Browse Prior Art Database

Digital Halftoning Using Biased Error Diffusion

IP.com Disclosure Number: IPCOM000042135D
Original Publication Date: 1984-Mar-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 4 page(s) / 55K

Publishing Venue

IBM

Related People

Anastassiou, D: AUTHOR

Abstract

To enhance the compressibility of a bilevel image produced by the error-diffusion halftoning process, the corrected gray-scale intensity value of the current pixel (discrete picture element) yielded by the error-diffusion algorithm is compared with a preselected range of biased threshold values, and if this corrected intensity value falls within the biased threshold range, then the decision to print a black or white dot is made in accordance with an arithmetic coding technique whereby the pattern of black and white dots in a template of previously printed neighboring pixels is analyzed by statistical count, without reference to gray-scale intensities, for determining the probability of a black or white dot occurring at the current pixel location.

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Digital Halftoning Using Biased Error Diffusion

To enhance the compressibility of a bilevel image produced by the error- diffusion halftoning process, the corrected gray-scale intensity value of the current pixel (discrete picture element) yielded by the error-diffusion algorithm is compared with a preselected range of biased threshold values, and if this corrected intensity value falls within the biased threshold range, then the decision to print a black or white dot is made in accordance with an arithmetic coding technique whereby the pattern of black and white dots in a template of previously printed neighboring pixels is analyzed by statistical count, without reference to gray-scale intensities, for determining the probability of a black or white dot occurring at the current pixel location. On the other hand, if the corrected gray- scale intensity value of the current pixel falls outside of the biased threshold range, then the decision to print a white or black dot depends simply upon whether this intensity value is above or below the threshold range. Digital halftoning is a method of producing a bilevel image in which different arrangements of black and white dots are printed in small areas each containing a number of pixels so as to simulate the gray-scale appearance of the corresponding areas in the original continuous-tone image. In a high-quality halftone image, the proportion of black to white dots in each small area is approximately equal to the gray-scale darkness of the original image at the corresponding location. However, the quality of reproduction is not the only criterion which governs a halftoning process. Another desirable criterion is the digital compressibility of the output image. If the image contains many isolated small spots of black or white, the digital storage or transmission of such an image will require many more bits than would be the case if the black and white contrasts were less frequent so that fewer runs of black and white pixels would need to be encoded. The present procedure addresses the problem of producing a high-quality bilevel image without undue sacrifice of compressibility. It utilizes features of an error-diffusion halftoning process aimed at high-quality reproduction combined with features of an arithmetic coding technique designed to achieve high compression. This results in a modified error-diffusion process which eliminates fuzzy thresholding decisions that tend to reduce compressibility of the output image. In the error-diffusion method of digital halftoning 1,2 Ùpixels are assigned gray-scale intensity values represented by 8 bits per pixel on a gray scale of 0 to 255. As this method has been used previously (but not in the present procedure), a fixed threshold at the midpoint value of 127 has been established as the black-white separation level. Pixels whose intensity values are above this threshold are printed white, and all others are printed black. The difference between the...