大规模多分辨率地形实时绘制算法研究

为解决海量地形数据无法实现直接、连续地实时绘制的问题,根据小波分析的特点,提出了基于小波分析的多分辨率地形模型生成算法.该算法利用小波分析对数字高程模型(DEM,digital elevation model)数据进行简化压缩,采用四叉树结构来表示地形,构建视点相关多分辨率地形模型.实验结果表明,该算法使需要的地形数据量大为减少,能高效地生成地形的多分辨率模型,并且地形的逼真程度高.因此,该算法结构简单、便于实现大规模地形实时绘制.     

ROAM动态地形渲染算法的改进

三维真实感地形的生成一直是计算机图形学领域中的焦点课题,研究了基于层次细节的实时优化自适应网格动态地形渲染算法,采用了基于地形块包围盒的可见性投影剔除技术的实时优化策略,提出了嵌套包围球方法和屏幕空间误差法相结合的优化算法来改进误差判据,以提高地形绘制的快速性,给出了该层次细节模型在地形渲染中的实现方法。实验证明,通过对实时优化自适应网格算法的实现和优化,在保证一定的地形渲染效果的前提下,减少了开销同时提高了实时渲染速度。     

一种基于四叉树的大规模地形实时生成算法

论文分析了现有地形简化算法,提出了一种基于地形四叉树实时构建地形多分辨率模型的优化算法,该算法采用分层分块的思想。首先将大规模高程数据进行分块,然后对块内数据按照分辨率的大小分层存储,并给出一种与视点相依赖的对地形节点误差进行评价的方法,在网格的生成中只使用一次四叉树遍历,从而大大提高了渲染速度。     

一种基于四又树的大规模地形实时生成算法

论文分析了现有地形简化算法,提出了一种基于地形四叉树实时构建地形多分辨率模型的优化算法,该算法采用分层分块的思想。,首先将大规模高程数据进行分块,然后对块内数据按照分辨率的大小分层存储,并给出一种与视点相依赖的对地形节点误差进行评价的方法,在网格的生成中只使用一次四叉树遍历,从而大大提高了渲染速度。     

用约束四叉树实现地形的实时多分辨率绘制

在地形可视化领域，实时绘制复杂地形场景的最有效工具是LOD技术，在研究和实验的基础上，提出了一种基于地形四叉树实时构建地形多分辨率模型的优化算法，该算法引入视相关的概念，给出一种与视点相依赖的对地形结点误差进行评价的方法，改进了LOD模型“裂缝”效应消除方法。实验结果表明，该算法能实时动态地生成地形的连续多分辨率模型，实现地形场景的平滑绘制。     

GPU构网的GeoMipMap地形无缝绘制算法

针对传统CPU实时构网算法和预处理阶段构网算法速度较慢问题,提出一种GPU构网的GeoMipMap地形渲染算法.算法的构网阶段由GPU实现,将CPU从繁重的构网工作中解放出来,并大幅度减少CPU向GPU传输的数据量,提高地形的渲染速度.整个地形分成大小相等的若干地形块,每个地形块又分为内部及四条边共五部分,对这五部分按分辨率不同分成多个细节层次,为每个细节层次计算空间误差,渲染时各部分根据屏幕投影误差选择细节层次,所构网格更加符合地形表面特征.考虑到GPU构网算法的高度并行性,采用一种新的裂缝处理方式,四条边的屏幕投影误差以边上顶点的空间误差计算,使得相邻块对于共享边的细节层次的计算结果相同,从而保证相邻块间无裂缝,且网格连续.实验结果表明该方法能够以较高的质量完成大规模地形的实时平滑漫游.     

LOD模型生成算法研究

细节层次模型(LOD)是指对同一个场景或场景中的物体,使用具有不同细节的方法得到一组模型,供绘制时使用.建立LOD模型能很有效地降低数据量和复杂度,实现三维场景的实时处理.文中通过对LOD模型生成算法进行分析,研究了基于点删除算法的多分辨率模型生成方法、递进网格的简化算法、基于二次误差度量的几何简化算法等三种算法的实现步骤和误差计算方法.为以后在地形可视化、虚拟现实等领域的应用提供了理论依据.     

