straight vs premultiplied for Flash import

[Steve Roberts]

It depends on what Flash wants. Why not do a quick test of each?

[Steven J casey]

I’m not sure what you mean by “what Flash wants”. And sure I’ll probably send them a test of each but I was hoping for an understanding of why one might work better than the other.

[Steve Roberts]

Well, Avid will only successfully import movies with straight alpha, so that’s what Avid wants.

– some apps just won’t accept straight
– straight looks like garbage in QT Player
– premult can be a bit of work to extract the colour fringe in some cases.

I generally render straight, unless it’s a test going to a client who’ll view it in QT Player.

Here’s an article:
https://www.adobe.com/designcenter/dialogbox/talkinghead/

It uses Premiere to create the movie with alpha, but it doesn’t specify straight or premultiplied. My guess is that it is premultiplied, because premult movies don’t look like garbage when opened in alpha-dumb apps like Quicktime Player, as I wrote. That frightens people who don’t know better. However, apps that know alpha well can open straight alpha movies and make them look good.

One is not necessarily better than the other, but many apps prefer to import movies with straight alpha, because of the way the alpha is calculated.

This is more of a Flash question, but if you want to find out more about straight and premultiplied, there should be a COW article on it. Check the COW search engine.

[Erik Pontius]

I did a hundred or so small clips that had an actor keyed out of the background that were then handed off to a flash developer to be incorporated into a long training piece. In my case the flash developers told me the exact dimensions of the clips they needed. I pulled the keys from the clips, created comps the required size and then output directly to flash 8 using the On2 Technologies Flix 8 Exporter. It has an option to keep or preserve the alpha channel. The resulting flash clips looked a lot like they were straight mattes and not premultiplied. In the end the developer just had to drop these flash clips into their flash project. Best thing to do is to discuss it with the flash developer and find out what they need specifically. In my case I was able to control the quality of the clips since I was pulling the keys using keylight from uncompressed footage, resizing, etc… and then exporting directly from AE to flash preserving as much quality as possible.

Erik

[Steven J casey]

Thanks guys, both good answers and a lot of information for me to absorb. Now back to keying!

steven