Method for predicting subblock-based temporal motion vector and apparatus therefor

An image encoding method performed by an encoding apparatus, the method comprising:

• determining whether to use an advanced temporal motion vector predictor (ATMVP) candidate of a sub-block unit for a current block;
• generating a flag indicating whether to use the ATMVP candidate of the sub-block unit for the current block;
• specifying a corresponding block related to the current block in a reference picture based on a motion vector for a spatial neighboring block of the current block in case of the ATMVP candidate of the sub-block unit being used;
• deriving the ATMVP candidate of the sub-block unit for the current block based on motion vectors of corresponding sub-blocks related to current sub-blocks split from the current block;
• constructing a merge candidate list including the ATMVP candidate of the sub-block unit for the current block;
• generating a candidate index indicating a candidate applied to the current block among the candidates in the merge candidate list;
• and
• encoding image information including the flag and the candidate index to output a bitstream,
• wherein the ATMVP candidate of the sub-block unit is determined not to be used for the current block based on a size of the current block being smaller than a fixed sub-block size which is pre-defined as a minimum sub-block size of 8×8.

A non-transitory computer-readable digital storage medium storing a bitstream generated by an image encoding method, the method comprising:

• determining whether to use an advanced temporal motion vector predictor (ATMVP) candidate of a sub-block unit for a current block;
• generating a flag indicating whether to use the ATMVP candidate of the sub-block unit for the current block;
• specifying a corresponding block related to the current block in a reference picture based on a motion vector for a spatial neighboring block of the current block in case of the ATMVP candidate of the sub-block unit being used;
• deriving the ATMVP candidate of the sub-block unit for the current block based on motion vectors of corresponding sub-blocks related to current sub-blocks split from the current block;
• constructing a merge candidate list including the ATMVP candidate of the sub-block unit for the current block;
• generating a candidate index indicating a candidate applied to the current block among the candidates in the merge candidate list;
• and
• encoding image information including the flag and the candidate index to output the bitstream,
• wherein the ATMVP candidate of the sub-block unit is determined not to be used for the current block based on a size of the current block being smaller than a fixed sub-block size which is pre-defined as a minimum sub-block size of 8×8.