Method and apparatus for video coding

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

decoding coded information of a transform block (TB) from a coded video bitstream, the coded information indicating a region of the TB on which a secondary transform is applied, the region including a first sub-region having transform coefficients calculated by the secondary transform and a second sub-region that is a zero-out region;

in response to a determination that a neighboring transform coefficient is in the second sub-region and a current transform coefficient in the TB is not in the second sub-region, determining the current transform coefficient according to a default value for the neighboring transform coefficient; and

reconstructing a sample in the TB based on the current transform coefficient for the sample.

2. The method of claim 1 , wherein

the current transform coefficient in the TB is one of a plurality of transform coefficients in a first coefficient group (CG), a first CG flag for the first CG indicating whether at least one of the plurality of transform coefficients in the first CG is a non-zero transform coefficient;

a second CG including transform coefficients is previously entropy decoded and is a neighbor of the first CG; and

the method further includes:

determining a location of the second CG; and

when the second CG is determined to be located in the second sub-region, determining the first CG flag based on a default value for a second CG flag for the second CG.

3. The method of claim 1 , wherein

the current transform coefficient in the TB is one of a plurality of transform coefficients in a first coefficient group (CG), a first CG flag for the first CG indicating whether at least one of the plurality of transform coefficients in the first CG is a non-zero transform coefficient;

a second CG includes a first transform coefficient and a second transform coefficient, the second CG being previously entropy decoded and a neighbor of the first CG; and

the method further includes:

determining a location of the second CG; and

when a portion of the second CG including the second transform coefficient is located in the second sub-region and the first transform coefficient is a non-zero transform coefficient, determining the first CG flag based on a second CG flag for the second CG.

4. The method of claim 1 , further comprising:

based on the current transform coefficient being in the second sub-region, determining that the current transform coefficient is not signaled and is zero.

5. The method of claim 1 , wherein the determining the current transform coefficient comprises:

determining a syntax element of the current transform coefficient, the syntax element indicating one of: whether the current transform coefficient is a non-zero transform coefficient; a parity of the current transform coefficient; whether the current transform coefficient is larger than 2; and whether the current transform coefficient is larger than 4.

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

decoding coded information of a transform block (TB) from a coded video bitstream; and

in response to the coded information indicating that a secondary transform is to be performed on a first region of the TB,

performing the secondary transform on the first region of the TB, the first region including a first sub-region having transform coefficients calculated by the secondary transform and a second sub-region, transform coefficients of the second sub-region being set to zero; and

determining that transform coefficients in a second region in the TB are zero, the second region being outside the first region.

7. The method of claim 6 , wherein a size and a location of a coefficient unit that includes multiple transform coefficients in the TB are determined based on the first region, and transform coefficients outside the coefficient unit are zero.

8. The method of claim 7 , wherein the first region is a top-left 8×8 region in the TB, the coefficient unit is the first region, and the second region is adjacent to the top-left 8×8 region.

9. The method of claim 7 , wherein the first sub-region is a top-left 4×4 region in the TB, the coefficient unit is the first sub-region in the first region, and transform coefficients in a combined region including the second region and the second sub-region are zero.

10. The method of claim 7 , wherein the first region is a top-left 4×4 region in the TB, the coefficient unit is the first region, and the second region is adjacent to the top-left 4×4 region.

11. An apparatus for video decoding, comprising processing circuitry configured to:

decode coded information of a transform block (TB) from a coded video bitstream, the coded information indicating a region of the TB on which a secondary transform is applied, the region including a first sub-region having transform coefficients calculated by the secondary transform and a second sub-region that is a zero-out region;

in response to a determination that a neighboring transform coefficient is in the second sub-region and a current transform coefficient in the TB is not in the second sub-region, determine the current transform coefficient according to a default value for the neighboring transform coefficient; and

reconstruct a sample in the TB based on the current transform coefficient for the sample.

12. The apparatus of claim 11 , wherein

the current transform coefficient in the TB is one of a plurality of transform coefficients in a first coefficient group (CG), a first CG flag for the first CG indicating whether at least one of the plurality of transform coefficients in the first CG is a non-zero transform coefficient;

a second CG including transform coefficients is previously entropy decoded and is a neighbor of the first CG; and

the processing circuitry is further configured to:

determine a location of the second CG; and

when the second CG is determined to be located in the second sub-region, determine the first CG flag based on a default value for a second CG flag for the second CG.

13. The apparatus of claim 11 , wherein

the current transform coefficient in the TB is one of a plurality of transform coefficients in a first coefficient group (CG), a first CG flag for the first CG indicating whether at least one of the plurality of transform coefficients in the first CG is a non-zero transform coefficient;

a second CG includes a first transform coefficient and a second transform coefficient, the second CG being previously entropy decoded and a neighbor of the first CG; and

the processing circuitry is further configured to:

determine a location of the second CG; and

when a portion of the second CG including the second transform coefficient is located in the second sub-region and the first transform coefficient is a non-zero transform coefficient, determine the first CG flag based on a second CG flag for the second CG.

14. The apparatus of claim 11 , wherein the processing circuitry is further configured to:

based on the current transform coefficient being in the second sub-region, determine that the current transform coefficient is not signaled and is zero.

15. The apparatus of claim 11 , wherein the processing circuitry is further configured to:

determine a syntax element of the current transform coefficient, the syntax element indicating one of: whether the current transform coefficient is a non-zero transform coefficient; a parity of the current transform coefficient; whether the current transform coefficient is larger than 2; or whether the current transform coefficient is larger than 4.

16. The apparatus of claim 11 , wherein the processing circuitry is further configured to:

determine that transform coefficients of a second region in the TB are zero, the second region being outside the region on which the secondary transform is applied.

17. The apparatus of claim 16 , wherein a size and a location of a coefficient unit that includes multiple transform coefficients in the TB are determined based on the region and transform coefficients outside the coefficient unit are zero.

18. The apparatus of claim 17 , wherein the region is a top-left 8×8 region in the TB, the coefficient unit is the region, and the second region is adjacent to the top-left 8×8 region.

19. The apparatus of claim 17 , wherein the first sub-region is a top-left 4×4 region in the TB, the coefficient unit is the first sub-region in the region, and transform coefficients in a combined region including the second region and the second sub-region are zero.

20. The apparatus of claim 17 , wherein the region is a top-left 4×4 region in the TB, the coefficient unit is the region, and the second region is adjacent to the top-left 4×4 region.