Video encoding/decoding method and device for deriving weight index for bidirectional prediction of merge candidate, and method for transmitting bitstream

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

obtaining information on a subblock merge candidate;

constructing a subblock merge candidate list for a current block;

selecting one subblock merge candidate from the subblock merge candidate list based on the information on the subblock merge candidate;

deriving motion information of the current block based on motion information of the selected subblock merge candidate; and

generating prediction samples of the current block based on the motion information of the current block,

wherein the constructing the subblock merge candidate list comprises deriving a constructed subblock merge candidate, and the deriving the constructed subblock merge candidate comprises deriving a weight index for bi-prediction of the constructed subblock merge candidate,

wherein the deriving the constructed subblock merge candidate is performed based on motion information of each candidate control point (CP) included in a combination of predefined candidate CPs among a plurality of candidate CPs for the current block,

wherein the predefined candidate CPs have a predetermined order in the combination,

wherein the weight index of the constructed subblock merge candidate is derived based on the order in the combination of the predefined candidate CPs, and

wherein the weight index of the constructed subblock merge candidate is derived as a weight index of the first candidate CP among the candidate CPs in the combination.

2 . The image decoding method of claim 1 ,

wherein the motion information of the candidate CP is derived based on motion information of a candidate block for the candidate CP, and

wherein the candidate block is an available candidate block among at least one candidate block for the candidate CP.

3 . The image decoding method of claim 1 , wherein the motion information of the candidate CP comprises a weight index for bi-prediction, and the weight index for bi-prediction is derived based on the candidate CP being a top-left candidate CP or top-right candidate CP for the current block.

4 . The image decoding method of claim 1 , wherein the motion information of the candidate CP comprises a weight index for bi-prediction, and the weight index for bi-prediction is not derived based on the candidate CP being a bottom-left candidate CP or bottom-right candidate CP for the current block.

5 . The image decoding method of claim 2 , wherein the candidate CP is determined to be unavailable based on an available candidate block among at least one candidate block for the candidate CP being not present.

6 . The image decoding method of claim 5 , wherein the deriving the constructed subblock merge candidate is performed based on all candidate CPs included in the combination of the predefined candidate CPs being available.

7 . The image decoding method of claim 1 ,

wherein the weight index of the constructed subblock merge candidate is derived based on whether a prediction direction for the combination is available, and

wherein whether the prediction direction for the combination is available is derived based on motion information of the candidate CPs included in the combination.

8 . The image decoding method of claim 7 , wherein, based on whether the prediction direction for the combination being available for both an L0 direction and an L1 direction, the weight index of the constructed subblock merge candidate is derived as the weight index of the first candidate CP.

9 . The image decoding method of claim 8 , wherein, based on whether the prediction direction for the combination being unavailable for at least one of the L0 direction or the L1 direction, the weight index of the constructed subblock merge candidate is derived as a predetermined weight index.

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

constructing a subblock merge candidate list for a current block;

generating prediction samples of the current block based on motion information of a subblock merge candidate included in the subblock merge candidate list; and

encoding information on the subblock merge candidate,

wherein the constructing the subblock merge candidate list comprises deriving a constructed subblock merge candidate, and the deriving the constructed subblock merge candidate comprises deriving a weight index for bi-prediction of the constructed subblock merge candidate,

wherein the deriving the constructed subblock merge candidate is performed based on motion information of each candidate control point (CP) included in a combination of predefined candidate CPs among a plurality of candidate CPs for the current block,

wherein the predefined candidate CPs have a predetermined order in the combination,

wherein the weight index of the constructed subblock merge candidate is derived based on the order in the combination of the predefined candidate CPs, and

wherein the weight index of the constructed subblock merge candidate is derived as a weight index of the first candidate CP among the candidate CPs in the combination.

11 . A method of transmitting a bitstream generated by an image encoding method, the image encoding method comprising:

constructing a subblock merge candidate list for a current block;

generating prediction samples of the current block based on motion information of a subblock merge candidate included in the subblock merge candidate list; and

encoding information on the subblock merge candidate,

wherein the constructing the subblock merge candidate list comprises deriving a constructed subblock merge candidate, and the deriving the constructed subblock merge candidate comprises deriving a weight index for bi-prediction of the constructed subblock merge candidate,

wherein the deriving the constructed subblock merge candidate is performed based on motion information of each candidate control point (CP) included in a combination of predefined candidate CPs among a plurality of candidate CPs for the current block,

wherein the predefined candidate CPs have a predetermined order in the combination,

wherein the weight index of the constructed subblock merge candidate is derived based on the order in the combination of the predefined candidate CPs, and

wherein the weight index of the constructed subblock merge candidate is derived as a weight index of the first candidate CP among the candidate CPs in the combination.

