A displacement detection method for a portable autonomous device having an integrated image sensor. The method comprises several steps. In the first step a current digital image is received from the image sensor; the current digital image depicting a background scene. Then, the position of a first area within the current digital image is chosen. Afterwards, a later digital image is received from the image sensor depicting an overlapping portion of the background scene. Then, the position of a second area matching to the first area within the later digital image is identified. After a match has been identified, the first and second areas depict approximately the same portion of the background scene. Then, a current motion vector of the portable autonomous device is calculated, according to a displacement between the first area and the second area.