From: mccool@dgp.toronto.edu (Michael David McCool)
Subject: Re: Superscape 3D Construction Kit Demo report
Date: 	Wed, 4 Dec 1991 16:42:58 -0500
Organization: University of Toronto/Computer Science/Playfair Neuroscience Unit



dstamp@watserv1.waterloo.edu (Dave Stampe-Psy+Eng) writes:

>An interesting feature is the use of spheres as well as polygons, and 
>semi-transparent polygons for windows (using halftones).  These are simple 
>tricks which may be useful to remember.  Spheres look like circles from any
>angle, so you just add a filled-circle drawing routine and compute a radius
>(based on depth) and a center point coordinate.  Transparancy is simple and
>I may add it to my poly blitter in a month or so.

Just a note:  spheres project to ellipses, not circles.  Consider the
intersection of a cone with a flat plane representing the screen.  The
point of the cone is the eye, and the sphere fits inside the
cone.  If this is done, and the geometry is right (ie the eye is where
the computation expected it), then the sphere will APPEAR to be a 
circle from the eye.  Ellipse drawing routines are almost as 
fast/easy as circle routines anyways.

Another trick for spheres: draw several concentric ellipses
to simulate shading on the sphere.

In a low-end VR system probably using the circle approximation is
a reasonable thing to do, as long as it doesn't become a habit :-)
Actually, painters often use a circle anyways because the geometry
for viewing a painting is almost never right anyways, and a circle
looks better from arbitrary viewpoints.  But in VR using eyephones,
you KNOW where the eye will be, so there's no excuse.

Nothing personal, it's just that this assumption comes up again and
again, and seems perfectly reasonable, but just happens to be wrong.

Michael McCool
mccool@dgp