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As I've been learning Blender and practicing a PBR workflow, the general consensus I get from others here and on different communities is this:

  1. Keep the specular value to 0.500 for most objects.
  2. Utilize a node group to convert IOR to the appropriate specular value.

What I'm really after though is a happy medium between the above two approaches. Sort of "general guidelines" to follow. What kind of materials generally have high, or low specular values? Is this something I can look up for both dielectrics and metals?

Any guidance would help. Thanks.

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    $\begingroup$ As far as I'm aware, the specular value is a creative slider that isn't physically accurate when used with an otherwise correct PBR workflow. You can use it to tweak the PBR look past what is correct. $\endgroup$
    – Jeff Lange
    Commented May 25, 2021 at 0:33
  • $\begingroup$ That isn't exactly true. While it can be used creatively, certain materials require different specular values. See this. $\endgroup$
    – fmotion1
    Commented Aug 9, 2021 at 22:00
  • $\begingroup$ @Jay isn't specular something the you could get with : Specular = ((ior - 1)/(ior +1))² and searching for a list of the material's index of refraction (ior) on the web, to get the correct specular for every object? $\endgroup$
    – Emir
    Commented Aug 9, 2021 at 22:50

2 Answers 2

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The formula to calculate what real life materials have which specular value done with following formula: $$ \mathrm{specular}=12.5\cdot\left(\frac{\mathrm{ior}−1}{\mathrm{ior}+1}\right)^2 $$ (see link)

Example Water: IOR of 1.33 => so specular is 0.25

You can find the IOR values of many materials on this site: http://forums.cgsociety.org/t/a-complete-ior-list/1070401

So you don't have to use the formula for everything, a guideline is the lower the IOR the less specular. The specular value is very similar to gloss(inverted roughness) just that is has no rough state as you can see in the image below: enter image description here enter image description here

The specular value/map is part of the specular workflow, which isn't really used anymore and has been replaced with the metallic workflow.

Specular workflow

  • Diffuse Albedo map/ Base Color map, Specular, Gloss map, Normal, Height, Ambient Occlusion

Metallic workflow

  • Base Color, Metallic, Roughness map, Normal, Height, Ambient Occlusion

Edit:

Materials like cloths wood sand should have a specular of 0, also note that the specular very tightly plays together with roughness. Without specular roughness has no effect.

I have also noticed that often times in stylized artworks tweaking it a bit can have a great effect. With realistic textures one has to be a bit more careful because this can quite quickly make things look incorrect when using bad values.

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  • $\begingroup$ Thank you very much for your detailed explanation. I am aware that you can derive a specular value via IOR (the formula you show), and you can look values up. However most of the values that you can look up are metals, liquids, glass, highly reflective objects etc. But for instance, what's the IOR of wood? Rubber? Leather? Cotton? Etc. Do all of these examples have a specular value of .5? This is what I mean by a general guide. Sometimes I creatively lower the specular value on fabrics and really rough surfaces to get a darker tone. $\endgroup$
    – fmotion1
    Commented Aug 15, 2021 at 22:28
  • $\begingroup$ (continued) I'm curious to know if other people do the same. I'd also like to know if IOR / Specular even matters when it comes to dielectrics? $\endgroup$
    – fmotion1
    Commented Aug 15, 2021 at 22:31
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    $\begingroup$ Many materials are not as simple as a few formulas. Surface structure, coatings, various contaminants, mixes of materials, dust, dirt, age and other factors have great impact on their look. Those formulas are just a starting point, a reference. You always have to observe how the actual material looks like. Wood is great example. There are tens of thousands of species of trees and every tree may also be different, the wood may be sanded or brushed differently, stained and lacquered and sealed in many ways - there is infinite amount of ways it can look. $\endgroup$
    – Martynas Žiemys
    Commented Aug 16, 2021 at 11:58
  • $\begingroup$ I watched someone explain this and he laughed as he saw the value of sand somewhere, so probably there are materials where the specular value is totally irrelevant. Materials like wood cloth, sand and so on $\endgroup$
    – Sidney
    Commented Aug 22, 2021 at 14:32
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just to add to the comprehensive answer by @Epicrex

Q: What materials should have a different specular value in a PBR workflow?
A: Very few, but there are some edge cases where it's useful


Some tips to use the Specular value intuitively


Specular values between 0 to 1 should all be physically feasible

  • this is one of the 'principles' of Principled BSDF
  • it's good to stay between 0,25 to 1 (only liquid gasses go below 0,25)
  • in this range, it's your artistic choice, you can't go wrong


Specular value of 0,5 actually is a really good default

  • most dielectrics have Specular value of 0,4—0,6 with 0,5 being the most common
  • the Specular slider isn't linear, the difference between 0,4—0,6 is tiny
  • when in doubt, Specular 0,5 will work just fine


Glass-like materials don't use the Specular value

  • When Transmission is set to 1, Specularity is ignored and the IOR value is used instead
  • This covers materials like water, glass, diamond and liquid gasses


For Transmission = 0, Specular value is used, IOR ignored enter image description here

For Transmission = 1, IOR value is used, Specular ignored enter image description here



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    $\begingroup$ I have some doubt about "0,25 to 1" Pure materials - sure... but when there is some surface structure it's all different. Vantablack paint for example has no meaningful specular at all. Most of us will never see it live, but the way it works gives some insight. It's surface structure is made of carbon nanotubes that absorb light when it bounces between them. So, I think some materials like fabrics can easily work the same way, only not to those extremes. It's best to observe. Viewing distance is also interesting - trees in a forest observed from an airplane are "surface structure". $\endgroup$
    – Martynas Žiemys
    Commented Aug 16, 2021 at 12:21
  • $\begingroup$ Oh, that's a good point, thanks :). I was mainly going from this long IOR list linked by @Epicrex. But there absolutely are cases where values below 0,25 are appropriate, Vantablack being a good example. $\endgroup$
    – jachym michal
    Commented Aug 16, 2021 at 15:34

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