Automatic registration of color separation films

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 mm² (0.25 in 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 μm (0.5 mil) for binary patterns and 25 μm (1 mil) for true halftones. This speed is obtained by using an off-the-shelf Mercury array processor attached to an IBM personal computer