When you put a track point to a feature, you can notice the white crosshair in the middle of the tracking marker. That is the tracking point. If your tracked feature is covered by something else or moves out of frame, you can re position your marker before that happens. Just before that moment:
- Place your cursor over the marker, but not the crosshair.
ALT on your keyboard, your cursor arrow will turn white.
ALT is pressed, hold your marker and move to a new feature. You can notice that the crosshair remains at it's initial position. When you place the marker, release
- Continue tracking, After Effects will keep your tracking point's crosshair in it's initial position, while tracking your new feature. So even your initial tracking point moves out of the frame or obscured by something else it will be OK.
But selection of the second feature is important. It must be on the same surface or plane as the first one. If your first point is on the wall, choosing the second point on the ground or another feature in the foreground, will cause a lot of problems, as there will be plenty of parallax motion (Foreground will move faster than background etc.)
Or if your footage is shot with a wide lens, features on the edge of the frame will have a different motion when compared to a feature in the middle of the frame, due to the lens distortion.
See the explained steps in a screencast video.