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A panel of Style 7 Martis Complex petroglyphs at Donner Pass in the Sierra Nevada range in California was recently documented using a combination of photogrammetry and DStretch, a plug-in for ImageJ that uses Principal Components Analysis (PCA) to enhance color contrasts. Although DStretch has been used effectively to enhance rock art, wall paintings, and other art works, it has been assumed that it would not be effective for petroglyphs because they're created by carving or pecking, rather than using pigments. However, the Style 7 petroglyphs at Donner Pass were pecked into pink granite (Lake Mary tonalite, dated to approximately 95-120 million years before present). There is enough contrast between the pecked glyphs and weathered pink granite to allow DStretch to work. Photogrammetry was used to create a textured 3D model of the panel of petroglyphs using RGB images. The images were enhanced using the LRD setting in DStretch, and the model was re-textured using the DStretched images. Orthophotos from both versions of the model were then produced. A comparison of orthophotos without DStretch and with DStretch to enhance the contrast of the petroglyphs is here. A higher resolution orthophoto of the petroglyphs with DStretch can be found here (7 Mb).
I've been exploring the use of an open-source image processing program developed by the National Institutes of Health (NIH) called ImageJ, and a related plug-in called DStretch. While RTI is very effective for examining texture, ImageJ and DStretch provide tools for analyzing color information, in addition to other capabilities, which can complement the use of RTI. (Image J and DStretch are Java-based programs, and you should check that you have installed the latest Java security update.) Both programs can be found easily by a simple search, or you can use the following links: ImageJ: http://rsb.info.nih.gov/ij/index.html DStretch: http://www.dstretch.com/ DStretch is short for "decorrelation stretch," an image enhancement technique originally developed at JPL for remote sensing applications. It has been successfully applied at rock art sites for identifying pictographs, where the composition consists primarily of pigments, as distinct from petroglyphs, which are defined by texture (apologies to archeologists for my loose definitions). An advantage of these tools over a proprietary program such as Photoshop is that they allow users to modify color-spaces in very specific, controlled ways, using defined algorithms. The Dstretch algorithm allows one to "reset" the image back to its original color values, so you can "undo" the effects of any filter you apply using the software. A more detailed explanation of the algorithms in DStretch is provided here: http://www.dstretch.com/AlgorithmDescription.html The approach I'm using to integrate DStretch into RTI workflows is to process the RTIs first (creating a .ptm or .rti file), then use the RTIViewer to relight the image and apply PTM/RTI algorithms to bring out textural details. When I'm happy with the RTI image, I save a snapshot from the viewer. The snapshot can then be opened in ImageJ, and the DStretch algorithms can be applied to enhance color features. A limitation of this approach is that it's generally best to leave the colors unchanged when saving the image in the RTI Viewer. This can be done either by using the "default" setting or, if using Diffuse Gain or Specular Enhancement, by setting the "gain" or "Kd" controls at the positions that leave the most color in the image. This constrains the application of algorithmic enhancements in the RTIViewer somewhat, but it allows both textural and color features to be rendered in ways that are complementary. Another approach, which might be preferable from a scientific perspective, is to process the images for color and texture using ImageJ/DStretch and RTI separately, and compare or combine the images using fade features available in other software. Keeping a log of how the images are processed would be very important, in any case. (Thanks to CHI Forums member Dr. George Bevan for pointing me to ImageJ and to Dr. Jon Harman for the DStretch plug-in.)