How do I extract a color from part of an image and use in keying? How do I extract a color from part of an image and use in keying?

I want to pull out a rect from the corner, like 10x10 to 100x100 and feed it into the key color of the keying node. This is so I can auto detect if its a green screen or blue screen input.
You just needed to scale that to the image size