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Friday, March 18, 2016

My buggy implimentation of Schlick approximation

One of the most used approximations in all of graphics is the Schlick approximation by Christophe Schlick.    It says the Fresnel reflectance of a surface can be approximated by a simple polynomial of cosine(theta).    For dielectrics, a key point is that this theta is the bigger of the two regardless of which direction the ray is traveling.
Regardless of light direction, the blue path is followed.   Regardless of light direction, the larger of the two angles (pictured as theta) is used for the Schlick approximation.

A sharp reader of my mini-book pointed out I have a bug related to this in my code.   I was surprised at this because the picture looked right (sarcasm).      The bug in my code and my initial fix is shown below.
The old code shown in blue is replaced with the next two lines.
My mistake was to pretend that if snell's law applies to sines,  n1*sin(theta1) = n2*sin(theta2), it must also apply to cosines.   It doesn't.    (The fix is just to use cos^2 = 1 - sin^2 and do some algebra) I make mistakes like this all the time, but usually the wonky-looking picture, but in this case it didn't.    It only affected internal reflections, and it just picked some somehwhat arbitrary value that was always between 0 and 1.   Since real glass balls look a little odd in real life, this is not something I can pick up.   In fact I am not sure which picture looks right!

Old picture before bug fix.
New picture after bug fix.

I am reminded of spherical harmonic approximation to diffuse lighting.   It looks different than the "correct" lighting, but not worse.  (In fact I think it looks better).    What matters about hacks is their robustness.    It's best to do them on purpose though...

1 comment:

Paco Martinez said...

Are you bugfixing the kindle book? if I buy you now your book in amazon, have you updated all the bugs that readers have said to you?.