Sylvia C. Pont

Haptic curvature discrimination at several regions of the hand

Static haptic discrimination of the curvature of convex, concave, or straight 20-cm-long strips was investigated for nine placements on the hand. In one condition, the strips were touched with the palmar side of the hand, and in the other condition, with the dorsal side. The influence of the lengths of the strips, and thus of contact lengths, was also investigated. For all placements, discrimination was poorer in the dorsal than in the palmar condition, owing to poorer cutaneous resolution on the dorsal side of the hand (the kinesthetic stimulation was the same in both conditions). Thus cutaneous stimulation is important. In both experiments, performance appeared to depend primarily on contact length. Moreover, the discrimination thresholds for all different placements and contact lengths followed the same trend. We conclude that in these experiments the effective stimulus for the discrimination of curved strips is the total difference of local surface attitude—that is, the slope difference over the far ends of the stimulus.

The visual light field.

Human observers are sensitive to the ‘(physical) light field’ in the sense that they have expectations of how a given object would appear if it were introduced in the scene in front of them at some arbitrary location. Thus the ‘visual light field’ is defined even in the ‘empty space’ between objects. In that sense the light field is akin to visual space considered as a ‘container’. The visual light field at any given point can be measured in psychophysical experiments through the introduction of a suitable ‘gauge object’ at that position and letting the observer adjust the appearance of that gauge object (eg through suitable computer rendering) so as to produce a ‘visual fit’ into the scene. The parameters of the rendering will then be considered as the measurement result. We introduced white spheres as gauge objects at various locations in stereoscopically presented photographic scenes. We measured the direction (‘direction of the light’), diffuseness (‘quality of the light’ as used by photographers and interior decorators), and intensity of the light field. We used three very different scenes, with very different physical light fields. The images were geometrically and photometrically calibrated, so we were in a position to correlate the observations with the physical ‘ground truth’. We report that human observers are quite sensitive to various parameters of the physical light field and generally arrive at close to veridical settings, although a number of comparatively minor systematic deviations from veridicality can be noted. We conclude that the visual light field is an entity whose existence is at least as well defined as that of visual space, despite the fact that the visual light field hardly appears as prominently in vision science as it does in the visual arts.

Illusory gloss on Lambertian surfaces

It has recently been shown that an increase of the relief height of a glossy surface positively correlates with the perceived level of gloss (Y.-H. Ho, M. S. Landy, & L. T. Maloney, 2008). In the study presented here we investigated whether this relation could be explained by the finding that glossiness perception correlates with the skewness of the luminance histogram (I. Motoyoshi, S. Nishida, L. Sharan, & E. H. Adelson, 2007). First, we formally derived a general relation between the depth range of a Lambertian surface, the illumination direction and the associated image intensity transformation. From this intensity transformation we could numerically simulate the relation between relief stretch and the skewness statistic. This relation predicts that skewness increases with increasing surface depth. Furthermore, it predicts that the correlation between skewness and illumination can be either positive or negative, depending on the depth range. We experimentally tested whether changes in the depth range and illumination direction alter the appearance. We indeed find a convincingly strong illusory gloss effect on stretched Lambertian surfaces. However, the results could not be fully explained by the skewness hypothesis. We reinterpreted our results in the context of the bas-relief ambiguity (P. N. Belhumeur, D. J. Kriegman, & L. Yuille, 1999) and show that this model qualitatively predicts illusory highlights on locations that differ from actual specular highlight locations with increasing illumination direction.

Irradiation direction from texture

We present a theory of image texture resulting from the shading of corrugated (three-dimensional textured) surfaces, Lambertian on the micro scale, in the domain of geometrical optics. The derivation applies to isotropic Gaussian random surfaces, under collimated illumination, in normal view. The theory predicts the structure tensors from either the gradient or the Hessian of the image intensity and allows inferences of the direction of irradiation of the surface. Although the assumptions appear prima facie rather restrictive, even for surfaces that are not at all Gaussian, with the bidirectional reflectance distribution function far from Lambertian and vignetting and multiple scattering present, we empirically recover the direction of irradiation with an accuracy of a few degrees.

A comparison of material and illumination discrimination performance for real rough, real smooth and computer generated smooth spheres

The appearance of objects in natural scenes is determined by their reflectance, their 3D texture, their shape and by the nature of the illumination. Results of previous experiments using computer generated images of spheres with different reflectance modes and under different canonical illuminations suggested that perception of reflectance mode and illumination are basically confounded. In the present study we investigate whether the conclusions from the experiments with simplified rendered spheres can be extended to ecologically valid images. We use two sets of photographs of real spheres, the first set is taken from the Dror Database [Dror et al. 2001], with simple reflectance modes but complex natural illumination (e.g. desklamp, lab, foodcourt). The second set is taken from the Utrecht Oranges Database [Pont and Koenderink 2003], with simple canonical illumination but material consisting of both reflectance and 3D texture differences (e.g. orange, golf ball, christmas decoration).We find that, although to a lesser extent, even in images of complex objects, perception of material and illumination are basically confounded. Overall, illumination and material are confounded most when we present rendered spheres that differ only in reflectance mode under simple canonical illumination conditions. Interestingly, adding complex natural illumination containing higher order angular frequencies helps to disambiguate this confound in material judgments, but not in illumination judgments. Most helpful was the addition of 3D texture.

