Validation of Predictive Dynamics Tasks - Human Motion Simulation: Predictive Dynamics - page 207

Another advantage is that the markers are passive sensors, meaning that they are
only reflective surfaces and can be attached easily to any area on the body of the sub-
ject without the need for wires to connect them to a data collection system. In addi-
tion, theoretically, only three markers are required to define the three-dimensional
(3D) velocity and acceleration of each body segment. Another advantage is that there
are many well-accepted professional packages, such as Visual3D by C-Motion
( http://www.c-motion.com/products/visual3d/ ) that can be used to post-process the
motion capture data for general and specific applications. Finally, real-time data flow
and avatar animation can be done easily with most optical motion capture systems,
and there are numerous post-processing options to digitally filter and differentiate
the motion data.
A potential problem with passive markers, though, is , where the

markers do not appear in enough of the camera shots due to blockage of the line
of sight by objects in the scene or by other parts of the subject's body. Most com-
mercial post-processing packages have the capability to deal with occluded mar-
kers. They may create virtual markers to help fill in the occluded markers, or they
may use redundant markers (more than the minimum required in the standard pro-
tocol) to compensate for occluded markers. Methodologies are available to fill in
the motion data from occluded markers with information from the redundant
markers.
Markerless motion capture systems such as those by organic motion ( http://
www.organicmotion.com/ ) may present an alternative new approach to motion
capture technology; however, their validity as an effective tool still needs to be
shown.

occlusion

9.3.2 Optical motion capture systems
A 12-camera Vicon System (infrared SVcam cameras with a resolution of
0.3 megapixels per frame and a peak capture rate of 200 Hz) and a 16-camera
Motion Analysis system (with four megapixels per frame and a peak capture rate
of 500 Hz) are used for data collection at the 3D Bio-Motion Research Lab
(3DBMRL) at the Center for Computer-Aided Design at The University of Iowa.
The basic idea behind the function of optical systems is that each camera in the
system sends infrared light that will be reflected back when it collides with reflec-
tive surfaces. The Cartesian position of the center of a reflective marker (normally
with a spherical or hemispherical shape) can be identified by the intersection of
the infrared lights of three cameras. Therefore, with more cameras, there is a bet-
ter chance that a marker will be seen by more than three cameras during the
experiments.
Figure 9.1 illustrates the procedure of using the motion capture system to
acquire human motion data for a person wearing a motion capture suit with a
number of markers attached to their body. The placement of the markers on the
human body is called a marker protocol. There are several standard marker proto-
cols, such as the plug-in gait for gait analysis, that are used in various labs, and