Extrinsic chessboard calibration trials are used to determine the position and orientation of every camera in your camera system relative to your desired global coordinate system. As with lens calibration trials, users with wand calibration-capable systems (e.g. Qualisys Miqus, Vicon Vue or FLIR Blackfly S systems) are not required to complete chessboard calibration. A minimum of one chessboard calibration trial should be collected every time you set up your camera system, but collecting multiple is recommended. A new calibration is required any time a camera is moved, so having multiple calibration trials collected throughout a long data collection session can help prevent data loss due to accidental or unnoticed camera movements.

Chessboard calibrations must be performed at the resolution that will be used to record your movement data. If you will be using more than one resolution during your data collection, you must record chessboard calibration trials at each resolution.

Recommendations:

• Record chessboard calibration trials at a low frame rate (20-30 Hz) to reduce file size and processing time.
• Keep the chessboard as flat as possible during the calibration trial.
• Focus on achieving groupings of 3+ cameras that can see the chessboard at all times.
• Align the chessboard with marks on the ground so you can confirm that the global coordinate system is positioned correctly when processing movement trials.

Recording Chessboard Calibrations:

1. Set up your camera system as desired for your data collection, following the recommendations in Data Collection Principles.
2. Choose or place visible marks on the ground which will be aligned with the global coordinate system, for confirmation when processing data.
3. Check that the chessboard is fully visible in at least 3 camera views when it is placed at the desired global coordinate system origin position.
4. Begin the recording.
5. Slowly ‘show’ the chessboard to groupings of 3 or more cameras while varying the position and orientation of the chessboard slightly. Ensure the chessboard is visible to cameras that overlap between groupings, ideally so that no fewer than 3 cameras can see the chessboard at all times. Focus on achieving groupings of 3+ cameras at all times.
6. When you are confident that all cameras have had sufficient views of the chessboard, slowly place the chessboard on the ground, aligned with your preselected marks.
7. Ensure you are not obstructing the view of the chessboard in any cameras during this localization phase.
8. End the recording.

To process extrinsic chessboard calibrations, see Chessboard Calibration.