Adjusting quantization to preserve non-zero AC coefficients

• US 8,130,828 B2
• Filed: 04/07/2006
• Issued: 03/06/2012
• Est. Priority Date: 04/07/2006
• Status: Active Grant

• Current Assignee: Microsoft Technology Licensing LLC (Microsoft Corporation)
• Original Assignee: Microsoft Corporation
• Inventors: Hsu, Pohsiang, Lin, Chih-Lung, Zuo, Xiping
• Primary Examiner(s): Puente, Eva
• Assistant Examiner(s): MCKIE, GINA M
• Application Number:
  US11/400,741
• Publication Number:
  US 20070237221A1
• Time in Patent Office: 2,160 Days
• Field of Search: None
• US Class Current: 375/240.03
• CPC Class Codes: H04N 19/126 Details of normalisation or weighting functions, e.g. normalisation matrices or variable uniform quantisers
• CPC Class Codes: H04N 19/61 in combination with predictive coding

Abstract

A video encoder identifies one or more AC coefficients of each of plural blocks in the picture. The encoder identifies a threshold quantization step size such that the identified AC coefficient(s) of each of the plural blocks are nonzero after quantization according to the threshold quantization step size. The threshold quantization step size is such that quantization according to the next higher quantization step size would result in at least one of the identified AC coefficient(s) of at least one of the plural blocks being zero. For example, identifying the threshold quantization step size comprises identifying n top AC coefficients in each of four blocks of a macroblock, determining the smallest AC coefficient among the identified n top AC coefficients of the four blocks, and iteratively evaluating the smallest AC coefficient with respect to candidate quantization step sizes until the threshold quantization step size is identified.

Claims

1. In a video encoder, a method comprising:

identifying one or more AC coefficients of each of plural blocks in a video picture;

identifying a threshold quantization step size, such that the identified one or more AC coefficients of each of the plural blocks are nonzero after quantization according to the threshold quantization step size, wherein identifying comprises determining a smallest AC coefficient and iteratively evaluating the smallest AC coefficient with respect to candidate quantization step sizes until the threshold quantization step size is identified and, for each of the plural blocks, the one or more AC coefficients are the one or more highest amplitude AC coefficients of the block, and wherein determining the smallest AC coefficient comprises determining the smallest AC coefficient of the highest amplitude AC coefficients;

compressing at least a portion of the video picture comprising the plural blocks using the threshold quantization step size; and

outputting results of the compression.

2. The method of claim 1 wherein quantization according to a next higher quantization step size relative to the threshold quantization step size results in at least one of the identified one or more AC coefficients of at least one of the plural blocks being zero.

3. The method of claim 1 wherein the video picture is an intra picture.

4. The method of claim 1 wherein the video picture is a predicted picture.

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

6. In a video encoder, a method comprising:

identifying one or more AC coefficients of each of plural blocks in a video picture;

for a macroblock comprising four blocks of the plural blocks, identifying a threshold quantization step size for the macroblock such that, for each the four blocks of the macroblock, the identified one or more AC coefficients of each of the plural blocks are nonzero after quantization according to the threshold quantization step size; and

compressing the macroblock using the threshold quantization step size for the macroblock; and

outputting results of the compression;

wherein the identifying the threshold quantization step size comprises;

determining an AC coefficient count n, where n is an integer greater than or equal to 1;

identifying n highest amplitude AC coefficients in each of the four blocks of the macroblock;

determining the smallest AC coefficient among the identified n highest amplitude AC coefficients of the four blocks.

7. The method of claim 6 wherein the threshold quantization step size for the macroblock is smaller than a default quantization step size for the video picture.

8. The method of claim 6 wherein the quantization uses a uniform quantizer.

9. The method of claim 6 wherein the quantization uses a non-uniform quantizer.

10. The method of claim 6 wherein the AC coefficient count n is 4 or fewer.

11. One or more computer-readable storage having stored thereon computer-executable instructions to cause a computer to perform the method of claim 6.

12. In a video encoder, a method comprising:

identifying a smooth region of a video picture;

for a portion of the smooth region, selecting a threshold quantization step size so as to preserve a gradient across the smooth region by retaining one or more AC coefficients for the portion that would otherwise be quantized to zero according to a next higher quantization step size, wherein the one or more AC coefficients for the portion include the n highest amplitude AC coefficients of each of plural blocks in the portion, where n is an integer greater than or equal to 1;

compressing the portion using the threshold quantization step size; and

outputting results of the compressing;

wherein selecting further comprises determining a smallest AC coefficient of the n highest amplitude AC coefficients and iteratively evaluating the smallest AC coefficient with respect to candidate quantization step sizes until the threshold quantization step size is selected.

13. The method of claim 12 wherein the video picture is an intra picture.

14. The method of claim 12 wherein the video picture is a predicted picture.

15. The method of claim 12 wherein the portion of the smooth region is identified using a texture map generated by the video encoder.

16. The method of claim 12 wherein the smooth region is a gradient slope region.

17. One or more computer-readable storage having stored thereon computer-executable instructions to cause a computer to perform the method of claim 12.