Physically-based deformations: copy and paste

In contrast with purely geometric approaches, physically-based deformation techniques usually afford greater realism in the animation of soft objects and characters, due to the consideration of the inherent physical properties of the materials. In this paper, we present a novel physically-based technique allowing versatile deformation effects to be created using simple copy and paste operations. The copy process involves the extraction of the deformation behaviour from a source object, and the paste operation applies it to a different object (target), resulting in the target object being deformed in a physically plausible manner. Our technique defines the deformation of an object as a linear combination of a set of carefully chosen fundamental solutions from classic mechanics, which separates the deformation data from the geometry. The deformation of the source object can be computed using any existing physically-based technique. To copy the deformation from an object is to formulate the weighting coefficients to the fundamental solutions. To paste the deformation to another object is to apply our deformation model to the target object with the derived weighting coefficients. Repeated copy and paste operations allow various local and global deformation effects to be created. In addition to visual realism, the main advantages of our technique compared with other existing physically-based methods are its capability to reuse the deformation data, computational efficiency and friendliness to animators.     

基于DDA的碰撞检测及碰撞仿真

碰撞检测是基于物理的动画,计算机辅助没计,计算机辅助制造,计算几何,虚拟现实,机器人等领域必须解决的关键问题,目前仍是研究热点.非连续变形分析方法是一种较新的土木工程领域的数值模拟技术,可以分析不连续块体的运动.将非连续变形分析方法引入到基于物理动厕领域.在计算辅助设计软件设计的虚拟物理场景下,仿真了汽车撞墙和车辆相撞.仿真结果显示:应用非连续变形分析方法可以成功的实现精确的碰撞检测和模拟真实的碰撞响应,在不连续变形、多块体碰撞仿真方面有较大优势.     

Efficient Collision Detection among Moving Spheres with Unknown Trajectories

Collision detection is critical for applications that demand a great deal of spatial interaction among objects. In such applications the trajectory of an object is often not known in advance either since a user is allowed to move an object at his/her will, or since an object moves under the rules that are hard to describe by exact mathematical formulas. In this paper we present a new algorithm that efficiently detects the collisions among moving spheres with unknown trajectories. We assume that the current position and velocity of every sphere can be probed at any time although its trajectory is unknown. We also assume that the magnitude of the acceleration of each sphere is bounded. Under these assumptions, we represent the bounding volume of the sphere as a moving sphere of variable radius, called a time-varying bound. Whenever the time-varying bounds of two spheres collide with each other, they are checked for actual collision. Exploiting these bounds, the previous event-driven approach for detecting the collisions among multiple moving spheres with ballistic trajectories is generalized for those with unknown trajectories. The proposed algorithm shows an interactive performance for thousands of moving spheres with unknown trajectories without any hardware help.     

Collision avoidance in cloth animation

For cloth modeling and animation, we use a physically based model to simulate the dynamic formation of folds, pleats, and wrinkles and the final static appearance of cloth draped over a rigid object. To simulate the behavior of the cloth and its final static appearance on the model, we propose a new collision and self-collision avoidance method to prevent penetration between the cloth and rigid objects and between parts of the cloth. At each time step, we enforce constraints over those grid points about to penetrate other objects. Our method is easier and more robust than conventional methods at representing interaction between the cloth and various objects.     

Animation of deformable models

Although kinematic modelling methods are adequate for describing the shapes of static objects, they are insufficient when it comes to producing realistic animation. Physically based modelling remedies this problem by including forces, masses, strain energies and other physical quantities. The paper describes a system for the animation of deformable models. The system uses physically based modelling methods and approaches from elasticity theory for animating the models. Two different formulations, namely the primal formulation and the hybrid formulation, are implemented so that the user can select the one most suitable for an animation depending on the rigidity of the models. Collision of the models with impenetrable obstacles and constraining of the model points to fixed positions in space are implemented for use in the animations.     

实时形状匹配变形体动画

给出了一个仿真变形体的方法。物体被几何方法推动变形,操纵基于点的物体并且不需要连接信息,这是一个不需要任何预处理,计算简单,并提供无条件稳定的动态仿真方法。主要思想是通过几何约束替换能量和通过当前位置到目标位置的距离替换力。这些目标位置通过一个通用的无变形的静止状态和点云的当前变形状态之间的形状匹配来决定,因为点总是在定义好的位置被绘制,显式积分方法的过度不稳定问题被消除。相关物体表现方法的灵活性能被控制,相关内存和计算是有效的,动态仿真上的无条件稳定性让这个方法特别适合游戏开发。     