Material — Illumination Ambiguities and the Perception of Solid Objects

The appearance of objects depends on their material, shape, and on the illumination conditions. Conversely, object appearance provides us with cues about the illumination and the material. This so-called inverse problem is basically underdetermined and therefore we expect that material and illumination perception are confounded. To gain insight into the relevant mechanisms, we rendered a set of artificial spheres for vastly different canonical light fields and reflectance functions. We used four physics-based bidirectional reflectance distribution functions (BRDFs) representing glossy, pitted, velvety, and matte material. The six illumination conditions were collimated illumination from four directions, hemispherical diffuse illumination, and fully diffuse (Ganzfeld) illumination. In three sub-experiments we presented pairs of stimuli and asked human observers to judge whether the material was the same, whether the illumination was the same, and for a subset in which either the illumination or the material was the same to judge which of the two was constant. We found that observers made many errors in all sub-experiments. In experiment 2 the illumination direction was chosen at random. Using an interactive interface, we asked human observers to match the illumination direction of a sphere of one of the four materials with that of a Lambertian sphere. We found systematical material-dependent deviations from veridical performance. Theoretical analysis of the radiance patterns suggests that judgments were based mainly on the position of the shadow edge. In conclusion, we found no evidence for ‘material constancy’ for perception of smooth rendered spheres despite vast quantitative and qualitative differences in illumination and in BRDF between the stimuli. Although human observers demonstrated some ‘illumination constancy’, they made systematic errors depending on the material reflectance, suggesting that they used mainly the location of the shadow edge. Our results suggest that material perception and light-field perception are basically confounded.

Bidirectional Texture Contrast Function

Three dimensional surface corrugations on globally smooth surfaces give rise to brightness modulations of global shading patterns. We study systematic variations of such 3D image texture as a function of illumination and viewing geometry. The 3D texture is especially noticeable near the shadow terminator (for collimated illumination) or near the dark pole (for hemispherical diffuse illumination). We find that a simple micro-facet model, assuming locally Lambertian scattering, suffices to robustly describe texture contrast gradients of a large variety (measured and rendered textures; laboratory and field conditions) in a semi-quantitative manner. Robust statistical measures of the texture allows one to draw inferences concerning the nature of the light field (collimated to diffuse) and of surface roughness parameters, which can be used as input to the simplest BRDF models.

Illumination direction from texture shading

We investigate the ability of human observers to judge the direction of illumination from image texture. Photographs of 61 real surfaces were used, taken from the Columbia-Utrecht Reflectance and Texture (Curet) database (http:/www.cs.columbia.edu/CAVE/curet). All samples were normally viewed but obliquely illuminated, the elevation of the source being 22.5 degrees, 45.0 degrees, or 67.5 degrees. The illumination was with a collimated, parallel beam. Stimuli were presented in random orientation, and observers had to judge both the elevation and the azimuth of the source. Observers judged the azimuth within approximately 15 degrees, except for the fact that they committed random (with approximately 50% probability) sign flips (180 degrees flips). Connected with this finding is the fact that observers judged the illumination to be from above rather than below in the overwhelming majority of cases, despite the fact that each case occurred with equal probability. The elevation of the illumination can be judged to some extent but is not far above chance level. The data are in good agreement with a simple model that bases the estimate of illumination direction on the second-order statistics of local luminance gradients. This locates the locus of the probable mechanism very early in the visual stream.

Light Direction from Shad(ow)ed Random Gaussian Surfaces

Three human observers estimated the illumination direction for samples of random Gaussian surfaces illuminated by a collimated beam from random directions. These stimuli appear as ‘texture’ due to shading and shadowing (the surface on the microscale was Lambertian of uniform albedo; thus texture appears only through shading and shadowing). We found that observers were able to estimate the azimuth of the source with remarkable accuracy. In the shading regime (no shadows) the observers committed 180° azimuth errors with 50% probability, whereas in the shadow-dominated regime they were able to avoid this convex/concave confusion to a large extent. They evidently relied on second-order statistics in the shading regime and used an unidentified first-order cue in the shadow regime. The elevations of the source were also estimated with remarkable precision. We attribute this to the statistical homogeneity of the sample which can apparently be exploited by the observers. Likely cues are the fraction of shadowed surface, average intensity and rms contrast. The ability of human observers to estimate the illumination direction from surface texture no doubt contributes to the ability to estimate the light field in scenes, which is a prerequisite to the photometric parsing of scenes (shape from shading, and so forth).

Matching illumination of solid objects

The appearance of objects is determined by their surface reflectance and roughness and by the light field. Conversely, human observers might derive properties of the light field from the appearance of objects. The inverse problem has no unique solution, so perceptual interactions between reflectance, roughness, and light field are to be expected. In two separate experiments, we tested whether observers are able to match the illumination of spheres under collimated illumination only (matching of illumination direction) and under more or less diffuse illumination (matching of illumination direction and directedness of the beam). We found that observers are quite able to match collimated illumination directions of two rendered Lambertian spheres. Matching of the collimated beam directions of a Lambertian sphere and that of a real object with arbitrary reflectance and roughness properties resulted in similar results for the azimuthal angle, but in higher variance for the polar angle. Translucent objects and a tennis ball were found to be systematic outliers. If the directedness of the beam was also varied, the direction settings showed larger variance for more diffuse illumination. The directedness settings showed an overall quite large variance and, interestingly, interacted with the polar angle settings. We discuss possible photometrical mechanisms behind these effects.