Method for CCITT compression of image data
AssigneeUnisys Corporation
InventorWilliam L. Kozlowski
Life of this patent
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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.
Claims10
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.
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.
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:
- 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.
Dependencies are inferred from the claim text.