Crack‐free rendering of dynamically tesselated B‐rep models

We propose a versatile pipeline to render B‐Rep models interactively, precisely and without rendering‐related artifacts such as cracks. Our rendering method is based on dynamic surface evaluation using both tesselation and ray‐casting, and direct GPU surface trimming. An initial rendering of the scene is performed using dynamic tesselation. The algorithm we propose reliably detects then fills up cracks in the rendered image. Crack detection works in image space, using depth information, while crack‐filling is either achieved in image space using a simple classification process, or performed in object space through selective ray‐casting. The crack filling method can be dynamically changed at runtime. Our image space crack filling approach has a limited runtime cost and enables high quality, real‐time navigation. Our higher quality, object space approach results in a rendering of similar quality than full‐scene ray‐casting, but is 2 to 6 times faster, can be used during navigation and provides accurate, reliable rendering. Integration of our work with existing tesselation‐based rendering engines is straightforward.     

Pearling: Stroke segmentation with crusted pearl strings

We introduce a novel segmentation technique, called Pearling, for the semi-automatic extraction of idealized models of networks of strokes (variable width curves) in images. These networks may for example represent roads in an aerial photograph, vessels in a medical scan, or strokes in a drawing. The operator seeds the process by selecting representative areas of good (stroke interior) and bad colors. Then, the operator may either provide a rough trace through a particular path in the stroke graph or simply pick a starting point (seed) on a stroke and a direction of growth. Pearling computes in realtime the centerlines of the strokes, the bifurcations, and the thickness function along each stroke, hence producing a purified medial axis transform of a desired portion of the stroke graph. No prior segmentation or thresholding is required. Simple gestures may be used to trim or extend the selection or to add branches. The realtime performance and reliability of Pearling results from a novel disk-sampling approach, which traces the strokes by optimizing the positions and radii of a discrete series of disks (pearls) along the stroke. A continuous model is defined through subdivision. By design, the idealized pearl string model is slightly wider than necessary to ensure that it contains the stroke boundary. A narrower core model that fits inside the stroke is computed simultaneously. The difference between the pearl string and its core contains the boundary of the stroke and may be used to capture, compress, visualize, or analyze the raw image data along the stroke boundary.     

Local and global analysis of parametric solid sweeps

In this work, we propose a structured computational framework for modelling the envelope of the swept volume, that is the boundary of the volume obtained by sweeping an input solid along a trajectory of rigid motions. Our framework is adapted to the well-established industry-standard brep format to enable its implementation in modern CAD systems. This is achieved via a “local analysis”, which covers parametrizations and singularities, as well as a “global theory” which tackles face-boundaries, self-intersections and trim curves. Central to the local analysis is the “funnel” which serves as a natural parameter space for the basic surfaces constituting the sweep. The trimming problem is reduced to the problem of surface–surface intersections of these basic surfaces. Based on the complexity of these intersections, we introduce a novel classification of sweeps as decomposable and non-decomposable. Further, we construct an invariant function θ on the funnel which efficiently separates decomposable and non-decomposable sweeps. Through a geometric theorem we also show intimate connections between θ, local curvatures and the inverse trajectory used in earlier works as an approach towards trimming. In contrast to the inverse trajectory approach of testing points, θ is a computationally robust global function. It is the key to a complete structural understanding, and an efficient computation of both, the singular locus and the trim curves, which are central to a stable implementation. Several illustrative outputs of a pilot implementation are included.     

Correct resolution rendering of trimmed spline surfaces

Current strategies for real-time rendering of trimmed spline surfaces re-approximate the data, pre-process extensively or introduce visual artifacts. This paper presents a new approach to rendering trimmed spline surfaces that guarantees visual accuracy efficiently, even under interactive adjustment of trim curves and spline surfaces. The technique achieves robustness and speed by discretizing at a near-minimal correct resolution based on a tight, low-cost estimate of adaptive domain griding. The algorithm is highly parallel, with each trim curve writing itself into a slim lookup table. Each surface fragment then makes its trim decision robustly by comparing its parameters against the sorted table entries. Adding the table-and-test to the rendering pass of a modern graphics pipeline achieves anti-aliased sub-pixel accuracy at high render-speed, while using little additional memory and fragment shader effort, even during interactive trim manipulation.     

A Layered Particle‐Based Fluid Model for Real‐Time Rendering of Water

We present a physically based real‐time water simulation and rendering method that brings volumetric foam to the real‐time domain, significantly increasing the realism of dynamic fluids. We do this by combining a particle‐based fluid model that is capable of accounting for the formation of foam with a layered rendering approach that is able to account for the volumetric properties of water and foam. Foam formation is simulated through Weber number thresholding. For rendering, we approximate the resulting water and foam volumes by storing their respective boundary surfaces in depth maps. This allows us to calculate the attenuation of light rays that pass through these volumes very efficiently. We also introduce an adaptive curvature flow filter that produces consistent fluid surfaces from particles independent of the viewing distance.     

