Sure - use an image mask. First, create a shape and fill it with a black and white gradient running vertically. The, select your rectangle, and choose Object > Add Image Mask. Drag the gradient to the well in the HUD (or even directly onto the image mask in the Layers tab). Set it to Luminance. Adjust the gradient to taste using the interactive gradient tool in the Canvas.
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Mark Spencer
Freelance Editor/Producer
Apple-certified instructor, FCP, DVDSP and Motion
Author, Motion Visual Quickstart Guide from Peachpit Press
http://www.applemotion.net