Inverse perspective mapping and optic flow: A calibration method and a quantitative analysis

In most imaging devices (including the human eye), the mechanisms that govern the projection of a 3D scene onto a 2D surface render the extraction of useful 3D information difficult. We investigate the effects of perspective on optic flow in a driver assistance application in which a camera is mounted on the wing mirror in order to observe a driver's so called ‘blind spot’. A car travelling toward the camera appears to increase in speed and size on the projected image although its real speed and size are constant. We show that the inverse perspective mapping, previously used for obstacle detection, can also help in the problem of extracting real world speed from 2D optic flow data. We provide a quantitative analysis that shows precisely to what degree speed uniformity in the 3D world can be recovered by the mapping. To determine some mapping parameters, we devised a calibration method adapted to our specific situation that can be performed on-line and unsupervised. Its simplicity lends itself to fast software or hardware implementation.