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to Premultiply or Use Separate Alpha Renders?

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Rusty Robbins
to Premultiply or Use Separate Alpha Renders?
on Oct 13, 2009 at 7:50:17 pm

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. 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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Dave LaRonde
Re: to Premultiply or Use Separate Alpha Renders?
on Oct 13, 2009 at 8:05:14 pm

It's not that big a deal.

Does your application call for Matte & fill files? Make sure you have 'em.
Does it accept files with embedded alpha channels? Make sure you get the right kind -- straight or premultiplied -- and use it.
Do you only have Matte & fill files? You either use them or you use a different application to turn them into a file with an embedded alpha channel.

There really isn't any difference in quality.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Rusty Robbins
Re: to Premultiply or Use Separate Alpha Renders?
on Oct 13, 2009 at 10:17:05 pm

Thanks for the quick response, Dave. So, if you were working with an animation department and they asked you what kind of files you would like to do your comps with, would you choose "RGB files with separate alpha channel", "premultiplied with integrated alpha" or "unpremultiplied with alpha channel"? Or, like you said, is it not that big a deal and let the animation people choose?
Thanks again


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Dave LaRonde
Re: to Premultiply or Use Separate Alpha Renders?
on Oct 13, 2009 at 10:24:59 pm

I find that the application often does the choosing. For example, it can get confusing to change the Preferences in Final Cut Pro accept Premultiplied alpha channels, so you simply create Straight alpha channels.

AE is flexible: it doesn't care. It'll guess for you if you tell it to do so, and 999 times out of a thousand, it's right.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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