  Commercial head-mounted eye trackers provide useful features to customers in
industry and research but are expensive and rely on closed source hardware and
software. This limits the application areas and use of mobile eye tracking to
expert users and inhibits user-driven development, customisation, and
extension. In this paper we present Pupil -- an accessible, affordable, and
extensible open source platform for mobile eye tracking and gaze-based
interaction. Pupil comprises 1) a light-weight headset with high-resolution
cameras, 2) an open source software framework for mobile eye tracking, as well
as 3) a graphical user interface (GUI) to playback and visualize video and gaze
data. Pupil features high-resolution scene and eye cameras for monocular and
binocular gaze estimation. The software and GUI are platform-independent and
include state-of-the-art algorithms for real-time pupil detection and tracking,
calibration, and accurate gaze estimation. Results of a performance evaluation
show that Pupil can provide an average gaze estimation accuracy of 0.6 degree
of visual angle (0.08 degree precision) with a latency of the processing
pipeline of only 0.045 seconds.
