## Monday, December 7, 2020

### Debugging refraction in a ray tracer

Almost everybody has trouble with getting refraction right in a ray tracer.  I have a previous blog post on debugging refraction direction if you support boxes.  But usually your first ray tracers just supports spheres and when the picture just looks wrong and/or is black, what do you do?

So if you are more disciplined than I usually am, write a visualization tool that shows you in 3D the rays generated for a given pixel and you will often see what the problem is.  Like this super cool program for example.

But if you are a stone age person like me:

Then you have exactly two debugging tools: 1) printf() and 2) output various frame buffers.  For debugging refraction, I like #1.  First, create some model where you know exactly what the behavior of a ray and its descendants should be.  Real glass reflects and refracts.  Let's get refraction right.  So comment out any possibility of reflection.  A ray goes in, and refracts (or if that is impossible, prints that).

Now let's set up the simplest ray and single sphere possible.  The one I usually use is this:

The viewing ray A from the camera starts at the eye and goes straight long the minus Z axis.  I assume here it is a unit length vector but it may not be depending on how you generate them.

How do you generate that ray?  You could hard-code it, or you could instrument your program to take a parameter or command line argument or whatever for which pixel to trace (like -x 250 -y 300 or whatever).  If you do that you may need to be careful to get the exact center-- like what are the pixel offsets?   That is why I usually just hard code it.  Then let the program recurse and make sure that you get:

A hits at Q which is point (0,0,1)

The surface normal vector at Q is (0,0,1)

The ray is refracted to create B with origin Q and direction (0,0,-1)

B hits at R is is point (0,0,-1)

The surface normal at R is (0,0,-1)

The ray is refracted to create C with origin R and direction (0,0,-1)

The ray C goes and computes a color H of the background in that direction.

The recursion returns that color H all the way to the original caller

That will find 99% of bugs in my experience

#### 1 comment:

Phil Dutré said...

What I sometimes recommend to students when debugging refraction (I might have mentioned this before), is to make an animation of a static scene/static camera, but with changing refraction index over the frames.
It isn't a full debugging (you do not know whether refraction is computed correctly), but at least you can see whether the computed refraction behaves continuously when changing the refraction index a tiny amount.