RotoPaint - Nuke 101: Professional Compositing and Visual Effects - page 271

Both images have the same actor. What you need to do is create one shot that has
the same actor twice in it! How novel!
To see the overlap a little better, use a handy tool called the Dissolve node.
3. Select the fg##.png Read node and insert a Dissolve node from the Merge tool-
box.
4. Connect the input called 1 to the bg##.png Read node's output ( FIGURE 6.39 ).
Figure 6.39. Your tree should look like this.
5. Set the Which property to 0.5 to blend evenly between the foreground and back-
ground (or in this case inputs 0 and 1).
6. View Dissolve1 in the Viewer.
7. Toseeitplaying,eitherpressAlt/Option-FtoloadthesequenceintoFramecycler,
or press Play in the Viewer ( FIGURE 6.40 ).