Biomedical Engineering Reference
In-Depth Information
perpendicular to the camcorder axis) or distances (after
calibration using the image of an object with a known size) .
Using several camcorders allows a plane-by-plane analysis
(front, side, etc.). This solution has the advantage of being
very simple to implement and not very expensive, but is still
dedicated to the overall analyze of movements, for example,
in the case of sporting actions
in situ,
because the accuracy of
the results is highly dependent on the context and
monitoring of many targets that may be long and tedious.
Optical capture systems
require that markers (plastic
balls covered in a reflective material) are positioned on
points of interest on the subject [FUR 97]. The diodes
surrounding the camera lenses emit stroboscopic radiation,
red to infrared, reflected in the incident direction by markers
whose surface is made of a retro-reflective material. A filter
positioned on the lense makes the cameras sensitive to a
specific wavelength, and only the markers are, therefore,
detected. After processing an image of a minimum of two
cameras, the positions of the markers on the overall subject
are calculated by triangulation. Current systems allow, after
the markers are identified on a reference image or a
pre-established model is used, the automatic tracking of
their trajectories in real time even for a high-sampling
frequency (500-2,000 Hz) and with no limit with regard to
the number of markers. The field volume analyzed can be
easily adapted to different needs by adjusting the size of the
markers, and the best performing systems guarantee
accuracy with regard to the position of the markers to
approximately 1I3,000th of the diagonal of the field. The
operator, in retrospect, has algorithms to handle possible
occlusions (when a limb hides the marker from one or several
cameras) and problems with swapping (confusion or
accidental misidentification of two markers, whose
trajectories cross, for example). Currently, the main
manufacturers of optical motion capture systems are Motion
Analysis, Vicon, Qualysis and BTS.
