一种倾斜摄影模型精细融合方法

现有三维模型融合方法多采用对原有三角面片顶点重新构网的方式，容易导致原有模型几何特征丢失及纹理失真等。本文针对倾斜摄影模型自身特点，提出了一种模型精细融合方法。首先，以一个模型作为中心模型，并以另一模型与中心模型外轮廓的空间关系计算融合区域；然后，以中心模型外轮廓作为约束，对原始模型三角面片进行精细剖分；最后，基于精细剖分的三角面片进行纹理坐标的插值计算。以实际倾斜摄影数据进行试验的结果表明，本文方法融合后的网格模型拓扑结构正确，融合边界不存在漏洞、裂缝和错位等情况，而且纹理过渡自然，颜色无明显差异。     

Developing an optimized texture mapping for photorealistic 3D buildings

Texture mapping generates photorealistic representations of three‐dimensional (3D) geometric objects and enhances the spatial perception of areas of interest. Over the past two decades, even though various approaches for 3D urban models have been investigated, their use has been limited because of the lack of spatial accuracy, details, and the complex processes. It is difficult to maintain highly detailed texture information without using a hybrid of aerial image and ground‐based imaging techniques, which are costly. Furthermore, it is hard to develop a fully automated process for 3D urban mapping that achieves high spatial accuracy. With regard to the issues, this research aims to develop a semi‐automated process for 3D building models that would help image‐based approaches. It helps acquire qualified texture information and improve the appearance of building façades in a large city. In particular, this research first investigates an optimal overlap of consecutive aerial images that generates sufficient information to texture each façade, thus making this process more cost‐effective. Second, this research develops an application to semi‐automatically build 3D buildings and textured 3D buildings. The application is developed in C++. The textured 3D building models are quantitatively and qualitatively assessed to determine the usability of the semi‐automated process.     

城市场景结构感知的网格模型简化算法

针对二次误差测度(QEM)网格简化算法全局几何特征信息损失严重的缺点,提出一种具有结构感知功能面向城市三维模型重建的网格简化算法。该算法顾及城市影像中平面结构特征,以代理平面为全局特征约束条件,使模型在简化过程中全局结构特征更多地被保持,以利于多层次细节模型(LOD技术)、网格优化提速等模型后续操作。以倾斜摄影获取影像生成的初始三角网格模型为试验数据,采用所给算法对其进行网格简化并与QEM算法进行对比。结果表明:所给算法简化精度及简化效率均优于QEM算法。     

一种顾及多重约束的三维地形简化方法

三维模型在许多领域内均有着越来越深入的应用,为保证三维模型在不同存储空间、不同图形绘制能力、不同显示分辨率的终端电子设备上显示的统一性和一致性,在很多情况下需要对三维模型进行适度的简化。对于三维地形而言,纹理颜色、地形区域边界和重要顶点是其非常重要的视觉特征,本文将这三项特征作为约束因子引入到三维地形的简化过程中,并根据这三项特征对现有的二次误差测度算法和边折叠代价的计算方法进行了改进,使其在三维地形简化过程中不会出现明显退化。本文还将改进后的算法与原来算法进行了实验对比,结果表明,改进后的简化算法不仅能够在不同精度上保持三维地形视觉上的一致性,而且产生的几何误差也相对较小。     

顾及地物类别的倾斜摄影三维模型简化方法

基于倾斜摄影测量的三维建模已成为城市级实景三维建设的发展趋势。在城市三维模型中,由于植被表面不平整,需要用大量的三角面来刻画,但这并不是实景三维数据记录和表达的重点,并且大量的数据会给模型展示和应用带来很大的困难。因此,本文提出了一种顾及地物类别的倾斜摄影三维模型简化方法。首先,计算三维模型的纹理信息和几何信息,结合马尔可夫随机场(MRF)顾及空间一致性的优点提取植被;然后,采用二次误差测度(QEM)算法简化特定的植被区域;最后,对简化后的白膜模型进行纹理重映射。试验结果表明,本文方法能够准确提取并有效简化植被区域,纹理重映射的结果在外观上也与原始模型相差无几,取得了预期的良好效果。     