跃移层内沙颗粒空中碰撞的模型仿真

研究风沙运动的碰撞性建模问题,对控制雾霾污染有着重要的意义.风沙运动时,风沙呈现空间的非均匀分布,存在动态波动,受到外界影响,呈现较强不稳定状态.针对风沙运动碰撞分析的模型,通常假设风沙可以均匀分布且稳定状态,模型中的参数无法随着风沙流结构等因素不断变化,无法对跃移层内沙颗粒的碰撞进行有效分析,存在一定的缺陷.提出了一种颗粒流体动力学的跃移层中沙颗粒空中碰撞仿真模型,通过颗粒流体动力学方法,分析沙粒与床面碰撞、沙颗粒的空中碰撞及沙粒与气流的耦合作用,获取跃移沙粒在空中发生碰撞的概率,按照跃移层内沙颗粒空中碰撞运动的特征,塑造基于单颗沙粒动力学的沙颗粒空中碰撞仿真模型,对沙颗粒在风力作用下发生空中碰撞和达到动态稳定的过程进行仿真,为分析风沙运动提供可靠的依据.实验结果表明,所提模型获取跃移层沙颗粒运动轨迹同已知实测结果具有较高的匹配度,能够实现跃移层沙颗粒空中碰撞的有效仿真.     

An overview of physically-based modelling techniques for virtual environments

A class of virtual environments is concerned with the representation of behaviour that is apparent in the real world. In order to model this behaviour, sophisticated physical models are required. The development of these models, classed asphysically-based modelling, is based upon the fundamental concepts of Newtonian dynamics. Considerable research into physically-based modelling has already been conducted by the computer graphics community, permitting realistic animation of object motion. The application of physical models to virtual environments poses further problems, not least that of real-time execution in a fully interactive environment. This paper gives an overview of the existing computer graphics research concerned with physically-based modelling, discussing the merits and problems of various techniques in terms of the requirements of virtual environment.     

Fast collision detection between complex solids using rasterizing graphics hardware

We present an efficient method for detecting collisions between complex solid objects. The method features a stable processing time and low sensitivity to the complexity of contact between objects. The algorithm handles both concave and convex objects; however, the best performance is achieved when at least one object is convex in the proximity of the collision zone (our techniques check the required convexity property as a byproduct of the calculations). The method achieves real-time performance when calculations are supported by the standard functionality of graphics hardware available on high-end workstations.     

A modelling system for complex deformable bodies suited to animation and collision processing

We propose an integrated set of methods for designing and animating bodies whose deformable flesh coats an articulated skeleton. The proposed methods provide good automatic positioning of the ‘skin’ after movements, and automatic deformation after collisions with rigid or deformable objects (dynamic or static). The response to collisions includes a feedback from the flesh to the skeleton, whose movement is adequately modified. Moreover, coating the skeleton gives an easy solution to closed loop collisions and angle constraints control. Our model is modular and gives some high level control to the user.     

Rapid pairwise intersection tests using programmable GPUs

Detecting self-intersections within a triangular mesh model is fundamentally a quadratic problem in terms of its computational complexity, since in principle all triangles must be compared with all others. We reflect the 2D nature of this process by storing the triangles as multiple 1D textures in texture memory, and then exploit the massive parallelism of graphics processing units (GPUs) to perform pairwise comparisons, using a pixel shader. This approach avoids the creation and maintenance of auxiliary geometric structures, such as a bounding volume hierarchy (BVH); but nevertheless we can plug in auxiliary culling schemes, and use stencils to indicate triangle pairs that do not need to be compared. To overcome the readback bottleneck between GPU and CPU, we use a hierarchical encoding scheme. We have applied our technique to detecting self-intersections in extensively deformed models, and we achieve an order of magnitude increase in performance over CPU-based techniques such as [17].     

基于约束的刚体碰撞响应仿真研究与应用

为了解决虚拟场景中刚体碰撞与穿透问题,采用了一种基于约束的碰撞响应方法。根据刚体之间的穿透深度和在碰撞点处的相对速度,判定不同的碰撞状态。通过将多刚体碰撞和多点碰撞分割为单个碰撞,以刚体在碰撞点相对速度法向分量构建法向约束,切向分量构建切向摩擦约束。根据不同的碰撞状态以及牛顿恢复系数碰撞模型求取法向误差,并以碰撞点相对速度切向分量作为切向摩擦误差。结合约束与误差,得到碰撞的约束方程,联合刚体的动力学方程,迭代求解,计算碰撞之后刚体的速度,更新刚体的坐标与姿态,并计算刚体顶点在世界坐标系中的坐标。在虚拟矿山场景中进行了应用,仿真效果较好。     

