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Exporting with Alpha Channel Looks Weird

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Ryan Davies
Exporting with Alpha Channel Looks Weird
on Aug 22, 2017 at 10:56:27 am

Hello,

I have some green screen footage of an actor marching which is going to be projected onto a stage background.
My problem is that when I export it with an alpha it looks like I have done a bad job at keying. It looks fine in premiere and also plays as it should when I bring the exported footage back into premiere.

I have tried using quicktime with animation/png codecs and AVI uncompressed. All of these have the same results. I've played the footage with VLC, windows media player, quicktime player, photo viewer and it's still the same.

This is driving me mad! I've had this problem for a few months now but usually I import the footage into another project so it works fine then. But now I need the keyed footage on it's own. I'll include a pic of how it plays when exported.

Can anyone help?

Thanks




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Todd Perchert
Re: Exporting with Alpha Channel Looks Weird
on Aug 22, 2017 at 6:58:34 pm
Last Edited By Todd Perchert on Aug 22, 2017 at 7:00:29 pm

What are you viewing it with if not PPro? QT will not show you the alpha, but you will see everything else used to premultiply. If you plan on projecting this, why not key over black and render flat - without the alpha? That will probably be your best bet.

http://www.spherevfx.com/written-training/miscellaneous-written-training/un...

TC


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Ryan Davies
Re: Exporting with Alpha Channel Looks Weird
on Aug 22, 2017 at 7:46:07 pm

If I render without the alpha won't the black be projected?

Thanks for the reply :)


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Bouke Vahl
Re: Exporting with Alpha Channel Looks Weird
on Aug 23, 2017 at 8:07:45 am

I've not clicked on Todds link, but I'm pretty sure it will explain everything.

There are two flavours of alpha, straight and premultiplied.
Premultiplied will look 'normal', and straight will look bad if the alpha is not applied.
(There could be a lot of garbage, but if the key signal says it's not to be used, it does not matter of course. It's the same that outside the set you see lights, flightcases and half empty coffee cups.)

However, both flavours contain the same data, only the math is different.

Most NLE's accept only straight alpha. AE lets you choose.
So, don't worry about it, just re-import in a NLE, overlay on 'something', then you can do QC.

Bouke
http://www.videotoolshed.com


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Ryan Davies
Re: Exporting with Alpha Channel Looks Weird
on Aug 23, 2017 at 9:09:01 am

I think I've gotten a little confused with the projection aspect. My first time doing anything with projection and I assumed I'd need to export with an alpha to avoid the black background being projected. Just tried projecting an mp4 version and worked how I wanted.

Thanks both for your replies much appreciated ☺ Please excuse my dull brain!!


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Todd Perchert
Re: Exporting with Alpha Channel Looks Weird
on Aug 23, 2017 at 2:37:48 pm

Alpha channels won't work with projection. The area you have cut out will be black. The alpha channel will only work in apps that can utilize alpha channels, such as an NLE (PPro/FCP), or AE/PS.
Good Luck!
TC


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Ryan Davies
Re: Exporting with Alpha Channel Looks Weird
on Aug 24, 2017 at 8:17:37 am

I tested it out and it worked okay for me. Exported it as an mp4 and projected that. The black didn't project so I'm quite happy now


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