Image decoding method and apparatus based on motion prediction using merge candidate list in image coding system

An encoding apparatus for an 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 an inherited candidate based on a block coded in an affine motion model among neighboring blocks of a current block;
• derive a constructed candidate based on a combination of candidate motion vectors for control points (CPs) of the current block;
• constitute a merge candidate list for the current block, wherein the merge candidate list includes the inherited candidate and the constructed candidate;
• derive control point motion vectors (CPMVs) for the CPs of the current block based on the merge candidate list;
• derive prediction samples for the current block by performing an inter prediction based on the CPMVs of the current block;
• derive residual samples for the current block based on the prediction samples;
• and
• encode image information including prediction information and residual information,
• wherein the inherited candidate is derived based on CPMVs of the block coded in the affine motion model among the neighboring blocks of the current block including a bottom-left corner neighboring block, a first left neighboring block adjacent to an upper side of the bottom-left corner neighboring block, a top-right corner neighboring block, a first top neighboring block adjacent to a left side of the top-right corner neighboring block and a top-left corner neighboring block,
• wherein the CPs includes CP0 related to a top-left corner of the current block, CP1 related to a top-right corner of the current block, CP2 related to a bottom-left corner of the current block and CP3 related to a bottom-right corner of the current block,
• wherein a candidate motion vector for the CP0 is derived based on a first neighboring block firstly confirmed to be available according to a first order in a first group, wherein the first order in the first group is an order of the top-left corner neighboring block, a second left neighboring block adjacent to a bottom side of the top-left corner neighboring block, and a second top neighboring block adjacent to a right side of the top-left corner neighboring block,
• wherein a candidate motion vector for the CP1 is derived based on a second neighboring block firstly confirmed to be available according to a second order in a second group, wherein the second order in the second group is an order of the first top neighboring block and the top-right corner neighboring block,
• wherein a candidate motion vector for the CP2 is derived based on a third neighboring block firstly confirmed to be available according to a third order in a third group, wherein the third order in the third group is an order of the first left neighboring block and the bottom-left corner neighboring block,
• wherein a candidate motion vector for the CP3 is derived based on a temporal neighboring block,
• wherein based on a reference picture of the first neighboring block, a reference picture of the second neighboring block and a reference picture of the third neighboring block being a same reference picture, the constructed candidate is derived based on a combination of the candidate motion vector for the CP0, the candidate motion vector for the CP1 and the candidate motion vector for the CP2,
• wherein based on the reference picture of the first neighboring block, the reference picture of the second neighboring block and the reference picture of the temporal neighboring block being the same reference picture, the constructed candidate is derived based on a combination of the candidate motion vector for the CP0, the candidate motion vector for the CP1 and the candidate motion vector for the CP3,
• wherein based on the reference picture of the first neighboring block, the reference picture of the third neighboring block and the reference picture of the temporal neighboring block being the same reference picture, the constructed candidate is derived based on a combination of the candidate motion vector for the CP0, the candidate motion vector for the CP2 and the candidate motion vector for the CP3, and
• wherein based on the reference picture of the second neighboring block, the reference picture of the third neighboring block and the reference picture of the temporal neighboring block being the same reference picture, the constructed candidate is derived based on a combination of the candidate motion vector for the CP1, the candidate motion vector for the CP2 and the candidate motion vector for the CP3.

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 an inherited candidate based on a block coded in an affine motion model among neighboring blocks of a current block, deriving a constructed candidate based on a combination of candidate motion vectors for control points (CPs) of the current block, constituting a merge candidate list for the current block, wherein the merge candidate list includes the inherited candidate and the constructed candidate, deriving control point motion vectors (CPMVs) for the CPs of the current block based on the merge candidate list, deriving prediction samples for the current block by performing an inter prediction based on the CPMVs of the current block, deriving residual samples for the current block based on the prediction samples, and encoding image information including prediction information and residual information;
• and
• a transmitter configured to transmit the data comprising the bitstream,
• wherein the inherited candidate is derived based on CPMVs of the block coded in the affine motion model among the neighboring blocks of the current block including a bottom-left corner neighboring block, a first left neighboring block adjacent to an upper side of the bottom-left corner neighboring block, a top-right corner neighboring block, a first top neighboring block adjacent to a left side of the top-right corner neighboring block and a top-left corner neighboring block,
• wherein the CPs includes CP0 related to a top-left corner of the current block, CP1 related to a top-right corner of the current block, CP2 related to a bottom-left corner of the current block and CP3 related to a bottom-right corner of the current block,
• wherein a candidate motion vector for the CP0 is derived based on a first neighboring block firstly confirmed to be available according to a first order in a first group, wherein the first order in the first group is an order of the top-left corner neighboring block, a second left neighboring block adjacent to a bottom side of the top-left corner neighboring block, and a second top neighboring block adjacent to a right side of the top-left corner neighboring block,
• wherein a candidate motion vector for the CP1 is derived based on a second neighboring block firstly confirmed to be available according to a second order in a second group, wherein the second order in the second group is an order of the first top neighboring block and the top-right corner neighboring block,
• wherein a candidate motion vector for the CP2 is derived based on a third neighboring block firstly confirmed to be available according to a third order in a third group, wherein the third order in the third group is an order of the first left neighboring block and the bottom-left corner neighboring block,
• wherein a candidate motion vector for the CP3 is derived based on a temporal neighboring block,
• wherein based on a reference picture of the first neighboring block, a reference picture of the second neighboring block and a reference picture of the third neighboring block being a same reference picture, the constructed candidate is derived based on a combination of the candidate motion vector for the CP0, the candidate motion vector for the CP1 and the candidate motion vector for the CP2,
• wherein based on the reference picture of the first neighboring block, the reference picture of the second neighboring block and the reference picture of the temporal neighboring block being the same reference picture, the constructed candidate is derived based on a combination of the candidate motion vector for the CP0, the candidate motion vector for the CP1 and the candidate motion vector for the CP3,
• wherein based on the reference picture of the first neighboring block, the reference picture of the third neighboring block and the reference picture of the temporal neighboring block being the same reference picture, the constructed candidate is derived based on a combination of the candidate motion vector for the CP0, the candidate motion vector for the CP2 and the candidate motion vector for the CP3, and
• wherein based on the reference picture of the second neighboring block, the reference picture of the third neighboring block and the reference picture of the temporal neighboring block being the same reference picture, the constructed candidate is derived based on a combination of the candidate motion vector for the CP1, the candidate motion vector for the CP2 and the candidate motion vector for the CP3.