REVISITING PENTLANDS ESTIMATOR OF LIGHT-SOURCE DIRECTION

We examine the pioneering method of Pentland [J. Opt. Soc. Am. 72, 448 (1982)] for automatically estimating the direction of the ''sun'' (light source) from a single image. It is shown that, under the assumptions used in the derivation of the method, the estimate of source direction is erroneous. Specifically, it is shown that an image-based expression used in calculating source direction diverges to infinity as the density of image points is increased and that the formula involving this expression is therefore incorrect. When the method is implemented, the flaw manifests itself in the undesirable dependence of the estimator on image resolution. Supporting experimental evidence is given for this. An alternative source-direction estimator that is free of these drawbacks is proposed.