Abstract: A technique for document edge detection in an optical scanning system. The document is placed against an over-sized neutral background. A linear optical sensor array is used to provide successive lines of pixel data representing the luminance of detected pixel areas. Commencing from one end of the line of pixel data, for successive current pixels being tested, a group of n adjacent pixel values is correlated with a successive group of n adjacent pixel values to provide a correlation value for the current pixel. One of the groups is assumed to contain only background pixels, and their luminance is approximated to be equal to the average luminance over the entire background. If the correlation value exceeds a threshold constant, the current pixel is chosen to be an edge in this line of pixels. Data from several lines is processed to find corresponding edges for each line, and the line edges are subjected to a least squares fit to calculate the location of the document edge.

Abstract: Each two-dimensional alignment mark on a printing medium is found by using information about the whole mark area, or with respect to each of two different directions relative to the medium by using a distinctive two-dimensional pattern or characteristic of the mark; or by finding the mark centroid, or by scanning the mark region to obtain a two-dimensional representation for analysis--and preferably by combinations of these procedures. An image is then aligned and printed by reference to the mark so located. A previous image may also be preprinted in known relation with the mark, so that the new image is aligned with the preprinted one--sometimes on the same piece of medium. Preferably information used includes the mark's intended size, shape, areal disposition and other properties; and a two-dimensional search template is defined which matches the mark in at least one of these.