Inter prediction method and apparatus in video coding system

An encoding apparatus for image encoding, the encoding apparatus comprising:

• a memory:
• and
• at least one processor connected to the memory, the at least one processor configured to:
• derive motion vectors for control points (CPs) for a current block;
• derive a motion vector for the current block based on the obtained motion vectors for the CPs;
• perform a prediction for the current block based on the motion vector for the current block;
• and
• encode video information including information on the prediction for the current block,
• wherein the CPs includes a first CP, a second CP and a third CP, wherein the first CP is for a top-left corner of the current block, the second CP is for a top-right corner of the current block and the third CP is for bottom-left corner of the current block,
• wherein the motion vectors for the CPs include a first motion vector for the first CP, a second motion vector for the second CP and a third motion vector for the third CP,
• wherein the first motion vector is derived based on a firstly-checked available block according to a first predefined priority order in a first block group,
• wherein the first block group comprises a top-left corner neighboring block of the current block, a first top neighboring block which is leftmost block among blocks adjacent to an upper side of the current block and a first left neighboring block which is an uppermost block among blocks adjacent to a left side of the current block,
• wherein the second motion vector is derived based on a firstly-checked available block according to a second predefined priority order in a second block group,
• wherein the second block group comprises a top-right corner neighboring block of the current block and a second top neighboring block which is a rightmost block among the blocks adjacent to the upper side of the current block,
• wherein the third motion vector is derived based on a firstly-checked available block according to a third predefined priority order in a third block group, and
• wherein the third block group comprises a bottom-left corner neighboring block of the current block and a second left neighboring block which is a lowermost block among the blocks adjacent to the left side of the current block.

An apparatus for transmitting data for an image, the apparatus comprising:

• at least one processor configured to obtain a bitstream for the image, wherein the bitstream is generated based on deriving motion vectors for control points (CPs) for a current block, deriving a motion vector for the current block based on the obtained motion vectors for the CPs, performing a prediction for the current block based on the motion vector for the current block, and encoding video information including information on the prediction for the current block;
• and
• transmitting the data comprising the bitstream,
• wherein the CPs includes a first CP, a second CP and a third CP, wherein the first CP is for a top-left corner of the current block, the second CP is for a top-right corner of the current block and the third CP is for bottom-left corner of the current block,
• wherein the motion vectors for the CPs include a first motion vector for the first CP, a second motion vector for the second CP and a third motion vector for the third CP,
• wherein the first motion vector is derived based on a firstly-checked available block according to a first predefined priority order in a first block group,
• wherein the first block group comprises a top-left corner neighboring block of the current block, a first top neighboring block which is leftmost block among blocks adjacent to an upper side of the current block and a first left neighboring block which is an uppermost block among blocks adjacent to a left side of the current block,
• wherein the second motion vector is derived based on a firstly-checked available block according to a second predefined priority order in a second block group,
• wherein the second block group comprises a top-right corner neighboring block of the current block and a second top neighboring block which is a rightmost block among the blocks adjacent to the upper side of the current block,
• wherein the third motion vector is derived based on a firstly-checked available block according to a third predefined priority order in a third block group, and
• wherein the third block group comprises a bottom-left corner neighboring block of the current block and a second left neighboring block which is a lowermost block among the blocks adjacent to the left side of the current block.