I Exampl^building 
! model generator 
Initial building 
model generator 
Figure 2: The main components as feature diagram. Component groups are colored white, concrete components are colored gray. 
The schema contains semantic entities, appearance entities and 
geometry/topology entities. The semantic part contains enti 
ties from the subpackages Core (namespace ’’core”), CityOb- 
jectGroup (namespace ”grp”), Generics (namespace ”gen”) and 
Building (namespace ”bldg”). It should be noted that in principle 
also other ontologies might be used for the semantic part. 
This grUML schema extends the tree-like XML schema of 
CityGML to a real graph-based schema, that (meta-)models the 
entities and relations of urban objects much more explicitly. 
In CityGML models multiple occurences of the same object can 
mostly be modeled by defining the object once and referencing 
it using XLink 10 . But this is not possible for every object. As 
an example, the GML specification offers the definition of the 
control points of a LinearRing (exterior of a Surface) using the 
types DirectPosition or PointProperty. The first is used, if 
the control points are used only in this geometry element, the 
second is used used, if the control points may be referenced from 
other geometry elements. CityGML restricts these possibilities 
to DirectPosition. This means in CityGML models for every 
occurence of the same real world point as control point of the 
surfaces of a building there is a new DirectPosition. And even 
if XLinks are used, they often can not be processed sequentially 
and their interpretation is time-consuming. 
Using the integrated model schema of figure 1, every entity in 
an urban object model exists only once as a node and all its uses 
and occurences are modeled by edges. This explicit, strongly 
linked representation reduces redundant information and enables 
automatic model processing by a very large class of algorithms. 
It is easy to import and export CityGML models using LODs 1 -3 
to and from an integrated model. 
Here, geometry and topology are modeled as another kind of 
boundary representation, namely as an extended vertex-edge- 
face-graph (v-e-f-graph) similar to the well-known and highly 
efficient Doubly Connected Edge List (DCEL) representation 
(Muller and Preparata, 1978). A geometric object consists of 3d 
points, 3d faces and 3d volumes, modeled as typed nodes con 
nected via edges. The geometric information is encoded in the 
attributes of the 3d points, and the topological information is rep 
resented by the edges between the geometric entities. 
3.2 Semantics schema part 
The semantics part of the integrated model schema is based on the 
CityGML modules Core, CityObjectGroup, Generics and Build 
ing. Thus, terms like ’’building”, ’’wall surface” and so forth can 
be used without further explanation in the following. Each of the 
mentioned modules is packed in its own subpackage. 
3.3 Appearance schema part 
The appearance part of the integrated model schema is oriented 
at the CityGML appearance module. But the different kinds of 
surface data (material, different kinds of textures) are directly re 
lated to the 3d faces they shall be applied to. The model allows 
static and dynamic textures, but dynamic textures are preferred. 
A dynamic texture consists of an image and a transformation ma 
trix containing values to compute 2d texture coordinates for ex 
isting 3d points concerning the given image. By using dynamic 
textures, texture coordinates can be updated during model export 
if their corresponding 3d points have changed during model im 
provement. 
4 INTEGRATED MODEL PROCESSING 
3.1 Geometry/topology schema part 
The geometry/topology part of the integrated model schema dif 
fers from the CityGML schema. CityGML uses a subset of GML 
to represent geometric entities as a boundary’ representation (Fo 
ley et al., 1990, Herring, 2001). But the geometry/topology part 
of the integrated model schema is not based on this GML subset, 
since entities and relations are not represented explicitly enough 
and the same geometric objects may appear more than once in the 
same model. 
10 http://www. w3.org/TR/xlink 
The integrated model schema defines the class of TGraphs that 
represent urban object models with all their aspects. There are a 
lot of possible processing activities for integrated models, which 
are introduced in the following. Figure 2 gives an overview over 
such processing activities and their dependencies in the form of a 
feature diagram (Czarnecki and Eisenecker, 2000). 
Here, the components constitute a product line (Pohl et al., 2005) 
where features are implemented by Java components (subsec 
tion 2.2). (The components are referenced by identifiers written 
in typewriter style.)