Here is a technique I used to clean up an area of my projection that wasn’t quite working. My shadow plate was taken from a slightly different angle from my main plate so the reflections didn’t quite line up, leaving a visible edge. I used the x channel from the position pass and clamped it to create a feathered alpha that was perfectly tracked in the scene to hide the edge. It’s not perfect, but it makes the edge a lot less obvious, especially in motion.