体数据的多精度等值面抽取

Ｍａｒｃｈｉｎｇ Ｃｕｂｅｓ算法是一个被广泛应用的体数据等值面抽取算法。本文提出的Ｍａｒｃｈｉｎｇ Ｂｏｘｅｓ算法结合显示所需精度，对ＭＣ算法作了优化，减少了由ＭＣ算法生成的三角面片数，使实时观察体数据成为可能。在保留图象细节的前提下，ＭＢ算法输出的三角面片经ＭＣ算法减少了一半以上，从而加快了体数据的面绘制速度。     

Marching Boxes：一个多精度等值面抽取算法

Ｍａｒｃｈｉｎｇ Ｃｕｂｅｓ算法是生成三维数据场等值面的经典算法，适用于数据密度较高的体数据。结合显示所需精度提出的Ｍａｒｃｈｉｎｇ Ｂｏｘｅｓ算法，对ＭＣ算法作了优化，减少了由ＭＣ算法生成的三角面片数，使实时观察体数据成为可能。     

MC算法在电磁场仿真的应用与实现

近年来可视化技术由于其优点得到了飞快的发展。Marching Cubes算法广泛应用于三维空间数据的等值面构建中。简要介绍MC算法的基本原理,在电磁场的仿真研究中引入该算法,针对电磁场数据和算法的特点,在实际的运用中做出改进,以满足实时仿真的要求。     

等值面抽取技术在超声探头声场模拟中的应用

为了更好地研究超声波声场,提出了以等值面方式来表现超声探头的声场分布;运用并改进了经典的MC（Marching Cubes）算法得出了圆盘和矩形两种超声探头声源的三维声场等值面描述.实验表明改进后的算法能获得较好的效果.     

应用于油藏数值模拟的MC改进算法

针对油藏数值模拟PEBI网格的特点及其关于网格划分的已知信息,提出了基于五面体特征体元的Marching Cubes算法,并用分块逐层遍历的方法来搜索有效体元,从而减少了数据场遍历的时间。该方法无须对数据进行预处理,避免了由预处理导致的误差所引起的井筒周围等值面的失真,同时也提高了数据处理的速度。实验表明该方法生成的等值面能满足实际需求。     

从大型体数据集中生成等值面的并行算法

从体数据集中生成等值面是体可视化的主要技术之一。当体数据集的数据量很大时，计算量也随之增大，单处理机的存储与计算能力难以胜任其可视化要求，基于并行与分布式计算环境设计并行可视化算法是有效的办法。本文基于工作站群机系统的ＰＶＭ环境，设计并实现了一种有效的、从大型体数据集中生成等值面的并行算法。     

一种改进的MC算法

为了对等值面与子等值面进行提取和分组,在MC算法原理的基础上,提出了一种改进的等值面提取与子等值面分组算法。该算法首先将数据场分解为点、棱边、面与体元的拓扑结构;然后在整个数据场范围内求所有棱边与等值面的交点,并在面内连接交点形成面与等值面的交线,交线在体元内连接生成空间多边形;接着通过三角化各个体元内的空间多边形得到由顶点表与三角形表组成的等值面数据;最后根据三角形在顶点处的连接关系,采用种子算法对属于同一子等值面的三角形与顶点进行标记,属于同一子等值面的顶点与三角形将被存放在独立的顶点表与三角形表中。实验结果表明,该算法可以高效地实现等值面提取与子等值面的分组。     

多分辨率并行等值面绘制

文中介绍一个多分辨率并行等值面绘制系统的设计与实现。通过显示调度策略实现基于多层次细节的实时等值面绘制。着重讲述并行处理、网格简化和多分辨率显示技术。实验结果表明，该系统能在并行分布式环境下动态生成等值面的多分辨率模型，实现等值面的远程快速绘制。     

