Rendering with alpha problems

[janlaiho]

Hi,

I have a scene where a plastic cup sits on a floor plane. I'd need to render out a version with the floor transparent, i.e. an alpha channel file with everything (incl. shadows) but not the floor, with transparency.

I've tried changing the floor material into a shadow catcher. I don't think this is a solution, as the shadows are much more pronounced and deep compared to when the floor material is a standard diffuse material. Also, I think the light bouncing from the floor is different or absent (the cup itself looks different).

I've also tried multipass rendering, with black shadows. Again, the resulting shadow layer is different from what the shadows look like when the diffuse floor material is there. Also, there are jagged edges on the cup itself.

I wonder what I'm after is possible - any help would be greatly appreciated.

Cheers,
Janne