Estimating the gradient direction of a luminance ramp

Erens, Roderik G. F.; Kappers, Astrid M. L.; Koenderink, Jan J.

doi:10.1016/0042-6989(93)90029-v

Cited by 7 publications

(4 citation statements)

References 12 publications

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“…However, we acknowledge that these are different tasks. A similar error of T6 deg was found when observers were asked to judge the gradient direction, but only when the luminance contrast (measured as the standard Michelson contrast) was 64% (Erens et al, 1993a). By this measure of luminance contrast, our reference scene had a luminance contrast of 2.2% and a 1.5% contrast difference yielded reliable discrimination.…”

Section: Discussionmentioning

confidence: 69%

“…For example, brightness discrimination ability does not predict but is superior to depth discrimination (Langer & Bülthoff, 2000). For luminance gradients, it has been found that both detection (Bijl, Koenderink, & Toet, 1989;McCann, Savoy, Hall, & Scarpetti, 1974) and the ability to determine gradient direction (Erens, Kappers, & Koenderink, 1993a) depend on within-image contrast and not on field size. Other studies (Curran & Johnston, 1996;Todd & Mingolla, 1983) suggest that small changes in gradients influence the human visual system's interpretation of shape even when other sources provide parallel or conflicting information.…”

Section: Introductionmentioning

confidence: 97%

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Sensitivity to luminance and chromaticity gradients in a complex scene

2008

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Image gradients--smooth changes in color and luminance--may be caused by intrinsic surface reflectance properties or extrinsic illumination phenomena, including shading, shadowing, and inter-reflections. In turn, image gradients may provide the visual system with information concerning the origin of these factors, such as the orientation of surfaces with respect to the light source. Color gradients induced by mutual illumination (MI) may play a similar role to that of luminance gradients in shape-from-shading algorithms; it has been shown that 3D shape perception modulates the influence of MI on surface color perception (M. G. Bloj, D. Kersten, & A. C. Hurlbert, 1999). In this study, we assess human sensitivity to changes in color and luminance gradients that arise from changes in the light source position, within a complex scene. In Experiment 1, we tested whether observers were able to discriminate between gradients due to different light source positions. We found that observers reliably detected a change in the gradient information when the light source position differed by only 4 deg from the reference scene. This sensitivity was mainly based on the luminance information in the gradient (Experiments 2 and 3). Some observers make use of the spatial distribution of chromaticity and luminance values within gradients when discriminating between them (Experiment 4). The high sensitivity to gradient differences supports the notion that gradients contain information that may assist in the recovery of 3D shape and scene configuration properties.

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Section: Discussionmentioning

confidence: 69%

Section: Introductionmentioning

confidence: 97%

Sensitivity to luminance and chromaticity gradients in a complex scene

2008

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“…Appendix, we might expect the local solid shape to bemore accurately determined at the attached shadow boundary. Erens Erens, Kappers and Koenderink, 1993a;Erens and de Haan, submitted showed that indeed, in regions of high contrast, observers perceive better the gradient direction and the curvature of the eld of isophotes.…”

