Chromostereopsis (Color Depth Illusion)

Flat colours at different depths

The demo shows saturated red and blue shapes on a black background. Everything is on the same flat screen, with no shading, perspective or patterns, yet for most people the red shapes seem to float a little in front of the blue ones. A minority see the reverse, with blue in front, and some see no depth at all. The grey version has exactly the same layout and looks perfectly flat.

It starts in the eye

Most illusions on this site are about how the brain interprets an image. Chromostereopsis starts earlier, in the optics of the eye itself. The eye's lens doesn't focus all colours at exactly the same point: shorter (blue) wavelengths are bent more than longer (red) ones. This is called chromatic aberration, the same flaw that gives cheap camera lenses coloured fringes around high-contrast edges.

On its own that would only cause slight blur. What turns it into depth is that the pupil isn't perfectly centred on the eye's optical axis, so red and blue light land in slightly different places on the retina, shifted sideways in opposite directions in your two eyes. The result is a small difference between the left and right eyes' images, which is exactly the kind of signal the brain uses to judge distance. The brain reads it as depth, and the red and blue shapes end up at different distances even though the image has no depth in it at all.

That's why closing one eye makes the effect disappear. With only one image, there's no difference between the eyes for the brain to read.

Why people disagree

Because it depends on the fine details of each person's eyes, especially where the pupil sits relative to the optical axis, chromostereopsis varies a lot from person to person. Willem Einthoven, better known today for inventing the electrocardiogram, made it the subject of his 1885 doctoral thesis and showed how much it depends on the pupil: covering the inner parts of both pupils makes red come forward, while covering the outer parts puts blue in front instead (Einthoven, 1885; Kitaoka, 2015). Later work examined in detail how the pupil's position and size shape the effect (Vos, 1960).

A problem for designers

Saturated red text on a blue background, or the other way round, can seem to shimmer or float in a way that's tiring to read, which is one reason designers avoid that combination. Like autostereograms, chromostereopsis creates depth from a difference between the two eyes' images, but here the difference comes from the optics of your eyes rather than from a pattern on the page. It's also a good example of the same stimulus producing different experiences in different people, like the dress.