Method for refining a motion vector derived under a merge mode using a difference vector

A method of decoding a video, the method comprising:

• determining whether a merge mode or a motion vector prediction mode is applied to a current block;
• and
• obtaining a motion vector of the current block,
• wherein, under the motion vector prediction mode, the motion vector of the current block is obtained by adding a motion vector difference to one of a plurality of motion vector predictors,
• wherein, under the merge mode, obtaining the motion vector of the current block comprises:
• obtaining an initial motion vector of the current block based on a merge candidate selected from a plurality of merge candidates;
• determining whether to refine the initial motion vector of the current block or not;
• when it is determined to refine the initial motion vector, obtaining a difference vector for
• refining the initial motion vector of the current block;
• and
• obtaining a refined motion vector of the current block by adding the difference vector to
• the initial motion vector,
• wherein the merge candidate is selected from the plurality of merge candidates based on first index information parsed from a bitstream, the first index information indicating an index of the merge candidate,
• wherein the difference vector is determined based on a difference value candidate selected from a plurality of difference value candidates,
• wherein the difference value candidate is selected from the plurality of difference value candidates based on second index information parsed from the bitstream, the second index information indicating an index of the difference value candidate,
• wherein the plurality of difference value candidates are pre-stored in a decoder, and
• wherein refining the initial motion vector based on the difference value candidate selected from the plurality of difference value candidates is allowed only for the merge mode but is not allowed for the motion vector prediction mode.