Alpha channel behind glass

[pafio81]

Hi to all, I'm newby with Corona, and I need some help in order to obtain an aplha channel that works behind the glass.
Anyone could suggest me how to resolve?

I have an interior scene with a glass window, outside the window I have placed a sphere object mapped with an HDR material.
My need is to extract the alpha channel  in order to replace in post production the background.
I've tried all the way but I have always a full white where I have glass panels.

Thanks.

Fabio

[Dex]

Hi, I'm a newbie too. Fabio, please use this reply as a starting point only!
Render settings -> Click the button "Effect" -> Corona Multi Pass -> Click the button "Add Pass" -> shading ->Alpha. Drag a material or object over to name field within the Render settings box.
That would give you something to select and Mask in photoshop.
I'm sure there is a better way! If anyone can give a correct reply, it would be very much appreciated.

I guess you need to check "Alpha Channel" in the Render Settings -> Save