【作者】 陆声链；

【导师】 赵春江； 黄丹枫；

【作者基本信息】 上海交通大学 ， 机械工程， 2008， 博士

【摘要】 植物形态是植物的外在表现形式,是植物在生长过程中自身生理规律和环境因素相互作用的结果。因此,要在计算机上逼真地模拟植物,不仅需要对植物的外部形态进行精确建模和绘制,同时也需要对植物的行为特性进行研究,并建立符合植物自身生理规律的行为模型,才能更真实地在虚拟空间中对植物的形态和运动进行仿真。另一方面,植物所表现出的行为特性是人类认识、分析和评价植物的最基本的方式和最直接的渠道。因此,运用计算机图形学和虚拟现实的技术与方法来定量化、可视化地描述植物的行为特性,并开发相应的软件工具和交互平台,通过视觉体验和三维互动的人机交互方式来揭示植物内在的遗传特性与环境的相互作用关系,具有重要的现实意义和广阔的应用前景。围绕植物的三维建模和运动模拟的研究已经有了几十年的历史,经过几代研究者的努力,目前已经取得了显著的成果。但由于植物形态的复杂性和多样性,而且植物的生长过程受环境的影响,这使得逼真重构植物的形态和模拟其在各种环境条件下的行为特性是一个巨大的挑战。特别是对农学研究中以植株个体为主要研究单位的园艺植物,其形态和行为特性的真实感模拟更有待加强。一方面,相对于其他植物,园艺植物尤其是瓜果类作物的相关研究还处于起步阶段。传统以树木为对象的植物形态建模方法重点在于树木的整体结构,而忽略器官上的细节,因此难以适用于以器官和个体为基本单位的园艺植物形态的建模。其次,虽然研究者提出了各种针对农作物器官的形态建模方法,但这些方法缺乏普适性,真实感效果有待加强。在植物模型的变形和运动方面,部分研究者已提出了各种模拟方法,但在这些研究中,往往以植株个体为对象,缺乏器官尺度上的变形模拟,而且只是考虑了植物在外力作用下的变形,很少考虑植物自主的变形和运动,如植物叶片在高温下的卷曲、失水条件下的萎蔫等现象。而对于园艺植物中常见的攀援行为,尚未有研究者从可视化模拟的角度进行研究。针对以上问题,本文拟运用计算机图形学和物理模型的理论和方法,研究园艺植物的三维形态建模及其典型行为特性可视化模拟的关键技术。本文以我国主要园艺植物黄瓜、西瓜和番茄为研究对象,重点是通过对品种特征的观测研究,借助计算机辅助设计思想,运用几何造型和三维可视化技术,构建主要园艺植物三维形态数字化设计工具;基于已有的农业模型和知识规则,构建参数化的植物器官精确几何模型;研究基于物理模型的植物变形和运动模拟技术;最终将上述各项技术有机集成,构建一个植物行为特性可视化仿真框架,逼真地虚拟显示主要园艺植物在器官、个体和群体水平上的自然行为特性。具体地,本文主要开展了以下几个方面的创新研究:1、园艺植物器官精确几何建模和植株结构交互式生成针对园艺植物器官形态复杂和多变的特点,提出了基于B样条曲线的植物器官骨架表示和网格生成方法,以B样条曲线的灵活性为形态多变的植物器官几何描述提供一种统一的表示方法;首次将Delaunay三角化方法和自适应细分方法应用于植物叶片几何曲面造型中,为植物叶片的精确几何建模提供了一种普适的方法。同时,结合已有的农业模型和知识规则提取了植物器官几何模型的主控参数。基于植物的器官模型开发了交互式的园艺植物形态结构辅助设计工具,采用器官模板和渐变技术生成植物器官不同生长阶段间的过渡效果。2、三维植物模型真实感绘制为增强三维植物模型的真实感效果,采用了多种细节描述技术。针对叶片和果实,采用纹理映射技术;针对茎、叶柄等圆柱型器官,使用过程纹理技术生成适用的纹理效果。开发了植物造型表面绒毛生成技术,采用Poisson-disk分布模式确定器官曲面上绒毛的分布,并根据网格曲面的面积确定绒毛的密度;绒毛的实体用样条曲线或圆柱体表示,绒毛的长度、半径、卷曲度、方向和密度等参数都考虑了植物器官的位置信息。该方法对植株个体模型达到了实时的处理要求,能够有效地生成各种具有真实感的植物表面绒毛效果。3、植物器官变形实时模拟提出了一种基于质点-弹簧系统的植物器官变形模拟方法,并根据交互式设计的要求开发了参数化的植物器官几何造型和弹簧模型生成方法,简化了质点-弹簧模型的构造。实验结果表明,该方法能够获得具有较高真实感的变形效果,适用于多种植物器官一般情况下的变形模拟;同时,参数化的几何曲面和物理模型生成能够满足于交互式实时模拟的需要。4、植物叶片卷曲和萎蔫过程的模拟首次为模拟植物叶片的卷曲和萎蔫运动建立了似然的物理模型。构建了一个由层次化弹簧构成的双层质点-弹簧系统,该弹簧系统被用来控制叶片的运动,叶片的卷曲通过收缩上层弹簧来实现,而叶片的萎蔫或展开则通过释放弹簧来驱动,同时提供了交互式的模拟界面,用户能够交互地控制该弹簧系统的运动,从而生成叶片卷曲或萎蔫的运动动画。针对双层质点-弹簧叶片模型难以构建的问题,提出了一个叶脉骨架驱动的植物叶片变形和运动控制模型,首先从叶片骨架中生成叶脉骨架,叶脉骨架由一条主脉和若干条侧脉构成,每条叶脉被分割为若干线段,构成一个骨架链;在此基础上,将叶片曲面上的点绑定到叶脉骨架中,并通过指定骨架链上每个顶点绕某个指定的矢量旋转来控制骨架链的变形和运动;最后将叶片网格曲面上的点根据变形后的叶脉骨架进行变形,从而实现叶脉骨架驱动的叶片曲面变形。该方法已被用来模拟植物叶片的卷曲和萎蔫过程,获得了较好的真实感效果。5、植物攀援特性建模和攀援行为模拟首次可视化地模拟了卷须类园艺植物藤蔓的形态发展过程。建立了卷须生长和寻找支持物的模拟模型;基于卷须的攀援能力,建立了园艺植物藤蔓的动态生长模拟模型,根据藤蔓在三维空间中的位置以及卷须的攀附情况确定藤蔓的空间形态,从而模拟了藤蔓在重力作用下的自然生长形态;开发了直线与圆柱体、直线与包围盒、圆柱体与包围盒间的碰撞检测技术,用来检测植物生长和下垂过程中器官与障碍物、以及器官之间的碰撞,并提供实时的碰撞避免处理机制。更多还原

【Abstract】 The morphological shape of a plant is its external representation, which may result from mutual influence between its underlying physiological process and the environmental factors. To simulate vividly the plant in computer, obviously, not only accurate modeling and rendering external appearances of a plant but also modeling its dynamic behaviors which are faithful to its underlying physiological mechanics is needed. Just in this way, the shape and motions of a plant can be simulated faithfully in virtual space. On the other hand, the behaviors characteristics of a plant are the most basic way and direct channel for human’s recognizing, analyzing and evaluating the plant. Therefore it is practical significance and broad application perspectives to reveal the mutual influence and relations between its internal heredity characteristics and the environment with visual experience and 3D human-machine interaction, by using techniques and methods in computer graphics and virtual reality to describe quantitatively and visually the behaviors characteristics of plant, and to develop relevant software and interactive platforms.There is a long history in modeling 3D shape of plant and simulating its motions, and remarkable achievements have been achieved thanks to many researchers’efforts. But due to the complexity and diversity of plant morphology, however, it is still a great challenge to reconstruct the shape of a plant and