"skitdev" wrote: This is the part that I think I'm having trouble understanding. When you specify a layer as "source" what is using as the source?
Specifying a CompositionMode of "Source" means that that layer's pixels will replace any pixels behind it. When set to "Source-Over" the pixels will be alpha blended together. Using "Source" on a layer with transparent pixels (e.g., #00000000), those pixels will "punch" through any layers below it, all the way through to the video plane.