A multi-sensor approach to creating accurate virtual environments

Creating virtual environment models often requires geometric data from range sensors as well as photometric data from CCD cameras. The model must be geometrically correct, visually realistic, and small enough in size to allow real-time rendering. We present an approach based on 3D range sensor data, multiple CCD cameras, and a colour high-resolution digital still camera. The multiple CCD cameras provide images for a photogrammetric bundle adjustment with constraints. The results of the bundle adjustments are used to register the 3D images from the range sensor in one coordinate system. The images from the high-resolution still camera provide the texture for the final model. The paper describes the system, the techniques for the registration of the 3D images, the building of the efficient geometric model, and the registration and integration of the texture with a simplified geometric model.     

Generalization of 3D building texture using image compression and multiple representation data structure

Textures are an essential part of 3D building models and often consume large portions of the data volume, thus making visualization difficult. Therefore, we propose a multi-resolution texture generalization method to compress the textures of 3D building models for dynamic visualization at different scales. It consists of two steps: image compression and texture coloring. In the first step, texture images are compressed using wavelet transformation in both the horizontal and the vertical direction. In the second step, a TextureTree is created to store building texture color for dynamic visualization from different distances. To generate a TextureTree, texture images are iteratively segmented by horizontal and vertical dividing zones, until each section is basically in the same color. Then the texture of each section is represented by their main color and the TextureTree is created based on the color difference between the adjacent sections. In dynamic visualization, the suitable compressed texture images or the TextureTree nodes are selected to generate 3D scenes based on the angle and the distance between the viewpoint and the building surface. The experimental results indicate that wavelet based image compression and the proposed TextureTree can effectively represent the visual features of the textured buildings with much less data.     

基于三维激光扫描点云数据的古建筑建模

针对传统古建筑数字化技术的不足和接触测量对建筑物造成"二次伤害",以西北民族大学蝴蝶厅异地重建项目为例,提出了一种结合三维激光扫描技术和现代测量技术以及本体思维进行室内外一体化三维建模的研究思路。首先给出了基于三维激光扫描技术的建模流程;其次,对点云数据处理和三维实体重建两个核心步骤,重点对点云数据的配准拼接、去噪简化和提取轮廓线、三维几何建模和纹理贴图等步骤进行详细论述。本体思维用于基于三维激光扫描的古建筑三维建模,可以有效指导古建筑构件分类建模,提高建模效率。结果表明:三维激光扫描技术适用于具有高复杂度几何特征的蝴蝶厅精细化、真实感建模,为西北民族大学蝴蝶厅古建筑文化遗产未来的开发、保护、维修、恢复重建与虚拟地理环境等提供高精度的数据基准。     

Change detection on LOD 2 building models with very high resolution spaceborne stereo imagery

Due to the fast development of the urban environment, the need for efficient maintenance and updating of 3D building models is ever increasing. Change detection is an essential step to spot the changed area for data (map/3D models) updating and urban monitoring. Traditional methods based on 2D images are no longer suitable for change detection in building scale, owing to the increased spectral variability of the building roofs and larger perspective distortion of the very high resolution (VHR) imagery. Change detection in 3D is increasingly being investigated using airborne laser scanning data or matched Digital Surface Models (DSM), but rare study has been conducted regarding to change detection on 3D city models with VHR images, which is more informative but meanwhile more complicated. This is due to the fact that the 3D models are abstracted geometric representation of the urban reality, while the VHR images record everything. In this paper, a novel method is proposed to detect changes directly on LOD (Level of Detail) 2 building models with VHR spaceborne stereo images from a different date, with particular focus on addressing the special characteristics of the 3D models. In the first step, the 3D building models are projected onto a raster grid, encoded with building object, terrain object, and planar faces. The DSM is extracted from the stereo imagery by hierarchical semi-global matching (SGM). In the second step, a multi-channel change indicator is extracted between the 3D models and stereo images, considering the inherent geometric consistency (IGC), height difference, and texture similarity for each planar face. Each channel of the indicator is then clustered with the Self-organizing Map (SOM), with “change”, “non-change” and “uncertain change” status labeled through a voting strategy. The “uncertain changes” are then determined with a Markov Random Field (MRF) analysis considering the geometric relationship between faces. In the third step, buildings are extracted combining the multispectral images and the DSM by morphological operators, and the new buildings are determined by excluding the verified unchanged buildings from the second step. Both the synthetic experiment with Worldview-2 stereo imagery and the real experiment with IKONOS stereo imagery are carried out to demonstrate the effectiveness of the proposed method. It is shown that the proposed method can be applied as an effective way to monitoring the building changes, as well as updating 3D models from one epoch to the other.     

