Hm.. I don't see why you can't just re-composite the sky back in using Foreground Alpha?
The alpha pass gets really tricky once you have glass, fog, other transparent materials, etc. How does your color pass then look to compensate for partially transparent pixels? In my experience the foreground alpha method works much better in these cases.
Also, you can extract the alpha channel from a PNG with transparency, which would give you the BW alpha pass.
Indeed, you are right, I forgot about that. But to composite aerial perspective you have to render scene without objects, right now no such option in blendigo and doing it manually you can make mistake like files overwrite.
Background re-composite (with rendered pure background) will let you make aerial perspective or clouds perfectly visible through glass (without refraction).
Right now steps involved to get aerial perspective:
- render image with background alpha enabled
- render background (objects turned off)
- render z-depth
- extract alpha mask from main image (for fog-on-objects compositing)
- composite fog on objects and that on background
It seems that to create complete and easy (no extracting) passes solution there should be "alpha-mask" (as in req) and "background pass" and like e.g. in cycles:http://blender.stackexchange.com/questi ... background