The first discrimination we should make is the difference between stereo (or indeed any multiview), 3D reconstruction using calibrated rigs, and SfM. While a rig of two or more cameras assume we already know what the motion between the cameras is, in SfM we don't actually know this motion and we wish to find it. Calibrated rigs, from a simplistic point of view, allow a much more accurate reconstruction of 3D geometry because there is no error in estimating the distance and rotation between the cameras—it is already known. The first step in implementing an SfM system is finding the motion between the cameras. OpenCV may help us in a number of ways to obtain this motion, specifically using the findFundamentalMat function.
Let us think for one moment of the goal behind choosing an SfM algorithm. In most cases we wish to obtain the geometry of the scene, for example, where objects are in relation to the camera and what their form is. Assuming we already know the...