交互式自然场景实时渲染的研究与实现
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摘要
自然场景实时交互技术是一种能表达自然场景的真实感,同时给用户极强的沉浸感的元素。自然场景的交互式实时渲染已成为目前的研究热点,尽管目前已取得了不少成果,但是早期的研究工作主要集中在草体外形的真实感表现上,而其它交互性和光影效果的模拟成果较少。直到2006年Kevin Boulanger实现了动态光照下的自然场景的实时真实感渲染,虽然真实感的实现几乎完美,但是没有涉及多少交互,而已有的交互实现还不是很完善,还有很大的研究前景。此外,很多自然景物还没有统一的建模方法,已经存在的建模方法各有其优缺点,还可以继续完善。因此,如何实现自然场景的真实感、可交互性和实时性渲染,是本文将要解决的问题。
首先,本文在irrlicht图形引擎的基础上设计了一个草地渲染系统。该系统以irrlicht图形引擎为平台,主要包括草体建模模块,草体交互模块,草体阴影模块等。具体每一模块的内容后续章节再一一进行详细介绍。
其次,本文实现了草地场景的LOD显示。虽然草体的几何建模方法很多,但是如果要实现对整个自然场景的几何建模,则数据量很大,重复性高。因此本文将整个草地根据离视点的距离分为三级LOD显示,其中第一级LOD作者借鉴了已有的改进的粒子系统草体建模的方法实现了近距离草体的几何建模,通过实验我们发现这种建模方法得到的草体真实感强,并且能方便地用于交互。在保证渲染质量的前提下,我们将星形布告板进一步简化作为第二级LOD的草体建模。
再次,本文针对基于改进的粒子系统建模的草体,提出了一种改进的基于过程的草体碾压算法,实现了球体碾压草体的动态过程。碾压效果的实现有两种方法:基于过程的方法和基于物理的方法。前者采用经验公式来模拟交互的过程,适用于实时应用;后者则采用精确的物理公式来计算,可以模拟出逼真的效果,但计算量大。本文通过采用基于过程的方法,达到了实时性的要求,同时,考虑了草体之间的碰撞检测,实验结果表明该方法能较真实地模拟球体与草体的交互过程,是实现草体碾压效果的一种简单有效的方法。
最后,本文利用一种适用于大规模虚拟环境中实时阴影生成算法,实现了草体阴影。该算法通过使用多张阴影图来代替一张代表整个场景的阴影图,既减少了系统开销,又改善了生成的阴影的质量。实验结果表明使用该算法生成的阴影能达到满意的视觉效果和生成速度。
Real-time rendering and interaction technology make the simulation of natural scenes realistic. The field about how to render real-time natural scenes interactively has been research highlight. Although we get much works, most of them were focused on the geometry of the grass, only a little of them are about the interaction and the light or shadow. Until 2006, Kevin Boulanger simulated real-time natural scenes with dynamic light. The scenes simulated by Kevin are very realistic, but he doesn’t handle interaction effects. The interaction effects currently realized are still not thoroughly, there is still much work to do. Besides, many natural objects, for example grass, haven’t a uniform modeling method. None of the existing modeling methods are perfect. Accordingly, how to rendering a natural scene, real-time, realistic and interactive, is the problem the research aims to solve.
Firstly, based on the graphical engine of irrlicht, the thesis designs a grass-rendering system. The system includes the module of grass modeling, interaction, shadow of grass and so on. The following parts will introduce them in detail.
Secondly, the thesis realizes the level of detail (LOD) of the grassland. As we know, although there are many ways to model the grass, for the whole grassland, there requires much data, and some of the data is not a must. Therefore, the thesis divides the grassland into three parts, according to the distance between the eye and the grass in the scene. The grass of the part that is closest to the eye is modeled by the method based on the theory of the improved particle system. The grass is verified to be realistic by our experiment, and it can be interacted easily. To reduce the geometry information but not affect the visual of the scene, the grass of the second part is modeled by simplified star billboard.
