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“Screen” mode – alpha issue
Hello goodpeople!
I am a After Effects supernovice having some problems solving an “alphaissue”. Think of the project being two layers on the timeline. The first and bottom layer is a flashlight moving about. The second one is the volumetric light coming from the flashlight. The flashlight has an alpha channel, and renders perfectly to a quicktime (with its alpha). But to get the desired effect of the volumetric light (top layer in AfterEffects timeline), I had to render (from the 3d software) all the frames of the volumetric sequence with solid black background. Then I turned the layer mode to “Screen” in AfterEffects and voila! Here comes the “problem”; The “Screen” mode renders black behind the whole movie instead of preserving the alpha. If i put a picture/movie under the bottom layer of the project, the screen mode acts as it should. But whitout anything there, the whole ting turns dark (when if exporting to and qicktime with alpha embedded).
I get the logic behind it, but is there any way to bypass this without going back to my 3d-software to render the volumetric with alpha? Any way preserving the alpha but still using “screen” mode on the volumetric etc.
Any help is deeply appreciated!
Have a great weekend!
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3 Replies
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You want to download a freebie from RedGiant ( I think that’s who owns it now) called Knoll UnMult.
You can google unmult or visit any of the dozens of plugin tracking sites.
Or you can search here for volumetric lighting tutorials, there are many ways to simulate crepuscular rays without resorting to 3d apps.
bogiesan
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[Sopwith] “Any way preserving the alpha but still using “screen” mode on the volumetric etc.”
Using the screen mode will not work. You should set the layer to use itself as a matte using the Channel > Set Matte effect. Set “Use For Matte” to Luminance (lighter areas = more opaque, darker = more transparent).
Then you’ll probably also want to apply a Channel > Remove Color Matting effect to remove the black fringing you will get around the beam.
Darby Edelen
DVD Menu Artist
Left Coast Digital
Aptos, CA -
Oh, and in the future, render with an alpha channel from your 3D app =)
Darby Edelen
DVD Menu Artist
Left Coast Digital
Aptos, CA
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