Video decoding method and device therefor, and video encoding method and device therefor

A video decoding apparatus comprising:

• an inter predictor configured to:
• determine whether to perform optical flow based compensation by using at least one of a size of a current block, flag information related to whether to perform the optical flow based compensation, a picture order count (POC) difference, and whether the current block is bi-predicted;
• if it is determined to perform the optical flow based compensation, obtain a first displacement vector in a horizontal direction and a second displacement vector in a vertical direction for a pixel group;
• and
• obtain a predicted pixel value of the current block based on a pixel value of a first reference block, a pixel value of a second reference block and a value obtained by using the first displacement vector and the second displacement vector for the pixel group, and
• a decoder configured to reconstruct the current block based on the predicted pixel value,
• wherein:
• the flag information is obtained from a bitstream,
• the POC difference is a difference between a POC of a reference picture and a POC of a current picture, and
• the pixel group is N×N block, N being equal to or greater than 2.

A video encoding method comprising:

• determining whether to perform optical flow based compensation by using at least one of a size of a current block, flag information related to whether to perform the optical flow based compensation, a picture order count (POC) difference, and whether the current block is bi-predicted;
• if it is determined to perform the optical flow based compensation, obtaining a first displacement vector in a horizontal direction and a second displacement vector in a vertical direction for a pixel group;
• obtaining a predicted pixel value of the current block based on a pixel value of a first reference block, a pixel value of a second reference block and a value obtained by using the first displacement vector and the second displacement vector for the pixel group;
• and
• encoding the current block based on the predicted pixel value,
• wherein:
• the flag information is included in a bitstream,
• the POC difference is a difference between a POC of a reference picture and a POC of a current picture, and
• the pixel group is N×N, N being equal to or greater than 2.

A video encoding apparatus comprising:

• an inter predictor configured to:
• determine whether to perform optical flow based compensation by using at least one of a size of a current block, flag information related to whether to perform the optical flow based compensation, a picture order count (POC) difference, and whether the current block is bi-predicted;
• if it is determined to perform the optical flow based compensation, obtain a first displacement vector in a horizontal direction and a second displacement vector in a vertical direction for a pixel group;
• obtain a predicted pixel value of the current block based on a pixel value of a first reference block, a pixel value of a second reference block and a value obtained by using the first displacement vector and the second displacement vector for the pixel group;
• and
• encode the current block based on the predicted pixel value, and
• a generator configured to generate a bitstream comprising the encoding results,
• wherein:
• the flag information is included in the bitstream,
• the POC difference is a difference between a POC of a reference picture and a POC of a current picture, and
• the pixel group is N×N, N being equal to or greater than 2.

A non-transitory computer-readable medium for recording a bitstream, the bitstream comprising:

• encoding results of a current block,
• wherein, the encoding results of the current block is obtained by:
• determining whether to perform optical flow based compensation by using at least one of a size of a current block, flag information related to whether to perform the optical flow based compensation, a picture order count (POC) difference, and whether the current block is bi-predicted;
• if it is determined to perform the optical flow based compensation, obtaining a first displacement vector in a horizontal direction and a second displacement vector in a vertical direction for a pixel group;
• obtaining a predicted pixel value of the current block based on a pixel value of a first reference block, a pixel value of a second reference block and a value obtained by using the first displacement vector and the second displacement vector for the pixel group;
• and
• encoding the current block based on the predicted pixel value,
• wherein:
• the flag information is included in the bitstream,
• the POC difference is a difference between a POC of a reference picture and a POC of a current picture, and
• the pixel group is N×N, N being equal to or greater than 2.