1. A system for displaying three-dimensional surface structures, said system comprising: means for storing three-dimensional signal patterns representative of a value of at least one physical property of a three-dimensional body at each of regularly spaced parallelopiped grid locations each defining a volume element within said body; means for retrieving signal pattern values associated with said parallelopiped volume elements; means for comparing signal pattern values associated with a volume element with at least two threshold values, to generate an indicia of the relationship between each volume element and internal surface features of said body as defined by said threshold values; means for generating a normal vector to one side at said surface at each of said grid locations; means for associating a three-dimensional location of each of said grid locations with said normal vector generated for the surface at that grid location; means for associating said volume element surface indicia with said normal vectors; and display processor means for receiving said locations and said normal vectors and providing a shaded image, said image representing at least one select surface.