Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!pasteur!ucbvax!decwrl!sgi!thant@horus.SGI.COM
From: thant@horus.SGI.COM (Thant Tessman)
Newsgroups: comp.graphics
Subject: Re: Ray tracing refraction
Summary: what is refraction
Message-ID: <25779@sgi.SGI.COM>
Date: 30 Jan 89 18:31:56 GMT
References: <65@sdcc10.ucsd.EDU>, <0XryqWy00Uo1875Ud-@andrew.cmu.edu>
Sender: daemon@sgi.SGI.COM
Distribution: usa
Organization: Silicon Graphics, Inc., Mountain View, CA
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In article <0XryqWy00Uo1875Ud-@andrew.cmu.edu>, po0o+@andrew.cmu.edu (Paul Andrew Olbrich) writes:
> Hi-ho,
> 
> I'm trying to add refraction to a ray tracing program I'm writing in C.  Could
> someone help me out a bit?

Warning: I'm doing this from memory, and I've never written a ray-tracer.


             \<-theta1->|
              \         |
               \        |
incoming ray->  \       | <- surface normal
                 \      |
                  \     |
                   \    |
                    \   |        <material with index of refraction n1> 
                     \  |
                      \ |
                       \|
-------------------------------------------------- <- surface
                        |
                        | \
                        |  \
                        |   \    <material with index of refraction n2>
                        |    \
                        |     \
                        |      \  <- outgoing ray
                        |       \
                        |        \
                        |         \
                        |<-theta2->\


sin(theta1)   sin(theta2)
----------- = ----------
    n1            n2

(theta1 and theta2 are angles)

If n1 is 1 (like for a vacuum) and n2 is bigger than 1 (like glass) then 
theta2 is smaller than theta1.  Note that if it is the other way around,
that is, the ray goes from an optically dense material to an optically
less dense material, (like a ray coming out of glass), then theta2 is 
bigger than theta1.  (Maybe this is where you are screwing up?)

Also, if you are solving for sin(theta2) and you get a number bigger than 1,
it means that the ray should be totally internally reflected.  This is why 
when you look up from the bottom of a pool, everything above the water looks 
like it is in a round window above your head.

There are other considerations.  The ratio of light reflected to  
transmitted is dependent on the angle but I can't remember that stuff.

Also, reflected and transmitted light is polarized (also dependent on the 
angle).  Pelicans have polarized filters in their eyes to see fish in the 
water better.  Bees have polarized eyes to navigate with sunlight on cloudy
days.

Some crystals have different indecies of refraction for light polarized in
different directions.  When you look through them you see two images.

And on and on...

Thant Tessman (thant@sgi.com)
"make money, not war"