Path-ology

A little planning before you shoot can make a huge difference in how easy it is to track them later, or even whether it's possible at all. Here, we show some examples of good and bad camera paths for 3-D tracking.

Case of the Wandering Cameraman

This is a typical plan for a cameraman (or -woman) with a hand-held camcorder, shooting a shot with "a moving camera." Unfortunately, it doesn't focus on anything long enough to get a 3-D track:

Attending to a Subject

This is a better plan—here the camera focuses on a subject as the camera rotates around it:

Shooting from a Tripod

Here's a shot from a tripod, done right. The camera is mounted on the camera so that the nodal point, up front between the lens and camera chips, is centered at the axis of the tripod. SynthEyes's tripod mode will solve it easily, producing a direction to each tracker, but mathematically it is impossible to determine a distance to anything. This won't necessarily prevent you from inserting objects into the shot, though:

Almost a Tripod

This is what you get using a person as a tripod—the camera is spinning around a point well behind the camera, nowhere near the nodal point. The shot will exhibit some perspective changes, and whether or not the shot is solvable depends on how close the trackable objects are, compared to the radius of the camera lens motion. If they are close enough, a 3-D solve might be obtained. If they are far away, a tripod-mode solve will work. But there is a no-solving zone in between, don't be this guy/gal:

Half-Right

Here, the camera travels forward through the scene, focusing on the scene to the left. As long as there are things to track over there, so far so good. However, the camera reaches the end, spins around, and comes back the same way. The returning section is trackable too. But, the turning-around part is not trackable; it is an in-between shot, neither 3-D nor tripod. Shots like this, but less obviously, are easy to achieve if the camera is allowed to spin rapidly. For this particular shot, it might be possible to break the shot into three pieces: the first move, the tripod pan, and the second move; and track each separately: