ON AUTOMATIC ORTHOPROJECTION AND TEXTURE-MAPPING
OF 3D SURFACE MODELS
L. Grammatikopoulos®, 1. Kalisperakis®, G. Karras“, T. Kokkinos', E. Petsa"
? Laboratory of Photogrammetry, Department of Surveying,
National Technical University of Athens (NTUA), GR-15780 Athens, Greece
S Department of Surveying, The Technological Educational Institute of Athens (TEI-A),
Ag. Spyridonos Str., GR-12210 Athens, Greece
E-mail: lazaros@central.ntua.er, 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
360
the casc 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, have also been presented
(Debevec et al., 1996).
On the other hand, range-based modeling (notably through laser
scanning) represents a powerful technology capable of sampling
vast numbers of surface points at very fast rates. In this sense, it
may well provide the required 3D support for orthorectification
(Monti et al., 2002). In a wider sense, the same also holds true
for creating photo-textured virtual models of real-world scenes,
chiefly in computer graphics applications, where visual quality
is a major concern (Bernardini et al., 2001; Corréa et al., 2002).
[n fact, high-resolution recording of cultural sites and possibili-
ties to promote them through virtual 3D visits, for instance, sti-
mulates research, notably regarding fusion of laser scanning and
colour imagery (Beraldin et al., 2002). Certain commercial 3D
systems provide model-registered colour texture but, neverthe-
less
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