Next I want to talk about Specular as it was key for me when I was trying to understand the difference between Superfly and Firefly.

Specular in Firefly is a hack - it is used because raytraced reflections in FF, especially blurred reflections, are extremely slow to render, so specular is a means of faking them. Superfly however does raytraced reflections on everything all the time, which is why when compared like for like it is much faster than FF. Specular in SF is base reflectance (this can also be called direct reflectance or F0).

Base reflectance is the amount of light that bounces back to you from a material if you shine a light (imagine holding a torch) straight at it. With metals, this is high, with values around 60-100%. With non-metals the range is around 2-8%, with a few gemstones going as high as 16%, so the average for the vast majority of materials is 4%.

The Specular chip on the Principled BSDF appears to map 0-1 as 0-8% base reflectance, so at 0.5 it should be set to 4% for non-metals which will be good enough for a wide range of materials. This reflected light will be white.

When the Metallic is set to 1 it will override this Specular setting. The reflectance will be set to 100% and the reflections will be coloured by the Diffuse chip.

What all this means in practice is that most of the time Specular on the Principled BSDF can be left at the default 0.5 - and if the material is a metal, set that to 1 and forget about Specular. Simple!

So here’s a simple metal -

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