After Effects: Sync an opacity to a background video

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Old 04 April 2012   #1
After Effects: Sync an opacity to a background video

I have this scene, where the lights are flickering wildly in the background from various directions, and I have this cartoon animated on top of it - however I did not animate the flickering lights. I decided to do it in After Effects, so I have dropped the animation into its own comp, and assigned masks to the various parts of the character. I have 3 layers for the character, all duplicated.

One represents the BRIGHT part of the character, the second represents the normal brightness of the character, and the last one represents the shadow.

I am wondering if there is a way to get the pixel information from the FLASHING background video, and have that information translated to the individual layers to adjust their opacities individually, without me having to manually keyframe it?

I am using After Effects CS5, thanks for any help at all!
 
Old 04 April 2012   #2
Sure, a sampleImage() expression is your friend. Though I don't really see the point of your setup. If the lights come from multiple directions and have different colors sampling all of them individually requires just as many "samplers" and they may need additional animation, too, plus the expression code would get complex and slow...

Mylenium
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Old 04 April 2012   #3
Originally Posted by Mylenium: Sure, a sampleImage() expression is your friend. Though I don't really see the point of your setup. If the lights come from multiple directions and have different colors sampling all of them individually requires just as many "samplers" and they may need additional animation, too, plus the expression code would get complex and slow...

Mylenium


Thanks,

How would you go about achieving the desired effect?
 
Old 04 April 2012   #4
The basic setup is sampleImage(coordinates, sample radius) and then you remap it using a linear() expression to you opacity. example:

sampler=sampleImage([X,Y],[width,height];

linear(sampler,0,1,0,100)

Fill in values as you see fit or tie e.g. the position to a point expression control...

Mylenium
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Old 04 April 2012   #5
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