Game Development Reference
In-Depth Information
When trying to determine the location and orientation of an actor, there
are three basic techniques: optical, magnetic and mechanical.
Optical suits use two or more cameras and highly reflective markers on an
actor's suit. The location of the cameras relative to each other is set to a
high degree of accuracy. Motion tracking software follows each marker
from a single camera viewpoint. Motion tracking is a form of pattern
recognition. Software scans the bitmap of a single frame from a video
camera. Then the next bitmap in the video sequence is scanned trying to
locate the same pattern. The location of the marker on the bitmap defines
a ray emanating from the camera, which goes through the marker and
then disappears off to infinity. Using the second and often a third camera,
the marker is tracked from other camera angles. Each camera provides a
line from the camera to infinity. Software can then determine the
intersection of these lines and using the knowledge of the camera world
position provide an x , y , z location in world space for a marker for each
frame of video. At certain times in the video sequences, markers will be
hidden due to the angle of the camera in relation to the performer; this is
described as occlusion. At such times knowledge of the topography of
markers on the performer helps the software to provide a best estimate of
marker location. This type of error checking can make optical motion
capture a very accurate form of analysis. Other benefits of optical capture
are the low cost of additional markers and the lack of restrictions to the
Figure 12.3 Optical motion capture.
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