3D motion analysis

A three-dimensional analysis has typically to be done for complex, rotating or simi- lar movements:

  • golf swing
  • hammer throwing
  • tennis
  • judo
  • martial arts

Scientists in sports biomechanics and medicine often use 3D analysis in order to describe a body's movements in three dimensional space. If you want to do a simi- lar motion analysis you have to use more than one camera, because each anatomic landmark (marker) has to be identified in at least two camera views.
Although 3D motion analysis can be done using only 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

For a static calibration, an object with known dimensions (e.g. a cube, cuboid or similar objects) 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 takes 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 takes a couple of minutes or more, depending on the system's detailed requirements and its real-time tracking capabilities.

See also
2D motion analysis

[3D calibration]
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