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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. 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 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 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Error level analysis」の詳細全文を読む スポンサード リンク
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