Very interesting write-up on stl format. I don't understand why the normal needs 3 coordinates, since it is only used to indicate which side of the face is inside the enclosed volume of the solid and which side is outside. It seems that the shape could be almost as accurately stored in a file 1/3 the size by using the normal coordinates as a fourth point to describe the vertex of a tetrahedron, projected along a normal vector at the center of the other three points. the direction of the surface normal needs only a single sign bit. the magnitude of the normal would be 0 if the face lies on a plane, otherwise it would lie on the curvature of the surface of the shape. Which side of the tetrahedral surface is interior to the solid and which side is outside is determined by the sign bit according to the right-hand rule applied to the three points.
Mike Doty
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