Specular reflection appears as a bright spot or highlight on any smooth glossy convex surface and is caused by a near mirror-like reflectance off the surface. Convex shapes always provide the ideal geometry for highlights, areas of very strong reflectance, regardless of the orientation of the surface or position of the receiver. Despite highlights and glossy appearance being common in chemically defended insects, their potential signalling function is unknown. We tested the role of highlights in warning colouration of a chemically defended, alpine leaf beetle, Oreina cacaliae. We reduced the beetles' glossiness, hence their highlights, by applying a clear matt finish varnish on their elytra. We used blue tits as predators to examine whether the manipulation affected their initial latency to attack, avoidance learning and generalization of warning colouration. The birds learned to avoid both dull and glossy beetles but they initially avoided glossy prey more than dull prey. Interestingly, avoidance learning was generalized asymmetrically: birds that initially learned to avoid dull beetles avoided glossy beetles equally strongly, but not vice versa. We conclude that specular reflectance and glossiness can amplify the warning signal of O. cacaliae, augmenting avoidance learning, even if it is not critical for it.
When planning interactions with nearby objects, our brain uses visual information to estimate shape, material composition, and surface structure before we come into contact with them. Here we analyse brain activations elicited by different types of visual appearance, measuring fMRI responses to objects that are glossy, matte, rough, or textured. In addition to activation in visual areas, we found that fMRI responses are evoked in the secondary somatosensory area (S2) when looking at glossy and rough surfaces. This activity could be reliably discriminated on the basis of tactile-related visual properties (gloss, rough, and matte), but importantly, other visual properties (i.e., coloured texture) did not substantially change fMRI activity. The activity could not be solely due to tactile imagination, as asking explicitly to imagine such surface properties did not lead to the same results. These findings suggest that visual cues to an object’s surface properties evoke activity in neural circuits associated with tactile stimulation. This activation may reflect the a-priori probability of the physics of the interaction (i.e., the expectation of upcoming friction) that can be used to plan finger placement and grasp force.
Perceptual Gloss Parameters Are Encoded by Population Responses in the Monkey Inferior Temporal Cortex
- The Journal of neuroscience : the official journal of the Society for Neuroscience
- Published over 3 years ago
There are neurons localized in the lower bank of the superior temporal sulcus (STS) in the inferior temporal (IT) cortex of the monkey that selectively respond to specific ranges of gloss characterized by combinations of three physical reflectance parameters: specular reflectance (ρs), diffuse reflectance (ρd), and spread of specular reflection (α; Nishio et al., 2012). In the present study, we examined how the activities of these gloss-selective IT neurons are related to perceived gloss. In an earlier psychophysical study, Ferwerda et al. (2001) identified a perceptually uniform gloss space defined by two axes where the c-axis corresponds to a nonlinear combination of ρs and ρd and the d-axis corresponds to 1 - α. In the present study, we tested the responses of gloss-selective neurons to stimuli in the perceptual gloss space defined by the c- and d-axes. We found that gloss-selective neurons systematically changed their responses in the perceptual gloss space, and the distribution of the tuning directions of the population of gloss-selective neurons is biased toward directions in which perceived gloss increases. We also found that a set of perceptual gloss parameters as well as surface albedo can be well explained by the population activities of gloss-selective neurons, and that these parameters are likely encoded by the gloss-selective neurons in this area of the STS to represent various glosses. These results thus provide evidence that the IT cortex represents perceptual gloss space.
Humans are able to estimate the reflective properties of the surface (albedo) of an object despite the large variability in the reflected light due to shading, illumination and specular reflection. Here we first used a physically based rendering simulation to study how different statistics (i.e, percentiles) based on the luminance distributions of matte and glossy objects predict the overall surface albedo. We found that the brightest parts of matte surfaces are good predictors of the surface albedo. As expected, the brightest parts led to poor performance in glossy surfaces. We then asked human observers to sort four (2 matte and 2 glossy) objects in a virtual scene in terms of their albedo. The brightest parts of matte surfaces highly correlated with human judgments, whereas in glossy surfaces, the highest correlation was achieved by percentiles within the darker half of the objects' luminance distributions. Furthermore, glossy surfaces tend to appear darker than matte ones, and observers are less precise in judging their lightness. We then manipulated different bands of the virtual objects' luminance distributions separately for glossy and matte surfaces. Modulating the brightest parts of the luminance distributions of the glossy surfaces had a limited impact on lightness perception, whereas it clearly influenced the perceived lightness of the matte objects. Our results demonstrate that human observers effectively ignore specular reflections while evaluating the lightness of glossy objects, which results in a bias to perceive glossy objects as darker.
