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Alan L. Gilchrist, Ana Radonjic; Lightness computation in the simplest images. Journal of Vision 2005;5(8):239. doi: 10.1167/5.8.239.
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© ARVO (1962-2015); The Authors (2016-present)
The simplest image consists of two different gray surfaces that fill the entire visual field. Perceived lightness is determined by both photometric and geometric factors. As for photometric relations, Li and Gilchrist (1999) found that the region with the highest luminance appears white. As for the geometric factor, the larger a region, the whiter it appears. We mapped the exact function of this effect. Nine opaque acrylic hemispheres (diam = 74 cm) were created. Each was divided into two radial sectors of dark and light gray (dark sector: 11°, 22.5°, 45°, 90°, 180°, 270°, 305°, 327°, 355°; reflectances: light: 36%, dark: 7.7%, luminance ratio 4.7:1). Each hemisphere was viewed by a separate group of 20 observers. Results: the lighter region always appeared white, while the lightness of the darker region moved from middle gray to very light gray as its area moved from 11 degrees to 355 degrees, but most of this change occurred when the darker region was more than half the total area. These results can be understood in terms of the photometric/geometric conflict. When the darker area is less than half, the lighter region has both the highest luminance and the largest area. Thus it makes a stable anchor and area plays little role. But as the darker region grows larger a conflict emerges. Each region now has a claim on white; one as the highest luminance and the other as the largest area. In addition to this area-lightness effect, three other phenomena occur in this zone that, while unusual, are also consistent with the conflict: range compression, brightness indention and perceived luminosity. The area function that we found is the same as that reported in 9 other studies using complex images, except for a weaker in the latter case, consistent with the Kardos principle of co-determination.
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