Inter prediction-based image coding method and apparatus

An encoding apparatus for image encoding, the encoding apparatus comprising:

• a memory;
• and
• at least one processor connected to the memory, the at least one processor configured to:
• construct motion vector predictor candidate lists of a current block;
• determine motion vector predictors of the current block based on the motion vector predictor candidate lists;
• generate selection information indicating the motion vector predictors among the motion vector predictor candidate lists;
• determine motion vector differences for the current block based on the motion vector predictors;
• derive reference picture lists including reference pictures, wherein the reference picture lists include an L0 reference picture list and an L1 reference picture list;
• determine whether an L0 reference picture in the L0 reference picture list is a short-term reference picture;
• derive a first picture order count (POC) difference between the L0 reference picture in the L0 reference picture list and current picture;
• derive an L0 symmetric motion vector difference reference index;
• determine whether an L1 reference picture in the L1 reference picture list is a short-term reference picture;
• derive a second POC difference between the L1 reference picture in the L1 reference picture list and the current picture;
• derive an L1 symmetric motion vector difference reference index;
• generate predicted samples based on the motion vector differences, the L0 symmetric motion vector difference reference index and the L1 symmetric motion vector difference reference index;
• generate residual information based on the predicted samples;
• and
• encode image information including the residual information and prediction related information including the selection information and information on the motion vector difference,
• wherein bi-prediction is applied to the current block,
• wherein the motion vector differences include an L0 motion vector difference for an L0 prediction and an L1 motion vector difference for an L1 prediction,
• wherein the L0 symmetric motion vector difference reference index is derived based on the first POC difference and a determination that the L0 reference picture in the L0 reference picture list is the short-term reference picture not a long-term reference picture,
• wherein the L0 symmetric motion vector difference reference index is derived by using a reference index for the L0 reference picture which is determined as the short-term reference picture,
• wherein the L1 symmetric motion vector difference reference index is derived based on the second POC difference and a determination that the L1 reference picture in the L1 reference picture list is the short-term reference picture not a long-term reference picture, and
• wherein the L1 symmetric motion vector difference reference index is derived by using a reference index for the L1 reference picture which is determined as the short-term reference picture.

An apparatus for transmitting data for an image, the apparatus comprising:

• at least one processor configured to obtain a bitstream for the image, wherein the bitstream is generated based on constructing motion vector predictor candidate lists of a current block, determining motion vector predictors of the current block based on the motion vector predictor candidate lists, generating selection information indicating the motion vector predictors among the motion vector predictor candidate lists, determining motion vector differences for the current block based on the motion vector predictors, deriving reference picture lists including reference pictures, wherein the reference picture lists include an L0 reference picture list and an L1 reference picture list, determining whether an L0 reference picture in the L0 reference picture list is a short-term reference picture, deriving a first picture order count (POC) difference between the L0 reference picture in the L0 reference picture list and current picture, deriving an L0 symmetric motion vector difference reference index, determining whether an L1 reference picture in the L1 reference picture list is a short-term reference picture, deriving a second POC difference between the L1 reference picture in the L1 reference picture list and the current picture, deriving an L1 symmetric motion vector difference reference index, generating predicted samples based on the motion vector differences, the L0 symmetric motion vector difference reference index and the L1 symmetric motion vector difference reference index, generating residual information based on the predicted samples, and encoding image information including the residual information and prediction related information including the selection information and information on the motion vector difference;
• and
• a transmitter configured to transmit the data comprising the bitstream,
• wherein bi-prediction is applied to the current block,
• wherein the motion vector differences include an L0 motion vector difference for an L0 prediction and an L1 motion vector difference for an L1 prediction,
• wherein the L0 symmetric motion vector difference reference index is derived based on the first POC difference and a determination that the L0 reference picture in the L0 reference picture list is the short-term reference picture not a long-term reference picture,
• wherein the L0 symmetric motion vector difference reference index is derived by using a reference index for the L0 reference picture which is determined as the short-term reference picture,
• wherein the L1 symmetric motion vector difference reference index is derived based on the second POC difference and a determination that the L1 reference picture in the L1 reference picture list is the short-term reference picture not a long-term reference picture, and
• wherein the L1 symmetric motion vector difference reference index is derived by using a reference index for the L1 reference picture which is determined as the short-term reference picture.