Placing Tracking Points

Once a background movie is loaded, the next step is to place tracking points on the main points of those objects that you want to follow. The easiest objects to track are those that have a strong contrast to the background and that are easy to distinguish. Corners and tips of objects make good tracking points.

To place tracking points in the project, you’ll need to click the Video Tracking tool in the Layer section of the Tools palette. With the Video Tracking tool selected, you’ll be able to click the background movie at a point to track. The tracking points are identified by two circles within each other. The inner circle identifies the item to track, and the outer circle marks the area within which the item moves. Figure 28.1 shows two tracking points.

Figure 28.1. Tracking points.

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Each tracking point can be named by typing a name in the text field on the Options bar. Each tracking point is given a name by default, but you can enter a custom name. If you plan on tracking multiple points, then giving each tracking point an easy-to-identify name is a good idea.

When placing tracking points, you can place multiple points on the video, but for each layer, you can set two points, a primary and a secondary point. These points are selected in the Motion Tracking dialog box when you set which layer to follow the tracking. The primary tracking point always defines the movement of the layer, and the secondary tracking point can be used to define the rotation or the scale of the layer relative to the primary point.

Imagine that you have a video of a friend dancing the robot dance with swinging arms and all. If you were to track this video, then you could place a tracking point at the hand to keep track of the hand’s movement and a second tracking point for the elbow to keep track of the rotation of the arm. Then when you create an arm layer, you can set the hand tracking point to the primary tracking point and the elbow tracking point to the secondary tracking point.

Another way to use a secondary tracking point is to keep track of the size of an object. If you had a video that zooms in and out of a scene, then a secondary point could be placed at the edge of the object to keep track of the scale of the object as it changes size.

After you’ve place the tracking points, you simply need to click the Play button or the Step Forward buttons to progress through the movie frames. Tracking points will place a keyframe in the Timeline palette for every frame of the animation. If the points are easy to track on the background, they will follow the point as the movie plays, but more than likely you’ll need to adjust the tracking points.