[rusteberg]

is there a way to render out color + alpha and still have background appear in render?

doesn't seem like there is, but maybe i'm missing something?

[zdeno]

Official answer is : NO

[Bubbaloo]

The anti-aliased edges of the objects will be affected by the background color if it's not black, resulting in a poor result when compositing another background. Are you making an alpha to use for a background composite or something different?

[zdeno]

as Bubbaloo wrote, replacing background with black color is good, and it is always better to paste background image in postproces.

but if You use 2.0 version

You can :

set Material ID for every material as white 255,255,255 (in materials properities)
and render with background + material ID pass which will be Your alpha chanell.

It has some limitation of course. refraction is not seen in this mode. only plain opacity is visible. So if there is not too much glass You can try this way.

but It is not recomended.

[rusteberg]

thanks for explaining that to me. it makes perfect sense why it does what it does now...