Deals with scene sequences showing objects in motion. Gray values of an object surface element, traced through the sequence, will exhibit systematic variations due to changing lighting and viewing conditions. Much of today's understanding of the underlying image formation process is due to Horn, who showed in particular that given sufficient knowledge about imaging conditions, gray values may be used to infer object shape. Assuming certain idealized but unknown object surface properties and light source characteristics it is shown that in general rotation angle increments and projected object surface angles but no absolute angle values may be computed from the observed data. Given additional knowledge about unknown illumination parameters, however, object shape may be determined.