Thirdly, based on the grass modeled by the theory of the improved particle system, an improved procedural approach for simulating the dynamic process of treading grass is designed and implemented. To simulate the process of treading grass, there are two means: the procedural approach and the physical approach. The former simulates the process by the experienced formula, so it is adapted to the real-time application. The latter can simulate realistic effect by the exactly physical formula, but much time is needed. So the thesis adapts the procedural approach for the requirement of real-time. Besides, we consider the conflict in grasses. The result of experiment shows that the approach can simulate the process of grass treading effectively and realistically.
Finally, based on the algorithm of real-time shadow rendering for large-scale virtual environments, we realize the shadow of the grassland. Instead of using a big shadow map for the whole grassland, the algorithm uses several small shadow maps. It requires less texture memory and improves the quality of shadow.
首先,本文在irrlicht图形引擎的基础上设计了一个草地渲染系统。该系统以irrlicht图形引擎为平台,主要包括草体建模模块,草体交互模块,草体阴影模块等。具体每一模块的内容后续章节再一一进行详细介绍。
其次,本文实现了草地场景的LOD显示。虽然草体的几何建模方法很多,但是如果要实现对整个自然场景的几何建模,则数据量很大,重复性高。因此本文将整个草地根据离视点的距离分为三级LOD显示,其中第一级LOD作者借鉴了已有的改进的粒子系统草体建模的方法实现了近距离草体的几何建模,通过实验我们发现这种建模方法得到的草体真实感强,并且能方便地用于交互。在保证渲染质量的前提下,我们将星形布告板进一步简化作为第二级LOD的草体建模。
再次,本文针对基于改进的粒子系统建模的草体,提出了一种改进的基于过程的草体碾压算法,实现了球体碾压草体的动态过程。碾压效果的实现有两种方法:基于过程的方法和基于物理的方法。前者采用经验公式来模拟交互的过程,适用于实时应用;后者则采用精确的物理公式来计算,可以模拟出逼真的效果,但计算量大。本文通过采用基于过程的方法,达到了实时性的要求,同时,考虑了草体之间的碰撞检测,实验结果表明该方法能较真实地模拟球体与草体的交互过程,是实现草体碾压效果的一种简单有效的方法。
最后,本文利用一种适用于大规模虚拟环境中实时阴影生成算法,实现了草体阴影。该算法通过使用多张阴影图来代替一张代表整个场景的阴影图,既减少了系统开销,又改善了生成的阴影的质量。实验结果表明使用该算法生成的阴影能达到满意的视觉效果和生成速度。
Real-time rendering and interaction technology make the simulation of natural scenes realistic. The field about how to render real-time natural scenes interactively has been research highlight. Although we get much works, most of them were focused on the geometry of the grass, only a little of them are about the interaction and the light or shadow. Until 2006, Kevin Boulanger simulated real-time natural scenes with dynamic light. The scenes simulated by Kevin are very realistic, but he doesn’t handle interaction effects. The interaction effects currently realized are still not thoroughly, there is still much work to do. Besides, many natural objects, for example grass, haven’t a uniform modeling method. None of the existing modeling methods are perfect. Accordingly, how to rendering a natural scene, real-time, realistic and interactive, is the problem the research aims to solve.
Firstly, based on the graphical engine of irrlicht, the thesis designs a grass-rendering system. The system includes the module of grass modeling, interaction, shadow of grass and so on. The following parts will introduce them in detail.
Secondly, the thesis realizes the level of detail (LOD) of the grassland. As we know, although there are many ways to model the grass, for the whole grassland, there requires much data, and some of the data is not a must. Therefore, the thesis divides the grassland into three parts, according to the distance between the eye and the grass in the scene. The grass of the part that is closest to the eye is modeled by the method based on the theory of the improved particle system. The grass is verified to be realistic by our experiment, and it can be interacted easily. To reduce the geometry information but not affect the visual of the scene, the grass of the second part is modeled by simplified star billboard.
Thirdly, based on the grass modeled by the theory of the improved particle system, an improved procedural approach for simulating the dynamic process of treading grass is designed and implemented. To simulate the process of treading grass, there are two means: the procedural approach and the physical approach. The former simulates the process by the experienced formula, so it is adapted to the real-time application. The latter can simulate realistic effect by the exactly physical formula, but much time is needed. So the thesis adapts the procedural approach for the requirement of real-time. Besides, we consider the conflict in grasses. The result of experiment shows that the approach can simulate the process of grass treading effectively and realistically.
