Why does the color black pixalate when removing the green from a chroma key shot? I sometimes notice this even when the scene is lit well.

My guess is that dark areas are used for "codec tricks" whereby it's much easier to hide a whole bunch of image compression there without it being visually noticed. It's only when you perform additional functions such as chroma-key based colour removal that you see the difference.

Andrew

Also depending on the camera the imager can be very noisy in the dark areas even when using low or no gain.

I find this particularly true in cameras with very small imagers and small photosites.

So is the answer to this in the chroma key software or does the answer have more to do with color correcting the black before doing the chroma key?

Chris, yes the color black frequently appears to be noisy. I wonder if applying noise reduction in post would help?

If you have a camera with noisy blacks you may want to crush them down before applying the key if its causing problems with edges.