ABSTRACT
Some surfaces are easy to describe without equations. A sphere is the locus of points at a fixed distance from a given point; a cylinder is the locus of points at a fixed distance from a given line. Thus, a sphere can be represented simply by a center point and a radius; a cylinder by an axis line and a radius. For these simple surfaces, we shall see that we can compute surface normals and raysurface intersections directly from their geometry, without resorting to implicit or parametric equations. Thus ray tracing these surfaces is particularly easy.
