A square contains a linear luminance gradient from white at the top to black at the bottom. Pinching this in at the waist forms an hourglass figure of two tip-to-tip triangles. The pinch reduces the perceptual gradient within each triangle, making the pale upper triangle and the dark lower triangle both look more uniform in brightness We attribute this to undersampling of the mid-greys.
AndersonS. J.HessR. F. (1990).
Post-receptoral undersampling in normal human peripheral vision. Vision Research,
30, 1507–1515.
2.
EvansD. W.WangY.HaggertyK. M.ThibosL. N. (2010).
Effect of sampling array irregularity and window size on the discrimination of sampled gratings. Vision Research,
50, 20–30. doi:10.1016/j.visres.2009.10.001
3.
FankhauserF.KwasniewskaS. (2009).
Cortical mechanisms of normal and abnormal processing in the visual system, Part 1. Spatial vision, amblyopia, hyperacuity, modal assumptions: A review. Technology and Health Care,
17, 77–97. doi:10.3233/THC-2009-0531
4.
GeorgesonM. A. (1994).
From filters to features: Location, orientation, contrast and blur. Ciba Foundation Symposium,
184, 147–165; discussion 165–169, 269–271.
5.
WattR. J.MorganM. J. (1983).
The recognition and representation of edge blur: Evidence for spatial primitives in human vision. Vision Research,
23, 1465–1477.