Getting Rid of Links in Hierarchical Radiosity

Hierarchical radiosity with clustering has positioned itself as one of the most efficient algorithms for computing global illumination in non-trivial environments. However, using hierarchical radiosity for complex scenes is still problematic due to the necessity of storing a large number of transport coefficients between surfaces in the form of links. In this paper, we eliminate the need for storage of links through the use of a modified shooting method for solving the radiosity equation. By distributing only unshot radiosity in each step of the iteration, the number of links decreases exponentially. Recomputing these links instead of storing them increases computation time, but reduces memory consumption dramatically. Caching may be used to reduce the time overhead. We analyze the error behavior of the new algorithm in comparison with the normal gathering approach for hierarchical radiosity. In particular, we consider the relation between the global error of a hierarchical radiosity solution and the local error threshold for each link.     

A Vector Approach for Global Illumination in Ray Tracing

This paper presents a method taking global illumination into account in a ray tracing environment. A vector approach is introduced, which allows to deal with all the types of light paths and the directional properties of materials. Three types of vectors are defined: Direct Light Vectors associated to light sources, Indirect Light Vectors which correspond to light having been diffusely reflected at least once and Caustic Light Vectors which are associated to light rays emitted by sources and reflected and/or transmitted by specular surfaces. These vectors are estimated at a small number of points in the scene. A weighted interpolation between known values allows to reconstruct these vectors for the other points, with the help of a gradient computation for the indirect component. This approach also allows to take uniform area light sources (spherical, rectangular and circular) into account for all the types of vectors. Computed images are thus more accurate and no discretizing of the geometry of the scene is needed.     

Multiresolution Isosurface Extraction with Adaptive Skeleton Climbing

An isosurface extraction algorithm which can directly generate multiresolution isosurfaces from volume data is introduced. It generates low resolution isosurfaces, with 4 to 25 times fewer triangles than that generated by marching cubes algorithm, in comparable running times. By climbing from vertices (0-skeleton) to edges (1-skeleton) to faces (2-skeleton), the algorithm constructs boxes which adapt to the geometry of the true isosurface. Unlike previous adaptive marching cubes algorithms, the algorithm does not suffer from the gap-filling problem. Although the triangles in the meshes may not be optimally reduced, it is much faster than postprocessing triangle reduction algorithms. Hence the coarse meshes it produces can be used as the initial starts for the mesh optimization, if mesh optimality is the main concern.     

A Two-Pass Hardware-Based Method for Hierarchical Radiosity

Finite elements methods for radiosity are aimed at computing global illumination solutions efficiently. However these methods are not suitable for obtaining high quality images due to the lack of error control. Two-pass methods allow to achieve that level of quality computing illumination at each pixel and thus introducing a high computing overhead. We present a two-pass method for radiosity that allows to produce high quality images avoiding most of the per-pixel computations. The method computes a coarse hierarchical radiosity solution and then performs a second pass using current graphics hardware accelerators to generate illumination as high definition textures.     

Maximum Intensity Projection Using Splatting in Sheared Object Space

In this paper we present a new Maximum Intensity Projection (MIP) algorithm which was implemented employing splatting in a shear-warp context. This algorithm renders a MIP image by first splatting each voxel on two intermediate spaces called "worksheet" and "shear image". Then, the maximum value is evaluated between worksheet and shear image. Finally, shear image is warped on the screen to generate the result image. Different footprints implementing different quality modes are discussed. In addition, we introduced a line encoded indexing speed-up method to obtain interactive speed. This algorithm allows for a quantitative, predictable trade-off between interactivity and image quality.     

The Hybrid World of Virtual Environments

Much of the work concerned with virtual environments has addressed the development of new rendering technologies or interaction techniques. As the technology matures and becomes adopted in a wider range of applications, there is, however, a need to better understand how this technology can be accommodated in software engineering practice. A particular challenge presented by virtual environments is the complexity of the interaction that is supported, and sometimes necessary, for a particular task. Methods such as finite-state automata which are used to represent and design dialogue components for more conventional interfaces, e.g. using direct manipulation within a desktop model, do not seem to capture adequately the style of interaction that is afforded by richer input devices and graphical models. In this paper, we suggest that virtual environments are, fundamentally, what are known as hybrid systems. Building on this insight, we demonstrate how techniques developed for modelling hybrid systems can be used to represent and understand virtual interaction in a way that can be used in the specification and design phases of software development, and which have the potential to support prototyping and analysis of virtual interfaces.     

Seamless Integration of Databases in VR for Constructing Virtual Environments

An approach for authoring virtual environments within the virtual environments themselves is presented, integrating a database containing arbitrary components that are used to construct the 3D scene. The issues important to a seamless integration of the database, such as multimedia data storage and database linkage are discussed, with the focus on the human-computer interaction component. A concept for a 3D database interface is described for query, presentation and usage of query results in the virtual environment. Finally, an implementation of the concept using VRML and Java is presented.     

