Image encoding and decoding method using bidirectional prediction, and image encoding and decoding apparatus

An image encoding method using bi-prediction, the image encoding method comprising:

• obtaining a first prediction block by using a first reference image, and obtaining a second prediction block by using a second reference image, for bi-prediction of a current block;
• selecting a candidate value from among candidate values included in a weight candidate group;
• obtaining a pair value of the candidate value, wherein the candidate value is applied to the first prediction block, and the pair value is applied to the second prediction block;
• obtaining a third prediction block by combining the first prediction block to which the candidate value is applied with the second prediction block to which the pair value is applied;
• performing entropy encoding on a weight index indicating the selected candidate value;
• and
• generating a bitstream including the entropy-encoded weight index,
• wherein a first binary value corresponding to the weight index is entropy-encoded based on a context model,
• wherein a remaining binary value corresponding to the weight index is entropy-encoded by a bypass method, and
• wherein a number of candidate values indicatable by the weight index in the weight candidate group when a picture order count (POC) of the first reference image and a POC of the second reference image are smaller than or equal to a POC of a current image is larger than a number of candidate values indicatable by the weight index in the weight candidate group when the POC of the first reference image and the POC of the second reference image are greater than the POC of the current image.

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

• an entropy-encoded weight index;
• and
• an encoding result of a current block,
• wherein the encoding result is obtained by:
• obtaining a first prediction block by using a first reference image, and obtaining a second prediction block by using a second reference image, for bi-prediction of the current block;
• selecting a candidate value from among candidate values included in a weight candidate group;
• obtaining a pair value of the candidate value, wherein the candidate value is applied to the first prediction block, and the pair value is applied to the second prediction block;
• obtaining a third prediction block by combining the first prediction block to which the candidate value is applied with the second prediction block to which the pair value is applied;
• and
• encoding the current block by using the third prediction block,
• wherein a first binary value corresponding to a weight index indicating the selected candidate value is entropy-encoded based on a context model,
• wherein a remaining binary value corresponding to the weight index is entropy-encoded by a bypass method,
• wherein a number of candidate values indicatable by the weight index in the weight candidate group when a picture order count (POC) of the first reference image and a POC of the second reference image are smaller than or equal to a POC of a current image is larger than a number of candidate values indicatable by the weight index in the weight candidate group when the POC of the first reference image and the POC of the second reference image are greater than the POC of the current image.