Method and apparatus for inter prediction in video processing system
Disclosed is an inter prediction method which includes deriving control points (CPs) for the current block, wherein the CPs include a first CP and a second CP, deriving a first motion vector predictor (MVP) for the first CP and a second MVP for the second CP based on neighboring blocks of the current block, decoding a first motion vector difference (MVD) for the first CP, decoding a difference of two MVDs (DMVD) for the second CP, deriving a first motion vector (MV) for the first CP based on the first MVP and the first MVD, deriving a second MV for the second CP based on the second MVP and the DMVD for the second CP, and generating a predicted block for the current block based on the first MV and the second MV.
1 . An image decoding method performed by a decoding apparatus, the method comprising:
obtaining residual information from a bitstream;
deriving a first motion vector predictor (MVP) for a first control point (CP), a second MVP for a second CP and a third MVP for a third CP of a current block based on neighboring blocks of the current block,
wherein the first CP is located at a top-left position of the current block, the second CP is located at a top-right position of the current block, and the third CP is located at a bottom-left position of the current block;
decoding information on a first motion vector difference (MVD) for the first CP to derive the first MVD;
decoding information on a difference of two MVDs (DMVD) for the second CP, which is a difference between a second MVD for the second CP and the first MVD, to derive the DMVD for the second CP;
decoding information on a DMVD for the third CP, which is a difference between a third MVD for the third CP and the first MVD, to derive the DMVD for the third CP;
deriving a first motion vector (MV) for the first CP based on the first MVP and the first MVD;
deriving a second MV for the second CP based on the second MVP and the second MVD;
deriving a third MV for the third CP based on the third MVP and the third MVD;
generating prediction samples for the current block based on the first MV, the second MV and the third MV;
generating residual samples for the current block based on the residual information; and
generating reconstructed samples based on the prediction samples and the residual samples,
wherein the second MVD is derived based on the first MVD and the DMVD for the second CP, and
wherein the third MVD is derived based on the first MVD and the DMVD for the third CP.
2 . An image encoding method performed by an encoding apparatus, the method comprising:
deriving a first motion vector predictor (MVP) for a first control point (CP), a second MVP for a second CP and a third MVP for a third CP of a current block based on neighboring blocks of the current block, wherein the first CP is located at a top-left position of the current block, the second CP is located at a top-right position of the current block, and the third CP is located at a bottom-left position of the current block;
deriving a first motion vector difference (MVD) for the first CP;
deriving a difference of two MVDs (DMVD) for the second CP based on a second MVD for the second CP and the first MVD;
deriving a DMVD for the third CP based on a third MVD for the third CP and the first MVD;
generating prediction samples for the current block based on a first motion vector (MV), a second MV and a third MV;
generating residual samples of the current block based on the prediction samples, and
encoding image information including information on the first MVD, information on the DMVD for the second CP, information on the DMVD for the third CP, and residual information related to the residual samples to output a bitstream,
wherein the second MVD is derived based on the second MV for the second CP and the second MVP for the second CP, and
wherein the third MVD is derived based on the third MV for the third CP and the third MVP for the third CP.
3 . A non-transitory computer readable storage medium storing the bitstream generated by the image encoding method of the claim 2 .
4 . A transmission method of data for an image, the method comprising:
obtaining a bitstream for the image, wherein the bitstream is generated based on deriving a first motion vector predictor (MVP) for a first control point (CP), a second MVP for a second CP and a third MVP for a third CP of a current block based on neighboring blocks of the current block, wherein the first CP is located at a top-left position of the current block, the second CP is located at a top-right position of the current block, and the third CP is located at a bottom-left position of the current block, deriving a first motion vector difference (MVD) for the first CP, deriving a difference of two MVDs (DMVD) for the second CP based on a second MVD for the second CP and the first MVD, deriving a DMVD for the third CP based on a third MVD for the third CP and the first MVD, generating prediction samples for the current block based on a first motion vector (MV), a second MV and a third MV, generating residual samples of the current block based on the prediction samples, and encoding image information including information on the first MVD, information on the DMVD for the second CP, information on the DMVD for the third CP, and residual information related to the residual samples; and
transmitting the data comprising the bitstream,
wherein the second MVD is derived based on the second MV for the second CP and the second MVP for the second CP, and
wherein the third MVD is derived based on the third MV for the third CP and the third MVP for the third CP.