Methods and apparatus of video coding in 4:4:4 chroma format

1. A method of video decoding, comprising:

obtaining, from a bitstream, a first syntax element in one slice header associated with a slice, wherein the first syntax element indicates whether luma mapping with chroma scaling (LMCS) is applied to coding units in the slice;

obtaining, from the bitstream, a second syntax element for one coding unit in the slice, wherein the second syntax element indicates whether the coding unit has been coded using adaptive color-space transform (ACT);

in accordance with a determination that the second syntax element has a non-zero value, applying inverse ACT to convert luma and chroma residuals of the coding unit from one transformed color space to an original color space of video data; and

after applying the inverse ACT to convert luma and chroma residuals of the coding unit, in accordance with a determination that the first syntax element has a non-zero value, deriving intermediate luma samples based on the luma residuals of the coding unit in the original color space, performing inverse luma mapping using the intermediate luma samples and performing inverse scaling using the chroma residuals of the coding unit in the original color space.

2. The method of claim 1 , further comprising:

before obtaining the second syntax element, obtaining a third syntax element from the bitstream, wherein the third syntax element indicates whether the video data has a predefined chroma format.

3. The method of claim 2 , wherein the predefined chroma format is 4:4:4 chroma format.

4. The method of claim 2 , wherein the second syntax element is present in the bitstream only when the video data has the predefined chroma format.

5. The method of claim 1 , wherein the transformed color space is YCgCo color space.

6. The method of claim 1 , wherein steps of deriving intermediate luma samples based on the luma residuals of the coding unit in the original color space, performing inverse luma mapping using the intermediate luma samples and performing inverse scaling using the chroma residuals of the coding unit in the original color space comprises:

generating the intermediate luma samples in a mapped domain of the coding unit by adding the luma residuals in the original color space to predicted luma samples of the coding unit;

converting the intermediate luma samples in the mapped domain to reconstructed luma components in a pre-mapped domain using one or more adaptive piecewise linear models;

inverse scaling the chroma residuals in the original color space according to corresponding intermediate luma samples in the mapped domain; and

reconstructing chroma components of the coding unit using the scaled chroma residuals and predicted chroma samples of the coding unit.

7. An electronic apparatus comprising:

one or more processing units;

memory coupled to the one or more processing units; and

a plurality of programs stored in the memory that, when executed by the one or more processing units, cause the electronic apparatus to perform a method of decoding video data, the method comprising:

obtaining, from a bitstream, a first syntax element in one slice header associated with a slice, wherein the first syntax element indicates whether luma mapping with chroma scaling (LMCS) is applied to coding units in the slice;

obtaining, from the bitstream, a second syntax element for one coding unit in the slice, wherein the second syntax element indicates whether the coding unit has been coded using adaptive color-space transform (ACT);

in accordance with a determination that the second syntax element has a non-zero value, applying inverse ACT to convert luma and chroma residuals of the coding unit from one transformed color space to an original color space of video data; and

after applying the inverse ACT to convert luma and chroma residuals of the coding unit, in accordance with a determination that the first syntax element has a non-zero value, deriving intermediate luma samples based on the luma residuals of the coding unit in the original color space, performing inverse luma mapping using the intermediate luma samples and performing inverse scaling using the chroma residuals of the coding unit in the original color space.

8. The electronic apparatus of claim 7 , wherein the method further comprises:

before obtaining the second syntax element, obtaining a third syntax element from the bitstream, wherein the third syntax element indicates whether the video data has a predefined chroma format.

9. The electronic apparatus of claim 8 , wherein the predefined chroma format is 4:4:4 chroma format.

10. The electronic apparatus of claim 8 , wherein the second syntax element is present in the bitstream only when the video data has the predefined chroma format.

11. The electronic apparatus of claim 7 , wherein the transformed color space is YCgCo color space.

12. The electronic apparatus of claim 7 , wherein steps of deriving intermediate luma samples based on the luma residuals of the coding unit in the original color space, performing inverse luma mapping using the intermediate luma samples and performing inverse scaling using the chroma residuals of the coding unit in the original color space comprises:

generating the intermediate luma samples in a mapped domain of the coding unit by adding the luma residuals in the original color space to predicted luma samples of the coding unit;

converting the intermediate luma samples in the mapped domain to reconstructed luma components in a pre-mapped domain using one or more adaptive piecewise linear models;

inverse scaling the chroma residuals in the original color space according to corresponding intermediate luma samples in the mapped domain; and

reconstructing chroma components of the coding unit using the scaled chroma residuals and predicted chroma samples of the coding unit.

13. A non-transitory computer readable storage medium configured to store therein a bitstream comprising video information to be decoded by a method of video decoding, the method comprising:

obtaining, from a bitstream, a first syntax element in one slice header associated with a slice, wherein the first syntax element indicates whether luma mapping with chroma scaling (LMCS) is applied to coding units in the slice;

obtaining, from the bitstream, a second syntax element for one coding unit in the slice, wherein the second syntax element indicates whether the coding unit has been coded using adaptive color-space transform (ACT);

in accordance with a determination that the second syntax element has a non-zero value, applying inverse ACT to convert luma and chroma residuals of the coding unit from one transformed color space to an original color space of video data; and

after applying the inverse ACT to convert luma and chroma residuals of the coding unit, in accordance with a determination that the first syntax element has a non-zero value, deriving intermediate luma samples based on the luma residuals of the coding unit in the original color space, performing inverse luma mapping using the intermediate luma samples and performing inverse scaling using the chroma residuals of the coding unit in the original color space.

14. The non-transitory computer readable storage medium of claim 13 , wherein the method further comprises:

before obtaining the second syntax element, obtaining a third syntax element from the bitstream, wherein the third syntax element indicates whether the video data has a predefined chroma format.

15. The non-transitory computer readable storage medium of claim 14 , wherein the predefined chroma format is 4:4:4 chroma format.

16. The non-transitory computer readable storage medium of claim 14 , wherein the second syntax element is present in the bitstream only when the video data has the predefined chroma format.

17. The non-transitory computer readable storage medium of claim 13 , wherein the transformed color space is YCgCo color space.

18. The non-transitory computer readable storage medium of claim 13 , wherein steps of deriving intermediate luma samples based on the luma residuals of the coding unit in the original color space, performing inverse luma mapping using the intermediate luma samples and performing inverse scaling using the chroma residuals of the coding unit in the original color space comprises:

generating the intermediate luma samples in a mapped domain of the coding unit by adding the luma residuals in the original color space to predicted luma samples of the coding unit;

converting the intermediate luma samples in the mapped domain to reconstructed luma components in a pre-mapped domain using one or more adaptive piecewise linear models;

inverse scaling the chroma residuals in the original color space according to corresponding intermediate luma samples in the mapped domain; and

reconstructing chroma components of the coding unit using the scaled chroma residuals and predicted chroma samples of the coding unit.