Method and apparatus for video coding

1. A method for video decoding in a decoder, comprising:

decoding prediction information of a current block in a current picture from a coded video bitstream, the prediction information being indicative of inter prediction;

determining, for the current block, motion information including a first motion vector (MV) that has a x component and a y component wherein each of the x and y components has a fractional MV precision that is 2 −N of a sample size in the current block and has one of 2 L+1 MV values with the fractional MV precision, N being 4 and indicating the fractional MV precision, L being a positive integer and indicating a fractional MV range, and each of the x and y components being within the fractional MV range with the fractional MV precision; and

reconstructing at least one sample of the current block based on the motion information.

2. The method of claim 1 , wherein each of the x and y components of the first MV is represented by (L+1) bits including L bits for magnitude and 1 bit for direction, and the 2 L+1 MV values are within the fractional MV range of [−2 L , 2 L −1].

3. The method of claim 2 , further comprising:

storing the motion information in a memory space, a size of the memory space being at least J bits reserved for the first MV and a second MV, a first reference index, a second reference index, and an indicator for an inter prediction direction, wherein

each of x and y components of the second MV is represented by (L+1) bits,

the first reference index is represented by K bits and indicates a first reference picture in a first reference list associated with the first MV,

the second reference index is represented by K bits and indicates a second reference picture in a second reference list associated with the second MV,

the indicator is represented by I bits,

the inter prediction direction is one of: a forward uni-directional prediction associated with the first MV and the first reference picture, a backward uni-directional prediction associated with the second MV and the second reference picture, and a bi-directional prediction associated with the first and second MVs and the first and second reference pictures,

K is a positive integer,

I is one of: 0 and 2, and

J is equal to (4(L+1)+2K+I).

4. The method of claim 3 , wherein L is 17, K is 4, I is 2, and J bits reserved for the memory space are 82, 18 bits being used for each of the x and y components of the first and second MVs.

5. The method of claim 3 , wherein L, N, K, and I correspond to one of:

Lis 18, K is 4, and I is 2;

L is 18, K is 5, and I is 0;

L is 19, K is 4, and I is 2;

L is 22, K is 4, and I is 2; and

L is 22, K is 5, and I is 0.

6. The method of claim 3 , wherein the motion information further includes the second MV, the first reference index, the second reference index, and the indicator that indicates the bi-directional prediction.

7. The method of claim 2 , wherein the determining the motion information further comprises:

determining the x and y components of the first MV based on corresponding x and y components of a MV predictor and a MV difference, respectively.

8. The method of claim 7 , wherein

the prediction information indicates a merge mode for the inter prediction; and

the determining the x and y components of the first MV includes determining the x and y components based on the corresponding x and y components of the MV predictor without the MV difference.

9. The method of claim 2 , wherein

the motion information further includes a first reference index indicating a first reference picture in a first reference list associated with the first MV; and

the determining the motion information further comprises:

obtaining an initial MV for the current block from a temporal candidate of the current block, a first temporal distance being between the first reference picture and the current picture, a second temporal distance being between a third reference picture of the temporal candidate and the current picture;

scaling, based on the first and second temporal distances, x and y components of the initial MV to obtain the x and y components of the first MV, respectively; and

for each of the x and y components of the first MV, when the respective component is outside the fractional MV range of [−2 L , 2 L −1], clipping the respective component to be within the fractional MV range of [−2 L , 2 L −1].

10. The method of claim 2 , wherein an integer MV range having an integer MV precision is [−2 M , 2 M −1] and corresponds to the fractional MV range, 2 N of the 2 L+1 MV values in the fractional MV range correspond to one of 2 M+1 MV values in the integer MV range, the integer MV precision is the sample size, and M is (L−N).

11. The method of claim 5 , wherein 19 bits are used for each of the x and y components of the first and second MVs, 4 bits are used for each of the first and second reference indices, 2 bits are used for the indicator, and J bits reserved for the memory space are 86, L being 18, K being 4, and I being 2.

