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Suppose the shot is from a helicopter circling a highway interchange you need to track, and there is a large truck driving around. You want to put a garbage matte around it before autotracking. If the helicopter is bouncing around a bit and only loosely locked onto the interchange, you might have to add a fair number of keys to the spline for the truck.
Alternatively, you could track the truck and import its path into the spline, using the Import Tracker to CP mode of the rotoscoping panel.
To do this, begin by adding a supervised tracker for the truck. At the start of the shot, create a rough spline around the truck, with its initial center point located at the
tracker. Turn on Import Tracker to CP, select the tracker, then click on the center control point of the spline. The tracker’s path will be imported to the spline, and it will follow the truck through the shot. (You'll be asked a question first, see below.) You can animate the outline of the spline as needed, and you’re done.
If the truck is a long 18-wheeler, and you’ve tracked the cab, say, the back end of the truck may point in different directions in the shot, and the whole truck may change in size as well.
You might simplify animating the truck’s outline with the next wrinkle: track something on the back end of the truck as well. Before animating the truck’s outline at all, import that second tracker’s path onto the rotation/scale control point. Now your spline will automatically swivel and expand to track the truck outline.
IMPORTANT TRICK: Since the rotate/scale control point is hidden, you must import to it by holding down shift or control and clicking the center origin point.
You may still need to add some animation to the outline control points of the truck for fine tuning. If there is an exact corner that can be tracked, you can add a tracker for it, and import the tracker’s path directly onto spline’s individual control points.
When you import a tracker into a control point, you'll be asked whether you want to import the relative motion, or the absolution position.
If you import the relative motion, the control point will stay in its exact same position on the current frame. Use this mode when the control point is in its desired location at present, and you want to use the motion of a nearby tracker to guide the motion of the control point.
If you import the absolute position, the control point will jump to exactly the location of the tracker, on the current frame and all others. Use this mode when the tracker is already in the desired location for the control point.
The tracker import capability gives a very flexible capability for setting up your splines, with a little thought. Here are a few more details. The import takes place when you click on the spline control point. Any subsequent changes to the tracker are not “live.” If you need them, you should import the path again. The importer creates spline keys only where the tracker is valid. So if the tracker is occluded by an actor for a few frames, there will be no spline keys there, and the spline’s linear control-point interpolation will automatically fill the gap. Or, you can add some more keys of your own. You’ll also want to add some keys if your object goes off the edge of the screen, to continue its motion.
Finally, the trackers you use to help animate the spline are not special. You can use them to help solve the scene, if they will help (often they will not), or you can delete them or change them into zero-weighted trackers (ZWTs) so that they do not affect the camera solution. And you should turn off their Exportable flag on the Coordinate System panel. If they are sitting alone on a camera or moving object that doesn't have other trackers, you should set the camera or moving object to Disabled, so that the solver doesn't attempt to solve it.
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