A digital image-processing technique originally developed by researchers at NASA’s Jet Propulsion Laboratory to study the rocky surface of Mars is now being used to uncover faded and hidden ancient artwork on Earth.
How NASA’s Mars Image Processing Techniques Reveal Ancient Rock Art on Earth
The method, known as a decorrelation stretch, was first created at JPL in 1978 to improve the interpretation of high-correlation remote sensing and satellite imagery. Jim Soha, a supervisor of a digital image processing group at JPL, proposed applying the Karhunen–Loève Transform—a statistical theorem used to maximize energy compaction—to digital imagery for color enhancement. Ronald Alley, a member of Soha’s group at JPL, later refined the process in 1996 to make it faster and more accurate.
In 2005, archaeologist and rock art enthusiast Jon Harman attended a rock art conference where he saw images from NASA’s Mars Exploration Rover webpage featuring the decorrelation stretch. Harman, who holds a PhD in mathematics and has a background in medical imaging, searched online and found a NASA research paper explaining the algorithm. Recognizing its potential for studying ancient, faded images, he developed a software plug-in called Dstretch, which eventually expanded into apps for Android and iOS.
Decoding the Decorrelation Stretch Technology
Rather than simply raising the contrast between colors, a decorrelation stretch functions as a visualization tool designed to enhance color variations among objects within a scene. According to JPL, the process maps original colors to a different, expanded range. This yields an output image whose pixels are well distributed among all possible colors while preserving the relative sense of hue, saturation, and intensity of the input image.
Harman tailored the technique specifically for rock art by experimenting with different color spaces, which are ranges of numerically represented colors such as the RGB color space commonly used on screen devices. By changing the working color space, he achieved striking results across various images. Harman first applied the tool to photographs he took in Baja California, Mexico, where a yellow figure in the Cave of San Borjitas appeared seemingly out of nowhere.
Uncovering Global Archaeological Discoveries
Since its creation, Dstretch has been used by Harman and archaeologists worldwide to enhance hundreds of images of ancient paintings, undocumented figures, and artwork across locations such as Egypt, Canada, Greece, and more. The software has even been utilized to enhance images of tattoos on mummified remains.
Notable applications of the plug-in include:
- Baja California, Mexico: Revealed a previously hidden yellow figure in the Cave of San Borjitas.
- Writing-on-Stone Provincial Park, Alberta, Canada: Uncovered a pictograph of a horse and rider believed to be a calling card from a Crow warrior taunting his Blackfoot enemies, representing an early form of graffiti tagging.
- Årsand 1 site, western Norway: Exposed about 15 previously undocumented figures and revealed new details in 28 others.
Despite the technological advancements helping researchers visualize these ancient creations, Harman noted the enduring mystery surrounding the sites. No one really knows why people made the rock art they made, at least in many cases,
Harman said, And the symbols are quite strange.
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