Simple Masks Tutorial
Compositing Compositing the separate renders requires a multilayer image editor such as Photoshop or GIMP. Simply load each render as a separate layer and then cut away the masked portions. It's a good idea to feather the edges slightly to avoid visible seams.
Masks are a very powerful tool. They make it possible to maximize quality while minimizing render time, salvage unfinished renders, avoid memory errors, correct stupid mistakes, and replace imperfect sections of the image. And, best of all, they make it very easy to do these things.
99