RESUMO
Two dilemmas arise in inferring shape information from shading. First, depending on the rendering physics, images can change significantly with (even) small changes in lighting or viewpoint, while the percept frequently does not. Second, brightness variations can be induced by material effects-such as pigmentation-as well as by shading effects. Improperly interpreted, material effects would confound shading effects. We show how these dilemmas are coupled by reviewing recent developments in shape inference together with a role for colour in separating material from shading effects. Aspects of both are represented in a common geometric (flow) framework, and novel displays of hue/shape interaction demonstrate a global effect with interactions limited to localized regions. Not all parts of an image are perceptually equal; shape percepts appear to be constructed from image anchor regions.
RESUMO
One of the main functions of vision is to estimate the 3D shape of objects in our environment. Many different visual cues, such as stereopsis, motion parallax, and shading, are thought to be involved. One important cue that remains poorly understood comes from surface texture markings. When a textured surface is slanted in 3D relative to the observer, the surface patterns appear compressed in the retinal image, providing potentially important information about 3D shape. What is not known, however, is how the brain actually measures this information from the retinal image. Here, we explain how the key information could be extracted by populations of cells tuned to different orientations and spatial frequencies, like those found in the primary visual cortex. To test this theory, we created stimuli that selectively stimulate such cell populations, by "smearing" (filtering) images of 2D random noise into specific oriented patterns. We find that the resulting patterns appear vividly 3D, and that increasing the strength of the orientation signals progressively increases the sense of 3D shape, even though the filtering we apply is physically inconsistent with what would occur with a real object. This finding suggests we have isolated key mechanisms used by the brain to estimate shape from texture. Crucially, we also find that adapting the visual system's orientation detectors to orthogonal patterns causes unoriented random noise to look like a specific 3D shape. Together these findings demonstrate a crucial role of orientation detectors in the perception of 3D shape.
Assuntos
Processamento de Imagem Assistida por Computador/métodos , Imageamento Tridimensional/métodos , Anisotropia , Encéfalo/fisiologia , Mapeamento Encefálico/métodos , Percepção de Forma , Humanos , Conformação Molecular , Distribuição Normal , Orientação , Percepção Espacial , Propriedades de Superfície , Visão Ocular , Córtex Visual/fisiologiaRESUMO
The association of borders with "figure" rather than "background" provides a topological organizing principle for early vision. Such global influences have recently been shown to have local effects, with neuronal activity modulated by stimulus properties from well outside the classical receptive field. We extend the theoretical analysis of such phenomena by developing the geometry of interaction between shading, boundaries, and boundary ownership for smooth surfaces. The purely exterior edges of smooth objects enjoy a fold-type relationship between shading and boundary, due to foreshortening, while the background is cut off transversely. However, at cusp points in the image mapping the exterior boundary ends abruptly. Since such singular points are notoriously unstable, we conjecture that this process is regularized by a natural quantization of suggestive contours due to physiological boundary-detection mechanisms. The result extends a theorem about how contours must end to one that characterizes surface (Gaussian) curvature in the neighborhood of where they appear to end. Apparent contours and their interaction with local shading thus provide important monocular shape cues.