Multi-level halftoning providing improved texture uniformity

• US 20050195438A1
• Filed: 03/05/2004
• Published: 09/08/2005
• Est. Priority Date: 03/05/2004
• Status: Active Grant

• Current Assignee: Eastman Kodak Company
• Original Assignee: Eastman Kodak Company
• Inventors: Spaulding, Kevin E., Couwenhoven, Douglas W., Braun, Gustav J.
• Granted Patent: US 7,369,276 B2
• Time in Patent Office: Days
• US Class Current: 358/3.05
• CPC Class Codes: H04N 1/4052 by error diffusion, i.e. transferring the binarising error to neighbouring dot decisions

Abstract

A method for multi-level halftoning an input digital image to form an output digital image which includes a multi-level error diffusion halftoning process incorporating periodic dither signals whose amplitudes are adjusted as a function of the error-diffusion texture characteristics.

Claims

1. A method for multi-level halftoning an input digital image having input pixels with corresponding input pixel values and N_i input levels to form an output digital image having output pixels with corresponding output pixel values and N_o output levels, where 2≦

N_o≦

N_i, comprising;

a) determining a periodic dither signal by modularly addressing a dither signal array with the row and column address of an input pixel in the input digital image, wherein the dither signal array is designed to produce periodic dither signals having low visibility to a human observer;

b) adjusting the amplitude of the periodic dither signal in response to the input pixel value of the input pixel wherein the amplitude adjustment is a function of the texture characteristics that would result from the application of a multi-level error diffusion halftoning process to a uniform image region having the particular input pixel value, the amplitude of the adjusted periodic dither signal being larger for input pixel values that would produce visually objectionable texture characteristics than for input pixel values that would produce visually pleasing texture characteristics;

c) combining the adjusted periodic dither signal with the input pixel value to produce an adjusted pixel value;

d) applying the multi-level error diffusion halftoning process to the adjusted pixel value to determine the output pixel value; and

e) repeating steps a)-d) for multiple input pixels of the input digital image to thereby provide the output digital image.

2. The method according to claim 1 wherein the dither signal array is determined by reducing a visual cost function.

3. The method according to claim 1 wherein the dither signal array is designed to produce periodic dither signals having spatial frequency characteristics similar to those produced by the multi-level error diffusion halftoning process.

4. The method according to claim 1 wherein the dither signal array is a dither matrix storing periodic dither values.

5. The method according to claim 4 wherein large dither matrix elements are paired with and are arranged to be adjacent to small dither matrix elements in the dither matrix.

6. The method according to claim 1 wherein the dither signal array is an array of dither look-up tables.

7. The method according to claim 1 wherein the periodic dither signal is added to the input pixel value to form the adjusted pixel value.

8. The method according to claim 1 wherein the multi-level error diffusion halftoning process is a binary error diffusion halftoning process producing output pixel values with two output levels.

9. The method according to claim 1 wherein the texture characteristics correspond to a perceived granularity value for the halftoning pattern produced by multi-level error diffusion halftoning process.

10. The method according to claim 1 wherein the texture characteristics correspond to a texture visibility value for the halftoning pattern produced by multi-level error diffusion halftoning process.

11. The method according to claim 1 wherein the amplitude of the adjusted periodic dither signal has a local maximum for one or more input pixel values that are intermediate to the output levels.

12. The method according to claim 1 wherein the amplitude of the adjusted periodic dither signal is adjusted to produce an output digital image having more uniform texture characteristics.

13. The method according to claim 1 wherein the periodic dither signal has a mean value of zero.

14. The method according to claim 1 wherein the distribution of periodic dither signal values is modified as a function of the input pixel value so that the multi-level halftoning process is mean preserving.

15. The method according to claim 1 wherein the multi-level error diffusion halftoning process in step d) includes:

i) applying a quantization operation to the adjusted pixel value to determine the output pixel value;

ii) determining an error signal by computing a difference between the output pixel value and the adjusted pixel value; and

iii) scaling the error signal by a set of error feedback weights and adding the scaled error weights to the input pixel values for nearby input pixels that have yet to be processed.

16. The method according to claim 1 wherein the multi-level error diffusion halftoning process in step d) includes producing an error feedback signal, and wherein the amplitude of error feedback signal is adjusted responsive to the input pixel value of the input pixel.

17. The method according to claim 16 wherein the amplitude of the error feedback signal is adjusted to be smaller for input pixel values that would produce visually objectionable texture characteristics when the multi-level error diffusion halftoning process is applied to a uniform image region having the particular input pixel value, and larger for input pixel values that would produce visually pleasing texture characteristics when the multi-level error diffusion halftoning process is applied to a uniform image region having the particular input pixel value.

18. The method according to claim 16 wherein the amplitude of the adjusted error feedback signal is smaller for input pixel values near the No output levels than for input pixel values intermediate to the No output levels.

19. The method according to claim 1 wherein the input digital image is a color input digital image having a plurality of color channels and the multi-level error diffusion halftoning process is a color error diffusion process.

20. The method according to claim 19 wherein different dither signal arrays are used for each color channel of the input digital image.

21. The method according to claim 20 wherein the dither signal arrays used for each color channels are jointly optimized.

22. The method according to claim 19 wherein the color multi-level error diffusion process is a vector error diffusion process.

23. A method for multi-level halftoning a color input digital image having a plurality of color channels where the method according to claim 1 is applied to at least one color channel of the color input digital image.

24. A computer program product, comprising a computer readable storage medium for practicing the method according to claim 1.