| OCR Text |
Show 133 their effect on the direction of the surface normal (and thus the light reflected) rather than their effect on the deviation of position of that portion of the surface from the average position. We would expect, therefore, to get a good effect from having a texturing function which performs a small perturbation on the direction of the surface normal before using it in the intensity formula. This perturbation must be specified in such a manner that it is independent of the orientation of the surface. That is, the normal must be nudged in the same direction and by the same amount (relative to the surface) independent of the orientation of the surface to the viewer. x X(u,v) Y Y(u,v) Z Z (u,v) The normal vector perturbation function will be derived in terms of a function which defines the ripples F(u,v). The value of this function will be negligibly small but thevalue of its derivatives (th@ prime component in the perturbation of the normals) might not be. The position of the base patch is defined by and the normal vector at any point is Xn Yu Zv - Zu Yv Yn Zu Xv - Xu Zv Zn Xu Yv - Xv Yu |
