Table of Contents
Introduction……………………………………………………………………………………… . 1
1.1 A Brief Historical Background……………. ……………. ……………………… 3
1.2 Problems……….. …………. ………….. ……………. ………………….. ……. 5
1.2.1 Shape and Geometry …………………………………………………. ……………… ….. 5
1.2.2 Behavior ………… …………………….. …………………….. ……………….. ……….. ….. 6
1.2.3 Interaction with Environment ……………………………….. ……………………….. 6
1.2.4 From Cloth to Garment ………………………………………… ……………………….. 7
1.2.5 Rendering …………………………………………………………………………………….. 7
1.3 Garment Design and Simulation System: An Example ……………. ……. 8
2 Simulation Models ……….. …………… ………………………………….. ….. 11
2.1 Introduction ………… ………………… ………… …………………… …………… 11
2.1.1 Dynamics …………………………………………………………………………………….. 11
2.1.2 Designing a Mechanical Simulation System ………….. ………………………… 12
2.2 Mechanical Properties of Fabric Materials………………………………………………….. 15
2.2.1 Fabric Mechanical Parameters………………………………………………………… 15
2.2.2 Experimental Analysis of Fabric Properties ……………………………………… 19
2.3 Implementing Mechanical Models……………. …………………………………… …………. 25
2.3.1 Defining Behavior Laws ………………………………………………………………… 25
2.3.2 Fundamental Laws of Mechanics…………………………………………………….. 28
2.3.3 Defining a Simulation Scheme ……………………………………………………….. 30
2.4 Mechanical Simulation Systems ……………………………………………………………….. 35
2.4.1 A Good Simulation System…………………………………………………………….. 35
2.4.2 Geometrical Models ………………………………………………………………………. 40
2.4.3 Continuum Mechanics Models ………. ………………………….. ………………….. 42
2.4.4 Particle System Models………………………………………………………………….. 52
2.4.5 A Fast Particle System for Irregular Meshes …………………………………….. 60
2.5 Numerical Integration …………………. ……………………………… …. …. 72
2.5.1 Integration Techniques …………………………………………………………………… 73
2.5.2 Choosing the Suitable Integration Method………………………………………… 98
3 Collision Detection……………. ………. …. ……………………………………… 103
3.1 The Collision Detection Problem ………………………………………………………………. 103
3.1.1 Introduction ………………………………………………………………………………….. 103
3.1.2 Mastering Complexity……………………………………………………………………. 106
3.1.3 An Overview of Different Techniques ……….. ………… …. …………….. ….. ….. 108
3.1.4 Robustness …………………………………………….. ………………………. ……………. 126
3.2 A Hierarchical Scheme for Polygonal Meshes …………………………………………….. 128
3.2.1 Collision Detection on Hierarchical Meshes ……………………………………… 128
3.2.2 Optimizing for Self-Collision Detection …………………………………………… 132
3.2.3 Efficiency …………………………………………………………………………………….. 142
4 Collision Response …………………………………………………. …. …………. … 145
4.1 Characterizing Collisions Geometrically…………. …………………. ……. …………….. … 145
4.1.1 Intersections and Proximities … …….. ………………… …… ………………………… 146
4.1.2 Collisions and Surface Orientation…………………………………………………… 150
4.2 Implementing Collision Response ……………………………………………………………… 155
4.2.l Collision Response on Polygonal Meshes …………………………………………. 156
4.2.2 Collision Models …………………. …… …… …….. ……………….. … …………………. 160
4.3 Constraints & Seaming …………………………………………………………………………….. 174
4.3.1 Elastics to Bring Objects Together …………………………………………………… 175
4.3.2 Controlling the Elastic Effect ………………………………………………………….. 176
4.3.3 Applications ………………………………………………………………………………….. 182
5 Smoothing & Wrinkles…………………………………………………………………………………….. 185
5.1 Multilayer Models……………………………. ………………………….. …. ………… …………… 185
5.2 A Simple Geometrical Interpolation Algorithm …………………………………………… 187
5.2.1 The Problem …………………………………………………………………………………. 187
5.2.2 Constructing the Surface ………………………………………………………………… 188
5.2.3 Results …………………………………………………………………………………………. 193
5.2.4 Texture as a Height Field ……………………………………………………………….. 195
5.2.5 Modulating Wrinkle Amplitude ………………………………………………………. 196
5.2.6 Multilayer Wrinkle Textures …………………………………………………………… 199
5.2.7 Rendering Wrinkles ………………………………………………………………………. 202
5.2.8 Applications …………………………………………………………………………………. 203
6 Rendering Garments……………………………………………………………….. 207
6.1 Rendering Techniques ……… …. …………… ………………………. ……… 207
6.1.1 Visualization Principles ……………. ……………… ………. ……… …….. …………… 207
6.1.2 Rendering Systems………………………………………………………………………… 220
6.2 Rendering Textiles ………………………………………………………………………………….. 224
6.2.1 Anisotropic Lighting of Textiles ……………………………………………………… 224
6.2.2 Volumetric Textile Models …………… …….. …… ………… ……. ………………….. 226
6.2.3 Rendering Choices for Realistic Garments ……………… ………… …………….. 227
7 The MIRACloth Software………… …… ……………….. …………………. ………. 231
7.1 Introduction……………………………………………………………………………………………. 231
7.2 Approach……………………………………………………………………………………………….. 234
7.2.1 Design of Garment Patterns …… ………………………. …………. ….. …….. ………. 234
7.2.2 Putting Patterns on Bodies ……………………………………………………………… 235
7.2.3 Seaming and Constructing Garments ……………………………………………….. 237
7.2.4 Animation of Garments ………. …… …… ……………………. …………….. …………. 239
7.2.5 Defining the Garment Materials and Textures …………………………………… 242
7.2.6 Cutting and Modifications ……………………………………………………………… 242
7.3 Software Description ……………………………………………………………………………….. 243
7.3.1 Program Features ………………………………………………………………………….. 243
7.3.2 Interface Description……………………………………………………………………… 245
7.3.3 V.R. Manipulation Tools ……………………………………………………………….. 248
7.4 MIRACloth at Work ………………………………………………………………………………… 249
7.4.1 Versatile Fabric Simulation ……………………………………………………………. 250
7.4.2 Computer Films and Fashion Shows ……………………………………………….. 253
7.4.3 Model Design ……………………………………………………………………………….. 256
7.4.4 Garment Prototyping ……………………………………………………………………… 257
8 Potential Applications……………………………………………………………… 261
