In this tutorial, Kenan Proffitt goes through the very simple process on how to use a matte key (picture 2) to key out a video with rotoscoping. I have duplicated his exact process by using a "Set Key" node with the video clip plugged into the "Image" input and the rotoscoped matte plugged into the "Alpha" input. (picture 1) It's fairly straightforward. But when I do that, the result is only half-way working. The rotoscope subject is perfectly still there while the background is only sort of transparent-not 100% transparent (picture 3). Why is this happening?