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Attenuation

The top layer is considered for both the outgoing and incoming rays, meaning that the ray is attenuated by the top layer twice: once when it leaves and once when it exits. This is the case for both fresnel attenuation at the interface, and for absorption through the thickness of the layer.

Refraction

Refraction is considered by refracting both the incoming and outgoing rays through the top layer, instead of passing them straight through as in the fresnel blend mode. This changes the look of the base layer as it receives these refracted rays. As an approximation, the top layer is considered to have a roughness of zero for the refraction calculation. This means that as the roughness of the top layer increases, the accuracy of the model decreases.

Scattering

To compensate for rough top layers, the roughness of the base layer is clamped to the roughness of the top layer, so that if the roughness of the top layer is greater, then the roughness of the base layer will be increased to match it. This approximates the scattering of rays as they pass through the rough top layer.

Relative IOR

The IOR of the top layer is also tracked, so that the relative IOR of the base layer is correct. For example, if there is a water layer on top of a skin layer, the relative IOR of the skin layer will be reduced, causing lower reflectivity.