Secondary Reflections

ghaztehschmexeh

Hey,

I was wondering what "Secondary Reflections" actually is in toolbag. Like, what does it physically simulate? It effects the entire mesh, so what sort of situations is it meant for? I've had a play with it, I'm just hesitant to use it without knowing what is actually going on!

Thanks.

EarthQuake

Secondary reflections adds another reflectivity pass to the material. This can be useful if you want to simulate complex layered materials where you have a base layer and a secondary clear coat on top of that. In that case, each layer would have different gloss values, say rough wood with a clear coat on top. Car paint is another example. This method is somewhat limited however, in that you can't load a secondary normal map as well.

Its also handy for hair, because hair tends to have a primary reflection (uncolored) and a secondary, offset reflection (typically tinted the same color as the hair).

ghaztehschmexeh

Aah, fantastic! Thanks for the info.

ZippZopp

it also works quite well when dealing with skin. i like to add a subtle secondary reflection

tach

ZippZopp wrote: »

it also works quite well when dealing with skin. i like to add a subtle secondary reflection

I was wondering about that! I've been using it with skin too and I was just like "oh sh*t, have I been using it wrong?" after reading the thread.

Secondary reflection = Fresnel, right? Or rather an added Fresnel effect in this case since it's already present under the primary reflection?

EarthQuake

No, secondary reflection does not = Fresnel. Fresnel is built into the standard reflectivity shaders and secondary reflection provides a secondary Fresnel input as well. The Fresnel input determines how much light is reflected at grazing angles, for which most materials is 100%.

The reason to use secondary reflection for skin would be to simulate the base layer of skin, which is quite rough, and then the secondary layer of oil on top of the skin, which is very smooth.

The GGX shader model does a better job of simulating the characteristics of skin with only the primary reflection enabled, when compared to Blinn-Phong at least.

instg8r

Is there a way to use a mask or another gloss/specular map for the secondary reflection?

EarthQuake

Unfortunately not at this time. One thing you can do if you need additional inputs is duplicate your mesh, give it another material, and set it to additive blending mode. This way you can give the second layer a unique normal, spec and gloss map.

More details on how to do this here (last section): http://www.marmoset.co/toolbag/learn/glass

instg8r

Cool. Thank you for the link! I will give that a try

EDIT: Went ahead and did the "add" trick and it worked pretty swell. Close enough to the look I was going for. Thanks again!