【作者】 张龙；

【导师】 彭群生；

【作者基本信息】 浙江大学 ， 应用数学， 2008， 博士

【摘要】 自然场景仿真是计算机图形学的一个重要研究领域,在三维视频游戏、影视特效、计算机动画、军事仿真以及虚拟现实等方面具有非常广泛的应用。树木的动态模拟是自然场景仿真中的重要问题。在虚拟场景中合成逼真的树木摇曳效果,能显著地增加场景的真实感。树木随风摇曳是由于树木在风场作用下产生形变的结果。由于树木具有复杂的几何外形,其形变效果相当复杂,准确地模拟非常费时。在现有的实时应用中,通常忽略树木的动态变形,或者采用简化的经验模型对树木形变做粗糙的模拟,效果不能令人满意。本文面向计算机游戏和虚拟现实等实时应用,研究大规模森林场景中树木动态摇曳的实时模拟方法。本文的目标是,既要保证算法的高效,又要实现动态摇曳效果的高质量。为此,本文对以下一些关键问题进行了研究和探索。一、研究最合适的树木摇曳形变模型,包括树木的几何模型以及树木形变的表达方法;二、研究基于数据驱动的树木动态摇曳效果模拟技术。将复杂的物理模拟放在预处理阶段完成。实时动态模拟通过对预计算摇曳动画的高效重用而快速实现;三、研究树木动态模拟的细节层次技术,对于距离较远的树木采用粗略的摇曳模拟以减少计算开销。采用提出的树木动态模拟方法,我们实时模拟了包含数万棵树、数十万棵草以及地形、水面等丰富景物的大规模森林场景在风场中的动态效果。场景绘制效果和树、草的随风摇曳效果达到较强的真实感。本文的主要内容如下。第一章介绍模拟树木摇曳效果的意义及其难点所在。对多个方面的相关工作进行了阐述,对其优点及不足之处进行了讨论。此外,还介绍了本文的研究目标、技术路线和文章组织。第二章提出混合式树木模型及形变模拟方法。树木的主枝结构使用几何数据表达,通过简化的动力学模型实现形变模拟。大量的细枝和树叶则先聚为叶簇,用billboard表达,使用简单的过程式方法模拟其局部扰动。结合几何与形变模拟的细节层次、两级实例化方法以及GPU加速等多种加速手段,实现了初步的大规模森林场景动态模拟。第三章提出基于预计算和数据驱动的树木动态摇曳效果模拟方法。在预处理阶段基于物理模型生成一段初始树木摇曳动画。通过对预计算的摇曳动画进行分析和处理,将它转化为易于重用的运动图结构。在实时运动合成时,通过在运动图中随机遍历快速地合成高质量的树木摇曳效果。实验结果表明,本章提出的树木摇曳模拟方法,其效果与物理模拟方法相当,而模拟效率快两、三个数量级。第四章对第三章的方法进行了一系列的扩展,实现了风力条件可控的树木摇曳效果模拟。首先,提出一种简单实用的运动采样算法,能够由预计算的树木摇曳动画实时合成出任意风力条件下的树木动态摇曳效果。其次,对建立运动图的算法进行了改进,使得生成的运动图结构更加合理。在实时运动合成时,用户可以在一定范围内任意改变风力条件。通过求解一个最优化问题,自动地合成最接近用户指定风力条件的树木摇曳效果。第五章将基于运动图算法的树木摇曳模拟技术应用到大规模森林场景中,实现了包含地形、水面、树木、花草等丰富景观元素的大规模森林场景的实时动态模拟。为增强场景真实感,提出一种伪光照方法以改进混合式树木模型的绘制效果;我们还提出一些高效率的方法以近似模拟水面的折射、反射等光学现象以及晴天、雨天等各种天气条件下的光照效果。为提高模拟效率,我们针对基于运动图算法的树木摇曳模拟提出一种细节层次技术,并采用了多种最新的GPU加速技术。第六章总结全文,并对未来工作进行展望。更多还原