- Proceedings. Biological sciences / The Royal Society
- Published almost 2 years ago
Visually identifying glossy surfaces can be crucial for survival (e.g. ice patches on a road), yet estimating gloss is computationally challenging for both human and machine vision. Here, we demonstrate that human gloss perception exploits some surprisingly simple binocular fusion signals, which are likely available early in the visual cortex. In particular, we show that the unusual disparity gradients and vertical offsets produced by reflections create distinctive ‘proto-rivalrous’ (barely fusible) image regions that are a critical indicator of gloss. We find that manipulating the gradients and vertical components of binocular disparities yields predictable changes in material appearance. Removing or occluding proto-rivalrous signals makes surfaces look matte, while artificially adding such signals to images makes them appear glossy. This suggests that the human visual system has internalized the idiosyncratic binocular fusion characteristics of glossy surfaces, providing a straightforward means of estimating surface attributes using low-level image signals.
Whether a surface is glossy or matte is physically independent of whether it is light or dark. Perceptually however, the albedo of specularly or diffusely reflecting surfaces can strongly influence perceived gloss (Pellacini et al., 2000, Proceedings of SIGGRAPH). Here we ask whether glossiness judgments are more constrained if there are variations of albedo within a single surface. To investigate the influence of different albedos on a single surface we first created a set of 45 real surfaces. Based on a 3D model, molds were cut out of polymer foam by the use of a CNC (Computerized Numerical Control) machine. Sheets of plastic (350mm x 350mm) were heated up inside a vacuum forming machine and then stretched onto the mold and later spray-painted in five colors and five gloss levels. Additionally, we also painted surfaces with combinations of two colors next to each other. 11 participants looked at each surface in a light box, through a viewing hole, and they were encouraged to rate the gloss on a scale from one to seven. In a second experiment participants rated only the left or the right half of each bi-colored surface. Our results confirm that perceived gloss varies with albedo. In addition, perceived global gloss of bicolored surfaces were close to the ratings of unicolored surfaces whose albedos are the average of the bicolored surfaces. However, results from the second experiment indicate that when participants rate gloss only on one side of the surface, judgments for lighter colors show simultaneous gloss contrast effects of the darker colors next to it. In conclusion we propose a fairly easy method to create mathematically defined bumpy, plastic surfaces with various colors and gloss levels. Our results from multi-colored surfaces show both averaging and simultaneous gloss contrast effects depending on the task. Meeting abstract presented at VSS 2015.
Separating the surface texture (changes in albedo) from surface shading is a fundamental task for the visual system (Kim et al., 2014). We investigated whether this task is possible only from the image statistics. Images of randomly generated “bumpy” surfaces with/without specular reflections were rendered as gloss/matte surfaces. Half of the images in each category were modified so as to have surface texture (pigmentation) by multiplicatively changing the intensities with binary noise patterns. These images of four categories (matte, gloss, textured-matte and textured-gloss; 1,800 images for each category) were normalized to have the same mean and variance of pixel intensity, and then 744 of PS statistics (Portilla & Simoncelli, 2000) were calculated for them. Next, we developed a statistical model to classify the images into four categories using their PS statistics by canonical discriminant analysis (CDA). It turned out that the trained classifier is able to clearly categorize the generated images, and the first two canonical coordinates correspond to gloss/matte and uniform/textured axes, respectively. With the trained classifier, we examined the hypothesis: computations for evaluating the geometric consistency of highlights on glossy surfaces are common with those for separation of changes in albedo from surface shading. Diffused matte components and specular highlight components were separated from original glossy surface images, and as the test images the matte surfaces were textured with the separated highlight components in the same manner as in the training “textured” image generation with various multiplicative factors (1.2 to 5.0). This pigmentation was applied to consistent and inconsistent locations (original highlight components were horizontally flipped) of surfaces (3,600 in total). The trained classifier was able to differentiate consistent/inconsistent categories, except when the highlight-shaped texture regions have very high or very low local contrasts, suggesting that inference of geometric consistency of specular highlights is partially done by albedo-shading computations. Meeting abstract presented at VSS 2015.
