Abstract: A method and apparatus for predicting the position of a register mark within an image on an elongate web relative to a reference position of the mark when the image is at a reference position is described. The apparatus includes a detector for obtaining successive samples of the image; a comparator for comparing the samples with a reference threshold so as to generate a binary sequence; and a comparator for comparing the binary sequence with a reference sequence. The apparatus is operable to carry out a series of comparisons between the two sequences with the sequences offset by successive one bit intervals and at each offset position, the number of coincidences between binary values is summed by counters. The resultant sums are then compared and the highest is taken to indicate the degree of offset between the two sequences and is used to predict that the register mark has the same offset.