Method and apparatus for block-based significance map and significance group flag context selection

1. A method of significance map context selection, the method comprising:

receiving transform coefficients associated with a (transform unit (TU), wherein the TU is divided into a plurality of sub-blocks, at least one first context set is used for non-DC transform coefficients in the TU and at least one second context set is used for a DC coefficient in the TU that is different from the at least one first context set;

coding, for each sub-block, all non-DC transform coefficients in the respective sub-block based on a single context set from the at least one first context set for significant map coding; and

coding the DC coefficient in one sub-block of the TU based on a single context set from the at least one second context set for significant map coding.

2. The method of claim 1 , wherein the single context set for the respective sub-block is determined based on a sub-block index in scan order, a horizontal sub-block index, a vertical sub-block index, a video component type, a TU width, a TU height, or a combination thereof.

3. The method of claim 2 , wherein the single context set for the respective sub-block is determined by comparing the sub-block index in scan order, the horizontal sub-block index, the vertical sub-block index, or a combination thereof with a threshold.

4. The method of claim 3 , wherein the threshold is related to the TU width, the TU height or a combination thereof.

5. The method of claim 4 , wherein the threshold is derived based on a maximum of the TU width and the TU height divided by 16.

6. The method of claim 2 , wherein a sum of the horizontal sub-block index and the vertical sub-block index of the respective sub-block is used to classify the respective sub-block into a class, wherein the single context set for the respective sub-block is determined according to the class.

7. The method of claim 6 , wherein the sum is compared with a threshold to classify the respective sub-block and the threshold is derived based on a maximum of the TU width and the TU height divided by 16.

8. The method of claim 1 , wherein the transform coefficients of each sub-block are converted into one-dimensional coefficients using a scanning order.

9. The method of claim 1 , wherein at least one sub-block has a size corresponding to 4×4, 4×8, 8×4, 8×8, 16×16, or 32×32.

10. An apparatus for significance map context selection, the apparatus comprising one or more electronics circuits configured for:

receiving transform coefficients associated with a transform unit(TU), wherein the TU is divided into a plurality of sub-blocks, at least one first context set is used for non-DC transform coefficients in the TU and at least one second context set is used for a DC coefficient in the TU that is different from the at least one first context set;

coding, for each sub-block, all non-DC transform coefficients in the respective sub-block based on a single context set from the at least one first context set for significance map coding; and

coding the DC coefficient in one sub-block of the TU based on a single context set from the at least one second context set for significant map coding.

11. The apparatus of claim 10 , wherein the single context set for the respective sub-block is determined based on a sub-block index in scan order, a horizontal sub-block index, a vertical sub-block index, a video component type, a TU width, a TU height, or a combination thereof.

12. The apparatus of claim 11 , wherein the single context set for the respective sub-block is determined by comparing the sub-block index in scan order, the horizontal sub-block index, the vertical sub-block index, or a combination thereof with a threshold.

13. The apparatus of claim 12 , wherein the threshold is related to the TU width, the TU height or a combination thereof.

14. The apparatus of claim 13 , wherein the threshold is derived based on a maximum of the TU width and the TU height divided by 16.

15. The apparatus of claim 11 , wherein a sum of the horizontal sub-block index and the vertical sub-block index of the respective sub-block is used to classify the respective sub-block into a class, wherein the single context set for the respective sub-block is determined according to the class.

16. The apparatus of claim 15 , wherein the sum is compared with a threshold to classify the respective sub-block and the threshold is derived based on a maximum of the TU width and the TU height divided by 16.

17. The apparatus of claim 10 , wherein the transform coefficients of each sub-block are converted into one-dimensional coefficients using a scanning order.

18. The apparatus of claim 10 , wherein at least one sub-block has a size corresponding to 4×4, 4×8, 8×4, 8×8, 16×16, or 32×32.

19. A non-transitory computer readable medium storing a computer-executable program, the computer-executable program, when executed, causing a decoder to perform the following steps:

receiving transform coefficients associated with a transform unit (TU), wherein the TU is divided into a plurality of sub-blocks, at least one first context set is used for non-DC transform coefficients in the TU and at least one second context set is used for a DC coefficient in the TU that is different from the at least one first context set;

coding, for each sub-block, all non-DC transform coefficients in the respective sub-block based on a single context set from the at least one first context set for significant map coding; and

coding the DC coefficient in one sub-block of the TU based on a single context set from the at least one second context set for significant map coding.