Automatic multimode continuous halftone line copy reproduction

• US 4,194,221 A
• Filed: 12/26/1978
• Issued: 03/18/1980
• Est. Priority Date: 12/26/1978
• Status: Expired due to Term

• Current Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Original Assignee: Xerox Corporation (Xerox Holdings Corp.)
• Inventors: Stoffel, James C.
• Primary Examiner(s): Britton, Howard W.
• Application Number:
  US05/973,821
• Time in Patent Office: 448 Days
• Field of Search: 358/280, 358/283, 358/298, 358/257, 364/515
• US Class Current: 358/3.08
• CPC Class Codes: H04N 1/40062 Discrimination between different image types, e.g. two-tone, continuous tone
• 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

An image data handling system to automatically detect and segregate from a stream of image pixels high frequency half-tone image input, continuous tone image input, low frequency half-tone image input, and line image input which may be present in the pixel stream. The image pixels are first autocorrelated in accordance with a predetermined algorithm to detect if high frequency half-tone image data is present. Data of this type found is processed by first descreening and then rescreening at a lower frequency to provide binary level pixels. The pixel stream is analyzed for the presence of continuous tone image data. Where found, such data is processed by a template screening process to provide binary level pixels. Remaining pixels comprising low frequency half-tone and line copy image data are thresholded to provide binary level pixels.

Claims

1. The method of processing automatically image pixels which may comprise high or low frequency half-tones, continuous tones, and/or lines to provide binary level output pixels, the steps which comprise:

(a) separating any high frequency half-tone image pixels from said image pixels;

(b) descreening any high frequency half-tone image pixels found to permit rescreening at a lower frequency;

(c) separating any continuous tone image pixels from said image pixels;

(d) screening any image pixels from steps b and c to produce binary level output pixels thereof; and

(e) thresholding the remaining low frequency half-tone and line image pixels to produce binary level output pixels.

2. The method according to claim 1 including the step of autocorrelating a preset block of said image pixels over a predetermined range to identify said high frequency half-tone image pixels.

3. The method according to claim 1 or 2 including the step of filtering said high frequency half-tone image pixels to descreen said high frequency half-tone image pixels.

4. The method according to claim 1 including the steps of:

(a) filtering a preset block of image pixels to determine the average gray value of said block of image pixels;

(b) comparing the gray value from step a with a preset gray value; and

(c) selecting a predetermined binary output signal pattern from various predetermined binary output signal patterns stored in memory in response to a first preset comparison.

5. The method according to claim 4, including the step of thresholding said preset block of image pixels to provide said binary output signals on a second preset comparison.

6. The method of automatically processing unknown image pixels, the steps comprising:

(a) identifying high frequency half-tone image pixels in said image pixels;

(b) identifying continuous tone image pixels in said image data;

(c) identifying low frequency half-tone and line pixels in said image pixels;

(d) descreening any image pixels found in step a;

(e) screening any image pixels produced by steps b or d; and

(f) thresholding any image pixels found in step c.

7. In the method of processing image data in the form of pixels to provide binary level output pixels, the steps which comprise:

(a) identifying continuous tone or high frequency half-tone pixels in said image data;

(b) descreening said high frequency half-tone pixels to provide descreened high frequency half-tone pixels;

(c) screening said continuous tone and said descreened high frequency half-tone pixels to provide said binary level output pixels; and

(d) thresholding the remaining pixels in said image data to convert said remaining pixels to binary level output pixels.

8. In an apparatus for processing a stream of image data, said image data being in the form of pixels whose signal levels represent the intensity of the discrete image areas represented by each pixel, the combination comprising:

(a) image data identifying means for identifying high frequency half-tone and continuous tone pixels in said stream of image data;

(b) descreen means for descreening any high frequency half-tone pixels found;

(c) screen means for screening any continuous tone and descreened high frequency half-tone pixels found; and

(d) threshold means for thresholding pixels remaining in said image data stream.

9. The apparatus according to claim 8 in which said image data identifying means includes:

(a) autocorrelator means for identifying any high frequency half-tone pixels in said stream of image data; and

(b) discriminator means for identifying any continuous tone pixels in said stream of image data.

10. The apparatus according to claim 8 in which said descreen means includes filter means for low pass filtering the signal levels of said high frequency half-tone pixels with the signal levels of a preset number of neighboring pixels to provide a descreened high frequency half-tone pixel output.

11. The apparatus according to claim 8 or 9 in which said descreen means includes filter means for averaging the signal levels of said image data pixels with the signal levels of a preset number of adjoining pixels to provide a signal reflecting continuous tone pixels.

12. The apparatus according to claim 8 in which said threshold means includes a comparator gate for comparing the signal levels of said remaining pixels with a preset reference signal level.

13. The apparatus according to claim 8 in which said screen means includes means for comparing said continuous tone and descreened high frequency halftone pixels with predetermined pixel threshold values.

15. The apparatus according to claim 8 including means for selectively bypassing said descreen means.

16. The apparatus according to claim 15 including an output bus for processed data, and control means for coupling the output of one of said screen means and threshold means with said output bus, said image data identifying means including means responsive to detection of high frequency halftone or continuous tone pixels for actuating said control means to couple said screen means output with said output bus, said last mentioned means actuating said control means to couple said threshold means output with said output bus when pixels other than said high frequency halftone or continuous tone pixels are detected.

14. In an apparatus for processing image data, said image data being in the form of a stream of image pixels whose signal levels represent the intensity of the discrete image areas represented by each pixel, the combination comprising:

(a) means for producing a first control signal in response to the presence of high frequency half-tone image pixels in said pixel stream, a second control signal in response to the presence of continuous tone image pixels in said pixel stream, and a third control signal in response to the presence of low frequency half-tone and line copy image pixels in said pixel stream;

(b) first image data processing means for processing high frequency half-tone image pixels;

(c) second image data processing means for processing continuous tone image pixels;

(d) third image data processing means for processing low frequency half-tone and line copy image pixels; and

(e) control means responsive to said first, second and third control signals for actuating said first, second and third image data processing means respectively whereof to process said image data regardless of the pixel content of said image data.

17. In an apparatus for processing image pixels which may represent continuous tone images, high frequency halftone images, low frequency halftone images, and/or line images, the combination of:

(a) descreening means for descreening high frequency halftone image pixels;

(b) screening means coupled to said descreening means output for rescreening descreened high frequency halftone image pixels output from said descreening means;

(c) thresholding means for thresholding low frequency halftone and line image pixels; and

(d) bypass means effective when actuated to bypass said descreening means to permit continuous tone image pixels to be routed directly to said screening means.

18. The apparatus according to claim 17 including:

(a) an output for said image pixels;

(b) selector means for selectively coupling one of said thresholding means and said screening means with said output; and

(c) control means for actuating said selector means to couple said thresholding means with said output when said image pixels comprise low frequency halftone image pixels or line image pixels, and to couple said screening means with said output when said image pixels comprise high frequency halftone image pixels or continuous tone image pixels.