Alpha Channel white balance ? - Video between AE and Vegas

Question

Hi Everyone

I changed my PC recently (better GPU) and I can't work out how to add a video with transparent background such that I use a Quicktime file with an alpha channel.

In Vegas you right click the file and go properties - media - alpha channel - straight unmatted. But I can't get the video white balance to reflect the same as in vegas - AE is adding a whiter alpha background. When I add AE text graphics over the video - the white balance difference is annoying. Please see picture which shows a mask I have applied in vegas around the AE exported text layer to show the different white balance. Before with my old PC it was so simple just to select an animation and RGB & Alpha.

Ive tried about 100 different renders but I am geussing now. I have made sure the video in AE is 8 bit depth and so too in Vegas - one setting in AE in Media encoder is 'HDR Graphics White (Nits)' under 'Basic settings'. I think this may be causing it but when I go into the tickbox next to it, and deselect it, to make sure no HDR settings is being applied, an HDR setting for HDR Graphics White still shows up in the output text in the summary box of media encoder. My Vegas project is just PAL 25FPS and has no HDR setting applied.

What am I doing wrong? and what settings should I use. Thank you for any help.

David Arbor · Answer

It sounds like you're overcomplicating things a bit. First, alpha channels have nothing to do with white balance; the level of white vs black is tied to transparency. You can use certain color correction tools to affect an alpha channel, but I don't think that's necessary here. All you need to know is that a fully white part of your alpha channel is opaque with zero transparency and a fully black part is completely transparent.

In After Effects, make your graphics with a transparent background (you can confirm this by showing the Transparency Grid in the Composition panel). After this, export your file as a format that supports an alpha channel. QuickTime is a container and doesn't say what codec you're using or whether your exported file has an alpha channel. ProRes 4444 is a cross-platform codec that supports an alpha channels, so I recommend you try that. It's not enough to choose ProRes 4444, though. You have to also make sure the alpha channel is included in your Output Module settings.

There's an Output Module preset that takes care of all of this for you. It's called "High Quality with Alpha."

If you choose that preset and then click on the blue text, you'll see the included channels are "RGB + Alpha" and the format is ProRes 4444.

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