The preceding chapters deal primarily with two-dimensional images. The subjects of the images may also be two dimensional, such as printed text on a page. Some images rep-resent things that can be treated as two dimensional, such as particulates scattered on a substrate, or the surface of the earth viewed from a satellite. And of course, many images are two-dimensional projections of three-dimensional scenes or sections through three-dimensional structures. In all cases, the 2D array of pixels can be processed and measured. It is the interpretation of the measurements that changes according to the nature of the subject.