Vector offset operators for deformable organic objects.

Many natural materials and most of living tissues exhibit complex deformable behaviours that may be characteriseda s organic. In computer animation, deformable organic material behaviour is needed for the development of characters and scenes based on living creatures and natural phenomena. This study addresses the problem of deformable organic material behaviour in computer animated objects. The focus of this study is concentrated on problems inherent in geometry based deformation techniques, such as non-intuitive interaction and difficulty in achieving realism. Further, the focus is concentrated on problems inherent in physically based deformation techniques, such as inefficiency and difficulty in enforcing spatial and temporal constraints. The main objective in this study is to find a general and efficient solution to interaction and animation of deformable 3D objects with natural organic material properties and constrainable behaviour. The solution must provide an interaction and animation framework suitable for the creation of animated deformable characters. An implementation of physical organic material properties such as plasticity, elasticity and iscoelasticity can provide the basis for an organic deformation model. An efficient approach to stress and strain control is introduced with a deformation tool named Vector Offset Operator. Stress / strain graphs control the elastoplastic behaviour of the model. Strain creep, stress relaxation and hysteresis graphs control the viscoelastic behaviour of the model. External forces may be applied using motion paths equipped with momentum / time graphs. Finally, spatial and temporal constraints are applied directly on vector operators. The suggested generic deformation tool introduces an intermediate layer between user interaction, deformation, elastoplastic and viscoelastic material behaviour and spatial and temporal constraints. This results in an efficient approach to deformation, frees object representation from deformation, facilitates the application of constraints and enables further development

Improvements to physically based cloth simulation

Physically based cloth simulation in computer graphics has come a long way since the 1980s. Although extensive methods have been developed, physically based cloth animation remains challenging in a number of aspects, including the efficient simulation of complex internal dynamics, better performance and the generation of more effects of friction in collisions, to name but a few. These opportunities motivate the work presented in this thesis to improve on current state of the art in cloth simulation by proposing methods for cloth bending deformation simulation, collision detection and friction in collision response. The structure of the thesis is as follows. A literature review of work related to physically based cloth simulation including aspects of internal dynamics, collision handling and GPU computing for cloth simulation is given in Chapter 2. In order to provide a basis for understanding of the work of the subsequent chapters of the thesis, Chapter 3 describes and discusses main components of our physically based cloth simulation framework which can be seen as the basis of our developments, as methods presented in the following chapters use this framework. Chapter 4 presents an approach that effectively models cloth non-linear features in bending behaviour, such as energy dissipation, plasticity and fatigue weakening. This is achieved by a simple mathematical approximation to an ideal hysteresis loop at a high level, while in textile research bending non-linearity is computed using complex internal friction models at the geometric structure level. Due to cloth flexibility and the large quantity of triangles, in a robust cloth system collision detection is the most time consuming task. The approach proposed in Chapter 5 improves performance of collision detection using a GPU-based approach employing spatial subdivision. It addresses a common issue, uneven triangle sizes, which can easily impair the spatial subdivision efficiency. To achieve this, a virtual subdivision scheme with a uniform grid is used to virtually subdivide large triangles, resulting in a more appropriate cell size and thus a more efficient subdivision. The other common issue that limits the subdivision efficiency is uneven triangle spatial distributions, and is difficult to tackle via uniform grids because areas with different triangle densities may require different cell sizes. In order to address this problem, Chapter 6 shows how to build an octree grid to adaptively partition space according to triangle spatial distribution on a GPU, which delivers further improvements in the performance of collision detection. Friction is an important component in collision response. Frictional effects include phenomena that are velocity dependent, such as stiction, Stribeck friction, viscous friction and the stick-slip phenomenon, which are not modelled by the classic Coulomb friction model adopted by existing cloth systems. Chapter 7 reports a more comprehensive friction model to capture these additional effects. Chapter 8 concludes this thesis and briefly discusses potential avenues for future work

