Inter prediction method and apparatus in video coding system

A video encoding method performed by an encoding device, the video encoding method including:

• 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,
• 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.

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

• 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 to output 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.

A transmission method of data for a video, the method comprising:

• obtaining a bitstream for the video, 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.