Is it possible to render with an alpha world material?

Exactly what the topic says.

If you save the rendered image with an alpha channel the backround will be transparent.

just set file type to png, and in render buttons, click ‘premul’, and ‘RGBA’

What exactly does ‘premul’ do? It seems that it preserves things like stars, but how exactly and on what other effects does it have?

if you use premul, you can get antialiased edges. if you use key i think it always leaves a hard edge. i just turn off osa when i want a hard edge though. i think that’s the deal.

thats SWEET!!! now i can super imposer (overlay) my 3d animations into a normal video!!!

That’s not quite it. Both Premul and Key uses antialiasing, but Key leaves the color channel intact and antialias the alpha instead (which is why the colors of halos look weird with Key unless you layer them on top of something else).
Premul oversamples both the alpha and color channels.

Martin