According to the wiki manual on material nodes, there should be a little green ‘a’ button for using the alpha of the second input on the ‘mix’ node. Only I can’t find it anywhere. I’ve searched for it as far back as blender 2.43 up to the current version 2.48a.
Is this just a mistake in the wiki manual? Seems like it would be useful to have an ‘a’ button on the mix node. Can anyone else find it?
As far as I can see, the button in question only exists on the compositing mix node. The material mix node doesn’t have this button. I assume this is because materials have separate color and alpha data (and all of that other stuff, too), while images have a per-pixel alpha channel built-in.
Thanks for the information TheGryphonRider. Perhaps someone should correct the wiki page as it’s a bit misleading.