This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: High in the central tower of Angkor Wat, an ancient Cambodian temple, paintings depict horseback riders and a traditional musical ensemble. Thousands of visitors pass these images daily without noticing them because they are faded to the point of invisibility.
They were discovered between 2010 and 2012, along with about 200 other paintings throughout the complex, by an archaeologist using a method conceived at NASA's Jet Propulsion Laboratory in Southern California. The technique, known as decorrelation stretch, heightens contrasts in digital imagery, making features easier to spot. It is especially popular for studying ancient rock art, partly because of the chance intersection of one man's hobby and professional background.
Around 2005, rock art enthusiast Jon Harman saw NASA images depicting the Martian surface with and without decorrelation stretch. Seeing how much detail the technique revealed, Harman, now retired in Pacifica, California, understood its implications for studying ancient, faded images. He also worked in medical imaging.
"I Googled it and found a NASA paper that explained how to do the algorithm," he said. "I knew from my medical imaging experience that I could do it, so I did." The paper was written in 1996 by Ronald Alley, a JPL employee developing applications for the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), a Japanese imaging instrument on NASA's Terra satellite. One of Alley's former supervisors at JPL had coinvented decorrelation stretch, and Alley had recognized its potential for gleaning information from ASTER imagery.
Harman made his plug-in for use with ImageJ, an open-source program developed by the National Institutes of Health. He said he fulfills about 200 requests for Dstretch per year. Around 2010, he also created smartphone apps that use a shortcut to mimic decorrelation stretch.
The apps have been downloaded thousands of times, and papers have been published describing Dstretch's usefulness in archaeology. It has been used to spot and clarify imagery at ancient sites beneath a cliff in Norway, in an Egyptian tomb, at a park in Canada and in many other locations. It has also helped archaeologists find buried remains of ancient Greek buildings and examine tattoos on mummified human remains, among its non-rock-art applications.
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