Automatic registration of color separation films

The problem of registration of four-color half tone separations for color printing is addressed. We present a method to performautomatically registration of color separation films, a step currently performed manually by highly skilled professionals in the graphic arts printing industry.An operator manually selects from the reference negative (generally CYAN) two windows with a digitizing cursor, and each window covering approximately 6 mm2 (0.25 in2) is digitized into a 640 × 640 array. On each negative and for each window, we extract the macro edges and approximate the contours by line segments. The segments from corresponding windows on different negatives are then matched with the reference ones to provide an estimate of the translation between them. The two translations (from the two windows) provide the parameters of the rigid planar transform between negatives (rotation and translation) and permit the punching of registration holes into the pictures for each negative.The system has been implemented in the RegiStar® machine 1 built to perform the mechanical tasks associated with the algorithm. It is able to handle a set of fourcolor separations in about 5 minutes, from image acquisition to the punching of registration holes in the films maintaining an accuracy of 12μ (0.5 mil) for binary patterns and 25μ (1 mil) for true halftones. This speed is obtained by using an off-the-shelf Mercury array processor attached to an IBM personal computer.

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