Barton L. Anderson

A Theory of Illusory Lightness and Transparency in Monocular and Binocular Images: The Role of Contour Junctions

A theory of illusory transparency and lightness is described for monocular and binocular images containing X-, T- and I-contour junctions. This theory asserts that the geometric and luminance relationships of contour junctions induce illusory transparency and lightness percepts by causing a phenomenal scission of a homogenous luminance into multiple contributions. Specifically, it is argued that a discontinuous change in contrast along aligned contours that preserve contrast polarity induces a scission of the lower contrast region into a near-transparent surface or an illumination change, and a more distant surface that continues behind this near layer. This scission is assumed to cause changes in perceived lightness and/or surface opacity. Discontinuous changes in contrast along contours also are assumed to induce end-cut illusory contours that run roughly perpendicular to the inducing orientation of the contour, both monocularly and binocularly. Binocular illusory contours are shown to be caused by the presence of unmatchable contour terminators. It is argued that the presented theory can provide a unified account of a variety of monocular and binocular illusions that induce uniform transformations in perceived lightness, including neon-color spreading, the Munker – White illusion, Benarys illusion, and illusory monocular and binocular transparency.

Image segmentation and lightness perception

The perception of surface albedo (lightness) is one of the most basic aspects of visual awareness. It is well known that the apparent lightness of a target depends on the context in which it is embedded, but there is extensive debate about the computations and representations underlying perceived lightness. One view asserts that the visual system explicitly separates surface reflectance from the prevailing illumination and atmospheric conditions in which it is embedded, generating layered image representations. Some recent theory has challenged this view and asserted that the human visual system derives surface lightness without explicitly segmenting images into multiple layers. Here we present new lightness illusions—the largest reported to date—that unequivocally demonstrate the effect that layered image representations can have in lightness perception. We show that the computations that underlie the decomposition of luminance into multiple layers under conditions of transparency can induce dramatic lightness illusions, causing identical texture patches to appear either black or white. These results indicate that mechanisms involved in decomposing images into layered representations can play a decisive role in the perception of surface lightness.

Image statistics do not explain the perception of gloss and lightness.

A fundamental problem in image analysis is to understand the nature of the computations and mechanisms that provide information about the material properties of surfaces. Information about a surfaces 3D shape, optics, illumination field, and atmospheric conditions are conflated in the image, which must somehow be disentangled to derive the properties of surfaces. It was recently suggested that the visual system exploits some simple image statistics-histogram or sub-band skew-to infer the lightness and gloss of surfaces (I. Motoyoshi, S. Nishida, L. Sharan, & E. H. Adelson, 2007). Here, we show that the correlations Motoyoshi et al. (2007) observed between skew, lightness, and gloss only arose because of the limited space of surface geometries, reflectance properties, and illumination fields they evaluated. We argue that the lightness effects they reported were a statistical artifact of equating the means of images with skewed histograms, and that the perception of gloss requires an analysis of the consistency between the estimate of a surfaces 3D shape and the positions and orientations of highlights on a surface. We argue that the derivation of surface and material properties requires a photo-geometric analysis, and that purely photometric statistics such as skew fail to capture any diagnostic information about surfaces because they are devoid of the structural information needed to distinguish different types of surface attributes.

Toward a general theory of stereopsis: binocular matching, occluding contours, and fusion.

Models of stereopsis have focused on developing strategies for identifying common features in the 2 half-images so that disparity may be computed. This emphasis ignores the unpairable features that arise at occluding contours (half-occlusions). Most models treat half-occlusions as noise or outliers that are interpreted after disparity processing is completed. A series of experiments reveal that occlusion relationships are sensed during the earliest stages of binocular processing. The authors hypothesize the existence of receptive field structures that sense the local structure of stereoscopic occlusion relationships to account for these findings. Finally, a simple theoretical framework is presented in which fusion, stereopsis, and occlusion are unified. This theory explains the co-occurrence of stereopsis and diplopia and how half-occlusions escape the suppression characteristic of binocular rivalry.

The Perception and Misperception of Specular Surface Reflectance

The amount and spectral content of the light reflected by most natural surfaces depends on the structure of the light field, the observers viewing position, and 3D surface geometry, particularly for specular (glossy) surfaces. A growing body of data has demonstrated that perceived surface gloss can vary as a function of its 3D shape and its illumination field, but there is currently no explanation for these effects. Here, we show that the perception of gloss can be understood as a direct consequence of image properties that covary with surface geometry and the illumination field. We show that different illumination fields can generate qualitatively different patterns of interaction between perceived gloss and 3D surface geometry. Despite the complexity and variability of these interactions, we demonstrate that the perception (and misperception) of gloss is well predicted by the way that each illumination field modulates the size, contrast, sharpness, and depth of specular reflections. Our results provide a coherent explanation of the effects of extrinsic scene variables on perceived gloss, and our methods suggest a general technique for assessing the role of specific image properties in modulating our visual experience of material properties.

