Chapter 87 Real-Time Rendering of Forest Scenes Based on LOD

Chapter 87 Real-Time Rendering of Forest Scenes Based on LOD Hao Li, Fuyan Liu and Shibo Yu Abstract Using the stochastic L-system for modeling the trees. Modeling the trees includes two sides, the trunk is modeled by using multi-level Levels of Detail (LOD) models, and the leaves use billboard technology. In order to achieve realtime rendering effect, we divide the LOD dynamic according to the location between the trees in space and viewpoint. Finally, we compare the rendering efficiency of different methods. Experiments show that this method can simulate the real forest scene, and also can achieve real-time rendering effect. Keywords Tree modeling L-system LOD technology Real-time rendering 87.1 Introduction In recent years, large-scale forest scenes modeling and rendering technology has been a hot research field of computer graphics. It was widely used in wartime simulation and game simulation. Many scholars dedicated to how to draw more efficiently forest scenes and achieve real-time effects. They use many methods, such as simplifying the modeling of trees, optimizing the generation algorithm, H. Li (&) F. Liu S. Yu School of Computer Engineering and Science, Shanghai University, Shanghai 200072, China e-mail: 676589932@163.com F. Liu e-mail: lfy@126.com S. Yu e-mail: Su47yuwenshu@gmail.com Z. Zhong (ed.), Proceedings of the International Conference on Information Engineering and Applications (IEA) 2012, Lecture Notes in Electrical Engineering 218, DOI: 10.1007/978-1-4471-4847-0_87, Ó Springer-Verlag London 2013 709

710 H. Li et al. and changing management of the scene. We use the stochastic L-system for modeling trees. In order to achieve real-time rendering result, we divides the levels of detail (LOD) dynamic according to the location between the trees in the space and viewpoint, for different levels of detail, we use different tree model to render. Then we generate large-scale forest scene by using instance technology. Experiment shows that the method can achieve real-time rendering effect. In Sect. 87.2, we introduce the related research. Section 87.3 describes the method of modeling trees. Section 87.4 presents the technology of LOD for forest scene rendering. Section 87.5 describes the experiment and analysis. We conclude this paper and describe the future work in Sect. 87.6. 87.2 Related Research At present, trees in 3D modeling and rendering method is mainly divided into two categories, the rule-based modeling and image-based modeling. Lindenmayer s L-system [1] and Reeves s particle system [2] stand for the rule-based modeling. The rule-based modeling approach can be a fine description of the organizational structure of trees with a strong sense of reality, but the rendering expenditure of time and space is huge. Billboard image-based method is the most commonly used method. It pastes the image to the billboard by the texture mapping method, that not only simplifies the calculation, but also improves the rendering efficiency. Many scholars have been working on the problem by generating large-scale forest efficiency through improving the tree modeling methods. ZhangShu and others improve the plant modeling method based on the L-system [3], so that the modeling of plants is more concise and efficient. Decaudin [4] and others achieve real-time rendering of forest scene by using volumetric textures. Fuhrmann [5] simplifies the Billboard Clouds algorithm, using 15 50 texture polygons to generate tree model and also achieve real-time rendering. For rendering, tens of thousands of triangle faces in the L-system algorithm, as well as billboard realistic performance is not high status, we use a mixture tree modeling approach. The trunk models use multi-level LOD models, and the leaves use billboard technology. In order to achieve real-time rendering, we divides the LOD dynamic according to the location between the trees in space and viewpoint combined with OpenGL. 87.3 Tree Modeling In 3D modeling of the trees, trees are composed of several separate unit space. The basic unit is divided into several parts, such as leaves, branches. Although the plants have many different forms of style, the external form reflects internal form of control mechanism. Here we will introduce the expression mechanism of trees L-system, and generate trees on the basis of stochastic L-system.

87 Real-Time Rendering of Forest Scenes Based on LOD 711 87.3.1 Trees Branching Pattern Branch of trees is generally divided into uniaxial branch and sympodial branch. The trunk of uniaxial branch has top edge, the lateral bud grows and generates collateral, and so on. The spindle is thicker than the collateral obviously. Sympodial branch does not have the obvious advantage of the top, after a period of growth, the trunk and lateral branches stop growing slowly, and then generates a new bud, instead of trunk and branch growth, to form a tortuous trunk. In this paper we uses the structure of the sympodial branch. 87.3.2 Random L-System The early L-system is an ordered triples L = \V, W, P[, V means the alphabet; V* is a collection of all words defined in the V; W [ V is non-empty word called axioms; P [ V 9 V* is a limited production rule set. If (x,y) [ P is a generation rule, that is for each x [ V, there is only one y [ V*, we call L-system identified. Because of the certainly rules, the image which is generated to determine L-system is single, and do not reflect the natural phenomena of the forest, so we use a random L-system. Stochastic L-system can be expressed as a four-tuple. L ¼ \V; W; P; F [ ð87:1þ V is the set of characters; W is the axiom to determine the string s initial iteration rule; P is the production rule set; the function F makes the production rules set subject to the probability distribution (0,1]. For example, a simple stochastic L-system is as follows: W : A½þAŠ½ AŠ P1 : A [ AA½þAŠA½ AŠ : 2 P2 : A [ A½þAŠA : 3 P3 : A [ A½ AŠA : 5 ð87:2þ As shown in Fig. 87.1, the meaning of the expression +,-, is shown in Fig. 87.2, graphical representation for the string AA [+A] A [-A]. In expression (87.2), P1, P2, P3 rules of probability are as follows: Probability ðp1þ ¼ 2= ð2 þ 3 þ 5Þ ¼ 0:2; Probability ðp2þ ¼ 3= ð2 þ 3 þ 5Þ ¼ 0:3; Probability ðp3þ ¼ 5= ð2 þ 3 þ 5Þ ¼ 0:5. So according to the rules, P3 has the maximum probability.

