Random-dot stereogram (RDS) is stereo pair of images of random dots which when viewed with the aid of a stereoscope, or with the eyes focused on a point in front of or behind the images, produces a sensation of depth, with objects appearing to be in front of or behind the display level.

Charles Wheatstone discovered the stereogram in 1838. He was looking for an understanding of binocular vision. Stereograms were created for the stereoscope. One would look at the stereogram through a stereoscope and see a 3-D image.

it is a picture that has a picture inside of it.

The Random Dot E test is a visual test of depth or distance perception. It is generally done to children from age three to eight (or 3rd grade).

It is of 3-D

A single image random dot stereogram (SIRDS) is a computer-generated image consisting of seemingly random dots or lines, which, when viewed properly, coalesce into a previously unseen 3-D illusion. To see the hidden illusion, the viewer must relax the eyes and focus on a point in space behind the image. SIRDS work in the same way that stereoscopes, anaglyphs, and Polaroid 3-D movies do. Each eye receives a slightly different image, which the brain integrates into a single 3-D illusion. SIRDS became popular in the early 1990s with the Magic Eye series of books.