Replacement halftoning

• US 6,851,783 B1
• Filed: 03/31/2003
• Issued: 02/08/2005
• Est. Priority Date: 03/31/2003
• Status: Expired due to Fees

• Current Assignee: Ricoh Company Limited
• Original Assignee: Ricoh Company Limited
• Inventors: Takahashi, Hiroshi, Gupta, Maya Rani
• Primary Examiner(s): Meier, Stephen D.
• Assistant Examiner(s): DUDDING, ALFRED E
• Application Number:
  US10/404,273
• Time in Patent Office: 680 Days
• Field of Search: 347/15, 347/43, 358/3.03, 358/3.06, 358/3.09, 358/3.13, 358/2.1, 358/535
• US Class Current: 347/15
• CPC Class Codes: H04N 1/52 Circuits or arrangements for halftone screening

Abstract

A method and an apparatus to perform replacement halftoning is disclosed. In one embodiment, the method comprises dithering multiple color channels of a pixel of an image to generate a plurality of halftone outputs. Each of the halftones corresponds to a distinct color channel of the pixel. The method further includes creating a virtual color channel from two or more color channels of the plurality of color channels and applying error diffusion to the virtual color channel to generate an error diffusion halftone outputs, and then replacing of the dither output error diffusion halftone outputs.

Claims

1. A method comprising:

applying a first halftoning technique to a plurality of color channels associated with a first pixel to generate a first plurality of halftone outputs, each of the plurality of halftone outputs corresponding to a color channel;

creating a virtual color channel from two or more of the plurality of color channels;

applying a second halftoning technique, different from the first halftoning technique, to the virtual color channel to generate a replacement halftone output; and

replacing at least one of the first plurality of halftone outputs for the color channels with a replacement halftone output for the virtual color channel.

2. A method comprising:

dithering a plurality of color channels associated with a first pixel to generate a plurality of dither halftone outputs, each of the plurality of dither halftone outputs corresponding to a color channel;

creating a virtual color channel from two or more of the plurality of color channels;

applying error diffusion to the virtual color channel to generate an error diffusion halftone output, and

replacing at least one dither halftone output for the color channels with the error diffusion halftone output for the virtual color channel.

3. The method of claim 2 further comprising selecting a color channel out of the plurality of color channel, and wherein replacing the at least one dither halftone output comprises replacing a dithered halftone for a selected color channel with the error diffusion halftone output for the virtual color channel if the pixel is at an edge.

4. The method of claim 3 further wherein creating the virtual color channel comprises selecting one of the color channels with the maximum value of C, M, Y or K color channels.

5. The method referred in claim 4 wherein selecting one of the color channels occurs if the pixel is at an edge.

6. The method of claim 3 further wherein creating the virtual color channel comprises selecting a previously selected virtual color channel used for the previous pixel.

7. The method defined in claim 6 wherein selecting the same virtual color channel used for the previous pixel occurs if the pixel is not at an edge.

8. The method of claim 3 further comprising detecting whether the pixel is at the edge.

9. The method of claim 8, wherein detecting whether the first pixel is at the edge comprises comparing the pixel with a second pixel above the first pixel and a third pixel to the left of the pixel.

10. The method of claim 2 further comprising comparing a selected color value of the virtual color channel with a predetermined threshold value, and wherein replacing the at least one dither halftone output with the error diffusion halftone output occurs if the selected color value of the virtual color channel exceeds the pre-determined threshold value.

11. The method of claim 2 further comprising:

selecting a color channel out of the plurality of color channels; and

determining whether the first pixel and a second pixel located above the first pixel and a third pixel to the left of the first pixel have substantially the same value and replacing the at least one dither halftone output with the one error diffusion output if the first, second, and third pixel values are substantially the same.

12. The method of claim 2, wherein creating the virtual color channel comprises choosing a color channel having a maximum value among the plurality of color channels.

13. The method of claim 2, wherein creating the virtual color channel comprises averaging values of the plurality of color channels.

14. The method of claim 2, wherein the plurality of color channels includes a C channel, a M channel, a Y channel, and a K channel, and the virtual color channel is created from a group of color channels consisting of the C channel, the M channel, and the K channel.

15. An article of manufacture having one or more machine-readable media storing instructions which, when executed by a machine, cause the machine to:

dither a plurality of color channels associated with a first pixel to generate a plurality of dither halftone outputs, each of the plurality of halftone outputs corresponding to a color channel;

create a virtual color channel from two or more of the plurality of color channels;

apply error diffusion to the virtual color channel to generate an error diffusion halftone output; and

replace at least one dither halftone output for the color channels with one error diffusion halftone output for the virtual color channel.

16. The machine-readable medium of claim 15, further comprising instructions which when executed cause the system to create a virtual color channel by selecting the one color channel of the plurality of color channels having the maximum value if the pixel is at an edge or the same virtual color channel used for the previous pixel if the first pixel is not at an edge.

17. The machine-readable medium of claim 16, further comprising instructions which when executed cause the system to detect whether the pixel is at the edge.

18. The machine readable-medium of claim 17, wherein the instructions to detect whether the first pixel is at the edge comprise instructions to compare the pixel with a second pixel above the first pixel and a third pixel to the left of the pixel.

19. The machine readable-medium of claim 15, further comprising instructions which when executed cause the system to compare a selected color value of the virtual color channel with a predetermined threshold value, and wherein replacing the at least one dither halftone output with the error diffusion halftone output occurs if the selected color value of the virtual color channel exceeds the pre-determined threshold value.

20. The machine-readable medium of claim 15, further comprising instructions which when executed causes the system to:

select a color channel out of the plurality of color channels; and

determine whether the first-pixel and a second pixel located above the first pixel and a third pixel to the left of the first pixel to have substantially the same value and replacing the at least one dither halftone output with the one error diffusion output if the first, second, and third pixel values are substantially the same.

21. The machine-readable medium of claim 20, wherein instructions to create the virtual color channel comprise instructions to set the virtual color channel to the maximum value of the plurality of color channels at each spatial location.

22. The machine-readable medium of claim 15, wherein instructions to create the virtual color channel comprise instructions to set the virtual color channel to the average value of the plurality of color channels.

23. The machine-readable medium of claim 15, wherein the plurality of color channels includes a C channel, a M channel, a Y channel, and a K channel, and the virtual color channel is created from a group of color channels consisting of the C channel, the M channel, and the K channel.

24. An apparatus comprising:

a first circuitry to dither a plurality of color channels associated with a first pixel to generate a plurality of dither halftone outputs, each of the plurality of halftone outputs corresponding to a color channel;

a second circuitry to create a virtual color channel from two or more of the plurality of color channels;

a third circuitry to apply error diffusion to the virtual color channel to generate an error diffusion halftone output; and

a fourth circuitry to replace at least one dither halftone output for the color channels with one error diffusion halftone output for the virtual color channel.

25. The apparatus of claim 24 further comprising a fourth circuitry to select a color channel out of replacing a dither halftone output for the color channels with one diffusion halftone output for the virtual color channel.

26. The apparatus of claim 25 further comprising an edge detector to detect whether the pixel is at an edge, wherein the fourth circuitry replaces the dithered halftone of the selected color channel with the error diffusion halftone value of the virtual color channel if the pixel is at the edge.

27. A method comprising:

receiving a pixel of an image, wherein the pixel is represented with a plurality of color channels;

performing error diffusion on a non-color separation channel to provide sharp edges for a printed version of the image; and

dithering remaining channels of the plurality of color channels.