# Correctly combining a two layer material in cycles

I am trying to create a material that has two physical layers. A thin top layer covering the second main body layer of the object, I have drawn a quick sketch of what I mean:

Light interacting with the material can

0) be reflected off its surface
1) be scattered by the top layer
2) be scattered by the lower layer
3) be scattered by both layers
4) be transmitted right through both layers.

My question is, how to combine these two layers. Say I have the properties of each layer set up in an individual shader, will a mix node give the correct results or do I need to combine them in a different manner?

• Looking for a solution that doesn't require extra geometry, I see two "hypothetical" options. [A] A surface shader (0) and two volume shader (that will cover everything else, (1)(2)(3)(4)), where their mixing factor depends on a "distance from the surface" coordinate, that - as far as I know - doesn't exist as a node. [B] Two surface shaders added together: one for (0) plus a subsurface scattering for (1) and (3); and a volume shader for (2) and (4). Unfortunately, though, the subsurface scattering doesn't have an entirely sharp falloff mode, so your red and yellow materials would blend. – Nicola Sap Jul 19 '17 at 17:37
• Is there a separate texture to the lower layer? If not, these various situations look like they could all be represented by a proper mix of SSS and translucency. The ratio of the two shaders determines the amount of category 4 rays and the radius of the SSS relates to the thickness of the top material. – PGmath Dec 25 '17 at 2:38
• Maybe give this a read google.com/url?sa=t&source=web&rct=j&url=http://… – Leo Nov 8 at 0:57

## 1 Answer

This link might be helpful. It has a cycles material that has thin film interference. Here is a Wikipedia article on what thin-film interference is.

Excerpt:

Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another, either enhancing or reducing the reflected light. When the thickness of the film is an odd multiple of one quarter-wavelength of the light on it, the reflected waves from both surfaces interfere to cancel each other. Since the wave cannot be reflected, it is completely transmitted instead. When the thickness is a multiple of a half-wavelength of the light, the two reflected waves reinforce each other, increasing the reflection and reducing the transmission.

Here is the blender file download link. I think it can be applied to what you want. It puts a bit of "thickness" on your object. If you want it to be more dramatic/visible, let me know and I'll try to brainstorm another solution.

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