12 . An image decoding apparatus comprising a memory and at least one processor, wherein the at least one processor is configured to perform:

obtaining information on a subblock merge candidate;

constructing a subblock merge candidate list for a current block;

selecting one subblock merge candidate from the subblock merge candidate list based on the information on the subblock merge candidate;

deriving motion information of the current block based on motion information of the selected subblock merge candidate; and

generating prediction samples of the current block based on the motion information of the current block,

wherein the constructing the subblock merge candidate list comprises deriving a constructed subblock merge candidate, and the deriving the constructed subblock merge candidate comprises deriving a weight index for bi-prediction of the constructed subblock merge candidate,

wherein the deriving the constructed subblock merge candidate is performed based on motion information of each candidate control point (CP) included in a combination of predefined candidate CPs among a plurality of candidate CPs for the current block,

wherein the predefined candidate CPs have a predetermined order in the combination,

wherein the weight index of the constructed subblock merge candidate is derived based on the order in the combination of the predefined candidate CPs, and

wherein the weight index of the constructed subblock merge candidate is derived as a weight index of the first candidate CP among the candidate CPs in the combination.

13 . The image decoding apparatus of claim 12 ,

wherein the motion information of the candidate CP is derived based on motion information of a candidate block for the candidate CP, and

wherein the candidate block is an available candidate block among at least one candidate block for the candidate CP.

14 . The image decoding apparatus of claim 12 , wherein the motion information of the candidate CP comprises a weight index for bi-prediction, and the weight index for bi-prediction is derived based on the candidate CP being a top-left candidate CP or top-right candidate CP for the current block.

15 . The image decoding apparatus of claim 12 , wherein the motion information of the candidate CP comprises a weight index for bi-prediction, and the weight index for bi-prediction is not derived based on the candidate CP being a bottom-left candidate CP or bottom-right candidate CP for the current block.

16 . The image decoding apparatus of claim 13 , wherein the candidate CP is determined to be unavailable based on an available candidate block among at least one candidate block for the candidate CP being not present.

17 . The image decoding apparatus of claim 16 , wherein the deriving the constructed subblock merge candidate is performed based on all candidate CPs included in the combination of the predefined candidate CPs being available.

18 . The image decoding apparatus of claim 12 ,

wherein the weight index of the constructed subblock merge candidate is derived based on whether a prediction direction for the combination is available, and

wherein whether the prediction direction for the combination is available is derived based on motion information of the candidate CPs included in the combination.

19 . An image encoding apparatus comprising a memory and at least one processor, wherein the at least one processor is configured to perform:

constructing a subblock merge candidate list for a current block;

generating prediction samples of the current block based on motion information of a subblock merge candidate included in the subblock merge candidate list; and

encoding information on the subblock merge candidate,

wherein the constructing the subblock merge candidate list comprises deriving a constructed subblock merge candidate, and the deriving the constructed subblock merge candidate comprises deriving a weight index for bi-prediction of the constructed subblock merge candidate,

wherein the deriving the constructed subblock merge candidate is performed based on motion information of each candidate control point (CP) included in a combination of predefined candidate CPs among a plurality of candidate CPs for the current block,

wherein the predefined candidate CPs have a predetermined order in the combination,

wherein the weight index of the constructed subblock merge candidate is derived based on the order in the combination of the predefined candidate CPs, and

wherein the weight index of the constructed subblock merge candidate is derived as a weight index of the first candidate CP among the candidate CPs in the combination.

20 . An apparatus comprising a memory and at least one processor, wherein the apparatus is configured to transmit a bitstream generated by an image encoding method, the image encoding method comprising:

constructing a subblock merge candidate list for a current block;

generating prediction samples of the current block based on motion information of a subblock merge candidate included in the subblock merge candidate list; and

encoding information on the subblock merge candidate,

wherein the constructing the subblock merge candidate list comprises deriving a constructed subblock merge candidate, and the deriving the constructed subblock merge candidate comprises deriving a weight index for bi-prediction of the constructed subblock merge candidate,

wherein the deriving the constructed subblock merge candidate is performed based on motion information of each candidate control point (CP) included in a combination of predefined candidate CPs among a plurality of candidate CPs for the current block,

wherein the predefined candidate CPs have a predetermined order in the combination,

wherein the weight index of the constructed subblock merge candidate is derived based on the order in the combination of the predefined candidate CPs, and

wherein the weight index of the constructed subblock merge candidate is derived as a weight index of the first candidate CP among the candidate CPs in the combination.