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

1 . A method of video decoding, comprising:

obtaining, from a bitstream, a plurality of syntax elements associated with a coding unit, wherein the syntax elements include a first coded block flag (CBF) for residual samples of a first chroma component of the coding unit, a second CBF for residual samples of a second chroma component of the coding unit, and a third syntax element indicating whether adaptive color transform (ACT) is applied to the coding unit or not, wherein the coding unit is associated with a partitioning method comprising at least one of quaternary partitioning, horizontal ternary partitioning, vertical ternary partitioning, horizontal binary partitioning or vertical binary partitioning;

determining whether to perform chroma residual scaling to residual samples of at least one of the first or second chroma components according to at least one of the first CBF, the second CBF, or the third syntax element; and

in response to a determination to perform the chroma residual scaling to the residual samples of the at least one of the first or second chroma components,

scaling the residual samples of the at least one of the first or second chroma components based on a corresponding scaling parameter; and

reconstructing chroma samples of the coding unit using chroma residual samples after scaling.

2 . The method according to claim 1 , further comprising:

in response to the third syntax element indicating that the ACT is applied to the coding unit:

applying an inverse ACT to luma and chroma residual samples of the coding unit; and

wherein the determining whether to perform the chroma residual scaling to residual samples of at least one of the first or second chroma components according to at least one of the first CBF, the second CBF, or the third syntax element comprises:

in response to the third syntax element indicating that the ACT is applied to the coding unit, determining to perform the chroma residual scaling to residual samples of the at least one of the first or second chroma components after the inverse ACT has been applied regardless of the first CBF and the second CBF.

3 . The method according to claim 1 , wherein the determining whether to perform the chroma residual scaling to residual samples of at least one of the first or second chroma components according to at least one of the first CBF, the second CBF, or the third syntax element comprises:

in response to the third syntax element indicating that the ACT is not applied to the coding unit:

determining to perform the chroma residual scaling to residual samples of one chroma component of at least one of the first or second chroma components of the coding unit when a corresponding CBF of the first CBF or the second CBF associated with said one chroma component is not zero; and

determining not to perform the chroma residual scaling to the residual samples of said one chroma component of the coding unit when the corresponding CBF associated with said one chroma component is zero.

4 . The method according to claim 2 , further comprising: applying an inverse transform to transform coefficients of the coding unit before applying the inverse ACT.

5 . The method according to claim 4 , further comprising: applying an inverse quantization to quantized transform coefficients of the coding unit before applying the inverse transform.

6 . The method according to claim 1 , wherein:

the first CBF is zero when non-zero chroma quantized transform coefficients are not present in quantized transform coefficients of the first chroma component; and

the second CBF is zero when non-zero chroma quantized transform coefficients are not present in quantized transform coefficients of the second chroma component.

7 . The method according to claim 1 , wherein the corresponding scaling parameter is derived from reconstructed luma samples.

8 . The method according to claim 2 , wherein an input to the inverse ACT is in YCgCo space.

9 . The method according to claim 2 , wherein an output of the inverse ACT is in RGB space.

10 . 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 acts comprising:

obtaining, from a bitstream, a plurality of syntax elements associated with a coding unit, wherein the syntax elements include a first coded block flag (CBF) for residual samples of a first chroma component of the coding unit, a second CBF for residual samples of a second chroma component of the coding unit, and a third syntax element indicating whether adaptive color transform (ACT) is applied to the coding unit or not, wherein the coding unit is associated with a partitioning method comprising at least one of quaternary partitioning, horizontal ternary partitioning, vertical ternary partitioning, horizontal binary partitioning or vertical binary partitioning;

determining whether to perform chroma residual scaling to residual samples of at least one of the first or second chroma components according to at least one of the first CBF, the second CBF, or the third syntax element; and

in response to a determination to perform the chroma residual scaling to the residual samples of the at least one of the first or second chroma components,

scaling the residual samples of the at least one of the first or second chroma components based on a corresponding scaling parameter; and

reconstructing chroma samples of the coding unit using chroma residual samples after scaling.

11 . The apparatus according to claim 10 , wherein the acts further comprising:

in response to the third syntax element indicating that the ACT is applied to the coding unit:

applying an inverse ACT to luma and chroma residual samples of the coding unit; and

wherein the determining whether to perform the chroma residual scaling to residual samples of at least one of the first or second chroma components according to at least one of the first CBF, the second CBF, or the third syntax element comprises:

in response to the third syntax element indicating that the ACT is applied to the coding unit, determining to perform the chroma residual scaling to residual samples of the at least one of the first or second chroma components after the inverse ACT has been applied regardless of the first CBF and the second CBF.

12 . The apparatus according to claim 10 , wherein the determining whether to perform the chroma residual scaling to residual samples of at least one of the first or second chroma components according to at least one of the first CBF, the second CBF, or the third syntax element comprises:

in response to the third syntax element indicating that the ACT is not applied to the coding unit:

determining to perform the chroma residual scaling to residual samples of one chroma component of at least one of the first or second chroma components of the coding unit when a corresponding CBF of the first CBF or the second CBF associated with said one chroma component is not zero; and

determining not to perform the chroma residual scaling to the residual samples of said one chroma component of the coding unit when the corresponding CBF associated with said one chroma component is zero.

13 . The apparatus according to claim 11 , wherein the acts further comprising: applying an inverse transform to transform coefficients of the coding unit before applying the inverse ACT.

14 . The apparatus according to claim 13 , further comprising: applying an inverse quantization to quantized transform coefficients of the coding unit before applying the inverse transform.

15 . The apparatus according to claim 10 , wherein:

the first CBF is zero when non-zero chroma quantized transform coefficients are not present in quantized transform coefficients of the first chroma component; and

the second CBF is zero when non-zero chroma quantized transform coefficients are not present in quantized transform coefficients of the second chroma component.

16 . The apparatus according to claim 10 , wherein the corresponding scaling parameter is derived from reconstructed luma samples.

17 . The apparatus according to claim 11 , wherein an input to the inverse ACT is in YCgCo space.

18 . The apparatus according to claim 11 , wherein an output of the inverse ACT is in RGB space.

19 . A method for transmitting a bitstream, comprising;

transmitting the bitstream to an image decoding apparatus, wherein the bitstream is to be decoded by performing acts comprising:

obtaining, from the bitstream, a plurality of syntax elements associated with a coding unit, wherein the syntax elements include a first coded block flag (CBF) for residual samples of a first chroma component of the coding unit, a second CBF for residual samples of a second chroma component of the coding unit, and a third syntax element indicating whether adaptive color transform (ACT) is applied to the coding unit or not, wherein the coding unit is associated with a partitioning method comprising at least one of quaternary partitioning, horizontal ternary partitioning, vertical ternary partitioning, horizontal binary partitioning or vertical binary partitioning;

determining whether to perform chroma residual scaling to residual samples of at least one of the first or second chroma components according to at least one of the first CBF, the second CBF, or the third syntax element; and

in response to a determination to perform the chroma residual scaling to the residual samples of the at least one of the first or second chroma components,

scaling the residual samples of the at least one of the first or second chroma components based on a corresponding scaling parameter; and

reconstructing chroma samples of the coding unit using chroma residual samples after scaling.

20 . The method according to claim 19 , wherein the acts further comprise:

in response to the third syntax element indicating that the ACT is applied to the coding unit:

applying an inverse ACT to luma and chroma residual samples of the coding unit; and

wherein the determining whether to perform the chroma residual scaling to residual samples of at least one of the first or second chroma components according to at least one of the first CBF, the second CBF, or the third syntax element comprises:

in response to the third syntax element indicating that the ACT is applied to the coding unit, determining to perform the chroma residual scaling to residual samples of the at least one of the first or second chroma components after the inverse ACT has been applied regardless of the first CBF and the second CBF.