Image decoding method and apparatus based on inter-prediction in image coding system

A decoding apparatus for performing video decoding, comprising:

• an entropy decoder to obtain information for inter prediction of a current block from a bitstream;
• and
• a predictor to construct a motion vector predictor (MVP) candidate list based on a neighboring block of the current block, derive costs of MVP candidates included in the MVP candidate list, derive a modified MVP candidate list based on the costs of the MVP candidates, derive an MVP of the current block based on the modified MVP candidate list, derive a motion vector of the current block based on the MVP of the current block and a motion vector difference (MVD), and perform prediction of the current block based on a reference picture index and the motion vector of the current block,
• wherein the information for the inter prediction includes the reference picture index and the MVD,
• wherein the cost of the MVP candidate is derived as the sum of a top cost of the MVP candidate and a left cost of the MVP candidate,
• the top cost of the MVP candidate is derived based on a top luma cost of the MVP candidate, a top chroma cost of the MVP candidate, a width of a luma block of the current block, a width of a chroma block of the current block and a weighting factor,
• the left cost of the MVP candidate is derived based on a left luma cost of the MVP candidate, a left chroma cost of the MVP candidate, a height of the luma block of the current block, a height of the chroma block of the current block and the weighting factor.

An encoding apparatus for performing video encoding, comprising:

• a predictor to construct a motion vector predictor (MVP) candidate list based on a neighboring block of a current block, derive costs of MVP candidates included in the MVP candidate list, derive a modified MVP candidate list based on the costs of the MVP candidates, derive an MVP of the current block based on the modified MVP candidate list, derive a motion vector of the current block based on the MVP of the current block, and perform prediction of the current block based on the motion vector of the current block;
• and
• an entropy encoder to encode information for inter prediction of the current block,
• wherein the information for the inter prediction includes a reference picture index indicating a reference picture of the current block and a motion vector difference (MVD),
• wherein the cost of the MVP candidate is derived as the sum of a top cost of the MVP candidate and a left cost of the MVP candidate,
• the top cost of the MVP candidate is derived based on a top luma cost of the MVP candidate, a top chroma cost of the MVP candidate, a width of a luma block of the current block, a width of a chroma block of the current block and a weighting factor,
• the left cost of the MVP candidate is derived based on a left luma cost of the MVP candidate, a left chroma cost of the MVP candidate, a height of the luma block of the current block, a height of the chroma block of the current block and the weighting factor.