Affine motion prediction-based image decoding method and device using affine merge candidate list in image coding system
An image decoding method performed by means of a decoding device according to the present disclosure comprises the steps of: configuring an affine merge candidate list with respect to a current block; deriving CPMVs with respect to CPs of the current block on the basis of the affine merge candidate list; deriving prediction samples with respect to the current block on the basis of the CPMVs; and generating a reconstructed picture with respect to the current block on the basis of the derived prediction samples, wherein the maximum number of inherited affine candidates is two, a first inherited affine candidate is derived on the basis of a left block group of the current block, and a second inherited affine candidate is derived on the basis of an upper block group of the current block.
1. An image decoding method, by a decoding apparatus, comprising:
constructing a merge candidate list for deriving motion information of subblock units of a current block, wherein the merge candidate list includes inherited affine candidates and constructed affine candidates;
deriving control point motion vectors (CPMVs) of a selected affine candidate in the merge candidate list as CPMVs for control points (CPs) of the current block;
deriving prediction samples for the current block based on the CPMVs for the CPs; and
generating a reconstructed picture for the current block based on the derived prediction samples,
wherein a maximum number of the inherited affine candidates is 2,
wherein a first inherited affine candidate is derived from a left block group including a bottom-left corner neighboring block and a left neighboring block of the current block,
wherein a second inherited affine candidate is derived from a top block group including a top-right corner neighboring block, a top neighboring block and a top-left corner neighboring block of the current block,
wherein the CPs include a CP0, a CP1, and a CP2,
wherein the CP0 is a point at a top left position of the current block, the CP1 is a point at a top right position of the current block, and the CP2 is a point at a bottom left position of the current block,
wherein the first inherited affine candidate is derived based on a neighboring block first checked as coded by an affine motion model in the left block group, neighboring blocks in the left block group are checked in a first order, and the first order is an order from the bottom left corner neighboring block to the left neighboring block, and
wherein the second inherited affine candidate is derived based on a neighboring block first checked as coded by the affine motion model in the top block group, neighboring blocks in the top block group is checked in a second order, and the second order is an order from the top right corner neighboring block, to the top neighboring block, and to the bottom left corner neighboring block.
2. The image decoding method of claim 1 , wherein
candidate motion vectors for the CPs are derived based on neighboring blocks of the current block, and
the constructed affine candidates are derived based on the candidate motion vectors for the CPs.
3. The image decoding method of claim 1 , wherein
the merge candidate list comprises a zero candidate when a number of the inherited affine candidates and the constructed affine candidates is smaller than a maximum candidate number of the merge candidate list, and the zero candidate comprises candidate motion vectors having a value of 0 and a reference picture index having a value of 0.
4. An image encoding method, by an encoding apparatus, comprising:
constructing a merge candidate list for deriving motion information of subblock units of a current block, wherein the merge candidate list includes inherited affine candidates and constructed affine candidates;
deriving control point motion vectors (CPMVs) of a selected affine candidate in the merge candidate list as CPMVs for control points (CPs) of the current block; and
encoding image information including prediction information for the current block,
wherein a maximum number of the inherited affine candidates is 2,
wherein a first inherited affine candidate is derived from a left block group including a bottom-left corner neighboring block and a left neighboring block,
wherein a second inherited affine candidate is derived from a top block group including a top-right corner neighboring block, a top neighboring block and a top-left corner neighboring block,
wherein the CPs include a CP0, a CP1, and a CP2,
wherein the CP0 is a point at a top left position of the current block, the CP1 is a point at a top right position of the current block, and the CP2 is a point at a bottom left position of the current block,
wherein the first inherited affine candidate is derived based on a neighboring block first checked as coded by an affine motion model in the left block group, neighboring blocks in the left block group are checked in a first order, and the first order is an order from the bottom left corner neighboring block to the left neighboring block, and
wherein the second inherited affine candidate is derived based on a neighboring block first checked as coded by the affine motion model in the top block group, neighboring blocks in the top block group is checked in a second order, and the second order is an order from the top right corner neighboring block, to the top neighboring block, and to the bottom left corner neighboring block.
5. A non-transitory computer-readable storage medium storing a bitstream generated by a method, the method comprising:
constructing a merge candidate list for deriving motion information of subblock units of a current block, wherein the merge candidate list includes inherited affine candidates and constructed affine candidates;
deriving control point motion vectors (CPMVs) of a selected affine candidate in the merge candidate list as CPMVs for control points (CPs) of the current block;
encoding image information including prediction information for the current block; and
generating the bitstream including the image information,
wherein a maximum number of the inherited affine candidates is 2,
wherein a first inherited affine candidate is derived from a left block group including a bottom-left corner neighboring block and a left neighboring block,
wherein a second inherited affine candidate is derived from a top block group including a top-right corner neighboring block, a top neighboring block and a top-left corner neighboring block,
wherein the CPs include a CP0, a CP1, and a CP2,
wherein the CP0 is a point at a top left position of the current block, the CP1 is a point at a top right position of the current block, and the CP2 is a point at a bottom left position of the current block,
wherein the first inherited affine candidate is derived based on a neighboring block first checked as coded by an affine motion model in the left block group, neighboring blocks in the left block group are checked in a first order, and the first order is an order from the bottom left corner neighboring block to the left neighboring block, and
wherein the second inherited affine candidate is derived based on a neighboring block first checked as coded by the affine motion model in the top block group, neighboring blocks in the top block group is checked in a second order, and the second order is an order from the top right corner neighboring block, to the top neighboring block, and to the bottom left corner neighboring block.