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.     

自适应LOD在三维雷达终端仿真中的应用

自适应视相关LOD简化算法是目前仿真和虚拟现实中研究的热点,它不同于以前的简化技术,将地形作为一个整体进行简化,而是针对视点的位置,对地形的局部进行简化.该文主要讨论该算法的原理、节点评价函数、算法流程和实现技术,解决了多分辨率地形网格拼接中的裂缝问题.并将该算法首次用于三维雷达终端显示器仿真系统中,大大提高了该仿真系统的实时绘制能力,有效地解决了该系统的实时绘制和硬件发展限制之间的矛盾,并取得了良好的实验效果.     

一种基于TIN的大规模地形实时可视化算法研究

在地形可视化领域中,如何对大规模地形的数据进行处理和可视化一直是一个关键问题.本文提出一种有效的实现方法.采用平均分割方法对大规模地形数据进行分割.其中重点解决地形块简化时相邻地形块问的边界处理问题;通过基于半边收缩和顶点分裂操作获得多分辨率TIN模型,并改进顶点分裂操作方法.实验结果表明,采用本方法可以实时生成大规模地形,提高了地形漫游的效率与可视化效果.     

Multi-resolution terrain rendering with GPU tessellation

GPU tessellation is very efficient and is reshaping the terrain-rendering paradigm. We present a novel terrain-rendering algorithm based on GPU tessellation. The planar domain of the terrain is partitioned into a set of tiles, and a coarse-grained quadtree is constructed for each tile using a screen-space error metric. Then, each node of the quadtree is input to the GPU pipeline together with its own tessellation factors. The nodes are tessellated and the vertices of the tessellated mesh are displaced by filtering the displacement maps. The multi-resolution scheme is designed to optimize the use of GPU tessellation. Further, it accepts not only height maps but also geometry images, which displace more vertices toward the higher curvature feature parts of the terrain surface such that the surface detail can be well reconstructed with a small number of vertices. The efficiency of the proposed method is proven through experiments on large terrain models. When the screen-space error threshold is set to a pixel, a terrain surface tessellated into 8.5 M triangles is rendered at 110 fps on commodity PCs.     

基于动态LOD 四叉树算法的虚拟地形可视化

在前人的基础上,改进了基于四叉树的LOD模型,通过建立节点判断准则来决定当前视区的细节程度.同时也提出了一种新的裂缝消除方法,即当视点到节点中心距离较远时看到的细节较少,裂缝消除可通过缩减边的方式实现;而当距离较近时需要观察到的细节较多,通过剖分方式实现可使细节变得更丰富,这样就可以在增强简化效果的同时提高实时渲染的速度.     

基于相对特征的越野地形可通行性分析

自主地面车在越野环境下导航面临最困难的问题之一是对地形的理解，分析感知到的越野地形，作适合于自主车导航的可通行性分析。本文提出了越野高程地形的相对不变性概念，并利用这种性质提取出在一定尺度范围内地形具有的相对不变特征，如地形坡度、起伏度和粗糙度，最后基于模糊规则组合各特征对地形的可通行性进行评估。自主车越野导航实验表明，本文算法稳定有效，能满足自主地面车越野导航的需要。     

基于ROAM的实时动态地形可视化研究

为了获得逼真的三维漫游效果,在对ROAM算法进行研究和实验的基础上,实现基于二叉树的地形模型连续多分辨率渲染;在地形数据管理上,设计一种分块策略;综合考虑地形粗糙度和视距,提出一种合理的节点评价准则;为有效地消除地形块间的裂缝,设计一种基于邻接关系的强制剖分方法。实验结果表明,该算法在保证真实感的前提下能获得较好的帧绘制速率,实现三维地形的实时漫游。     

第四代地理信息系统研究中的尺度转换数字模型

分析了空间插值模型,数字地面模型和它们与地理信息系统集成的研究进展,存在的缺陷和需要解决的理论问题,论述了建立基于曲面论和遥感反演方法数字模型及实现其与地理信息系统有效集成的必要性和可行性。     

