Contrast effects and local white distribution

Contrast is the well-known observation in which a gray in a white surround appears darker than in a black one. An example is a 32 × 32 pixel gray area subtending 1.5 deg on a black background--256 pixels on a side. The gray appears dark when surrounded by a band of white--12 pixels wide. The white band is made up of 2112 individual white pixels. If these 2112 white pixels are redistributed uniformly in the black background, the gray changes its appearance. In this distributed pattern of white pixels the gray patch looks lighter. We measured the appearance of the gray patch as influenced by 2112 white pixels in a great many different spatial configurations. The sets of different spatial patterns of white pixels that generate the same matching lightness for gray are defined as equivalent backgrounds. We then analysed the spatial properties of equivalent backgrounds, in terms of frequency, position, distance, etc. The effects of local presence of white are presented together with an analysis of the spatial properties of different intermediate surrounds in terms of changes of visual appearance.