Graphics Reference
In-Depth Information
for production-quality animation since it is generally less accurate than markered
techniques. We give an overall formulation of the problemand reviewmethods based
on image silhouettes and visual hulls. This general approach is strongly related to the
techniques for 3D data acquisition discussed in more detail in Chapter
8
.
7.1
THE MOTION CAPTURE ENVIRONMENT
Today, there are two primary types of production-quality motion capture technol-
ogy. The first approach is
magnetic
: the performer wears a suit instrumented with
small receivers that can accurately determine their three-dimensional position and
orientation with respect to an external electromagnetic field. The second approach is
optical
: in this case, the performer's suit is fitted with special markers whose three-
dimensional position is inferred by an array of surrounding cameras. Whilemagnetic
motion capture systems are relatively inexpensive and each receiver is always “visi-
ble,” they are sensitive tometal in their environment (commonly found in soundstage
walls and computer monitors), which can degrade the tracking output. Earlier mag-
netic systems also required wires and cables snaking around the performer and a
In this chapter, we focus exclusively on optical motion capture, exemplified by the
industry-standard systems produced by Vicon Motion Systems. The performance
to be recorded takes place in a large room containing a defined
capture volume
,
a space several meters in each dimension. The capture volume is surrounded by
between six and fifty cameras, each of which is circled by a strobe light of infrared
LEDs, as illustrated in Figure
7.2
. These LEDs strongly illuminate the capture volume
with light not visible to the human eye. The cameras are all temporally synchronized
with each other and with the strobe lights.
(a)
(b)
Figure 7.2.
(a) A typical motion capture volume. (b) A motion capture camera surrounded by
infrared LEDs.
2
Other possibilities formotion capture include inertial systems based on gyroscopes and accelerom-
eters, or exoskeletons that directly measure joint angles. Neither are commonly used to collect data
for animation and visual effects.
