Correct value of Specular for PBR Materials

Development Rendering
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Hi all,

I am new to UE4 and come from a Film/VFX background having worked on several offline render engines such as Renderman, Arnold, Vray, Mantra.

I have a question pertaining to creating PBR Materials in UE4

As per the UE4 documentation for PBR Materials, it says the default specular value for say plastic should be set to 0.5 (Refer the Specular section - Table of Measured Specular Values)

However, if I scroll down on that same page to the References and open up the first reference link which points to the blog from Lagarde, it states that for PBR materials, specular values are actually fresnel reflectance at normal incidence i.e. f0. Which means for most plastics and dielectrics, the specular value will be 0.04 (based on IOR of 1.5). Additionally this page also has a table for specular values for different PBR materials.
Now this specular = f0 theory is what is used in almost all offline render engines for PBR materials

As per this understanding, the specular values for glass, plastic should be 0.04 while for water/ice should be 0.02

However on the UE4 Specular values documentation, the specular values for glass, plastic is specified as 0.5 while for water/ice is specified as 0.2

So I’m confused as to if the UE4 Specular values documentation is incorrect OR am I missing something here ?

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Just came across the question as I was researching Ray Tracing in UE4 4.26. This may be too late for you @viduttam but just in case someone else comes along after me…

If I interpret the below ArtStation post (h/t Iri Shinsoj) correctly it sounds like Epic re-normalizes the inputs in UE4 to fit the range that Lagarde describes:

“In Unreal Engine the specular is a value between 0 and 1 and is used to scale the amount of specularity on non-metallic surfaces. It has no effect on metals. In most cases just let the value by default (0.5) which is good for 99% of the materials. Most of the materials reflect 4% of light, when looking strait at the surface. Specular value of 0.5 represents 4% reflection, that said, the specular range 0-1 represents 0-8%. Engine handles the fresnel effect, rising the reflectivity at the angles.”

https://www.artstation.com/blogs/shinsoj/Q9j6/albedo-chart

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