Graphics Reference
In-Depth Information
(3) Recognize the top surfaces
Firstly, a directed graph expressing the connecting topology of the reconstructed
lines is created. As demonstrated by Figure 4, the lines are delegated by the edges of
the graph, and the nodes are represented as the vertices of the graph, which also stores
a chain set to record all the edges connected to it.
a)
The line segments b)The derived directed graph
Fig. 4. The creation of the directed graph
Secondly, a recursive depth priority search algorithm is developed to extract the
roof surfaces according to the directed graph. Given a random edge, the algorithm
searches other edges along one direction and records coplanar edges continuously
until a closed surface is formed or no surface is achieved. After all the edges are
processed, all the roof surfaces are found.
At the end, the roof surfaces are sorted by counter clockwise and the vertical ones
are eliminated for satisfying the requirements of creating solid models.
2.3
Create Building Solid Models
The building solid modes can be created utilizing the identified roof surfaces and
DEM generated by the digital photogrammetric technique. The process consists of
two main steps as following.
(1) Query the ground elevation
Each roof surface is projected onto the ground vertically and the DEM is queried
inside the projected region by a specified resolution. Consequently, a series of ground
elevation values are obtained, and there the minimum value is considered as the
building ground elevation.
(2) Creating the solid models
The solid models are created by stretching the roof surfaces down to the ground
elevation. While at this moment the solid models constituting a building are
separated, some further processing is still needed to get the right model. Figure 5 is an
example to create solid models by stretching.
 
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