Image decoding method and apparatus based on affine motion prediction in image coding system

1. A video decoding method performed by a decoding apparatus, comprising:

obtaining motion prediction information for a current block from a bitstream;

constructing an affine motion vector predictor (MVP) candidate list including affine MVP candidates for the current block;

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 vector differences (CPMVDs) for the CPs of the current block based on the motion prediction information;

deriving control point motion vectors (CPMVs) for the CPs of the current block based on the CPMVPs and the CPMVDs;

deriving prediction samples for the current block based on the CPMVs; and

generating a reconstructed picture for the current block based on the derived prediction samples,

wherein the affine MVP candidates include a first affine MVP candidate and a second affine MVP candidate,

wherein CPMVs derived based on an affine motion model of a first block in a left block group including a bottom-left corner neighboring block and a left neighboring block are derived as CPMVP candidates of the first affine MVP candidate,

wherein the first block is coded with the affine motion model of the first block and a reference picture of the first block is same as a reference picture of the current block,

wherein CPMVs derived based on an affine motion model of a second block in a top block group including a top-right corner neighboring block, a top neighboring block, and a top-left corner neighboring block are derived as CPMVP candidates of the second affine MVP candidate, and

wherein the second block is coded with the affine motion model of the second block and a reference picture of the second block is same as the reference picture of the current block.

2. The method of claim 1 , wherein the first block is a first block verified to be satisfying a condition when checking neighboring blocks within the left block group according to a specific order.

3. The method of claim 1 , wherein the second block is a first block verified to be satisfying a condition when checking neighboring blocks within the top block group according to a specific order.

4. The method of claim 3 , wherein the specific order is an order starting from the top neighboring block to the top-right corner neighboring block and the top-left corner neighboring block.

5. The method of claim 1 , wherein the motion prediction information includes an affine MVP candidate index for the current block, and

wherein the CPMVPs for the CPs of the current block are derived based on CPMVP candidates of an affine MVP candidate being indicated by the affine MVP candidate index.

6. The method of claim 1 , wherein the step of constructing an affine motion vector predictor (MVP) candidate list, comprises:

dividing motion vectors of neighboring blocks of the current block into a first group, a second group, and a third group; and

deriving a CPMVP candidate for CP0 of the current block from the first group, a CPMVP candidate for CP1 of the current block from the second group, and a CPMVP candidate for CP2 of the current block from the third group, and deriving a constructed affine MVP candidate including CPMVP candidates for the CPs.

7. The method of claim 6 , wherein the neighboring blocks include neighboring block A, neighboring block B, neighboring block C, neighboring block D, neighboring block E, neighboring block F, and neighboring block G, and

wherein, in case a size of the current block is W×H, and an x component and a y component of a top-left sample position of the current block are 0, the neighboring block A is a block including a sample of coordinates (−1, −1), the neighboring block B is a block including a sample of coordinates (0, −1), the neighboring block C is a block including a sample of coordinates (−1, 0), the neighboring block D is a block including a sample of coordinates (W−1, −1), the neighboring block E is a block including a sample of coordinates (W, −1), the neighboring block F is a block including a sample of coordinates (−1, H−1), and the neighboring block G is a block including a sample of coordinates (−1, H).

8. The method of claim 7 , wherein the first group includes a motion vector of the neighboring block A, a motion vector of the neighboring block B, and a motion vector of the neighboring block C,

wherein the second group includes a motion vector of the neighboring block D and a motion vector of the neighboring block E, and

wherein the third group includes a motion vector of the neighboring block F and a motion vector of the neighboring block G.

9. The method of claim 8 , wherein the CPMVP for CP0 is a motion vector having a reference picture first verified to be the same as a reference picture of the current block, when checking motion vectors within the first group according to a specific order,

wherein the specific order is an order starting from the neighboring block A to the neighboring block B and the neighboring block C.

10. The method of claim 8 , wherein the CPMVP for CP1 is a motion vector having a reference picture first verified to be the same as a reference picture of the current block, when checking motion vectors within the second group according to a specific order,

wherein the specific order is an order starting from the neighboring block D to the neighboring block E.

11. The method of claim 8 , wherein the CPMVP for CP2 is a motion vector having a reference picture first verified to be the same as a reference picture of the current block, when checking motion vectors within the third group according to a specific order, wherein the specific order is an order starting from the neighboring block F to the neighboring block G.

12. A video encoding method performed by an encoding apparatus, comprising:

constructing an affine motion vector predictor (MVP) candidate list including affine MVP candidates for a current block;

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; and

encoding motion prediction information including information on the CPMVDs,

wherein the affine MVP candidates include a first affine MVP candidate and a second affine MVP candidate,

wherein CPMVs derived based on an affine motion model of a first block in a left block group including a bottom-left corner neighboring block and a left neighboring block are derived as CPMVP candidates of the first affine MVP candidate,

wherein the first block is coded with the affine motion model of the first block and a reference picture of the first block is same as a reference picture of the current block,

wherein CPMVs derived based on an affine motion model of a second block in a top block group including a top-right corner neighboring block, a top neighboring block, and a top-left corner neighboring block are derived as CPMVP candidates of the second affine MVP candidate, and

wherein the second block is coded with the affine motion model of the second block and a reference picture of the second block is same as the reference picture of the current block.

13. The method of claim 12 , wherein the first block is a first block verified to be satisfying a condition when checking neighboring blocks within the left block group according to a specific order.

14. The method of claim 12 , wherein the second block is a first block verified to be satisfying a condition when checking neighboring blocks within the top block group according to a specific order.

15. The method of claim 14 , wherein the specific order is an order starting from the top neighboring block to the top-right corner neighboring block and the top-left corner neighboring block.

16. A non-transitory computer-readable storage medium storing a bitstream, the bitstream, when executed, causing a decoding apparatus to perform the following steps:

obtaining motion prediction information for a current block from the bitstream;

constructing an affine motion vector predictor (MVP) candidate list including affine MVP candidates for the current block;

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 vector differences (CPMVDs) for the CPs of the current block based on the motion prediction information;

deriving control point motion vectors (CPMVs) for the CPs of the current block based on the CPMVPs and the CPMVDs;

deriving prediction samples for the current block based on the CPMVs; and

generating a reconstructed picture for the current block based on the derived prediction samples,

wherein the affine MVP candidates include a first affine MVP candidate and a second affine MVP candidate,

wherein CPMVs derived based on an affine motion model of a first block in a left block group including a bottom-left corner neighboring block and a left neighboring block are derived as CPMVP candidates of the first affine MVP candidate,

wherein the first block is coded with the affine motion model of the first block and a reference picture of the first block is same as a reference picture of the current block,

wherein CPMVs derived based on an affine motion model of a second block in a top block group including a top-right corner neighboring block, a top neighboring block, and a top-left corner neighboring block are derived as CPMVP candidates of the first affine MVP candidate, and

wherein the second block is coded with the affine motion model of the second block and a reference picture of the second block is same as the reference picture of the current block.