结构化骨架求取和内窥可见性计算

提出了包含距离和拓扑信息的结构化骨架提取方法和骨架指导的内窥可见性计算方法,并将其有机地应用于虚拟漫游系统中.结构化骨架提取算法结合了并行细化算法和距离变换算法,使得骨架能够有效地控制虚拟内窥镜的视点移动和漫游位置的跟踪,有利于准确地观察病变位置.内窥可见性算法利用人体器官的封闭管状特征,将其分割成许多网格单元,并在预处理中使用深度缓存计算单元间的可见性,同时对每个单元建立可见性树.虚拟漫游时,通过当前视点信息可进一步动态地删减所在单元的可见性树,从而得到实时可见性,最终实现实时漫游并保持漫游图像的质量.     

视点依赖的温室场景模拟与交互式漫游

为对包含日光温室的农业场景进行实时、逼真的绘制,采用基于视点的连续细节层次（LOD）来减少实际需要绘制的网格数据量,并基于包围盒技术实现了视点的碰撞检测,同时使用可见性剔除算法来加速场景渲染速度。在渲染温室内植物时,通过几何变换实现了大规模植物群体的快速构建,并在场景中使用阴影体算法来渲染阴影,生成了具有真实感的大规模温室农业场景。模拟结果表明该方法能有效地减少渲染的面元数目,大大提高了绘制速率,场景有较高的真实感,能够满足交互式实时漫游的要求。     

层次可见性与层次细节地表模型相结合的快速绘制

大规模地表模型的实时绘制是虚拟现实技术中重要的研究课题之一。为了加速地表模型的绘制，人们采用视点相关的动态多分辨率层次细节模型方法，但是算法效率依然有待提高。该文提出一种层次可见性与层次细节地表模型相结合的快速地形绘制方法。算法旨在利用地表模型所具有的horizon特性在预处理中为地表多分辨率块模型建立相应的层次“块”可见性结构，快速判定地形块相对于当前视点的可见性，以减少多分辨率模型中模型细节的处理和绘制三角形的数目。同时为消除地表模型层次变化所带来的可见性错误，算法提出一种层次结构可见性计算方法，以修正多分辨率模型所带来的可见性动态变化。实验结果表明算法有效地提高了绘制效率，是可行的。     

HLODs模型在场景快速绘制中的应用

论述了一种基于视相关分级细节层次的场景简化的算法，加速场景绘制速度。预处理阶段利用八叉树构造场景的分层次结构；实时渲染阶段，采用多线程进行场景的实时绘制：根据视点进行可见性剔除，根据屏幕误差选择合理的分级细节层次，利用数据管理调度进行数据的实时更新。在保持图像的保真度条件下，可实现场景的快速绘制。     

基于多线程并行的大规模场景交互漫游研究

针对大规模三维场景的交互漫游,提出了一种基于多线程并行调度解决方案,并给出了相应的交互漫游算法.该方法使用离散层次细节技术结合视点相关的动态连续层次细节选择和过渡的批LOD技术.在预处理阶段,对大规模场景进行分层分块处理;在实时漫游阶段,采用多线程并行技术:绘制线程利用四又树层次进行可见性剔除和视点相关简化获取当前可绘地形,并将其提交给GPU进行绘制.预取线程通过预测视点的位置,从外存预取相关的几何数据并调入内存.将该方法应用于具体实例,取得了良好效果,证明了该并行方法的有效性.     

高精度高速度曲面建模的3维地表模拟和漫游算法

三维真实感地形场景是生态系统模拟、虚拟地理环境和三维GIS不可或缺的要素,而高精度高速度曲面建模方法可以动态实时构建高精度曲面模型。论文引入高精度高速度曲面建模方法,提出了一种可以动态生成地表模型并实时绘制和漫游三维真实感地表的方法。该方法实时动态建立高精度DEM,结合改进的视点依赖多分辨率绘制方法,随视点改变动态重建并绘制当前DEM。首先建立当前可见场景的DEM,然后通过依赖于视点的简化等处理建立层次细节模型,进行实时场景绘制和漫游,随视点改变,在当前场景基础上动态加点和减点更新DEM及层次细节模型,生成当前可见场景,以次循环。实例测试表明,算法可产生真实感较强的场景,实时漫游效率较高,模拟精确。     

基于OpenGL几何变换的虚拟漫游视点控制及应用

灵活自然的视点控制和精确实时的碰撞检测可以大幅提高虚拟漫游系统的真实感.探讨并计算漫游交互基本控制动作中视点位置变化,基于OpenGL中的三维几何变换实现漫游时视点控制,创建八叉树进行碰撞检测,最后在基于粒子系统的土壤可视化漫游中予以应用.漫游效果逼真、实时性强,表明所采用的方法具有一定的科研及应用价值.     

