‘As he was developing the Dstretch plug-in, Harman applied it to this image from the Cave of San Borjitas in Baja California, Mexico. When the yellow figure appeared in the middle of the picture, he knew he had something useful.’ | Credit: Jon Harman
A colour-processing technique developed to expose subtle variations on Mars is now helping archaeologists recover ancient images on Earth. Known as Dstretch, the method has revealed faded rock art that can be almost invisible in ordinary photographs.
The Discovery
The technique behind these archaeological discoveries began with a problem that sounds more planetary than archaeological: how to make small colour differences visible in images of Mars.
In 2005, NASA's Jet Propulsion Laboratory described decorrelation stretch as a visualisation tool designed to enhance colour variations between objects in a scene. Instead of simply increasing contrast, the process transforms the original colours into an expanded range while retaining their relative hue, saturation and intensity.
For researchers examining the Martian surface, such changes can expose geological variations that would otherwise be difficult to distinguish.
But when rock art researcher Jon Harman encountered an example of the technique at a rock art conference, he immediately recognised another possible application.
A photograph of the Martian surface was shown with and without decorrelation stretch. The difference revealed so much additional detail that Harman began considering what the same process might do to faded prehistoric imagery.
He subsequently located a NASA research paper explaining the algorithm and created the Dstretch plug-in for an open-source image-processing programme.
The result was an unexpected bridge between planetary imaging and archaeological investigation.
What Archaeologists Found
Harman first tested Dstretch on photographs he had taken of rock art in Baja California, Mexico.
The results were striking. An image-processing procedure originally associated with studying Mars revealed a yellow figure that was effectively invisible in the unprocessed photograph.
“That convinced me of the usefulness,” Harman recalled.
He then experimented with different colour spaces, discovering that changing the way an image's colours were mathematically represented could produce useful results across different photographs.
The technology soon attracted the attention of archaeologists and rock art researchers.
Dstretch has since been used to enhance images of ancient artwork in several countries, including Egypt, Canada and Greece. The technique has also been applied to photographs of tattoos preserved on mummified remains.
Its value lies in recovering colour differences that ordinary photographs can conceal. An image that appears to contain little more than a weathered rock surface can, after processing, reveal outlines, pigments or figures that are difficult to see directly.
One of the clearest demonstrations came from Cambodia.
Between 2010 and 2012, Singaporean archaeologist Noel Hidalgo Tan used Dstretch to identify more than 200 faded paintings around the Angkor Wat temple complex.
Many of these paintings were almost invisible to the naked eye.
One example, found in a chamber near the top of the temple's central tower, depicts a traditional Cambodian musical ensemble known as a pinpeat. In an ordinary photograph, the painting is extremely difficult to distinguish. Processing the image makes its surviving colours and forms far more apparent.
Historical Background
Rock art and ancient painted surfaces frequently deteriorate without disappearing completely.
Pigments can fade, surfaces can weather and layers of later material can obscure earlier images. In photographs, these subtle remnants may be compressed into colours that appear almost identical to the surrounding stone.
That creates a particular challenge for archaeologists. A painting that has become difficult to see does not necessarily mean that its pigment has completely vanished.
Digital image processing offers another way of examining these surfaces.
The connection to NASA began with planetary imaging. Researchers studying Mars need to distinguish subtle variations across landscapes, and enhanced colour imagery can make geological features easier to recognise.
Harman's insight was that the same principle could work on a completely different kind of surface.
Instead of looking for geological variations on Mars, researchers could use the technique to search for differences between ancient pigments and the rock around them.
The approach did not require the original artwork to be recreated. It worked with photographs, allowing researchers to examine existing documentation in a different way.
Scientific Analysis
Decorrelation stretch is more sophisticated than simply increasing image contrast.
NASA explains that the technique maps the original colours into a different, expanded colour range. The process includes mathematical operations such as diagonalising colour matrices, allowing subtle variations that may be difficult to distinguish in the original image to become more visually separated.
Harman's background helped him recognise how the method could be adapted. He had experience in medical imaging and held a PhD in mathematics, giving him both the imaging experience and mathematical foundation to work from the NASA research.
After finding the relevant NASA paper, he was able to implement the algorithm and create the Dstretch plug-in.
The importance of changing colour space is also central to its archaeological usefulness. Harman found that different images could respond better to different colour-space settings.
This means Dstretch is not simply an automatic switch that makes every faded painting visible. Researchers can experiment with how colour information is represented to bring out different details within an image.
That flexibility has helped make the technique useful across archaeological photography.
Why This Discovery Matters
The story of Dstretch demonstrates how a technical method developed for one scientific problem can find an entirely different application decades later.
NASA's original concern was understanding variations on another planet. Archaeologists are using the same underlying principle to examine traces of human activity on Earth.
The Cambodia discoveries show the potential scale of the approach. More than 200 faded paintings were identified around Angkor Wat during Noel Hidalgo Tan's work between 2010 and 2012.
The method can also be valuable where physical intervention is difficult or undesirable. Researchers can process existing photographs, potentially extracting information from images that already form part of archaeological records.
It has even been used beyond rock art, including photographs of tattoos on mummified remains.
For archaeology, that raises an important question: how many ancient images already photographed by researchers contain details that conventional viewing has failed to reveal?
Dstretch cannot restore pigment that has completely disappeared, but where traces of colour remain within a photograph, processing may make those differences considerably easier to recognise.
What's Next?
Dstretch's continuing use by archaeologists suggests that digital image processing will remain an important part of documenting fragile and faded ancient imagery.
Researchers can revisit existing photographs and test whether enhanced colour separation reveals figures or motifs that were previously overlooked.
The approach also allows specialists in different fields to build on techniques developed for unrelated scientific purposes. In this case, a NASA imaging method moved from planetary science into archaeology through a researcher who recognised its potential.
Harman himself has said he has been surprised by the range of applications people have found for the technique.
As archaeological photography becomes increasingly extensive, image-processing tools may provide another way to examine archives without physically altering the original surfaces.
Conclusion
A NASA imaging technique created to enhance colour variations in photographs of Mars has found an unexpected second life in archaeology.
Jon Harman adapted the decorrelation stretch method into Dstretch after seeing its effect on Martian imagery and recognising its potential for faded rock art. Tests in Baja California demonstrated that apparently empty sections of photographs could contain hidden figures, while later archaeological work applied the technique to ancient imagery around Angkor Wat and elsewhere.
The discovery is a reminder that archaeological evidence can sometimes remain present in a photograph even when the human eye can barely see it.
FAQ
What is Dstretch?
Dstretch is an image-processing plug-in created by Jon Harman based on NASA's decorrelation stretch technique. It expands colour differences in photographs to make subtle variations easier to distinguish.
Was the technique originally developed for archaeology?
No. Decorrelation stretch was used as a visualisation method for enhancing colour variations in imagery, including photographs used to study the Martian surface. Harman later recognised its potential for ancient rock art.
Where has Dstretch been used?
The technique has been used to study ancient imagery in places including Mexico, Cambodia, Egypt, Canada and Greece. It has also been used to enhance photographs of tattoos on mummified remains.
What did Dstretch reveal at Angkor Wat?
Between 2010 and 2012, archaeologist Noel Hidalgo Tan used Dstretch to discover more than 200 faded paintings around the Angkor Wat temple complex. One processed image revealed a depiction of a traditional Cambodian pinpeat musical ensemble.
