The above picture is a composite of two layers for a scene I am working on. You can see clearly that emission shaders for the sparks work only if there is RGB behind but not if they are in front of an alpha channel. The back wall sparks do not show across the floor because the rest of the room is front most layer, so that is understandable, but the blue sparks from the side walls lose their emissivity when they cross into the space that was held out. The reason I am layering this way is because there is footage to sandwich in between them.
I checked this with cycles and the problem is there too, but I do not know how to resolve it. Ideally I would render only the sparks and have the room as separate composite layer, but I need the reflections caused by the sparks as part of the effect.
There must surely be a solution? I am sure I have used emission shaders with alpha channels successfully before, but I think it may have something to do with the hold out function.
