Exercise-4-7 Setting a custom, external rotation point
Sometimes you want to set a specific point around which an object rotates. But you need to specify a rotation point outside the bounding box of the object. Fortunately, there is a trick for creating invisible objects that you can use to specify your own custom rotation point. This is the trick I’m most proud of in this book.
The finished file for this exercise is Exercise-4-7-finished.indd.
Open the file Exercise-4-7-start.indt. This file is a prop plane and a globe. We want the plane to circle around the globe (A).
The ordinary transformation points in the Animation panel won’t work as we need the plane to rotate around the globe, outside the plane.
Select the plane group. Choose the Rotation tool from the Tools panel.
Hold the Opt/Alt key and click in the center of the globe (B). This opens the Rotate dialog box (C).
Enter 180° in the Angle field. Click the Copy button. A clone of the plane appears on the other side of the globe (D).
With the duplicate selected, apply the None swatch to both the fill and the stroke. You should now have a “ghost” plane on the other side of the globe (E).
Group the original and the ghost planes together. Name this Plane group in the Animation panel.
You have just created a larger object (F) that just happens to have its center right on the center of the globe. This is where the default rotation point is located.
If you look very carefully, you’ll see a small green dot in the center of the globe. This is the transformation point for the animation.
Change the setting in the Rotate field in the Properties area to -360° rotation (G).
You set the Rotate field to a negative number so the plane circles counterclockwise.
Leave the rotation point at the center of the Transformation Reference Points. Use the EIP panel to preview the animation.
By creating the ghost object, you made the center of the animated object line up with the center of the globe. The plane circles the globe correctly.