倾斜摄影三维模型Unreal Engine 4渲染的数据转换方法

对三维地理场景真实还原是数字孪生的核心技术要求之一，地理信息系统与游戏引擎的结合为城市实景三维建设提供了新思路。倾斜摄影数据是实景三维建设中重要的数字底板要素，针对游戏引擎加载倾斜摄影数据问题，以Unreal Engine 4渲染为例提出一种倾斜摄影三维模型数据转换方法。该方法利用原始数据的文件组织，通过数据解析获取转换信息构建数据读取树；迭代树节点时分别对几何数据与纹理数据进行转换，转换过程兼顾游戏引擎渲染资源所需物理属性，最终完成几何和纹理重组。将该方法应用于深圳市福田区倾斜摄影数据，实验结果表明，所提方法能准确实现倾斜摄影三维模型数据至Unreal Engine 4渲染资源的转换，可为类似三维模型数据转换问题提供理论与技术参考。

A Data Conversion Method from Oblique Photogrammetric 3D Models to Renderable Assets in Unreal Engine 4

  Objectives  The digital twin city seeks to restore virtually the geographic scene that is close to reality and provide the audience an immersive experience. Three dimensional (3D) real urban scene is one of the key technologies to achieve the goal. Nowadays, the combination of geographic information system and game engine becomes an industrial trend and provides new opportunities for 3D real urban scene. Oblique photogrammetric data, as being a fundamental data source for 3D urban modeling, is able to represent large-scale and high-precision scene. The game engine plays an excellent role in offering realistic effects and post-production systems that are greatly useful for geospatial data visualization. However, the diversity of specifications and formats of oblique photogrammetric data has caused many different data loading and transformation methods, resulting in a problem to be solved in 3D real urban scene.  Methods  This paper aims to propose a data transformation approach from oblique photogrammetric data to rendering resources in game engine. To stress, the proposed approach can allow us to load instantly massive amount of oblique photogrammetric data in Unreal Engine, which is one of the most popular game engines. We further compared oblique photogrammetric data with rendering resources in game engine and discussed the significance of such a data transformation. The approach reorganizes the original structure of the data by parsing data into a tree structure that better suits transformation purposes. Also, the proposed approach transforms respectively the geometry and texture data when iterating tree nodes, as well as considers the essential physical properties of rendering assets. All these features are combined into a package data format.  Results  In the case study of Futian district, Shenzhen, the results show that the proposed approach successfully converts oblique photogrammetric data to rendering resources in Unreal Engine 4. In the transformation process, the geometrical structure and collision properties of triangular mesh are guaranteed, and color, bump and other visual elements of original data are kept in materials. The rendering resources are loaded to simulate the real urban scenarios at different time periods of a day in Unreal Engine 4.  Conclusions  This paper devises a novel approach which can convert the oblique photogrammetric data into rendering resource for a game engine to load and render, and meantime maintain the consistency of data's geometry and texture. For its scalability, it also supports parallel processing of large-scale data transformation when data is organized in the same directory. The proposed approach may be of great theoretical and practical value for other related 3D model data transformation problems.