I have an animated lower third with shadow effects (so one edge blends into blackness). The problem is that this lower third exists only as a QuickTime file with pure black background--the original Photoshop layers are no longer available, and we don't know what app was used to make this.
So I tried a couple of tips I saw online - Shift Channels to set the Luminance to the Alpha channel, for example, but that didn't get me anywhere near a clean alpha channel background. What's the best way to handle this?
Without seeing it nobody can tell you much. Chances are that it would be quicker to rebuild. Otherwise multiple channel operations may be necessary.
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If that doesn't work then you'll have to figure out how to generate a procedural matte or possibly use a difference matte. It's hard to say without seeing at least a frame of your lower third.