Camera Projection - Nuke 101: Professional Compositing and Visual Effects - page 476

The composite is looking better, but the edges of the buildings still need a little
work. You want to slightly erode the matte. Because this will change the relation-
ship between the RGB and alpha channels, you will need to break up the premul-
tiplied buildings image first.
4. Select ScanlineRender1 (the original one creating the buildings image) and insert
an Unpremult node from the Merge toolbox.
5. Insert an Erode (filter) after Unpremult1 and a Premult node after that
( FIGURE 11.28 ) .
Figure 11.28. Inserting an Erode node in between Unpremult and Premult
nodes.