Creating a pass/render layer of refracted objects only

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Hi there,

I am placing some synthetic objects in to a back plate and these objects pass behind a drinking glass in the photograph. After setting up some geometry to represent the glass in the plate and creating a glass shader (mia materials), I can change my shader set up so that my RGB render looks correct, but I can't completely get the alpha channel correct. Any thoughts would be appreciated...

Here is a simplfied example I have been testing:

beautyRender.jpeg

Two spheres, one glass and one opaque placed behind a glass cylinder. These are lit with one spotlight, an IBL and final gather.

I have changed the scene set up to give me this result. Isolating only the refractions and reflections of refractions.

RefractedObjectsOnly_Colour.jpeg

To get the above image I created a new render layer, changed a few render stats, turned off the IBL, created some duplicate materials and used mib_lookup_sphericals for the material environment shaders. This basically gives me the RGB result I want (ignoring the floor for now, which is hidden to stop it appearing in the glass cylinder).

The problem however is in the alpha channel, with propagate alpha turned on I get this result:

RefractedObjectsOnly_Alpha.jpeg

This is not going to composite correctly for me.

I hope this simplified example makes sense. I haven't quite worked out exactly where the problem lies or where to begin fixing this, any thoughts would be appreciated.

I have also attached a maya project with the scene file, textures and example images

Thanks for your time

James