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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