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{{Short description|Type of document with features that distinguish copies from an original}}
'''Copy-evident''' documents have features that make it detectable that a copy is not the original.

In [[Security printing#Copy-evident|security printing]], various methods such as [[Void pantograph|void pantograph]] are used to create patterns that are hard to copy exactly and when copied inexactly produces an easily detectable image. This can include [[Halftone|halftone screens]] that are affected by the copying process, fine line patterns in different directions that are differently affected by copying and hence produce a mark, metallic inks causing diffraction or reflection.<ref>George K. Phillips. New Digital Anti-Copy/Scan and Verification Technologies. In: Optical Security and Counterfeit Deterrence Techniques V, San Jose, California; June 3, 2004; p. 133-141; {{ISBN|0-8194-5213-0}} http://www.verifyfirst.com/Documents/AntiCopy12-12-03.pdf {{Webarchive|url=https://web.archive.org/web/20131002233200/http://www.verifyfirst.com/Documents/AntiCopy12-12-03.pdf |date=2013-10-02 }}</ref>

Copy-evident computer image files are also possible, for example by embedding a high-frequency pattern in a [[JPEG]] image that is imperceptible, but produces an obvious message when re-compressed with a different quality factor.<ref>Andrew B. Lewis, Markus G. Kuhn. Towards copy-evident JPEG images. Digitale Multimedia-Forensik, 39. Jahrestagung der Gesellschaft für Informatik 2009, Lübeck, Germany, GI-Edition: Lecture Notes in Informatics, Volume P154, pp 171;1582–91. http://www.cl.cam.ac.uk/~mgk25/informatik09-jpeg.pdf</ref>

Some forms of steganography, paper watermarking, and [[Digital watermarking|digital watermarking]] can be regarded as copy-evidence, although detecting that a document or file has been copied typically requires special inspection, tools or software. For example, [[3D printing|3D printed objects]] can be equipped with marks of genuinity that can be identified using a measurement device but are hard to deduce from sample objects; copies will lack these marks.<ref>Daniel G. Aliaga, Mikhail J. Atallah. Genuinity Signatures: Designing Signatures for Verifying 3D Object Genuinity. Computer Graphics Forum Volume 28, Issue 2, pages 437–446, April 2009 http://www.cs.purdue.edu/cgvlab/papers/aliaga/eg2009.pdf</ref>
As another example, a [[Copy detection pattern|copy detection pattern]].

==References==
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[[Category:Authentication methods]]

{{Sourceattribution|Copy-evident document}}