Method and apparatus for inter prediction in video processing system

1. An inter prediction method performed by a decoding apparatus, the method comprising:

deriving a first motion vector predictor (MVP) for a first control point (CP) and a second MVP for a second 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 and the second CP is located at a top-right position 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,

wherein the DMVD for the second CP is for a difference between a second MVD for the second CP and the first MVD, and

wherein the deriving the second MV for the second CP comprises:

deriving the second MVD for the second CP based on the first MVD and the DMVD for the second CP; and

deriving the second MV for the second CP based on the second MVP and the second MVD which is derived based on the DMVD for the second CP.

2. The inter prediction method according to claim 1 , wherein the inter prediction method further comprises deriving a third MVP for a third CP of the current block, wherein the third CP is located at a bottom-left position of the current block.

3. The inter prediction method of claim 2 , wherein the third MVP is derived based on the first MVP and the second MVP.

4. The inter prediction method of claim 2 , wherein the third MVP is derived based on the following equation,

{

mvp

2

⁢

x

=

mvp

0

⁢

x

-

h

*

(

mvp

1

⁢

y

-

mvp

0

⁢

y

)

w

mvp

2

⁢

y

=

mvp

0

⁢

y

+

h

*

(

mvp

1

⁢

x

-

mvp

0

⁢

x

)

w

where mvp 0x and mvp 0y are x and y components of the first MVP, mvp 1x and mvp 1y are x and y components of the second MVP, mvp 2x and mvp 2y are x and y components of the third MVP, and h and w are the height and width of the current block.

5. The inter prediction method of claim 2 , wherein the third MVP is derived based on neighboring blocks of the third CP.

6. The inter prediction method of claim 2 , wherein the DMVD for the second CP is decoded when a median value derived based on the first MVP, the second MVP, and the third MVP is same as the first MVP.

7. The inter prediction method of claim 2 , further comprising:

decoding a DMVD for the third CP; and

deriving a third MV for the third CP based on the third MVP and the DMVD for the third CP,

wherein the predicted block is further generated based on the third MV, and

wherein the DMVD for the third CP is for the difference between the first MVD and the third MVD for the third CP.

8. 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) and a second MVP for a second 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 and the second CP is located at a top-right 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; and

encoding image information including information on the first MVD and information on the DMVD for the second CP to output a bitstream,

wherein the DMVD for the second CP is for a difference between a second MVD for the second CP and the first MVD, and

wherein the deriving the DMVD for the second CP comprises:

deriving the second MVD for the second CP based on the second MV for the second CP and the second MVP for the second CP; and

deriving the DMVD for the second CP based on the second MVD and the first MVD.

9. The image encoding method according to claim 8 , wherein the image encoding method further comprises deriving a third MVP for a third CP of the current block, wherein the third CP is located at a bottom-left position of the current block.

10. The image encoding method according to claim 9 , further comprising deriving a DMVD for the third CP,

wherein the image information further includes information on DMVD for the third CP, and

wherein the DMVD for the third CP represents the difference between the first MVD and the third MVD for the third CP.

11. A non-transitory computer readable storage medium storing encoded information causing a decoding apparatus to perform an inter prediction method, the method comprising:

deriving a first motion vector predictor (MVP) for a first control point (CP) and a second MVP for a second 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 and the second CP is located at a top-right position 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,

wherein the DMVD for the second CP is for a difference between a second MVD for the second CP and the first MVD, and

wherein the deriving the second MV for the second CP comprises:

deriving the second MVD for the second CP based on the first MVD and the DMVD for the second CP; and

deriving the second MV for the second CP based on the second MVP and the second MVD which is derived based on the DMVD for the second CP.