Advanced modelling and design of a tennis ball

Modern tennis has been played for over a hundred years, but despite significant improvements in the design and manufacture of tennis balls to produce a long-lasting and consistent product, the design of a tennis ball has barely changed in the last century. While some work has been done to better understand the dynamic behaviour of a tennis ball, no structured analysis has been reported assessing how the typical constructions of the inner rubber core and cloth panels affect its behaviour and performance. This research describes the development of an advanced and validated finite element (FE) tennis ball model which illustrates the effects of the viscoelastic and anisotropic materials of a tennis ball on ball deformation and bounce during impacts with the ground and the racket,representative of real play conditions. The non-linear strain rate properties exhibited by the materials of a tennis ball during high velocity impacts were characterised using a series of experiments including tensile and compressive tests as well as low and high velocity impact tests. The impacts were recorded using a high speed video (HSV) camera to determine deformation, impact time, coefficient of restitution (COR) and spin rate. The ball material properties were tuned to match the HSV results, and the ball s model parameters were in good agreement with experimental data for both normal and oblique impacts at velocities up to 50 m/s and 35 m/s, respectively. A time sequenced comparison of HSV ball motion and FE model confirmed the accuracy of the model, and showed significant improvement on previous models. Although the existing construction of tennis ball cores was found to provide a sufficiently uniform internal structure to base competition standard tennis balls, the anisotropic nature of the cloth panels resulted in deviation angles as high as 1.5 degrees in ball bounce. Therefore, new cloth panel configurations were modelled which allowed the cloth fibre orientations around the ball to be adjusted resulting in better bounce consistency. The effect of cloth seam length on ball flight was explored through wind tunnel tests performed on solid balls made by additive manufacturing (AM) and on actual pressurised tennis ball prototypes. A reverse Magnus effect was observed on the AM balls, however, this phenomenon was overcome by the rough nature of the cloth cover on the real tennis ball prototypes. A ball trajectory simulation showed that there was no obvious dependence between seam length and shot length or ball velocity. Finally, a basic panel flattening method was used to determine the 2Dsize of the cloth panel patterns corresponding to the new configurations, and tiling methods were designed to estimate cloth wastage. The traditional dumbbell design appeared to result in the minimum amount of waste. The work reported in this thesis represents a significant improvement in the modelling of tennis ball core, cloth and seams, as well as the ball s interaction with the ground and racket strings. While this research focused on woven cloth, needle cloth is also widely used in the manufacture of balls in the US. The modelling of needle cloth could therefore be part of a future study. Additionally, details such as the depth and roughness of the cloth seam could be included in the model to study their effect on spin generation. Also, including cloth anisotropy in the flattening method would allow a better prediction of cloth wastage which could then have an influence on the configuration of the cloth panels

The application of three-dimensional mass-spring structures in the real-time simulation of sheet materials for computer generated imagery

Despite the resources devoted to computer graphics technology over the last 40 years, there is still a need to increase the realism with which flexible materials are simulated. However, to date reported methods are restricted in their application by their use of two-dimensional structures and implicit integration methods that lend themselves to modelling cloth-like sheets but not stiffer, thicker materials in which bending moments play a significant role. This thesis presents a real-time, computationally efficient environment for simulations of sheet materials. The approach described differs from other techniques principally through its novel use of multilayer sheet structures. In addition to more accurately modelling bending moment effects, it also allows the effects of increased temperature within the environment to be simulated. Limitations of this approach include the increased difficulties of calibrating a realistic and stable simulation compared to implicit based methods. A series of experiments are conducted to establish the effectiveness of the technique, evaluating the suitability of different integration methods, sheet structures, and simulation parameters, before conducting a Human Computer Interaction (HCI) based evaluation to establish the effectiveness with which the technique can produce credible simulations. These results are also compared against a system that utilises an established method for sheet simulation and a hybrid solution that combines the use of 3D (i.e. multilayer) lattice structures with the recognised sheet simulation approach. The results suggest that the use of a three-dimensional structure does provide a level of enhanced realism when simulating stiff laminar materials although the best overall results were achieved through the use of the hybrid model

