Customizing Nuke with Gizmos - Nuke 101: Professional Compositing and Visual Effects - page 513

You should now see the Safe Area guides composited over the color bars. You
can't read the text well in this case because there is a white box where the text is
supposed to appear. Maybe another operation (instead of over) would be better in
this case.
4. Change the Operation property in Merge2 properties from “over” to “difference.”
You can now see the text better.
Let's try a different resolution for the ColorBars1 node.
5. In the ColorBars1 Properties panel, choose HD from the menu.
Boo-hoo. It's all broken. There is no Action Safe guide on the right, and nothing
at the top ( FIGURE 12.30 ). This is because—although you made the box a deriv-
ative of the resolution—I didn't show you how to connect the Rectangle nodes to
the source resolution.
Figure 12.30. Changing the resolution broke the Safe Area.