多源点云数据融合的建筑物精细化建模

三维建模技术能够实现建筑物的数字化存档，在古建筑保护与修复和现代建筑规划与改造中具有不可替代的作用。针对倾斜摄影测量和三维激光扫描建模技术中建筑物模型存在的问题，本文提出了一种倾斜摄影测量和三维激光扫描生成三维点云模型相融合的建筑物精细化建模方法。选用无人机和三维激光扫描仪作为试验设备，利用ContextCapture、SCENE软件完成点云拼接、生产和编辑，通过ICP算法完成点云精细匹配，实现多源点云数据融合建模；对比单一建模方法模型，从纹理结构和模型精度两方面对融合建模模型进行质量评价。结果表明，融合建模模型纹理清晰，几何结构完整，模型距离中误差和高差中误差的均值均低于倾斜摄影测量模型的值，接近三维激光扫描模型。     

3D reconstruction from a single image using geometric constraints

Photogrammetry has many advantages as a technique for the acquisition of three-dimensional models for virtual reality. But the traditional photogrammetric process to extract 3D geometry from multiple images is often considered too labour-intensive. In this paper a method is presented with which a polyhedral object model can be efficiently derived from measurements in a single image combined with geometric knowledge on the object. Man-made objects can often be described by a polyhedral model and usually many geometric constraints are valid. These constraints are inferred during image interpretation or may even be extracted automatically. In this paper different types of geometric constraints and their use for object reconstruction are discussed. Applying more constraints than needed for reconstruction will lead to redundancy and thereby to the need for an adjustment. The redundancy is the basis for reliability that is introduced by testing for possible measurement errors. The adjusted observations are used for object reconstruction in a separate step. Of course the model that is obtained from a single image will not be complete, for instance due to occlusion. An arbitrary number of models can be combined using similarity transformations based on the coordinates of common points. The information gathered allows for a bundle adjustment if highest accuracy is strived for. In virtual reality applications this is generally not the case, as quality is mainly determined by visual perception. A visual aspect of major importance is the photo-realistic texture mapped to the faces of the object. This texture is extracted from the same (single) image. In this paper the measurement process, the different types of constraints, their adjustment and the object model reconstruction are treated. A practical application of the proposed method is discussed in which a texture mapped model of a historic building is constructed and the repeatability of the method is assessed. The application shows the feasibility of the method and the potential of photogrammetry as an efficient tool for the production of 3D models for virtual reality applications.     

Context-based automatic reconstruction and texturing of 3D urban terrain for quick-response tasks

Highly detailed 3D urban terrain models are the base for quick response tasks with indispensable human participation, e.g., disaster management. Thus, it is important to automate and accelerate the process of urban terrain modeling from sensor data such that the resulting 3D model is semantic, compact, recognizable, and easily usable for training and simulation purposes. To provide essential geometric attributes, buildings and trees must be identified among elevated objects in digital surface models. After building ground-plan estimation and roof details analysis, images from oblique airborne imagery are used to cover building faces with up-to-date texture thus achieving a better recognizability of the model. The three steps of the texturing procedure are sensor pose estimation, assessment of polygons projected into the images, and texture synthesis. Free geographic data, providing additional information about streets, forest areas, and other topographic object types, suppress false alarms and enrich the reconstruction results.     

