>>2007,DU Zhiqiang, Perception-Driven Simplification Methodology for Complex 3D Building Models

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Abstract

Virtual Geographic Environments (VGEs) are designed to describe unlimited complex geographic features and phenomena. If we do not effectively abstract and generalize the 3D geographic features and phenomenon according to application requirements, the data volume will be infinite. Moreover, the spatial and temporal resolution of the eyes is finite in real life. Based on this fact, the details not be perceived by human eyes should be ignored in modeling process. It is possible to obtain the building features and entire acknowledge of geographic environment through the finite and discrete LOD models (LODs). Creating the LODs that can meet the requirement of visual continuity becomes a key element for dramatically decreasing the intensity of manual modeling and it is one of the important bases for real-time visualization. Therefore, how to effectively reduce the redundancy of 3D data model becomes the basis of 3D modeling, real-time visualization, and high-efficient spatial analysis. It is a problem that needs to be resolved in 3D Geographic Information System (GIS).

Complex building models are widely used in various fields, such as VGEs, cyber city, and culture heritage documentation, with well development of the virtual reality and 3DGIS technologies. A complex building model means it has highly topological and geometric complexity. In order to obtain such complicated structure and precise texture, the model usually assembled by more than ten thousand pieces of components. But such high complexity and huge data volume hamper the real-time and interactive application requirements of complex building models in virtual environments.

Aiming at the characteristics, the traditional simplification methods have two drawbacks. One is that the most traditional simplification methods in computer graphics are restricted to simplify a single 3D object with continuous surface mesh but not a set of meshes, but a complex building model is just a set of mesh because each component is a mesh. The other one is due to the simplification methods do not take human perception information to drive the simplification operations, so that the simplified results can not accord with the rule of human perception, and the LOD models derived from the simplification methods are difficult to ensure the continuity on visual effect. The main contents of this research include:

(1) After reviewing the various automatic simplification methodologies, technologies, and implements, the existing simplification methodologies are analyzed. Moreover, the states of art and the development tendency of the technologies and tools are indicated.

(2) Aimed at the characteristics of complex building models, perception-driven simplification methodology is proposed, which means interpreting the perceptual features of the image displayed on the screen, and locating the component and geometric primitives that will be decimated, accordingly, providing pilot acknowledge for simplification operations. This methodology includes five main steps, and simulation of the perceptual image on retina, clustering according to the perceptual difference of pixels, and projection of the perception information are the key points in the five steps.

(3) Perception-driven simplification algorithm, which includes three aspects:
A new half-edge data structure is proposed, which can meet the requirement for the simplification operation of the complex building. In this research, the data structure is expanded on semantic information, so that the semantic information are used as foundations of spatial partition and this can effectively overcome the unstructured states among building components. Furthermore, the semantic information kept in the simplified models can serve the spatial attribute query in VGEs.

Semantic-based simplification for building components can remove the concave features on the surface of component models. After discrete signal method is used to describe the surface features of component models, levels of vertex and the rule of vertex transition lead to half-edge collapse operations until the all the concave features are eliminated.

Designing operators for perception-driven simplification and for preserving the appearance attributes are main contents in this algorithm. On the simplification operator, creating new vertex and calculating the cost of vertex pair contract are key elements. For overcoming the drawback of the pure geometrical cost, calculating the intension change between new vertex and original vertex can be weighted with the geometrical cost. This method helps to balance the geometrical error and perceptual error. On the appearance attributes, the operator for preserving appearance attributes is developed, aimed at various changes of appearance attributes caused by geometrical simplification.

Based on the theoretical results and VGEGIS software made by LIESMARS, taking a typical Chinese ancient building as a case, a prototype of automatic simplification for complex building models is built on personal desktop computer, which can prove that these research works are feasible. This prototype can not only dramatically reduce the number of triangles, but also keep the appearance of models well. Moreover, the prototype provides multi-levels simplification to create LOD models according to users’ conditions, such as hardware, system configuration, for scarifying their requirements. It facilitates creating LOD models of complex building in Virtual Geographic Environments.

Key Words

Scale; Virtual Geographic Environments，LOD Modeling，Complex 3D Building model，Visual Perception，Automatic Simplification，half-edge data structure，semantics