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Error level analysis (ELA) is the analysis of
compression artifacts A compression artifact (or artefact) is a noticeable distortion of media (including Image, images, Sound recording, audio, and video) caused by the application of lossy compression. Lossy data compression involves discarding some of the medi ...
in digital data with
lossy compression In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...
such as
JPEG JPEG ( , short for Joint Photographic Experts Group and sometimes retroactively referred to as JPEG 1) is a commonly used method of lossy compression for digital images, particularly for those images produced by digital photography. The degr ...
.


Principles

When used, lossy compression is normally applied uniformly to a set of data, such as an image, resulting in a uniform level of compression artifacts. Alternatively, the data may consist of parts with different levels of compression artifacts. This difference may arise from the different parts having been repeatedly subjected to the same lossy compression a different number of times, or the different parts having been subjected to different kinds of lossy compression. A difference in the level of compression artifacts in different parts of the data may therefore indicate that the data has been edited. In the case of JPEG, even a composite with parts subjected to matching compressions will have a difference in the compression artifacts. In order to make the typically faint compression artifacts more readily visible, the data to be analyzed is subjected to an additional round of lossy compression, this time at a known, uniform level, and the result is subtracted from the original data under investigation. The resulting difference image is then inspected manually for any variation in the level of compression artifacts. In 2007, N. Krawetz denoted this method "error level analysis". Additionally, digital data formats such as JPEG sometimes include
metadata Metadata (or metainformation) is "data that provides information about other data", but not the content of the data itself, such as the text of a message or the image itself. There are many distinct types of metadata, including: * Descriptive ...
describing the specific lossy compression used. If in such data the observed compression artifacts differ from those expected from the given metadata description, then the metadata may not describe the actual compressed data, and thus indicate that the data have been edited.
File:Felis silvestris silvestris small gradual decrease of quality.png, A composite image, where the different parts have different JPEG compression levels File:Felis silvestris silvestris small gradual decrease of quality - JPEG compression.jpg, The same image with a uniform 90% quality JPEG compression File:Felis silvestris silvestris small gradual decrease of quality - difference image.png, The difference between the two images shows a variation of the JPEG compression artifacts


Limitations

By its nature, data without
lossy compression In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...
, such as a PNG image, cannot be subjected to error level analysis. Consequently, since editing could have been performed on data without lossy compression with lossy compression applied uniformly to the edited, composite data, the presence of a uniform level of compression artifacts does not rule out editing of the data. Additionally, any non-uniform compression artifacts in a composite may be removed by subjecting the composite to repeated, uniform lossy compression. Also, if the image color space is reduced to 256 colors or less, for example, by conversion to
GIF The Graphics Interchange Format (GIF; or , ) is a Raster graphics, bitmap Image file formats, image format that was developed by a team at the online services provider CompuServe led by American computer scientist Steve Wilhite and released ...
, then error level analysis will generate useless results. More significant, the actual interpretation of the level of compression artifacts in a given segment of the data is subjective, and the determination of whether editing has occurred is therefore not robust.
File:The commissar vanishes - difference image.png, As such, the JPEG compression artifacts whether deemed uniform or not are unrelated to the Soviet-era photo manipulation


Controversy

In May 2013, Dr Neal Krawetz used error level analysis on the 2012
World Press Photo of the Year The World Press Photo of the Year award is part of the World Press Photo Awards, organized by the Dutch foundation World Press Photo. Considered one of the most prestigious and coveted awards in photojournalism, the World Press Photo of the Ye ...
and concluded on his ''Hacker Factor'' blog that it was "a composite" with modifications that "fail to adhere to the acceptable journalism standards used by
Reuters Reuters ( ) is a news agency owned by Thomson Reuters. It employs around 2,500 journalists and 600 photojournalists in about 200 locations worldwide writing in 16 languages. Reuters is one of the largest news agencies in the world. The agency ...
,
Associated Press The Associated Press (AP) is an American not-for-profit organization, not-for-profit news agency headquartered in New York City. Founded in 1846, it operates as a cooperative, unincorporated association, and produces news reports that are dist ...
,
Getty Images Getty Images Holdings, Inc. (stylized as gettyimages) is a visual media company and supplier of stock images, editorial photography, video, and music for business and consumers, with a library of over 477 million assets. It targets three mark ...
, National Press Photographer's Association, and other media outlets". The
World Press Photo World Press Photo Foundation is an independent, non-profit organization based in Amsterdam, Netherlands. Founded in 1955, the organization holds the annual World Press Photo Contest for press photography. Since 2011, World Press Photo has orga ...
organizers responded by letting two independent experts analyze the image files of the winning photographer and subsequently confirmed the integrity of the files. One of the experts, Hany Farid, said about error level analysis that "It incorrectly labels altered images as original and incorrectly labels original images as altered with the same likelihood". Krawetz responded by clarifying that "It is up to the user to interpret the results. Any errors in identification rest solely on the viewer". In May 2015, the
citizen journalism Citizen journalism, also known as collaborative media, participatory journalism, democratic journalism, guerrilla journalism, grassroots journalism, or street journalism, is based upon members of the community playing an active role in the pro ...
team
Bellingcat Bellingcat (stylised bell¿ngcat) is a Netherlands-based investigative journalism group that specialises in fact-checking and open-source intelligence (OSINT). It was founded by British citizen journalist and former blogger Eliot Higgins in Ju ...
wrote that error level analysis revealed that the
Russian Ministry of Defense The Ministry of Defence of the Russian Federation (; MOD) is the governing body of the Russian Armed Forces. The President of Russia is the Supreme Commander-in-Chief of the Armed Forces of the Russian Federation, Commander-in-Chief of the forces ...
had edited satellite images related to the
Malaysia Airlines Flight 17 Malaysia Airlines Flight 17 (MH17/MAS17) was a scheduled passenger flight from Amsterdam Airport Schiphol, Amsterdam to Kuala Lumpur International Airport, Kuala Lumpur that was shot down by Russian-backed forces with a Buk missile system, Bu ...
disaster. In a reaction to this, image forensics expert Jens Kriese said about error level analysis: "The method is subjective and not based entirely on science", and that it is "a method used by hobbyists". On his Hacker Factor Blog, the inventor of error level analysis Neal Krawetz criticized both Bellingcat's use of error level analysis as "misinterpreting the results" but also on several points Jens Kriese's "ignorance" regarding error level analysis.


See also

*
Image analysis Image analysis or imagery analysis is the extraction of meaningful information from images; mainly from digital images by means of digital image processing techniques. Image analysis tasks can be as simple as reading barcode, bar coded tags or a ...


References

* {{cite web, url= http://fotoforensic.com/tutorial-ela.php , website= fotoforensic.com , title= Tutorial: Error Level Analysis , accessdate= 2015-07-23


External links


Image Forensics : Error Level Analysis

FotoForensics

Fake Image Detector using Error Level Analysis
Data analysis Data compression Computer vision