This problem has been absolutely killing me.
Here is an example .blend file: http://www.pasteall.org/blend/21564
When I render out passes for an object, those render pass images have an alpha value of 1. The “combined” image has an alpha of 0 where it should. Now, if I want to use the “Glossy Direct” pass to drive glare streaks, I’m doing it to an image that has a solid alpha. How do I composite this over a BG?
This is not just for Glossy or Diffuse passes, but also ID passes. When I want a particular Material ID to have some Fog Glow to it, I can’t seem to use it very well, since the edges of the alpha kill the effect beyond the boundaries of the alpha. I imagine it’s something very simple I’m missing, but I have no idea what it is.