QP derivation and offset for adaptive color transform in video coding

1. A method of decoding video data, the method comprising:

receiving an offset value in the video data;

determining for one or more blocks of the video data that adaptive color transform is enabled;

determining a red, green, and blue (RGB)-color space quantization parameter for the one or more blocks, wherein determining the RGB-color space quantization parameter comprises adding the offset value to a luma-chroma (YCbCr)-color space quantization parameter;

in response to a value of the RGB-color space quantization parameter being below a threshold, modifying the RGB-color space quantization parameter to determine a modified RGB-color space quantization parameter; and

dequantizing transform coefficients of a block from the one or more blocks based on the modified RGB-color space quantization parameter.

2. The method of claim 1 , wherein determining for the one or more blocks of the video data that adaptive color transform is enabled comprises determining a chroma format for the video data.

3. The method of claim 2 , wherein determining for the one or more blocks of the video data that adaptive color transform is enabled comprises:

in response to determining a chroma format for the one or more blocks of the video data is 4:4:4, determining that adaptive color transform is enabled for the one or more blocks.

4. The method of claim 2 , further comprising:

for one or more additional blocks, in response to determining a chroma format of the one or more additional blocks is other than 4:4:4, determining that adaptive color transform is disabled for the one or more additional blocks.

5. The method of 1 , wherein determining for the one or more blocks of the video data that adaptive color transform is enabled comprises parsing a syntax element in the video data, wherein the syntax element indicates if adaptive color transform is enabled.

6. The method of claim 5 wherein the syntax element is signalled in a picture parameter set (PPS).

7. The method of claim 1 , wherein the threshold is zero and wherein a value of the modified quantization parameter is greater than or equal to zero.

8. The method of claim 1 , wherein the modified RGB-color space quantization parameter is less than or equal to 51 plus the offset value.

9. The method of claim 1 , further comprising:

receiving a flag indicating the offset value is to be added to the quantization parameter.

10. The method of claim 1 , further comprising:

based on the dequantized transform coefficients, determining a residual block for the video data;

determining a predictive block;

adding the residual block to a predictive block to determine a reconstructed block of video data.

11. The method of claim 1 , wherein the method of decoding the video data is performed as part of a video encoding process.

12. The method of claim 1 , further comprising:

receiving the video data at a receiver of a wireless communication device;

storing the video data in a memory of the wireless communication device; and

processing the video data on one or more processors of the wireless communication device.

13. A device for decoding video data, the device comprising:

a video data memory;

one or more processors configured to:

receive an offset value in the video data;

determine for one or more blocks of the video data that adaptive color transform is enabled;

determine a red, green, and blue (RGB)-color space quantization parameter for the one or more blocks, wherein determining the RGB-color space quantization parameter comprises adding the offset value to a luma-chroma (YCbCr)-color space quantization parameter;

in response to a value of the RGB-color space quantization parameter being below a threshold, modify the RGB-color space quantization parameter to determine a modified RGB-color space quantization parameter; and

dequantize transform coefficients of a block from the one or more blocks based on the modified RGB-color space quantization parameter.

14. The device of claim 13 , wherein to determine for the one or more blocks of the video data that adaptive color transform is enabled, the one or more processors are further configured to determine a chroma format for the video data.

15. The device of claim 14 , wherein to determine for the one or more blocks of the video data that adaptive color transform is enabled, the one or more processors are further configured to:

in response to determining a chroma format for the one or more blocks of the video data is 4:4:4, determine that adaptive color transform is enabled for the one or more blocks.

16. The device of claim 14 , wherein the one or more processors are further configured to:

for one or more additional blocks, in response to determining a chroma format for the one or more additional blocks is other than 4:4:4, determine that adaptive color transform is disabled for the one or more additional blocks.

17. The device of claim 13 , wherein to determine for the one or more blocks of the video data that adaptive color transform is enabled, the one or more processors are further configured to parse a syntax element in the video data, wherein the syntax element indicates if adaptive color transform is enabled.

18. The device of claim 17 , wherein the syntax element is signalled in a picture parameter set (PPS).

19. The device of claim 13 , wherein the threshold is zero and wherein a value of the modified quantization parameter is greater than or equal to zero.

20. The device of claim 13 , wherein the modified RGB-color space quantization parameter is less than or equal to 51 plus the offset value.

21. The device of claim 13 , wherein the one or more processors are further configured to receive a flag indicating the offset value is to be added to the quantization parameter.

22. The device of claim 13 , wherein the one or more processors are further configured to:

based on the dequantized transform coefficients, determining a residual block for the video data;

determining a predictive block;

adding the residual block to a predictive block to determine a reconstructed block of video data.

23. The device of claim 13 , where the one or more processors are further confiugred to decode the video data, and wherein the device further comprises a display configured to display the decoded video data.

24. The device of claim 13 , where the one or more processors are further confiugred to encode the video data, and wherein the device further comprises a camera configured to acquire the video data.

25. The device of claim 13 , wherein the device comprises a wireless communication device, the device further comprising a receiver configured to receive the video data.

26. The device of claim 25 , wherein the wireless communication device comprises a telephone handset and wherein the receiver is configured to demodulate, according to a wireless communication standard, a signal comprising the video data.

27. An apparatus for decoding video data, the apparatus comprising:

means for receiving an offset value in the video data;

means for determining for one or more blocks of the video data that adaptive color transform is enabled;

means for determining a red, green, and blue (RGB)-color space quantization parameter for the one or more blocks, wherein determining the RGB-color space quantization parameter comprises adding the offset value to a luma-chroma (YCbCr)-color space quantization parameter;

means for modifying the RGB-color space quantization parameter to determine a modified RGB-color space quantization parameter in response to a value of the RGB-color space quantization parameter being below a threshold; and

means for dequantizing transform coefficients of a block from the one or more blocks based on the modified RGB-color space quantization parameter.

28. A non-transitory computer-readable storage medium storing instructions that when executed by one or more processors cause the one or more processors to:

receive in the video data an offset value;

determine for one or more blocks of the video data that adaptive color transform is enabled;

determine a red, green, and blue (RGB)-color space quantization parameter for the one or more blocks, wherein determining the RGB-color space quantization parameter comprises adding an offset to a luma-chroma (YCbCr)-color space quantization parameter;

modify the RGB-color space quantization parameter to determine a modified RGB-color space quantization parameter in response to a value of the RGB-color space quantization parameter being below a threshold; and

dequantize transform coefficients of a block from the one or more blocks based on the modified RGB-color space quantization parameter.