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transmitter.
Photo courtesy of Ascension Technology
.
host computer is aware of the timing of the signals.
When the receivers pick up the signals, the host knows
the distance from the transmitter by the time elapsed
and the orientation of the receiver by the changes in the
signal caused by tilting of the magnetic fields.
Optical systems are preferred for high-res work. An
optical system can collect hundreds of data points,
though you seldom need more than about thirty. It works
by visually tracking small reflective markers attached to
the performer at key points. Optical systems are also first
choice when you have lots of action. You can’t have wires
getting in the way with your actors bounding around, so
you need optical tracking (see Figure 2).
The more data points you track, the less extrapolation
the software has to do. So for an animation of a
character that doesn’t look much like a human being—
such as those dancing credit cards—you only need
enough points to get the basic motion into a file. The animators
then use that as a framework. They do plenty of
work to make those points fit their character.
The object in most MoCap sessions is simple—save a
data set that represents the subject’s motion with the
optimal level of detail and the least amount of noise. The
optimal detail varies according to the project. For
ergonomic study, you want fine resolution of the motion
and highly repeatable measurements. For animation of
a fantasy character like Moxie, the animated MTV host,
you need relatively low resolution and repeatability.
Sports games such as EA Sports Madden NFL 99, or
Knockout Kings, where the movements are fast and accuracy
is essential to realism, would fall somewhere in
between, since the motion animates a relatively realistic
human figure. For example, a setup to capture a martial
arts sequence might use 20 to 30 sensors, and a
full-body ergonomic study could use 100 or more.
Another issue is the speed of the motion being captured.
Optical systems can run at 240 Hz or more—critical
for discerning very fine movements, or when the
motion is quick. In addition, high-speed systems typically
multiplex their data capture channel among the
sensors. So an optical system with 100 tracking points
would need to operate about four times faster than a
system tracking just 25 points to obtain the same temporal
resolution. Magnetic systems operate at about 140
Hz and can support fewer sensors than optical systems.
Of course, there are other considerations as well.
IEEE Computer Graphics and Applications 15
World Wide Web URLs for MoCap
You can find out more about the following motion capture
houses and equipment manufacturers by visiting their sites on the
World Wide Web.
Adaptive Optics Associates: https://www.aoainc.com
Analogus: https://www.analogus.com
Ascension Technology: https://www.ascension-tech.com
BioVision: https://boris.biovision.com
Digital Domain: https://www.d2.com
5DT: https://www.5dt.com
GameTek: https://www.gametek.com
House of Moves: https:// www.moves.com
MediaLab: https://www.demon.co.uk/mlab
Motion Analysis: https://www.motionanalysis.com
Performance Animation Society: https://www.pasociety.org
Polhemus: https://www.polhemus.com
Protozoa: https://www.protozoa.com
Richard Cray’s Performance Animation Page:
https: