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I'd like a simple color material that has for example 80% front face opacity, 20% back face.

If somehow this is not possible, I'd like to at least achieve semi-transparent front face and invisible back face.

Test models sample enter image description here

Blender 2.91

Test file:

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  • $\begingroup$ There is a lot of useful information here about differing materials/transparencies on front/back face orientations. $\endgroup$
    – james_t
    Dec 16 '20 at 17:42
  • $\begingroup$ (late for me..GMT) could you share your misbehaving file on blend-exchange.giantcowfilms.com? $\endgroup$ Dec 16 '20 at 22:35
  • $\begingroup$ @RobinBetts File uploaded. $\endgroup$
    – Slava
    Dec 17 '20 at 18:51
  • $\begingroup$ I'm getting a reasonable result with some tweaks, which I hope you don't think is cheating! I was bearing in mind: 1. Lighting.. the spheres absorb light and shadow the plane.. the distinction is there in your version, but not very visible. 2. Caustics.. don't seem very happy in this unreal context, so I switched them off. 3. Roughness of surface changes contributions: refract/reflect. You might want to use the Backfacing switch to affect Alpha instead? It depends, how physical you want to be. $\endgroup$ Dec 19 '20 at 8:26
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For a face without thickness, you could just use the Backfacing output of a Geometry node to make a switch between values:

enter image description here

(If you don't want unrealistic 1-sided refraction, set the marked IOR field to 1.. you could also use the switch to control, say, Alpha,which may work better for you.. or any other property in the material)

These are simple planes: the faces differ only in their normals-to-camera:

enter image description here

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  • $\begingroup$ Hi Robin, I realize that I forgot to mention being a total beginner. Could you please show the full nodes including Principled BSDF and beyond? I can't get the results without that. $\endgroup$
    – Slava
    Dec 16 '20 at 20:00
  • $\begingroup$ Hi, @Slava .. is that better? $\endgroup$ Dec 16 '20 at 20:29
  • $\begingroup$ Hi Robin, as much as I'd like to say yes, I couldn't replicate the effect by adding notes as your screenshot shows 1:1. My line from Geometry>Backface doesn't have that split though. I just have two lines. My example result is attached to the question. $\endgroup$
    – Slava
    Dec 16 '20 at 21:26
  • $\begingroup$ The split is just an organizational tool. Shift + A -> Layout -> Reroute $\endgroup$ Dec 16 '20 at 23:54
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My scene is the default scene with a suzanne added in. Default light has been converted to sun with a strength of 3.

Expanding on the backfacing output, we can use it as a factor for a mix shader to do what you ask on a mesh. We can use two principled BSDF nodes with different transmission values.

Here's the principled BSDF on its own for reference.

enter image description here

Here is a setup reflecting your request. Light has about a 50% chance of entering the mesh, once it's in there, it's exiting immediately out of whatever face it hits.

enter image description here

We can add in the Transparent BSDF to produce this effect.

enter image description here

And here it is with IOR set to 1, no Transparent BSDF.

enter image description here

And with:

enter image description here

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