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After Effects Spline Tricks, Part 2

Keyframing position with masks By Ko Maruyama
There's more to splines than simple image extraction. Sure they're great for cutting mattes, but mask data in After Effects is based on Bezier paths that contain specific values which can be used for more than alpha channels. In this tutorial, we take a quick look at "lifting" the shape data for use in less obvious tasks, including position keyframing.

If you haven't had a chance to read the article regarding After Effects' new Auto-trace feature in the latest release, check out http://www.creativemac.com/2004/01_jan/tutorials/ko18aetrace1040113.htm. If you've glanced at the manual, or have completed a handful of projects with After Effects, you're probably familiar with the masking process. If you're new to Mask Shapes in AE, here are some basics for the purposes of this lesson:


1. Masks in AE are any user-created splines on layers.
2. Closed splines can be used as alpha channels for the layer by applying various mask modes to the spline.
3. Masks do not obey transformation keyframe data, such as Position, Scale and Rotation. Instead, they are animated by editing the vertices (points that make the spline): Mask Shape.

Keyboard Shortcuts:Keyboard Shortcuts:

  • Q: I remember "Quickmask," make a quick shape. Q and Shift-Q toggle the shape between rectangular and oval.
  • G: "Garbage matte." When I used Discreet Commotion for desktop rotoscoping, the only thing I'd use AE's pen tool was for G-mattes. G=pen tool for creating and editing mask shapes.
  • M: Reveal the Mask shape in the timeline for selected layer(s). Good for editing keyframes.
  • Option-M (Mask selected): Sets/deletes a keyframe for mask shape at that time.
  • MM: Reveals all Mask parameters in timeline for selected layer(s).

Masks for Position
To use mask splines for position keyframe data, all you need to do is select the mask, copy the data, select the Position parameter and paste. After Effects will create a linear keyframe for the first and last vertices. The keyframes between those two will exist as "roving keyframes."

Naturally, because you can import Adobe Illustrator paths from the clipboard into After Effects to use as masks, you can also copy from Illustrator and paste paths directly into the After Effects position track.

Easy, huh?

Continue to the next page for the first example of this Mask to Position procedure.

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