simulating its’behavior characteristics under different environments vividly. On the one hand, horticultural plant, especially melons and fruits, were less researched. Traditional methods for modeling plant shape generally focused on tree, in which the whole structure of tree was more important than the details on organs. Therefore, these methods would be not suitable for the modeling of horticultural plant that takes organs and individuals as basic unit. Secondly, although a various methods for modeling the morphology of crop organs have been proposed, these methods lack universality, and the realistic effects of the generated models need to be improved. In aspect of modeling deformation and motions of plant organs, some researchers have proposed various simulation methods, but they generally modeled at individual plant scale, not at organ scale. Moreover, they just simulated deformation of plant derived by extern forces, the automatically deformation and motions of a plant, such as curling of plant leaves under high temperature, are not considered yet. On the other hand, the climbing behavior which can be found commonly in horticultural plants has not been simulated from visualization perspective. Basing on the above analysis, this dissertation will focus on research the key techniques for modeling the 3D shape and simulating its typical behaviors of horticulture plants, by using theory and methodology from both computer graphics and physical modeling.This dissertation presents a methodology, firstly, for modeling and designing the 3D geometric shapes of main horticultural crops including cucumber, watermelon and tomato, by using techniques from computer-aid design (CAD), geometric modeling and 3D visualization; secondly, for creating parameterized geometric models of plant organs basing on existed agronomic models and knowledge rules. It is also a goal of this dissertation to develop physical models for simulating deformation and motions of plant. Lastly, the above techniques are integrated systematically to develop a visual platform for simulating typical behaviors of plants, and demonstrating vividly the natural life behaviors of main horticultural crops in virtual environment, at organic, individual and crops scales.Specifically, the main contributions and innovations of this dissertation can be listed as follows:1 Modeling accurate geometry of organs and interactive designing plant structure of horticultural cropsTo handle the diversity of species and irregularity of morphogenesis in plant, a B-splines curves-based method which using a skeleton description of organs shape and constructing the geometry of an organ from the skeleton was proposed. With this model users could create a wide range of crop organs with high realistic shape. This method aims to provide a common representation of irregular outlines of plant organs by utilizing the flexibility of B-splines curves. The principal control parameters of these geometric models were then extracted from existed agronomic models and knowledge rules. Delaunay triangulation was used to mesh irregular polygon in organs surface, such as lobed leaf surface. Furthermore, an adaptive subdivision scheme was used to smooth the generated mesh of a leaf blade. Basing on the geometric models of organs, an interface for designing crops structure interactively was developed, in which techniques such as organs templates and morphing were used to generate transitional shapes between two growth stages of an organ.2 Techniques for realistic rendering 3D plant modelsFor improving the appearance of the computer generated plant geometric models, several details description techniques were used in this dissertation. Texture mapping was used to leaf and fruit models, while process texture generation was introduced to cylinder-like organs such as caudex and petiole. Moreover, a method for generating plant hairs