System and method for enhancement of document images

• US 20040042680A1
• Filed: 03/24/2003
• Published: 03/04/2004
• Est. Priority Date: 08/29/2002
• Status: Active Grant

• Current Assignee: Palo Alto Research Center, Inc. (Xerox Holdings Corp.)
• Original Assignee: Palo Alto Research Center, Inc. (Xerox Holdings Corp.)
• Inventors: Saund, Eric
• Granted Patent: US 7,177,483 B2
• Time in Patent Office: Days
• US Class Current: 382/274
• CPC Class Codes: G06T 2207/10008 from scanner, fax or copier
• CPC Class Codes: G06T 2207/10024 Color image
• CPC Class Codes: G06T 2207/20036 Morphological image processing
• CPC Class Codes: G06T 2207/30176 Document
• CPC Class Codes: G06T 5/94 based on local image properties, e.g. for local contrast enhancement
• CPC Class Codes: G06T 7/194 involving foreground-background segmentation
• CPC Class Codes: H04N 1/4074 using histograms
• CPC Class Codes: H04N 1/6027 Correction or control of colour gradation or colour contrast H04N1/6058 takes precedence

Abstract

A method for enhancing electronic images of a document, in which the electronic images contain a background with foreground markings, includes receiving an electronic image containing pixels and assigning an initial provisional foreground/background status to the pixels. The initial foreground/background estimate is revised and a determination is made as to whether the revised foreground/background estimate is to be the final status assignment for the pixels of the image. If a final pixel status assignment has not been achieved, the estimate is again revised until a final foreground/background status is reached for the image pixels prior to creation of an output image.

Claims

1. A method for enhancement of electronic images of a document, the electronic images having a background with foreground markings placed thereon, the method comprising:

receiving an electronic image comprised of pixels;

assigning initial provisional foreground/background status to said pixels to form an initial foreground/background estimate;

revising said initial foreground/background estimate to form a revised foreground/background estimate;

determining whether said revised foreground/background estimate is the final foreground/background assignment for said pixels;

repeating revising said revised foreground/background estimate until said final foreground/background assignment for said pixels is achieved; and

providing said final foreground/background assignment for said pixels.

2. The method for enhancement of electronic images of a document according to claim 1, wherein assigning said initial foreground/background estimate comprises:

quantizing image colors to yield a color table for said pixels wherein said color table is comprised of a plurality of color table bins;

identifying not less than one of said color table bins most likely to represent white; and

assigning provisional foreground status to a first plurality of said pixels based on an HSV comparison.

3. The method for enhancement of electronic images of a document according to claim 1, wherein revising said initial foreground/background estimate comprises:

assigning provisional foreground status to a second plurality of said pixels based on high-pass filtering;

assigning provisional background status to pixels not assigned provisional foreground status; and

estimating a white background pixel array.

4. The method for enhancement of electronic images of a document according to claim 2, wherein quantizing image colors comprises utilizing means for pestering images.

5. The method for enhancement of electronic images of a document according to claim 2, wherein identifying not less than one of said color table bins most likely to represent white is based on RGB value and the number of pixels having that value.

6. The method for enhancement of electronic images of a document according to claim 2, wherein assigning provisional foreground status to a first plurality of said pixels based on an HSV comparison comprises identifying image pixels having HSV values dissimilar to the not less than one color table bin estimated to represent white.

7. The method for enhancement of electronic images of a document according to claim 3, wherein assigning provisional foreground status to a second plurality of said pixels based on high-pass filtering comprises determining locally darker pixels.

8. The method for enhancement of electronic images of a document according to claim 3, wherein estimating a white background pixel array comprises smoothing and interpolating provisional background pixel RGB values.

9. The method for enhancement of electronic images of a document according to claim 1, wherein providing final foreground/background status for said pixels comprises:

merging foreground pixel assignments produced by high-pass filtering with foreground pixel assignments resulting from an HSV comparison; and

labeling as background pixels all pixels not labeled as foreground pixels.

