*4.4. Feature-Tracking Length Comparison*

Feature-tracking length is another important indicator for feature tracking especially in VO or SLAM since the improved continuity of features can give more constraints for estimating poses or constructing maps. Without re-detecting features, feature-tracking length is quantified by the time period between the timestamp of first frame and the time when the number of tracked features decreases to 10% of the initial one. Due to the high computational complexity, HTRT is not further compared in its feature-tracking length.

The result is shown in Table 4: PDAT shows limited feature-tracking ability on tracking length, which means 90% of initialized feature can only last for 0.35 s. Its main difference with the other two methods is it purely uses event information for feature tracking, which can easily lose tracked features.


**Table 4.** Average duration of one track for PDAT/EKLT/WF-MHT-BP methods (unit: s).

WF-MHT-BP and EKLT show similar feature-tracking length statistics, showing the superiority in integrating the measurement from two sensors. The commonality between the two methods is that intensity information is used to correct the feature-tracking drifts from event information, which is helpful to continue tracking features.