【Abstract】 Realistic simulation of the natural scene is one of the most important research areas in computer graphics.It has wide applications in 3D video games, special effects in movies,computer animations,military emulations and virtual reality,etc.Dynamic simulation of trees plays an significant role in simulating the nature.Synthesizing tree movements vividly in a virtual scene remarkably enhances the scene reality.As trees have complex appearances,their deformation is also very complicated. As a result,it’s very time-consuming to simulate the swaying effects of trees in an exact way.In recent real-time applications,such dynamic effects are either ignored,or synthesized by simple empirical methods,whose simulation quality is not so satisfactory.This thesis orients real-time applications such as computer games and virtual reality,and focuses on algorithms to simulate the swaying movements of trees in large-scale forest scenes.Our goal lies in reaching both high efficiency and high quality.To this end,we have explored the following problems.First,we seek to find out the most suitable tree deformation model,including the representation of the tree models and their movements.Second,we adapt data-driven methods to the simulation of swaying trees.The complex physical simulation is performed during the preprocessing time,and real-time motion synthesis is efficiently done by reusing the pre-computed tree animation.Third,we ineorpbrate the dynamic simulation with the level of details technique.The deformation of trees that are far away is computed in a coarse level to reduce the simulation cost.With the proposed algorithms for simulating swaying trees,we can animate large-scale forest scenes containing rich scene elements,including tens of thousands of trees,hundreds of thousands of grasses,natural terrain and lakes,etc, in real time.Realistic effects are achieved in terms of both scene rendering and dynamic simulation of trees and grasses.The main contents of the thesis is as follows. Chapter 1 introduces the importance and difficulties of the dynamic simulation of trees.Current approaches in practical applications as well as their deficiencies are discussed.Next,we introduce the goal,the technical route and the organization of the thesis.The chapter ends with discussions of related work in multiple aspects.Chapter 2 proposes the hybrid tree model and deformation model.Major branches are represented by geometric data,and their deformation is simulated via a simplified dynamic model.Numerous small twigs and leaves are firstly clustered and each cluster is represented by a billboard.A simple procedural method is used to simulate their local vibrations.By combining various acceleration methods including level of details techniques concerning both geometry and dynamic simulation,a two-level instancing method and GPU accelerations, this chapter realizes preliminary effects in the dynamic simulation of large-scale forest scenes.Chapter 3 proposes precomputation-based and data-driven methods for the simulation of swaying trees.During the preprocessing stage,complex and precise physical simulation is performed to generate a finite tree animation.The animation data is then analyzed and transformed into a motion graph which is more suitable for motion reuse.In the real-time motion synthesis stage,high-quality tree movements are efficiently synthesized by a random walk on the pre-computed motion graph.Experimental results show that the presented method achieves a similar simulation quality to that generated by physical simulation,while has a faster simulation efficiency in two to three orders of magnitude.Chapter 4 extends the algorithm presented in Chapter 3,and realizes the simulation of swaying trees under controllable wind conditions.First,we propose a simple yet effective motion sampling algorithm,resulting in a set of tree animation data from which rich tree movements under various wind conditions can be synthesized.Second,we improve the motion graph construction scheme which leads to better graph structure.At runtime,the user can arbitrarily change the current wind condition within a given scope.Appropriate tree movements that best meet the specified wind condition are synthesized by solving an optimization problem.Chapter 5 applies the proposed simulation methods of swaying trees,based on the motion graph algorithm,to the simulation of forest scenes.We achieve the goal of real-time dynamic simulation of large-scale forest scenes containing rich scene elements including terrain,water body,trees,grasses and flowers.In order to enhance the reality of the scene,we propose a fake lighting method to improve the rendering quality of trees with the hybrid geometry/image representation.We also present several efficient algorithms to approximately simulate the refraction and reflection effects of the water surface as well as the lighting effects under various weather conditions such as sun and rain.To improve the simulation efficiency,we propose a level of details technique for the simulation of swaying trees based on the motion graph algorithm,and we take advantage of many modern abilities of the GPU.In Chapter 6,we conclude the thesis and discuss several possible future work.更多还原