Touring the Interface with a Basic Composite - Nuke 101: Professional Compositing and Visual Effects - page 88

6. Go to frame 43 in the Timebar.
7. From the Composite button/menu at the center of the Viewer, choose Wipe
( FIGURE 2.61 ) .
Figure 2.61. Turning on the Wipe option in the Viewer Composite controls.
With the Composite controls set in this way, there's a new axis on the View-
er—the image to the left of this axis is Write1 and the image to the right of this
axis is Read4.
You can move the axis using the controls shown in FIGURE 2.62 .