The perception of gloss depends on highlight congruence with surface shading

Studies have shown that displacing specular highlights from their natural locations in images reduces perceived surface gloss. Here, we assessed the extent to which perceived gloss depends on congruence in the position and orientation of specular highlights relative to surface shape and the diffuse shading from which surface shape is recovered. The position and orientation congruence of specular highlights with diffuse shading was altered while preserving their compatibility with physical surface shape (Experiment 1). We found that perceived gloss diminished as the position of highlights became incompatible wit h the surfaces global diffuse shading maxima. In a subsequent experiment, we constrained highlight proximity near the global luminance maxima in diffuse shading. When we disrupted the consistency in the local position and orientation of specular highlights with respect to the diffuse shading and local surface meso-structure, a decline in perceived gloss was still observed (Experiment 2). This decline in perceived gloss caused by misaligning the positions and orientations of specular highlights relative to diffuse surface shading could not be explained by differences in orientation fields alone (Experiments 3 and 4). These results suggest the visual system assesses both position and orientation congruence between specular highlights and diffuse shading to estimate surface gloss.

A theoretical analysis of illusory contour formation in stereopsis

A theoretical analysis of how illusory contours are formed in untextured stereograms is presented. This analysis focuses on the role of contour junctions in initiating surface interpolation processes. More specifically, this theory asserts that the matching geometry of stereoscopic junctions that is, the pattern of matchable and unmatchable features present at the intersection of binocularly viewed contours initiates processes of illusory contour formation (“modal” completion) and the connection of partially occluded objects (“amodal” completion). The matching geometry of three forms of stereoscopic junctions that elicit percepts of illusory surfaces is derived under the assumption that only horizontal disparities are matched. This analysis reveals the presence of two distinct kinds of monocular features generated by binocular viewing. Experiments and demonstrations are presented that reveal that: 1) these monocular features play a critical role in modal and amodal completion in stereopsis; and 2) that the difference in these two kinds of monocular features is responsible for asymmetries in the perceptual stability of modally and amodally completed surfaces.

The role of brightness and orientation congruence in the perception of surface gloss

The perception of surface gloss depends on specular highlights but little is understood about how the visual system distinguishes specular highlights from other luminance maxima generated by variations in pigmentation or illumination. It has been argued that diffuse shading gradients provide information for identifying specular highlights. Specular highlights typically share the orientation of the diffuse shading locally. Specular highlights are typically proximal to the brightest region of the diffuse shading locally. We compared the contributions of these two relationships to perceived gloss. Highlight orientation relative to the diffuse shading was varied by rotating highlights. Highlight distance from the brightest region of the diffuse shading was varied by translating highlights in displays that preserved the orientations of highlights relative to their surrounds. Both manipulations reduced perceived gloss. Rotations reduced perceived gloss more than translations, even though translations displaced highlights into darker regions than rotations. The same reductions in perceived gloss occurred when highlights were matched in perceived contrast across conditions (Experiment 2b). The results provide evidence that the perception of gloss depends on highlight distance from the luminance maxima of the surrounding intensity gradient (brightness congruence) in addition to the shared orientation of highlights with their surrounds (orientation congruence).

The role of occlusion in the perception of depth, lightness, and opacity.

A theory is presented that explains how the visual system infers the lightness, opacity, and depth of surfaces from stereoscopic images. It is shown that the polarity and magnitude of image contrast play distinct roles in surface perception, which can be captured by 2 principles of perceptual inference. First, a contrast depth asymmetry principle articulates how the visual system computes the ordinal depth and lightness relationships from the polarity of local, binocularly matched image contrast. Second, a global transmittance anchoring principle expresses how variations in contrast magnitudes are used to infer the presence of transparent surfaces. It is argued that these principles provide a unified explanation of how the visual system computes the 3-D surface structure of opaque and transparent surfaces.

Motion direction, speed and orientation in binocular matching

The spatial differences between the images seen by the two eyes, called binocular disparities, can be used to recover the volumetric (three-dimensional) aspects of a scene. The computation of disparity depends upon the correct identification of corresponding features in the two images. Understanding what image features are used by the brain to solve this matching problem is one of the main issues in stereoscopic vision. Many cortical neurons in visual areas V1 (ref. 2), MT (refs 3, 4) and MST (refs 5, 6) that are tuned to binocular disparity are also tuned to orientation, motion direction and speed. Although psychophysical work has shown that motion direction can facilitate binocular matching, the psychophysical literature on the role of orientation is mixed, and it has been argued that speed differences are ineffective in aiding correspondence. Here we use a different psychophysical paradigm to show that the visual system uses similarities in orientation, motion direction and speed to achieve binocular correspondence. These results indicate that cells that multiplex orientation, motion direction, speed and binocular disparity may help to solve the binocular matching problem.