# Methodology for the Efficient Progressive Distribution and Visualization of 3D Building Objects
## 2 Related Work
### Generalization of City Models  
There has also been extensive research on generalization methods, such as for populating multiple representation databases with geodata. Much of this work has concentrated on the generalization of single buildings [28]. [PERSON] and [PERSON] [29] segmented a building in boundary representation into several parts where the parts are stored in a CSG representation. In later work, [PERSON] and [PERSON] [30] developed a method to optimize the size of the parts by adapting templates using the least squares method. [PERSON] [31] and [PERSON] [32] created a scale-space technique partly based on the morphological operators opening and closing to simplify the 3D building model. A half space model is used by [PERSON] [33] to detect the main outline of a building. The half-space-based segmentation method is then extended in [PERSON] [34] to include the roof structures, utilizing pre-defined roof types. [PERSON] et al. [35] suggested using a transition polygon to represent 3D buildings, but this method can only support flat roof structures. [PERSON] and [PERSON] [36] generated 3D building exterior representation by combining the roof and ground plan. Their method is based on: (i) the ground plan being generated from the 3D building object and simplified; (ii) the roof polygons being merged and typified depending on their spatial relationships; and (iii) the building exterior shell being constructed by increasing the ground plan in height and intersecting with the roof structure. This approach is efficient for many simple structured buildings but will fail if there is one non-vertical wall structure. There have also been studies, for example by [PERSON] and [PERSON] [37], on creating generalization routines to convert buildings from the different levels of detail in CityGML.  
For building groups, [PERSON] and [PERSON] [38] developed a method to generalize all buildings within a city block, or other structures created by a hierarchical network, to a common building object. Their method pays particular attention to landmark buildings to improve the orientation capability of the model. [PERSON] et al. [39] developed a methodology of typification of building groups, based on minimum spanning tree structures, and [PERSON] et al. [40] designed an aggregation method for buildings, based on optimization using mixed integer programming.  
A specific case of building generalization is the conversion of building construction models [41]. This conversion includes both semantic and geometric conversions which have to be treated simultaneously. [PERSON] and [PERSON] [42] provided a framework of how IFC objects (_IfcVall_, _IfcWindow_, etc.) can be used to generate CityGML objects in different LODs. For the semantic conversion, [PERSON] et al. [43] proposed the use of a unified building model which encapsulates both the CityGML and IFC models.  
The evaluation of the quality of 3D geodata generalization has not received the same attention as 2D [44]. One study by [PERSON] et al. [39] which is based on methods in pattern recognition develops a methodology to compare the original and generalized 3D data, based on Attributed Relational Graphs.