Hasta La Motion!

I’m working with some green screen footage at the moment and I’ve managed to get what looks like a decent key when working with the still image, but evidently is not when I play the sequence. The problem is that every keying technique I try, produces a slightly jagged, pixelated edge (mainly around the guys head – see pic) which can be fixed with an edge blur and erode. This looks fine in the still image but when played as a sequence the edges kinda ‘crawl’ as the pixels move… can anyone help with this??