Scaling depth from shadow offset

Abstract

When an object casts a shadow on a background surface, both the offset of the shadow and the blur of itspenumbra are potential cues to the distance betweenthe object and the background. However, the shadowoffset and blur are also affected by the direction andangular extent of the light source and these are oftenunknown. This means that the observer must makesome assumptions about the illumination, the expecteddistribution of depth, or the relation between offset anddepth in order to use shadows to make distancejudgments. Here, we measure human judgments ofperceived depth over a range of shadow offsets, blurs,and lighting directions to gain insight into this internalmodel.We find that distance judgments are relativelyunaffected by blur or light direction, whereas theshadow offset has a strong and linear effect. The dataare consistent with two models, a genericshadow-to-depth model and a Bayesian model