Physically Based Rendering (hereafter PBR) is a process by which 3D assets can be created for use in any lighting environment. Given that there’s a fairly in-depth explanation as to how this happens in the citations of this post, what I write here aims to give a rather layman’s understanding, and hopefully to show I understood what I read as well. To start…

Albedo as opposed to Diffuse – if we’re not being too technical really the terms are interchangeable. The idea is though that while in a more traditional texturing/ rendering format you would paint in a good deal of the lighting detail relevant to reflection/refraction of coloured light and general shadows into what would then become the diffuse map (that is to say, the diffused colours for the area you’re rendering to) in PBR this isn’t required, as the basis of the entire process is that the renderer will handle all the light calculations based upon values presented in other maps for values like roughness (or gloss, in the inverse), metalness, ambient occlusion, cavity maps etc.  The bottom line is that an albedo map should be showing colour values for unmolested, 100% white light with 0 reflective sources, in general.

How Metal Are You? – There are a few exceptions to the rule but in general most of the world’s substances fall into 2 categories: conductors and insulators. To consider this in the every day sense it means does it conduct electrical energy, and by extension light? This is from a physical rendering standpoint probably one of if not the most defining factor of most materials, as it allows in engines that list a metal map the ability to do away with old specular rendering techniques entirely.

Defining Material Properties with Metal Maps - Source: http://www.marmoset.co/toolbag/learn/pbr-conversion
Defining Material Properties with Metal Maps – Source: http://www.marmoset.co/toolbag/learn/pbr-conversion

Central to most PBR systems for this reason is the roughness map, the purpose of which it is to dictate to the renderer the general reflective nature the material possesses, referring to the level of light either passing through an object entirely, being reflected entirely, or being reflected after partially passing through from a different point within, by which you begin to achieve simulated subsurface scattering (I want to go more in detail into that in another post). Most aforementioned conductive metals will for instance fall within a reflective range of 70-100%, and conversely in general most insulators as a rule of thumb will not exceed a reflective range of 20%, with the majority even of those materials not passing 5%. For this reason there are in fact two methods to go about rendering out differing conductive material values. The aforementioned metal mapping being tweaked by the values presented in the roughness is the first, and the second is a more traditional approach by which you still bake in some of the specular data to your diffuse and communicate a more exact specular value via a separate map using knowledge of the properties of the material taken from real-world observation (which you should still use for both methods, but it’s far harder to mess up in general with a metal shader).

Fresnel – Light reflected from grazing angles (outer extremes). From a real-world physical standpoint all materials irregardless of a conductive or insulative nature will have reflective properties if light is striking in an indirect fashion, increasing directly for how indirect it happens to be. The Fresnel effect is obviously still affected by the nature of the material in question, but for more generally reflective materials it will be far more distinct, and generally begin being so closer to the point of direct contact. Normally for this reason there is little reason not to have a Fresnel present, controlling the properties indirectly via a gloss (roughness in unreal 4) map across all individual materials.

An example of Fresnel effect and the curve that defines it. Fresnel is present in all materials to some degree. - Source: http://www.marmoset.co/toolbag/learn/pbr-theory
An example of Fresnel effect and the curve that defines it. Fresnel is present in all materials to some degree. – Source: http://www.marmoset.co/toolbag/learn/pbr-theory

References

Docs.unrealengine.com,. (2015). Physically Based Materials | Unreal Engine. Retrieved 2 November 2015, from https://docs.unrealengine.com/latest/INT/Engine/Rendering/Materials/PhysicallyBased/index.html
Quixel,. (2015). Megascans. Retrieved 2 November 2015, from http://www.quixel.se/dev/megascans
Russel, J. (2014). PBR Theory. Marmoset. Retrieved 2 November 2015, from http://www.marmoset.co/toolbag/learn/pbr-theory
Wilson, J. (2014). PBR In Practice. Marmoset. Retrieved 2 November 2015, from http://www.marmoset.co/toolbag/learn/pbr-practice#micro
Wilson, J. (2014). PBR Texture Conversion. Marmoset. Retrieved 2 November 2015, from http://www.marmoset.co/toolbag/learn/pbr-conversion

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