Section: Attached Shadow Boundarymentioning

confidence: 99%

Categorical Local-Shape Perception

1996

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How well do observers perceive the local shape of an object from its shaded image? We address this complex problem by rst deriving a potential representation of local solid shape, and by presenting the results of a simple psychophysical experiment. Our descriptor of local solid shape, called shape characteristic, provides a viewpoint independent continuum between hyperbolic saddle-shaped and elliptic egg-shaped points. We then study the ability of human observers to make categorical judgments of local solid shape. We investigated this question using a smooth croissant", a simple object made of two connected regions of elliptic and hyperbolic points. Observers decided whether the surface was locally elliptic or hyperbolic at various points on the object. The task was natural, and the observers could reliably partition the shaded image of the object into one elliptic and one hyperbolic region. The ability of observers to perform this partition shows that they can, at least implicitly, localize the parabolic curves on a surface. This ability to locate the parabolic curve can in turn beexploited for other purposes, in particular to segment an object into its parts.Categorical Local Shape Perception 1 A central issue in object perception is how the shape of an object is represented by the visual system. Shape may be represented in a variety of ways that depend on the visual task and the stages of processing in a given task. However, several factors might be common to all these representations. In particular, we take as a fundamental property of any shape representation that it should be viewpoint invariant. This property alone precludes traditional representations in terms of depth values or local surface orientation Marr, 1982. We suggest here that a natural local representation of solid shape is given by the dichotomy b e t ween hyperbolic and elliptic patches.Hyperbolic points are the points on the surface where the shape can locally be described as a saddle, while elliptic points correspond to egg-shaped regions. The boundaries between hyperbolic and elliptic regions are the parabolic curves, at which loci the surface is locally cylindrical. Parabolic curves are associated with important events occuring on a surface, such as the formation of a new occluding contour segment, a new shadow o r a n e w s p e c u l a r point Koenderink, 1990;Mamassian, 1995;Longuet-Higgins, 1960.In this paper, we take the rst steps towards characterizing human ability to categorize local solid shape in terms of view-independent descriptions. In the rst section, we derive a local representation of solid shape based on the hyperbolic-elliptic dichotomy. We then describe a psychophysical experiment whose purpose was to see how reliably human observers could partition a simple surface into hyperbolic and elliptic regions. Encouraged by the results of the experiment, we discuss the advantages to have a local solid shape representation such as the one built from our viewpoint i n variant descriptor. Local Solid Shape SpaceIn this se...

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“…They are less sensitive to obliqueness, but in non-planar objects they are immediately aware of the obliqueness variations due to local surface attitude variations which cause modulations of texture contrast (not just shading). Human observers can use pure shading in the absence of texture as a shape cue, but they are dependent on complementary cues (e.g., contour) to deploy the shading cue effectively, 41 because of the large group of ambiguity transformations in the case the flow direction is not specified.…”

Section: D Textures -The Global Structure Of the Illuminance Flowmentioning

confidence: 99%

Ecological optics of natural materials and light fields

Pont

2009

Human Vision and Electronic Imaging XIV

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The appearance of objects in scenes is determined by their shape, material properties and by the light field, and, in contradistinction, the appearance of those objects provides us with cues about the shape, material properties and light field. The latter so-called inverse problem is underdetermined and therefore suffers from interesting ambiguities. Therefore, interactions in the perception of shape, material, and luminous environment are bound to occur.Textures of illuminated rough materials depend strongly on the illumination and viewing directions. Luminance histogram-based measures such as the average luminance, its variance, shadow and highlight modes, and the contrast provide robust estimates with regard to the surface structure and the light field. Human observers performance agrees well with predictions on the basis of such measures. If we also take into account the spatial structure of the texture it is possible to estimate the illumination orientation locally. Image analysis on the basis of second order statistics and human observers estimates correspond well and are both subject to the bas-relief and the convex-concave ambiguities. The systematic robust illuminance flow patterns of local illumination orientation estimates on rough 3D objects are an important entity for shape from shading and for light field estimates. Human observers are able to match and discriminate simple light field properties (e.g. average illumination direction and diffuseness) of objects and scenes, but they make systematic errors, which depend on material properties, object shapes and position in the scene. Moreover, our results show that perception of material and illumination are basically confounded. Detailed analysis of these confounds suggests that observers primarily attend to the low-pass structure of the light field. We measured and visualized this structure, which was found to vary smoothly in natural scenes in-and outdoors.

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Estimating the gradient direction of a luminance ramp

Cited by 7 publications

References 12 publications

Sensitivity to luminance and chromaticity gradients in a complex scene

Sensitivity to luminance and chromaticity gradients in a complex scene

Categorical Local-Shape Perception

Ecological optics of natural materials and light fields

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