Checking for drift

Analyzing and applying tracking

Now that the search and feature regions are defined, you can apply the tracker.

1 Click the Analyze Forward button () in the Tracker panel. Watch the analysis to ensure the track point stays with the metronome weight. If it doesn’t, stop the analysis and reposition the feature region. (See the sidebar, “Checking for drift.”)

2 When the analysis is complete, click the Apply button.

3 In the Motion Tracker Apply Options dialog box, click OK to apply the tracking to the x and y dimensions.

_ Note: The tracking analysis may take quite a while. The larger the search and feature regions, the longer After Effects takes to analyze tracking.

As an image moves in a shot, the lighting, surrounding objects, and angle of the object can all change, making a once-distinct feature no longer identifiable at the subpixel level. It takes time to develop an eye for choosing a trackable feature. Even with careful planning and practice, you will often find that the feature region drifts away from the desired feature. Readjusting the feature and search regions, changing the tracking options, and trying again are all standard parts of digital tracking. When you notice drifting occurring, try the following:

1 Immediately stop the analysis by clicking the Stop button in the Tracker panel.

2 Move the current-time indicator back to the last good tracked point. You can see this in the Layer panel.

3 Reposition and/or resize the feature and search regions, being careful to not accidentally move the attach point. Moving the attach point will cause a noticeable jump in your tracked layer.

4 Click the Analyze Forward button to resume tracking.

The motion-tracking data is added to the Timeline panel, where you can see that the track data is in the metronome layer, but the results are applied to the Position property of the Shape Layer 1 layer. This is another difference between tracking and stabilization: The resulting data is applied to a layer’s position, rather than to its anchor point.

4 Watch a RAM preview. The star not only follows the pendulum; it moves with the camera’s movement.

5 When you’re ready, press the spacebar to stop playback.

6 Hide the properties for both layers in the Timeline panel, choose File > Save, and then choose File > Close Project.

Motion tracking an element onto background footage can be fun. As long as you have a stable feature to track, single-point motion tracking can be quite easy.

Using multipoint tracking

After Effects also offers two more advanced types of tracking that use multiple tracking points: parallel corner-pinning and perspective corner-pinning.

When you track using parallel corner-pinning, you simultaneously track three points in the source footage. After Effects calculates the position of a fourth point to keep the lines between the points parallel. When the movement of the points is applied to the target layer, the Corner Pin effect distorts the layer to simulate skew, scale, and rotation, but not perspective. Parallel lines remain parallel, and relative distances are preserved.

When you track using perspective corner-pinning, you simultaneously track four points in the source footage. When applied to the target footage, the Corner Pin effect uses the movement of the four points to distort the layer, simulating changes in perspective.

You’ll attach an animation to a computer monitor using perspective corner-pinning. If you haven’t already watched the sample movie for this exercise, do so now.

Дата добавления: 2014-11-20