Render the Inside of a Balloon?

Hello all,
A show that I’m working on requires a CGI balloon. I’m well on my way with it, but I’ve run into a problem I haven’t been able to solve with tutorials.

Here’s what I’m trying to accomplish:
Since the layers of a balloon are semi transparent, I need to be able to not just see through the balloon to the background, but through the exterior layer to the interior layer to the background.

My current approach is this:
I’d like to render it in 2 passes, the exterior, and the interior. I will composite them in After Effects.

The Problem:
I cannot figure out how to render just the interior of the balloon. For this render pass I want the exterior layer to be invisible. I’ve played around a little bit with flipping face normals, but didn’t seem to be able to get anything to work.

Does anyone have any idea how I can render just the interior of my object, looking through the invisible exterior? Either that or another solution that I haven’t thought of?

I’d appreciate any help,
~Seth

Hi,

Have you tried modelling the interior of the balloon?

Had a go with just material settings:

Something to possibly think about, in this object I extruded in to give it 2 layers as you wanted. You could perhaps
give the inside layer an extra material and ramp up ray mirror, there is some on this atm which is partly why the right hand side is grey.

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Thanks for the thoughts. No, I can’t model the interior as a separate mesh, the balloon has to be able to freely tumble. If I have a weird half sphere representing the interior surface that always has to be oriented properly, the animation process will be drastically complicated. Really… I’m just asking if there’s a way that I can I can make the exterior skin of a mesh invisible so I can see through to the interior surface. If anyone knows how to do that by any method, I’d appreciate the info.

I posted a very rough comp of what I’m trying to do: http://vimeo.com/10547069 … You can see the interior specular highlights, that’s what I’m trying to get. This setup is a hack job, the way I accomplished it was with a half ‘sphere’ of the back side of the balloon, but this method is prohibitively limited when animating.

Any ideas?

I couldn’t get it to work well, but ramping translucency up and turning on sss should help
With our problems on the interior of the balloon, you could do it by extruding all the faces of the outside and scale them in, then constrain the inside to the outside so whatever deformations occur to the outside effect the inside too.

Have you got double sided on or off?

Had another go, this is with one layer only as you asked with material settings:
If you look at the sides, you can see how it travels around…

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Is it a fresnel effect you’re after? That’s in ray-transparency settings.

If you’re after just the single pass showing inside / reverse faces only (I’m assuming by your use of After Effects you’re a compositor and would have no problems mixing) then a quick’n’nasty hack would be to set the camera clipping so that it begins at the centre of the balloon. You’d be pretty limited in terms of rotating the balloon because its long shape would mean it could tilt past the line, meaning you would alwy have to have the balloon flat to screen, so it’s probably not much better than your current shots.

Another method (equally bad but we’re all smoke’n’mirrors here) would be to light the non-transparent balloon from the reverse angle, creating opposite speculars thus shading as though it was the insides.