Method and system for improving compressed image chroma information

1. A method for decoding a color video image at a decoder using quantization parameters, the color video image comprising a plurality of macroblocks, the method comprising:

at an encoder:

utilizing a variable quantization step size and a quantization parameter (QP) to represent a size of a step where an increase in the QP corresponds to a larger quantization step size;

accessing, for a first macroblock of the color video image, a first QP value;

generating, for the first macroblock, a first bias value, wherein the first bias value is specific to at least one of a first chroma channel and a second chroma channel of the color video image for the first macroblock and is not used in dequantizing a luminance channel of the color video image for the first macroblock;

encoding the first macroblock;

transmitting, from the encoder to a decoder, the first QP value for the first macroblock and the first bias value for the first macroblock;

at the decoder:

receiving, from the encoder, the first QP value for the first macroblock and the first bias value for the first macroblock;

decoding the first macroblock of the color video image, the decoding comprising:

dequantizing the luminance channel of the first macroblock utilizing the first QP value without using the first bias value; and

dequantizing at least one of the first chroma channel and the second chroma channel of the color video image for said first macroblock utilizing a second QP value derived from the first QP value and the first bias value, and

wherein the first chroma channel is a first color difference channel,

wherein the second chroma channel is a second color difference channel, and

wherein said second QP value is derived by at least applying the first bias value to the first QP value to provide a first intermediate result.

2. The method of claim 1 , wherein said second QP value is also determined by comparing the first intermediate result to a predetermined value.

3. The method of claim 2 , wherein said second QP value is also determined by, under a first condition that the first intermediate result falls below the predetermined value, setting the first intermediate result equal to the predetermined value.

4. The method of claim 1 , wherein receiving, from the encoder, the QP value for the first macroblock and the first bias value for the first macroblock comprises receiving said first QP value and said first bias value as part of a compressed bit stream.

5. The method of claim 1 , further comprising determining an upsizing resolution factor for the first chroma channel of the first macroblock.

6. The method of claim 1 , wherein the first macroblock is an MPEG-2 macroblock or an MPEG-4 macroblock.

7. The method of claim 1 , wherein the first macroblock is a 16×16 pixel grouping.

8. A method for decoding a color video image at a decoder using quantization parameters, the color video image comprising a plurality of blocks, the method comprising:

receiving, from an encoder, a first QP value for a first block of a color video image and a first bias value for the first block, wherein the first bias value is specific to at least one of a first chroma channel and a second chroma channel of the color video image for the first block and is not used in dequantizing a luminance channel of the color video image for the first block;

decoding the first macroblock of the color video image, the decoding comprising:

dequantizing the luminance channel of the first block utilizing the first QP value without using the first bias value; and

dequantizing at least one of the first chroma channel and the second chroma channel of the color video image for said first block utilizing a second QP value derived from the first QP value and the first bias value, and

wherein the first chroma channel is a first color difference channel,

wherein the second chroma channel is a second color difference channel, and

wherein said second QP value is derived by application of at least the following operations:

(a) applying the first bias value to the first QP value to provide a first intermediate result;

(b) comparing the first intermediate result to a predetermined value; and

(c) under a first condition that the first intermediate result falls below the predetermined value, setting the first intermediate result equal to the predetermined value.

9. The method of claim 8 , further comprising dequantizing the first chroma channel or the second chroma channel of the color video image for said first block utilizing a third QP value, wherein said third QP value is determined by application of at least the following operations:

(a) applying a second bias value to the second QP value to provide a second intermediate result;

(b) comparing the second intermediate result to the predetermined value; and

(c) under a second condition that the second intermediate result falls below the predetermined value, setting the second intermediate result equal to the predetermined value.

10. The method of claim 9 , wherein applying the second bias value to the second QP value to provide the second intermediate result comprises subtracting a constant value from the second QP value.

11. The method of claim 8 , wherein the second QP value is further determined by applying a non-linear look-up table to the first intermediate result.

12. The method of claim 11 , further comprising utilizing a spatial resolution of the luminance channel that is greater than a spatial resolution of the first chroma channel and the second chroma channel.

13. The method of claim 12 , wherein the second QP value is less than the first QP value.

14. The method of claim 8 , further comprising determining an upsizing resolution factor for the first chroma channel of the first block.

15. The method of claim 8 , wherein the first block is a 16×16 block.

16. A method for decoding a color video image at a decoder using quantization parameters, the color video image comprising a plurality of macroblocks, the method comprising:

at an encoder:

utilizing a variable quantization step size and a quantization parameter (QP) to represent a size of a step where an increase in the QP corresponds to a larger quantization step size;

accessing, for a first macroblock of the color video image, a first QP value;

generating, for the first macroblock, a first bias value, wherein the first bias value is specific to at least one of a first chroma channel and a second chroma channel of the color video image for the first macroblock and is not used in dequantizing a luminance channel of the color video image for the first macroblock;

encoding the first macroblock;

transmitting the first QP value for the first macroblock and the first bias value for the first macroblock from the encoder to a decoder;

at the decoder:

receiving, from the encoder, the first QP value for the first macroblock and the first bias value for the first macroblock;

decoding the first macroblock, said decoding comprising:

dequantizing the luminance channel of the first macroblock utilizing the first QP value without using the first bias value; and

dequantizing at least one chroma channel of the color video image for said first macroblock utilizing a second QP value derived from the first QP value and the first bias value, and

wherein the at least one chroma channel comprises at least one color difference channel;

and

wherein said second QP value is derived by applying the first bias value to the first QP value to produce a first intermediate result and then applying a limit function to the first intermediate result.

17. The method of claim 16 , further comprising utilizing a spatial resolution of the luminance channel that is greater than a spatial resolution of the at least one chroma channel.

18. The method of claim 17 , wherein utilizing the second QP value further comprises applying a non-linear look-up table to the first intermediate result.

19. The method of claim 18 , wherein the second QP value is less than the first QP value.

20. The method of claim 16 , further comprising determining an upsizing resolution factor for the first chroma channel of the first macroblock.

21. A method for decoding a color video image at a decoder using quantization parameters, the color video image comprising a plurality of macroblocks, the method comprising:

at an encoder:

utilizing a variable quantization step size and a quantization parameter (QP) to represent a size of a step where an increase in the QP corresponds to a larger quantization step size;

accessing, for a first macroblock of the color video image, a first QP value;

generating, for the first macroblock, a first bias value, wherein the first bias value is a difference between the first QP value used to dequantize a luminance channel of the first macroblock and a second QP value used to dequantize at least one of a first chroma channel and a second chroma channel of the first macroblock;

encoding the first macroblock;

transmitting, from the encoder to a decoder, the first QP value for the first macroblock and the first bias value for the first macroblock;

at the decoder:

receiving, from the encoder, the first QP value for the first macroblock and the first bias value for the first macroblock;

decoding the first macroblock of the color video image, the decoding comprising:

dequantizing the luminance channel of the first macroblock utilizing the first QP value without using the first bias value; and

dequantizing at least one of the first chroma channel and the second chroma channel of the color video image for said first macroblock utilizing the second QP value derived from the first QP value and the first bias value, and

wherein the first chroma channel is a first color difference channel,

wherein the second chroma channel is a second color difference channel, and

wherein said second QP value is derived by at least applying the first bias value to the first QP value to provide a first intermediate result.