Compositing CGI with Bigger Node Trees - Nuke 101: Professional Compositing and Visual Effects - page 125

Using the ShuffleCopy Node
Now that you have finished building the lemming's beauty pass, you need to premultiply
your composite to get rid of those nasty pixelated edges ( FIGURE 3.38 ) and return to a
premultiplied image that can be composited with other elements. To do this, you need to
make sure you have the right alpha channel.
Figure 3.38. Time to get rid of those nasty edges.