Finally, based on the algorithm of real-time shadow rendering for large-scale virtual environments, we realize the shadow of the grassland. Instead of using a big shadow map for the whole grassland, the algorithm uses several small shadow maps. It requires less texture memory and improves the quality of shadow.
引文
[1] W. T. Reeves and R. Blau.,Approximate and probabilistic algorithms for shading and rendering structured particle systems. Computer Graphics,1985,19(3): 313-322.
[2] Frank Perbet and Marie-Paule Cani.“Animating Prairies in Real-Time.”In S. N. Spencer,editor,Proceedings of the Conference on the 2001 Symposium on interactive 3D Graphics. ACM,Eurographics,ACM Press,2001.
[3] Brook Bakay,Paul Lalonde,and Wolfgang Heidrich.“Real Time Animated Grass.”In Eurogr- aphics 2002,2002.
[4] Kurt Pelzer. GPU Gems,chapter7– Rendering Countless Blades of Waving Grass,pp.107-121. Addison-Wesley,March 2004.
[5] Philippe Decaudin and Fabrice Neyret.“Rendering Forest Scenes in Real-Time.”In A. Keller H. W. Jensen,editor,Eurographics Symposium on Rendering,2004.
[6] Sylvain Guerraz,Frank Perbet,David Raulo,and Marie-Paule Cani,et al.“A Procedural Appr- oach to Animate Interactive Natural Sceneries."In CASA '03:Proceed- ings of the 16th Internat- ional Conference on Computer Animation and Social Age- nts (CASA 2003),p. 73,Washington,DC,USA,2003. IEEE Computer Society.
[7] Milan Ikits,Joe Kniss,Aaron Lefohn,and Charles Hansen. GPU Gems,chapter 39– Volume Rendering Techniques,pp. 667-692. Addison-Wesley,March 2004.
[8] Alexandre Meyer and Fabrice Neyret.:“Interactive Volumetric Textures." In George Drettakis and Nelson Max,editors,Eurographics Rendering Workshop 1998,pp. 157-168,July 1998.
[9] Fabrice Neyret.“Synthesizing Verdant Landscapes using Volumetric Textures." In X. Pueyo and P. Schro?oder,editors,Rendering Techniques 96,pp. 215-224 and 291. Springer-Verlag,1996.
[10] Fabrice Neyret.“Modeling Animating and Rendering Complex Scenes using Volumetric Te- xtures." In IEEE Transactions on Visualization and Computer Graphics,volume 4(1),1998.
[11] Musawir A. Shah,Jaakko Konttinen,and Sumanta Pattanaik.“Real-time rendering of realisti- c-looking grass." In Proceedings of GRAPHITE '05,pp. 77-82,November 2005.
[12] David Whatley. GPU Gems 2,chapter 1 - Toward Photorealism in Virtual Botany,pp. 7-25. Addison-Wesley,March 2005.
[13] O. Deussen,P. Hanrahan,M. Pharr,B. Lintermann,R. Mˇech,and P. Prusinkiewicz. Realistic modeling and rendering of plant ecosystems. Computer Graphics,32(3),SIGGRAPH 98 Co- nference Proceedings.
[14] Deussen,O.,Colditz,C.,Stamminger,M.,and Drettakis,G. 2002.Interactive visualization of complex plant ecosystems. In Proceedings of the IEEE Visualization Conference,IEEE.
[15] James T. Kajiya and Timothy L. Kay.“Rendering fur with three dimensional textures." In C- omputer Graphics (SIGGRAPH 89 Proceedings),volume 23(3),pp. 271-280. ACM,July 1989.
[16] Jerome Lengyel,Emil Praun,Adam Finkelstein,and Hugues Hoppe.“Real-Time Fur over A- rbitrary Surfaces." In ACM Symposium on Interative 3D Graphics,pp . 227-232. ACM Marc- h 2001.
