The goal of the proposed project is to determine how the human visual system computes the black, gray and white values of object surfaces represented in the optical image projected onto the retina. This problem has not yet been solved for machine vision. For example, there is currently no program that can determine the surface colors of objects within a video image. A key part of the problem lies in disentangling variations in image intensity that are due to light and shadow from those that are due to light and dark gray surfaces. The work proceeds using the method of psychophysics. Visual images or displays are created that, when viewed under controlled conditions, allow a test between competing theories or hypotheses. Ideally, for example, an image might be created so that a given target surface within the image should appear white according to Theory A but black according to Theory B. The image is then viewed by human observers, who report the appearance of the target surface. Three years ago, the PI proposed a new theory based on the concept of anchoring. According to this theory, a complex image is parsed by the visual system into frameworks, or perceptual groups, based on rules of grouping. For any given surface, several lightness (gray scale) values are computed, one for each group to which it belongs. The shade of gray actually perceived for that surface is a weighted value of each of these computed values. The work proposed here will extend the theory into a class of illusions that involve spatial intensity gradients. The work will also focus on certain experience effects: how the perceived gray value is influenced by prior exposure to related scenes. Finally, a series of experiments will examine how the relative area of surfaces influences their perceived gray level, especially in more complex images. The project will also include the creation of a web page containing a comprehensive collection of visual illusions in lightness. This publicly accessible collection is likely to be well used by educators and other researchers.
