Cloth Modeling And Animation

Cloth modeling and simulation based on physical principles is widely used in many applications, such as computer animation, film industry, computer game, virtual reality, garment industry, and so on. Although cloth animation has achieved important progresses in past 20 years, it is yet one of the most important challenging problems in computer graphics community. Efficiency, stability, quality and reality are the main issues to be considered in cloth simulation. Compared with other researches for simulating general elastic objects, various problems will arise when we try to solve above issues.Cloth modeling using continuum mechanics has suffered serious difficulties and problems because it involves solving complex mechanical problems such as large displacements, large rotations and large strains. Moreover, cloth has complex micro-interweaved structures compared with continuum materials and this leads to significantly different mechanical behavior. Furthermore, simulating thin flexible materials such as cloth has to solve stiff problems with dynamic systems, which makes the numerical solution of the governing equation hard to maintain stability. In addition, interactive cloth simulation needs to consider complex interactions between cloth and other objects or self interactions, such as constraint, collision, friction and so on, as they have great influences on the efficiency and reality of cloth animation systems. Until now, it is still a difficult problem to produce fast, stable and realistic cloth animation.This thesis focuses on some key techniques on improving the performance and reality for cloth animation, and presents some practical solutions. The main contributions are listed in the followings.An efficient and stable method for real time cloth animation is proposed. In our method, the large sparse linear system is transformed into several smaller equations using the features of our mass-spring system, and they are then solved by an efficient and fast iterative scheme. When employing this scheme to solve our simplified and improved cloth model, a near linear time complexity is achieved.An efficient and stable solution to simulate cloth undergoing large rotations is proposed. Forced vibrations are introduced into the cloth system by the widely used semi-implicit method, where the vibrator is the mass-spring system itself, and the vibration source is the neglected nonlinear term. The vibration will bring instability, inaccuracy and inefficiency when simulating cloth simulations undergoing large rotations. We solve this problem by eliminating the vibration source and avoiding being the vibrator.One important characteristic of fabric different from other soft sheet materials is its discontinuous micro-structure, and this greatly affects the behavior of fabrics. However, it has been neglected in previous cloth modeling. We analyze the rotation deformation between yarns under the shear loadings, and develop a new physical model considering the micro-interweaved-structure in woven fabric and provide its implementation on spring-mass systems, which can model the in-plane deformation and out-plane shearing buckling more accurately.We decouple the buckling deformation into two different types: shearing buckling and structural buckling, where the previous one is usually arisen by stretching and the latter is mainly produced by compression. The structural buckling is the most important factor to affect the generation and vanishing of wrinkles and folds in cloth simulation. A new dynamic bending model is derived from the thin-shell theory to describe the structural buckling. An implementation of the dynamic bending model by the dynamic stiffness method has been developed on the widely used spring-mass systems. The animations generated by our technique are with wrinkles and folds appear and vanish in a more natural way than other approaches.