[17] Jakulin,A. 2000. Interactive vegetation rendering with slicing and blending. Eurographics (short presentations).
[18] Przemyslaw Prusinkiewicz and Aristid Lindenmayer. The algorithmic beauty of plants. Spring- er-Verlag New York,Inc.,New York,NY,USA,1990.
[19] Kevin Boulanger.“Real-Time Realistic Rendering of Nature Scenes With Dynamic Lighting”: [博士学位论文]. Orlando,Florida: University of Central Florida,2005.
[20] William T. Reeves.“Particle Systems- A Technique for Modeling a Class of Fuzzy Objects.”, Objects.",July 1983.
[21] Tomas Akenine-M?oller and Eric Haines. Real-Time Rendering - Second Edition,chapter 12.1. 3,pp. 492{494. A K Peters,2002.
[22] Neyret,Fabrice,“Animated Texels”,Eurographics Workshop on Rendering’96 pp. 97–103 (September 1995).
[23] COHEN F.,DECAUDIN P.,NEYRET F.GPU-based lighting and shadowing of complex nat- ural scenes. Siggraph'04 Poster (conf. select CD-ROM and conf. DVD-ROM). Los Angeles, USA (2004).
[24]王长波。基于物理模型的自然景物真实感绘制:[博士学位论文]。浙江大学理学院。2006
[25]张淮声。大尺度地形/植被的实时绘制技术:[博士学位论文]。浙江大学。2006。
[26]谭同德,许绘香等。大规则虚拟环境中实时阴影生成技术。计算机应用。郑州大学2008
[27] Michael Wimmer,Daniel Scherzer and Werner Purgathofer. Light space perspective shadow maps. Eurographics Symposium on Rendering,2004.
[28]李恋。图形引擎中阴影算法的研究与实现[硕士学位论文]。电子科技大学,2009
[29]凌飞。图形引擎中大规模草本植被渲染研究与实现。电子科技大学,2009
[30]谭同德,许绘香,赵红领,等。大规模虚拟环境中实时阴影生成技术。计算机应用,2008,28(7)。
[31]刘明,徐飞,刘玉。基于GPU的大规模波动草叶实时渲染技术。图像处理,2008,24(5-3)。
[32]付建林,周美玉。不规则三角形网格(TIN)地形的生成。Railway Computer Application. 2001.12.
[33] Terrain Rendering Using GPU-Based Geometry Clipmaps. 2004.
[2] Frank Perbet and Marie-Paule Cani.“Animating Prairies in Real-Time.”In S. N. Spencer,editor,Proceedings of the Conference on the 2001 Symposium on interactive 3D Graphics. ACM,Eurographics,ACM Press,2001.
[3] Brook Bakay,Paul Lalonde,and Wolfgang Heidrich.“Real Time Animated Grass.”In Eurogr- aphics 2002,2002.
[4] Kurt Pelzer. GPU Gems,chapter7– Rendering Countless Blades of Waving Grass,pp.107-121. Addison-Wesley,March 2004.
[5] Philippe Decaudin and Fabrice Neyret.“Rendering Forest Scenes in Real-Time.”In A. Keller H. W. Jensen,editor,Eurographics Symposium on Rendering,2004.
[6] Sylvain Guerraz,Frank Perbet,David Raulo,and Marie-Paule Cani,et al.“A Procedural Appr- oach to Animate Interactive Natural Sceneries."In CASA '03:Proceed- ings of the 16th Internat- ional Conference on Computer Animation and Social Age- nts (CASA 2003),p. 73,Washington,DC,USA,2003. IEEE Computer Society.
[7] Milan Ikits,Joe Kniss,Aaron Lefohn,and Charles Hansen. GPU Gems,chapter 39– Volume Rendering Techniques,pp. 667-692. Addison-Wesley,March 2004.
[8] Alexandre Meyer and Fabrice Neyret.:“Interactive Volumetric Textures." In George Drettakis and Nelson Max,editors,Eurographics Rendering Workshop 1998,pp. 157-168,July 1998.
