1. An apparatus for optically correlating two images, comprising: means for providing an input signal corresponding to a two-dimensional input image capable of being displayed in a raster-scanned format; an optically transmissive mask having a pattern of optical transmissivity corresponding to a two-dimensional reference image; buffer means for storing and serially shifting said input signal; a subimage frame in said buffer means corresponding to a predetermined raster segment in said raster-scanned format for storing successive portions of said input signal as said input signal is shifted by said buffer means, each of said portions corresponding to a respective subimage of said input image; optical means responsive to said successive portions stored in said subimage frame for projecting on said mask a succession of optical signals, each representing a respective subimage of said input image; and optical detection means for receiving light transmitted through said mask by the projection of said succession of optical signals, and for producing, based upon light transmitted by the projection of an optical signal, a correlation signal representative of the degree of correlation between said reference image and the subimage represented by said optical signal.