To premultiply or to render separate alphas?

[Rusty Robbins]

Hi there
I’m doing a research paper on compositing and I’m finding that the issue of premultiplying seems to be coming up a lot. I understand that CG images can be brought into a compositing package with an integrated alpha channel and used as either straight (unpremultiplied)or premultiplied. The issue out there seems to be in making sure the system is expecting the right version of what has been rendered. If the system is not set up correctly, weird black or white fringing could result.
I was wondering if, in the industry, it might be conceivable for artists to use separate alpha renders that are brought in independently of a primary, RGB-only image sequence. This is also known as Matte & Fill. That way, with no integrated alpha channel, premult issues might not happen and time could be saved in troubleshooting comps. In AE I would have to run track mattes with each render, right? Could it be made easier if you precomp each element with its imported alpha channel? In a node-based package like Combustion, it seems like it might be easier to manage many elements like this. Does anyone out there have thoughts concerning this? I’d love to hear from industry professionals. Thanks!

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