基于 OSG 的漫游系统的设计与实现

虚拟现实技术的提出,凭借真实的沉浸感、可交互性以及可构想性等这些优势,轻而易举地把现实世界通过计算机数字化中呈现出来。在城市建设、室内设计、工业设计、道路桥梁规划、旅游教学、房地产、古迹恢复、电力水利、地质灾害、虚拟校园等各个领域广泛应用虚拟现实技术,并对相应领域的发展产生了极大的帮助和推动作用。基于OSG的漫游系统正是基于此技术上的一项应用,该系统具备模型加载,点、线、面渲染,导航,漫游,雨、雪、雾特效,等功能。最终目标是希望实时呈现了一个逼真的、立体化的,能给人视觉、听觉、触觉感受的虚拟漫游系统。该漫游系统的实现有利于景区旅游资源充分开发利用,对广西师范大学建设规划具有重要的现实意义。同时,也是弘扬和传播广西师范大学校园文化的重要途径。     

Filling trim cracks on GPU-rendered solid models

We present an algorithm for improving the rendering appearance of CAD models with trimmed freeform surfaces when evaluated on graphics processing units (GPUs). Rendering on client GPUs allows mechanical CAD to embrace cloud computing by storing a single auto-synchronized model file in the cloud and transferring only minimal data (control points, trim curves, etc.) to the client nodes for local evaluation/rendering. However, current parallel algorithms that directly evaluate and render trimmed surfaces by masking the trims, without tessellating along the trim curves, suffer from “cracks” along the trim boundaries. We have developed a hybrid CPU–GPU algorithm to remove these artifacts in the rendering stage for a smooth, color- and shading-matched appearance. After dynamically detecting the cracks, our algorithm selectively fills in the affected pixels using a GPU fragment program, while avoiding artifacts at silhouettes. We have implemented this algorithm to demonstrate improvements in the appearance of solid models directly evaluated and rendered on the GPU.     

Efficient surface reconstruction method for distributed CAD

This paper describes a new fast Reverse Engineering (RE) method for creating a 3D computerized model from an unorganized cloud of points. The proposed method is derived directly from the problems and difficulties currently associated with remote design over the Internet, such as accuracy, transmission time and representation at different levels of abstraction. With the proposed method, 3D models suitable for distributed design systems can be reconstructed in real time. The mesh reconstruction approach is based on aggregating very large scale 3D scanned data into a Hierarchical Space Decomposition Model (HSDM), realized by the Octree data structure. Then, a Connectivity Graph (CG) is extracted and filled with facets. The HSDM can represent both the boundary surface and the interior volume of an object. Based on the proposed volumetric model, the surface reconstruction process becomes robust and stable with respect to sampling noise. Moreover, the data received from different surface/volume sampling devices can be handled naturally. The hierarchical structure of the proposed volumetric model enables data reduction, while preserving significant geometrical features and object topology. As a result, reconstruction and transmission over the network are efficient. Furthermore, the hierarchical representation provides a capability for extracting models at desired levels of detail, thus enabling designers to collaborate at any product development stage: draft or detailed design.     

Adaptive generation of hexahedral element mesh using an improved grid-based method

An improved grid-based algorithm for the adaptive generation of hexahedral finite element mesh is presented in this paper. It is named as the inside-out grid-based method and involves the following four steps. The first step is the generation of an initial grid structure which envelopes the analyzed solid model completely. And the elements size and density maps are constructed based on the surface curvature and local thickness of the solid model. Secondly, the core mesh is generated through removing all the undesired elements using even and odd parity rules. The third step is to magnify the core mesh in an inside-out manner through a surface node projection process using the closest position approach. To match the mesh to the characteristic boundary of the solid model, a minimal Scaled Jacobian criterion is employed. Finally, in order to handle the degenerated elements and improve the quality of the resulting mesh, two comprehensive techniques are employed: the insertion technique and collapsing technique. The present method was applied in the mesh construction of different engineering problems. Scaled Jacobian and Skew metrics are used to evaluate the hexahedral element mesh quality. The application results show that all-hexahedral element meshes which are well-shaped and capture all the geometric features of the original solid models can be generated using the inside-out grid-based method presented in this paper.     

