ELi5 – How do magic eye pictures work?
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Spoiler: they don’t work when you only see out of one eye. Nor do 3D glasses for that matter
The way we see in 3D in the real world is by combining the two slightly different images produced by each of our eyes. A type of picture called a stereogram uses this principle to create 3D pictures by presenting slightly different pictures to each eye. Sometimes this involves special glasses to make it easier for the viewer to see the image.
Magic Eye pictures are a specific type of stereogram called an autostereogram. Instead of using two images, it uses one. The way this works is that our brains have a hard time combining images with repeating patterns. If you let your eyes converge on a point beyond the image, it can create the illusion of a flat surface “behind” the real image. What Magic Eye pictures do is slightly change the spacing between elements of the repeating pattern, which changes the perceived distance of the illusory flat surface. This creates an image of a flat surface with a closer silhouette on top of it.
Try this: Look at something that’s at least a few meters away from you, for example a lightswitch that’s on the far side of the room. Now, close one eye and hold up your thumb so it covers the thing you’re looking at. Then switch which eye is open and which is closed. Suddenly, the lightswitch (or whatever) is no longer covered by your thumb! Why?
Your thumb is directly between your first eye and the lightswitch, thus blocking its view of the lightswitch. But your second eye is in a slightly different place, and the line that goes from it to the lightswitch is not blocked by your thumb, so it can see the lightswitch.
This is called [parallax](https://alex.strinka.net/blog/parallax.html) and it’s one of the ways we can tell how far away something is from us.
[Magic eye pictures](https://en.wikipedia.org/wiki/Autostereogram) use this idea to work. When you don’t have your eyes crossed, they both see the same image, so there’s no apparent parallax. But when you cross your eyes, your two eyes see slightly different images that overlap in such a way that mimics parallax, so some parts of the image look like they’re closer or farther from you than other parts of the image.
The 3-d image only pops out when your vision is blurred in a certain way, so that your right and left eye overlap more than they normally do.
If you have a hard time doing it, try this: hold out your finger 👆 in front of you, like 18 inches away from your face. Gaze past your finger, and you should see two blurry fingers. Try to adjust until you see the two fingers a little more clearly. That’s the way you need to focus your eyes for the Magic Eye to work.
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