How Does RealEye Compute Fixations?
Fixation is a sequence of gaze points that remain close in both time and space. RealEye uses an algorithm similar to the I-VT (Velocity-Threshold Identification) with the following criteria:
- Maximum gaze duration is 33 ms, ending at the raw gaze timestamp.
- Instead of angular velocity, RealEye uses velocity as a percentage of item size.
- Fixation time expands if the gap between gazes is ≤50 ms.
- Fixations require at least two gaze points.
- Fixation coordinates are calculated as the average of all included gaze points.
- Calculations use IEEE 754 double-precision format (14 decimal places).
- Sampling Rate Requirement – The fixation filter works with data sampled at over 20 Hz.
- AOI Fixation Inclusion – A fixation is considered part of an AOI (Area of Interest) if it begins within the AOI’s time range.
To determine whether a cluster of gazes qualifies as a fixation, we define several parameters:
- Noise Reduction Window: the TOI (Time of Interest) window used to denoise raw data by applying a moving median algorithm. Each gaze point is replaced by the median of nearby points within the TOI window, reducing noise influence. Default: 200 ms.
- Minimum Fixation Duration: the shortest time a gaze cluster (eye pause) must last to be considered a fixation. Typically ranges between 80–300 ms. Default: 80 ms.
- Maximum Fixation Duration: the longest time a gaze cluster can be considered a fixation before being split into multiple fixations. Default: 400 ms.
- Gaze Velocity Threshold: data below this threshold is classified as a fixation, while higher-speed data is a saccade. This filter can be set to 0 if an appropriate noise reduction level is applied. Default: 150%/s.
- Maximum Saccade Duration: the upper time limit for saccades. Default: 150 ms.
Check the Data Chart to see how different filter settings affect your results.
💌 If you have any questions or need further assistance, feel free to reach out to our team at contact@realeye.io