Finite-horizon dynamic optimization of nonlinear systems in real time

A novel scheme for constructing and tracking the solution trajectories to regular, finite-horizon, deterministic optimal control problems with nonlinear dynamics is devised. The optimal control is obtained from the states and costates of Hamiltonian ODEs, integrated online. In the one-dimensional case the initial costate is found by successively solving two first-order, quasi-linear, partial differential equations, whose independent variables are the time-horizon duration T and the final penalty coefficient S. These PDEs should in general be integrated off-line, the solution rendering not only the missing initial condition sought in the particular (T,S)-situation, but additional information on the boundary values of the whole two-parameter family of control problems, which can be used for designing the definitive objective functional. Optimal trajectories for the model are then generated in real time and used as references to be followed by the physical system.  Numerical improvements are suggested for accurate integration of naturally unstable Hamiltonian dynamics, and strategies are proposed for tracking their results, in finite time or asymptotically, when perturbations in the state of the system appear. The whole procedure is tested in models arising in aero-navigation optimization.     

Parallel‐optimizing SPH fluid simulation for realistic VR environments

In virtual environments, real‐time simulation and rendering of dynamic fluids have always been the pursuit for virtual reality research. In this paper, we present a real‐time framework for realistic fluid simulation and rendering on graphics processing unit. Because of the high demand for interactive fluids with larger particle set, the computational need is becoming higher. The proposed framework can effectively reduce the computational burden through avoiding the computation in inactive areas, where many particles with similar properties and low local pressure cluster together. While in active areas, the computation is fully carried out; thus, the fluid dynamics are largely preserved. Here, a robust particle classification technique is introduced to classify particles into either active or inactive. The test results have shown that the technique improves the time performance of fluid simulation largely. We then incorporate parallel surface reconstruction technique using marching cubes to extract the surfaces of the fluid. The introduced histogram pyramid‐based marching cubes technique is fast and memory efficiency. As a result, we are able to produce plausible and interactive fluids with the proposed framework for large‐scale virtual environments. Copyright © 2013 John Wiley & Sons, Ltd.     

A virtual test facility for the efficient simulation of solid material response under strong shock and detonation wave loading

A virtual test facility (VTF) for studying the three-dimensional dynamic response of solid materials subject to strong shock and detonation waves has been constructed as part of the research program of the Center for Simulating the Dynamic Response of Materials at the California Institute of Technology. The compressible fluid flow is simulated with a Cartesian finite volume method and treating the solid as an embedded moving body, while a Lagrangian finite element scheme is employed to describe the structural response to the hydrodynamic pressure loading. A temporal splitting method is applied to update the position and velocity of the boundary between time steps. The boundary is represented implicitly in the fluid solver with a level set function that is constructed on-the-fly from the unstructured solid surface mesh. Block-structured mesh adaptation with time step refinement in the fluid allows for the efficient consideration of disparate fluid and solid time scales. We detail the design of the employed object-oriented mesh refinement framework AMROC and outline its effective extension for fluid–structure interaction problems. Further, we describe the parallelization of the most important algorithmic components for distributed memory machines and discuss the applied partitioning strategies. As computational examples for typical VTF applications, we present the dynamic deformation of a tantalum cylinder due to the detonation of an interior solid explosive and the impact of an explosion-induced shock wave on a multi-material soft tissue body.     

Multiple tool path planning for NC machining of convex pockets without islands

This work focuses on machining of pockets with convex angles and containing no islands. The approach is a hybrid contouring – staircasing one. Contouring aims at clearing enough space around the boundary to allow efficient staircasing in the interior. Three tools are foreseen: two for contouring and one for staircasing. The algorithm for contouring is based on creating offsets of the pocket boundary and checking them for self-crossing. Staircasing is based on parametrisation of the tool path with respect to its orientation to the pocket, so as to minimise the tool path length. Machining strategy is complemented by choosing the best combination of diameters of the three tools used. This is simply done by first enumerating all available combinations and then excluding the non-feasible ones, in order to compare the rest according to tool path length (or to machining time when individual feed values are known). The output of the program, which was implemented in Fortran, is the tool path, the CNC part program, which is created automatically, and a numerical comparison of all the tool-angle combinations tried out. A machining simulation based on the CNC program output is conducted on commercial CAM software to demonstrate validity of the result.     

基于球面全景图的虚拟场景实时漫游系统

设计并实现一种基于球面全景图的虚拟场景实时漫游系统。虚拟场景的实时漫游可以通过对球面全景图进行重投影完成。通过重投影球面全景图的可视部分到视平面上,可以生成虚拟场景在不同视线方向上的透视视图。使用球面全景图的重投影算法可以模拟相机的旋转运动,通过改变相机的视域,可以模拟相机的变焦运动。针对直接使用重投影不能满足球面全景图实时绘制的问题,在对球面全景图的重投影算法进行仔细分析的基础上,提出使用查找表和增量计算进行加速的策略。实验结果表明,优化后的系统能够对基于球面全景图的虚拟场景进行实时漫游     

