void d3scale (x, y, z, xp, yp) D3PLT.C Level 3
float x, y, z The coordinates of a point in 3-D space in user units.
int *xp, *yp The projected 2-D location of the point in GraphiC pixel space.
d3scale() converts 3-D x, y, z coordinates into projected xp, yp coordinates in Tektronix 8192 space. The arguments *xp and *yp are the addresses of the xp and yp variables passed in, not arrays. d3scale() allows you to determine the two-dimensional position of an object in three-dimensional space. This facility is used, for example, by d3pltfnt() to 
determine the location at which to plot the string.
This routine will be obsolete in the future and usertopix() should be used instead.