Method and apparatus for affine based inter prediction of chroma subblocks

1. A method of chroma motion vector derivation used in an affine based inter prediction of a current image block including a luma block and a co-located chroma block, comprising:

determining chroma scaling factors in horizontal and vertical directions respectively based on chroma format information, wherein the chroma format information indicates a chroma format of a current picture which the current image block belongs to;

determining a set (S) of luma subblocks of the luma block based on values of the chroma scaling factors; and

determining a chroma motion vector for a respective chroma subblock of chroma subblocks in the chroma block based on motion vectors of one or more luma subblocks in the set (S) of luma subblocks,

wherein, when both SubWidthC and SubHeightC are equal to 1, the set (S) of luma subblocks comprises a luma subblock indexed by

S 0=(xSbIdx,ySbIdx);

when at least one of SubWidthC and SubHeightC is not equal to 1, the set (S) of luma subblocks comprises: a first luma subblock indexed by

S 0=((xSbIdx>>(SubWidthC−1)<<(SubWidthC−1)),(ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))), and a second luma subblock indexed by

S 1=((xSbIdx>>(SubWidthC−1)<<(SubWidthC−1))+(SubWidthC−1),(ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))+(SubHeightC−1))

wherein SubWidthC and SubHeightC represent the chroma scaling factors in the horizontal and vertical directions, respectively;

xSbIdx and ySbIdx represent a subblock index in the horizontal direction and a subblock index in the vertical direction, respectively, for a luma subblock in the set (S);

“<<” represents a left arithmetic shift;

“>>” represents a right arithmetic shift;

xSbIdx is an integer number ranging from 0 to numSbX−1 and ySbIdx is an integer ranging from 0 to numSbY−1;

numSbX indicates the number of luma subblocks in the luma block along the horizontal direction; and

numSbY indicates the number of luma subblocks in the luma block along the vertical direction.

2. The method of claim 1 , wherein each of the one or more luma subblocks in the set (S) is represented by a subblock index in a horizontal direction and a subblock index in a vertical direction.

3. The method of claim 1 , wherein the number of the chroma subblocks in the chroma block along the horizontal direction and the vertical direction are the same with the number of the luma subblocks in the luma block along the horizontal and vertical directions, respectively.

4. The method of claim 1 , wherein when the chroma format is 4:4:4, the set (S) comprises one luma subblock co-located with the chroma subblock;

when the chroma format is 4:2:2, the set (S) comprises two luma subblocks horizontally adjacent to each other; or

when the chroma format is 4:2:0, the set (S) comprises two luma subblocks which are diagonal.

5. The method of claim 1 , wherein, when there are more than one luma subblocks in the set (S), the determining the chroma motion vector for the chroma subblock based on the motion vectors of one or more luma subblocks in the set S of luma subblocks, comprises:

averaging motion vectors of the luma subblocks in the set S to generate an averaged luma motion vector (mvAvgLX); and

deriving the chroma motion vector (mvCLX[xSbIdx][ySbIdx]) for the chroma subblock based on the averaged luma motion vector (mvAvgLX).

6. The method of claim 5 , wherein the averaging motion vectors of the luma subblocks in the set S comprise one or more of:

averaging horizontal component of the motion vectors of the luma subblocks in the set S; or

averaging vertical component of the motion vectors of the luma subblocks in the set S.

7. The method of claim 5 , wherein the averaging motion vectors of the luma subblocks in the set S to generate an averaged luma motion vector comprises:

mvAvgLX=Σ i mvLX[ S i x ][S i y ]

when mvAvgLX[0] is greater or equal to 0, mvAvgLX[0]=(mvAvgLX[0]+(N>>1)−1)>>log 2(N), otherwise,

mvAvgLX[0]=−((−mvAvgLX[0]+(N>>1)−1)>>log 2(N));

when mvAvgLX[1] is greater or equal to 0, mvAvgLX[1]=(mvAvgLX[1]+(N>>1)−1)>>log 2(N), otherwise,

mvAvgLX[1]=−((−mvAvgLX[1]+(N>>1)−1)>>log 2(N)),

wherein:

mvAvgLX is a resulting motion vector of the averaging;

mvAvgLX[0] is a horizontal component of the resulting motion vector mvAvgLX, and

mvAvgLX[1] is a vertical component of the resulting motion vector mvAvgLX;

S i x and S i y are horizontal and vertical indices of subblock Si in the set S of luma subblocks in a motion vector array; and

mvLX[S i x ][S i y ] is a motion vector of a luma subblock with indices S i x and S i y , N is a number of elements in the set S of luma subblocks, log 2(N) represents the logarithm of N to the base 2 and is the power to which the number 2 is raised to obtain the value N, and “>>” represents a right arithmetic shift.

