Real-time Simulation Of Swaying Trees In Large-scale Forest Scenes

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.