Touring the Interface with a Basic Composite - Nuke 101: Professional Compositing and Visual Effects - page 43

Because the Merge node expects a premultiplied image as its foreground input, you need
to premultiply the image first.
1. While Read2 is selected, press Tab and type pre . A drop-down menu displays
withthePremultoption.UsethedownarrowkeytonavigatetoitandpressEnter/
Return. Alternatively, you can choose the Premult node from the Merge toolbox
( FIGURE 2.11 ) .
Figure 2.11. The Premult node sits in between Read2 and Merge1.
There's now a Premult node called Premult1 connected to Read2. The area out-
side the doll's outline is now black ( FIGURE 2.12 ). It is now a premultiplied im-
age. You can proceed to place this image over the background.
Figure 2.12. The area that was light purple is now black.