Editing Optics (section)

====Visual effects<!--linked from 'Practical effect'-->====
{{Main|Optical illusion|Perspective (graphical)}}
{{for|the visual effects used in film, video, and computer graphics|visual effects}}
[[File:Ponzo illusion.gif|right|thumb|The Ponzo Illusion relies on the fact that parallel lines appear to converge as they approach infinity.]]
Optical illusions (also called visual illusions) are characterized by visually perceived images that differ from objective reality. The information gathered by the eye is processed in the brain to give a [[percept]] that differs from the object being imaged. Optical illusions can be the result of a variety of phenomena including physical effects that create images that are different from the objects that make them, the physiological effects on the eyes and brain of excessive stimulation (e.g. brightness, tilt, colour, movement), and cognitive illusions where the eye and brain make [[unconscious inference]]s.<ref>{{cite web|url=http://www.livescience.com/strangenews/080602-foresee-future.html|title=Key to All Optical Illusions Discovered|author=J. Bryner|publisher=LiveScience |date=2008-06-02|url-status=live|archive-url=https://web.archive.org/web/20080905122802/http://www.livescience.com/strangenews/080602-foresee-future.html|archive-date=2008-09-05}}</ref>

Cognitive illusions include some which result from the unconscious misapplication of certain optical principles. For example, the [[Ames room]], [[Hering illusion|Hering]], [[Müller-Lyer illusion|Müller-Lyer]], [[Orbison's illusion|Orbison]], [[Ponzo illusion|Ponzo]], [[Sander illusion|Sander]], and [[Wundt illusion]]s all rely on the suggestion of the appearance of distance by using converging and diverging lines, in the same way that parallel light rays (or indeed any set of parallel lines) appear to converge at a [[vanishing point]] at infinity in two-dimensionally rendered images with artistic perspective.<ref>[http://mathdl.maa.org/convergence/1/?pa=content&sa=viewDocument&nodeId=477&bodyId=598 Geometry of the Vanishing Point] {{webarchive|url=https://web.archive.org/web/20080622055904/http://mathdl.maa.org/convergence/1/?pa=content&sa=viewDocument&nodeId=477&bodyId=598 |date=2008-06-22 }} at [http://mathdl.maa.org/convergence/1/ Convergence]  {{webarchive|url=https://web.archive.org/web/20070713083148/http://mathdl.maa.org/convergence/1/ |date=2007-07-13 }}</ref> This suggestion is also responsible for the famous [[moon illusion]] where the moon, despite having essentially the same angular size, appears much larger near the [[horizon]] than it does at [[zenith]].<ref>[http://facstaff.uww.edu/mccreadd/ "The Moon Illusion Explained"] {{webarchive|url=https://web.archive.org/web/20151204212728/http://facstaff.uww.edu/mccreadd/ |date=2015-12-04 }}, Don McCready, University of Wisconsin-Whitewater</ref> This illusion so confounded [[Ptolemy of Alexandria|Ptolemy]] that he incorrectly attributed it to atmospheric refraction when he described it in his treatise, ''[[Optics (Ptolemy)|Optics]]''.<ref name=Ptolemy />

Another type of optical illusion exploits broken patterns to trick the mind into perceiving symmetries or asymmetries that are not present. Examples include the [[Café wall illusion|café wall]], [[Ehrenstein illusion|Ehrenstein]], [[Fraser spiral illusion|Fraser spiral]], [[Poggendorff illusion|Poggendorff]], and [[Zöllner illusion]]s. Related, but not strictly illusions, are patterns that occur due to the superimposition of periodic structures. For example, [[Transparency (optics)|transparent]] tissues with a grid structure produce shapes known as [[moiré pattern]]s, while the superimposition of periodic transparent patterns comprising parallel opaque lines or curves produces [[line moiré]] patterns.<ref>{{cite book|title=Energy Minimization Methods in Computer Vision and Pattern Recognition|author1=A.K. Jain |author2=M. Figueiredo |author3=J. Zerubia |publisher=Springer
|year=2001|url = https://books.google.com/books?id=yb8otde21fcC&pg=RA1-PA198|isbn=978-3-540-42523-6}}</ref>