View source for Spot the difference

{{Short description|Image-based puzzle}}
{{about|a type of puzzle|the Squeeze album|Spot the Difference}}
{{unref|date=October 2018}}
[[File:Spot the difference.png|thumb|upright=1.35|Example of a children's spot the difference puzzle. There are 15 differences between the two pictures.]]

'''Spot the difference''' is a type of [[puzzle]] where players must find a set number of differences between two otherwise similar images.

== Description ==
Spot the difference games are found in various media including activity books for children, [[newspaper]]s, and [[video game]]s. They are a type of [[puzzle]] where players must find a set number of differences between two otherwise similar images, whether they are illustrations or photographs that have been altered with [[photo manipulation]].

== Solving techniques ==
[[Visual comparison]] may be used to solve the puzzle. The solution to the puzzle is often listed nearby or in an accompanying answer page of a puzzle book. Additionally, one can cross one's eyes and merge the two pictures into one, in the much the same way as when viewing an [[autostereogram]] picture. The differences will appear to blink in and out of one's vision. This is a very effective method of solving these puzzles.

=== Overlay ===
[[File:Spot the difference inverse overlay.png|thumb|Solving "Spot the difference" by overlaying the left image (top left) with an inverse image (bottom left) of the right one (top right). Differences appear as non grey parts (bottom right)]]

A way to solve a spot the difference puzzle is to digitally compute the difference pixel by pixel. This requires both images to be perfectly aligned. In the presence of distortions, e.g. when taken from a photo, the [[Correspondence_problem|correspondence problem]] has to be solved before overlay is possible.

To compute a pixelwise image difference <math>p_1-p_2</math> an inverse version of the first image <math>1-p_1</math> can be overlayed with 50% of the other, i.e. <math>\frac{1}{2}(1-p_1)+\frac{1}{2}p_2</math>.{{clarify inline|date=February 2024}}

== See also ==
* ''[[Photo Hunt]]''
* ''[[The Exit 8]]''

[[Category:Comparisons]]
[[Category:Puzzles]]
[[Category:Stereoscopy]]

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