12. The method of claim 2 , wherein the determining the motion information further comprises:

scaling an MV that is obtained from a temporal candidate of the current block; and

clipping one of an x component and a y component of the scaled MV to be within the fractional MV range of [−2 L , 2 L −1] based on the one of the x component and the y component being outside the fractional MV range of [−2 L , 2 L −1] to determine the first MV.

13. The method of claim 1 , wherein L is larger than or equal to 17.

14. An apparatus, comprising processing circuitry configured to:

decode prediction information of a current block in a current picture from a coded video bitstream, the prediction information being indicative of inter prediction;

determine, for the current block, motion information including a first motion vector (MV) that has a x component and a y component wherein each of the x and y components has a fractional MV precision that is 2 −N of a sample size in the current block and has one of 2 L+1 MV values with the fractional MV precision, N being 4 and indicating the fractional MV precision, L being a positive integer and indicating a fractional MV range, and each of the x and y components being within the fractional MV range with the fractional MV precision; and

reconstruct at least one sample of the current block based on the motion information.

15. The apparatus of claim 14 , wherein each of the x and y components of the first MV is represented by (L+1) bits including L bits for magnitude and 1 bit for direction, and the 2 L+1 MV values are within the fractional MV range of [−2 L , 2 L −1].

16. The apparatus of claim 15 , wherein the processing circuitry is configured to:

store the motion information in a memory space, a size of the memory space being at least J bits reserved for the first MV and a second MV, a first reference index, a second reference index, and an indicator for an inter prediction direction, wherein

each of x and y components of the second MV is represented by (L+1) bits,

the first reference index is represented by K bits and indicates a first reference picture in a first reference list associated with the first MV,

the second reference index is represented by K bits and indicates a second reference picture in a second reference list associated with the second MV,

the indicator is represented by I bits,

the inter prediction direction is one of: a forward uni-directional prediction associated with the first MV and the first reference picture, a backward uni-directional prediction associated with the second MV and the second reference picture, and a bi-directional prediction associated with the first and second MVs and the first and second reference pictures,

K is a positive integer,

I is one of: 0 and 2, and

J is equal to (4(L+1)+2K+I).

17. The apparatus of claim 16 , wherein L is 17, K is 4, I is 2, and J bits reserved for the memory space are 82, 18 bits being used for each of the x and y components of the first and second MVs.

18. The apparatus of claim 16 , wherein L, N, K, and I correspond to one of:

Lis 18, K is 4, and I is 2;

L is 18, N is 4, K is 5, and I is 0;

Lis 19, K is 4, and I is 2;

L is 22, K is 4, and I is 2; and

L is 22, K is 5, and I is 0.

19. The apparatus of claim 16 , wherein the motion information further includes the second MV, the first reference index, the second reference index, and the indicator that indicates the bi-directional prediction.

20. The apparatus of claim 15 , wherein the processing circuitry is configured to:

determine the x and y components of the first MV based on corresponding x and y components of a MV predictor and a MV difference, respectively.

21. The apparatus of claim 15 , wherein

the motion information further includes a first reference index indicating a first reference picture in a first reference list associated with the first MV; and

the processing circuitry is configured to:

obtain an initial MV for the current block from a temporal candidate of the current block, a first temporal distance being between the first reference picture and the current picture, a second temporal distance being between a third reference picture of the temporal candidate and the current picture; and

scale, based on the first and second temporal distances, x and y components of the initial MV to obtain the x and y components of the first MV, respectively; and

for each of the x and y components of the first MV, when the respective component is outside the fractional MV range of [−2 L , 2 L −1], clip the respective component to be within the fractional MV range of [−2 L , 2 L −1].

22. A non-transitory computer-readable storage medium storing a program executable by at least one processor to perform:

decoding prediction information of a current block in a current picture from a coded video bitstream, the prediction information being indicative of inter prediction;

determining, for the current block, motion information including a first motion vector (MV) that has a x component and a y component wherein each of the x and y components has a fractional MV precision that is 2 −N of a sample size in the current block and has one of 2 L+1 MV values with the fractional MV precision, N being 4 and indicating the fractional MV precision, L being a positive integer and indicating a fractional MV range, and each of the x and y components being within the fractional MV range with the fractional MV precision; and

reconstructing at least one sample of the current block based on the motion information.