Best practices for masking image layers for compositing? I'm looking for a efficient way to do image masking within Blender (using the masking tool) &agrave; la After Effects.  Basically, I want to cut out my images from within Blender instead of doing it destructively in an image editor.

Typically, in AE, I would precomp an image then do the masking in that precomp.  I'd like to mimic this workflow in Blender; however, it always seems overly complicated.  

I have considered creating a separate scene with the same dimensions as the image and masking there, then bringing it into a master scene to assemble in 3D space, but I'm not sure I can map this to a plane.

Sorry that I may not be explaining this very well. If anyone is familiar with AE, then you'll be familiar with what I'm trying to achieve.
go to node editor and enable use nodes
add mask node and mix node
in uv editor change from paint to mask (if you want a mask image, upload that image to uv editor, the mask will be placed over your image)

LMB+Ctrl  to add point on the curve, close curve by enabling toggle cyclic
use this mask as input for mask node and plug mask node to factor of the mix node


for rotoscoping you could use this add on, see this for more info.