Image encoding/decoding method and device for performing prediction, and method for transmitting bitstream involving weighted prediction and bi-directional optical flow

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

• deriving a first flag for Bi-Directional Optical Flow (BDOF) on a current block of a picture;
• based on the first flag specifying that the BDOF is to be applied to the current block:
• performing a BDOF process on the current block;
• based on the first flag specifying that the BDOF is not to be applied to the current block:
• checking (i) a second flag for weighted prediction on the current block and (ii) a weight index BcwIdx for performing bi-prediction with CU-level weight (BCW) for the current block;
• and
• based on the second flag and the weight index BcwIdx, determining whether to perform default weighted prediction or explicit weighted prediction on the current block,
• wherein the default weighted prediction is performed on the current block, based on the second flag being equal to 0 or the weight index BcwIdx not being equal to 0,
• wherein the explicit weighted prediction is performed on the current block, based on the second flag being equal to 1 and the weight index BcwIdx being equal to 0, and
• wherein the second flag is determined independently of weight prediction flags for a reference picture of the current block;
• encoding information related to the first flag, the second flag and the weight index BcwIdx,
• wherein the default weighted prediction performs the average sum on the current block based on the weight index BcwIdx being equal to 0,
• wherein the default weighted prediction performs the BCW on the current block based on the weight index BcwIdx not being equal to 0, and
• wherein the explicit weighted prediction is performed based on weighting parameters (weight and offset) for a reference picture of the current block.

A method for transmitting a bitstream, comprising:

• transmitting a bitstream generated by the steps of:
• deriving a first flag for Bi-Directional Optical Flow (BDOF) on a current block of a picture;
• based on the first flag specifying that the BDOF is to be applied to the current block:
• performing a BDOF process on the current block;
• based on the first flag specifying that the BDOF is not to be applied to the current block:
• checking (i) a second flag for weighted prediction on the current block and (ii) a weight index BcwIdx for performing bi-prediction with CU-level weight (BCW) for the current block;
• and
• based on the second flag and the weight index BcwIdx, determining whether to perform default weighted prediction or explicit weighted prediction on the current block,
• wherein the default weighted prediction is performed on the current block, based on the second flag being equal to 0 or the weight index BcwIdx not being equal to 0,
• wherein the explicit weighted prediction is performed on the current block, based on the second flag being equal to 1 and the weight index BcwIdx being equal to and
• wherein the second flag is determined independently of weight prediction flags for a reference picture of the current block;
• encoding information related to the first flag, the second flag and the weight index BcwIdx,
• wherein the default weighted prediction performs the average sum on the current block based on the weight index BcwIdx being equal to 0,
• wherein the default weighted prediction performs the BCW on the current block based on the weight index BcwIdx not being equal to 0, and
• wherein the explicit weighted prediction is performed based on weighting parameters (weight and offset) for a reference picture of the current block.