  Vision-based localization is a cost-effective and thus attractive solution
for many intelligent mobile platforms. However, its accuracy and especially
robustness still suffer from low illumination conditions, illumination changes,
and aggressive motion. Event-based cameras are bio-inspired visual sensors that
perform well in HDR conditions and have high temporal resolution, and thus
provide an interesting alternative in such challenging scenarios. While purely
event-based solutions currently do not yet produce satisfying mapping results,
the present work demonstrates the feasibility of purely event-based tracking if
an alternative sensor is permitted for mapping. The method relies on geometric
3D-2D registration of semi-dense maps and events, and achieves highly reliable
and accurate cross-modal tracking results. Practically relevant scenarios are
given by depth camera-supported tracking or map-based localization with a
semi-dense map prior created by a regular image-based visual SLAM or
structure-from-motion system. Conventional edge-based 3D-2D alignment is
extended by a novel polarity-aware registration that makes use of signed
time-surface maps (STSM) obtained from event streams. We furthermore introduce
a novel culling strategy for occluded points. Both modifications increase the
speed of the tracker and its robustness against occlusions or large view-point
variations. The approach is validated on many real datasets covering the
above-mentioned challenging conditions, and compared against similar solutions
realised with regular cameras.