基于微机环境的三维数据场多等值面快速显示算法

直接体绘制技术能够利用半透明效果显示三维数据场，提供了比等值面绘制方法更为丰富的信息，但是，由于数据场中所有体素都参与了图象生成过程，使得该技术的计算开销昂贵，远远无法达到交互式操作的要求。事实上，如果用边界表示法来表示三维数据场，就可以利用三维空间连续性来大幅度缩短绘制时间。边界表示法只关心有值面穿过的边界体元，用Ｏ内存单元来表示大小的原始数据场，从而产生大规模数据压缩。本文在此基础上提出一种基     

等值面的高质量显示方法

等值面是描述体数据中所含特定器官表面的有效方式，因此对等值面进行快速高质量的显示就显得非常重要。与以往的等值面显示算法不同，本文提出的方法不需要用三角形网格去逼近等值面然后再对三角形网格进行显示，而是直接对原始等值面进行显示，从而避免了三角化过程中的计算损耗和精度损失等问题，保证了等值面轮廓不会出现锯齿状现象，使等值面的显示既快速又准确。     

A type-merging algorithm for extracting an isosurface from volumetric data

A new approach for reducing the number of triangles representing an isosurface in volumetric data is presented. The basic idea is to classify the configurations of the marching cubes approach into types. Surface patches traversing neighboring cubes of the same type can be merged into patches, which can be approximated with fewer and larger triangles. Experimental results show that the number of triangles is about 50% of that obtained with the marching cubes algorithm, with comparable image quality. The execution time is somewhat longer than that of the marching cubes algorithm.     

对体可视化Marching Cube算法的改进

提出一个由三维数据计算等值面中点的算法,它在两方面对标准Ｍａｒｃｈｉｎｇ Ｃｕｂｅ算法进行了改进。第一个改进是：在等值面上样本点的状态依赖于它所连接的边同等值面相交的数目;第二个改进是：两相邻样本点中等值面多边形顶点被定位于中点,使得共面三角片合并为一个多边莆,减少了生成多边形的数量,提高了算法效率。     

Fast Isosurface Rendering on a GPU by Cell Rasterization

This paper presents a fast, high‐quality, GPU‐based isosurface rendering pipeline for implicit surfaces defined by a regular volumetric grid. GPUs are designed primarily for use with polygonal primitives, rather than volume primitives, but here we directly treat each volume cell as a single rendering primitive by designing a vertex program and fragment program on a commodity GPU. Compared with previous raycasting methods, ours has a more effective memory footprint (cache locality) and better coherence between multiple parallel SIMD processors. Furthermore, we extend and speed up our approach by introducing a new view‐dependent sorting algorithm to take advantage of the early‐z‐culling feature of the GPU to gain significant performance speed‐up. As another advantage, this sorting algorithm makes multiple transparent isosurfaces rendering available almost for free. Finally, we demonstrate the effectiveness and quality of our techniques in several real‐time rendering scenarios and include analysis and comparisons with previous work.     

Optimizing the topological and combinatorial complexity of isosurfaces

Since the publication of the original Marching Cubes algorithm, numerous variations have been proposed for guaranteeing water-tight constructions of triangulated approximations of isosurfaces. Most approaches divide the 3D space into cubes that each occupy the space between eight neighboring samples of a regular lattice. The portion of the isosurface inside a cube may be computed independently of what happens in the other cubes, provided that the constructions for each pair of neighboring cubes agree along their common face. The portion of the isosurface associated with a cube may consist of one or more connected components, which we call sheets. The topology and combinatorial complexity of the isosurface is influenced by three types of decisions made during its construction: (1) how to connect the four intersection points on each ambiguous face, (2) how to form interpolating sheets for cubes with more than one loop, and (3) how to triangulate each sheet. To determine topological properties, it is only relevant whether the samples are inside or outside the object, and not their precise value, if there is one. Previously reported techniques make these decisions based on local—per cube—criteria, often using precomputed look-up tables or simple construction rules. Instead, we propose global strategies for optimizing several topological and combinatorial measures of the isosurfaces: triangle count, genus, and number of shells. We describe efficient implementations of these optimizations and the auxiliary data structures developed to support them.     

