 from a viewing point, is the boundary
   of the portion of the body's surface that is visible from that
   viewing point.  In this definition, we consider a surface point
   to be `visible' if it can be connected to the viewing point by a
   line segment that doesn't pass through the body.  This is a purely
   geometrical definition that ignores the matter of which portions
   of the surface are illuminated, or whether the view is obscured by
   any additional objects.

   If a body is modeled as a triaxia