A Procedural Modeling Method With Hierarchical Spatial Constraints For Generating High-speed Railway Scenes

In China,high-speed railways involve knowledge in multiple fields and are characterized by dense network and long mileage. Therefore, it is necessary to construct virtual high-speed railway scenes that integrate multi-field data under the same space-time datum for visual display and management analysis, which is helpful for improving management standards and information construction for high-speed railways. However,existing modeling methods for generating 3D high-speed railway scenes are faced with the following difficulties: (1) In terms of modeling process, lots of professional knowledge is required,so it is difficult to standardize the complex modeling process. (2) In terms of modeling knowledge and modeling operation, there is no unified expression and storage mechanism, so modeling knowledge and modeling operation cannot be efficiently reused. As a result, it is difficult to generate 3D scenes for different types of railways. (3) In terms of scene rendering, modeling output and graphics engine are integrated fixedly, so application of 3D scenes is limited. Therefore, to overcome the preceding difficulties, it is urgent to put forward a procedural modeling method for high-speed railways, which can not only ensure standardized modeling process, simplified modeling operation and reused modeling knowledge, but can also adapt to multiple scene types and different application scenarios.In this dissertation,a procedural modeling method with hierarchical spatial semantic constraints is proposed for generating high-speed railway scenes. To solve the problem that traditional modeling methods are complex, difficult and nonstandard, hierarchical semantic constraint rules are proposed to guide the complex modeling process. To solve the problem that modeling knowledge involved in different types of railway scenes cannot be efficiently reused, we have designed a kind of knowledge template and proposed its reusable method.To solve the problem that rendering engines for 3D scenes are fixed, a scene mapping and instantiation method is proposed to separate modeling output and scene rendering. The following is an overview of this dissertation.(1) A high-speed railway primitive model base and its explicit expression diagram were designed. The diagram was used to give an accurate definition and clear description of the primitive models, which can better support the standardized modeling operation and efficient management analysis. First, the spatial characteristics and relationship attributes of each domain object were analyzed. In consideration of computer visualization constraints, scene objects of high-speed railways were classified at multiple levels and a primitive model base was created subsequently. Then, storage and schedule mechanism were designed to store and manage the primitive models using spatial database technologies. Finally, the spatial semantic relationships between the primitive models were further clarified to construct primitive model ontology, which helps to clearly describe scene objects and their relationships in the high-speed railway scene.(2) Hierarchical semantic constraint rules were proposed to guide and restrain the complex modeling process automatically. By using the rules, we separated the professional domain knowledge from modeling operation, which helps to reduce the difficulty and complexity of the modeling process. First, the interactive operation characteristics in the primitive model combination process was analyzed to extract and express the spatial layout,combinational modeling and spatial attitudes. Then we obtained hierarchical semantic constraint rules, which were used to describe the logical and motion relationships between the primitive models in the modeling process. Finally, we proposed a modeling method under hierarchical semantic constraint rules to judge, position and connect these primitive models. Using the hierarchical modeling method with semantic constraint rules, we generated a conceptual high-speed railway scene.(3) In view of the poor reusability of modeling knowledge, the low flexibility of scene configuration and the weak extensibility of scene objects, we designed a knowledge template structure and proposed its reuse method. First, the expression method of modeling knowledge and modeling operation factors were analyzed to design the structure of modeling knowledge template. The contents of modeling knowledge template were described in detail from the aspects as semantic information, geographic information, linear reference and topologic information. A method for instantiating modeling knowledge template was proposed to express and store modeling knowledge in a parametrized way. Second, a scene configuration template was designed using a soft tree structure to classify and organize the modeling knowledge templates as well as scene objects. By using the scene configuration tenplate, we can dynamically invoke different modeling knowledge templates as needed and flexibly organize the high-speed railway scene. Finally, the two kinds of templates were analyzed from the aspects including scene organization, semantic information, geographic information, linear reference and topologic information. A definite understanding and clear description of the conceptual scene were achieved.(4) A scene mapping and instantiation method was proposed to analyze diverse modeling requirements and generate high-speed railway scenes automatically. Using the instantiation method, we can separate modeling output from scene rendering to extend application scenarios of 3D scenes. First, the relationships between conceptual scene, scene mapping and scene generation were analyzed to establish the mapping relationship between conceptual scene and 3D scene. All the line objects in the scene were decomposed into a series of discrete points, every point can be seen as a 3D symbol corresponding with a primitive model. Then the relationship between conceptual scene and mapping scene was built. Finally, scene mapping results were output for the instantiation in different graphic engines to generate high-speed railway scenes.(5) By integrating the preceding research achievements, we have developed a prototype system for automatically generating high-speed railway scenes. Typical cases were selected for experiments, including the effect and accuracy of modeling results, the reusability of modeling knowledge and visual presentation on different graphic engines. Experimental results show that, the proposed method can be used to automatically generate high-speed railway scenes, separate professional knowledge from modeling operation, and store the modeling knowledge by using templates. The advantages of the proposed method lie in: It reduces the difficulty of complex modeling operation, ensures standardization in modeling process,improves modeling efficiency,and expands scene application scenarios.