[9] Fabrice Neyret.“Synthesizing Verdant Landscapes using Volumetric Textures." In X. Pueyo and P. Schro?oder,editors,Rendering Techniques 96,pp. 215-224 and 291. Springer-Verlag,1996.
[10] Fabrice Neyret.“Modeling Animating and Rendering Complex Scenes using Volumetric Te- xtures." In IEEE Transactions on Visualization and Computer Graphics,volume 4(1),1998.
[11] Musawir A. Shah,Jaakko Konttinen,and Sumanta Pattanaik.“Real-time rendering of realisti- c-looking grass." In Proceedings of GRAPHITE '05,pp. 77-82,November 2005.
[12] David Whatley. GPU Gems 2,chapter 1 - Toward Photorealism in Virtual Botany,pp. 7-25. Addison-Wesley,March 2005.
[13] O. Deussen,P. Hanrahan,M. Pharr,B. Lintermann,R. Mˇech,and P. Prusinkiewicz. Realistic modeling and rendering of plant ecosystems. Computer Graphics,32(3),SIGGRAPH 98 Co- nference Proceedings.
[14] Deussen,O.,Colditz,C.,Stamminger,M.,and Drettakis,G. 2002.Interactive visualization of complex plant ecosystems. In Proceedings of the IEEE Visualization Conference,IEEE.
[15] James T. Kajiya and Timothy L. Kay.“Rendering fur with three dimensional textures." In C- omputer Graphics (SIGGRAPH 89 Proceedings),volume 23(3),pp. 271-280. ACM,July 1989.
[16] Jerome Lengyel,Emil Praun,Adam Finkelstein,and Hugues Hoppe.“Real-Time Fur over A- rbitrary Surfaces." In ACM Symposium on Interative 3D Graphics,pp . 227-232. ACM Marc- h 2001.
[17] Jakulin,A. 2000. Interactive vegetation rendering with slicing and blending. Eurographics (short presentations).
[18] Przemyslaw Prusinkiewicz and Aristid Lindenmayer. The algorithmic beauty of plants. Spring- er-Verlag New York,Inc.,New York,NY,USA,1990.
[19] Kevin Boulanger.“Real-Time Realistic Rendering of Nature Scenes With Dynamic Lighting”: [博士学位论文]. Orlando,Florida: University of Central Florida,2005.
[20] William T. Reeves.“Particle Systems- A Technique for Modeling a Class of Fuzzy Objects.”, Objects.",July 1983.
[21] Tomas Akenine-M?oller and Eric Haines. Real-Time Rendering - Second Edition,chapter 12.1. 3,pp. 492{494. A K Peters,2002.
[22] Neyret,Fabrice,“Animated Texels”,Eurographics Workshop on Rendering’96 pp. 97–103 (September 1995).
[23] COHEN F.,DECAUDIN P.,NEYRET F.GPU-based lighting and shadowing of complex nat- ural scenes. Siggraph'04 Poster (conf. select CD-ROM and conf. DVD-ROM). Los Angeles, USA (2004).
[24]王长波。基于物理模型的自然景物真实感绘制:[博士学位论文]。浙江大学理学院。2006
[25]张淮声。大尺度地形/植被的实时绘制技术:[博士学位论文]。浙江大学。2006。
[26]谭同德,许绘香等。大规则虚拟环境中实时阴影生成技术。计算机应用。郑州大学2008
[27] Michael Wimmer,Daniel Scherzer and Werner Purgathofer. Light space perspective shadow maps. Eurographics Symposium on Rendering,2004.
[28]李恋。图形引擎中阴影算法的研究与实现[硕士学位论文]。电子科技大学,2009
[29]凌飞。图形引擎中大规模草本植被渲染研究与实现。电子科技大学,2009
[30]谭同德,许绘香,赵红领,等。大规模虚拟环境中实时阴影生成技术。计算机应用,2008,28(7)。
[31]刘明,徐飞,刘玉。基于GPU的大规模波动草叶实时渲染技术。图像处理,2008,24(5-3)。
[32]付建林,周美玉。不规则三角形网格(TIN)地形的生成。Railway Computer Application. 2001.12.
[33] Terrain Rendering Using GPU-Based Geometry Clipmaps. 2004.