Fast Spheres

A new method for generating sphere‐like images, using parabolic surfaces delimited by Bresenham's circle‐ generation algorithm, is presented. In many cases the resultant images are indistinguishable from spheres illuminated from a given single direction. The main form of the algorithm uses first and second integer differences to minimise computation and uses typically not more than one floating‐point calculation per sphere. Two variants of the algorithm are given, one optimized for the special case of the light‐source being behind the view‐point, and one in which values are calculated for a z‐buffer hidden‐surface algorithm at the same time as the pixel values. The difference formulae can be exploited by differencing hardware or digital signal processors although very little arithmetic, typically five low‐weight integer operations, including address calculation operations, is required per pixel on a conventional architecture.