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In PBS context what is the metalness map? Are my interpretation of Normal, Roughness and Albedo map correct?

I know almost for sure that:

Normal map : Models the normals per differential region in our material (kind of).

Roughness map : Together with the normal map allow to define a microfacet distribution in a differential region of our material.

Albedo map : This models the percentage of light that is diffusely reflected.

Normal + Roughness allow to define the amount of energy that is specularly reflected.

I struggle to understand the physical meaning of the metalness though, I understand it's related to have a material look like metal of course, but physically speaking I don't get what it models.

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Those parameters have no direct physical equivalent. They were chosen to be intuitive and to range from 0-1. See the Burley's Physically-Based Shading at Disney for the full rationale and derivation of what is now commonly referred to as "PBR".

The "metalness" parameter is a simple blend between two models - one dielectric BSDF with a coloured diffuse and white specular, and a metallic BSDF without diffuse and a coloured specular component.

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  • $\begingroup$ Is my interpretation of the other parameters correct? $\endgroup$
    – user8469759
    Commented Apr 27, 2018 at 9:56
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    $\begingroup$ Not quite. Albedo changes its meaning depending on whether the model is set to be a metal or not. When it's a metal, albedo determines the color of the specular component. When it's not a metal (it's dielectric then, in most implementations with a fixed IOR of 1.4), albedo drives the color of the diffuse component. Roughness is a parameter that determines how wide or narrow the specular lobe is. Neither normal map or roughness have any influence on the amount of energy that the specular component reflects. Only albedo controls energy loss. $\endgroup$
    – Stefan Werner
    Commented Apr 27, 2018 at 14:47
  • $\begingroup$ Is the document you provided a reference for all of these? $\endgroup$
    – user8469759
    Commented Apr 27, 2018 at 14:53
  • $\begingroup$ To my knowledge, pretty much every shader that calls itself "PBR" these days is derived from that paper, one way or the other. $\endgroup$
    – Stefan Werner
    Commented Apr 27, 2018 at 19:03

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