In this report, an simple and new estimator is developed for the tilt angle of the rotationaxis of a rigid object that is illuminated by a point light source. The tilt angle defines the orientation of the 2D projection of the 3D object unit rotation axis in the image plane. The angle is one of the 6 degrees of freedom of 3D rigid motion. It is analytically shown that the tilt angle can be estimated just by exploiting the error of motion compensation between two images dueto object shading. Required input data are a dense displacement vector field for motion compensationand a 2D binary object mask. The estimator assumes rigid, lambertian objects with equally distributed surface normals as well as a scene illumination by a distant point light source and ambient light. Experiments with head-and-shoulder video sequences yield qualitatively good resultst hat are derived fully automatically. A performance analysis shows that the estimator works best if light comes fromviewing direction.finally, an application to video indexing shows that head-and-shoulder sequences can be fully automatically segmented into temporal segments of homogeneous object motion. A demo can be found at http://www.irisa.fr/temics/Demos.