The central difference along a selected axis through a point (u, v) of a rectangularly-sampled luminance field and at an arbitrary angle .alpha. is computed by interpolation. The octant in which the selected axis lies is identified, and first and second pixels through which the lines that limit the identified...http://www.google.com/patents/US4809070?utm_source=gb-gplus-sharePatent US4809070 - Method and apparatus for adding texturing highlights to a video signal
