Scene geometry from moving objects

It has been observed that in most videos recorded by surveillance cameras the image size of an object is a linear function of the y coordinate of its image location. This simple linear relationship holds in the most common surveillance camera configurations, where objects move on a planar surface and the camera´s X axis is parallel to that plane. This linear relationship enables us to easily perform and enhance several geometric tasks based on tracking an object over a few frames: (i) computing the horizon; (ii) computing the relative real world sizes of objects in the scene based on their image appearance; (iii) improving tracking by constraining an object´s location and size. When the the camera´s X axis is not parallel to the ground plane, after tracking a couple of objects it is possible to find the rotation which rectifies the video so that its new X axis is parallel to the ground plane.