Local luminance amplitude modulates the interpretation of shape-from-shading in textured surfaces

Abstract

The pattern of illumination on an undulating surface can be used to infer its 3-D form (shape-from-shading). But the recovery of shape would be invalid if the luminance changes actually arose from changes in reflectance. So how does vision distinguish variation in illumination from variation in reflectance to avoid illusory depth? When a corrugated surface is painted with an albedo texture, the variation in local mean luminance (LM) due to shading is accompanied by a similar modulation in local luminance amplitude (AM). This is not so for reflectance variation, nor for roughly textured surfaces. We used depth mapping and paired comparison methods to show that modulations of local luminance amplitude play a role in the interpretation of shape-from-shading. The shape-from-shading percept was enhanced when LM and AM co-varied (in-phase) and was disrupted when they were out of phase or (to a lesser degree) when AM was absent. The perceptual differences between cue types (in-phase vs out-of-phase) were enhanced when the two cues were present at different orientations within a single image. Our results suggest that when LM and AM co-vary (in-phase) this indicates that the source of variation is illumination (caused by undulations of the surface), rather than surface reflectance. Hence, the congruence of LM and AM is a cue that supports a shape-from-shading interpretation.

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

“…The uniform texture condition faithfully represented the shading of the curved step that participants were required to climb while the low-contrast condition masked the shape of the kerb by presenting a texture amplitude profile more consistent with a simulated reflectance change. However, when presented alone in this fashion the two treatments are known to appear rather similar (Schofield et al, 2006;2010). Indeed participants were unaware of the difference between the two kerb treatments in the stepping task as evidenced by both subjective report and the lack of any effect for texture type in the kerb height estimation part of Experiment 2.…”

“…This manipulation is useful as it leaves the primary luminance cue unaffected yet can produce quite distinct precepts based on a relatively simple and easily controlled manipulation. In the first experiment we employed both the shading and reflectance simulations in a plaid configuration that has been shown to maximise the difference in appearance between the two cues (Schofield et al, 2006;2010). Further, the efficacy of the manipulation employed is known to depend on high-frequency (fine) components in the underlying texture (Sun & Schofield, 2011).…”

“…The right leaning component is paired with a positively correlated change in texture amplitude (regions of high amplitude texture align with regions of high luminance) and thus simulates, and is perceived as, a shaded, corrugated surface. The left leaning component has been paired with a negatively correlated change in texture amplitude and appears as a flat reflectance change: like strips of material laid across the undulations (Schofield et al, 2006;2010;Schofield, Rock & Georgeson, 2011). This apparent distinction between undulations and material changes is especially strong in the plaid configuration depicted in Figure 1b.…”

“…In the first (Figure 3c) the contrast of the texture was unchanged such that any shading due to illumination would produce the normal, yoked (in-phase), combination of luminance and texture amplitude (but not contrast) changes that results from multiplicative (real) shading of a matte surface: we call this the Uniform condition. Due to the natural properties of shaded textures (see Schofield et al, 2006Schofield et al, , 2010 this condition resulted in the peak of the luminance highlight caused by the spotlight to co-inside with a peak in texture amplitude (LM+AM; cf Experiment 1). In the second treatment ( Figure 3d) the contrast of the texture was reduced on the riser such that once illuminated the luminance and texture amplitude varied in anti-phase across the riser.…”

“…Schofield and colleagues (Schofield, Heese, Rock & Georgeson, 2006;Schofield, Rock, Sun, Jiang & Georgeson, 2010;Sun & Schofield, 2011) have recently shown that modulations in the luminance amplitude of visual textures (texture amplitude) can be used to discriminate between changes in illumination -such as shading and shadows -and changes in the reflectance properties of a surface.…”