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  1. 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).
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