Quantization adjustments for DC shift artifacts

1. In a video encoder, a method comprising:

computing plural transform coefficients for each of plural blocks in a video picture;

identifying one or more portions of the video picture that contain DC shift blocks by identifying one or more gradient slope regions in the video picture and analyzing quantization effects on DC coefficients for one or more of the plural blocks in the one or more gradient slope regions;

adjusting quantization for the video picture to reduce contouring artifacts when the video picture is reconstructed; and

outputting encoded data for the video picture, wherein the identifying the one or more portions of the video picture that contain DC shift blocks includes checking a predetermined list of DC coefficient values exhibiting DC shift artifacts at a given quantization step size.

2. The method of claim 1 wherein the video encoder analyzes a texture map to identify the one or more gradient slope regions in the video picture.

3. The method of claim 1 wherein the adjusting quantization for the video picture comprises reducing a quantization step size for one or more macroblocks containing one or more of the DC shift blocks.

4. The method of claim 1 wherein the quantization comprises bi-level differential quantization.

5. The method of claim 1 wherein the quantization comprises multi-level differential quantization.

6. The method of claim 1 wherein the identifying the one or more portions of the video picture that contain DC shift blocks includes identifying the DC shift blocks using DC coefficients of the plural blocks or mean sample values of the plural blocks, and wherein the quantization uses a uniform quantizer or a non-uniform quantizer.

7. The method of claim 1 wherein the identifying the one or more portions of the video picture that contain DC shift blocks includes:

identifying the DC shift blocks;

identifying one or more gradient slope regions; and

for one of the DC shift blocks in one of the one or more gradient slope regions, checking if any neighbor block both is smooth and has a lower or higher DC coefficient value.

8. The method of claim 1 wherein the adjusting quantization further comprises increasing quantization for a high-texture portion of the video picture.

9. One or more computer-readable memory media having stored thereon computer executable instructions to cause a computer to perform the method of claim 1 .

10. The method of claim 1 , wherein the identifying one or more gradient slope regions includes using a gradient slope mask that indicates gradient slope decisions for regions in the video picture.

11. In a video encoder, a method comprising:

computing plural transform coefficients for each of plural blocks in a video picture;

identifying one or more portions of the video picture that contain DC shift blocks by identifying one or more gradient slope regions in the video picture and analyzing quantization effects on DC coefficients for one or more of the plural blocks in the one or more gradient slope regions;

adjusting quantization for the video picture to reduce contouring artifacts when the video picture is reconstructed; and

outputting encoded data for the video picture, wherein the identifying the one or more portions of the video picture that contain DC shift blocks includes:

identifying the DC shift blocks;

identifying one or more gradient slope regions; and

for one of the DC shift blocks in one of the one or more gradient slope regions, checking if any neighbor block both is smooth and has a lower or higher DC coefficient value.

12. The method of claim 11 , wherein the video encoder analyzes a texture map to identify the one or more gradient slope regions in the video picture.

13. The method of claim 11 , wherein the adjusting quantization for the video picture comprises reducing a quantization step size for one or more macroblocks containing one or more of the DC shift blocks.

14. The method of claim 11 , wherein the quantization comprises bi-level differential quantization.

15. The method of claim 11 , wherein the quantization comprises multi-level differential quantization.

16. The method of claim 11 , wherein the identifying the one or more portions of the video picture that contain DC shift blocks includes identifying the DC shift blocks using DC coefficients of the plural blocks or mean sample values of the plural blocks, and wherein the quantization uses a uniform quantizer or a non-uniform quantizer.

17. The method of claim 11 , wherein the identifying one or more gradient slope regions includes using a gradient slope mask that indicates gradient slope decisions for regions in the video picture.

18. A computer-readable memory device for executing a method, the method comprising:

computing plural transform coefficients for each of plural blocks in a video picture;

identifying one or more portions of the video picture that contain DC shift blocks by identifying one or more gradient slope regions in the video picture and analyzing quantization effects on DC coefficients for one or more of the plural blocks in the one or more gradient slope regions;

adjusting quantization for the video picture to reduce contouring artifacts when the video picture is reconstructed; and

outputting encoded data for the video picture, wherein the identifying the one or more portions of the video picture that contain DC shift blocks includes checking a predetermined list of DC coefficient values exhibiting DC shift artifacts at a given quantization step size.

19. The computer-readable memory device of claim 18 , further including analyzing a texture map to identify the one or more gradient slope regions in the video picture.

20. The computer-readable memory device of claim 18 , wherein the adjusting quantization for the video picture comprises reducing a quantization step size for one or more macroblocks containing one or more of the DC shift blocks.