that cover many plant organs was proposed. This method employed Poisson-disk pattern to assign the distribution of hairs on the surface of each organ, while the density of hairs was adjusted based on the area of organ surface. This can avoid generating irregular pattern of hairs on irregular surface. Individual hairs were then mapped onto the organ surfaces according to the generated attachment points. Hair properties including length, radius, direction and density were specified and adjusted according to positional information of the organ surface, and the geometry of individual hair can be represented as curve or cylinder. This allows generating a wide range of hairs styles. Sample results showed that the proposed method can render realistic hairs in real-time, and can represent various styles of hairy plant effectively.3 Real-time simulating deformation of plant organsA method for modeling the deformation of plant organs based on mss-spring system was proposed, and parameterized methods for generating the 3D geometry and the spring model of plant organs were developed to satisfy the needs from interactive design. This could simplify the creation of mass-spring model. Results on several plant organs demonstrated that the proposed approach is applicable for simulating deformation of plant organs with realistic effects. Further, parameterized generation of geometric surface and physical model can be used in interactive simulation at real-time speed.4 Simulating curling and wilting of plant leavesTwo approximate physical kinetic models for simulating motions of plant leaves including curling and wilting were proposed in this dissertation. Firstly, a bi-layered mass-spring model consists of hierarchical spring was developed which was used to control the motions of leaves. Curling of a leaf blade was simulated by animating the spring tensions, while unfolding and wilting were simulated by animating the spring relaxation. With a suitably designed interface, animations can be obtained by manual interaction, which allowed for animating plant motions especially curling and wilting process of a leaf surface. Basing on the fact that the bi-layered mass-spring of a leaf model is difficult to be constructed, a venation skeleton-driven method for deforming plant leaves was proposed for this reason. Firstly, the leaf skeleton was used to generate a detail mesh for the leaf surface, and a venation skeleton was also generated interactively from the leaf skeleton. Each vein in the venation skeleton consisted of a segmented vertices string. Secondly each vertex in the leaf mesh was banded to the nearest vertex in the venation skeleton. We then deformed the venation skeleton by controlling the movement of each vertex in the venation skeleton by rotating it around a fixed vector. Finally the leaf mesh was mapped to the deformed venation skeleton, as such the deformation of the mesh followed the deformation of the venation skeleton. The proposed techniques have been applied to simulate leaf surface deformation resulted from biological responses of plant including wilting and curling.5 Modeling climbing characteristics and simulating climbing behavior of plantA model for simulating searching for support and growth of tendril was proposed, with this model, dynamic growth model of vine was developed. This model defined the special shape of the simulated vine according to its position in space with a considering if the tendrils on the vine have found support. By this way, the natural growth shape of a vine in gravity can be simulated. To detect collisions between organ and obstacle, organ and organ occur in the process of growth and drooping of the vine, techniques for detecting collisions between line and cylinder, line and bounding-box, cylinder and bounding-box were developed, and mechanics for avoiding collisions in real time were introduced simultaneously.更多还原