Method and apparatus of latency reduction for chroma residue scaling

1. A method of video coding, the method comprising:

determining a plurality of first segments, wherein a luma sample range corresponding to luma samples in a first signal domain is divided into the plurality of first segments, and wherein each first segment corresponds to one line piece mapping the luma samples from the first signal domain to a second signal domain;

determining a plurality of second segments, wherein the luma sample range corresponding to the luma samples in the second signal domain is divided into the plurality of second segments, and wherein all second segments have a same size equal to a target interval value;

determining or deriving one or more first pivot points, wherein each first pivot point is associated with one first segment, and wherein said one or more first pivot points are determined or derived so that: if a target mapped pivot point corresponding to one first pivot point in the second signal domain has a multiple of the target interval value, the target mapped pivot point and a next mapped pivot point are allowed to be located in a same second segment; and otherwise, the target mapped pivot point and the next mapped pivot point are not allowed to be located in the same second segment; and

performing video encoding or decoding of the luma samples using a piece-wise linear mapping to map a target luma sample from the first signal domain to the second signal domain, or from the second signal domain to the first signal domain.

2. The method of claim 1 , wherein the piece-wise linear mapping is associated with the plurality of first segments, the plurality of second segments and said one or more first pivot points.

3. The method of claim 1 , wherein all of the first segments have a same first interval value.

4. The method of claim 3 , wherein the first interval value corresponds to 2 to a power of a first pre-defined number and the first pre-defined number is an integer.

5. The method of claim 4 , wherein the first pre-defined number is equal to (bitDepth−4) and the bitDepth represents bit depth of the luma samples.

6. The method of claim 1 , wherein the target interval value corresponds to 2 to a power of a second pre-defined number and the second pre-defined number is an integer.

7. The method of claim 6 , wherein the second pre-defined number is equal to (bitDepth−5) and the bitDepth represents bit depth of the luma samples.

8. The method of claim 1 , wherein each second segment has at most two mapped pivot points.

9. The method of claim 1 , wherein the luma sample range is equal to 0 to 1023, inclusive, and the target interval value is equal to 32.

10. An apparatus of video coding, the apparatus comprising one or more electronic circuits or processors arranged to:

determine a plurality of first segments, wherein a luma sample range corresponding to luma samples in a first signal domain is divided into the plurality of first segments, and wherein each first segment corresponds to one line piece mapping the luma samples from the first signal domain to a second signal domain;

determine a plurality of second segments, wherein the luma sample range corresponding to the luma samples in the second signal domain is divided into the plurality of second segments, and wherein all second segments have a same size equal to a target interval value;

determine or derive one or more first pivot points, wherein each first pivot point is associated with one first segment, and wherein said one or more first pivot points are determined or derived so that: if a target mapped pivot point corresponding to one first pivot point in the second signal domain has a multiple of the target interval value, the target mapped pivot point and a next mapped pivot point are allowed to be located in a same second segment; and otherwise, the target mapped pivot point and the next mapped pivot point are not allowed to be located in the same second segment; and

perform video encoding or decoding of the luma samples using a piece-wise linear mapping to map a target luma sample from the first signal domain to the second signal domain, or from the second signal domain to the first signal domain.

11. The apparatus of claim 10 , wherein the piece-wise linear mapping is associated with the plurality of first segments, the plurality of second segments and said one or more first pivot points.

12. The apparatus of claim 10 , wherein all of the first segments have a same first interval value.

13. The apparatus of claim 12 , wherein the first interval value corresponds to 2 to a power of a first pre-defined number and the first pre-defined number is an integer.

14. The apparatus of claim 13 , wherein the first pre-defined number is equal to (bitDepth−4) and the bitDepth represents bit depth of the luma samples.

15. The apparatus of claim 10 , wherein the target interval value corresponds to 2 to a power of a second pre-defined number and the second pre-defined number is an integer.

16. The apparatus of claim 15 , wherein the second pre-defined number is equal to (bitDepth−5) and the bitDepth represents bit depth of the luma samples.

17. The apparatus of claim 10 , wherein each second segment has at most two mapped pivot points.

18. The apparatus of claim 10 , wherein the luma sample range is equal to 0 to 1023, inclusive, and the target interval value is equal to 32.