Lightness computation in simple and complex images

Rutgers University New Brunswick, New Brunswick NJ

Investigators

Abstract

How we distinguish black, white and gray surfaces has yet to be explained scientifically. With NSF support, Dr. Alan Gilchrist studies how the visual system determines the shades of gray in the world we see. Consider the follow: Under the same lighting, a white paper reflects more light to the eye than a black paper. But a black paper under brighter illumination can reflect much more light to the eye than a less- illuminated white paper. Nevertheless we see black as black and white as white. This is the kind of puzzle that Dr. Gilchrist attempts to understand. His studies progress from the simplest possible scenes. An observer's head is placed inside a large, opaque hemisphere that fills the observer's visual field. The inside surface divides vertically into two gray shades. Regardless of which shades are used, the lighter one always appears white. This is an important clue to the visual system. In these simple circumstances, it appears to assign white to the brightest part of the scene, but the relative area of the two regions, and the difference between their actual gray shades also play a role. Next to be studied are laboratory scenes that contain more than one region of illumination. Here the rules become more complex. The final studies in the series include complex, natural scenes using a new apparatus that can insert an artificial patch of gray onto any surface of the scene being viewed. This innovative use of technology sets the funded work apart from previous efforts. It allows investigators of lightness perception to conduct research on the richly complex surfaces that constitute real-world scenes. One broad impact of the work will be to make this technology and an illusion archive available to other scientists. In addition, understanding how gray shades are computed correctly in human vision will lead to programming advances in machine vision.