Let a hidden object affect a render

Hey guys!

I created a fluid simulation that I want to include in a premade video later on. To ensure that the fluid behaves like it should, I used another object as an obstacle. Now I want to render only the fluid without the obstacle - no problem. But certain parts of the fluid are hidden behind the other object, and these parts shouldn’t show up in the render as well.
How do I accomplish that the hidden areas won’t show up, even if the object that hides them isn’t visible in the render?

I have one more question, that tends in the same direction… The fluid should look like water, so I used a glass material and changed the IOR. Obviously Water relates strongly on the environment when it comes to its appearance, because it’s translucent.
How is it possible that the environmental influences are taken into account even if only the water is rendered?

Thanks,

Jazuac

you can get this by using layers. Put your obstacles in different layer from the fluid and ant renderable objects, and set it as a mask in the RenderLayers tab.

About the refraction/reflections, it will be a bit tricky. I think you’ll have to play a lot in compositer, and the it can go from modeling and texturing all your scene according to the base footage, to just deform the footage, by some normals/refraction/reflection vectors of your fluid object.