I am trying to composite some footage of a model landscape.
It consists of three layers. The background layer is a shot of a swamp. The middle layer will be a sunbeam. The top layer is the swamp again, but with a card just behind the trees which frame the foreground. The final shot would look like a sunbeam in a swamp that is framed by trees in the foreground. The card is too unevenly lit to get anything like a decent key.
I had hoped to be able to matte out the parts of the foreground shot which are different from the background shot, leaving just the frame of trees, but when I tried to do so with a difference matte I find it eliminates the part that is the same, and leaves the card, which is the exact inverse of what I need.
Is there a way to invert the part that the difference matte eliminates, or to use something like a combination of a difference matte and luma matte to get the desired result, or some other solution that hasn't occurred to me?