I'm helping a friend with a workflow for alpha channel lower thirds. When they come into FCP 7 they are gray instead of white. I've tried exporting alpha channels separately, all bit levels. It is something to do with the way FCP is interpreting the alpha because if I render with a temp background and bring that in the whites look perfect. (White bar has about 85% opacity).
A white bar with 85% opacity over a darker background should appear gray.
There are two relevant settings in FCP that may alter the appearance here. One is alpha interpretation. This can be straight or premultiplied, and this should match your output settings from Ae.
The other is gamma. FCP may incorrectly guess gamma for RGB-encoded video (like the Animation codec). Import the clip into FCP. Select the clip, then hit Cmd-9 to open Item Properties. Right-click the gamma value (normally set to Source) and set it to 2.2.
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Thanks so much Walter. I couldn't change the value for the gamma, mostly because it didn't show any value but then when I looked at the alpha options (I'm not a fcp guy) it looks correct when set to Black.
I'm not sure I understand why this is the solution in fcp but I'm glad it works.