Some properties of the motion of rigid objects are presented. The motion of a rigid object can be recovered if three vanishing points and two additional points are put in correspondance in two successive frames. Moreover it is possible to obtain relevant features of the motion of wiewed objects by the analysis of singular points of the 2-D velocity field. The main result is that pure rotation gives always nodes and as a consequence singular points associated to focuses imply rotations.
"On The Understanding Of Motion Of Rigid Objects", Proc. SPIE 0726, Intelligent Robots and Computer Vision V, (27 March 1987); doi: 10.1117/12.937730; https://doi.org/10.1117/12.937730