A STEP-compliant process planning system for CNC turning operations

Over the last 50 years, there have been many significant enhancements in computer aided systems which have influenced the CNC technology. One area that can be considered as a bottleneck to these CNC enhancements, and in particular to interoperability in CNC manufacturing is G&M part programming (ISO 6983). To overcome this bottleneck, the new standard ISO 14649, known as STEP-NC, is being developed to provide detailed information on component design, process planning and machining strategies to manufacture parts for the next generation of intelligent CNCs. This standard forms the basis of a new paradigm shift in the CNC domain to support digital modelling of CNC manufacturing resources. The research in this paper aims to identify major issues and develop new software tools to demonstrate the feasibility of interoperable CNC manufacturing based on STEP-NC. Besides the literature review on recent research and development on STEP-NC, this paper proposes a Process Planning System (PPS) with surface roughness chosen as the process planning objective. PPS consists of five modules: program reader, process planner, STEP-NC CAD viewer, STEP-NC CAM viewer and program writer. The reader is responsible for interpreting the geometry and the manufacturing data from a STEP-NC text file into a stored data list. The process planner uses this data list and enables users to evaluate surface roughness based on a mathematical model. Through the STEP-NC CAD viewer, the part geometry can be shown and via the STEP-NC CAM viewer the toolpath can be verified. Finally, the writer converts the stored STEP-NC data of the system into an updated STEP-NC file. An example case study component is used to demonstrate the PPS and show the interfacing of the STEP-NC data.     

Navigating large terrains using commodity mobile devices

We describe a hybrid client-server technique for remote adaptive streaming and rendering of large terrains in resource-limited mobile devices. The technique has been designed to achieve an interactive rendering performance on a mobile device connected to a low-bandwidth wireless network. The rendering workload is split between the client and the server. The terrain area close to the viewer is rendered in real-time by the client. The terrain located far from the viewer is portrayed as view-dependent impostors, rendered by the server on demand. A prototype has been built and an exhaustive set of experiments covering several platforms, wireless networks and a wide range of viewer velocities has been conducted. Results show that the approach is feasible, effective and robust.     

三维地形可视化技术研究

为了获得较好的三维可视化效果,在对地形数据组织和实时绘制技术进行研究和实验的基础上,实现了基于四叉树结构的地形模型的连续多分辨率渲染;在考虑视点和地形粗糙度的基础上,设计了一种合理的节点评价系统;提出了一种快速自适应的三角网剖分方法,有效消除了不同分辨率节点间的裂缝。实验结果表明,在保证地形真实感的前提下,该技术实现三维地形显示可以获得较好的图形质量和显示速度。     

Ocean colour comes of age

Abstract

In complex terrain, secondary reflection from neighbouring surfaces may contribute to the total upwelling radiance at a given location. Existing work has assessed the effect of such upwelling on the sensor using simulation models based on the assumption that the surfaces reflect isotropically. Over bare surfaces, this approach may lead to errors since the surface roughness with its range of particle sizes may, under certain conditions, create anisotropic behaviour. Radiance measurements on a V-shaped surface covered by sand revealed that secondary reflection can be high in certain geometrical configurations. To gain a better understanding of the phenomenon, a simulation model was built for surfaces with a particulate nature. Results from the simulation were compared with the experimental data and a good agreement was found. The effect of facet-to-facet reflection contributed substantially to the sensor response. This occurred mainly under conditions of anisotrophic reflection and high surface reflectivity. The effect of varying the valley orientation is discussed.     

Fuzzy-based speed estimation for navigation of unmanned robots

This study presents a framework for assessing the navigation speed of unmanned robots by combining information extracted from the 3D world model of natural terrain with regional traversability based on the fuzzy technique. The proposed method divides the world model into several patches, extracts the slope and roughness of each terrain patch along four heading directions, and then uses them to evaluate the level of difficulty associated with the traversal. The slope is estimated through curved surface fitting, and roughness is obtained using fractal-based analysis together with another two RMS metrics. As navigation systems can cope with the imprecision and uncertainty of input data, we modify the Seraji’s fuzzy-based measure to assess the traversability and navigation speed of each patch for path planning. The proposed method is tested on both fractal and real terrain to verify its effectiveness.