Advanced techniques to improve the design of tennis ball cores

Over the past century tennis balls have seen little development, despite issues with durability and recyclability. Over the same period rackets have seen several development iterations through the use of wood, aluminium and carbon fibre reinforced composites frames. Players physical capabilities have dramatically improved and even line-calling has been automated. In professional tennis, balls are used for as little as nine games before being discarded, whilst recreational players demand a long-lasting product at minimal cost. Balls are comprised of a vulcanised rubber core, which is pressurised, and woven felt covering. Similarities in materials, combined with strict performance limits defined by the International Tennis Federation (ITF) and consumer pressures culminates in a product with low profit margins and little market differentiation. The work presented in this thesis focussed on the elastomeric material used to manufacture the core of tennis balls, presenting a scientific means of assessing alternative ball core materials that could benefit players and brands alike. Ball tracking data collected during professional tournaments, spanning the major court surfaces used in professional tennis, was analysed and used to determine the impact frequency and conditions a ball is subjected to during play. The range of impact conditions determined were replicated in the laboratory and subjected to digital image correlation techniques (GOM Correlate Professional), which were applied to measure the surface strains and strain rates present during impact. The results of which enabled the transformation of typical ball impact conditions in professional tennis into mechanical testing conditions representative of what occurs during impact. Current pressurised and pressureless ball core rubber were subjected to tensile testing, matching as closely as was possible, the strains and strain rates measured during impact. Dynamic mechanical analysis (DMA) was also utilised to characterise the viscoelastic properties of current ball core rubber. The characterisation of current materials provided a benchmark against which alternatives could be compared and enabled the implementation finite element (FE) simulations of ball cores during impact. FE modelling utilised advanced viscoplasticity material models (Bergstrӧm-Boyce model) enabling the viscoelastic and strain rate dependent behaviour of rubber to be incorporated, eradicating the need to artificially tune model coefficients, as seen in previous examples of tennis ball modelling. Having quantified the conditions required for materials characterisation testing and developed a methodology for the simulation of pressurised ball core impacts, alternative materials were identified and assessed. Thermoplastic elastomers (TPEs) were identified as materials with potential for replacing vulcanised rubber. TPEs offered potential improvements to pressure retention properties, recyclability as well as the opportunity to utilise thermoplastic manufacturing processes. When subject to the same materials characterisation testing and FE modelling as ball core materials, TPEs exhibited, in part, comparable properties to ball core rubber, with simulations estimating similar ball core performance for melt processible rubber TPE. The work presented in this thesis implies TPE materials are worthy of further investigation for use as tennis ball cores

Study on 3- Dimension Simulation for Loop Structure of Weft- Knitted Fabric Considering Mechanical Properties of Yarn

13301甲第4561号博士（学術）金沢大学博士論文本文Full 以下に掲載予定：Journal of Fiber Science and Technolog

A study of low force fabric characteristics and vibrational behaviour for automated garment handling

One of the fundamental concepts in automated garment assembly is that the orientation of a fabric panel should never be lost. However, if a panel does become distorted, several techniques, such as vision, air flotation tables, and vibratory conveyors are available to restore the orientation. This thesis has investigated the behaviour of a fabric panel on a vibratory table. Several table parameters such as amplitude of vibration, frequency and angle of inclination, together with some important fabric properties as friction and compressibility are required to understand the behaviour. However, most work on friction in textiles considers fibre-fibre or fabric-fabric friction, which is not appropriate to this and so low force frictional properties between unloaded fabric and engineering surfaces (i.e., aluminium, Formica and rubber) have been studied. The influence of several experimental variables on friction is demonstrated, in particular, the effect of humidity and velocity. Further, an in depth study is made on the stick-slip of fabric panels wherein a novel measuring technique is introduced. An estimate of the damping, which is required to model the fabric, has been obtained from an in-plane vibration test.The second significant fabric property to be studied is the compression both static and impact. Again, only low-force compression tests are carried out since these are the typical forces experienced by fabrics on a vibrating table. The static compressibility of knitted and woven materials is verified with van Wvk's equation. which gives a near indistinguishable fit with the experimental data