PBR Specular question

Development Rendering
1

I know there are various approaches to PBR, and I’ve read about several of them in the various PBR guides that are scattered about the internet. I’m working on a scene which in which all the materials are dielectrics, which should (according to several PBR guides) mean an F0 value of around 4%. However, UE 4 defaults to a specular value of 50%. The UE wiki recommends leaving this untouched and using roughness to control things, but with an F0 at 50%, that makes every surface awash with reflections, which no amount of roughness (even pure white) can temper at all.

Having read that dielectrics should be between 2 and 4%, I plugged a value of 0.04 into the UE4 spec channel and things looked hugely better. This goes against the recommendation in the UE wiki though.

Am I missing something?

2

It’s quite complicated. As far as I understand, it works this way:

The default value in Unreal Engine 4 is 0.04, so 4%.
In the material editor, it’s equivalent to 0.5 of Specularity.

Which means if you plug a 1.0 constant in your Specular Output, it’s going to be 0.08, or 8%.
If you plug a 2.0 Constant it will be 0.16 or 16%.

3

I wonder why when I don’t plug a value into spec then, that my scene is awash with crazy amount of reflectivity? Even with roughness set to white the scene is massively reflective. Is there anything else which controls this?

4

Specular amount for non metals is 2% to 4% linjear unreal automatically deals with fresnel, some plastics might show metal like reflections so you might want to create slight blend. Also for metals you usually want more specular something around 0.6