International Archives of the Photogrammetry, Remote Sensing

International Archives of the Photogrammetry, Remote Sensing & Spatial Information Sciences, 35(5):360-365, 2004.

ON AUTOMATIC ORTHOPROJECTION AND TEXTURE-MAPPING

OF 3D SURFACE MODELS

L. Grammatikopoulos a, I. Kalisperakis a, G. Karras a, T. Kokkinos a, E. Petsa b

a Laboratory of Photogrammetry, Department of Surveying,

National Technical University of Athens (NTUA), GR-15780 Athens, Greece

b Department of Surveying, The Technological Educational Institute of Athens (TEI-A),

Ag. Spyridonos Str., GR-12210 Athens, Greece

E-mail: lazaros@central.ntua.gr, ilias_k@central.ntua.gr, gkarras@central.ntua.gr, tk97010@survey.ntua.gr, petsa@teiath.gr KEY WORDS: Orthorectification, DEM/DTM, Laser scanning, Texture, Visualization, Automation, Heritage Conservation ABSTRACT

Photo-textured 3D surface models, and orthophotography in particular, are most important photogrammetric products, notably in he-ritage 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-pro-jected 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 ex-clude 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 com-bining geometric accuracy and visual quality with speed. Finally, future tasks and further elaborations are outlined.

1. INTRODUCTION

Among all photogrammetric products for the documentation of cultural heritage, digital orthomosaics – a combination of geo-metric accuracy with textured detail – are perhaps the most pro-minent. This, of course, is not intended to understate the signifi-cance of other related products, such as digital developments or cartographic projections, drapings or photorealistic visualisation and animation. In fact, orthophoto generation stands here as the paradigm for a core problem of photogrammetry, which incor-porates both surface modeling and photo-texturing. Compared to conventional aerial mapping, orthoimaging of cul-tural monuments often faces a number of significant problems. For instance, as discussed in Mavromati et al. (2002), these may include use of amateur cameras on unstable camera platforms; related problems concerning control over image configurations; resulting difficulties in bundle adjustment. However, a matter of primary importance is accurate surface modeling. It needs to be underlined here that a 3D model is not simply a prerequisite for orthoprojection or realistic rendering. Actually, in cases where only photo-realism or animated visualizations are required, then image-based rendering techniques may provide a direct solution (Beraldin et al., 2002). But photogrammetry typically relies on model-based texturing, as it is mostly asked to also produce ex-plicit 3D data and representations for the purposes of geometric or morphological documentation and analysis.

In many close-range applications object shapes may indeed be complex. As a rule, this implies significant occlusion problems. Thus, surface modeling is a key factor for producing orthophoto results, which will be geometrically reliable and visually correct (no ‘melting’ or ‘stretching’). Conventionally, all surface points are collected manually with stereoscopic viewing (the commer-cial matching algorithms usually require considerable editing in the case of an archaeological object). It has been demonstrated by Mavromati et al. (2003) that suitable collection strategies, as regards breaklines in particular, are capable of providing results of high quality. Notwithstanding its merits, however, this course is indeed tedious and time-consuming. Its limitations also in-clude registration problems among stereopair-based 3D models in the case of images all around the object. At the other far end of image-based modeling, powerful techniques are being deve-loped, notably in computer vision, for the automatic extraction of 3D surface models from an image sequence without any prior information about objects or camera. Although models of high visual quality can be thus produced, it appears that the obtained accuracies are not yet in position to meet the requirements for most mapping applications (Pollefeys et al., 2000). The metric potential of advanced techniques for an automatic dense recon-struction from small numbers of multiple wide-baseline images (Strecha et al., 2003) also remains to be further assessed. For certain objects classes, semi-automatic (hybrid) methods, based on a basic volumetric model of the scene which is subsequently exploited to constrain stereo matching, …… 此处隐藏：21935字，全部文档内容请下载后查看。喜欢就下载吧 ……