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Patrick Cavanagh, Roberto Casati, James Elder; Tight shadows shrink depth. Journal of Vision 2018;18(10):493. doi: 10.1167/18.10.493.
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When an object casts a shadow on the surface behind it, we get a strong impression of depth separating the object and the rear surface. Kersten and Mamassian (1996) demonstrated this with a cast shadow that moved in and out below a static square, making the square appear to approach and recede in depth. There was no visible light source and the shadow motion could have been caused by a moving light source. Instead, viewers assumed a fixed light and a moving object where the depth of the object scaled with the offset of its shadow. Here we report a particularly strong light source bias in the case of "tight shadows", shadows that have very small offsets from the object casting them, as is often seen with flash photography. The small offset between object and shadow is due to the light source being near the point of view of the observer or camera. However, rather than attributing the tight shadow to the light source location, observers instead attribute it to a very small separation between the object and the rear surface. This bias – "tight shadow equals small separation" – leads to paradoxical impressions of objects impossibly close to their backgrounds. However, it is consistent with the expected object to background separation averaged over all possible light source directions, e.g., lights to the left for right shadows. In this framework, tight shadows are most likely to be caused by shallow relief.
Meeting abstract presented at VSS 2018
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