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The auto mode is identical to rough coating in most cases, but switches to fresnel blend for diffuse layers. This is to accommodate for the common Diffuse < Specular < Clearcoat material configuration, where the diffuse and specular materials are thought of as one single base material, with the clearcoat sitting on top.
Comparisons
Weidlich & Wilkie surface types
These images show a collection of simple and common material configurations, inspired by Weidlich and Wilkie's arbitrarily layered micro-facet surfaces. These have been rendered with both the fresnel blend mode, and rough coating mode.
Smooth gold with a rough/bumpy purple coating
These images show a smooth gold base layer with a bump mapped dielectric with absorption.
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| fresnel blend | rough coating |
Rough/bumpy gold with a smooth purple coating
These images are of a similar material configuration, but with the roughness and bump map on the base layer instead of the top layer. This configuration demonstrates that even when the top layer is smooth, it still creates a difference in look due to the refraction and compression of rays that reach the base layer.
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