Translucent roughness: Only for specular but not diffuse? #31

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bhouston opened this Issue Sep 24, 2015 · 1 comment

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@bhouston

The simplified pseudo code for the "translucent" material is this:

bsdfs.add( new LambertianReflection( reflectance * diffuse ) );
bsdfs.add( new LambertianTransmission( transmittance * diffuse ) );

var microfacetDistribution = new TrowbridgeReitzDistribution( roughness, roughness );
var fresnel = new FresnelDielectric( 1, 1.5 );
bsdfs.add( new MicrofacetReflection( reflectance * specular, microfacetDistribution, fresnel ) );
bsdfs.add( new MicrofacetTransmission( transmittance * specular, microfacetDistribution, 1.0, 1.5 ) );

You can see that roughess is only used for the specular. But if one assumes a microfacet model of the surface for specular, shouldn't one similarly assume microsurface model for the diffuse?

Second issue, doesn't this lead to a microfacet model conflict: Oren-Nayar model is derived from the older Cook-Torrence / Beckmann microfacet distribution. But the specular model here is based on the more modern Trowbridge-Reitz microfacet distribution. Thus there may be a roughness parameterization conflict between these two models as it is -- although it also may be so slight it doesn't matter, or maybe diffuse roughness in this case has no perceptible result?

@mmp
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mmp commented Sep 26, 2015

Ah, interesting. I agree it'd probably make sense to use the roughness for both the specular and diffuse models, though, as you point out, it's not obvious how to relate those two.

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