Green Screen Auto Levels Expression

[Timber_Wolf_2]

I'm doing some Green Screen work for someone who used auto exposure when shooting. It's not terrible, but it's not awesome, as you can imagine.

Is there an expression that can sample a region of pixels and return an numerical value for its average luminance (I assume between 0 and 255)?

My thinking is, if I can use frame 1 as my master brightness level, then every frame after that will have a definite numerical value that is determined by how much brighter or darker it is from frame 1. I could then use that number to drive an exposure effect.

I could create a perfect auto-levels effect using expressions!

So... can it be done?

[Michael_Szalapski]

I mean, there's sampleImage(). Resource here.