Given 3-by-anything coordinates XYZ, return X and Y in viewer's coordinate system (set by rot3, mov3, orient3, etc.; see "Functions For Setting Viewing Parameters" on page 56). If the second argument is present and non-zero, also return Z (for use in sort3d or get3_light, for example; see "Sort z Coordinates: sort3d" on page 89 and "Find 3D Lighting: get3_light" on page 87.). If the camera position has been set to a finite distance with setz3 (see "Physical orientation" on page 57), the returned coordinates will be tangents of angles for a perspective drawing (and Z will be scaled by 1/zc). x, y, and z can be either 1D or 2D, so this routine is written in two cases.