8. The method of claim 5 , wherein the averaging of motion vectors of the luma subblocks in the set S comprises:

mvAvgLX=mvLX[(xSbIdx>>(SubWidthC−1)<<(SubWidthC−1))][(ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))]+mvLX[(xSbIdx>>(SubWidthC−1)<<(SubWidthC−1))+(SubWidthC−1)][(ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))+(SubHeightC−1)]

if mvAvgLX[0]>=0 then

mvAvgLX[0]=(mvAvgLX[0]+1−(mvAvgLX[0]>=0))>>1

if mvAvgLX[1]>=0 then

mvAvgLX[1]=(mvAvgLX[1]+1−(mvAvgLX[1]>=0))>>1

wherein mvAvgLX is an averaged motion vector, mvAvgLX[0] is a horizontal component of the averaged motion vector mvAvgLX, mvAvgLX[1] is a vertical component of the averaged motion vector mvAvgLX; wherein SubWidthC and SubHeightC represent the chroma scaling factors in horizontal and vertical directions respectively; xSbIdx and ySbIdx represent a subblock index in horizontal direction and a subblock index in the vertical direction, respectively, for a luma subblock in the set S, “<<” is a left arithmetic shift and “>>” is a right arithmetic shift.

9. The method of claim 1 , wherein the determining chroma scaling factors in horizontal and vertical directions respectively based on chroma format information, comprises:

determining the chroma scaling factors in the horizontal and vertical directions based on a mapping between the chroma format information and a pair of chroma scaling factors in the horizontal and vertical directions respectively.

10. The method of claim 1 , further comprising: generating a prediction of the chroma subblock based on the determined motion vector.

11. The method of claim 1 , wherein the chroma format comprises one of a YUV 4:2:2 format, a YUV 4:2:0 format or a YUV 4:4:4 format.

12. A non-transitory computer-readable medium storing program instructions which, when executed by one or more processors, cause the one or more processors to perform operations of chroma motion vector derivation used in an affine based inter prediction of a current image block comprises a luma block and a co-located chroma block, the operations comprising:

determining chroma scaling factors in horizontal and vertical directions respectively based on chroma format information, wherein the chroma format information indicates a chroma format of a current picture which the current image block belongs to;

determining a set (S) of luma subblocks of the luma block based on values of the chroma scaling factors; and

determining a chroma motion vector for a respective chroma subblock of chroma subblocks in the chroma block based on motion vectors of one or more luma subblocks in the set (S) of luma subblocks,

wherein, when both SubWidthC and SubHeightC are equal to 1, the set (S) of luma subblocks comprises a luma subblock indexed by

S 0=(xSbIdx,ySbIdx);

when at least one of SubWidthC and SubHeightC is not equal to 1, the set (S) of luma subblocks comprises: a first luma subblock indexed by

S0=((xSbIdx>>(SubWidthC−1)<<(SubWidthC−1)), (ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))), and a second luma subblock indexed by

S1=((xSbIdx>>(SubWidthC−1)<<(SubWidthC−1))+(SubWidthC−1), (ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))+(SubHeightC−1))

wherein SubWidthC and SubHeightC represent the chroma scaling factors in the horizontal and vertical directions, respectively;

xSbIdx and ySbIdx represent a subblock index in the horizontal direction and a subblock index in the vertical direction, respectively, for a luma subblock in the set (S);

“<<” represents a left arithmetic shift;

“>>” represents a right arithmetic shift;

xSbIdx is an integer number ranging from 0 to numSbX−1 and ySbIdx is an integer ranging from 0 to numSbY−1,

numSbX indicates the number of luma subblocks in the luma block along the horizontal direction; and

numSbY indicates the number of luma subblocks in the luma block along the vertical direction.

