Compositing CGI with Bigger Node Trees - Nuke 101: Professional Compositing and Visual Effects - page 140

The Mask input limits the area where the node operates. It receives a single channel, and
makes the node operate only where that channel is white. Gray areas in the mask image
are going to make the node operate in that percentage—a 50% gray means 50% effect,
and black means no effect. This makes it really easy to make a node affect only a specific
area.
Note that the Mask input has nothing to do with alpha channels and merging streams to-
gether. Its only purpose is to limit the area a node operates in.
You'll use the Mask input with a Grade node, darkening the background where the drop
shadow should be.
1. Press G on your keyboard to insert a Grade node after Read2.
2. Drag Grade2's Mask input to the Dot that supplied you with the uncropped CGI
renders ( FIGURE 3.59 ) .
Figure 3.59. Make sure you connected the Mask input in the correct way.