Image decoding method and device for deriving weight index information for generation of prediction sample

1. An image decoding method performed by a decoding apparatus, the method comprising:

receiving image information comprising inter prediction mode information through a bitstream;

generating a merge candidate list for a current block based on the inter prediction mode information;

deriving motion information on the current block based on a candidate selected from among candidates in the merge candidate list;

generating L0 prediction samples and L1 prediction samples of the current block based on the motion information; and

generating prediction samples of the current block based on the L0 prediction samples, the L1 prediction samples, and a weight index for the current block, wherein the weight index is derived based on a weight index for the selected candidate,

wherein the candidates comprise a constructed affine merge candidate, and the constructed affine merge candidate comprises control point motion vectors (CPMVs),

wherein based on a case that the constructed affine merge candidate comprises a CPMV 0 for a control point 0 (CP 0 ), a weight index for the constructed affine merge candidate is set equal to a weight index for the CP 0 , wherein the CP 0 is related to a top-left corner of the current block, and

wherein based on a case that the constructed affine merge candidate does not comprise the CPMV 0 for the CP 0 , the weight index for the constructed affine merge candidate is set equal to a weight index for the CP 1 , wherein the CP 1 is related to a top-right corner of the current block.

2. The image decoding method of claim 1 ,

wherein the weight index for the CP 0 is equal to a weight index for a block that is used for deriving the CPMV 0 among neighboring blocks of the CP 0 , and

wherein the neighboring blocks of the CP 0 comprises a top-left corner neighboring block of the current block, a left neighboring block adjacent to a bottom side of the top-left corner neighboring block, and a top neighboring block adjacent to a right side of the top-left corner neighboring block.

3. The image decoding method of claim 1 ,

wherein the weight index for the CP 1 is equal to a weight index for a block that is used for deriving the CPMV 1 among neighboring blocks of the CP 1 , and

wherein the neighboring blocks of the CP 1 comprise a top-right corner neighboring block of the current block and a top neighboring block adjacent to a left side of the top-right corner neighboring block.

4. The image decoding method of claim 1 ,

wherein the candidates comprise a sub-block-based temporal motion vector prediction (SbTMVP) candidate, and

wherein a weight index for the SbTMVP candidate is derived based on a weight index for a left neighboring block of the current block.

5. The image decoding method of claim 1 ,

wherein the candidates comprise a sub-block-based temporal motion vector prediction (SbTMVP) candidate, and

wherein a weight index for the SbTMVP candidate is derived as 0.

6. The image decoding method of claim 1 ,

wherein the candidates comprise a sub-block-based temporal motion vector prediction (SbTMVP) candidate,

wherein a weight index for the SbTMVP candidate is derived based on a weight index for a center block in a collocated block,

wherein the collocated block comprises a block at a same position as a position of the current block in a reference picture different from a current picture in which the current block is positioned, and

wherein the center block comprises a bottom-right sub-block among four sub-blocks positioned in a central area of the collocated block.

7. The image decoding method of claim 1 , wherein based on the case that the constructed affine merge candidate comprise the CPMV 0 for the CP 0 , the weight index for the constructed affine merge candidate is set equal to the weight index for the CP 0 regardless of the weight index for the CP 1 and a weight index for a control point 2 (CP 2 ), wherein the CP 2 is related to a bottom-left corner of the current block.

8. The image decoding method of claim 1 , wherein based on the case that the constructed affine merge candidate comprise the CPMV 0 for the CP 0 , the weight index for the constructed affine merge candidate is set equal to the weight index for the CP 0 without comparing the weight index for the CP 0 to the weight index for the CP 1 or a weight index for a control point 2 (CP 2 ), wherein the CP 2 is related to a bottom-left corner of the current block.

9. An image encoding method performed by an encoding apparatus, the method comprising:

determining an inter prediction mode of a current block and generating inter prediction mode information indicating the inter prediction mode;

generating a merge candidate list for the current block based on the inter prediction mode;

generating selection information indicating one candidate among candidates included in the merge candidate list; and

encoding image information comprising the inter prediction mode information and the selection information,

wherein the candidates comprise a constructed affine merge candidate, and the constructed affine merge candidate comprises control point motion vectors (CPMVs),

wherein based on a case that the constructed affine merge candidate comprises CPMV 0 for a control point 0 (CP 0 ), a weight index for the constructed affine merge candidate is set equal to a weight index for the CP 0 , wherein the CP 0 is related to a top-left corner of the current block, and

wherein based on a case that the constructed affine merge candidate does not comprise the CPMV 0 for the CP 0 , the weight index for the constructed affine merge candidate is set equal to a weight index for a control point 1 (CP 1 ), wherein the CP 1 is related to a top-right corner of the current block.

10. The image encoding method of claim 9 , wherein based on the case that the constructed affine merge candidate comprise the CPMV 0 for the CP 0 , the weight index for the constructed affine merge candidate is set equal to the weight index for the CP 0 regardless of the weight index for the CP 1 and a weight index for a control point 2 (CP 2 ), wherein the CP 2 is related to a bottom-left corner of the current block.

11. The image encoding method of claim 9 , wherein based on the case that the constructed affine merge candidate comprise the CPMV 0 for the CP 0 , the weight index for the constructed affine merge candidate is set equal to the weight index for the CP 0 without comparing the weight index for the CP 0 to the weight index for the CP 1 or a weight index for a control point 2 (CP 2 ), wherein the CP 2 is related to a bottom-left corner of the current block.

12. A non-transitory computer readable digital storage medium storing a bitstream generated by an image encoding method, the method comprising:

determining an inter prediction mode of a current block and generating inter prediction mode information indicating the inter prediction mode;

generating a merge candidate list for the current block based on the inter prediction mode;

generating selection information indicating one candidate among candidates included in the merge candidate list; and

encoding image information to generate the bitstream, wherein the image information comprises the inter prediction mode information and the selection information,

wherein the candidates comprise a constructed affine merge candidate, and the constructed affine merge candidate comprises control point motion vectors (CPMVs),

wherein based on a case that the constructed affine merge candidate comprises CPMV 0 for a control point 0 (CP 0 ), a weight index for the constructed affine merge candidate is set equal to a weight index for the CP 0 , wherein the CP 0 is related to a top-left corner of the current block, and

wherein based on a case that the constructed affine merge candidate does not comprise the CPMV 0 for the CP 0 , the weight index for the constructed affine merge candidate is set equal to a weight index for a control point 1 (CP 1 ), wherein the CP 1 is related to a top-right corner of the current block.