Refinement of Facade Models Using Image Sequences

Three dimensional building models have become important during the past years for visualization of touristic or historic objects. Detailed facade models are essential for realistic impressions on a terrestial observer. Such models require both a geometric component that describes the shape of the building or parts of it as well as a texture component that provides color information and depicts details that have not been incorporated in the geometric model. Textures for building facades are typically extracted from images taken by ground based cameras. For very high demands on photorealistic quality, textures mapped on simple geometric models like polyhedra or regular surfaces are not sufficient because relief structure is not preserved. This leads to an unrealistic and artificial impression on close-up views. Relief structures are beneficial for large scale models that are closely inspected within a limited area in a virtual world. Facade relief information can be extracted by measurement of depth information using a terrestrial laser scanner or by photogrammetric analysis of the same images also acquired for texture mapping. Video sequences or photographs might be the only source of information available, if facades have been destroyed or otherwise changed or if costs are a limiting factor. In this paper the refinement of a coarse wire-frame model is described. Given several views of a building along with some outline polygons for its facades, textured relief maps will be added to the model. The depth information is recovered from the images by correlation, where the given polygons provide guidance to reduce the number of outliers. Results are given for a set of images taken by a hand held video camera. They show that relief structure due to window holes or ornaments can be retrieved but that the overall geometric quality is limited by the resolution of the camera.