Examples of how to use 6DOF bodies

How to use 6DOF bodies

The standard way to use the 6DOF bodies are to create a body for each separate object that you want to measure. In this case the markers on each subject have no relation to markers on the other subjects. Follow these instructions to use 6DOF bodies.

  1. To create a 6DOF body follow for example the instructions in chapters How to design a 6DOF body and Definition of 6DOF bodies.

  2. It is important to think about the definition of rotation and position of the local coordinate system, see chapter Definition of local coordinate system.

  3. It is also important to consider which Euler angles definition you want to use, because the calculated angles will differ depending on the definition, see chapter Definition of custom rotation axes.

  4. The points in the 6DOF definition can be renamed on the 6DOF bodies page.

  5. Remember to turn on the Calculate 6DOF option on the Processing page.

These are some tips on how you can improve the 6DOF data.

  • If you are streaming the data in real time it can help to activate the Do not require that the whole body is visible before identifying it the first time setting on the 6DOF bodies page so that the identification starts even if a marker is hidden.

  • The 6DOF data can sometimes be improved with marker filtering, see chapter 2D Preprocessing and filtering.

How to use 6DOF bodies in an AIM model

In this case the 6DOF bodies are placed on parts that move together, e.g. clusters placed on a human subject. Then the best approach is to use AIM model to identify the markers and then calculate the 6DOF bodies from the already calculated markers.

  1. Create the bodies and their definition normally. Make sure that you name the points of the bodies in the same way as they are named in the AIM model.

  2. Create an AIM model from the subject, see chapter Generating an AIM model. If you already have an AIM model with the correct marker setup you can add the current measurement to that AIM model.

    The AIM model can contain markers that are not included in 6DOF bodies.

  3. Activate both Apply the current AIM model and Calculate 6DOF on the Processing page. The AIM model will be applied first and then the 6DOF bodies will be calculated from the identified labels.

    If AIM fails to identify a marker, the 6DOF calculation will not try to identify it either even if you select the Reidentify all body markers in reprocessing. In most cases the best way to fix this is to manually identify the data and add it to the AIM model.

  4. There can still be 6DOF bodies in the file that are not included in the AIM models. These will only be identified and calculated when the Calculate 6DOF option is activated, i.e. if you reprocess the file and only apply the AIM model the trajectories of the separate bodies will not be reidentified.

How to use virtual markers in an AIM model

The virtual markers in the 6DOF functionality can be used in a regular AIM model if it contains markers that are actually rigid bodies. This example describes the case when the 6DOF data is only used to create virtual markers and therefore the actual 6DOF data is not really important.

  1. Create an AIM model as described in chapter Generating an AIM model. If you want to use a completely virtual point then you can add that as an empty label in the AIM model.

  2. Disable the display of the rigid bodies with the Show rigid bodies option on the 3D view settings page. Otherwise you will see for example the coordinate systems in the 3D view window.

  3. Increase the Bone tolerance to 20 mm on the 6DOF Tracking page. This will make sure that the 6DOF bodies can be calculated even if the marker positions are not completely rigid.

  4. Also activate the option Calculate missing markers in rigid bodies on the same page so that the virtual markers are calculated when a real marker is lost.

    There must be at least three real markers identified in a frame to calculate the virtual marker.

  5. When you measure a new subject:

    1. Unless you are really sure that markers on the rigid body have not moved, delete all of the bodies in the Rigid bodies list.

    2. Create the 6DOF bodies from a static file of the subject where AIM has identified the markers. Select at least three markers that do not move in relation to each other, and then use the Define rigid body option in the Trajectory info window menu. The trajectories will keep the labels they got from the AIM model.

      All of the real markers in the 6DOF body must be included the AIM model, the others will not be identified. However 6DOF bodies that are completely virtual will be calculated even if they are not in the AIM model.

  6. Make sure that both Apply the current AIM model and Calculate 6DOF are activated on the Processing page.