基于细分的地形生成方法研究

提出了一种基于水平面的投影为参考起点,来生成一个很小的原始数据集,进一步构成多分辨率四边形网格,用以确定地形基座的形状。并用分形技术中的中点位移法及四边形边线插值细分方法对地形进行递归细分,从而生成自然的地形。模拟实验结果表明所提出的算法可以方便生成具有真实感、可控性的山脉地形,较好地解决了传统方法对生成地形的形状无法预测和控制以及不真实的问题。通过实验,对分形断面和分形地貌的模拟生成进行了可视化实验研究,结果证实了该方法的有效性和实用性。     

层次可见性与层次细节地表模型相结合的快速绘制

大规模地表模型的实时绘制是虚拟现实技术中重要的研究课题之一。为了加速地表模型的绘制，人们采用视点相关的动态多分辨率层次细节模型方法，但是算法效率依然有待提高。该文提出一种层次可见性与层次细节地表模型相结合的快速地形绘制方法。算法旨在利用地表模型所具有的horizon特性在预处理中为地表多分辨率块模型建立相应的层次“块”可见性结构，快速判定地形块相对于当前视点的可见性，以减少多分辨率模型中模型细节的处理和绘制三角形的数目。同时为消除地表模型层次变化所带来的可见性错误，算法提出一种层次结构可见性计算方法，以修正多分辨率模型所带来的可见性动态变化。实验结果表明算法有效地提高了绘制效率，是可行的。     

视相关地形多分辨率模型在飞行仿真中的应用

对于飞行仿真中海量地形数据,现有的软硬件无法直接对其进行实时连续的绘制,采用视点相关的层次细节模型可以有效地提高实时绘制的性能.该文针对飞行仿真中大地形实时显示问题,对现有的视点相关多分辨率模型算法进行了改进,采用四叉树结构来表示地形,给出了节点的数据结构和构建四叉树的算法;提出了一种新的细节层次选择误差度量标准;实现了视点相关的连续多分辨率地形网格的简化和实时绘制.     

基于硬件细分的层次细节地形渲染算法

针对顶点着色器细分地形网格需要额外生成模板、计算细分层次复杂的不足,提出了一种利用细分着色器进行地形网格细分的层次细节(LOD)地形渲染算法。利用分块四叉树组织建立地形粗糙网格的分层结构,以LOD判别函数对活动地形块进行筛选;提出了在细分控制着色器中基于视点三维连续距离的细分因子计算方法,并针对外部细分因子进行处理消除了裂缝;实现在细分计算着色器上的置换贴图,对精细网格的高度分量进行位移。而且将四叉树结构存储至顶点缓冲区,减少中央处理器(CPU)与图形处理器(GPU)的资源交换;引入细分队列加速细分过程。实验证明,该算法具有平滑的细节层次过渡和良好的细分效果,能够有效提高GPU利用率和地形渲染效率。     

改进FBM的3D复杂场景实时显示方法

分形布朗运动FBM(Fractional Brownian Motion)能真实地表现自然场景,但计算量大,运行费时,在实际的应用中对硬件的要求比较高,难以满足虚拟场景实时生成需求。三角形边界细分法能很好地满足场景实时交互的要求,但在细节描述方面不如FBM。提出一种把两者结合起来生成地形的有效算法,在FBM的基础上对各边法线进行统一方向扰动,通过比较中心点与阈值关系来控制迭代次数,最后采用滤波技术解决运动时产生的闪烁问题。实验证明该算法既能充分表现地形表面的丰富细节,又具有速度快、方便快捷的特点。     

改进四点细分法及其应用

对经典的四点细分格式进行推广，提出了可通过对形状参数的适当选择来实现对极限曲线形状调整和控制的四参数四点细分曲线造型方法，并把该方法扩展到曲面上，对其连续性和收敛性进行了分析。把四参数四点细分法运用于山地模拟，由于其中四个参数选取的灵活性，可对生成的地形形状进行适当的调整，生成比较丰富的地貌形状。细分方法具有多尺度特点，所以可对地貌进行细节描述。试验证明能够较好地生成模拟山地地形，为山地地形模拟仿真提供了一种有效的方法。     

