Method for CCITT compression of image data
First Claim
Patent Images
1. A method for CCITT compression of image data comprising the steps of:
- a) creating and saving a scaled background of the image by calculating a background average for each array of pixels of a predetermined size;
b) thresholding each pixel to a black or white rendition and storing the thresholded pixel in a manner which automatically transposes the pixels to a predetermined order;
c) determining run lengths of adjacent pixels having like color and storing the run lengths as color transition pointers in a memory table; and
d) processing two rows of thresholded pixels simultaneously pursuant to CCITT Group 4 encoding principles using the color transition pointers from the memory table in place of retrieving and examining each pixel individually to determine transition locations in the two rows.
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Abstract
A method of compressing image data utilizing CCITT Group 4 coding criteria reduces compression time and the amount of required data transfer by combining pixel data thresholding and transposition steps through the capabilities of a customized processing chip. Additional economies are achieved by using black/white and white/black transition pointers rather than the conventional approach requiring examination of every pixel to locate the transitions in preparation for conventional CCITT Group 4 encoding.
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Citations
10 Claims
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1. A method for CCITT compression of image data comprising the steps of:
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a) creating and saving a scaled background of the image by calculating a background average for each array of pixels of a predetermined size;
b) thresholding each pixel to a black or white rendition and storing the thresholded pixel in a manner which automatically transposes the pixels to a predetermined order;
c) determining run lengths of adjacent pixels having like color and storing the run lengths as color transition pointers in a memory table; and
d) processing two rows of thresholded pixels simultaneously pursuant to CCITT Group 4 encoding principles using the color transition pointers from the memory table in place of retrieving and examining each pixel individually to determine transition locations in the two rows. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
determining a background threshold as a predetermined function of an input background for a pixel being thresholded;
comparing a grayscale level of the pixel being thresholded to the background threshold; and
setting the pixel being thresholded to a first predetermined color whenever its value is above the background threshold and setting the pixel being thresholded to a second predetermined color whenever its value is equal to or less than the background threshold.
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9. In a method for compressing document image data in accordance with CCITT wherein a document to be imaged is scanned in a first format and the resultant image data is compression processed in a second format, wherein image data pixels are transposed into the second format, wherein each image data pixel is next thresholded to a black or white value, and wherein the transposed and thresholded image pixels are examined to determine run lengths of a same color, the improvement comprising:
combining the steps of transposing and thresholding into a single step.
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10. In a method for compressing document image data in accordance with CCITT Group 4 wherein preselected run lengths of a same pixel color in two adjacent rows of image data are examined for each encoding step, the improvement comprising:
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alternately using a first color look-up table and a second color look-up table to determine locations in each pixel row of respective transitions between the first and second colors; and
using the determined transition locations in lieu of examining each pixel of the two adjacent rows to perform the CCITT Group 4 encoding process.
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Specification