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Cultural Heritage Acquisition: Geometry-Based Radiometry in the Wild

Measuring radiometric surface properties "in the wild" is challenging, because the appearance of a surface strongly depends on ambient illumination, especially when direct sunlight or cast shadows need to be handled, and when the scene must not be altered by shrouding devices or by reference targets. We present a purely photogram metric measurement method that integrates 3D surface geometry, camera poses, known artificial illumination, and calibration of the measurement device. For geometry measurement, we extend Structure-from-Motion with constrained bundle adjustment post processing to obtain geo-referenced Euclidean reconstructions. For radiometry measurement, we perform frame differencing of images with and without intense artificial illumination to eliminate the influence of ambient illumination, and we calculate radiometric surface properties based on known illumination configuration and radiometric calibration. Calculating the relations between surface normal, camera pose, and incident light leads to the final result of dense 3D point clouds that represent radiometric surface properties. In our experimental validation, we apply this method to the field of cultural heritage acquisition of prehistoric rock art and achieve excellent results compared to ground truth. Further, the measurement method itself is quite general and will be applicable to many research areas where both, precise surface geometry and radiometry is required.