Surface gloss information conveyed by image cues (i.e., highlights) has been shown to be processed in ventral and dorsal areas. In this study we used fMRI to distinguish the brain areas that selectively process 2D and 3D cues about surface gloss. We performed one experiment using 2D images of random objects with glossy surfaces where diffuse highlights could be presented rotated by 45 degree to make the object look matte. We also performed a second experiment with binocular cues where the specular reflections of the environmental on random shapes could have disparities coincident with the surface so to appear painted and thus making the object look matte. The same twelve participants took part in the two experiments where fMRI activations were measured over the whole brain with an Echo-Planar Imaging sequence (32 slices, TR 2000 ms, TE 35 ms, voxel size 2.5 × 2.5 × 3 mm). We performed Multi-Voxel Pattern Analysis to test whether a classifier trained to discriminate glossy vs. matte objects with 2D cues can still discriminate with 3D cues, and vice versa. We found transfer effects from 2D to 3D cues in early (V1, V2) and dorsal visual areas (V3d, V3A/B, V7, IPS). This transfer suggests the presence of circuits processing gloss independently of the type of cues in dorsal areas only. We did not find transfer from training with 3D cues to 2D cues, suggesting that stereoscopic information related to gloss has a pattern of activation that is additional to the representation of gloss. Meeting abstract presented at VSS 2015.
Abstract Objective. The purpose of this study was to determine the effects of two denture cleansing methods on 3-D surface roughness, gloss and color of denture base materials. Materials and methods. Thirty disks from nylon (Valplast) and 30 from heat-polymerized acrylic denture base material (Paladon 65) were made and 10 of each material were immersed in water (control), Val-Clean (peroxide cleanser) and Corega Extradent (peroxide cleanser) plus microwaving for a period simulating 30 days of daily cleansing. 3-D surface roughness, gloss and color parameters were measured before and after cleansing using an interferometric profilometer, a gloss meter and a colorimeter. The results were statistically analysed by regression, paired-t, Mann-Whitney and Kruskal-Wallis tests at α = 0.05 level of significance. Results. The results showed significant differences at baseline in L* and b* parameters between materials (p < 0.01), with a significantly lower gloss (p < 0.05) and higher roughness (p < 0.05) for Valplast. After cleansing, Δϵ* was significantly greater in Valplast than Paladon 65 (p < 0.05). Gloss of both materials decreased significantly within the Corega Extradent plus microwave solution (p < 0.05), while roughness increased significantly only for Paladon 65 (p < 0.05). Conclusions. Valplast was found to have a significantly lower gloss and a higher roughness than Paladon 65 before cleansing. After cleansing, ΔE* increased more in Valplast than in Paladon 65, gloss of both materials decreased and roughness only of Paladon 65 increased within the Corega extradent plus microwaving method.
A series of experiments were conducted to assess how the reflectance properties and the complexity of surface “mesostructure” (small-scale 3-D relief) influence perceived lightness. Experiment 1 evaluated the role of surface relief and gloss on perceived lightness. For surfaces with visible mesostructure, lightness constancy was better for targets embedded in glossy than matte surfaces. The results for surfaces that lacked surface relief were qualitatively different than the 3-D surrounds, exhibiting abrupt steps in perceived lightness at points at which the targets transition from being increments to decrements. Experiments 2 and 4 compared the matte and glossy 3-D surrounds to two control displays, which matched either pixel histograms or a phase-scrambled power spectrum, respectively. Although some improved lightness constancy was observed for the 3-D gloss display over the histogram-matched display, this benefit was not observed for phase-scrambled variants of these images with equated power spectrums. These results suggest that the improved lightness constancy observed with 3-D surfaces can be well explained by the distribution of contrast across space and scale, independently of explicit information about surface shading or specularity whereas the putatively “simpler” flat displays may evoke more complex midlevel representations similar to that evoked in conditions of transparency.