I’ve run into a problem getting an alpha channel for compositing from some dirty glass panes I’ve created in my scene. I need the alpha channel to composit the room seen through the panes.

I’m using mia x materials in my scene but since these don’t work directly with layered textures I’ve plugged my mia x glass material into a surface shader and this in turn into a layered texture node. On top of this layer I’ve placed a dirt texture. The layered texture outputs to another surface shader and this is applied to the window panes. I’m happy with the result of this set-up. However, I want to render separately the windows and the rooms behind them because I need to composit some live action elements between them. This is where the problem arises: I can’t seem to render out the alpha information to make the windows partially transparent. It appears the problem is created by the surface shader: if I apply the mia x material to the panes and render this out the alpha channel works correctly but if I apply the surface shader it’s plugged into and render this out, After Effects tells me the tiff has an alpha channel but it doesn’t allow me to make the panes transparent.

So I’m wondering if there’s any way of getting a useable alpha channel out of a surface shader, and if not, whether someone can suggest a workaround.

(Since I wrote this, I have tried using a transparent Blinn instead of the mia material and applying the layered texture to the panes via a second Blinn but can’t get any transparency in the render at all.)