What to do & see
On the right there a many greenish disks over a reddish background. Some disks are partially obscured, hinting at the outline of reclining figure. Since the figure is drawn in the same colour as the background, its outline is only partially visible.
Our gestalt perception manages to reconstruct the full person from these few partial contours, an amazing feat.
While you are watching, the luminance of the green disks varies from “darker than red” to “brighther than red”. In beetween it passes “equiluminance” where the two only differ in colour, not in luminance. Interestingly, in “equiluminance”, contour completion breaks down. Depending on your personal colour vision, and your computer settings, this should occur roughly in the middle position of the bottom slider.
You can narrow down the effect by manually adjusting the brightness with the slider. Also you can try different background colours – for instance the same hue as the figure.
The image itself was created by Gianni Sarcone and is particularly suitable for this demonstration.
I dimly recall the first demonstration I saw of gestalt perception breaking down at equiluminance was by Richard Gregory. Pat Cavanagh’s (1991) review alerted me of the particularly strong equiluminance effect with contours.
Cavanagh P (1991) Vision at Equiluminance. In: Kulikowski JJ, Murray IJ, Walsh V Vision and Visual Dysfunction Volume V: Limits of Vision. Boca Raton, FL: CRC Press; 234–250
Gianni Sarcone giannisarcone.com/Art.html [with thanks for graceful permission to use his image here]