Image decoding method and apparatus based on affine motion prediction in image coding system
According to the present disclosure, an image decoding method performed by a decoding apparatus comprises the steps of: acquiring motion prediction information on a current block from a bitstream; generating an affine MVP candidate list including affine motion vector predictor candidates for the current block; deriving CPMVPs for CPs of the current block on the basis of one affine MVP candidate among the affine MVP candidates included in the affine MVP candidate list; deriving CPMVDs for the CPs of the current block on the basis of the motion prediction information; deriving CPMVs for the CPs of the current block on the basis of the CPMVPs and the CPMVDs; and deriving prediction samples for the current block on the basis of the CPMVs.
1. A decoding apparatus for video decoding, the decoding apparatus comprising:
a memory; and
at least one processor connected to the memory, the at least one processor configured to:
obtain motion prediction information for a current block from a bitstream;
obtain residual information for the current block from the bitstream;
construct an affine motion vector predictor (MVP) candidate list including affine MVP candidates for the current block, wherein the affine MVP candidates include a first affine MVP candidate and a second affine MVP candidate;
derive control point motion vector predictors (CPMVPs) for control points (CPs) of the current block based on CPMVP candidates of one of the affine MVP candidates included in the affine MVP candidate list;
derive control point motion vector differences (CPMVDs) for the CPs of the current block based on the motion prediction information;
derive control point motion vectors (CPMVs) for the CPs of the current block based on the CPMVPs and the CPMVDs;
derive prediction samples for the current block based on the CPMVs;
derive residual samples of the current block based on the residual information;
generate a reconstructed picture for the current block based on the derived prediction samples and the derived residual samples; and
apply a deblocking filtering to the reconstructed picture,
wherein the first affine MVP candidate is derived based on a first block in a left block group including only a bottom-left corner neighboring block and a left neighboring block,
wherein the first block is coded with an affine motion model and a reference picture of the first block is same as a reference picture of the current block,
wherein the second affine MVP candidate is derived based on a second block in a top block group including only a top-right corner neighboring block, a top neighboring block, and a top-left corner neighboring block, and
wherein the second block is coded with the affine motion model and a reference picture of the second block is same as the reference picture of the current block.
2. An encoding apparatus for video encoding, the encoding apparatus comprising:
a memory; and
at least one processor connected to the memory, the at least one processor configured to:
construct an affine motion vector predictor (MVP) candidate list including affine MVP candidates for a current block, wherein the affine MVP candidates include a first affine MVP candidate and a second affine MVP candidate;
derive control point motion vector predictors (CPMVPs) for control points (CPs) of the current block based on CPMVP candidates of one of the affine MVP candidates included in the affine MVP candidate list;
derive control point motion vectors (CPMVs) for the CPs of the current block;
derive control point motion vector differences (CPMVDs) for the CPs of the current block based on the CPMVPs and the CPMVs;
generate motion prediction information including information on the CPMVDs; and
encode the motion prediction information and residual information for the current block,
wherein the first affine MVP candidate is derived based on a first block in a left block group including only a bottom-left corner neighboring block and a left neighboring block,
wherein the first block is coded with an affine motion model and a reference picture of the first block is same as a reference picture of the current block,
wherein the second affine MVP candidate is derived based on a second block in a top block group including only a top-right corner neighboring block, a top neighboring block, and a top-left corner neighboring block, and
wherein the second block is coded with the affine motion model and a reference picture of the second block is same as the reference picture of the current block.
3. An apparatus for transmitting data for a video, the apparatus comprising:
at least one processor configured to obtain a bitstream for the video, wherein the bitstream is generated based on constructing an affine motion vector predictor (MVP) candidate list including affine MVP candidates for a current block, wherein the affine MVP candidates include a first affine MVP candidate and a second affine MVP candidate, deriving control point motion vector predictors (CPMVPs) for control points (CPs) of the current block based on CPMVP candidates of one of the affine MVP candidates included in the affine MVP candidate list, deriving control point motion vectors (CPMVs) for the CPs of the current block, deriving control point motion vector differences (CPMVDs) for the CPs of the current block based on the CPMVPs and the CPMVs, generating motion prediction information including information on the CPMVDs, and encoding the motion prediction information and residual information for the current block; and
a transmitter configured to transmit the data comprising the bitstream,
wherein the first affine MVP candidate is derived based on a first block in a left block group including only a bottom-left corner neighboring block and a left neighboring block,
wherein the first block is coded with an affine motion model and a reference picture of the first block is same as a reference picture of the current block,
wherein the second affine MVP candidate is derived based on a second block in a top block group including only a top-right corner neighboring block, a top neighboring block, and a top-left corner neighboring block, and
wherein the second block is coded with the affine motion model and a reference picture of the second block is same as the reference picture of the current block.