Accurate camera and target calibration is an essential prerequisite for obtaining precise measurements from an optical tracker. An iotracker system requires three separate calibration steps before it becomes fully functional: Intrinsic Camera Calibration (performed by one of our technicians prior to shipment), Extrinsic Camera Calibration and Rigid-Body Target Calibration.
1. Intrinsic Camera Calibration
Intrinsic camera calibration is the process of obtaining the parameters of a projective camera model that are independent for each camera: focal length, principal point and, most importantly, compensation of the image distortion. In optics, distortion ("straight lines appear curved in the image") means divergence from rectilinear projection ("straight lines appear straight") and can be corrected by a non-linear warping function.
Once an iotracker camera has been fully assembled, its intrinsic parameters do not change. Thus, intrinsic camera calibration will be performed by one of our technicians prior to shipping new systems.
2. Extrinsic Camera Calibration
Extrinsic camera calibration, sometimes also known as "geometric calibration", is the process of finding the spatial transformation between all pairs of cameras in a given setup and must be repeated every time a camera is moved or reoriented. The iotracker software splits the procedure into two steps, both of which are conveniently accessible through an easy-to-use graphical user interface.
The first step requires you to record a set of image point correspondences by waving a single-marker calibration wand through the entire tracking volume.
Once a sufficient number of correspondences has been recorded, which should take no longer than two or three minutes, the iotracker software will optimize every camera's projection model for the given tracking setup.
In the second step, you can align your iotracker's virtual coordinate system to real-world structures by placing an L-shaped coordinate system proxy at the desired location inside the interaction volume.
With a bit of practice, the entire extrinsic calibration procedure should take less than five minutes.
3. Rigid-Body Target Calibration
After building or purchasing a new rigid-body target, your iotracker system requires a round of "training" to recognize it. The iotracker software provides an intuitive wizard-style interface specifically for this purpose. Just hold your new target into the tracking volume for a bit and let the training-wizard handle the rest.