Touring the Interface with a Basic Composite - Nuke 101: Professional Compositing and Visual Effects - page 39

The Merge node can connect two or more images together using various layering
operations such as Overlay, Screen, and Multiply. However, the default operation
called Over simply places a foreground image with an alpha channel over a back-
ground image. The A input, already connected, is the foreground input. The un-
connected B input is the background input. Let's connect the B input.
Note
ToseeallthedifferentlayeringoperationstheMergenodeiscapable
of,clicktheOperationpropertyintheMergenode'sPropertiespanel.
To see the math of the operations, hover your mouse pointer over the
Operationdrop-downmenuuntilatooltipappears.Tooltipsareavail-
able on practically every button in Nuke.
2. Click and drag Merge1's B input towards Read1 and release it over Read1
( FIGURE 2.7 ) .
Figure 2.7. Both inputs are now connected, creating a composite.