Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!rutgers!bpa!cbmvax!vu-vlsi!swatsun!gessel
From: gessel@cs.swarthmore.edu (Dan Gessel)
Newsgroups: comp.graphics
Subject: Matrix Operations on Normals
Message-ID: <2392@carthage.cs.swarthmore.edu>
Date: 2 Feb 89 17:56:44 GMT
Reply-To: gessel@swatsun.UUCP ()
Distribution: na
Organization: SUN Lab, Swarthmore College, PA
Lines: 24

I have a question about matrix operations on vertex lists defining objects
within a ray tracer. I use a triangular primatve that goes through a recursive
subdivision to either smooth out the surface or make it fractal during the ray
trace calculation. This allows for very fine detail over the surface
without having to store large amount of information.

Stored in the vertex list is not only the coordinates of the vertex, but also
the normal to the surface at that point. These normals define the smoothing
operations required. I use matrices to perform rotations and would like to for
scaling operations. The first is no problem, I rotate the vertices and the
normals through simple matrix multiplication, but if I want to stretch the 
object, the normals would be increased in length in the direction of the
stretch. If we think of a sphere, it will stretch to an oval shape. We can
figure that the opposite should happen, the normals should decrease in the
direction of the stretch to the oval shape.

That is, given a matrix operation to be performed on a  surface, how does this
operation affect the normals? Any solutions or sources would be appreciated.

Daniel Mark Gessel
gessel@cs.swarthmore.edu

I apologize if this went up  twice. I posted it yesterday and found it marked
unavailable this morning, not simply already read, so I assumed it didn't go
through.