It's actually a kind of visual illusion.
I saw such a "three-dimensional painting" full of tiled patterns and confused at first glance at home when I was a child, and this thing still appears on the Internet from time to time, moving my eyes to the right angle. you can see the three-dimensional relief-like object from the original plane picture:
(can you see what's in this? The answer is a shark)
this stereoscopic painting, called Autostereogram, can actually be seen as an optical illusion that uses binocular parallax to "deceive" depth perception. This kind of image is usually produced by computer, usually using a horizontal tiled repetitive pattern plus a little change.
to put it simply, the principle is something like this: the image of the three-dimensional world forms a plane projection on the retina, which determines that the original signal received by the human visual system is actually two-dimensional. The three-dimensional world we perceive requires the brain to reconstruct according to a variety of depth cues. Among them, an important clue is binocular parallax, which matches the appearance of an object seen by the left eye with the same object seen by the right eye, and judges the distance of the object by the difference between the two sides.
this method is originally very easy to use, but in stereo painting, there are a large number of tiled and repeated patterns, so that when matching "the same object", it may be easy to mismatch. If the "same object" matched by the brain is not the same, but two tiled patterns that look the same, then the resulting sense of depth will go wrong, thus revealing a three-dimensional sense that does not exist.
A simple schematic diagram:
the main point of looking at a stereoscopic picture is not to focus your eyes on the paper. There are two ways to look: parallel and crossover. Generally speaking, parallel is more common (such as the picture above). In this method, the focus of the eyes should be farther away than the actual picture, so that the two tile patterns seen by the left and right eyes can be regarded as the same in the appropriate position, resulting in a depth perception that the background is farther away than the picture. However, because it requires proper adjustment to match, it is possible to return to the paper focus as soon as you inadvertently focus on the struggle.
the same picture can be seen in both ways, but the bump effect will be the opposite.
it's hard to imagine how you see it when you don't see it, but as long as you get used to it, it becomes easy to see. One of the ways I've seen before is to put the image very close to me first, so that if the eyes don't meet anyway, you will relax your head view, and then keep this state and slowly stretch out. Just find the position where you can see clearly.
some places claim that this thing can improve your eyesight. I think this statement is unfounded. Because the movement of stereoscopic painting is actually more of the muscles that control eye movement, which has nothing to do with eyesight, but as a classic visual illusion, it is still very intentional.
(originally in fruit shell Q & A)
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