Stereograms.
A stereogram is a two-dimensional picture with embedded depth information for simulating a three-dimensional representation.
On the surface a stereogram looks like a two-dimensional random dot image or a tiled pattern. It is only a specific viewing method that gives access to its depth information. Stereograms became particularly popular in 1990-1994 under the name "Magic Eye".
To recognize the hidden image in the stereogram a special viewing technique is required. Choose your favorite method from the following:
01
Method 1
Put your face close to the stereogram. Allow your eyes to relax, and stare right through the stereogram as if your eyes were focused at a point behind the surface of the stereogram. Slowly move away from the stereogram without changing the position of your eyes.
02
Method 2
Look at your own reflection (or the reflection of another light source) in the screen. Slowly shift your attention to the stereogram on screen without changing the position of your eyes.
Please note that only 85 - 90 percent of people are able to see stereographic pictures in a three-dimensional way.
Final tip to try, But a difficult one.
Mark two small dots on the top of the stereogram 4cm apart, split the two dots into four by making your eyes cross(squint) and now join the two dots in the middle and make it one, concentrate on the middle dot between the two and now put your concentration down on the 3D image.