动态空间正图像透视投影正反解

卫星图像都是在动态情形下获取的。瞬间曝光获取的图像投影性质符合透视投影。本文针对卫星动态获取的正图像，建立其平面透视投影，利用矢量解法研究其正反解变换和星下点坐标计算方法，最后给出了算例。     

基于ICP方法缺乏纹理物体三维曲面模型的匹配

对缺乏纹理物体建立三维模型,关键的难度在于缺乏特征点和点的坐标获得。本文采用ICP算法来对物体进行匹配,当特征点不足时,ICP算法是一个很适用的匹配方法,只需要对物体的给定点寻找最近点。为了计算目标点的坐标,利用投影器投射清晰稳定的纹理到缺乏纹理物体的表面,然后CCD相机对物体进行拍摄。通过线检测和空间前方交会解算出物体部分模型表面的空间点;并使用无需控制点的ICP方法对相互重叠的部分模型进行匹配,从而完成物体的三维重建。实验证明ICP方法对缺乏纹理物体的三维曲面匹配有效。     

Precise photorealistic visualization for restoration of historic buildings based on tacheometry data

This paper puts forward a 3D reconstruction methodology applied to the restoration of historic buildings taking advantage of the speed, range and accuracy of a total geodetic station. The measurements representing geo-referenced points produced an interactive and photorealistic geometric mesh of a monument named ‘Neoria.’ ‘Neoria’ is a Venetian building located by the old harbor at Chania, Crete, Greece. The integration of tacheometry acquisition and computer graphics puts forward a novel integrated software framework for the accurate 3D reconstruction of a historical building. The main technical challenge of this work was the production of a precise 3D mesh based on a sufficient number of tacheometry measurements acquired fast and at low cost, employing a combination of surface reconstruction and processing methods. A fully interactive application based on game engine technologies was developed. The user can visualize and walk through the monument and the area around it as well as photorealistically view it at different times of day and night. Advanced interactive functionalities are offered to the user in relation to identifying restoration areas and visualizing the outcome of such works. The user could visualize the coordinates of the points measured, calculate distances and navigate through the complete 3D mesh of the monument. The geographical data are stored in a database connected with the application. Features referencing and associating the database with the monument are developed. The goal was to utilize a small number of acquired data points and present a fully interactive visualization of a geo-referenced 3D model.     

地形模型的多分辨率简化与压缩

在地形的三维表达中 ,海量的三维数据在显示、存储、传输等方面存在着很多困难。为了解决这些困难 ,本文提出了一种基于渐进简化的多分辨率几何简化与压缩方法。通过该种方法 ,复杂的地形模型能够很好的被简化成适于LOD连续表达的方式 ,并且在简化基础上的数据压缩可使地形模型的存储量减少、绘制速度增加、网络传输效率提高 ,其结果能够胜任地形模型在网络上的渐进传输     

Geometric structure simplification of 3D building models

The complexity of building models directly affects the application efficiencies of 3D urban maps. To address the challenges of building models with various structures, we propose a structural simplification method in this paper. The geometric structures of building models are classified into three categories: embedded structures, compositional structures, and connecting structures, which can be extracted separately through convex/concave analysis. Some specific rules are proposed for the simplification of geometric structures, and the building models are suggested to be simplified progressively. The robustness and efficiency of the method are demonstrated by experiments, and the applications of the Levels of detail of the building models are illustrated.     

地景模型的简化与快速绘制方法研究

在分析地景模型数据特点的基础上，提出了地景模型简化的判决准则，并根据该则提出了基于视点的区域数据抽取与简化方法以及基于法矢量的细节模型简化方法。实验结果表明，使用该方法数据压缩量大，绘制速度快，且逼真度无明显变化。     

三维城市模型中建筑物LOD模型研究

建筑物模型在三维城市模型(3DCM)中占有相当大的比重,建筑物模型多细节层次(LOD)的建立和表达直接关系到三维城市模型的整体显示效率和真实表现力,是三维城市模型的重要研究内容之一。本文从建筑物模型的纹理特征和几何结构出发,论述了建筑物LOD模型的建立原则,并从应用角度就建筑物各个细节层次模型的特点、应用范围与数据获取等方面进行了分析。     

基于加权网格简化的建筑物群渐进式典型化

提出了渐进式简化建筑物群目标的算法，该算法考虑了目标之间的分辨距离和密度，能够渐进式得到任意比例尺的地图，可以从几何和语义两方面最大限度地保持原有建筑物群的特征，同时可保持综合前后不同分区之间的密度差异。