Error level analysis
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Error Level Analysis is the analysis of compression artifacts in digital data with lossy compression such as JPEG.
How it works
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 have been edited.
In the case of JPEG, even a composite with parts subjected to matching compressions will have a difference in the compression artifacts.[1]
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'.[1]
Additionally, digital data format such as JPEG sometimes include metadata 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.
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A composite image, where the different parts have different JPEG compression levels -
The same image with a uniform 90% quality JPEG compression -
The difference between the two images shows a variation of the JPEG compression artifacts
Limitations
By its nature, data without lossy compression 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.[citation needed]
Worse still, the actual determination 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.[1][2]
As such, image forensics expert Jens Kriese describes Error Level Analysis as 'a method used by hobbyists',[2] while image analysis specialist Hany Farid has said of the method that 'It incorrectly labels altered images as original and incorrectly labels original images as altered with the same likelihood'.[3]
See also
References
- ^ a b c Wang, W.; Dong, J.; Tan, T. (October 2010). "Tampered Region Localization of Digital Color Images". Digital Watermarking: 9th International Workshop, IWDW 2010. Seoul, Korea: Springer. pp. 120–133.
We are hardly able to tell the tampered region from the unchanged one sometimes just by human visual perception of JPEG compression noise
- ^ a b Bidder, Benjamin (2015-06-04). "'Bellingcat Report Doesn't Prove Anything': Expert Criticizes Allegations of Russian MH17 Manipulation". Spiegel Online. Retrieved 2015-07-23.
- ^ Steadman, Ian (2013-05-16). "'Fake' World Press Photo isn't fake, is lesson in need for forensic restraint". Wired UK. Retrieved 2015-09-11.
- "Tutorial: Error Level Analysis". fotoforensic.com. Retrieved 2015-07-23.