Bibliography…………………….. …………………………… …… …………… …. …………. 263
1 Introduction
In this book, we investigate the problem of simulating clothes and clothing. A range of topics are addressed, from shape modeling of a piece of cloth to the realistic garments on virtual humans. Different situations demand different properties a cloth. Existing solutions, though useful for many applications, reveal that further improvements are required.
Cloth modeling has been a topic of research in the textile mechanics and engineering communities for a very long time. However, in the mid 1980s, researchers in computer graphics also became interested in modeling cloth in order to include it in the 3D computergenerated images and films. The evolution of cloth modeling and garment simulation in computer graphics indicates that it has grown from basic shape modeling to the modeling of its complex physics and behaviors. Chapter 2 provides a summary of the different methods developed in computer graphics over the last 15 to 20 years. In computer graphics, only the macroscopic properties of the cloth surface are considered. Physical accuracy is given less importance in comparison to the visual realism. However, a trend of employing a multidisciplinary approach has started, and the community of textile engineering and computer graphics have begun to combine their expertise to come up with solutions that can satisfy that of both communities. While the textile engineering offers precise details of modeling cloth, at a microscopic level, the computer graphics provides the framework for animation and visualization.
Since it is difficult to cover all the methods with adequate details in the book, additional details of the modeling of cloth and garment simulation have been included for the MlRACloth system as a case study. The current techniques in computer graphics enable the simulation of a piece of cloth, as well as a complete set of garments with the interaction with their environment.
The different schemes of collision detection are presented in Chapter 3. Collision detection and response is an integral part of cloth simulation. Collision determines the contact of the cloth with its environment, as in the case of a garment in contact with the body. Thus, collision detection is an extremely important aspect of cloth simulation, taking into account the cloth’s interaction with other objects. Collision response determines the feedback to the collision, which may depend on friction and other physical phenomena. Collision detection is conceived as geometrical processing in order to find potential interpenetration of the regions or individual elements of the corresponding surfaces. The methods to optimize the processes for both collision with other objects and self-collision are also studied. An efficient algorithm for collision detection for polygonal mesh representing cloth surface, is detailed with special attention to the issues relating to self-collision which is the employing of a curvature criteria to help resolve the problem of “false adjacency”. For collision response, a hybrid model is described where the geometrical basis (e.g., proximity) is used to determine the nature of the response. The notion of friction is added without affecting the efficiency of the mechanical simulation.
Seaming is another kind of constraint, which is required during mechanical simulation to construct a complete garment from the 20 patterns. The notion of “elastic” or “elastic lines” is used to join the points of the pattern boundaries collectively or individually. It is like putting some attraction forces between the boundaries to be joined or stitched.
For rendering clothes, many of the simulation models use classical rendering packages or modules. Not much research has been undertaken exclusively on the rendering of materials like cloth. We briefly present some conventional rendering methods, which are commonly used in cloth simulations. Some particular methods for rendering anisotropic reflections, the microscopic structures and weaving styles are also discussed. This book gives a limited coverage of the rendering within its present scope. However, we believe that the rendering of cloth requires special attention.
In order to further enhance the realism of cloth modeling, finer wrinkles are added. These wrinkles are not computed as the surface deformation based on physics, but instead these are created using the prescribed perturbation to the geometrical information – surface normal. In this way, we avoid the overheads on both the computation and the data.
A separate chapter is devoted to MIRACloth, a system developed at MIRALab, University of Geneva, for building and animating the garments on virtual actors. In fact, it is a general animation framework where different types of animation can be associated with the different objects – static, rigid, and deformable (key frame, mechanical simulation). The methodology for building garments relies on the traditional garment design in real-life. The 20 patterns are created through a polygon editor, which are then taken to the 3D simulator and placed around the body of a virtual actor. The process of seaming brings the patterns together. The garment can be animated with a moving virtual actor. Many applications are shown: the production of computer generated films, web based production suitable for E-commerce, and computer aided garments design.