ELi5 – How do magic eye pictures work?
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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.
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