On the perception of shape from shading

Abstract

The extraction of three-dimensional shape from shading is one of the most perceptually compelling, yet poorly understood, aspects of visual perception. In this paper, we report several new experiments on the manner in which the perception of shape from shading interacts with other visual processes such as perceptual grouping, preattentive search C'pop-out"), and motion perception. Our specific findings are as follows: (1) The extraction of shape from shading information incorporates at least two "assumptions" or constraints-first, that there is a single light source illuminating the whole scene, and second, that the light is shining from "above" in relation to retinal coordinates. (2) Tokens defined by shading can serve as a basis for perceptual grouping and segregation. (3) Reaction time for detecting a single convex shape does not increase with the number of items in the display. This "pop-out" effect must be based on shading rather than on differences in luminance polarity, since neither left-right differences nor step changes in luminance resulted in pop-out. (4) When the subjects were experienced, there were no search asymmetries for convex as opposed to concave tokens, but when the subjects were naive, cavities were much easier to detect than convex shapes. (5) The extraction of shape from shading can also provide an input to motion perception. And finally, (6) the assumption of "overhead illumination" that leads to perceptual grouping depends primarily on retinal rather than on "phenomenal" or gravitational coordinates. Taken collectively, these findings imply that the extraction of shape from shading is an "early" visual process that occurs prior to perceptual grouping, motion perception, and vestibular (as well as "cognitive") correction for head tilt. Hence, there may be neural elements very early in visual processing that are specialized for the extraction of shape from shading.We use three-dimensional (3-D) depth perception to find our way around the world and to manipulate objects that we encounter. Although the retinal image is two-dimensional, somehow the brain is able to use the information from this image to yield an experience of solidity and depth.Of the numerous mechanisms used by the visual system to recover the third dimension, the ability to use shading is probably phylogenetically one of the most primitive. One reason for believing this is that in the natural world, animals have often evolved the principle of countershading to conceal their shapes from predators; they have pale bellies that serve to neutralize the effects of the sun shining from above (Thayer, 1909). The prevalence of countershading in a variety of animals (including fishes) suggests that shading must be a very important source of information about 3-D shapes.Although artists have long recognized the importance of shading, there have been few studies of how the human visual system actually extracts and uses this information. Since the time when Leonardo da Vinci first

Efeitos da cor, iluminação e rotação na ilusão estereoscópica da máscara côncava

“…Ainda que a percepção da direção da fonte de iluminação vinda por cima dos objetos seja uma informação crucial para a percepção da sua profundidade ou relevo (Kleffner & Ramachandran, 1992;Ramachandran, 1988), na ilusão da máscara côncava a percepção da direção da fonte de iluminação se inverte para dar lugar à percepção da face convexa. Assim, uma máscara côncava posicionada verticalmente e iluminada objetivamente por cima da cabeça é percebida como convexa com fonte de iluminação por baixo (Hill & Bruce, 1993).…”

“…Diferente do que ocorre com as fi guras circulares quando se inverte a direção da fonte de iluminação que, de convexas passam a ser percebidas como côncavas (Champion & Adams, 2007;Kleffner & Ramachandran, 1992;Quaglia & Fukusima, 2006;Ramachandran, 1988;Thomas et al, 2010), a máscara côncava é percebida como convexa, independente da direção da fonte de iluminação. A ilusão da máscara côncava é um fenômeno robusto.…”

“…Este fenômeno possibilita a percepção de profundidade ou relevo na natureza (Kleffner & Ramachandran, 1992;Ramachandran, 1988). Por exemplo, a fotografi a de uma cratera se transforma em uma fotografi a de montanha se for girada em 180º.…”

“…Os hemisférios que aparentavam estar iluminados por cima foram percebidos como convexos, enquanto que os que pareciam estar iluminadas por baixo foram percebidos como côncavos. Quando a direção da fonte de iluminação é invertida, ocorre uma inversão na percepção da profundidade ou relevo das imagens hemisféricas (Champion & Adams, 2007;Kleffner & Ramachandran, 1992;Quaglia & Fukusima, 2006;Ramachandran, 1988;Thomas, Nardini, & Mareschal, 2010).…”

Why do we need perceptual content?

“…Nevertheless, thanks to perceptual psychology, we know that the visual system avoids these underdetermination problems by "making a lot of highly plausible assumptions about the nature of the environment and the conditions under which it is viewed." 20 Gilchrist et al (1999). 20 Palmer (1999: 23).…”