Calibration of the Camera

To achieve a high-accuracy tracking system one has to consider the camera's remaining lens distortions. ART uses a camera model with up to 11 parameters (“inner orientation”) to correct residual distortions during tracking.

ART adjusts each tracking camera's focus and aperture to the customer’s requirements before delivery. Lens distortions are determined and the set of parameters is stored in the camera’s memory. This kind of calibration can only be performed in the ART labs. When tracking is active, the parameters are read out and used for image correction.

The camera parameters are only valid for certain settings of focus and aperture. If these settings are changed, a recalibration has to be done in the ART labs.

Calibration of the Room

Calibration set

After setting up the tracking system, i.e. after camera mounting and cabelling, the exact position and orientation (“external orientation”) of each camera has to be determined. This is done by a simple calibration procedure that takes not more than five minutes in all. This “room calibration” process uses an L-shaped tool („angle tool“) with four markers, which defines the coordinate system, and a stick with two markers (“wand”) which has to be moved in the tracking volume.

If just an update of the calibration is wanted, the calibration process can be performed without the angle tool („recalibration“); the process then maintains the existing coordinate system.

Room calibration hardware is included in all standard packages. The calibration process can be controlled by the DTrack2 software.

Body Calibration

Tree target

In order to identify a three dimensional arrangement of markers as a “target” (or “rigid body”), i.e. as an object to be tracked, the geometry of this marker configuration has to be calibrated. In other words, the target is shown to the tracking system and the special geometry of marker positions is determined, which is the base for target identification and tracking.

This fast (30 sec) and simple calibration process is included in the DTrack2 software. DTrack2 also offers many customisation possibilities in adjusting the measured geometry to the customer's needs.

Calibration has to be redone if the target geometry had changed. For this a special calibration process can be performed that preserves the existing geometry as much as possible („recalibration“). Recalibration is not necessary if just the room calibration was refreshed. Body calibration data can be transferred from one tracking system to another.