Specifications

2-D motion tracking is the technology used in composi-
tors to affix a new element to a specific point in the frame.
A user generally will select one or two feature points, and
the computer then will follow the points around the frame
as the objects move within it. When the tracker slides off
the selected point, the artist gently will correct it to keep
the track from drifting. Two commonplace examples of this
process can be seen in blurring suspects’ faces on Cops!
and in placing virtual advertisements on infield walls at
baseball games. 2-D tracking tracks only the position of an
object within the frame, which gives it a double-barreled
Achilles’ heel: parallax and perspective.
Parallax is the phenomenon whereby foreground
objects seem to move faster than do background objects.
As your point of view moves, the angle at which you per-
ceive objects changes subtly, which is why you see a paral-
lax driving down the road. With 2-D tracking, your track
marks are pretty much all you get. This can be a problem
if, for instance, you’re moving over a greenscreen and the
digital set is supposed to extend for quite a ways down in
depth. As soon as you add depth to lateral movement,
particularly when your track marks are close to the camera,
you need to work in 3-D, or you have to fake parallax by
hand—a dubious and difficult undertaking that easily shatters
the illusion you’re trying to create. A really good artist can
pull it off, but it takes a lot of practice.
Perspective is the other wild card in the equation. Lenses
do not see the world as it is. Instead, every lens distorts the
world in certain mathematically predictable ways. This distor-
tion is closely related to focal length and aperture, and mea-
suring the distortion accurately is essential to tracking ele-
ments properly in the shot. This wild card gets even wilder
with zoom (extending the lens to get a closer shot) and
dolly-in (moving the camera toward a subject) movements,
which involve constantly changing perspective in one fashion
or another along the z axis, which is the axis that 2-D motion
tracking can’t cope with. Perspective changes also can be
faked, but it’s far more difficult than faking parallax and far
more time consuming.
This is where 3-D camera tracking comes in. Instead of
simply tracking the location of certain user-selected fea-
tures to create a good 2-D track, the computer attempts to
guess the position and motion of the camera based on the
footage. Pitch, yaw, roll and lens length are all calculated
based solely on the finished video (though any information
you have and can input manually will make it work faster).
The ability to reconstruct all these parameters accurately
means that the problems of parallax and perspective are
solved, even during dolly and zoom moves. Needless to
say, this is a mathematically complex process designed to
test the minds of even the most ardent effects artist who
wasn’t also a comp-sci or optics major at a university.
Nonetheless, the algorithms for pulling this off are well
known and included in most camera-tracking packages.
Although most 2-D motion trackers are built in to exist-
ing compositing systems (such as After Effects), 3-D cam-
era trackers operate on a standalone basis and export their
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