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

1 . A method of video encoding, comprising:

determining whether to perform chroma residual scaling to residual samples of at least one of first or second chroma components of a coding unit according to at least one of conditions, wherein the conditions comprise: whether quantized transform coefficients of the first chroma component contain a non-zero chroma quantized transform coefficient or not, whether quantized transform coefficients of the second chroma component contain a non-zero chroma quantized transform coefficients or not, and whether adaptive color transform (ACT) is applied to the coding unit or not;

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; and

encoding at least one of a first coded block flag (CBF) of the first chroma component of the coding unit, a second CBF of the second chroma component of the coding unit, or a third syntax element indicating whether ACT is applied to the coding unit or not, into a video bitstream.

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

wherein in response to a determination that the ACT is applied to the coding unit,

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

wherein determining whether to perform chroma residual scaling to residual samples of the at least one of the first or second chroma component of the coding unit comprises:

in response to a determination 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 component after the inverse ACT has been applied regardless of whether quantized transform coefficients of the first chroma component contain a non-zero chroma quantized transform coefficients or not, and whether quantized transform coefficients of the second chroma component contain a non-zero chroma quantized transform coefficients or not.

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

in response to a determination 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 the at least one of the first or second chroma components of the coding unit when the quantized transform coefficients of said one chroma component of the first CBF or the second CBF contain a non-zero chroma quantized transform coefficients; and

determining not to perform the chroma residual scaling to the residual samples of said one chroma component of the coding unit when the quantized transform coefficients of said one chroma component do not contain any non-zero chroma quantized transform coefficient.

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 . The method according to claim 1 , wherein the first CBF, the second CBF or the third syntax element is determined to be zero when not present in the video bitstream.

11 . 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:

determining whether to perform chroma residual scaling to residual samples of at least one of a first or second chroma component of a coding unit according to at least one of conditions, wherein the conditions comprise: whether quantized transform coefficients of the first chroma component contain a non-zero chroma quantized transform coefficient or not, whether quantized transform coefficients of the second chroma component contain a non-zero chroma quantized transform coefficients or not, and whether adaptive color transform (ACT) is applied to the coding unit or not;

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; and

encoding at least one of a first coded block flag (CBF) of the first chroma component of the coding unit, a second CBF of the second chroma component of the coding unit, or a third syntax element indicating whether ACT is applied to the coding unit or not, into a video bitstream.

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

wherein in response to a determination that the ACT is applied to the coding unit,

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

wherein determining whether to perform chroma residual scaling to residual samples of the at least one of the first or second chroma component of the coding unit comprises:

in response to a determination 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 component after the inverse ACT has been applied regardless of whether quantized transform coefficients of the first chroma component contain a non-zero chroma quantized transform coefficients or not, and whether quantized transform coefficients of the second chroma component contain a non-zero chroma quantized transform coefficients or not.

13 . The method according to claim 1 , wherein determining whether to perform chroma residual scaling to residual samples of the at least one of the first chroma component or the second chroma component of the coding unit comprises:

in response to a determination 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 the at least one of the first or second chroma component of the coding unit when the quantized transform coefficients of said one chroma component of the first CBF or the second CBF contain a non-zero chroma quantized transform coefficients; and

determining not to perform the chroma residual scaling to the residual samples of said one chroma component of the coding unit when the quantized transform coefficients of said one chroma component do not contain any non-zero chroma quantized transform coefficient.

14 . The apparatus according to claim 12 , wherein the acts further comprising: applying an inverse transform to transform coefficients of the coding unit before applying the inverse ACT.

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

16 . The apparatus according to claim 11 , 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.

17 . The apparatus according to claim 11 , wherein the corresponding scaling parameter is derived from reconstructed luma samples.

18 . The apparatus according to claim 12 , wherein an input to the inverse ACT is in YCgCo space.

19 . The apparatus according to claim 12 , wherein an output of the inverse ACT is in RGB space.

20 . A method for transmitting a bitstream, comprising:

generating the bitstream by performing acts of video encoding, the acts comprising:

determining whether to perform chroma residual scaling to residual samples of at least one of a first or second chroma component of a coding unit according to at least one of conditions, wherein the conditions comprise: whether quantized transform coefficients of the first chroma component contain a non-zero chroma quantized transform coefficient or not, whether quantized transform coefficients of the second chroma component contain a non-zero chroma quantized transform coefficients or not, and whether adaptive color transform (ACT) is applied to the coding unit or not;

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; and

encoding at least one of a first coded block flag (CBF) of the first chroma component of the coding unit, a second CBF of the second chroma component of the coding unit, or a third syntax element indicating whether ACT is applied to the coding unit or not, into a video bitstream; and

transmitting the bitstream to an image decoding apparatus.