Customizing Nuke with Gizmos - Nuke 101: Professional Compositing and Visual Effects - page 496

7. Choose Edit Expression from the Area property's Animation menu ( FIGURE
12.4 ) .
Figure 12.4. The Area property Expression panel.
Hereyoucanseefourlines,oneforeachofthepropertiesavailable. Youusedthe
ExpressionpanelbeforewhenyoulinkedtheCornerPintotheTrackerin Chapter
5 . This time, you're not going to link at this stage, just use basic math.
The X property is the distance from the left side of the frame. Let's say you want
your Action Safe area to be 5% away from the left side of the frame.
8. In the X expression field, enter width*5/100 ( FIGURE 12.5 ) .
Figure 12.5. Writing the first expression.
On the right you can see the result of the expression: 102.4.
Next, do the Y property, using the “height” expression.
9. In the Y expression field, enter height*5/100 .
The result here is 77.8.