ON AUTOMATIC ORTHOPROJECTION AND TEXTURE-MAPPING OF 3D SURFACE MODELS

Photo-textured 3D surface models, and orthophotography in particular, are most important photogrammetric products, notably in heritage conservation. However, conventional software typically uses surface descriptions obtained via 2D triangulation; additionally, it cannot handle image visibility. Ignoring multiple elevations and image occlusions is clearly too restrictive for a complex surface shape. Geometric accuracy and visual quality are then possible only with tedious human interaction during surface modeling but also orthoprojection. Yet, laser scanning allows today fast collection of accurate, dense surface point clouds and creation of 3D meshes. Close-range photogrammetry is obviously expected to take full advantage of this. The authors present their approach for an automated production of orthoimages from fully 3D surface representations derived from laser scanning. In a first step, the algorithm detects surface occlusions for the novel view. While common photogrammetric software needs operator-defined patches on individual original images as the source for image content, here all available images are combined for ‘viewer-independent’ texturing of the new image. To this end, bundle adjustment data allow all surface triangles to be back-projected onto all initial images to establish visibilities. Texture blending is performed with suitable weighting, which controls the local radiometric contribution of each original image involved. Given more than two values, a statistical test allows to automatically exclude outlying colour data. The implemented algorithm was tested at the example of a Byzantine church in Athens to indicate that this coupling of laser scanning with photogrammetry is capable to automatically create novel views from several images, while combining geometric accuracy and visual quality with speed. Finally, future tasks and further elaborations are outlined.