Calibration establishes the intrinsic and extrinsic parameters of your two cameras so that QOOM can reconstruct 3D positions from the two 2D video streams.
What is measured
- Intrinsic parameters: each camera's focal length, principal point, and lens distortion. These are properties of the phone itself and rarely change.
- Extrinsic parameters: the position and orientation of camera 2 relative to camera 1. These depend on where you place the phones.
Why it matters
Without calibration, QOOM has no way to determine absolute distances or 3D joint positions. Metrics like gait speed, hop distance, or knee flexion angle depend directly on calibration accuracy.
Poor calibration produces plausible-looking but inaccurate metrics. A few centimeters of calibration error can shift a metric from "green" to "red" or vice versa.
When to recalibrate
Recalibrate if any of the following are true:
- Either phone has moved since the last calibration.
- The calibration target has moved.
- More than 15 minutes have passed since calibration (for highest accuracy).
- You are starting a new capture session with the same subject.
Calibration target
QOOM supports:
- ChArUco boards — preferred. More robust to partial occlusion and varying angles.
- Checkerboards — also supported. Must be fully visible in both cameras.
Print the target on rigid backing (foam board or thick cardstock). A flexed target introduces calibration error.
Confirming calibration quality
After calibration, the app shows a reprojection error estimate. Values below 1 pixel are excellent; 1–2 pixels is acceptable; above 2 pixels should trigger a recalibration.