LuxRender Materials Glossycoating
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Glossycoating is an "empty gloss coating" material. It is used to add gloss layers on top of other materials. It is something of a special glossy material that has no base reflector of its own, but rather takes another material node as its base.
The material node that forms the base reflector beneath the coat. If you use the matte material, the result will be the same as the regular glossy material. The material specified here can be any material type, including null, mix, layered, or another glossycoating material. The only exception is that the initial node in the chain is not glossycoating, as all glossycoating materials need to have a base material specified.
Specular color and IOR
This is the color of the varnish coat. In many real world materials, all color is in the base rather than the gloss, so this color should often be left as gray. (For example, the paint on porcelain is applied before the glaze, so your paint texture should only be on the diffuse channel). Darker colors will make the material less shiny, especially at shallow angles. Optionally, you can specify an index of refraction for the coating material, however this will prevent you from coloring the coating, it will always be gray. This can be useful if you want to specify a specific material making up the coating. For example, a wet floor would have IOR = 1.333, the IOR of water.
The specular color should not be set higher than about .25, and many everyday materials will have much lower values, such as .03-.05. The IOR and specular color are related via the formula IOR = (sqrt(Ks)+1)/(1-sqrt(Ks)) (Ks is the proper term for specular color). This translates a specular color of .25 to an index of refraction of ~3. A specular color of .05 results in an IOR of approximately 1.6, which is still a bit on the high side. A specular color of 1.0 (full white) results in an IOR of infinity, which is impossible. Even the most extreme metamaterials would only have a specular color of approximately 0.9 (IOR = 40)
This determines how shiny the material is by varying the roughness of microfacets. If your exporter uses the exponent to control roughness, higher values are shinier, with 0 being matte. If your exporter uses the direct roughness control, lower values are shinier, with 0 being a perfect reflector and .8 being matte. Values between .8 and 1 are an unrealistic "super-matte" and should be avoided.
These allow you to specify the color and depth of light absorbtion by the varnish. Note that since this is an absorption color, it will seem to work "backwards". Setting it to blue will cause blue light to be absorbed, leaving you with a yellow-orange appearance. To defeat this option, set the color to full black (0.0).
The option will simulate light being scattered at the surface by things such as fuzz or fine hair. It is useful when using glossy for a skin or cloth material, and will give a soft, fuzzy appearance: