De-screening halftones using sigma filters

• US 6,947,178 B2
• Filed: 02/26/2001
• Issued: 09/20/2005
• Est. Priority Date: 02/26/2001
• Status: Expired due to Fees

• Current Assignee: Ricoh Company Limited
• Original Assignee: International Business Machines Corporation
• Inventors: Rao, Ravishankar, Kuo, Chungkui, Thompson, Gerhard R.
• Primary Examiner(s): Lee, Thomas D.
• Application Number:
  US09/791,653
• Publication Number:
  US 20020154339A1
• Time in Patent Office: 1,667 Days
• Field of Search: 382/260, 382/261, 382/262, 382/264, 358/3.08
• US Class Current: 358/3.08
• CPC Class Codes: G06T 2207/10008 from scanner, fax or copier
• CPC Class Codes: G06T 2207/10024 Color image
• CPC Class Codes: G06T 2207/20192 Edge enhancement; Edge preservation
• CPC Class Codes: G06T 2207/30176 Document
• CPC Class Codes: G06T 5/20 using local operators
• CPC Class Codes: G06T 5/70 Denoising; Smoothing
• 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

A digital imaging system and method uses a two-stage sigma filter to de-screen color images. This filter does not assume any a priori knowledge about the screening process using to produce the halftone image. The two-stage sigma filter may therefore be used to convert color halftone images into continuous-tone images irrespective of the screening process used. The two-stage sigma filter may be constructed, or emulated in software, using an O (N) algorithm which performs smoothing and preserves edge information simultaneously in the Red/Green/Blue color space. This system and method outperforms conventional approaches which, for example, use a Gaussian blur, because it satisfies the dual criteria of completely eliminating halftone screens while preserving edge information. When combined with halftone segmentation techniques, a complete document processing algorithm for gray-scale and color documents is created.

Claims

1. A method for converting halftone images into continuous-tone images, comprising:

obtaining a halftone image;

defining a two-stage filter, wherein a first stage is a smoothing filter and a second stage is a sigma filter connected to the smoothing filter;

defining parameters for the smoothing filter and the sigma filter, which includes an intensity difference delta;

inputting the halftone image into a smoothing filter to derive an intermediate image;

sweeping a window across said halftone image and the corresponding intermediate image, and within a window, selecting those pixels that have an intermediate gray value within said intensity difference delta of a center window pixel;

applying a sigma filter to pixels from said halftone image that correspond to pixels selected in said sweeping step; and

outputting from said sigma filter a continuous-tone image which is piecewise-smooth and has preserved edges.

2. The method of claim 1, wherein said smoothing filter is a low-pass filter.

3. The method of claim 2, wherein said low-pass filter includes a plurality of Gaussian low-pass filters.

4. The method of claim 1, further comprising:

printing said continuous-tone image.

5. The method of claim 1, wherein said halftone image is a color image.

6. The method of claim 1, wherein said halftone image is a gray-scale image.

7. The method of claim 1, wherein said smoothing filter is an isotropic low-pass filter.

8. The method of claim 1, wherein said intensity difference delta is determined by computing a set of edge pixels from said intermediate image and selecting those pixels that fall within a given spatial distance from said set of edge pixels to form a second set, and computing a standard deviation of the gray values of the pixels in the second set, and setting delta to be a multiple of said standard deviation.

9. The method of claim 8 wherein the multiple is 2.