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2.4 Photo realistic texture.
Photo realistic texture is another important point in
the process of constructing 3D city models. There
are discussions in the literature about the need of
photo realistic texture. The experimental work in ICG
Graz (cf. [2]) shows that more than 90% of data
belong to the phototextures. There are at least two
quite significant reasons:
e Photorealistic texturing applied to 3D models
gives a most realistic presentation of the real
world.
e Texture presents details and matereial
properties, which are not content of the
geoemtry.
One always has to take care about the degree of
resolution which is reasonably acceptable to
maintain. Some small details from the facades may
occur very expensive for collection in terms of time,
human efforts and money. Photo realistic texture
allows very complex elements, from geometrical
point of view, to be presented quite easily.
Essentialy there are two data sources which can be
used for photo texturing:
e aerial images for the terrain and roofs of the
buildings;
e terrestrial images, taken from street level for the
buildingsí facades and other vertical faces.
3. Merging the data from various data sets.
Bearing in mind the topics discussed above, the
following data sets are used in the work on
constructing 3D models of the cities:
e digital terrain model
e 2D information about surface features and
footprints of the buildings
e aerial and terrestrial images
A DTM is used for modelling the surface of the
constructed scene. Information from 2D maps is
used for deriving the footprints of the buildings.
Various images provide information about the height
of the buildings and data for phototexturing.
The suggested technology for construction of the
geometry can be separated into several important
steps:
1. Polygonization of the surface, applying the
Delaunay triangulation (cf. [11])
2. Computation of the footprints of the buildings
using the data from 2D maps and DTM. The
process, in fact, is an interpolation of z-
coordinates for each point.
3. Re-triangulation of the TIN using the footprints
313
as a constraints. The re-triangulation can be
carry out either for the whole surface or only for
restricted regions. An entire re-triangulation is
necessary for the initial constructing of the
model, while a partitioned re-triangulation can be
applied in case of local changes with the
geometric objects.
4. Creation of the building boxes and posing at the
minimum position of the footprints area (fig.1).
Intersection of the surface objects with the DTM and
modelling the horizontalness of the patches and
passways (fig. 2). This operation imposes changes
in the surface which require modelling the sides
slopes along the linear objects.
Fig. 1, Building boxes are posed at minimum hight
of the footprint area onto the binDTM
Fig. 2 Merging of terrain data and CAD data of a
pathway
The idea of the method is to handle and maintain
the reconstructed surface and every particular
building as a separate objects. It can be realized
that some points of the building occur below the
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996