How stereograms work

A stereogram turns a flat pattern into depth by exploiting how your two eyes work together. This guide explains the vision side and the image side, using the method that builds every puzzle on this site.

Your eyes measure depth by comparison

Your eyes sit about 6 to 6.5 cm apart, so each sees the world from a slightly different angle. Your brain matches features between the two views and turns the horizontal difference, called disparity, into distance. Close objects have large disparity; far objects have small disparity.

A stereogram supplies disparity without any real object. It repeats a pattern across the image. When you relax your eyes as if looking past the screen, the left eye locks onto one repeat while the right eye locks onto the next. Your brain matches them and sees a surface floating behind or in front of the screen.

Shifting the repeats creates the hidden shape

If every repeat is the same width, the fused pattern looks like a flat wall. To hide a shape, the generator makes the repeat slightly narrower wherever the shape should stand out. Narrower repeats mean your eyes converge a little more there, so your brain places that area closer. The shape exists only in those tiny width changes, which is why you cannot see it with normal vision.

From depth map to stereogram

1. Depth map. The answer is drawn as a grayscale image: white where the word or shape is, black for the background. Depth is kept in a few hard steps, because blurred edges create visible streaks.
2. Separation. For each pixel, the depth sets how far apart its two matching points must be. On this site the background repeat is 110 pixels on a laptop-size image and about 97 pixels where the answer stands out.
3. Linking. Each pixel is linked to the pixel that must share its color. This is the linked-pixel method published by Thimbleby, Inglis and Witten in 1994; it also handles places where one surface hides another.
4. Texture. A texture, here random dots in the week's colors, is poured through the links so that every linked pair gets the same color.

The archive shows each week's depth map next to its stereogram once the week closes, so you can compare the two. Browse the archive.

Why the image must be shown at its real size

The repeat width has to match the distance your eyes can comfortably separate on screen. Stretching or shrinking the image changes the repeat and makes the 3D picture harder to fuse. That is why this site serves each puzzle in four sizes, from phone to large monitor, and shows the one that fits your screen without scaling.

Keeping the answer hidden

A badly made stereogram can give its answer away: the rows that cross the hidden shape look slightly different from the rest. The generator here uses single-pixel random textures and scatters small specks at the answer's depth across the whole image, so every row carries the same kind of pattern. Each image is checked automatically and reviewed before it goes live.