Can a glass object be made to act completely transparent to only the object contained inside of it?

Question

Say I put Blender's iconic monkey inside a glass sphere (hollow, but solidified around its exterior). All lamps and camera(s) are outside the sphere. Using Cycles.

What I want to accomplish is for the sphere to get rendered as an "ordinary" glass object, whereas the monkey inside of it gets rendered identical to the way it would get rendered if the sphere around it did not exist at all. (Most of all, I want to avoid the dimming effect the glass would otherwise have on the monkey's (diffuse) surface brightness).

Is this doable? By using the Light Path input in the node editor perhaps? Or maybe by compositing two render actions into a single image?

Answer 1

Yes, this is possible.

To do this, we can make a glass material which allows some light rays through without refracting them based on certain conditions.

To get a good idea of which rays we want to let through, it's a good idea to try and visualize the possible ways rays could bounce through your scene (at least up to a few bounces):

_{This diagram doesn't show all possible light paths for three bounces, this is just for demonstration purposes.}

I've highlighted the ray which we want to ignore in order to illuminate Suzanne as if the glass sphere wasn't there in green. Keep in mind that rays travel from the camera.

So as a solution for this particular case, you could create a node setup that ignores rays which:

• Have bounced a maximum of once
• Are not transmission rays
• Are colliding with the inside of the glass

Result (200 samples):

For comparison, here's the same scene rendered with just a glass shader (2000 samples):