# Setting transparency on mesh from inside, but not outside

Horrible title, I know. I doubt transparency is even what I need. Hopefully I can clarify what I'm trying to experiment with:

Two ico spheres share the same position, subdivisions, and rotations. Only difference between them are their scales, so one is inside the other. Outer sphere has an opaque material applied. Is there a material, or effect I could apply to the inner sphere, that would allow a camera object at the centre to see outside of the two spheres?

Only thing I can think of is to put a camera object at the normal of each face on the external sphere, then assign individual materials to the corresponding inner sphere faces which would display the camera views. However, that seems like a time intensive task, and a nightmare to organise. Wondering if there's a more elegant way to achieve the same effect.

• maybe the Input > Geometry node > Backfacing output socket is what you're looking for? Once plugged into the factor of a Mix Shader it allows you to create 2 different materials for each side of a face, in your case it would be opaque/transparent May 20, 2020 at 8:29
• That would work, if I was only working with the external sphere. The inner sphere would block visibility to the outer sphere's faces, though. May 20, 2020 at 8:37
• Still unclear, to me... could you illustrate (however roughly) the desired end effect? May 20, 2020 at 9:24
• Yeah, that's the problem with this experiment. Best analogy I can come up with is to think of each face of the inner sphere as an individual TV screen, and each screen displays what is seen by a camera on each face of the outer sphere. So if you were inside the inner sphere, you should see an X-ray effect applied only to the inner and outer spheres, but if you viewed the outer sphere from the outside, you should see the material assigned to it. May 20, 2020 at 9:35

I want to thank you for this question because I learned something in researching the answer.

Things have changed. Here's how you can do this easily in 2.8.

There is a backface culling option when in Edit Mode from the Viewport Shading Menu pulldown, BUT this is NOT what you want. Leave it alone. :)

THere is another backface culling option in the Materials tab of the Properties Panel. THIS is what you want. Make sure it is checked.

Your camera will now render what is on the outside of any object with this material assigned to it, but not what is on the inside of it.

It feels good to have finally learned how this works just now. :)

• I didn't find that setting in Cycles, but it did lead me to a workaround that mimics the setting over here: katsbits.com/codex/backface-culling - That seems to have achieved the intended effect. Cheers. May 21, 2020 at 2:19
• Sorry about that. I forgot to check which renderer I was using. But now I know that this too is something to take into account. And thanks for the link. May 21, 2020 at 3:13