13. A non-transitory computer-readable medium for storing data associated with a video signal, comprising:

a bitstream for the video signal by including a plurality of syntax elements, wherein the plurality of syntax elements comprises a first flag, and operations of chroma motion vector derivation used in an affine based inter prediction of a current image block including a luma block and a co-located chroma block are performed on the basis of a value of the first flag, the operations comprising:

determining chroma scaling factors in horizontal and vertical directions respectively based on chroma format information, wherein the chroma format information indicates a chroma format of a current picture which the current image block belongs to;

determining a set (S) of luma subblocks of the luma block based on values of the chroma scaling factors; and

determining a chroma motion vector for a respective chroma subblock of chroma subblocks in the chroma block based on motion vectors of one or more luma subblocks in the set (S) of luma subblocks,

wherein, when both SubWidthC and SubHeightC are equal to 1, the set (S) of luma subblocks comprises a luma subblock indexed by

S 0=(xSbIdx,ySbIdx);

when at least one of SubWidthC and SubHeightC is not equal to 1, the set (S) of luma subblocks comprises: a first luma subblock indexed by

S0=((xSbIdx>>(SubWidthC−1)<<(SubWidthC−1)), (ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))), and a second luma subblock indexed by

S1=((xSbIdx>>(SubWidthC−1)<<(SubWidthC−1))+(SubWidthC−1), (ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))+(SubHeightC−1))

wherein SubWidthC and SubHeightC represent the chroma scaling factors in the horizontal and vertical directions, respectively;

xSbIdx and ySbIdx represent a subblock index in the horizontal direction and a subblock index in the vertical direction, respectively, for a luma subblock in the set (S);

“<<” represents a left arithmetic shift;

“>>” represents a right arithmetic shift;

xSbIdx is an integer number ranging from 0 to numSbX−1 and ySbIdx is an integer ranging from 0 to numSbY−1;

numSbX indicates the number of luma subblocks in the luma block along the horizontal direction; and

numSbY indicates the number of luma subblocks in the luma block along the vertical direction.

14. An apparatus, comprising:

a video data memory; and

a video coder, wherein the video coder is configured to:

determine chroma scaling factors in horizontal and vertical directions respectively based on chroma format information, wherein the chroma format information indicates a chroma format of a current picture which a current image block belongs to;

determine a set (S) of luma subblocks of the luma block based on values of the chroma scaling factors; and

determining a chroma motion vector for a respective chroma subblock of chroma subblocks in the chroma block based on motion vectors of one or more luma subblocks in the set (S) of luma subblocks,

wherein, when both SubWidthC and SubHeightC are equal to 1, the set (S) of luma subblocks comprises a luma subblock indexed by

S 0=(xSbIdx,ySbIdx);

when at least one of SubWidthC and SubHeightC is not equal to 1, the set (S) of luma subblocks comprises: a first luma subblock indexed by

S0=((xSbIdx>>(SubWidthC−1)<<(SubWidthC−1)), (ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))), and a second luma subblock indexed by

S1=((xSbIdx>>(SubWidthC−1)<<(SubWidthC−1))+(SubWidthC−1), (ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))+(SubHeightC−1))

wherein SubWidthC and SubHeightC represent the chroma scaling factors in the horizontal and vertical directions, respectively;

xSbIdx and ySbIdx represent a subblock index in the horizontal direction and a subblock index in the vertical direction, respectively, for a luma subblock in the set (S);

“<<” represents a left arithmetic shift;

“>>” represents a right arithmetic shift;

xSbIdx is an integer number ranging from 0 to numSbX−1 and ySbIdx is an integer ranging from 0 to numSbY−1;

numSbX indicates the number of luma subblocks in the luma block along the horizontal direction; and

numSbY indicates the number of luma subblocks in the luma block along the vertical direction.

15. The apparatus of claim 14 , wherein each of the one or more luma subblocks in the set (S) is represented by a subblock index in a horizontal direction and a subblock index in a vertical direction.

16. The apparatus of claim 14 , wherein the number of the chroma subblocks in the chroma block along the horizontal direction and the vertical direction are the same with the number of the luma subblocks in the luma block along the horizontal and vertical directions, respectively.

