The authors have developed a machine to perform the task of automatic registration of color separation films, a process manually performed by skilled professionals in the graphics arts printing industry. The development of such a machine requires overcoming significant challenges: designing a sound computer vision methodology while respecting hard timing constraints, transferring software across platforms and languages, validating the software, building the actual machine around the algorithms, testing the conformity to tolerances, educating operators on the use of such a machine, and having a system robust enough to operate around the clock with no technical supervision. The authors present a brief overview of the problem, followed by the answers they provided to the challenges above.>

The problem of registration of four-color halftone separations for color printing is addressed. A method is presented for automatic registration of color separation films. An operator manually selects two windows from the reference negative (generally cyan) with a digitizing cursor, and each window covering approximately 6 mm/sup 2/ (0.25 in/sup 2/) is digitized into a 640*640 array. On each negative and for each window, the macro edges are extracted, and the contours are approximated 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, built to perform the mechanical tasks associated with the algorithm. It is able to handle a set of four-color separations in about 5 min, from image acquisition to punching of registration holes on the films, maintaining an accuracy of 12 mu m (0.5 mil) for binary patterns and 25 mu m (1 mil) for true halftones. This speed is obtained by using an off-the-shelf Mercury array processor attached to an IBM personal computer.>