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Digital Heritage 2018 3rd International Congress & Expo
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Image-Based Rendering for Camera-Mounted Flash Photogrammetry

Booth Image-Based Rendering
4-192 Keller Hall 200 Union St. SE Minneapolis, MN 55455 United States
Image-Based Rendering for Camera-Mounted Flash Photogrammetry
by Michael Tetzlaff, Gary Meyer, Charles Walbridge and Daniel Dennehy

Photogrammetry is an important technique for scanning and archiving 3D cultural heritage objects, but the traditional texturing most often applied by existing photogrammetry software is insufficient for capturing the color appearance of shiny, inhomogeneous 3D objects. A 3D rendering application called IBRelight has been developed to address this gap. The 3D interaction controls in IBRelight are similar to other existing computer graphics software, but IBRelight better preserves the color appearance properties of shiny and inhomogeneous objects using a novel shading algorithm. This algorithm selectively blends the original photographs, using the geometry and camera calibrations from the photogrammetry software such as Agisoft PhotoScan to project the images into 3D space.

To complement this software, an alternative photographic strategy using a camera-mounted flash for illumination is demonstrated. This photographic configuration ensures that the light source is nearly coincident with the viewpoint. A key advantage of this photographic strategy over a diffuse lighting environment is its ability to capture the appearance of sharp specular highlights, not to mention its affordability and convenience in restrictive spaces. Not only can this imagery still be used to obtain 3D geometry by existing photogrammetry software, but IBRelight’s enhanced texturing capabilities can use the original photographs to create new images with novel illumination conditions, based on physically-based rendering principles.  

Using this new photogrammetry technique in conjunction with the same set of flash photographs, a 3D model of a shiny object may be virtually illuminated in computer software, either by a set of virtual spot lights or a high-dynamic-range photographic environment. Using these two illumination mechanisms either independently or in combination, the original flash photographs can be transformed into novel images depicting the object illuminated by a sunny or cloudy sky, by indoor candlelight, or in a modern gallery or studio, all while maintaining accurate color appearance for the object.

Several objects at the Minneapolis Institute of Art (Mia) have been photographed using this camera-mounted flash technique, and 3D models of these objects have been built using Agisoft PhotoScan. Two such objects captured using a robotic turntable at Mia include a bronze statue of Guan Yu from 16th century China, and a child’s tiger hat from China made out of silk and metallic thread. Another way of performing this kind of photography is to do unstructured photography in the gallery, which has been done at Mia for a bronze cast of a sculpture of Boreas. All of these models can be effectively rendered and relit using IBRelight.

Objects like Guan Yu, the child’s tiger hat, and Boreas are ideal for IBRelight in that they exhibit moderately shiny reflectance that is captured well by the flash photogrammetry technique and can be effectively relit by the shading algorithm. Additionally, the shininess of these objects is not completely uniform across the surface of the object, which would make them difficult to model with traditional 3D editing software such as Maya. Flash photogrammetry is thus believed to be a promising technique for documenting these kinds of objects for which traditional photogrammetry is insufficient.