The figures above should be free-fused, either by crossing or by diverging your eyes. When fused, two different illusions are generated by the blue diamond figures that appear in front of the circular discs. These figures generate vivid illusory contours, and appear to split into two layers: a near, grey transparent figure, and a more distant, opaque background. All of the grey figures are actually the same shade of grey, but when fused, the grey transparent diamonds appear to be made of very different colors. The top figure appears to take on a luminescent or "neon" appearance, whereas the bottom figure appears to be much darker film that attenuates the luminance of the far white discs. Note that the differences in the appearance of these grey regions is essentially the same as the color differences induced by monocular variants of this illusion, such as that presented here, or by colored variants of this illusion such as that presented here. A theory of what's underlying the lightness illusions in this figure is described in a paper to appear in a special issue of Perception ("Contextual effects on color appearance"), a preprint of which can be downloaded here. A theory the illusory contours formed in stereo images such as this is described in Anderson & Julesz (1995), a preprint of which is available here.