  Compact and low-cost devices are needed for autonomous driving to image and
measure distances to objects 360-degree around. We have been developing an
omnidirectional stereo camera exploiting two hyperbolic mirrors and a single
set of a lens and sensor, which makes this camera compact and cost efficient.
We establish a new calibration method for this camera considering higher-order
radial distortion, detailed tangential distortion, an image sensor tilt, and a
lens-mirror offset. Our method reduces the calibration error by 6.0 and 4.3
times for the upper- and lower-view images, respectively. The random error of
the distance measurement is 4.9% and the systematic error is 5.7% up to objects
14 meters apart, which is improved almost nine times compared to the
conventional method. The remaining distance errors is due to a degraded optical
resolution of the prototype, which we plan to make further improvements as
future work.
