Search for gross illumination discrepancies in images of natural objects

Abstract

Shadows may be "discounted" in human visual perception because they do not provide stable, lighting-invariant, information about the properties of objects in the environment. Using visual search, R. A. Rensink and P. Cavanagh (2004) found that search for an upright discrepant shadow was less efficient than for an inverted one. Here we replicate and extend this work using photographs of real objects (pebbles) and their shadows. The orientation of the target shadows was varied between 30 and 180 degrees. Stimuli were presented upright (light from above, the usual situation in the world) or inverted (light from below, unnatural lighting). RTs for upright images were slower for shadows angled at 30 degrees, exactly as found by Rensink and Cavanagh. However, for all other shadow angles tested, the RTs were faster for upright images. This suggests, for small discrepancies in shadow orientation, a switch of processing from a relatively coarse-scaled shadow system to other general-purpose visual routines. Manipulations of the visual heterogeneity of the pebbles that cast the shadows differentially influenced performance. For inverted images, heterogeneity had the expected influence: reducing search efficiency and increasing overall search time. This effect was greatly reduced when images were presented upright, presumably when the distractors were processed as shadows. We suggest that shadows may be processed in a functionally separate, spatially coarse, mechanism. The pattern of results suggests that human vision does not use a shadow-suppressing system in search tasks.

What makes cast shadows hard to see?

“…Once the orientation difference became small (30-), however, search was slower with the upright (light-from-above for both target and distractors) than the inverted displays, just as reported by Rensink and Cavanagh (2004). This is in keeping with shadow processing being coarsely scaled, so that only large shadow discrepancies can be readily perceived (Lovell et al, 2009;Mamassian, 2004).…”

“…That is, in the context of Rensink and Cavanagh's (2004) claims, an (easily perceived) inverted shadow "poppedout" among (hard-to-perceive) upright ones, whereas a (hard-to-perceive) upright shadow was well concealed among easily perceived inverted distractors (see also Rensink & Cavanagh, 1993). Similarly, Lovell, Gilchrist, Tolhurst, and Troscianko (2009) demonstrated faster search of natural images of heterogeneous pebbles with upright (light-from-above) shadows for one with a discrepant shadow inverted 180-(light-from-below) than vice versa. Lovell et al also showed how the orientation difference between target and distractors affects search speeds.…”

“…While all of these reports (Elder et al, 2004;Lovell et al, 2009;Rensink & Cavanagh, 2004) agree that the visual system has more difficulty in inspecting a shadow-like item than the exact same visual input inverted, the underlying mechanisms and processing levels remain unclear. Rensink and Cavanagh (2004) suggested that, since shadow-like status can slow down otherwise very fast search, shadow identification and discounting involves rapid, parallel mechanisms and thus probably early levels of vision.…”

Reduced sensitivity for visual textures affects judgments of shape-from-shading and step-climbing behaviour in older adults

“…We speculate that this may relate to our tendency to process shading and shadows differently from material cues to object identity (see for example, Lovell, Gilchrist, Tolhurst, & Troscianko, 2009). Perhaps this tendency results in the undulations -which are simulated here only via shading -being slightly harder to process than the simulated reflectance changes.…”

Shading and shadowing on Canaletto's Piazza San Marco

“…4 Although cast shadows give effective 3D position information, 8 differences between shading and shadowing are not very well noticed. 9 Furthermore, we showed previously 10 that light direction estimation of cast shadows do not follow the rules of linear perspective, much like the errors Canaletto made in the painting in figure 1.…”