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3D motion analysis |
3D motion analysis
A three-dimensional analysis has to be done for rotating or more complex movements, such as
- golf swing
- hammer throwing
- tennis
- judo
- martial arts
Each anatomic landmark (marker) has to be identified in at least two cameras. Therefore 3D motion analysis requires at least two cameras. Most movements require four or more cameras.
With 3D coordinates six degrees of freedom (6 DOF) can be computed, i.e. rotations around all three axes of a segment or a joint can be determined as well as the exact spatial position.
3D calibration
Two different methods are commonly used in commercial motion capture systems:
Static calibration frame
An object with known dimensions (e.g. a cube) is placed into the camera’s field of view.
The size of the calibration frame should cover the total movement space.
The object’s shape is then digitized using manual or automatic marker detection for
the calibration points.
For most algorithms it is not necessary that all cameras see the same set of calibration
points.
A static calibration needs approx. 1-3 minutes per camera.
Dynamic calibration (“wand calibration”)
During dynamic calibration the user moves a pole with attached markers which have to
be seen by all or a subset of the cameras.
The pole is moved until the complete movement space is covered.
A dynamic calibration needs a couple of minutes or more, depending on the system’s
detailed requirements and its real-time tracking capabilities.
See also
2D motion analysis
