Sorry, for some reason I thought you where talking about animating stills.
Create a Mask around the part of the tree that the text will touch.
Select the shape or mask you want to track, click the Add Behavior icon in the Toolbar,
then choose Shapes > Track Points from the pop-up menu.
The behavior is added to the shape, and trackers appear for each control point on the
shape. The trackers are ordered in the same order that the shape was drawn: Control
Point 1 is Track 1, Control Point 2 is Track 2, and so on.
Click the Analyze button in the HUD or Behaviors tab of the Inspector.
The mask control points are tracked to the reference patterns.
Hope this helps. It may be easier to animate the mask yourself.
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