Adaptive quantization of grayscale images

Details Adaptive quantization of grayscale images Adaptive quantization of grayscale images

1996-12-10

2001-11-27

Evans, Arthur G. (Department: 2622)

Facsimile and static presentation processing

Static presentation processing

Size, resolution, or scale control

C358S001130

Reexamination Certificate

active

06323956

ABSTRACT:

APPENDIX
The disclosure in the appendix of this patent document contains material to which a claim of copyright protection is made. The copyright owner has no objection to the facsimile reproduction of any one of the patent documents or the patent disclosure as it appears in the U.S. Patent and Trademark Office patent file or records, but reserves all other rights whatsoever.
FIELD OF THE INVENTION
The present invention relates to the field of digital image processing and more particularly to processing of document images to reduce the amount of data used to represent the image.
BACKGROUND OF THE INVENTION
Document images which are scanned using 8 bits per pixel require large amounts of storage space and expensive hardware for processing. Scanned documents are currently quantized using one bit per pixel, which reduces the hardware and processing requirements, but degrades the image quality, specifically when these images are displayed on display devices which support 4-8 bits per pixel
One known method of quantizing document images in a single-bit down is to use one-bit adaptive thresholding as disclosed in U.S. Pat. No. 5,583,659, entitled “A Multiwindowing Technique For Thresholding An Image Using Local Image Properties,” issued Dec. 10, 1996, to Y. Lee, J. Basile, and P. Rudak. This approach limits the display to one bit. The current method of quantizing document images in a multi-bit domain employs fixed threshold points for each gray value. This approach results in poor image quality when a small number of bits per pixel is used to reduce data storage and processing requirements. There is a need therefore for an improved method of quantizing document images to optimize image quality when displayed on a multilevel display device while minimizing data storage requirements.
SUMMARY OF THE INVENTION
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a method of generating an M-bit grayscale image from an N-bit grayscale image, where 1<M<N, includes the steps of: for each pixel in the N-bit image, determining a threshold, based on the values of neighboring pixels, for generating the most significant bit (MSB) of the M-bit image, and determining thresholds, based on the threshold determined in step i) and the values of surrounding pixels, for each successive less significant bit(s) (LSB) of the M-bit image. The thresholds are used to quantize the pixels in the N-bit image to produce the M-bit image. The resulting M-bit image can be displayed on a multilevel display device for good readability, and the most significant bit of the M-bit image can be archived and printed on a binary printer thereby minimizing long term storage requirements.
The proposed method quantizes the scanned images using two (or more) bits per pixel so that their display can be improved, while maintaining low requirements on storage and processing hardware. Additionally, the improvements in image quality can significantly improve character recognition accuracy especially for broken or partially formed characters.
ADVANTAGEOUS EFFECT OF THE INVENTION
Adaptive quantization of scanned images using two (or more) bits per pixel provides an image representation which is similar to the original image, while maintaining low storage and processing requirements. The invention provides:
Improved representation and display of scanned documents.
Low contrast text characters are better displayed.
Jaggedness around character contour is reduced.
Background is closer to original shade of gray.
Improved character recognition via OCR.


REFERENCES:
patent: 4682869 (1987-07-01), Itoh et al.
patent: 5070413 (1991-12-01), Sullivan et al.
patent: 5384646 (1995-01-01), Godshalk et al.
P. R. Roetling, “Visual Performance and Image Coding”,SPIE/OSA vol. 74(1976)Image Processing, pp. 195-199.
R. S. Gentile et al., “Quantization and multilevel halftoning of color images for near-original image quality”, vol. 7, No. 6/Jun. 1990/Optical Society of America, pp. 1019-1026.
R. Miller et al., “Mean-preserving multilevel halftoning algorithm”,SPIE vol. 1913, 1993, pp. 367-377.
Y. S. Ng et al., “Gray Level Printing Method with Embedded Non-Uniformity Correction Using a Multi-Bit LED Printhead”,IS&T's 47th Annual Conference/1CPS, 1994, pp. 622-625.

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