Follow the directions I posted above which was: I find this to be the best way to do it: Place your image in Motion. Then use the Bezier or B-spline tool to trace over the line you want to animate. Make sure the line is white. Add a Write On Behavior to the white line. Add an image mask to the original line. Make the Mask Source the white B-spline line you created. It will then write on your original line. If your line changes in thickness make your B-spline lines Brush type Airbrush instead of solid.
Next, we need to get that brush look. To do it go to the Shape tab in the Inspector. Click on the Shape Style pop up button. Choose the Liquid, Ink effect that you like. Some of them write on by default and don’t need the write on behavior. Then it will make the line below appear as if it is being painted on. Best of luck.
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Stephen Smith
Utah Video Productions
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