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COMPRESSION OF HALFTONED IMAGES BY SEPARATION OF THE SCREEN INTENSITY SAMPLES

IP.com Disclosure Number: IPCOM000025967D
Original Publication Date: 1989-Jun-30
Included in the Prior Art Database: 2004-Apr-04
Document File: 2 page(s) / 131K

Publishing Venue

Xerox Disclosure Journal

Abstract

Halftoning allows the imaging of gray-level pictures on two-tone (black or white) displays or printers. In printing, a halftone is formed by projecting the gray-level image through a screen. This results in a pattern of dots with the dot size varying with the image intensity. Digital halftoning simulates the halftone screen. The digital halftone screen is a pattern of intensity thresholds which is replicated over the image. Intensity samples from the image are compared against the intensity thresholds in the halftone screen at the corresponding positions. If the sampled intensity exceeds the screen value for a given position, then that position is colored white, otherwise it is black. The halftone pattern can be used to separate the picture into N smaller pictures, where N is the number of unique threshold values in a screen cell. There will be a small picture for each intensity value of the halftone screen pattern. Each small picture will contain one dot for each instance of the threshold in each replication of the halftone screen pattern. Each small picture will show the original picture thresholded at one of the halftone screen intensity values.

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COMPRESSION OF HALFTONED Proposed Classification IMAGES BY SEPARATION OFTHE
SCREEN INTENSITY SAMPLES
Steven J. Harrington

U.S. C1.346/140'13 Int. C1. Gold 15/16

Halftoning allows the imaging of gray-level pictures on two-tone (black or white) displays or printers. In printing, a halftone is formed by projecting the gray-level image through a screen. This results in a pattern of dots with the dot size varying with the image intensity. Digital halftoning simulates the halftone screen. The digital halftone screen is a pattern of intensity thresholds which is replicated over the image. Intensity samples from the image are compared against the intensity thresholds in the halftone screen at the corresponding positions. If the sampled intensity exceeds the screen value for a given position, then that position is colored white, otherwise it is black. The halftone pattern can be used to separate the picture into N smaller pictures, where N is the number of unique threshold values in a screen cell. There will be a small picture for each intensity value of the halftone screen pattern. Each small picture will contain one dot for each instance of the threshold in each replication of the halftone screen pattern. Each small picture will show the original picture thresholded at one of the halftone screen intensity values.

The advantage in dividing the picture in this manner is that the bit color changes much less rapidly for the thresholded pictures than it does for the full halftoned image. This makes it easier to predict what color the pictures elements will be. The result of applyin a predictor to each of the small

every point for which the predictor is correct, and black where an error is made. A good predictor will yield mostly white and black regions. Run-length encoding is an effective way of compressing the (mostly white) predictor results. Huffman encoding of the run lengths can be used to reduce the run- length data.

Therefore, the compressed image is represented by N run-length encoded streams representing the prediction results for N thresholded picture, each corresponding to one of the halftone screen intensity values. The image may be reformed by decoding the streams in parallel and using the result to build N-element blocks of the ima e. Note that both the encoding and decoding of the N su...