Visualisation of Origins, Destinations and Flows with OD Maps

We present a new technique for the visual exploration of origins (O) and destinations (D) arranged in geographic space. Previous attempts to map the flows between origins and destinations have suffered from problems of occlusion usually requiring some form of generalisation, such as aggregation or flow density estimation before they can be visualized. This can lead to loss of detail or the introduction of arbitrary artefacts in the visual representation. Here, we propose mapping OD vectors as cells rather than lines, comparable with the process of constructing OD matrices, but unlike the OD matrix, we preserve the spatial layout of all origin and destination locations by constructing a gridded two‐level spatial treemap. The result is a set of spatially ordered small multiples upon which any arbitrary geographic data may be projected. Using a hash grid spatial data structure, we explore the characteristics of the technique through a software prototype that allows interactive query and visualisation of 105‐106 simulated and recorded OD vectors. The technique is illustrated using US county to county migration and commuting statistics

Fast Collision Culling in Large-Scale Environments Using GPU Mapping Function

International audienceThis paper presents a novel and efficient GPU-based parallel algorithm to cull non-colliding object pairs in very large-scale dynamic simulations. It allows to cull objects in less than 25ms with more than 100K objects. It is designed for many-core GPU and fully exploits multi-threaded capabilities and data-parallelism. In order to take advantage of the high number of cores, a new mapping function is defined that enables GPU threads to determine the objects pair to compute without any global memory access. These new optimized GPU kernel functions use the thread indexes and turn them into a unique pair of objects to test. A square root approximation technique is used based on Newton's estimation, enabling the threads to only perform a few atomic operations. A first characterization of the approximation errors is presented, enabling the fixing of incorrect computations. The I/O GPU streams are optimized using binary masks. The implementation and evaluation is made on largescale dynamic rigid body simulations. The increase in speed is highlighted over other recently proposed CPU and GPU-based techniques. The comparison shows that our system is, in most cases, faster than previous approaches

Optimal parameters for a hierarchical grid data structure for contact detection in arbitrarily polydisperse particle systems.

The objective of this paper is to find the optimum number of hierarchy levels and their cell sizes for contact detection algorithms based on a versatile hierarchical grid data structure, for polydisperse particle systems with arbitrary distribution of particle radii. These algorithms perform as fast as O(N) for N particles, but the prefactor can be as large as N for a given system, depending on the algorithm parameters chosen, making a recipe for choosing these parameters necessary. We estimate theoretically the calculation time of two distinct algorithms for particle systems with various packing fractions, where the sizes of the particles are modelled by an arbitrary probability density function. We suggest several methods for choosing the number of hierarchy levels and the respective cell sizes, based on truncated power-law radii distributions with different exponents and widths. The theoretical estimations are then compared with simulation results for particle systems with up to one million particles. The proposed recipe for selecting the optimal hierarchical grid parameters allows to find contacts in arbitrarily polydisperse particle systems as fast as the commonly-used linked-cell method in purely monodisperse particle systems, i.e., extra work is avoided in presence of polydispersity. Furthermore, the contact detection time per particle even decreases slightly with increasing polydispersity or decreasing particle packing fraction

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

Interactive freeform editing techniques for large-scale, multiresolution level set models

Level set methods provide a volumetric implicit surface representation with automatic smooth blending properties and no self-intersections. They can handle arbitrary topology changes easily, and the volumetric implicit representation does not require the surface to be re-adjusted after extreme deformations. Even though they have found some use in movie productions and some medical applications, level set models are not highly utilized in either special effects industry or medical science. Lack of interactive modeling tools makes working with level set models difficult for people in these application areas.This dissertation describes techniques and algorithms for interactive freeform editing of large-scale, multiresolution level set models. Algorithms are developed to map intuitive user interactions into level set speed functions producing specific, desired surface movements. Data structures for efficient representation of very high resolution volume datasets and associated algorithms for rapid access and processing of the information within the data structures are explained. A hierarchical, multiresolution representation of level set models that allows for rapid decomposition and reconstruction of the complete full-resolution model is created for an editing framework that allows level-of-detail editing. We have developed a framework that identifies surface details prior to editing and introduces them back afterwards. Combining these two features provides a detail-preserving level set editing capability that may be used for multi-resolution modeling and texture transfer. Given the complex data structures that are required to represent large-scale, multiresolution level set models and the compute-intensive numerical methods to evaluate them, optimization techniques and algorithms have been developed to evaluate and display the dynamic isosurface embedded in the volumetric data.Ph.D., Computer Science -- Drexel University, 201