A catadioptric sensor with multiple viewpoints

Conventional cameras with a limited field of view often lose sight of objects when their bearings change suddenly due to a significant turn of the observer (robot), the object, or both. Catadioptric omnidirectional sensors, consisting of a camera and a mirror, can track objects and estimate their distances more robustly. The shapes of mirrors used by such sensors have differing merits. This paper discusses several advantages of the conical mirror over other shapes of mirrors in current use. A perspective projection unwarping of the conical mirror images is developed and demonstrated. This has hitherto been considered impossible for mirrors with multiple viewpoints. An estimation of distance (range) over a large surrounding area is crucial in mobile robotics. A solution is proposed here in the form of an omnidirectional stereo apparatus with two catadioptric sensors in a vertical coaxial arrangement. The coaxial stereo requires very simple matching since the epipolar lines are the radial lines of identical orientations in both omnidirectional images. The radial matching is supported by a novel polar edge finder which uses discrete cosine transform and returns image gradients expressed in polar coordinates. (c) 2004 Elsevier B.V. All rights reserved.