@TechReport{TR020,
  author =	 "Cornelia Fermueller and Yiannis Aloimonos",
  institution =	 "PRIP, TU Wien",
  number =	 "PRIP-TR-020",
  title =	 "Qualitative {E}gomotion",
  year =	 "1993",
  url =		 "https://www.prip.tuwien.ac.at/pripfiles/trs/tr20.pdf",
  abstract =	 "Due to the aperture problem, the only general
                  unambiguous motion measurement in images is normal
                  flow---the projection of image motion on the
                  gradient direction. In this paper we show how a
                  monocular observer can estimate its 3D motion
                  relative to the scene by using normal flow
                  measurements in a global and mostly qualitative
                  way. The problem is addressed through a search
                  technique. By checking constraints imposed by 3D
                  motion parameters on the normal flow field the
                  possible space of solutions is gradually reduced. In
                  the four modules that comprise the solution,
                  constraints of increasing restriction are
                  considered, culminating in testing every single
                  normal flow value for its consistency with a set of
                  motion parameters. The fact that motion is rigid
                  defines geometric relations between certain values
                  of the normal flow field. The selected values form
                  patterns in the image plane that are dependent on
                  only some of the motion parameters. These patterns,
                  which are determined by the signs of the normal flow
                  values, are searched for in order to find the axes
                  of translation and rotation. The third rotational
                  component is computed from normal flow vectors that
                  are only due to rotational motion. Finally, by
                  looking at the complete data set, all solutions that
                  cannot give rise to the given normal flow field are
                  discarded from the solution space.",
}