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Premultiplied alpha not working on freeze frames

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Paul Roper
Premultiplied alpha not working on freeze frames
on Apr 6, 2012 at 8:30:54 pm


I have some rendered animations from After Effects which I'm bringing into FCP. They're ProRes 4444 with a premultiplied (on black) alpha. When I import it, set the alpha to 'black' and then put it on the timeline of my ProRes 422 (HQ) sequence, it looks great.

The problems start if I want to create a freeze frame (shift-N). I have tried every combination of setting the alpha of the footage and/or the freeze frame in the browser and/or in the timeline to straight/black/white, but the freeze frame always looks crap - it ignores the fact that it's premultiplied with black and any translucent areas are too dark.

Is this just FCP being lousy? Do I need to render a great long hold in After Effects just so FCP doesn't get the chance to screw up the matting process if I make a freeze in FCP? I suppose one other workaround would be to do a time stretch on a few frames (FCP won't time stretch a single frame, tragically).

I've tried rendering out of After Effects with a straight matte, but this makes no difference.

Any suggestions would be greatly appreciated!

- Paul

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Sean Smith
Re: Premultiplied alpha not working on freeze frames
on Apr 7, 2012 at 1:50:07 pm

Final Cut is not great when it comes to alpha channels. A lot of 'personality' there. I had this same problem myself a year or so ago. We were using png sequences that I was making reference movies from in QT Pro then importing into FCP. Sometimes there was an alpha channel in these, premultiplied on black. My advice: save yourself a lot of trouble and take care of as much alpha stuff as you can in After Effects. Rendering a long hold there (or just copying the single frame over and over in FCP) is probably the easiest thing to do. I'm curious if anyone else has a different answer though.

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