8

Working with Motion-Capture Data 911
commonly used formats. This data can be
imported to the biped and used as is, or combined
with other motions with
Motion Flow (page
2–878)
or the Motion Mixer (page 2–581) .
Motion-capture data typically needs some
adjustment before it fits your biped or animation
perfectly. When motion-capture data is imported
to 3ds Max, it can be fi ltered to:
•Usefewerkeyframes
•Createfootstepmotion
• Use props in t he scene
In addition, some motion-capture files come with
a separate marker file, which can be used to match
thebipedposturetothemotion-captureactor.
HowMotion-CaptureDataisAcquired
Motion-capture data is typically acquired by one
of several means:
•Opticalsensingtechnology
Optical systems have become quite popular over
the last couple of years. These systems can offer
the performer the most freedom of movement
sincetheydonotrequireanycabling.Optical
systems incorporate directionally reflective
balls, referred to as
markers (page 3–1063) ,
that attach to the performer. Optic al systems
require at least three video cameras, each of
which is equipped with a light source that is
aligned to illuminate the field of view for that
camera.Eachcameraisinturnconnectedto
a synchronized frame buffer. The computer
ispresentedwitheachcameraviewinorder
to calculate a 3D position of each marker;
the resulting data stream therefore consists of
3D position data for each marker. This data
is typically applied to an inverse kinematics
system, to animate a skeleton.
• Electro-magnetic sensing technology
This is a popular method used for performance
capture. Magnet ic capture involves the use
of a centrally located transmitter, and a set of
receivers that are strapped on to various parts
of the per former’s body. T hese re ceivers are
capable of measuring their spatial relationship
to the transmitter. Each receiver is connected to
an interface that can be synchronized to prevent
data skew. The resulting data stream consists
of 3D positions and orientations for each
receiver. Thisdataistypicallyappliedtoan
inverse kinematics system to drive an animated
skeleton. This magnetic approach shares the
same lack of occlusion problems with the audio
method, but it also shares the same negative
factors, such as the hindrance of cables, lack
of sufficient receivers, and the limited capture
area. In addition, being magnetic, the system
is affected by any sizable areas of metal in the
vicinit y of the capture area, such as girders,
posts, and so on
• Prosthetic sensing technology
This is one of the ea rly methods for capturing
the motion from various par ts of human
anatomy. These methods include simple
"on/off " type of motion-detection systems
as well as complex motion-tracking systems.
The latter type of prosthetic motion capture
would be an ideal approach if it weren’t for
the complex mechanical requirements and the
performance-inhibiting qualities generally
associated with such designs. However, the
type o f data provided can be clean, rotational
data collected in real time without any
occlusion problems. This method is based
on a set of armatures that must be attached
all over the per former’s body. The armatures
are then connected to each other by using a
series of rotational and linear encoders. These
encoders are then connected to an interface
that can simultaneously read all the encoders
in order to prevent data skewing. Finally,