Use the following guidelines when rotoscoping:
Find the frame with the most detail showing in the subject you’re masking and use that as your starting frame. For example, if you’re masking someone walking, choose a frame where that person’s arms and legs are extended in mid-stride. Doing so lets you specify how many control points to start with. Using as few control points to achieve the necessary level of detail in your mask simplifies the animation process.
Use as few control points to achieve the necessary level of detail in your mask.
Animated masks trigger the same motion blur as any other keyframed parameter in Motion. For example, if you animate a layer’s position so it moves really fast, the layer is blurred when you enable motion blur. If you animate a layer’s mask so it also moves fast, the edges of the mask are blurred as well. This is important because an animated mask’s blur should match any blur present in the foreground subject.
When rotoscoping a moving subject, it’s a good idea to play the clip and examine its direction and speed to get a sense of how the mask must be animated. You may find it helpful to place markers indicating frames where the subject or camera stops, changes direction, or changes speed, because these are likely candidates for your first pass of keyframed mask changes. By noting these changes, you can attempt to reduce the amount of keyframing by making your first keyframed mask shape adjustments at these major changes in speed and direction. Because animated masks are interpolated to change from one keyframed shape to another, you can let Motion do some of your work for you.
All mask animation is stored in the Shape Animation channel in the Keyframe Editor. You can create, delete, and edit the timing of a mask’s Shape Animation channel keyframes. For simplicity, each change you make to a mask is recorded as a single keyframe, no matter how many control points you edited. The Shape Animation channel allows only keyframes set to Constant—you cannot use any other form of keyframe interpolation.
It may not be necessary to rotoscope an entire subject with a single mask. Motion lets you apply multiple masks to a single object, so you can rotoscope different parts of a subject with separate masks. For more information, see About combining multiple masks.
To pan the image in the Canvas while rotoscoping, press the Space bar. You can then pan in the Canvas without losing your mask selection or progress.
Use the Track Points behavior to apply tracking data to a shape or mask. For more information, see Track shapes, masks, and paint strokes.