a y component of the target sample is b, and the first intra-prediction mode is #m intra-prediction mode which is one of vertical directional intra-prediction modes, the second intra-prediction mode for the target sample is derived as #m-b intra-prediction mode.
3. The video decoding method of claim 1 , wherein
when a x component of a top left sample of the current block is 0 and a y component of the top left sample of the current block is 0, a x component of the target sample is a and a y component of the target sample is b, and the first intra-prediction mode is #m intra-prediction mode which is one of horizontal directional intra-prediction modes, the second intra-prediction mode for the target sample is derived as #m-a intra-prediction mode.
4. The video decoding method of claim 1 , wherein the step of deriving the second intra-prediction mode for the target sample based on the position of the target sample and the first intra-prediction mode includes:
deriving #m−1 intra-prediction mode as the another intra-prediction mode for the second intra-prediction mode for the target sample.
5. The video decoding method of claim 4 , wherein M is equal to N.
6. The video decoding method of claim 1 , wherein the step of deriving the second intra-prediction mode for the target sample based on the position of the target sample and the first intra-prediction mode includes:
deriving the second reference sample among the neighboring samples, wherein the second reference sample is located in a prediction direction of #m−1 intra-prediction mode based on the target sample; and
deriving as the another intra-prediction mode for the second intra-prediction mode based on the distance between the target sample and the second reference sample.
7. The video decoding method of claim 6 , wherein when the distance between the target sample and the second reference sample is not greater than the specific value, #m−1 intra-prediction mode is derived as the another intra-prediction mode for the second intra-prediction mode for the target sample, and
when the distance between the target sample and the second reference sample is greater than the specific value, #m−2 intra-prediction mode is derived as the another intra-prediction mode for the second intra-prediction mode for the target sample.
8. The video decoding method of claim 1 ,
wherein the step of deriving as the another intra-prediction mode for the second intra-prediction mode for the target sample based on the position of the target sample and the first intra-prediction mode includes deriving a reference sample index based on the position of the target sample and the first intra-prediction mode,
wherein the reference sample index represents a neighboring sample located in a prediction direction of the first intra-prediction mode based on the target sample,
when the first intra-prediction mode is one of vertical directional intra-prediction modes, the reference sample index is derived based on the following equation:
ildx =(( y+ 1)×intraPredAngle)>>5
wherein, ildx represents the reference sample index, y represents y component of the target sample, and intraPredAngle represents a prediction angle of the first intra-prediction mode.
9. The video decoding method of claim 1 , further comprising receiving a flag indicating whether to derive the second intra-prediction mode for the target sample,
when the flag indicates that the second intra-prediction mode for the target sample is derived, the second intra-prediction mode for the target sample is derived based on the position of the target sample of the current block and the first intra-prediction mode.
10. A video decoding method performed by a decoding apparatus, the method comprising:
deriving a first intra-prediction mode for a current block:
deriving neighboring samples including left neighboring samples and upper neighboring samples of the current block: and
determining whether to derive the second intra-prediction mode for the target sample based on a size of the current block,
when the size of the current block is greater than a specific size, the second intra-prediction mode for the target sample is derived based on the position of the target sample of the current block and the first intra-prediction mode,
wherein prediction of the target sample is performed based on the second intra-prediction mode for the target sample,
wherein the specific value is derived based the size of the current block, and
when the size of the current block is N×M and the first intra-prediction mode is one of vertical directional intra-prediction modes, the specific value is derived as N,
when the size of the current block is N×M and the first intra-prediction mode is one of horizontal directional intra-prediction modes, the specific value is derived as N, and
wherein the horizontal directional intra-prediction modes include #2 intra-prediction mode to #33 intra-prediction mode, and the vertical directional intra-prediction modes include #34 intra-prediction mode to #66 intra-prediction mode;
wherein the step of deriving the second intra-prediction mode for the target sample based on the position of the target sample and the first intra-prediction mode includes:
deriving a first reference sample located in a prediction direction of the first intra-prediction mode based on the target sample among the neighboring samples; and
when the first intra-prediction mode is #m intra-prediction mode, and a distance between the target sample and the first reference sample is greater than the specific value, deriving another intra-prediction mode as the second intra-prediction mode for the target sample.
11. A video encoding method performed by an encoding apparatus, the video encoding method comprising:
deriving a first intra-prediction mode for a current block;
deriving neighboring samples including left neighboring samples and upper neighboring samples of the current block;
deriving a second intra-prediction mode for a target sample based on a position of the target sample and the first intra-prediction mode; and
performing prediction of the target sample based on the second intra-prediction mode for the target sample,
wherein a specific value is derived based a size of the current block, and
when the size of the current block is N×M and the first intra-prediction mode is one of vertical directional intra-prediction modes, the specific value is derived as N,
when the size of the current block is N×M and the first intra-prediction mode is one of horizontal directional intra-prediction modes, the specific value is derived as N, and
wherein the horizontal directional intra-prediction modes include #2 intra-prediction mode to #33 intra-prediction mode, and the vertical directional intra-prediction modes include #34 intra-prediction mode to #66 intra-prediction mode;
wherein the step of deriving the second intra-prediction mode for the target sample based on the position of the target sample and the first intra-prediction mode includes:
deriving a first reference sample located in a prediction direction of the first intra-prediction mode based on the target sample among the neighboring samples; and
when the first intra-prediction mode is #m intra-prediction mode, and a distance between the target sample and the first reference sample is greater than the specific value, deriving another intra-prediction mode as the second intra-prediction mode for the target sample.
12. The video encoding method of claim 11 ,
when a x component of a top left sample of the current block is 0 and a y component of the top left sample of the current block is 0, a x component of the target sample is a and a y component of the target sample is b, and the first intra-prediction mode is #m intra-prediction mode which is one of vertical directional intra-prediction modes, the second intra-prediction mode for the target sample is derived as #m-b intra-prediction mode.
13. The video encoding method of claim 11 , wherein the step of deriving the second intra-prediction mode for the target sample based on the position of the target sample and the first intra-prediction mode includes:
deriving #m−1 intra-prediction mode as the another intra-prediction mode for the second intra-prediction mode for the target sample.
14. The video encoding method of claim 13 , wherein M is equal to N.