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Alpha Channel Not Showing Up

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Angel NievesAlpha Channel Not Showing Up
by on Aug 4, 2011 at 8:02:32 pm

I have a shot which is 32 seconds long. At about the 31 second mark there's an animation which uses the alpha channel. I've been rendering with the Animation codec, which supports alpha channel, and selecting the 'RGB & Alpha' option, but the alpha won't show up. When I lay the shot over the video in FCP, the alpha portion is black. Only when I render a portion of the shot, about the last 4 seconds, does the alpha channel show up, and I have to add that as a tag at the end of the FCP shot to get the desired effect. But why won't it render the alpha channel when I render the entire 32 second shot? Any ideas?


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Dave LaRondeRe: Alpha Channel Not Showing Up
by on Aug 4, 2011 at 8:12:34 pm

I'm not following what you're doing. Are you adding the animation in FCP, or in AE? And where was this animation made: FCP or AE?

Please clarify. At the moment, what you've written is awfully confusing.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Angel NievesRe: Alpha Channel Not Showing Up
by on Aug 4, 2011 at 8:32:24 pm

Sorry about that. I created the animation in After Effects, which lasts 32 seconds. At the tail end of the animation there's an effect (Twirl) which uses the alpha channel. But the alpha channel is not rendering when I render the entire 32 second shot. If I render only about the last 4 seconds of the animation, and not the entire 32 second animation, then I get the alpha channel.

My reference to FCP regards only the part in which I bring the animation into FCP to add to my film. I lay the animation over another shot which is revealed through the alpha channel once the animation plays out. But the alpha is black, which means the shot underneath is not revealed.

I have to render the last 4 seconds in AE and bring that additional quicktime into FCP, and edit it in to replace the last 4 seconds of the 32 second animation, and then I get the desired effect, because that 4 second render does show the alpha channel. I get the desired effect that way, but I would like to have it all in one shot.

I guess my question is, why is it rendering the alpha on the 4 second version of the animation and not the 32 second version?


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Dave LaRondeRe: Alpha Channel Not Showing Up
by on Aug 4, 2011 at 9:02:49 pm

Thanks for the clarification. Now I understand!

I would inspect your AE comp closely throughout its duration for transparency. You can do it by toggling on the comp window's familiar Photoshop-style checkerboard background, or you can change the comp window to see Alpha instead of RGB.

The most likely offender is a black solid in the comp. If that ain't it, you need to scrutinize that animation in the comp to discover why there's transparency only in the last 4 seconds. From your description, there's no way to tell how you did it.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Angel NievesRe: Alpha Channel Not Showing Up
by on Aug 4, 2011 at 10:05:59 pm

The comp has nothing except a precomp which contains all the elements. I used the precomp to apply the Twirl effect, which uses the alpha channel.

It's important to note that when I render only the tail end of the comp, I get the alpha channel. Only when I render the entire comp do I not get the alpha.


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Dave LaRondeRe: Alpha Channel Not Showing Up
by on Aug 4, 2011 at 10:16:04 pm

Okay, then if you're unwilling to dig into that animation to see what's cooking transparency-wise, just render out the tail end. Set the beginning of the work area to the point where transparency begins and have at it.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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