Visual shape perception in the case of transparent objects

Abstract

In order to estimate the shape of objects, the visual system must refer to shape-related regularities in the (retinal) image. For opaque objects, many such regularities have already been identified, but most of them cannot simply be transferred to transparent objects, because they are not available there at all or are available only in a substantially modified form. We here consider three potentially relevant regularities specific to transparent objects: optical background distortions due to refraction, changes in chromaticity and brightness due to absorption, and multiple mirror images due to specular reflection. Using computer simulations, we first analyze under which conditions these regularities may be used as shape cues. We further investigate experimentally how shape perception depends on the availability of these potential cues in realistic scenes under natural viewing conditions. Our results show that the shape of transparent objects was perceived both less accurately and less precisely than in the opaque case. Furthermore, the influence of individual image regularities varied considerably depending on the properties of both object and scene. This suggests that in the transparent case, what kind of information is usable as a shape cue depends on a complex interplay of properties of the transparent object and the surrounding scene. Figure 2. Conceptual analysis of the relationship between optical background distortions caused by a light-refracting surface and its shape. (a) Illustration of the light paths of six arbitrary light rays reaching an observer in a hexagonal configuration. The geometry of the underground depicted by the undistorted rays can be approximately described by a circle (blue dashed circle). Its radius r 0 is given by the eigenvalues of the covariance matrix of the reflection points on the underground. The background area depicted by the distorted rays can vary in size, position, and shape. For sufficiently small bundles of light, the form of this background patch can be approximated by an ellipse. Its half-axes a and b are related to the minimum and maximum magnifications M min and M max with which the ray bundle depicts the underground. More specifically, M min ¼À(a À r 0 ) and M max ¼À(b À r 0 ). (b) Illustration of how the geometry of an optically distorted background patch (bottom), and thus its minimum and maximum magnifications M min and M max , are related to the shape type of the refracting surface (top). Like in (a), the blue dashed circles denote the undistorted background patches, while the red circles/ellipses denote the background patches optically distorted by refraction. Specific patterns of minimum and maximum magnifications are related to qualitatively different surface shapes.

The Effect of Material Properties on the Perceived Shape of Three-Dimensional Objects

“…Why might purely refractive objects be perceived flatter than the objects with other material properties? It is known that the surface slant of transparent objects tends to be underestimated, and the result depends on a combination of transparent object properties and the surrounding environment ( Schlüter & Faul, 2019 ). It is possible that other sources of information may be relevant (e.g., Shape from texture).…”

“…Rather than observers matching internal experiences of refractivity, these researchers found that observers tended to match surfaces directly based on similarity in image cues: specular reflections and the distortion field. Further studies have used gauge figure tasks to estimate variations in perceived surface slant (i.e., surface curvature) and found that the 3D shape of objects with semiopaque reflectance properties tends to be perceptually underestimated ( Chowdhury et al., 2017 ; Schlüter & Faul, 2019 ). These interactions between perceived shape and material properties suggest that the perception of both shape and materials depends on similar sources of image-based information.…”

Translucency perception: A review

“…At the boundary of the media, the difference between their refractive indices defines the angle the light ray is refracted with. Therefore, the refractive index has a strong impact on the background distortion in see-through images (proposedly also contributing to shape perception Schlüter & Faul, 2019). Fleming et al (2011) have shown that humans are surprisingly good at estimating refractive indices of transparent materials, proposedly relying on a background distortion cue (although subject to biases owing to the object's thickness and distance to the background).…”

“…Regardless of these attempts, the role of the refractive index in the appearance of non-see-through materials remains understudied. Additionally, difference in the refractive indices of the two bounding media modulates the magnitude of the Fresnel reflection and transmission, more refractive objects usually appearing glossier (Fleming et al, 2011;Schlüter & Faul, 2019) (also impacting caustics; Kán & Kaufmann, 2012;Lynch et al, 2001). This is illustrated in Figure 7.…”

The joint role of geometry and illumination on material recognition

“…Material perception is a complex process that involves a large number of distinct dimensions (Mao et al, 2019;Obein et al, 2004;Sève, 1993) that, sometimes, are impossible to physically measure (Hunter et al, 1937). The illumination of a scene (Beck & Prazdny, 1981;Bousseau et al, 2011;Zhang et al, 2015) and the shape of a surface, are responsible for the final appearance of an object (Nishida & Shinya, 1998;Schlüter & Faul, 2019;Vangorp et al, 2007) and, therefore, for our perception of the materials it is made of (Olkkonen & Brainard, 2011). Humans are capable of estimating the reflectance properties of a surface (Dror et al, 2001b) even when there is no information about its illumination (Dror et al, 2001a;Fleming et al, 2001), yet we perform better under illuminations that match real-world statistics (Fleming et al, 2003).…”