Compositing CGI with Bigger Node Trees - Nuke 101: Professional Compositing and Visual Effects - page 103

Linking Properties with Expressions
The CurveTool does not apply any operations; it only accumulates data. To use the ac-
cumulated data, you need to use another node—in this case, a Crop node. You have one
already, you just need to tell the Crop node to use the properties from the CurveTool's
AutoCropData tab. You will create a link between CurveTool1's property called “auto-
cropdata” and Crop1's property called “box” using an expression.
Expressions are programmatic commands used to change a property's value. Expressions
can be simple math functions (such as 2+4), more complex math functions (sin[frame/2]),
or a set of available functions that may be included as part of an expression such as width
for the width of an image, or frame which is the current frame number. A few of these
functions are explained throughout the topic.
You can type an expression by hand (in fact, you will do this in Chapter 5 ) or you can
make one by clicking and dragging, as you will now do:
1. Double-click Crop1andthendouble-click CurveTool1todisplayboththeirProp-
erties panels, one on top of the other.
2. Ctrl/Cmd-click-dragdownfromCurveTool1'sAutoCropDataAnimationmenuto
Curve1's Box Animation menu and release the mouse button ( FIGURE 3.10 ).
Figure 3.10. Dragging from one Animation menu to another while holding
Ctrl/Cmd creates a linking expression.