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Can someone please explain the new 'Metallic reflection' parameter in the mantra surface shader to me? I've dug around in the network but it still doesn't seem to make sense to me. Is it affecting the original spec to make it more metallic, or is it adding another layer of reflection? it seems to have the same effect as using the specular minimum parameter, thus effectively mixing between the fresnel blending or not... is that right and the spec minimum is just deprecated, or is it something else?

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No, it is not the same as boosting spec minimum but it may look similar in some cases. Conductors displays a different response to the light than dielectrics and its reflections have different characteristics. Specular Metallic parameter blends between usual fresnel derived reflections and conductor-like reflections which should help you to render more natural looking metals. Also, it allows you to use Metallic Edge Tint parameter that is specific for conductors and has no effect when Specular Metallic = 0. I is not, however, a full complex fresnel equation to compute an accurate conductor appearance as that requires the extinction coefficient besides the surface refractive index. This complex fresnel is available as a separate node.

Spec minimum only acts as a minimum clamp for the surface reflectivity. 

in case you are interested in math background: https://seblagarde.wordpress.com/2013/04/29/memo-on-fresnel-equations/

Edited by davpe
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