实时三维图形平台BH_GRAPH

BH_GRAPH是一个面向视景仿真类应用系统开发人员、支持实时三维图形开发与运行的基础软件平台．它提供可扩展的软件体系结构、标准化的场景管理机制、高效率的场景处理方法、方便易用的应用程序接口，为三维图形应用系统的快速开发、高效运行提供完整的技术支撑．BH_GRAPH主要由三维视景绘制引擎、三维对象建模工具、三维场景布置工具以及一系列关键技术构成．概要介绍了BH—GRAPH各主要组成部分的软件结构、基本功能和技术特点．     

Interactive Rendering of Interior Scenes with Dynamic Environment Illumination

A rendering system for interior scenes is proposed in this paper. The light reaches the interior scene, usually through small regions, such as windows or abat‐jours, which we call portals. To provide a solution, suitable for rendering interior scenes with portals, we extend the traditional precomputed radiance transfer approaches. In our approach, a bounding sphere, which we call a shell, of the interior, centered at each portal, is created and the light transferred from the shell towards the interior through the portal is precomputed. Each shell acts as an environment light source and its intensity distribution is determined by rendering images of the scene, viewed from the center of the shell. By updating the intensity distribution of the shell at each frame, we are able to handle dynamic objects outside the shells. The material of the portals can also be modified at run time (e.g. changing from transparent glass to frosted glass). Several applications are shown, including the illumination of a cathedral, lit by skylight at different times of a day, and a car, running in a town, at interactive frame rates, with a dynamic viewpoint.     

A framework for quad/triangle subdivision surface fitting: Application to mechanical objects

In this paper we present a new framework for subdivision surface approximation of three‐dimensional models represented by polygonal meshes. Our approach, particularly suited for mechanical or Computer Aided Design (CAD) parts, produces a mixed quadrangle‐triangle control mesh, optimized in terms of face and vertex numbers while remaining independent of the connectivity of the input mesh. Our algorithm begins with a decomposition of the object into surface patches. The main idea is to approximate the region boundaries first and then the interior data. Thus, for each patch, a first step approximates the boundaries with subdivision curves (associated with control polygons) and creates an initial subdivision surface by linking the boundary control points with respect to the lines of curvature of the target surface. Then, a second step optimizes the initial subdivision surface by iteratively moving control points and enriching regions according to the error distribution. The final control mesh defining the whole model is then created assembling every local subdivision control meshes. This control polyhedron is much more compact than the original mesh and visually represents the same shape after several subdivision steps, hence it is particularly suitable for compression and visualization tasks. Experiments conducted on several mechanical models have proven the coherency and the efficiency of our algorithm, compared with existing methods.     

Regional Classes of Sea Ice Cover in the East Antarctic Pack Observed from Satellite and In Situ Data during a Winter Time Period

Ice concentration data alone are often of limited use in many process, and modeling studies as different ice règimes of approximately 100% concentration can have significantly different heat flux, albedo, and other surface properties. Current ice concentration algorithms perform poorly in regions of predominantly thin or highly fragmented ice, which constitute a significant proportion of the pack in East Antarctica. The impact of the sea ice cover on high latitude air–sea interactions and marine ecology depends not only on ice extent and concentration but also on the ice-type composition of the pack. An unsupervised ice classification scheme, using data from four channels of the SSM/I, is presented and tested as a means of gaining important additional, complementary information on surface type. Class interpretation is by comparison with AVHRR, ERS-1 SAR, and near-coincident digital aerial photography and in situ data. The classification does a reasonable job at consistently differentiating the large-scale constituent règimes, including the outer marginal ice zone, the interior pack, and a transition zone separating the two. Given the short period of data analyzed, the cluster maps appear to be generally coherent and consistent through time as the pack changes in response to synoptic-scale atmospheric forcing, although the robustness of the technique needs further testing over longer time periods. An observed crossover in the meridional brightness temperature profiles is a dominant and consistent feature which marks the transition from unconsolidated and wet ice in the marginal ice zone to more consolidated ice with a thicker and drier snow cover in the interior pack. Ambiguities occur at the boundaries of some of these règimes due to sensor resolution limitations and the mixing of different ice types and open water. Also, some ice classes (like brash ice) cannot be distinguished from forming pancake ice. Furthermore, although the signature of the inner pack is usually distinct from that of the outer pack, we observed one extraordinary swell propagation event which led to ice fracturing and surface wetting, and significantly altered the surface classification. The results of this multiparameter study underline the importance of using multisensor systems synergistically to improve interpretation of passive microwave data and better characterize the complex Antarctic pack.     

基于交线存储的快速实体边界生成算法

随着ＣＡＤ技术的日益普及，越来越复杂的设计对象进入系统，这就不可避免地导致了系统时空优化问题。目前实体造型中的主要表示方法，即边界模型及构造模型，由于各自特写的结构，无法较好地满足实际工程的需要。本文通过分析实体边界构造过程，提出了一个基个交线存储的实体存储方式及边界重构算法，该方法具有实体数据存储空间小，边界生成速度快等特点，较好地解决了系统存在的时空优化问题。