This paper addresses the issue of window estimation of a micro Air Vehicle (MAV) in unknown urban environments. The MAV is required to navigate from an initial and outdoor position to a final position inside a building. This paper develops two vision-based methods using the information provided by the onboard vision system. To effectively identify the target and estimate the distance between the camera carrier and target, firstly a stereo camera system is applied. Besides, we propose another approach using point cloud captured by a RGB-D camera.