GoKawiilBody
On the walls of a chamber near the top of the central tower in the ancient Cambodian temple Angkor Wat, paintings depict horseback riders and a traditional musical ensemble. Every day, thousands of visitors pass these images without noticing, because they’re faded to the point of invisibility.
They were discovered, along with about 200 other paintings throughout the sprawling complex, between 2010 and 2012 by a Singaporean 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 subtle differences clearer and features easier to spot. Originally used by NASA to extract more information from satellite imagery, it has proven useful in many fields but has come to be especially widely used for studying ancient rock art. This is partly because these images are commonly faded beyond recognition. But it’s also due to the chance intersection of one man’s hobby with his unrelated professional and educational background.
In the 1980s, one of Jon Harman’s friends brought him along on a few field trips with a local rock art group, and the mystery surrounding these ancient images intrigued him. “No one really knows why people made the rock art they made, at least in many cases,” he said. “And the symbols are quite strange.”
At a rock art conference around 2005, someone showed him images from NASA’s Mars Exploration Rover webpage, depicting the Martian surface with and without the application of decorrelation stretch. Seeing how much detail the technique revealed, he understood the implication for studying ancient, faded images.
He also happened to work 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.”
Thus was born the Dstretch plug-in and, later, Dstretch apps for Android and iOS.
Rather than simply raising the contrast between colors, decorrelation stretch maps the original colors to a different, expanded range of colors. It’s a complicated process that includes steps like diagonalizing color matrices. “Diagonalizing is algebra, and I have a PhD in math from Berkeley, where I studied algebra,” said Harman, now retired in Pacifica, California.
The technique is based on the Karhunen–Loève Transform, a theorem used in statistical analysis and coinvented by Michel Loève, one of Harman’s former math professors.
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