基于GPU的烟雾数据体面混合绘制技术研究

研究烟雾等大规模体数据在虚拟场景中实时可视化的技术是一种新的体面混合绘制技术,在灾害现象仿真中有着重要的应用。在处理体面混合绘制时,已有的绘制算法难以实时地处理体数据的遮挡和裁剪,并在内窥绘制中出现片元空洞。针对以上问题,提出一种基于GPU的体面混合绘制算法,将体数据视为参与介质,同时将面绘制的结果作为体绘制的积分域约束条件,从而把面绘制中的裁减算法推广到体数据的绘制中,实现了体面混合绘制的无缝融合。仿真结果表明算法能够实时地绘制虚拟场景中的大规模体数据,并且能够处理复杂的内窥效果。     

Large-Scale Terrain Simplification and Visualization Based on TIN

In the preprocessing phase, the global terrain model is partitioned into blocks with their feature points being picked out to generate TIN model for each terrain block, then the multi-resolution models of terrain organized in the form of quad-tree is created bottom-up. Cracks between terrain blocks are avoided by inserting vertices to form common boundaries. At run-time, a view-dependent LOD algorithm is used to control the loading and unloading of the proper blocks by an additional synchronous thread. To e...     

基于手势的交互式虚拟环境

交互式虚拟现实系统能够为用户提供直观自然的用户接口。使用手势,用户可以更加高效地与虚拟环境进行交互。但随着系统中识别手势的增多,正确手势的识别率和系统的性能会跟着下降。文章提出用交互状态解决手势识别率和系统性能下降问题,并给出了实现方案。     

一种基于松散八叉树的复杂场景可见性裁剪算法

针对传统八叉树方法的不足,在采用松散八叉树组织场景、利用八叉树空间划分优点的同时弥补其局限性.为提高遮挡查询效率,将子节点依视点排序,针对复杂场景采用双层裁剪技术以进一步提高性能.实验结果表明,文中算法对深度复杂度高、面片数量大的复杂场景具有较好的裁剪效率,能够很好地满足实时绘制的要求.     

基于边折叠的网格简化算法研究

在计算机图形学和几何造型中,物体表面常用多边形网格模型来描述.一个复杂的场景可能会包含大量的多边形,不利于存储、传输及绘制.网格简化技术用于处理计算机图形学领域中广泛使用的多边形网格数据,主要应用领域有科学可视化、实时显示和虚拟现实等.文章给出了网格简化方法的分类,研究了基于边折叠的简化算法,对相关技术及其特点进行了介...     

基于流媒体的虚拟地理环境研究

通过将流媒体技术和虚拟地理环境相结合,解决了协同角色不能真实可视化,交流信息延迟等问题。该文根据流媒体技术的原理,设计了基于流媒体的虚拟地理环境系统结构与功能模块,并以虚拟地理研讨室为例建立了原型试验系统,实现了视频音频的本地播放和网络传输功能。研究表明,将流媒体技术与虚拟地理环境相结合,可以有效地增强虚拟地理环境中协同工作者之间交流,提高地理认知能力。     

虚拟现实场景中基于八叉树的网格简化方法

在基于顶点聚类的网格简化算法中,通常对网格模型进行八叉剖分来建立一棵顶点树以表示整个网格模型的数据结构;但顶点在空间上分布的不均匀导致了顶点树的不平衡,增加了顶点树的深度。针对上述问题,论文提出了基于紧凑八叉树的剖分方法。该方法在虚拟现实场景中表现了较好的效果。     

基于快速求交的自适应光线投射法

光线投射是一个重要的体绘制算法,但直线上采样点的确定是非常费时的,也是影响体绘制实时性的重要因素。该文利用直线和平面族的交点可以快速求得直线上的采样点及其特征值,该方法充分利用了直线的相关性,避免了大量的三线性插值计算。该文还基于快速求交提出了自适应采用方法,以提高绘制图像的质量。     

网络环境中虚拟树木的建模和实时渲染研究

在网络环境中虚拟树木表现技术上,如何达到网络传输的实时性和视觉效果的平衡一直是国内外学者研究的热点.论文摹于Java3D技术,提出了一种web环境下虚拟树木的实时建模和渲染方法.该方法以树木特性为依据设计了虚拟树木场景结构以及基于web的虚拟树木建模算法.在树木场景创建以及渲染过程中采用多种优化技术如面片建模、LOD方法等既保证视觉效果又有效减少场景面片数以加速场景的漫游显示速率.经过测试,构建的基于Web的虚拟树木场景不但具有较强的真实感及良好的交互性,而且满足网络实时传输的要求.     

虚拟装配中基于导纳控制的力觉渲染技术

传统的力触觉渲染多采用阻抗控制,不能很好地满足虚拟装配的应用要求,相比之下导纳控制模式更适用这一领域.为此提出一种基于导纳控制的双线程力觉渲染构架,并给出相应的力觉渲染算法.首先建立用于导纳控制的动力学模型,并讨论了碰撞和约束这2个状态下的力觉渲染;为了使用力觉交互接口进行虚拟装配中的小间隙装配,提出物理约束与几何约束结合的力觉渲染方法;最后针对物理计算和力反馈循环2个线程刷新频率不匹配的问题,利用二次拉格朗日多项式进行数值插值,实现了力觉交互接口的平稳输出.通过力反馈设备与自主开发的虚拟装配原型系统VAPP的连接与应用,验证了所提出的算法满足虚拟装配系统中力觉交互的应用要求.     

协同虚拟环境下基于时间包围盒的状态一致性维护

首先分析了DR算法应用到协同虚拟环境下可能会引起状态不一致的两个因素——网络延时和用户交互．然后提出了基于时间包围盒的补偿方法。实验结果证明,该方法能有效地维护系统运行过程中实体运动状态的一致性。