Render background on separate layer


I’d like to know if there was a way to render a backdrop (or basically any object) with it being transparent but reflections and shadows visible on it? Preferably on a separate layer so that I can work with them later on. Sorry if I’m asking something obvious or stupid, I love working with material nodes but have been missing out on compositing.

Thanks, cheers!

Some days ago I’ve found a video that explains this:

skip to 12:49, it’s when the RenderLayers explanation starts

Thanks, still can’t accomplish what I’d like to, but it’s a good starting point, maybe I can work something out with this and some passes.
Meantime I’ve figured out what I really need is actually an inverse Light Path Node, which precalculates the type of the next reflection (since Blender starts calculating from the camera, not from light source) - That’s impossible I guess.

well, I really don’t know how to do this… but good luck :slight_smile:

Is that what they mean by bi-directional path tracing?

That phrase rings a bell, yes maybe, but AFAIK Cycles does not work that way.