Graphics Reference
In-Depth Information
7
Motion Capture
Motion capture
(often abbreviated as
mocap
) is probably the application of
computer vision to visual effects most familiar to the average filmgoer. As illustrated
in Figure
7.1
, motion capture uses several synchronized cameras to track the motion
of special
markers
carefully placed on the body of a performer. The images of each
marker are triangulated and processed to obtain a time series of 3D positions. These
positions are used to infer the time-varying positions and angles of the joints of an
underlying skeleton, which can ultimately help animate a digital character that has
the same mannerisms as the performer. While the Gollum character from the
Lord of
the Rings
trilogy launched motion capture into the public consciousness, the tech-
nology already had many years of use in the visual effects industry (e.g., to animate
synthetic passengers inwide shots for
Titanic
). Today,motion capture is almost taken
for granted as a tool to helpmap an actor's performance onto a digital character, and
In addition to creating computer-generated characters for feature films, motion
capture is pervasive in the video game industry, especially for sports and action
games. The distinctive mannerisms of golf and football players, martial artists,
and soldiers are recorded by video game developers and strung together in real
time by game engines to create dynamic, reactive character animations. In non-
entertainment contexts, motion capture is used in orthopedics applications to
analyze a patient's joint motion over the course of treatment, and in sports medicine
applications to improve an athlete's performance.
Our focus in this chapter is on the computer vision aspects ofmotion capture, from
acquiring the data tofitting it to a 3Dskeleton.Whilewe discuss how to string different
motion capture sequences together with fluid transitions, we stop short of extensively
modifying the motion for production-quality computer animation. The latter prob-
lem involves application-specific decisions — both technical and artistic — outside
this chapter's scope. Motion capture data is routinely modified by animators, both in
small and large part, in order to achieve a director's vision for a computer-generated
character or scene.While thepopular perceptionmaybe that amotioncapture actor's
performance is mapped directly onto a digital character without modification, this is
rarely the case.
1
The end-to-end process from suiting up a performer to animating a character is sometimes called
performance capture
. The term
motion capture
generally refers to the specific technology of 3D
marker acquisition, independent of its subsequent use.
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