Color Correction - Nuke 101: Professional Compositing and Visual Effects - page 173

9. View Merge1 ( FIGURE 4.23 ) .
Figure 4.23. All that white on the dashboard shouldn't to be there.
You can see here that this is not the desired result ( FIGURE 4.23 ). You still need
to change the Merge2 operation to something that will cut the B image with the
A image. This operation is called Stencil. Stencil is the reverse operation from
Mask, which you used in Chapter 3 . Mask held image B inside the alpha channel
of image A, and Stencil will hold image B outside image A.