I would guess that the output format your using involves compression, like JPGs, yes? If that's so, then you'll end up with all kinds of problems around the edges of things, as that is the area that a JPG compressor will anti-alias. And since anti-aliasing smudges the colors together to make it 'kinda' smooth, then those areas are no longer 100% white. I would agree with the others, use TIFs instead, set the background to NONE, activate the G-buffer so you have an alpha-channel, and then adjust in Photoshop, or your editor of choice. If this is an animation, then output as sequencial-TIFs, and use a batch process in photoshop to do the edits, or After-effects, whatever compositing program your using. Hope this helps! :>