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After Effects Alpha Channel
I am in need of some help with after effects. I am trying to do a basic alpha channel mask and I am having so much trouble. I bought a video from istockphoto which contained an alpha channel of the same video. The link to the video I bought is below so you can see it.
I need the black background of the flag to be transparent but I can’t get the alpha channel mask to work at all. I am not sure if I am doing something wrong or not. Could someone please help me with the steps or point me to a tutorial that shows me how to do it. I need to have this video done by tonight and I didn’t think I was going to have so many problems with this. Any help would be greatly appreciated! Thank you in advance for your time.
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4 Replies
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I just took a look at the iStockPhoto page that you linked to, and the page says that the footage comes with a luma matte (not a mask, as you said) for this footage.
The flag layer will take its opacity values from the luminance values of the matte layer, so black parts of the matte will cause 0% opacity (100% transparency).
Put the matte layer in the Timeline panel above the footage (flag) layer. Then, in the TrkMat (track matte) column for the flag layer, select the Luma Matte “[matte layer name]” option.
These instructions and more details are in the “Convert a layer into a track matte” section of After Effects Help. You can search for “luma matte” in After Effects Help for more information.
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Todd Kopriva, technical editor, Adobe Systems Incorporated
putting the ‘T’ back in ‘RTFM’ : After Effects Help on the Web
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Todd, that worked! Thank you so much!
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Since the element has some bright white patches on a black background you might also want to apply a Channel > Remove Color Matting effect to the footage before you render. Or just make sure that you interpret the footage as Premultiplied in whatever application it’s destined for.
Darby Edelen
DVD Menu Artist
Left Coast Digital
Aptos, CA -
I was given a number of clips with similar properties (one of the actual subject, one of a luma matte of the same clip) and have been able to key it using the track matte set to luma as was suggested above, but now I need to export the footage as video with an alpha channel. How do I do that in AE? What are my codec options? I’m working on a mac with quicktime files but the final deliverables are supposed to be AVI files. Can I deliver AVI files with alpha channels from AE 7 pro on a mac machine?
Thanks,
Michael Escher
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