Shape-from-Shading Depends on Visual, Gravitational, and Body-Orientation Cues

Abstract

The perception of shading-defined form results from an interaction between shading cues and the frames of reference within which those cues are interpreted. In the absence of a clear source of illumination, the definition of 'up' becomes critical to deducing the perceived shape from a particular pattern of shading. In our experiments, twelve subjects adjusted the orientation of a planar disc painted with a linear luminance gradient from one side to the other, until the disc appeared maximally convex-that is, until the luminance gradient induced the maximum perception of a three-dimensional shape. The vision, gravity, and body-orientation cues were altered relative to each other. Visual cues were manipulated by the York Tilted Room facility, and body cues were altered by simply lying on one side. The orientation of the disc that appeared maximally convex varied in a systematic fashion with these manipulations. We present a model in which the direction of perceptual 'up' is determined from the sum of three weighted vectors corresponding to the vision, gravity, and body-orientation cues. The model predicts the perceived direction of 'up', contributes to our understanding of how shape-from-shading is deduced, and also predicts the confidence with which the 'up' direction is perceived.

The human visual system's assumption that light comes from above is weak

“…Studies that used complex illuminated objects with strong lighting direction cues concluded that lighting cues guide shape-from-shading (6, 7), whereas studies that used weaker lighting cues concluded that they have little effect (8)(9)(10)(11)(12). We discuss the lighting cues in previous studies in further detail in SI Discussion.…”

“…The weight ratio w prior /w cue determines whether the prior or the cues have a greater influence on the effective lighting direction. This model is well established in the literature on the subjective vertical (11,17). We have recently shown that it is largely equivalent to a Bayesian cue combination model that assigns reliability weights to noisy directional cues [here the prior, which we treat as just another cue (18,19), and the lighting cues] and combines the cues by making a maximum-likelihood direction estimate (16).…”

“…The evidence is inconclusive: some researchers have argued that visual lighting cues completely override the prior (6, 7), whereas others have maintained that lighting cues have either no effect at all on the perceived lighting direction that guides shape-fromshading (8-10), or less influence than nonvisual factors such as head orientation and the direction of gravity (11,12). This question is pivotal, however, for understanding 3D shape perception.…”

“…To examine this strategy more closely, we used a vector sum model of how observers combine information from two or more cues to estimate a direction (11,16,17). In this model the prior direction and the lighting cue direction are represented by unit vectors v prior and v cue , respectively, and the effective lighting direction is a weighted sum of the two vectors, w cue v cue .…”

Does gravity influence the visual line bisection task?

“…A leftward bias has also been observed for the assumed direction of illumination in shaded images (Howard et al 1990) (Jenkin et al 2003;Jenkin et al 2004;McManus et al 2004), and it has been demonstrated that this leftward bias persists when tilted in the roll plane (Barnett-Cowan et al 2013;Jenkin et al 2003). Likewise, leftward biases were reported for spatial perception of the body (Barnett- Cowan et al 2013;Barnett-Cowan and Harris 2008;Tarnutzer et al 2012), perceived direction of gravity (Schuler et al 2010), and the perceptual upright (Dyde et al 2006).…”