Vicon Body Builder

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Vicon Labelling Template Builder: How to create rigid bodies for real time tracking.

In this tutorial you will learn

  • How to place markers onto physical objects for tracking
  • How to use Body Builder to name Markers and create Bodies
  • How to track Bodies models in Vicon Nexus

Placing Markers onto physical objects

When placing Markers onto physical objects it is important to avoid symmetrical placement. That is, markers that look the same from both sides will confuse the system and cause the body to unexpectedly flip during real time tracking. Body builder calculates the exact position differences between Markers for accurate body recognition.

Another important point is that independent objects should be created as separate bodies. For example, if you want to track two separate wands, create a body model for each. Also, make sure that the placement of markers is different for each configuration. You do not need to use more markers, the Vicon can easily determine each object based on the different placements alone.

Use the following steps to place reflective markers on physical objects

  1. Determine how many markers you need. A rigid object typically needs at least 3 markers to determine yaw, pitch and roll.
  2. Determine the size of the markers that you will use. Large spheres are best when the markers are placed far away. They can get merged into a single Marker if placed too close.
  3. Determine where and how to place the markers on your object. If you have multiple objects you'll need to think about how you might vary the placement of number of markers for each object. Also some markers come with a black plastic base tip. You can remove these if you don't need them.
  4. Cut appropriate sized squares of double sided tape for each marker (found in the black life sciences box).
  5. Place the tape squares on the physical object and then place the reflective marker on the tape.
  6. Look at your configuration in the Vicon Nexus software. Make sure all reflective markers are visible through each camera.

Please note: For objects with joints it is generally reccomended that you place markers on or around each joint. The Vicon system is designed to adjust for the Marker movements around joints. Putting three markers on the thumb tip, finger tip, and palm will likely not work because the position change of the thumb and finger is too large for accurate recognition.

Using The Labelling Template Builder

Label Template Builder is a wizard that allows you to create new bodies by making the new body visible to all cameras and naming each individual Marker and specifying Body points and Joints. The end product of using body build is a model file saved to your hard disk that can be loaded and tracked in real time.

To create a new body use the following steps:

  1. Place your new body so that it is visible to all MX-F40 cameras. You can check this in the Vicon Nexus perspective view. Make sure that there are no suprious 3D points.
  2. In the left system panel disable the tracking of any bodies.
  3. In the bottom panel underneath the perspective view click on the Body Builder link.
  4. If the 3D points are clearly visible, click the pause live button to take a snapshot of the current 3D points. Chose next.
  5. First we need to label each Marker of our new body. This is done by first clicking on a 3D point in the perspective window, then clicking the textbox underneath and providing an apropriate name. Repeat this step for each marker placed on the new body.
  6. Next, we need to specify a Body for these markers. Do this by clicking on the create body button and then clicking two or more Markers. You should see a yellow box appear around these marks, the first point you clicked is the default body position. This can be changed later using Vicon scripting see tutorial.
  7. (Optional) If you click next, you will now have the option of creating joints that connect Bodies together. For a hand model each segement of the finger would be considered a Body. These Bodies would be linked using a joint. Free joints provide the most flexibility, while other joints restrict motion if appropriate.
  8. Press next, and now you will have the option to save this model to a file on the local hard drive. It is reccomended that you save this to a directory you will be able to find at a later time.
  9. Check the output window to see if the model was actually saved. If not, there was an error in the Model specification. Try again, removing joints has been know to be beneficial. You can also now unpause live for real time tracking.

Tracking Bodies in Real Time.

The last step is to make sure that your new body can be tracked in real time using the Vicon Nexus software. The new body must first be added to the list of tracked objects the body may need to be moved out of the tracked volume and then moved back in.

To track bodies in real time use the following steps:

  1. Place your body file in the default Vicon nexus Body directory (c:\program files\Vicon Nexus\Bodies)
  2. On the left system panel, click on the add body button that has a '+' on it. Select your new body.
  3. Click the checkbox to enable tracking of your new object, it is also reccomended that you disable other objects when intially testing.
  4. Object should now be tracked, and the real time data should now show the newly added body.