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— Combining Tracking and Reference Frames: Since each frame that’s motion tracked is effectively keyframed, manually dragging a stroke on a frame that’s already been tracked overrides the tracking data at that frame with a reference frame that records the new position of the stroke at that frame. If there’s no more tracking data, either before or after that reference
frame, then the stroke disappears until you do more tracking. If there’s tracking data immediately after a reference frame, then the stroke jumps to the position dictated by that tracking data.
— Deleting Stroke Tracking at Specific Frames: To accommodate cases where a subject you’re isolating moves behind something and becomes hidden for a range of frames, you have the ability to delete stroke tracking for one or more frames. This lets you eliminate the stroke for however many frames the subject you’re tracking is obscured and cannot be sampled. This makes it easy
to deal with features that come and go over the duration of a clip, since you can delete stroke tracking over however many ranges of frames the feature disappears. If you’ve muted frames by mistake, you can either undo or re-track the stroke over those frames.
To delete tracking frames, draw a bounding box on the stroke tracks over the range you want to delete, and then choose Clear Selected Track Data from the Magic Mask option menu.
Option Menu Commands for Removing Strokes and Tracking
The Option menu of the Magic Mask palette presents a number of commands for clearing either strokes or stroke tracking data.
— Clear All Strokes: Deletes every stroke in the Stoke list, along with tracking.
— Clear All Strokes of Current Frame: Deletes the tracking data at the current position of the playhead on all strokes in the list.
— Clear All Strokes from In to Out: If you use the I and O keys to set In and Out points in the Stroke Timeline, this command deletes all tracking data on all strokes within that range.
— Clear Selected Stroke from In to Out: If you select a stroke in the Stroke list, and then use the I and O keys to set In and Out points in the Stroke Timeline, this command deletes the tracking data of selected strokes within that range.
— Clear Selected Track Data: If you drag a bounding box around tracking data for one or more strokes, this command deletes all tracking data within the box.
An Example Stroke Tracking Workflow
By combining motion tracking, manual reference frames, and stroke muting, you’ll be able to make short work of isolating most moving subjects. The following procedure illustrates how you might use these techniques together to make the strokes you’ve drawn follow along with the features you’re isolating.
1 If necessary, move the playhead to the frame where you want to begin isolating the subject you want to mask. For example, if you’re isolating a person walking into the room through a door, you probably want to begin your work on a frame where the person is already in the room, and work your way backward and forward from that frame. If your subject is already fully visible at the first frame of the shot, you can start there.
2 Draw at least one stroke identifying the subject or feature you want to isolate. You cannot track a stroke to follow the features unless there’s at least one stroke.
3 Choose whether you want to track all strokes or only selected strokes. By default, all existing strokes with at least one frame in the Stroke Timeline at the position of the playhead will be tracked. However, you can choose to track only selected strokes in the Stroke list by choosing Track Selected Stroke Only in the Magic Mask palette’s Option menu.
4 Click the Track Forwards to End or Track Backwards to Beginning buttons if you want to motion track the strokes over the remaining duration of the clip.
By default, each stroke is tracked to follow whatever image detail immediately surrounds it; you don’t have to define a tracking region, this is done automatically. As tracking proceeds, the tracking bar for each stroke will fill up showing you which frames have been tracked, and which
have yet to be tracked. Meanwhile, each stroke being tracked will transform to follow along with simple position and rotation changes in the subject. However, things that obscure the subject, as well as more extreme movements, may cause problems.
Strokes that are tracking a feature that moves off screen are automatically excluded from tracking at the frame where the feature is completely off screen, even if that feature returns onscreen in a later frame.
5 (Optional) If a problem occurs during the track, such as the subject being occluded behind something else in the frame (for example, a person’s face moves behind a tree as they walk), click the Pause Track button. You can deal with problems in the track in the following ways:
a) If a stroke has completely moved off of the subject: For example, you’re tracking a stroke to follow a person and they turn around 180 degrees, so the stroke moves off onto the background. To fix this, pause the track, move the playhead to the first frame where the stroke hasn’t followed the subject properly, then use the pointer to drag the stroke onto another part of the subject that’s clearly visible. On each frame where you drag the stroke to a new position, a new reference frame will be automatically added. Continue this process until the subject’s motion is consistent enough to be trackable again, and you can then resume tracking.
b) If a stroke has moved onto something that’s partially obscuring the subject: Pause the track, drag a bounding box over the frames where the track is having a problem, and choose Delete Selected Tracking from the Magic Mask Option menu to clear the bad tracking data. Then, move the playhead to the first frame of the deleted tracking data, and use the pointer to drag the stroke onto any part of the subject that’s visible to either side of the occlusion. Move the playhead to each successive frame, and drag the stroke onto whichever part of the subject is visible from behind the occlusion, until the subject moves past the occlusion. At that point, you can use the Track Forwards to End or Track Backwards to Beginning buttons to continue tracking as usual.
c) If the subject becomes completely hidden: Clips where the subject appears and disappears over the duration of a clip can be resolved by deleting the stroke or strokes on that subject over all frames where the subject is obscured, so that strokes only sample the image when the subject is visible and thus able to be isolated. You can delete one or more strokes by dragging a bounding box over a range of frames, and then choosing Clear Selected Track Data from the Magic Mask Option menu.
d) If a stroke is having trouble following the subject: If the subject you’re tracking is moving too fast, too irregularly, or is changing shape too greatly for the tracker to give a good result (for example, someone lifts their arm towards the camera), you can track through the problem area of the clip one frame by frame using the Track Forward 1 Frame or Track Backward 1 Frame buttons, and then use the pointer to manually drag the stroke to follow along with the subject as you go. Once the problem motion stops and the motion of the subject becomes more regular, you can use the Track Forwards to End or Track Backwards to Beginning buttons to continue tracking as usual.
e) If the subject is moving off the frame: If you’ve drawn a stroke in the middle of someone’s face, and their face is moving off screen, you can manually reposition the stroke on the last few frames before the subject completely exits so that stroke continues to identify what parts of the face are still visible even though the part of the face it was originally following becomes hidden. Once the face completely exits the frame, you can mute the stroke for the rest
of the shot.
6 (Optional) If you only tracked the stroke’s movement over part of a shot, you can click the Jump to Start or Jump to End buttons to move the playhead to the first or last tracked frame, in preparation for tracking from that frame onward.