Decoding method and apparatuses with candidate motion vectors

An image decoding apparatus comprising a processor and a memory, the processor, working together with the memory, being configured to:

• decode information indicative of a fixed candidate number to be used for decoding a plurality of merge-mode blocks;
• and
• for a current block among the plurality of merge-mode blocks:
• derive a plurality of first merging candidates, wherein each merging candidate is derived based on a combination of a prediction direction, a motion vector, and a reference picture index that has been used to decode a first block;
• determine whether or not a total number of first merging candidates is less than the fixed candidate number;
• when the total number of first merging candidates is less than the fixed candidate number, derive at least one second merging candidate that is different from each of the first merging candidates until the total number of first merging candidates and second merging candidates is equal to the fixed candidate number,
• wherein each of the one or more second merging candidates has a different combination of a prediction, a motion vector, and a reference picture index from each of the first merging candidates;
• and
• decode a coded index using the fixed candidate number regardless of the total number of first merging candidates,
• wherein the coded index corresponds to one of the plurality of first merging candidates or one of the at least one second merging candidate.