---
title: Fixation Filter
description: Fixation Filter
---

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# Fixation Filter

## Explore how RealEye identifies fixations by analyzing gaze duration, movement speed, and noise-filtered data.

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### How Does RealEye Compute Fixations?

[Fixation](https://support.realeye.io/eye-tracking-glossary?hsLang=en) 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](https://www.researchgate.net/publication/339174160_A_Custom_Implementation_of_the_Velocity_Threshold_Algorithm_for_Fixation_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](https://support.realeye.io/what-is-aoi?hsLang=en))** if it begins within the AOI’s time range.

![](https://support.realeye.io/hs-fs/hubfs/undefined-Feb-23-2025-10-51-29-5445-PM.png?width=403&height=283&name=undefined-Feb-23-2025-10-51-29-5445-PM.png)

 

To determine whether a cluster of gazes qualifies as a fixation, we define several parameters:

![](https://support.realeye.io/hs-fs/hubfs/image-png-Mar-05-2025-11-25-21-8222-AM.png?width=258&height=468&name=image-png-Mar-05-2025-11-25-21-8222-AM.png)

**- 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. Range: 20–1000 ms. **Default: 200 ms.**

\- **Minimum Fixation Duration**: the shortest time a gaze cluster (eye pause) must last to be considered a fixation. Range: 60–3000 ms. **Default: 80 ms.**

\- **Maximum Fixation Duration**: the longest time a gaze cluster can be considered a fixation before being split into multiple fixations. Range: 60–3000 ms. **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. Range: 1–400 %/s. **Default: 150%/s.**

\- **Maximum Saccade Duration:** the upper time limit for saccades. Range: 20–300 ms. **Default: 150 ms.**

 

Check the [**Data Chart**](https://support.realeye.io/data-chart?hsLang=en) to see how different filter settings affect your results.

 

---

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