Combining error diffusion, dithering and over-modulation for smooth multilevel printing

• US 6,271,936 B1
• Filed: 12/11/1998
• Issued: 08/07/2001
• Est. Priority Date: 12/11/1998
• Status: Expired due to Term

• Current Assignee: Eastman Kodak Company, University Of Kentucky Research Foundation (University of Kentucky)
• Original Assignee: Eastman Kodak Company
• Inventors: Spaulding, Kevin E., Yu, Qing
• Primary Examiner(s): ROGERS, SCOTT A
• Application Number:
  US09/210,311
• Time in Patent Office: 970 Days
• Field of Search: 358/1.9, 358/456, 358/458, 358/457, 358/466, 358/298, 382/252, 382/270, 382/237
• US Class Current: 358/3.04
• CPC Class Codes: H04N 1/40087 Multi-toning, i.e. converting a continuous-tone signal for reproduction with more than two discrete brightnesses or optical densities, e.g. dots of grey and black inks on white paper
• CPC Class Codes: H04N 1/4053 with threshold modulated relative to input image data or vice versa

Abstract

A new multitoning technique is proposed that combines error diffusion, blue-noise dithering and over-modulation in an adaptive algorithm to achieve high quality multilevel printing with smooth texture transition. A periodic dither signal is first added to an input digital image wherein the amplitude of the periodic dither signal is a function of the input pixel value for each input pixel. The amplitude of the periodic dither signal is larger for input pixel values near the N output levels, and the amplitude of the periodic dither signal is smaller for input pixel values intermediate to the N output levels to produce a modified input image. Then, a multi-level error diffusion halftoning algorithm is applied to the modified input image wherein the error diffusion halftoning algorithm uses a set of error feedback weights which are adjusted according to the original input pixel value for each input pixel. The sum of the error feedback weights is smaller for input pixel values near the N output levels, and the sum of the error feedback weights is larger for input pixel values intermediate to the N output levels to produce an output multi-level digital image.

Claims

1. A method for multi-level halftoning an input digital image having an x,y array of input pixel values to produce an output image having N output levels comprising the steps of:

a) adding a periodic dither signal to the input digital image wherein the amplitude of the periodic dither signal is a function of the input pixel value for each input pixel, the amplitude of the periodic dither signal being larger for input pixel values near the N output levels, and the amplitude of the periodic dither signal being smaller for input pixel values intermediate to the N output levels to produce a modified input image; and

b) applying a multi-level error diffusion halftoning algorithm to the modified input image wherein the error diffusion halftoning algorithm uses a set of error feedback weights, the error feedback weights being adjusted according to the original input pixel value for each input pixel, such that the sum of the error feedback weights is smaller for input pixel values near the N output levels, and the sum of the error feedback weights is larger for input pixel values intermediate to the N output levels to produce an output multi-level digital image.

2. The method of claim 1, wherein the periodic dither signal is supplied by a dither matrix having dither matrix elements which is modularly addressed by the x,y address of the input pixel, the periodic dither signal having a frequency spectra similar to that produced by the multi-level error-diffusion halftoning algorithm.

3. The method of claim 2, wherein the dither matrix is constructed by minimizing a visual cost function.

4. The method of claim 2, wherein large dither matrix elements of the dither matrix are paired with and are arranged to be adjacent to small dither matrix elements of the dither matrix.

5. The method of claim 1, wherein the amplitude of the periodic dither signal for input pixel values near the N output levels is large enough so that the resulting output multi-level digital image will contain more than two output levels for those input pixel values near the N output levels.

6. The method of claim 1, wherein a first periodic dither signal is added to the input digital image, and an additional periodic dither signal is added to the input digital image for input pixel values near the N output levels.

7. The method of claim 6, wherein the amplitude of the additional periodic dither signal is chosen so that the resulting output multi-level digital image will contain more than two output levels for those input pixel values near the N output levels.

8. The method of claim 6, wherein the additional periodic dither signal is generated by scaling the first periodic dither signal according to the difference between the input pixel value and the nearest output levels.

9. The method of claim 4, wherein the amplitude of the periodic dither signal for input pixel values at the N output levels is chosen to produce multi-level halftone patterns having a similar visibility to multi-level halftone patterns produced for input pixel values intermediate to the N output levels.

10. The method of claim 1, wherein the amplitude of the periodic dither signal is adjusted by scaling dither matrix elements of a fixed dither matrix, the scale factor being a function of the input pixel value.

11. The method of claim 1, wherein the error feedback weights are adjusted by scaling a set of fixed error feedback weights, the scale factor being a function of the input pixel value.

12. The method of claim 1, wherein the sum of the error feedback weights for input pixels values at the N output levels is zero.