Method for encoding/decoding image using bi-directional optical flow prediction

A method of encoding an image, the method comprising:

• dividing a current block to acquire a subblock of the current block;
• determining whether to perform a subblock based BI-DIRECTIONAL OPTICAL FLOW (BIO) prediction on the subblock;
• deriving a first direction prediction sample and a second direction prediction sample of the subblock;
• deriving a BIO offset of the subblock;
• generating a final prediction sample of the subblock using the first direction prediction sample, the second direction prediction sample and the BIO offset;
• and
• encoding a residual sample based on the final prediction sample,
• wherein whether to perform the subblock based BIO prediction determines based on a result of comparing a threshold value and a difference between the first direction prediction sample and the second direction prediction sample,
• wherein the threshold value is set using on a size of the current block,
• wherein the deriving a BIO offset comprising:
• deriving a gradient using neighboring samples;
• deriving a motion refining vector;
• and
• deriving the BIO offset based on the gradient and the motion refining vector,
• wherein the gradient is, when a position of the neighboring sample is a subpixel position, derived using to an integer pixel position close to the subpixel position.

A method for transmitting a bitstream, the method comprising:

• transmitting the bitstream generated by an encoding method,
• wherein the encoding method comprising:
• dividing a current block to acquire a subblock of the current block;
• determining whether to perform a subblock based BI-DIRECTIONAL OPTICAL FLOW (BIO) prediction on the subblock;
• deriving a first direction prediction sample and a second direction prediction sample of the subblock;
• deriving a BIO offset of the subblock;
• and
• generating a final prediction sample of the subblock using the first direction prediction sample, the second direction prediction sample and the BIO offset;
• and
• encoding a residual sample based on the final prediction sample,
• wherein whether to perform the subblock based BIO prediction in units of subblock determines based on a result of comparing a threshold value and a difference between the first direction prediction sample and the second direction prediction sample,
• wherein the threshold value is set using on a size of the current block,
• wherein the deriving a BIO offset comprising:
• deriving a gradient using neighboring samples;
• deriving a motion refining vector;
• and
• deriving the BIO offset based on the gradient and the motion refining vector,
• wherein the gradient is, when a position of the neighboring sample is a subpixel position, derived using to an integer pixel position close to the subpixel position.