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Computer simulated stimuli can provide a flexible method for creating artificial scenes in the study of visual perception of material surface properties. Previous work based on this approach reported that the properties of surface roughness and glossiness are mutually interdependent and therefore, perception of one affects the perception of the other. In this case roughness was limited to a surface property termed bumpiness. This paper reports a study into how perceived gloss varies with two model parameters related to surface roughness in computer simulations: the mesoscale roughness parameter in a surface geometry model and the microscale roughness parameter in a surface reflectance model. We used a real-world environment map to provide complex illumination and a physically-based path tracer for rendering the stimuli. Eight observers took part in a 2AFC experiment, and the results were tested against conjoint measurement models. We found that although both of the above roughness parameters significantly affect perceived gloss, the additive model does not adequately describe their mutually interactive and nonlinear influence, which is at variance with previous findings. We investigated five image properties used to quantify specular highlights, and found that perceived gloss is well predicted using a linear model. Our findings provide computational support to the 'statistical appearance models' proposed recently for material perception.

In dynamic scenes, relative motion between the object, the observer, and/or the environment projects as dynamic visual information onto the retina (optic flow) that facilitates 3D shape perception. When the object is diffusely reflective, e.g. a matte painted surface, this optic flow is directly linked to object shape, a property found at the foundations of most traditional shape-from-motion (SfM) schemes. When the object is specular, the corresponding specular flow is related to shape curvature, a regime change that challenges the visual system to determine concurrently both the shape and the distortions of the (sometimes unknown) environment reflected from its surface. While human observers are able to judge the global 3D shape of most specular objects, shape-from-specular-flow (SFSF) is not veridical. In fact, recent studies have also shown systematic biases in the perceived motion of such objects. Here we focus on the perception of local shape from specular flow and compare it to that of matte-textured rotating objects. Observers judged local surface shape by adjusting a rotation and scale invariant shape index probe. Compared to shape judgments of static objects we find that object motion decreases intra-observer variability in local shape estimation. Moreover, object motion introduces systematic changes in perceived shape between matte-textured and specular conditions. Taken together, this study provides a new insight toward the contribution of motion and surface material to local shape perception. (C) 2015 The Authors. Published by Elsevier Ltd.

We investigated whether specular highlights facilitate the perception of shape from shading in a search paradigm and how highlights interact with shading to facilitate this perception. Our results indicated that stimuli containing highlights led to shorter searching time with the dependence on the light source direction (top lights make searching faster), suggesting that highlights indeed facilitate shape-from-shading processing. To examine how highlight processing interacts with shading processing, we tested unnatural stimuli for which the lighting directions for shading and highlights were inconsistent. The results indicated that unnatural highlights (bright spots) placed in a direction inconsistent with the shading either decrease or do not alter searching time. This suggests that highlights may facilitate, and not suppress, shading processing. With more physically plausible highlights generated from image-based lighting, we also observed facilitation with consistent highlights, but no change with inconsistent highlights. Finally, we examined whether highlights indeed work to facilitate depth perception in a discrimination task. The results showed that correct discrimination of depth increases when highlights are added to shading even when their lighting directions are inconsistent. These results indicate that specular highlights facilitate shading processing, and do not suppress it even when the highlights are placed in a direction inconsistent with shading. The results also elucidate the lighting constraints of the visual system.

Two experiments are reported in which observers judged the sign and magnitude of surface curvature from shaded images of an indoor scene. The depicted surfaces were illuminated by an area light in the ceiling or floor, and the illumination was attenuated with distance in a physically correct manner. The displays were presented both with and without cast shadows, specular highlights and surface inter-reflections in all possible combinations. The overall pattern of results revealed a strong perceptual bias to interpret the images as convex rather than concave, and a weaker bias to prefer illumination from above rather than from below, though there were large individual differences in the relative strengths of these biases. For displays that did not contain cast shadows or specular highlights, the accuracy of observers' judgments about the sign of surface curvature was no greater than chance, but performance was significantly improved when those aspects of normal shading were included in the rendering model. An analysis of the apparent depth magnitudes revealed that convex surfaces produce much greater perceived depth than concave surfaces with comparable relief.

Binocular disparity is an aspect of natural viewing. This research investigates whether disparity affects surface color perception. Achromatic settings were obtained and compared for two stereograms of a scene with specular reflections, one stereogram with binocular disparity and one without it (cyclopean view). Binocular disparity was found to improve color constancy. Next, the geometry of specular highlights, which is distorted without binocular disparity, was specifically examined. Measurements compared color constancy with specular reflections that were either normal (with stereo disparity) or distorted (cyclopean view of the specularities). No significant change in constancy was found due to the geometrical distortion of specular highlights that occurs without stereo disparity, suggesting that constancy depends on other features of the percept affected by disparity. The results are discussed in terms of illuminant estimation in surface color perception.

Many recent computational models of surface color perception presuppose information about illumination in scenes. The models differ primarily in the physical process each makes use of as a cue to the illuminant. We evaluated whether the human visual system makes use of any of three of the following candidate illuminant cues: (1) specular highlight, (2) full surface specularity [Lee, H. C. (1986). Method for computing the scene-illuminant chromaticity from specular highlights. Journal of the Optical Society of America A, 3(10), 1694–1699; D’Zmura, M., & Lennie, P. (1986). Mechanisms of color constancy. Journal of the Optical Society of America A, 3(10), 1662–1672], and (3) uniform background. Observers viewed simulated scenes binocularly in a computer-controlled Wheatstone stereoscope. All simulated scenes contained a uniform background plane perpendicular to the observer’s line of sight and a small number of specular, colored spheres resting on the uniform background. Scenes were rendered under either standard illuminant D65 or standard illuminant A. Observers adjusted the color of a small, simulated test patch to appear achromatic. In a series of experiments we perturbed the illuminant color signaled by each candidate cue and looked for an influence of the changed cue on achromatic settings. We found that the specular highlight cue had a significant influence, but that the influence was asymmetric: greater when the base illuminant, CIE standard Illuminant A, was perturbed in the direction of Illuminant D65 than vice versa. Neither the full surface specularity cue nor the background cue had any observable influence. The lack of influence of the background cue is likely due to the placement of the test patch in front of the background rather than, as is typical, embedded in the background. © 2001 Elsevier Science Ltd. All rights reserved.