步进立方体算法的SOB数据结构的改进

为了对步进立方体算法进行加速,首先分析了在等值面生成时此算法出现蜕变点的原因1,并提出了相应的解决方法;然后论述了一种加速步进立方体方法的数据结构——基于层和对象(SOB)的结构,并同八叉树和行程编码方法进行了比较.实验数据表明当数据体中存在一个对象时,SOB方法和八叉树方法的绘制时间没有大的差别,而当数据体中存在多个对象或等值面时,虽然八叉树方法的绘制时间是SOB方法的几倍,但SOB方法的存储容量要比八叉树方法大一些.可见,在数据量不是很大的情况下,以差别不大的存储空间来换取较少的等值面生成时间是可行的,其比八叉树方法有较高的效率,尤其在用户需要选择感兴趣的对象进行绘制时,比八叉树和行程编码更具有灵活性.     

Data perturbation for fewer triangles in marching tetrahedra

A simple efficient method is proposed to reduce the total number of triangles in an isosurface extraction method based on tetrahedral decomposition. We slightly perturb the input volumetric data so that useless small and thin triangles are removed. The perturbed volumetric data contain the exact isovalues from which a mesh is extracted. Since the proposed method is a pre-process of an isosurface extraction, it is not necessary to modify the mesh structure unlike the other similar methods.     

Adaptive generation of surfaces in volume data

A widespread approach to generating polygonal approximations of iso-surfaces or contour surfaces in volume data is the socalled marching-cubes algorithm. This algorithm, however, has the disadvantage that the number of polygonal chains generated is considerable. The splitting-box algorithm presented here reduces the number of polygonal chains by adapting their size to the shape of the surface. The resulting polygonal chains offer a wide spectrum for representing the contour surface. An exact representation is achieved by a new type of generic patches calculated from the polygonal chains. Approximations of different quality may be obtained by combining the algorithm generating the patches with simple triangulations.     

The Use of Geometric Algebra for 3D Modeling and Registration of Medical Data

We present a new approach to model 2D surfaces and 3D volumetric data, as well as an approach for non-rigid registration; both are developed in the geometric algebra framework. The approach for modeling is based on marching cubes idea using however spheres and their representation in the conformal geometric algebra; it will be called marching spheres. Note that before we can proceed with the modeling, it is needed to segment the object we are interested in; therefore, we include an approach for image segmentation, which is based on texture and border information, developed in a region-growing strategy. We compare the results obtained with our modeling approach against the results obtained with other approach using Delaunay tetrahedrization, and our proposed approach reduces considerably the number of spheres. Afterward, a method for non-rigid registration of models based on spheres is presented. Registration is done in an annealing scheme, as in Thin-Plate Spline Robust Point Matching (TPS-RPM) algorithm. As a final application of geometric algebra, we track in real time objects involved in surgical procedures.

Advanced interactive medical visualization on the GPU

Interactive visual analysis of a patient’s anatomy by means of computer-generated 3D imagery is crucial for diagnosis, pre-operative planning, and surgical training. The task of visualization is no longer limited to producing images at interactive rates, but also includes the guided extraction of significant features to assist the user in the data exploration process. An effective visualization module has to perform a problem-specific abstraction of the dataset, leading to a more compact and hence more efficient visual representation. Moreover, many medical applications, such as surgical training simulators and pre-operative planning for plastic and reconstructive surgery, require the visualization of datasets that are dynamically modified or even generated by a physics-based simulation engine.     

首例男女人体数据的自动分割和体透明分析

在研究体绘制技术的基础上，提出了体透明分析的新思想，给出了基于广义模糊粗糙集GFRS的体数据分割算法和体数据对比度增强算法，研究了这些技术在体透明分析系统中的应用，并利用第4代交互数据语言IDL6.2设计实现了体透明分析系统。中国和美国首例男女人体数据在该系统上的成功验证，标志着该系统有一定的理论价值、实用价值及广阔的市场前景。