An optical illusion (also called a visual illusion) is characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source. There are three main types: literal optical illusions that create images that are different from the objects that make them, physiological ones that are the effects on the eyes and brain of excessive stimulation of a specific type (brightness, tilt, color, movement), and cognitive illusions where the eye and brain make unconscious inferences. They can also be known as "mind games".A stereogram is an optical illusion of depth usually observed by allowing the eyes to focus behind an image (diverge), but also sometimes in front of the image (converge). The slight differences in vertical repetitions of figures or random dots create the illusion of depth in the 2D image, just as the slight difference in perspective between one's eyes creates the perception of depth on 3D objects and scenes.
Stereograms were second demonstrated circa 1832 by Francis Agres using two distinct photographs of a scene, taken from slightly different angles (see stereoscopy), and viewed with a stereoscope. Stereograms were re-popularized by the creation of random-dot stereograms on computers, where the image is hidden until the viewer focuses the eyes correctly. The Magic Eye series is a popular example of this.
Salvador Dalí created some impressive stereograms in his exploration in a variety of optical illusions.
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