3D BUILDING GENERALIZATION BY ROOF SIMPLIFICATION AND
TYPIFICATION
M. Kada
martin.kada@ifp.uni-stuttgart.de
The article
presents an automatic generalization approach for 3d building models with
regard to the cartographic visualization of urban landscapes. Important
application areas are urban planning, city marketing, location-based services
and navigation. These applications generally do not run on dedicated graphics
workstations, but rather on commodity PCs or even mobile devices like PDAs or
cell phones. Here, the use of generalized building models speed up the
rendering process and also improve the visual impression and perceptibility especially
on small displays.
In order to
reduce the complexity of building models, traditional mesh simplification techniques
known from the field of computer graphics are not applicable. These algorithms
are designed to reduce the complexity of arbitrarily shaped meshes that can
consist of millions of triangles. The single buildings in a reconstructed 3d
city model, however, typically feature a much smaller number of polygons.
Furthermore, the regularities (e.g. rectangularity, coplanarity and
parallelism) that are inherent to buildings must be preserved during simplification.
Generalization need not only be limited to single buildings, but it can also be
the merging of buildings in order to have a single model that forms a building
block.
Based on a
3d building generalization technique that uses cell decomposition, a new
approach is presented for the simplification of elements that feature curved
ground plans. Generalization with cell decomposition first creates a
decomposition of the ground plan from approximating planes. Since these planes
can not maintain the features of curved elements, special care has to be taken.
So before the actual generalization takes place, the line elements that
approximate curved elements are detected in the ground plan polygon. This is
e.g. the case for the towers of churches and castles which are important
structures for these types of buildings. As a result, additional curved cells
are generated for the aforementioned features.
Once the
horizontal extension of the structure is known, then the roof type can be
evaluated both for the rectangular and the curved cells. Here, an explicit rule
set helps to ensure that neighbouring primitives fit together. From the roof
types, parameterized primitives are computed that describe the building parts
of each cell. For simplification purposes, the parameters for shed, saw tooth
and parallel roof types can be altered in order to create fewer roof elements.
This results in a typification of the roof. But also the ground plan of curved
cells can be simplified by using fewer line segments. In order to gain the
resulting 3d building model, the cells are combined with a combined gluing and
Boolean intersection operation that are known from cell decomposition and
constructive solid geometry.