10. The method for enhancement of electronic images of a document according to claim 1, further comprising normalizing foreground pixel RGB values.

11. The method for enhancement of electronic images of a document according to claim 10, wherein normalizing foreground pixel RGB values comprises dividing each said pixel'"'"'s original RGB value by the RGB value of the estimated white background at said pixel'"'"'s location.

12. The method for enhancement of electronic images of a document according to claim 1, further comprising determining whether said image is bimodal.

13. The method for enhancement of electronic images of a document according to claim 12, wherein determining whether said image is bimodal comprises detecting whether the color histogram for said image contains not more than two clusters of pixel lightness values.

14. The method for enhancement of electronic images of a document according to claim 12, further comprising setting light pixels as transparent for said bimodal image.

15. The method for enhancement of electronic images of a document according to claim 1, further comprising determining whether an image is of a type, said type being a solid graphics image.

16. The method for enhancement of electronic images of a document according to claim 15, wherein determining whether an image is of said solid graphics type comprises examining the statistics of said pixels'"'"' errors in RGB values compared to said pixels'"'"' assigned quantized color table bins.

17. The method for enhancement of electronic images of a document according to claim 15, further comprising assigning a background white estimate to images determined to be of said solid graphics type.

18. The method for enhancement of electronic images of a document according to claim 15, further comprising, for images found to be not of said solid graphics type:

assigning provisional foreground status to pixels based on an HSV comparison;

assigning provisional foreground status based on high-pass filtering;

assigning provisional background status; and

estimating a white background array.

19. The method for enhancement of electronic images of a document according to claim 1, further comprising performing contrast equalization.

20. The method for enhancement of electronic images of a document according to claim 19, wherein performing contrast equalization comprises:

estimating local contrast; and

normalizing foreground pixel RGB values so as to equalize contrast.

21. A system adapted to enhance electronic images of a document, the electronic images having a background with foreground markings placed thereon, the system comprising:

means for inputting an electronic image comprised of pixels;

means for inputting algorithm parameters;

a processor, connected for receiving requests for image enhancement and for accessing a memory structure;

program memory means including not less than one foreground/background separation module, wherein said foregroundfbackground separation module assigns initial provisional foreground/background status to said pixels, revises said initial foreground/background status, determines whether said revised foreground/background status is the final foreground/background assignment for said pixels, and repeats revising said revised foreground/background status until a final foreground/background assignment for said pixels is obtained; and

data memory means.

22. The system adapted to enhance electronic images of a document according to claim 21, wherein said electronic image comprises an image stored in digital form in said data memory.

23. The system adapted to enhance electronic images of a document according to claim 21, wherein said algorithm parameters are received from a previously stored file.

24. The system adapted to enhance electronic images of a document according to claim 21, wherein said algorithm parameters are received from a graphical user interface.

25. The system adapted to enhance electronic images of a document according to claim 21, wherein said program memory means further comprises a local contrast estimation module.

26. The system adapted to enhance electronic images of a document according to claim 21, further comprising image output means.

27. The system adapted to enhance electronic images of a document according to claim 21, wherein said processor is configured as a windows-type system.

28. The system adapted to enhance electronic images of a document according to claim 21, wherein said program memory module further comprises a color normalization module.

29. An article of manufacture comprising a computer usable medium having computer readable program code embodied in said medium which, when said program code is executed by said computer causes said computer to perform method steps for enhancement of electronic images of a document, the electronic images having a background with foreground markings placed thereon, said method comprising:

receiving an electronic image comprised of pixels;

assigning initial provisional foregroundfbackground status to said pixels to form an initial foreground/background estimate;

revising said initial foregroundlbackground estimate to form a revised foreground/background estimate;

determining whether said revised foreground/background estimate is the final foregroundibackground assignment for said pixels;

repeating revising said revised foreground/background estimate until said final foreground/background assignment for said pixels is achieved; and

providing said final foreground/background assignment for said pixels.