基于场景结合的大规模动态群体可见性计算方法

动态场景的可见性计算对于大规模场景的实时渲染具有重要意义,其中运动中的大规模群体更给可见性计算带来了很大的开销.针对大规模动态群体在建筑物场景内部运动的情况,提出一种与场景结合的动态群体可见性计算方法.在预处理时,根据个体在不同仿真时刻的位置,将其绑定到相应的场景节点中;在实时绘制时,结合场景的可见性判断结果对动态群体中的个体进行可见性判断.实验结果表明,该方法能高效地剔除动态群体中的不可见个体,使大规模动态场景的实时绘制效率得到明显提高.     

多边形中的点可见性快速算法

针对点的可见性计算这一计算几何中的基础问题,提出一种支持任意查询点的可见多边形快速计算的基于多边形Voronoi图的点可见性算法.以与Voronoi骨架路径对应的Voronoi通道概念,以及相应的局部最短路径概念为基础,按照深度优先策略对Voronoi图进行遍历,在计算Voronoi骨架路径的同时计算局部最短路径,并基于局部最短路径计算所遍历的多边形边的可见部分.该算法可以处理“带洞”多边形,而且只对多边形进行局部访问;对于“带洞”多边形,由于该算法的数据结构比较简单、剖分空间合理且易于实现,因此仅需O(n)空间和O(nlgn)预处理时间.最后给出了在三维室内虚拟场景设计与漫游系统中的应用实例,结果表明文中算法是实际可行,且运行时间与点的可见多边形的边数和多边形的边数均呈线性关系.     

基于可见性裁剪的地形数据流式传输策略

为实现地形场景数据的实时传输和有效显示,根据高空和地面视点对视野区域的要求,提出基于可见性裁剪的数据流式传输策略,当视点位置移动时,能有效确定视野区域节点的可见性和关键节点的层次细节度。实验结果表明,经过服务器端预处理后的地形数据可快速传输到客户端进行精确显示。     

VLOD: high-fidelity walkthrough of large virtual environments

We present visibility computation and data organization algorithms that enable high-fidelity walkthroughs of large 3D geometric data sets. A novel feature of our walkthrough system is that it performs work proportional only to the required detail in visible geometry at the rendering time. To accomplish this, we use a precomputation phase that efficiently generates per cell vLOD: the geometry visible from a view-region at the right level of detail. We encode changes between neighboring cells' vLODs, which are not required to be memory resident. At the rendering time, we incrementally construct the vLOD for the current view-cell and render it. We have a small CPU and memory requirement for rendering and are able to display models with tens of millions of polygons at interactive frame rates with less than one pixel screen-space deviation and accurate visibility.     

基于可见性预处理和细节简化的虚拟环境快速漫游算法

本文针对复杂多边形场早的漫游提出了一个新的预处理方法，并给出了相应的快速漫游算法。     

A fast algorithm for an envelope construction of a huge set of topologically consistent polygons

This paper describes an efficient algorithm for the determination of an envelope of a set of polygons. The polygons have to be aligned along their borders – the case which, for example, appears in geographic information systems. The edges, which belong to two neighbouring polygons, are called twin edges, and are eliminated first. To accelerate the geometric search, a two-level uniform plane subdivision structure is proposed. The remaining non-twin edges belong to the envelope, and they have to be joined to form the simple polygons at the end. For this task, a new algorithm has been developed. At the end, spatial relationships between resulting polygons are established. The whole algorithm for envelope determination works in expected time O(n log n) using O(n) memory space, where n is the total number of edges belonging to the input polygons. In the last part of the paper, practical results using data from a geographical database are considered.     

Recognizing polygons,or how to spy

A new class of so-called pseudo-starshaped polygons is introduced. A polygon is pseudo-star-shaped if there exists a point from which the whole interior of the polygon can be seen, provided it is possible to see through single edges. We show that the class of pseudo-star-shaped polygons unifies and generalizes the well-known classes of convex, monotone and pseudostar-sphaped polygons. We give algorithms for testing whether a polygon is pseudostar-shaped from a given point in linear time, and for constructing all regions from which the polygon is pseudo-star-shaped in quadratic time. We show the latter algorithm to be worst-case optimal. Also, we give efficient algorithms solving standard geometrical problems such as point-location and triangulation for pseudo-starshaped polygons.A preliminary version of this paper has been presented at the 24 th Allerton Conference on Communication, Control and Computing, Monticello, Ill, October 1986Research for this paper was done while the author was at Carleton UniversityResearch for this paper was done in part while the author was visiting Carleton UniversityThis research was supported in part by NSERC and by Carleton University     

