I'm trying to replicate the "polarisation" effect - so if you look through a piece of glass that's polarised you can see an image behind that image that normally would be invisible or white as it's been polarised too. So, I want to have a see-through material I can apply to something, so that the thing behind it is only visible if seen through that material. Is that possible? If so, how?
Material seen through window:
Viewport render showing Suzanne through window:
Viewport render showing invisible Suzanne next to window:
What I have is a Glass BSDF window, slightly off white in color so to be seen, and with a 1.00 IOR to be in all rights except one a Transparent BSDF. Glass BSDFs refract rays like glass refracts light, and I can test for that refraction (called a Transmission Ray).
My Suzanne has a material (red Diffuse BSDF, but can be anything) that is mixed with a pure white (clear) Transparent BSDF. The mix Fac:, whaddoyouno, just happens to be a Transmission Ray :)