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Improving Reproduction of Images Containing Continuous Tone and Alphanumeric Text

IP.com Disclosure Number: IPCOM000087625D
Original Publication Date: 1977-Feb-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 33K

Publishing Venue

IBM

Related People

Stucki, P: AUTHOR [+2]

Abstract

In document reproduction, the input images can be either text, continuous tone or halftone pictures or a combination of these. It is essential that the reproduction device is capable of copying these different types of images with adequate print quality. Text materials can be characterized by sharp boundaries, and the scanned signals contain fast transients. Its proper reproduction calls for high spatial resolution requirements.

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Improving Reproduction of Images Containing Continuous Tone and Alphanumeric Text

In document reproduction, the input images can be either text, continuous tone or halftone pictures or a combination of these.

It is essential that the reproduction device is capable of copying these different types of images with adequate print quality. Text materials can be characterized by sharp boundaries, and the scanned signals contain fast transients. Its proper reproduction calls for high spatial resolution requirements.

A moderate number of amplitude quantization levels is sufficient to guarantee adequate reproduction quality if the scanning resolution is high. Halftone materials can be treated in a similar way as text. Continuous-tone pictures, however, are quite different from text. In areas with slowly varying gray levels, the issue of amplitude quantization is crucial and fine quantization is required, otherwise artificial contouring will appear. At the same time, moderate spatial resolution requirements usually guarantee adequate reproduction quality.

In digital halftoning, the generation of gray-level "illusions" is obtained by varying the number of dots to be printed per unit area. The scanned value, after digitization, is compared with a threshold value. If the scanned value is less than the threshold value, a dot will be printed in the output halftone image. By placing different threshold values in the threshold matrix, different halftone patterns can be created.

There are many possible ways of designing threshold matrices. Basically, however, one can distinguish between two main classes of half-tone dot patterns: "uniformly distributed" and "clustered" ones.

A uniformly distributed halftone dot pattern is designed to have adjacent gray level threshold values as far apart as possible in the matrix. his type of pattern has the capability of reproducing high spatial r resolutions, but amplitude rendition fidelity is poor, particularly when an on-off printer with overlapping dots is used.

Clustered halftone dot patterns, which include the "Super-Circle" method, are designed to have high amplitude rendition fidelity. Its spatial resolution capability, however, is naturally limited. Although decreasing the size of the threshold matrix will improve the spatial resolution capability of the "Super-Circle" method, amplitude resolution is lost at the same time. Hence, there exists no single dot pattern design which provides high spatial as well as high amplitude resolution capabilities.

The o...