三维地形可视化技术的研究

由于三维地形可视化有广阔的应用前景,近来越来越受到人们的关注。该文介绍了地形可视化涉及到的数字地形模型、三维地形的显示、地形的多分辨率模型等领域的一些概念和研究成果,重点介绍了层次细节技术在：维地形可视化中的应用。最后使用一个简单高效的节点评价函数,编程实现了基于四叉树的动态多分辨率三维地形。试验结果表明,LOD方法可以很好地满足三维地形可视化和虚拟现实应用中地形实时简化和显示的要求。     

一种基于多区域并行的网格模型简化的两步法

针对三角形网格模型简化中保持细节特征的要求，引入了图像的区域分割原理，提出了一种基于区域生长的网格模型区域分割方法．在此基础之上，提出了一种基于多区域并行的自适应三角网格模型简化的两步法，对于不同的区域采取按照比例进行简化的并行机制．此方法的特点是保持模型细节的基础上，大大地加快了模型的简化速度．另外，还提出了一种基于多区域并行的累进网格模型的实现方法，有效地实现了保持细节特征的多分辨率模型，并实现了具有细节特征的多分辨模型间的层次过渡．经实验证明，该方法能够快速有效地对三角形网格模型进行简化，并具有保持模型的三角形网格密度分布的特点．     

大型网格模型多分辨率的外存构建与交互绘制

结合多分辨率、网格排布和基于视点的绘制技术,提出一种外存多分辨率构建和绘制算法.采用适应性八叉树对模型的包围盒进行划分,自顶向下构建模型的多分辨率层次结构,较好地保持了原模型的细节分布;并对多分辨率结构中每个节点所包含的三角形片段进行网格排布优化,降低了缓存的平均失效率;在实时绘制时,采用基于视点的细节层次选择策略进行模型的细化;最后通过引入数据预取机制来隐藏磁盘I/O延时,进一步提高绘制性能.实验结果表明,该算法在绘制速度与细节保留上均优于同类MRMM算法.     

基于聚类和动态LOD的三维地形建模方法

为提高基于DEM高程数据的三维大地形绘制效率,提出基于K-means聚类和动态LOD的三维地形建模方法。对高程数据集使用K-means聚类分析方法,发现地形数据的自相似性,建立数据采样点与地貌特征分类的自动关联;依据采样点的地貌属性值完成粗化处理,构建不规则三角网模型;采用局部优化算法进行有限域内的三角形分裂以实现三角网的动态更新,结合克里金插值法实现插入点的精确计算,形成LOD层次细节多分辨率模型。实验结果表明,该方法对不同类型地形都获得了较高的数据简化率,提高了三维模型的绘制效率,具有较好的地形特征保持性。     

Using the Dempster–Shafer method for the fusion of LIDAR data and multi-spectral images for building detection

A method for the detection of buildings in densely built-up urban areas by the fusion of first and last pulse laser scanner data and multi-spectral images is presented. The method attempts to achieve a classification of land cover into the classes “building”, “tree”, “grassland”, and “bare soil”, the latter three being considered relevant for the subsequent generation of a high-quality digital terrain model (DTM). Building detection is accomplished by first applying a hierarchical rule-based technique for coarse DTM generation based on morphological filtering. After that, data fusion based on the theory of Dempster–Shafer is used at two different stages of the classification process. We describe the algorithms involved, giving examples for a test site in Fairfield (New South Wales).     

基于数字地面模型的公路实时三维动画

研究了一种用数字地面模型和公路设计数据实现实时三维动画的方法。首先利用Delauany三角化方法构造数字地面模型，然后将公路设计数据与数字地面模型进行叠加。为解决叠加过程中的相容性问题，提出了先将地界约束边界线插入Delauany三角形网格，然后重新三角化的方法。为加快渲染速度，提出了多分辨率显示和网格数据结构相结合的加速措施。文中的方法在实践中得到了验证。     

Bare-earth extraction and DTM generation from photogrammetric point clouds including the use of an existing lower-resolution DTM

A method of extracting bare-earth points from photogrammetric point clouds by partially using an existing lower resolution digital terrain model (DTM) is presented. The bare-earth points are extracted based on a threshold defined by local slope. The local slope is estimated from the lower resolution DTM. A gridded DTM is then interpolated from the extracted bare-earth points. Five different interpolation algorithms are implemented and evaluated to identify the most suitable interpolation method for such non-uniformly scattered data. The algorithm is tested on four test sites with varying topographic and ground cover characteristics. The results are evaluated against a reference DTM created using aerial laser scanning. The deviations of the extracted bare-earth points, and the interpolated DTM, from the reference DTM increases with increasing forest canopy density and terrain roughness. The DTM created by the method is significantly closer to the reference DTM than the lower resolution national DTM. The ANUDEM (Australian National University Digital Elevation Modelling) interpolation method is found to be the best performing interpolation method in terms of reducing the deviations and in terms of modelling the terrain realistically with minimum artefacts, although the differences among the interpolation methods are not considerably large.     

