Rehalftoning of gray halftones with optimal screen frequency for robust suppression of moire

• US 20030218779A1
• Filed: 05/22/2002
• Published: 11/27/2003
• Est. Priority Date: 05/22/2002
• Status: Active Grant

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Loce, Robert Paul, Xu, Beilei
• Granted Patent: US 7,133,161 B2
• Time in Patent Office: Days
• US Class Current: 358/3.07
• CPC Class Codes: H04N 1/40075 Descreening, i.e. converting a halftone signal into a corresponding continuous-tone signal; Rescreening, i.e. combined descreening and halftoning

Abstract

Optimal rehalftone screen frequencies are found by searching frequency space for points that are maximally spaced from significant frequency components of an input image halftone screen or screens. Selecting a rehalftone screen having a maximally spaced frequency produces moiré of the highest frequency possible. High frequency moiré are visually unobjectionable. Optimal rehalftone frequencies may be found near the maximally spaced points where system or other constraints limit the usefulness of the maximally spaced points. Rehalftone screen frequencies in the range of about 1.4 to about 1.8 times the fundamental frequency of the input image halftone screen are often optimal. A rehalftone screen frequency of 1.5 times the fundamental frequency of the input image halftone screen is often optimal when the input image is monochrome and uses a dot screen.

Claims

1. A method of rendering an image based on an original halftone image, the method comprising:

determining a frequency of a significant frequency component of the original halftone image;

selecting a rehalftone frequency, based on the frequency of the significant frequency component, the selection being directed toward producing moiré

of unobjectionably high frequency;

rehalftoning the original halftone image at the selected rehalftone frequency; and

,

rendering the image based on the rehalftoning.

2. The method of rendering an image of claim 1 wherein determining a halftone frequency comprises:

segmenting the image; and

,

classifying the image in regard to at least one halftone screen frequency component, based on the segmentation.

3. The method of rendering an image of claim 1 wherein selecting the rehalftone frequency comprises:

searching frequency space by minimizing a function of distances between candidate rehalftone frequency points and frequency components of interest; and

,

selecting a rehalftone frequency from within a region of frequency space near a local minimum of the function of distances.

4. The method of rendering an image of claim 3 wherein selecting a rehalftone frequency from within a region of frequency space near a local minimum further comprises:

selecting a rehalftone frequency that is achievable with an associated rendering device.

5. The method of rendering an image of claim 3 wherein selecting a rehalftone frequency from within a region of frequency space near a local minimum further comprises:

selecting a rehalftone frequency that yields moiré

having a frequency above 50 cpi.

6. The method of rendering an image of claim 1 wherein selecting the rehalftone frequency comprises:

selecting the rehalftone frequency to be in a range of about 1.4 to about 1.8 times the a fundamental halftone frequency associated with the original halftone image.

7. The method of rendering an image of claim 1 wherein selecting the rehalftone frequency comprises:

selecting the rehalftone frequency to be one of 1.5, 1.64 and 2.5 times the a fundamental halftone frequency associated with the original halftone image.

8. The method of rendering an image of claim 1 wherein selecting the rehalftone frequency comprises:

selecting the rehalftone frequency to be in a range of about 2.4 to about 2.8 times the a fundamental halftone frequency associated with the original halftone image.

9. The method of rendering an image of claim 1 wherein rendering the image comprises using a xerographic print engine to print the image.

10. The method of rendering an image of claim 1 wherein rendering the image comprises using one of a lithographic print engine and an ink jet print engine to print the image.

11. A method of rendering an image based on an original halftone image, the method comprising:

determining a halftone frequency of the original halftone image;

selecting a rehalftone frequency to be 1.4 to 1.8 or 2.4 to 2.8 times the fundamental frequency of the original halftone image;

rehalftoning the original halftone image at the selected rehalftone frequency; and

,

rendering the image based on the rehalftoning, thereby ensuring that no visually objectionable moiré

occur.

12. The method of rendering an image of claim 11 wherein determining a halftone frequency comprises:

segmenting the image; and

,

classifying the image in regard to at least one halftone screen frequency component, based on the segmentation.

13. The method of rendering an image of claim 11 wherein selecting a rehalftone frequency comprises:

selecting a rehalftone frequency that yields moiré

having a frequency above 50 cpi.

14. The method of rendering an image of claim 11 wherein rendering the image comprises using a xerographic print engine to print the image.

15. The method of rendering an image of claim 11 wherein rendering the image comprises using one of a lithographic print engine and an ink jet print engine to print the image.

16. A system operative to render an image based on an original halftone image, the system comprising:

a halftone frequency classifier operative to determine a significant frequency component of the original halftone image;

a screen selector operative to select a rehalftone screen having a rehalftone frequency based on the significant frequency component, the selection being directed toward producing moiré

of unobjectionably high frequency;

a rehalftoner operative to rehalftone an image, based on the original halftone image, with the selected rehalftone screen; and

,

a rendering device operative to render the image.

17. The system operative to render an image of claim 16 wherein the halftone frequency classifier comprises:

an image segmenter; and

,

a halftone frequency classifier.

18. The system operative to render an image of claim 17 wherein the halftone frequency classifier comprises:

an operator interface operative to receive information regarding a significant frequency component of the original halftone image from a system operator.

19. The system operative to render an image of claim 16 wherein the halftone frequency classifier is operative to extract halftone frequency information from tag information associated with the image.

20. The system operative to render an image of claim 16 wherein screen selector is operative to select a rehalftone screen having a rehalftone frequency of between approximately 1.4 to 1.8 or of between approximately 2.4 to 2.8 times the frequency of the significant frequency component of the original image.

21. The system operative to render an image of claim 16 wherein the screen selector is operative to select a rehalftone screen having a rehalftone frequency of approximately one of 1.5, 1.64 and 2.5 times the original halftone frequency.

22. The system operative to render an image of claim 16 wherein the screen selector is operative search frequency space by minimizing a function of distances between candidate rehalftone frequency points and frequency components of interest, and select a rehalftone frequency from within a region of frequency space near a local minimum of the function of distances.

23. The system operative to render an image of claim 16 wherein the rendering device comprises a xerographic print engine.

24. The system operative to render an image of claim 16 wherein the rendering device comprises one of a lithographic print engine and an ink jet print engine.

25. The system operative to render an image of claim 16 wherein the screen selector is operative to select a rehalftone screen that yields moiré

having a frequency above 50 cpi.