The role of texture amplitude in shape from shading

The pattern of illumination on an undulating surface can be used to infer its 3-D form (shape from shading). But the recovered shape would be invalid if the luminance pattern actually arose from reflectance variation. When the surface has a reflectance texture, variation in local mean luminance (LM) due to shading is accompanied by similar modulation in texture amplitude (AM) in-phase with the LM. This does not hold for reflectance variation, nor for roughly textured surfaces. We presented LM+AM and LM-AM cues in a plaid configuration co-registered with a haptic surface, using a ReachIn workstation. In experiment 1 observers adjusted the position of the haptic surface until its peak in depth aligned with the visually perceived one. Haptic settings varied with plaid orientation and were consistent with a lighting-from-above prior. In experiment 2, AM modulation depths were 0.1, 0.2 or 0.4 and the haptic surface amplitude was adjusted to match the visual one. Observers chose relatively large amplitude settings when the haptic surface was aligned with the LM+AM cue, independently of modulation depth. When the haptic surface was aligned with the LM-AM cue, haptic amplitude settings decreased as modulation depth increased. The weakest stimuli were close to the detection threshold for AM but well above the threshold for LM. We conclude that when a visible AM signal is out-of-phase with its LM partner this is a cue to reflectance variation, which is seen as flat. An experiment using visual depth probes was consistent with the haptic results.