How to design a 6DOF body

A rigid body is a configuration of points at fixed relative positions. For the 6DOF tracking function to work the rigid body must be defined by at least three points, which should not be on the same line. When designing a 6DOF body, consider the following aspects.

Physical design of the rigid body

When choosing or designing a rigid body, it is important to consider the type of markers that you want to use, as well as their placement. The rigid bodies should allow for optimal placement of the markers. This often depends both on the shape of the rigid body, as well as the positioning of the cameras. Generally, there should be clear lines of sight from the cameras to the markers for optimal tracking.

In case you need to have an exact definition of the local coordinate system of the rigid body, you can design the body in a way that the markers can be placed at known positions.

For more detailed information about the choice of markers and their configuration, see the chapters below.

Choice of markers

When designing a rigid body, take the following in consideration when choosing markers:

  • Marker size

    The markers should be large enough for tracking, see chapter Marker size. Furthermore, the markers should be small enough so that there is sufficient separation between the markers to minimize the occurrence of merging of the markers in the camera views.

  • Active vs passive markers

    In most situations, passive markers are recommended. Active markers can be considered in the following situations:

    • In large volume applications, where the distance between markers and cameras are too large for tracking passive markers, for example marine applications. In this case, it is recommended to use long-range active markers (LRAM).

    • In case you need to track many rigid bodies simultaneously, you can consider the use of Active or Naked Traqrs. The active markers use sequential coding for reliable rigid body identification of multiple rigid bodies. Due to the automatic marker identification there is no need to use an asymmetric or unique marker configuration. Active markers can also be useful for robust real-time tracking of drones.

      For Crazyflie drones, dedicated active marker decks for Qualisys can be purchased via Bitcraze.

Marker configuration

The following aspects are important for the tracking of rigid bodies.

  • Marker placement

    The rigid body must consist of at least 3 markers, which should not be in a straight line. In many cases, it can be an advantage to use 4 or more markers for more redundancy, for example in case markers may get occluded during the measurement.

  • The markers should be placed in a way that they are clearly visible and well separated in the camera views for optimal tracking.

  • Marker distribution

    Generally, the accuracy of the 6DOF data increases with the distance between the markers. Try to place the markers as far apart as possible in different directions, spanning up a plane or a volume.

  • Asymmetry

    Marker should be applied in an asymmetric configuration, so that the orientation of the rigid body can be uniquely determined. When the markers are placed symmetrically, the measured orientation of the rigid body may flip.

    This limitation does not apply when using active markers with marker ID.

  • Uniqueness

    When tracking multiple rigid bodies simultaneously, they should have unique configurations so that they can be identified. If the configurations are the same, it is not possible to distinguish between the rigid bodies.

    This limitation does not apply when using active markers with marker ID.

Using extra markers for the rigid body definition

The marker placement can also be important for the definition of the rigid bodies. For example, markers can be placed in a way that they can be associated with important positions, axes or planes for the definition of a models geometry.

In some cases, these landmark positions may not be suitable for tracking the rigid body during a measurement. In that case, you can temporarily place extra markers at these positions and use them for the definition of the rigid body. After completing the definition, these extra markers can be removed, while keeping the markers used for tracking.

Here are some guidelines for how to use extra markers for creating a rigid body definition:

  1. Apply the markers for tracking and the extra markers for the definition of the rigid body.

  2. Place the model in a way that all markers are visible.

    • If it is not possible to see all markers in a single measurement, you can also make several recordings with the model in different orientations.

  3. Make an initial rigid body definition, see chapter Definition of 6DOF bodies.

    • In case you have multiple recordings, use the Add to rigid body (6 DOF) function from the Trajectory info window menu to add selected markers to the rigid body definition.

    • If needed, you may even add virtual markers to the definition, for example a point in between two markers.

  4. Use the Translate body and Rotate body methods to change the local coordinate system of the rigid body according to your specifications.

  5. You can now remove the extra markers from the physical rigid body.

    • You must either remove the corresponding points from the rigid body definition, or check their Virtual option, so that QTM will not try to track the point as a marker. When using the virtual option, the point will be added as a virtual trajectory when measuring the rigid body in QTM, which can for example be useful for visualization purposes.