Some of Angkor Wat’s art has been hiding in plain sight.

High in the central tower of the ancient Cambodian temple, paintings of horseback riders and a traditional musical ensemble are so faded that thousands of visitors pass them every day without noticing. An archaeologist found those images between 2010 and 2012, along with about 200 other paintings across the complex, using a method first conceived at NASA’s Jet Propulsion Laboratory in Southern California.

The technique is called decorrelation stretch. It heightens contrasts in digital imagery, making features easier to spot.

It became especially popular in the study of ancient rock art after rock art enthusiast Jon Harman saw NASA images of the Martian surface around 2005, with and without decorrelation stretch. Harman, now retired in Pacifica, California, said the difference in detail made the method’s use for faded ancient images obvious to him. 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.”

That paper was written in 1996 by Ronald Alley, a JPL employee who was developing applications for the Advanced Spaceborne Thermal Emission and Reflection Radiometer, or 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 seen its potential for pulling information from ASTER imagery.

Harman later made a plug-in for ImageJ, an open-source program developed by the National Institutes of Health. He said he now fulfills about 200 requests for Dstretch each 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 described Dstretch’s usefulness in archaeology.

The method has been used to spot and clarify imagery at ancient sites under a cliff in Norway, in an Egyptian tomb, at a park in Canada, and in many other places. It has also helped archaeologists find buried remains of ancient Greek buildings and examine tattoos on mummified human remains.

Harman said he expected the rock art community to pick it up, but its broader use still caught him off guard.

“But I’ve been surprised by a lot of the different applications people have found. So that’s been cool.”

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