Sigma?

General questions about Indigo, the scene format, rendering etc...
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CTZn
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Re: Sigma?

Post by CTZn » Thu Dec 17, 2009 6:25 am

Thanks polinalkrimizei, image inspired by fused's btw.

Zom-B: A quote from http://www.the123d.com/tutorial/general ... s2-1.shtml (chapter 7):
I've seen a lot of different implementations of Oren-Nayar models, so it's hard for me to tell you some facts. Most of them do not use masking and self-shadowing and most of them also do not take different wave-lenght computations into account. Most of the time this model looks like a lambertian diffusor combinated with some darkening in the viewing direction and some backscattering.
So darkening in the wiewing direction seems to belong to the "global" model. I explain that to myself by laws of energy conservation, it's just the lambertian dark rims blending into the surface (that's the explanation to myself, nothing I can affirm really).
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lycium
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Re: Sigma?

Post by lycium » Thu Dec 17, 2009 9:22 am

energy conservation was my guess too; although i don't see how doing a wavelength-dependent simulation will affect the result.

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pixie
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Re: Sigma?

Post by pixie » Thu Dec 17, 2009 9:47 am

lyc wrote:energy conservation was my guess too; although i don't see how doing a wavelength-dependent simulation will affect the result.
I have no idea what you both are talking about! :P

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Re: Sigma?

Post by gagar » Thu Dec 17, 2009 10:40 pm

Oren-Nayar does model masking, shadowing and interreflections. That is why when sigma increases, the surface appears darker as seen in the Wikipedia article.
As for the dark edges, when sigma is 0 the model is Lambertian (the surface is completely flat and diffuse) and thus there is no backscattering. When sigma increases the surface gets rougher and backscattering increases.

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CTZn
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Re: Sigma?

Post by CTZn » Fri Dec 18, 2009 12:20 am

Thank you for clarifying the topic, gagar !

Said alternately, the effect with higher sigma values is similar to sound in a recording studio, wich walls are absorbing waves into their internal structure. The higher sigma goes, the deeper (more absorbing) is the structure.

Another illustration in the field of radio frequencies could be the F-117A.
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