712 H. Li et al. Fig. 87.1 The meaning of the expression +,- Fig. 87.2 Graphical representation for the string AA[+A]A[-A] 87.3.3 Expression of the Tree L-system We use stochastic L-system, according to the formula (87.1), (87.2) in 3.2, can get the expression of tree L-system. The modeling trees by using stochastic L-system are shown as follows. As shown in Fig. 87.3. By controlling the number of iterations, we can generate tree models of different fine degree. When the number of iterations is 5 6 times, we can describe the general characteristics of a tree. When the number of iterations is more than ten times, we can describe the tree fine. 87.4 Forest Rendering There are many trees in a forest scene which are generated by using random L-system. In order to achieve real-time rendering with the large amount of data, and show good rendering effect when the viewpoint is moving, this is a large conflict between the physical hardware devices. Therefore, we also need to use the forest accelerated algorithm. 87.4.1 Forest Rendering Algorithm Forest scene rendering is divided into the following phases: Drawing the tree models. Adding the leaves placeholder, and output multiple LOD tree model. Leaves pretreatment, and the placeholder is replaced with the billboard.

87 Real-Time Rendering of Forest Scenes Based on LOD 713 Fig. 87.3 The process of 3D tree modeling Leaves texture to texture space. Selecting the LOD model in accordance with the viewpoint. 87.4.2 Forest Rendering Based on the LOD Technology As already mentioned, we need to accelerate the forest algorithm for real-time rendering. The first can be improved by the invisible region of the scene, we do not need to draw this region. In addition, we can also determine the viewable area; the block part within the viewable area cannot be drawn. Within the frustum, forest rendering efficiency has something with the number of iterations and the viewpoint position. According to the current viewpoint and the scenery location calculate a measure, and select an appropriate LOD details. We usually choose the distance from the object to the viewpoint as shown in Fig. 87.4. h is the perspective size of the viewpoint; D is the actual size of the object; d stands for the object size which is projected to the screen; R is the distance between the viewpoint and trees; r is the distance of the viewpoint to the screen; and W is the width of the viewport. According to the geometrical relationship, we know the following: tan h 2 ¼ W=2r ð87:3þ

714 H. Li et al. Fig. 87.4 The relationship between the viewpoint and the scene R ¼ D d r ¼ D d W 2 tan h ð87:4þ = 2 Therefore, we need to make a grading for the distance from the viewpoint to the subject based on R, and set R1 and R2 to divide the forest into distant, medium, and close. For the distant subject, we need to use the Billboard map. In order to make the subject more lifelike, cross method is employed to get better rendering effects. For the middle distance subject, we can improve the rendering efficiency properly by adjusting L-system modeling the iteration number of trees. The close subject can be shown in elaborate trees model to be more vivid. 87.5 Experimental Result and Analysis According to the methods above, the author made a successful real-time rendering of forest scene on the PC with the help of VS2010 and OpenGL. Three-dimensional real-time rendering of forest scene was completed based on a Microsoft Windows 7 Professional 32-bit operating system, Intel(R) Core(TM) i3-380m 2.53 GHz CPU, 2,048 MB RAM (Hynix DDR3 1,333 MHz), Nvidia GeForce GT540M VIDOE CARD (1,024 MB) and a 5,400 rpm ST9500325AS 500 GB hard disk. As shown in Fig. 87.5. In order to prove the good effect, we compared the simplex L-system trees with the combination between L-system and LOD technology in the same hardware conditions. Results are shown in Fig. 87.6. The horizontal axis showing the number of trees and the vertical axis showing rendering efficiency (frames per second). L-system is shown in green while the combination between L-system and LOD technology is manifested in red.

87 Real-Time Rendering of Forest Scenes Based on LOD 715 Fig. 87.5 The forest scene rendering effect Fig. 87.6 Rendering efficiency comparisons From the figure, we can see why we render only a few trees. Both the methods have a relatively high rendering efficiency. But with the growing number of trees, the rendering efficiency of simplex L-system begins to fall. When the number of trees reaches a certain scale, the efficiency is quite low. The latter method achieves real-time rendering effects. 87.6 Conclusions In the above the method of random L-system and combining LOD technology, according to the position of the trees scene and viewpoint, he dynamically divide the level of detail, realizing the forest scene of the real-time drawing. The future work is to further strengthen the efficiency in the render; we can pass the modeling method of fine trees and optimize the scene management, using the technology CPU parallel processing to improve the rendering efficiency.

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