17. The apparatus of claim 14 ,

wherein when the chroma format is 4:4:4, the set (S) comprises one luma subblock co-located with the chroma subblock;

when the chroma format is 4:2:2, the set (S) comprise two luma subblocks horizontally adjacent to each other; or

when the chroma format is 4:2:0, the set (S) comprise two luma subblocks which are diagonal.

18. The apparatus of claim 14 , wherein when there are more than one luma subblocks in the set S of luma subblocks, the video coder is further configured to:

average motion vectors of the luma subblocks in the set S to generate an averaged luma motion vector (mvAvgLX); and

derive the chroma motion vector (mvCLX[xSbIdx][ySbIdx]) for the chroma subblock based on the averaged luma motion vector (mvAvgLX).

19. The apparatus of claim 18 , wherein the video coder is further configured to:

average horizontal component of the motion vectors of the luma subblocks in the set S; or

average vertical component of the motion vectors of the luma subblocks in the set S.

20. The apparatus of claim 18 , wherein the video coder is further configured to average motion vectors of the luma subblocks in the set S to generate an averaged luma motion vector as follows:

mvAvgLX=Σ i mvLX[ S i x ][S i y ]

when mvAvgLX[0] is greater or equal to 0, mvAvgLX[0]=(mvAvgLX[0]+(N>>1)−1)>>log 2(N), otherwise,

mvAvgLX[0]=−((−mvAvgLX[0]+(N>>1)−1)>>log 2(N));

when mvAvgLX[1] is greater or equal to 0, mvAvgLX[1]=(mvAvgLX[1]+(N>>1)−1)>>log 2(N), otherwise,

mvAvgLX[1]=−((−mvAvgLX[1]+(N>>1)−1)>>log 2(N)),

wherein mvAvgLX is a resulting motion vector of the averaging;

mvAvgLX[0] is a horizontal component mvAvgLX of the resulting motion vector, and

mvAvgLX[1] is a vertical component mvAvgLX of the resulting motion vector;

S i x and S i y are horizontal and vertical indices of subblock Si in the set S of luma subblocks in a motion vector array; and

mvLX[S i x ][S i y ] is a motion vector of a luma subblock with indices S i x and S i y , N is a number of elements in the set S of luma subblocks, log 2(N) represents the logarithm of N to the base 2 and is the power to which the number 2 is raised to obtain the value N, and “>>” represents a right arithmetic shift.

21. The apparatus of claim 18 , wherein the video coder is further configured to average motion vectors of the luma subblocks in the set S to generate an averaged luma motion vector as follows:

mvAvgLX=mvLX[(xSbIdx>>(SubWidthC−1)<<(SubWidthC−1))][(ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))]+mvLX[(xSbIdx>>(SubWidthC−1)<<(SubWidthC−1))+(SubWidthC−1)][(ySbIdx>>(SubHeightC−1)<<(SubHeightC−1))+(SubHeightC−1)]

if mvAvgLX[0]>=0 then

mvAvgLX[0]=(mvAvgLX[0]+1−(mvAvgLX[0]>=0))>>1

if mvAvgLX[1]>=0 then

mvAvgLX[1]=(mvAvgLX[1]+1−(mvAvgLX[1]>=0))>>1

wherein mvAvgLX is an averaged motion vector, mvAvgLX[0] is a horizontal component mvAvgLX of the averaged motion vector, mvAvgLX[1] is a vertical component mvAvgLX of the averaged motion vector; wherein SubWidthC and SubHeightC represent the chroma scaling factors in horizontal and vertical directions respectively; xSbIdx and ySbIdx represent a subblock index in horizontal direction and a subblock index in the vertical direction, respectively, for a luma subblock in the set (S), “<<” is a left arithmetic shift and “>>” is a right arithmetic shift.

22. The apparatus of claim 14 , wherein the video coder is further configured to determine the chroma scaling factors in the horizontal and vertical directions based on a mapping between the chroma format information and a pair of chroma scaling factors in the horizontal and vertical directions.

23. The apparatus of claim 14 , wherein the video coder is further configured to generate a prediction of the chroma subblock based on the determined motion vector.

24. The apparatus of claim 14 , wherein the chroma format comprises one of a YUV 4:2:2 format, a YUV 4:2:0 format, or a YUV 4:4:4 format.