Touring the Interface with a Basic Composite - Nuke 101: Professional Compositing and Visual Effects - page 74

Look at Merge2's Properties panel ( FIGURE 2.44 ). It shows that the Operation
property is still Over. You need to change that to something that uses the A input
to punch a hole in the B input.
Figure 2.44. The Operation drop-down menu.
The Stencil operation does exactly that. It creates a hole in the B input where the
alpha channel of the A input is white. Let's change the operation to Stencil.
8. Change Merge2's Operation property from Over to Stencil.
9. Select Merge1 and press 1 on the keyboard to view it.
Look at the image in the Viewer ( FIGURE 2.45 ) . It appears unchanged. The doll's
feet are still visible in front of the artist's toolbox. This is because the Stencil op-
eration uses the A input's alpha channel, but your matte is in the red channel.
To solve this, move the red channel into the alpha channel using a node called
Shuffle.
Figure 2.45. The doll's feet are still in front.