  In literature, several approaches are trying to make the UAVs fly
autonomously i.e., by extracting perspective cues such as straight lines.
However, it is only available in well-defined human made environments, in
addition to many other cues which require enough texture information. Our main
target is to detect and avoid frontal obstacles from a monocular camera using a
quad rotor Ar.Drone 2 by exploiting optical flow as a motion parallax, the
drone is permitted to fly at a speed of 1 m/s and an altitude ranging from 1 to
4 meters above the ground level. In general, detecting and avoiding frontal
obstacle is a quite challenging problem because optical flow has some
limitation which should be taken into account i.e. lighting conditions and
aperture problem.
