Imagine the following scene, top down view with a simple object emitting light (cycle) on a large floor (plane).
Now what i'd like to achieve is to only render the object and the emission of light but not the plane (tranparent bg). of course if I hide the plane there is no emission visible since there is no surface to refect it.
I think maybe using green screen effect (making the plane green) could do the trick but I don't think it will be very clean for a effect like that. I wonder if there is any other way it could be done ? I tested transparent material but again no emmision visible then.
edit : picture to illustrate what I do what i see what i'd like to see in a way less ugly version (done with my awesome skill in gimp) (but since its a white glow on a white background you can't see it here damnit)