the semantic context to simplify the automatic
reconstruction.
The following section gives background
information on projects that aim to reconstruct a 3D
model of a building from survey data. In these
projects the semantic information that describes the
context of the building takes an important place.
Section 3 describes our approach inspired from these
projects. Section 4 focuses on this method by
explaining all the important parts of the
reconstruction process.
2 BACKGROUND
Today, computer-driven evaluation of spatial data
sets is limited by the complexity of the objects to be
extracted. As a matter of fact it is complicated and
time consuming to formulate rules in order to detect
and extract objects geometrically correct. It is due to
one essential reason that the objects are broken
down into many small geometrical pieces. Even if
each piece can be treated in an isolated way, it is not
possible to treat all data at one time. Therefore, the
use of knowledge and its introduction into the
process of evaluation is promising for global
interrelations. The impact of semantic information
on the reconstruction process depends on the
structure of the raw data that has to be handled.
Therefore, it is necessary to study those structures
and reconstruction processes. A short survey is
given in the two following subsections. The first
subsection is concerned with reconstruction methods
based on photogrammetric data and the second
considers reconstruction methods based on scanning
data. Each method has its own characteristics and
advantages but the best choice depends on the
material available, the object to be captured, the
required precision, and the time available (Grün,
2002), (Bryan, 1999), (Balletti, 2004), (Boehler,
2004).
2.1 Photogrammetry
Reconstruction methods based on photogrammetric
data are of two kinds. The semi-automatic methods
consist of the interaction with the user during the
whole process. The automatic methods consist in the
initiation of the process by the user at the beginning
so that later the process runs without user
interaction. Semi-automatic reconstruction methods
can be found in the projects: Realise (Zitova, 2003),
TotalCalib (Robert, 1995), (Bougnoux, 1997),
(Faugeras, 1997), Marina (Cantzler, 2002),
(Nüchter, 2003) and Rekon (Frasson, 1999),
(Loscos, 1999), (Poulin, 1998). Automatic
reconstruction methods have been developed by
Pollefeys et al. (Pollefeys, 2000) and Zisserman et
al. (Werner, 2002). They use the projective
geometry on non-calibrated images. Pollefeys'
system combines various algorithms from computer
vision, like projective reconstruction, auto-
calibration and depth map estimation. Of special
interest for our work was the project Aida (Weik,
1996) because it uses a semantic network to guide
the reconstruction. This method opens a new way by
using semantic information. The automatic
reconstruction remains a difficult task in spite of
many years of research (Backer, 1981), (Fleet,
1991), (Grimson, 1981), (Jones, 1992), (Marr,
1979), (McMillan, 1995). The major problems are
the impact of the viewpoint onto the appearance of
the object in the image. This is due to the changes
with respect to geometry, radiometry, occlusions and
the lack of texture. Strong variations of the
viewpoint may destroy the adjacency relations of
points, especially when the object surface shows
considerable geometrical variations. This
dissimilarity causes confusion in the determination
of correspondence and it is worse when partial
occlusions result in a disappearance of object parts.
In cases of weak texture the algorithms do not have
sufficient information to solve the correspondence
problem correctly. Usually, this is the reason why
the reconstruction fails.
2.2 Scanning
Accurate reconstruction of a surface model from
unorganized points of clouds provided by scanning
systems are complex and are still not completely
solved. Problems arise from the fact that the points
are generally not organized, contain noise and do not
reflect directly the object characteristics, for
example. Computer-based processes of object
extraction are therefore limited in their efficiency. F.
Remonido gives a good overview of existing
algorithms (Remondino, 2003). Close attention is
given to the work of Cantzler et al. (Cantzler, 2002)
and to the work of Nüchter et al. (Nüchter, 2003)
because these projects use semantic information.
Planes which are being reconstructed are associated
to a semantic interpretation which has to fit to a
network model (Grau,1997). A tree of
“backtracking” allows to find the best mapping
between the scene interpretation and the semantic
network model. A coherent labelling exists if all
surfaces are labelled.
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