Blender 3D tip — Realistic Specular value in Principled shader

Tutorial / 02 novembre 2018

I love Blender 3D. It's a deservedly popular and successful open source 3D editor. Together with ZBrush it's my most important tool for 3D creation. Since version 2.79, Blender's Cycles renderer includes the Principled shader, enabling you to create most material types using a single shader node. I'm happy to share a little tip for a slightly more realistic result.

The reflectivity of a material created with the Principled shader node is usually a balance between three values: Specular, Roughness and IOR. IOR is an abbreviation of Index Of Refraction, and indicates how much light rays are bent when they are reflected or refracted.

Every material has an average IOR value. For really accurate results each light wavelength reflecting from or refracting through a real-world material has its own IOR value, but an average IOR value usually suffices in the world of 3D.

Higher IOR values increase the light bending effect. In the case of reflections, this causes a surface to generally become more reflective. For example, shiny metals usually have a higher average IOR value than plastics.

A great online resource of IOR values for all kinds of materials is the IOR List at Pixel and Poly.

The Specular value of Blender's Principled shader is usually left at the default generic value of 0.5, but for a more realistic result there's a formula to convert a material's IOR value to the corresponding Specular value:

Specular = ( ( IOR − 1 ) / ( IOR + 1 ) ) ² / 0.08

Some examples, applying this formula:

  • Water: IOR = 1.33, Specular value = 0.25
  • Glass: IOR = 1.5, Specular value = 0.5
  • Diamond: IOR = 2.417, Specular value = 2.15

To automate this formula for the Principled shader I've made a little node setup that processes an IOR input value to the correct Specular value:

As shown in the screenshot:

  1. Group the formula node setup to a single group node with a value input slot, and label it 'IOR to Specular'.
  2. Connect the IOR to Specular formula node group to the Principled shader's Specular input slot.
  3. Connect an Input ➔ Value node to both the node group's input slot and the Principled shader's IOR input slot. For clarity, enter 'IOR' as the Value input node's label.

For clarity: this node setup translates a material's IOR value to the Principled shader's Specular value, while the Principled shader's own IOR value only applies to refraction (the Transmission setting in the shader), but it's convenient to already plug the IOR value into the IOR slot as well, in case you want to add refraction (Transmission).

Now all you have to do is look up the correct IOR value for each of your materials and enter that in the IOR Value input node. Note that you'll still need to adjust the Roughness value, but this node setup adds a little bit of realism to the Principled shader.

Below you can see two test renderings I made using Blender's Suzanne test-monkey, with a Roughness value of 0.5. As you can see, the interaction between the IOR value and the Specular value causes a sheen of specularity to appear, without using the Principled shader's Metallic or Sheen option.

I hope this helps you achieve slightly more realistic renderings!

— Metin Seven, metinseven.nl