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Stitching of Prescreened Images Disclosure Number: IPCOM000010903D
Publication Date: 2003-Jan-30

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The Prior Art Database


This paper describes an experimental method for seamless stitching of two prescreened bitmap images.

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Stitching of Prescreened Images


Version:                                               1.0

Author:                                                Mike Slone-Murphy

Document No.         R312

Date:                                                     01/12/00

                                                                                                 Copyright FUJIFILM Electronic Imaging Ltd,  2000



1. Introduction

This paper describes an experimental method for seamless stitching of two prescreened bitmap images.

2. The Problem

Celebrant imposition software works by storing prescreened bitmap subimages that are used to construct complete multiple pages. Many other imposition products store contone images that are then put together and rescreened. The Celebrant process generates final results much more quickly than those obtained by rescreening, but with a penalty. If final complete images are to be contiguous, that is, with no borders or margins, as would be the case with a centrefold, for example, the join between the subimages is visible as a thin speckled line.

The reason for this effect is that the screening changes phase suddenly at the join. The subimage to the left of the join has been cut at some arbitrary scanline in the threshold array that defines the positions of the dots, and the subimage to the right of the join restarts the screening at some other undetermined scanline.

Figs 1 and 2 show magnified examples of a pair of butted flat tints before and after correction by the algorithm described in this paper. The snapshot is simulated and the strip across which the correction is applied is much narrower than would be the case in practice for reasons of clarity.

Fig 1. Region around the join of two incompatible images

Fig 2. Region around the join of the same two images after correction (exaggerated for clarity)

3. Proposed Solution

The method described here is fairly simple and involves defining a rectangular strip down the vertical extent of the final image, with the join between the subimages down its centre. The two halves of the strip are stretched by an appropriate amount to achieve a gradual phase change across the strip, instead of the sudden change in the uncorrected image.

The method works by selecting a small rectangle in the left half of an obviously screened part of the strip as in Fig 3. In practice, this rectangle would be much bigger than this. The rectangle is placed over the join as in Fig 4 and moved around in a half-spiral path shown in Fig 8 until a displac...