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– Stroke Timeline Area with Stroke Tracks: The stroke track for each stroke lets you keep track
of and manipulate the animation of strokes to follow camera and subject motion. Blue or red
frames in this track let you see at a glance how much of each stroke has been motion tracked;
blue frames for positive strokes, red frames for negative strokes. You can also use these tracks
to identify frames where you’d like to mute that stroke’s contribution to the final mask and to keep
track of which frames have been muted. You can draw a bounding box within the stroke track
around however many frames of however many strokes for which you want to delete motion
tracking, delete keyframes, or mute stroke contributions to the mask.
Stroke Duration
When you first draw a stroke, that stroke has a duration of one frame, located at the timecode position
of the playhead when you drew it. If you move the playhead to the left or right, you’ll see the stroke
disappear. If you want one or more strokes to continue analyzing the subject for the duration of that
shot, you need to use the Motion Tracking controls to track the stroke to follow along with the motion
of whatever it’s drawn on top of. As a stroke is tracked, its duration increases to cover the entire range
of tracked frames. If at any point a stroke outlasts its usefulness, such as when a person walks off
frame, you can stop tracking that stroke when it’s no longer necessary and it will stop contributing to
the analysis.
This is useful, because often you’ll want to place strokes to deal with analysis issues that only last for a
few frames, to fix a hole in the mask that only appears briefly. Just be aware, you can’t simply draw a
stroke and move on to the next shot; you must track at least one stroke to last for the duration of a
subject’s time on screen for that subject to be continuously masked.
Tracking and Keyframing Strokes to
Follow Subject Motion
You can think of each stroke you’ve drawn as a persistent eyedropper that samples the image that
overlaps it. The mask that results from all strokes’ collective analysis of the image is generated live
over each frame of the clip. This means that if the camera or the subject moves, you need to motion
track or otherwise adjust the position of each stroke to follow along with the motion, so the subject
continues to be correctly identified. You also need to make sure that each stroke is able to analyze
what it’s supposed to, and turn off strokes that can’t for whatever reason.
TIP: Magic Mask is a processor-intensive operation, so to accommodate users of less
powerful workstations, there is a Use Fast Tracking option available in the Magic Mask
palette’s Option menu, which speeds up the process of tracking at the expense of potentially
less accurate tracking for fast or erratically moving subjects.
Methods of Moving Strokes to Follow Subject Motion
For these reasons and more, there are multiple methods of manipulating strokes to refine the
final analysis.
– Motion Tracking: A set of tracking controls at the top of the Magic Mask palette let you motion
track one or more strokes to follow the camera and subject motion in a clip. Every frame that’s
motion tracked effectively has a motion-tracked keyframe placed on it that moves the stroke to a
new position.
Chapter 135Magic Mask 2698
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