Image decoding method and apparatus based on motion prediction in sub-block unit in image coding system

1. A decoding apparatus for image 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;

configure an affine motion vector predictor (MVP) candidate list for the current block;

derive control point motion vector predictors (CPMVPs) for control points (CPs) of the current block based on the affine MVP candidate list;

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

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

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

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

wherein the at least one processor further configured to:

check whether a first affine MVP candidate is available, wherein the first affine MVP candidate is available based on a first block in a left block group being coded with an affine motion model and a reference picture of the first block being same as a reference picture of the current block;

check whether a second affine MVP candidate is available, wherein the second affine MVP candidate is available based on a second block in a top block group being coded with the affine motion model and a reference picture of the second block being same as the reference picture of the current block,

wherein the left block group includes a bottom-left corner neighboring block of the current block, and a first left neighboring block adjacent to an upper side of the bottom-left corner neighboring block, and

wherein the top block group includes a top-right corner neighboring block of the current 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; and

check whether a third affine MVP candidate available based on a number of the available affine MVP candidate being less than 2,

wherein a 6-parameter affine model is used to an inter prediction,

wherein for the 6-parameter affine model being used to the inter prediction, the third MVP affine candidate is available based on a first motion vector for CP0, a second motion vector for CP1 and a third motion vector for CP2 of the current block being derived from a top-left block group of the current block, a top-right block group of the current block and the left block group respectively, and

wherein the top-left block group includes the top-left corner neighboring block of the current 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, and the top-right block group includes the top-right corner neighboring block and the first top neighboring block.

2. An encoding apparatus for image encoding, the encoding apparatus comprising:

a memory: and

at least one processor connected to the memory, the at least one processor configured to:

configure an affine motion vector predictor (MVP) candidate list for a current block;

derive control point motion vector predictors (CPMVPs) for control points (CPs) of the current block based on 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 CPs of the current block based on the CPMVPs and the CPMVs; and

encode motion prediction information including information on the CPMVDs,

wherein the at least one processor further configured to:

check whether a first affine MVP candidate is available, wherein the first affine MVP candidate is available based on a first block in a left block group being coded with an affine motion model and a reference picture of the first block being same as a reference picture of the current block;

check whether a second affine MVP candidate is available, wherein the second affine MVP candidate is available based on a second block in a top block group being coded with the affine motion model and a reference picture of the second block being same as the reference picture of the current block,

wherein the left block group includes a bottom-left corner neighboring block of the current block, and a first left neighboring block adjacent to an upper side of the bottom-left corner neighboring block, and

wherein the top block group includes a top-right corner neighboring block of the current 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; and

check whether a third affine MVP candidate available based on a number of the available affine MVP candidate being less than 2,

wherein a 6-parameter affine model is used to an inter prediction,

wherein for the 6-parameter affine model being used to the inter prediction, the third MVP affine candidate is available based on a first motion vector for CP0, a second motion vector for CP1 and a third motion vector for CP2 of the current block being derived from a top-left block group of the current block, a top-right block group of the current block and the left block group respectively, and

wherein the top-left block group includes the top-left corner neighboring block of the current 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, and the top-right block group includes the top-right corner neighboring block and the first top neighboring block.

3. 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 configuring an affine motion vector predictor (MVP) candidate list for a current block, deriving control point motion vector predictors (CPMVPs) for control points (CPs) of the current block based on 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 CPs of the current block based on the CPMVPs and the CPMVs, and encoding motion prediction information including information on the CPMVDs; and

a transmitter configured to transmit the data comprising the bitstream,

wherein the configuring the affine MVP candidate list comprises:

checking whether a first affine MVP candidate is available, wherein the first affine MVP candidate is available based on a first block in a left block group being coded with an affine motion model and a reference picture of the first block being same as a reference picture of the current block;

checking whether a second affine MVP candidate is available, wherein the second affine MVP candidate is available based on a second block in a top block group being coded with the affine motion model and a reference picture of the second block being same as the reference picture of the current block,

wherein the left block group includes a bottom-left corner neighboring block of the current block, and a first left neighboring block adjacent to an upper side of the bottom-left corner neighboring block, and

wherein the top block group includes a top-right corner neighboring block of the current 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; and

checking whether a third affine MVP candidate available based on a number of the available affine MVP candidate being less than 2,

wherein a 6-parameter affine model is used to an inter prediction,

wherein for the 6-parameter affine model being used to the inter prediction, the third MVP affine candidate is available based on a first motion vector for CP0, a second motion vector for CP1 and a third motion vector for CP2 of the current block being derived from a top-left block group of the current block, a top-right block group of the current block and the left block group respectively, and

wherein the top-left block group includes the top-left corner neighboring block of the current 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, and the top-right block group includes the top-right corner neighboring block and the first top neighboring block.