非线性折射和反射平面场景的实时光线跟踪

该文利用光学映射虚对象的概念，提出了实时计算平面折射虚物体的新方法，给出了在不同情况下实时计算平面折射虚顶点的计算公式，实现了基于三维图形硬件（加速卡）加速的实时光线跟踪算法。该算法能够处理非线性平面折射和反射问题，极大地提高了真实感图形绘制和显示的速度。这对于建筑物实时漫游、实时动画和虚拟现实等领域具有非常广阔的应用前景。     

Interactive Line Art Rendering of Freeform Surfaces

In recent years, synthetically created line art renderings have reached quality levels that are aesthetically pleasing. Moreover, the sketch based approach was found to be quite capable at conveying geometrical information in an intuitive manner. While a growing interest in this type of rendering method has yielded successful and appealing results, the developed techniques were, for the most part, too slow to be embedded in real time interactive display. This paper presents a line art rendering method for freeform polynomial and rational surfaces that is capable of achieving real time and interactive display. A careful preprocessing stage that combines an a-priori construction of line art strokes with proper classification of these strokes, allows one to significantly alleviate the computational cost, in real time, of the sketch based rendering, and enables interactive line art display.     

基于图像的实时漫游

提出了一个点和多边形模型混合的场景表达方式,从而实现了对复杂场景的视点不受限制的实时漫游.它从多幅带深度的参考图像出发,在预处理阶段区别对待场景中平面物体和曲面物体在参考图像中的对应像素.对于在参考图像所占区域较大的空间平面,用传统的多边形模型方式对其进行表达,试图恢复出其平面方程,然后通过采样密度比较和重采样过程,将该空间平面在所有参考图像中的出现合并成一个均匀采样的纹理图像;而对于空间曲面或在参考图像中所占面积较小的平面,使用点的形式对其进行表达,通过采样密度的比较去掉冗余的点,将保留下来的点按其空间位置进行聚类.同时,对于场景中那些不被所有参考图像所拍摄到而在漫游过程中可能形成空洞的部分,提出一个空洞预填补技术,在预处理阶段即对这类空洞进行填补,从而大大减少了漫游阶段出现空洞的几率.在漫游阶段则使用纹理映射和点Warping的方式进行绘制,以充分利用图形硬件的加速功能.     

An automatic rotoscopy system for human motion based on a biomechanic graphical model

A system for analysis and synthesis of human motion is presented. The whole system consists of an analysis part and a synthesis part. The analysis part includes the preprocessing phase, modeling phase, matching phase, and interpretation phase. The synthesis part reconstructs the same motion of the human body. A biomechanic graphical model is defined to represent the human body in 3D space which is matched with the real images to discover the motion. The synthesis part uses the result of the analysis part to reconstruct the same motion of the human body which can be viewed from any viewpoint and from any distance.     

Ray tracing general parametric surfaces using interval arithmetic

This paper describes an algorithm for ray tracing general parametric surfaces. After dividing the surface adaptively into small parts, a binary tree of these parts is built. For each part a bounding volume is calculated with interval arithmetic. From linear approximations and intervals for the partial derivatives it is possible to construct parallelepipds that adapt the orientation and shape of the surface parts very well and form very tight enclosures. Therefore we can develop an algorithm for rendering that is similar to that used with Bèzier and B-spline surfaces, where the bounding volumes are derived from the convex hull property. The tree of enclosures (generated once in a preprocessing step) guarantees that each ray that hits the surface leads to an iteration on a very small surface part; this iteration can be robustly (and very quickly) performed in real arithmetic.     

Efficient Algorithms for Computing Conservative Portal Visibility Information

The number of polygons in realistic architectural models is many more than can be rendered at interactive frame rates. Typically, however, due to occlusion by opaque surfaces (e.g., walls), only small fractions of such models are visible from most viewpoints. This fact is used in many popular methods for preprocessing visibility information which assume a scene model subdivided into convex cells connected through convex portals. These methods need to establish which cells or parts thereof are visible to a generalized observer located within each cell. The geometry of this information is termed a 'visibility volume' and its computation is usually quite complex. Conservative approximations of viewing volumes, however, are simpler and less expensive to compute. In this paper we present techniques and algorithms which permit the computation of conservative viewing volumes incrementally. In particular, we describe an algorithm for computing the viewing volumes for a given cell through a sequence of ' m ' portals containing a total of ' n ' edges in O m n time.