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# Ashikhmin-Shirley lightning model

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4 replies to this topic

### #1Lambo  Members

Posted 02 April 2012 - 08:42 AM

Hi, I'm trying to implement an anisotropic Phong BRDF lightning model http://www.cs.utah.e...dfs/jgtbrdf.pdf. Where specular component is defined as: . I.m having problems with dot(n, h) exponent, because its numeric value is to big. Right now I think there is a problem with equation or my interpretation: according to the document Nu and Nv are freely chosen constants and can be equal, so if we chose Nu=Nv, exponent is equal to Nv whose value can range 1000, with is obviously to big and returns NaN in pow() function. Maybe someone can pinpoint my mistake?

### #2Tasty Texel  Members

Posted 02 April 2012 - 04:20 PM

1000 is usually not too big for an exponent, so your mistake must be somewhere else located.
Best is you post your source code to make it easier to hunt down the prob.

### #3Bacterius  Members

Posted 03 April 2012 - 03:40 AM

I.m having problems with dot(n, h) exponent, because its numeric value is to big

This worries me. If your n and h are normalized, then dot(n, h) is in the range [-1,+1] so what you describe shouldn't happen.
Are you sure your vectors are normalized?

“If I understand the standard right it is legal and safe to do this but the resulting value could be anything.”

### #4jischneider  Members

Posted 03 April 2012 - 08:06 AM

Why not isolate the different parts of the equation?

Also, the pow function does not work with values below or equal 0 because the implementation is done with the equation exp(log(value) * value). Try to put something like this: max(dot(…), 0.0001f);

Good luck!!!

Project page: < XNA FINAL Engine >

### #5Lambo  Members

Posted 05 April 2012 - 01:37 PM

Thanks guys! It simply was not normalized view vector, so my half vector was wrong... Paniced because if exponent would be 10000, base should not be smaller than 0.9913 or else outcome could not be expressed in sp float.

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