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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.