基于OBB包围盒的碰撞检测研究与应用

碰撞检测是计算机仿真、虚拟现实等领域内的研究重点之一,其中基于包围盒的碰撞检测算法是一种比较方便有效的方法。为了达到仿真系统中所需要的碰撞检测的精确性和实时性,对方向包围盒(OBB)的计算方法进行了分析,并针对其不足之处进行了改进和优化。利用改进后的OBB包围盒碰撞检测技术实现了JQ900型下导梁架桥机中部件间碰撞的精确快速检测,并最终在VC++和Open Inventor API平台上实现了系统的逼真的仿真效果。     

基于Jack的VR环境下碰撞检测问题的研究

在虚拟现实系统中,碰撞检测是影响系统实时性的重要因素。该文对基于Jack的VR环境下的碰撞检测算法的特点和适用范围作了分析和介绍,并从场景中对象间的碰撞检测的管理角度,提出了使用碰撞队列来对对象间的碰撞进行组织和管理的方法,并提出了通过对碰撞队列中对象进行控制,以及对碰撞队列进行过滤等途径,来提高碰撞检测效率的方法。     

真实感游戏中的碰撞检测方法研究

为了增强游戏场景的真实性,主要研究Unity3D中的碰撞检测方法。分析组件碰撞、射线碰撞以及距离碰撞这三种碰撞检测方法的原理,对比其使用条件、范围及优缺点,最终实现三维生态庄园游戏的开发。该碰撞检测方法已经应用在实际游戏开发中,可提高碰撞检测准确率及运行效率,增强游戏真实感。     

碰撞检测在服装CAD排版系统中的实现

在对计算机动画的研究中,碰撞检测问题是比较热的一个话题,而服装CAD排版系统中的排版也正是一个碰撞检测问题,本文通过排版的实现来提供一个碰撞检测算法。     

基于碰撞响应的三维空间多刚体实时运动仿真

该文将计算机科学与理论力学相结合,以刚性球体为例介绍了多刚体物理运动仿真的步骤和方法。首先论述了单个及多个刚体的运动仿真步骤以及应用叠加原理计算合力及合力矩的方法。然后,针对刚体运动中的不同碰撞响应类型：碰撞接触与临时接触,提出分别建立不同的碰撞响应力学模型,前者采用动量及动量矩定理建立响应模型,后者通过分析刚体的受力和运动状态建立相应的动力学模型。最后,详细讨论了刚性球体的碰撞仿真,包括它的碰撞测定、碰撞响应模型的建立及求解等,实现了三维空间中刚性球体实时、精确、快速的运动仿真,为以后多刚体大规模仿真奠定基础。     

d-Dimensional parametric models for dynamic animation of deformable objects

This paper introduces an accurate, efficient, and unified engine dedicated to dynamic animation of d-dimensional deformable objects. The objects are modelled as d-dimensional manifolds defined as functional combinations of a mesh of 3D control points, weighted by parametric blending functions. This model ensures that, at each time step, the object shape conforms to its manifold definitions. The object motion is deduced from the control points dynamic animation. In fact, control points should be viewed as the degrees of freedom of the continuous object. The chosen dynamic equations (Lagrangian formalism) reflect this generic modelling scheme and yield an exact and computationally efficient linear system.     

Meshless methods for physics-based modeling and simulation of deformable models

As 3D digital photographic and scanning devices produce higher resolution images, acquired geometric data sets grow more complex in terms of the modeled objects’ size, geometry, and topology. As a consequence, point-sampled geometry is becoming ubiquitous in graphics and geometric information processing, and poses new challenges which have not been fully resolved by the state-of-art graphical techniques. In this paper, we address the challenges by proposing a meshless computational framework for dynamic modeling and simulation of solids and thin-shells represented as point samples. Our meshless framework can directly compute the elastic deformation and fracture propagation for any scanned point geometry, without the need of converting them to polygonal meshes or higher order spline representations. We address the necessary computational techniques, such as Moving Least Squares, Hierarchical Discretization, and Modal Warping, to effectively and efficiently compute the physical simulation in real-time. This meshless computational framework aims to bridge the gap between the point-sampled geometry with physics-based modeling and simulation governed by partial differential equations. Supported by the National Science Foundation (Grant Nos. CCF-0727098, IIS-0710819)     

Graceful Degradation of Collision Handling in Physically Based Animation

Interactive simulation is made possible in many applications by simplifying or culling the finer details that would make real-time performance impossible. This paper examines detail simplification in the specific problem of collision handling for rigid body animation. We present an automated method for calculating consistent collision response at different levels of detail. The mechanism works closely with a system which uses a pre-computed hierarchical volume model for collision detection.