Need expression help with matching CG to flickering footage

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Old 10 October 2012   #1
Need expression help with matching CG to flickering footage

Hey everyone,

so the scene I'm working on right now is an actress in the foreground getting lit by flickering/changing lights, both in brightness and in colour. The background (and floor in some shots) is completely CG.

How can I match the CG to the flickering/illumination changes in the plate without having to painstakingly animate everything manually?

Is there a way to sample an area of the footage, take the average value of that area for every channel and then look at the changes in value from one frame to the next and apply that difference to the CG background? And how would I go about building a tool like this?

I'm more or less completely inexperienced when it comes to using expressions so far - apart from the most basic stuff. How can I sample the average of an area (that I should be also able to animate so it tracks with the footage, they're all shots with a moving camera) and how can I then make Nuke compare these values from one frame to the next (or to one initial "reference frame")?

It would be awesome if somebody could help me out here.

Cheers,
Chris
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Old 10 October 2012   #2
What software are you using?
 
Old 10 October 2012   #3
I'm sorry, I should have mentioned that of course. I'm using Nuke.
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Old 10 October 2012   #4
I think what you are looking for is the curve tool
Within that node you can set an area to sample (put that on a spot that flickers in the footage), set it to sample the average intensities and link that data to a grade nodes gain for example.
That way you should get reasonable results with a bit of tweaking.
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Old 11 November 2012   #5
Originally Posted by julsVFX: I think what you are looking for is the curve tool
Within that node you can set an area to sample (put that on a spot that flickers in the footage), set it to sample the average intensities and link that data to a grade nodes gain for example.
That way you should get reasonable results with a bit of tweaking.


Perfect, thank you - that was exactly what I was looking for.
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Old 11 November 2012   #6
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