Smooth reverse subdivision

In this paper we present a new multiresolution framework that takes into consideration reducing the coarse points’ energy during decomposition. We start from initial biorthogonal filters to include energy minimization in multiresolution. Decomposition and reconstruction are main operations for any multiresolution representation. We formulate decomposition as smooth reverse subdivision, based on a least squares problem. Both approximation of overall shape and energy are taken into account in the least squares formulation through different weights.Using this method, significant smoothness in decomposition of curves and tensor product surfaces can be achieved; while their overall shape is preserved. Having smooth coarse points yields details with maximum characteristics. Our method works well with synthesizing applications in which re-using high-energy details is important. We use our method for finding the smooth reverse of three common subdivision schemes. We also provide examples of our method in curve synthesis and terrain synthesis applications.     

三维地球仪的建模与实现

随着地球科学的发展，大量观探测数据不断出现，要求各种数据场在同一可视化环境中有层次、直观地显示。本文提出制作三维地球仪作为可视化显示环境，在此基础上分别介绍了基于经纬度的三角划分方法和基于单位球上的Renka三角划分两种不同建模方法，并对Renka算法作出改进，建立了地形数据与图形空间的映射关系。此模型已在Visual C ／OpenGL环境下编程实现．     

基于Hamiltonian三角剖分的三角网格多分辨率表示

三角网格模型的多分辨率表示是几何模型绘制与传输的基础，本文通过三角形之间的拓扑相邻关系将三角网格划分为广义三角形带的集合，然后利用Hamiltonian三角剖分的性质构造三角网格的多分辨率表示。该方法统一了单分辨率网格和多分辨率网格的表示方法，当模型有c个不同分辨率表示时，其编码效率为（logc＋5）bit／vertex。     

基于分块和包围球误差函数的地形绘制方法

针对大规模地形数据庞大、绘制速度慢的问题,提出一种基于数据分块和包围球误差函数的地形绘制方法。该方法对数据进行分块组织,按行列顺序对数据块编号,实现对地形数据的部分读取。依据视点可见性判断,实时调入可见数据块,设计一种基于包围球的误差函数,通过三角形二叉树构建层次细节模型,实现大规模地形实时绘制。实验结果表明该方法可以取得较高的帧速率和较好的绘制效果。     

An Optimization Approach to Improving Collections of Shape Maps

Finding an informative, structure‐preserving map between two shapes has been a long‐standing problem in geometry processing, involving a variety of solution approaches and applications. However, in many cases, we are given not only two related shapes, but a collection of them, and considering each pairwise map independently does not take full advantage of all existing information. For example, a notorious problem with computing shape maps is the ambiguity introduced by the symmetry problem — for two similar shapes which have reflectional symmetry there exist two maps which are equally favorable, and no intrinsic mapping algorithm can distinguish between them based on these two shapes alone. Another prominent issue with shape mapping algorithms is their relative sensitivity to how “similar” two shapes are — good maps are much easier to obtain when shapes are very similar. Given the context of additional shape maps connecting our collection, we propose to add the constraint of global map consistency, requiring that any composition of maps between two shapes should be independent of the path chosen in the network. This requirement can help us choose among the equally good symmetric alternatives, or help us replace a “bad” pairwise map with the composition of a few “good” maps between shapes that in some sense interpolate the original ones. We show how, given a collection of pairwise shape maps, to define an optimization problem whose output is a set of alternative maps, compositions of those given, which are consistent, and individually at times much better than the original. Our method is general, and can work on any collection of shapes, as long as a seed set of good pairwise maps is provided. We demonstrate the effectiveness of our method for improving maps generated by state‐of‐the‐art mapping methods on various shape databases.