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When modelling / rendering a glass with liquid inside (in Cycles), which direction do the normals for the glass only surfaces, shared glass-liquid surfaces, and liquid only surfaces point? I have looked into this myself but seem to get conflicting answers from different sources. See here for an example.

Also, should each of the surface types mentioned above need a separate material / IOR?


Autodesk has information about these here, but I'm not sure it applies for Cycles / path tracing: Autodesk, normals and IOR for glass-liquid interfaces

Note that Blender SE has some related questions:
How to illuminate the darkness inside glass objects in Cycles?

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The answer seems to be yes. This is a fairly commonly used technique, it works with LuxRender and Yafaray as well.

Here is an example rendered in cycles with the normals and materials setup like the autodesk example:

  • Air-Glass interface (IOR = $\frac{1.5}{1} = 1.5$)
  • Air-Liquid interface (IOR = $\frac{1.33}{1} = 1.33$)
  • Glass-Liquid interface (IOR= $\frac{1.33}{1.5} = 0.8$)

It is evident that in the most common case of an interface with air, the IOR to use is the IOR of the media (since the IOR of air is 1.0), whereas in an interface between two different media, the situation is different.

enter image description here

It seems that the basic thing to keep in mind is that the interface IOR should be equal to:

$$ \frac {\textrm{IOR of medium with normals facing }\textbf{away}} {\textrm{IOR of medium with normals facing }\textbf{towards}} $$

If you set up the normals the other way around, (e.g. here) it is the opposite:

$$ \frac {\text{IOR of medium with normals facing }\textbf{towards}} {\text{IOR of medium with normals facing }\textbf{away}} $$

In the autodesk example, Liquid (with an IOR of 1.33) is divided by Glass (IOR of 1.5), because the normals are facing the Glass medium.

enter image description here

Yes, you do need to use separate materials and IOR values for each surface type. Assign the materials to the appropriate faces:

The Air to Glass material, the Air to Liquid material, and the Glass to Liquid material:
enter image description here enter image description here enter image description here

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