1. A photometric system for describing an edge within a field of view, comprising: an offset patch of pixels in an image, the offset patch of pixels taken across a width of the edge, the patch having a single row of a predetermined number of pixels; and a binary edge descriptor determined by comparing pixel pairs within the offset patch under a predetermined rule comparing at least one of greyscale and contrast of one or more pixel pairs in the offset patch of pixels, the result of the comparisons under the predetermined rule setting bits within the binary descriptor.