The calibration process estimates parameters of the user's eyes required to perform gaze estimation and other eye tracker functions, and is needed once per individual user.
FOVE includes a default calibration profile that will allow basic eye tracking for most people, but high precision eye tracking requires information about the geometry of the user's eye. Because each person's eyes are unique, calibration is required on a per-user basis.
Once calibrated, the profile functionality built into the FOVE VR Platform can persist the profile data to disk, so that person does not need to calibrate again. You can even switch between multiple users without recalibrating, simply by switching profiles.
There is a small amount of accuracy lost when remounting the headset (due to drift in the mount position), bit it is minor enough to not matter in casual situations such as gaze-based UI control, assuming the headset is mounted well. In situations where you need the highest possible accuracy, such as a research study, we recommend calibrating every time the headset is mounted.
How does calibration work?
During calibration the eye tracker is tasked with estimating the following primary data points:
- The size of the cornea
- Whether or not eye glasses are present
- The optical-to-visual-axis transform
- Compensation parameters (offset & scale)
The size of the cornea is computed by a mathematical optimization function to determine what cornea radius would produce the observed glints, given the 3D geometry.
Glasses presence is detected by searching for additional reflections that would normally not be present. When glasses are present, additional computations are enabled in the eye tracker to mitigate the effects of having extra reflection artifacts in the eye image, and additional lens distortion from the extra lens.
The remaining parameters are computed by comparing the points displayed on the screen during calibration with the gaze estimation generated by the eye tracker from eye image taken from the camera.
In the case of smooth-pursuit calibration (commonly known as spiral calibration), a combination of spatial and temporal algorithms are used to match which eye image corresponds to which point on the scene.
Typically, the calibration is run simultaneously for both eyes, but the calibrations for each are calculated independently. There is an additional feature in FOVE Pro to allow showing the calibration dot for one eye at a time.