eosacro demonstrates one of the basic concepts in compositing, premultiply and unmultiply, and provides a great example of exactly what is happening when an image with RGBA is premultiplied.
If you have ever wondered what premultiply is and what it does to an image, then in this video I will show you understand it
PreMultiply basically refers to an image with an alpha channel, where it is commonplace to multiply the color of the image by its alpha channel to save having to do this process during compositing.
eosacro also look as when you would want to se a premultiply in NUKE as well as how to use unmultiply while working on a composition, providing some examples.