Image decoding method and image decoding apparatus using candidate motion vectors

An image decoding apparatus that decodes a current block, the image decoding apparatus comprising:

• a processor;
• a memory storing instructions, which when executed by the processor cause the apparatus to:
• decode information from a slice header indicative of a maximum number of merging candidates, wherein the slice header corresponds to a slice containing a plurality of blocks including the current block;
• derive a plurality of merging candidates for decoding the current block from a corresponding plurality of blocks that are neighbor or co-located blocks of the current block, each of the plurality of merging candidates having a motion vector for decoding a corresponding one of the plurality of blocks, a prediction direction that corresponds to the motion vector for decoding the corresponding one of the plurality of blocks, and a reference picture index that identifies a reference picture corresponding to the motion vector for decoding the corresponding one of the plurality of blocks;
• determine whether a number of the plurality of merging candidates is less than the maximum number of merging candidates;
• when it is determined that the number of the plurality of merging candidates is less than the maximum number of merging candidates, derive a combined merging candidate for bi-directional prediction, whereby the memory comprises instructions which, when executed by the processor cause the apparatus to:
• assign the motion vector and the reference picture index of a first merging candidate of the plurality of merging candidates to a first prediction direction of the combined merging candidate, and
• assign the motion vector and the reference picture index of a second merging candidate of the plurality of merging candidates to a second prediction direction of the combined merging candidate;
• and
• decode the current block by using a candidate selected from the plurality of merging candidates and the combined merging candidate.