Image processor with smooth transitioning between dither and diffusion processes
First Claim
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1. An image processor comprising:
- image memory means for storing an image comprising a frame of M columns and N rows of multi-gray level pixels;
adder means for forming successive input amplitudes from successive ones of said pixel so as to form a current input amplitude corresponding to the current one of said pixels;
means for comparing said current input amplitude with a dithered threshold so as to generate;
a. a binary print/no print decision,b. a feedback error;
means for attenuating said feedback error so as to produce an attenuated error, said attenuating means responsive to the average gray level of a local neighborhood of pixels surrounding said current pixel so as to attenuate said feedback error proportionately as said average gray level approaches a light or dark limit;
means for transmitting said attenuated error to said adder means so that said adder means adds said attenuated error to the next one of said successive pixels to form the next one of said successive input amplitudes; and
means for transmitting said dithered threshold to said comparator means, comprising;
a. means for storing a plurality of ordered dither matrices, each of said matrices defining plural patterns of dithered thresholds as individual elements in each of said matrices, each of said patterns characterized by dither threshold amplitudes for each of said M ×
N pixels spatially modulated with a contour of a characteristic sharpness,b. means for sensing the amplitude of the high spatial frequency content of said local neighborhood,c. means for selecting one of said plurality of ordered dither matrices whereby the characteristic sharpness of the one ordered dither matrix so selected is inversely proportional to the amplitude of the high spatial frequency content of said local neighborhood, andd. means for selecting for said dithered threshold an element within the one selected ordered dither matrix corresponding to the position of the current one of said successive pixels stored in said image storing means, whereby an ordered dither process is provided which is attenuated in proportion to the amplitude of the high spatial frequency content of the local neighborhood surrounding each successive one of said M ×
N pixels.
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Abstract
An image processor converts a multiple gray-level image to a bi-tonal image using both error diffusion and ordered dither to enhance image quality. The error diffusion process is attenuated smoothly as the local area intensity approaches a light or dark maximum, while the ordered dither process is attenuated smoothly as the local area high spatial frequency content increases.
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Citations
9 Claims
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1. An image processor comprising:
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image memory means for storing an image comprising a frame of M columns and N rows of multi-gray level pixels; adder means for forming successive input amplitudes from successive ones of said pixel so as to form a current input amplitude corresponding to the current one of said pixels; means for comparing said current input amplitude with a dithered threshold so as to generate; a. a binary print/no print decision, b. a feedback error; means for attenuating said feedback error so as to produce an attenuated error, said attenuating means responsive to the average gray level of a local neighborhood of pixels surrounding said current pixel so as to attenuate said feedback error proportionately as said average gray level approaches a light or dark limit; means for transmitting said attenuated error to said adder means so that said adder means adds said attenuated error to the next one of said successive pixels to form the next one of said successive input amplitudes; and means for transmitting said dithered threshold to said comparator means, comprising; a. means for storing a plurality of ordered dither matrices, each of said matrices defining plural patterns of dithered thresholds as individual elements in each of said matrices, each of said patterns characterized by dither threshold amplitudes for each of said M ×
N pixels spatially modulated with a contour of a characteristic sharpness,b. means for sensing the amplitude of the high spatial frequency content of said local neighborhood, c. means for selecting one of said plurality of ordered dither matrices whereby the characteristic sharpness of the one ordered dither matrix so selected is inversely proportional to the amplitude of the high spatial frequency content of said local neighborhood, and d. means for selecting for said dithered threshold an element within the one selected ordered dither matrix corresponding to the position of the current one of said successive pixels stored in said image storing means, whereby an ordered dither process is provided which is attenuated in proportion to the amplitude of the high spatial frequency content of the local neighborhood surrounding each successive one of said M ×
N pixels. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. An image processor for receiving a multiple gray level image comprising an array of image pixels and producing therefrom a bitonal image, said image processor comprising:
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error diffusion means for diffusing among neighboring pixels the difference between each image pixel in said array and a binarization threshold so as to produce a succession of error diffused pixels; ordered dither means for dithering said binarization threshold in accordance with a spatially variant ordered dither pattern of threshold values dithered in accordance with a given contour having a characteristic amplitude excursion; first attenuation means for attenuating the difference diffused among said neighboring pixels by said error diffusion means proportionally as the average value of a neighborhood of pixels surrounding a corresponding image pixel approaches a light of dark limit; and second attenuation means for attenuating the amplitude excursion of said ordered dither pattern in proportion to the amplitude of the high spatial frequency content of said neighborhood of pixels.
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Specification