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1、 摘 要 体绘制算法直接对三维数据场进行绘制，避免了构造中间几何图元，绘制的图像可以反映物体的内部细节，逐渐成为三维可视化算法的主流。不过由于体绘制算法的计算量很大，一般难以满足实时绘制的要求，限制了体绘制技术的大规模应用。错切变形算法通过将体绘制中最耗时的重采样计算从三维降低到二维，大大提高了绘制速度，基本实现了实时绘制，是目前为止最快的体绘制算法，具有广阔的应用前景。但是，错切变形算法由于重采样步长相对过大，造成图像质量下降，同时要求体数据各向同性，同性化的过程也可能导致数据信息的丢失。 本文研究了主流的体绘制算法，重点研究了错切变形算法，针对该算法存在的问题提出改进方案，实现了改进后的算
2、法，验证了方案的有效性；并将实现的算法集成到实验室医学图像处理软件系统中，对软件系统进行了扩充和完善。 本文的主要内容有： 1. 研究和总结了主流的三维可视化算法，讨论了可视化技术的数学背景；对错切变形算法的基本原理、矩阵分解、坐标系统定义及变换等做了深入研究，总结了算法的具体流程；介绍了将屏幕二维坐标转化为空间三维坐标的虚拟球面模型，以实现通过鼠标在屏幕上的二维移动控制三维物体的空间旋转； 2. 详细研究了图像合成理论，针对错切变形算法图像质量下降的问题，提出一种改进的图像合成方法。该方法中，用于合成的光线段的取值由两端的样点值决定，引入了后端样点对光线段取值的影响，而不是传统方法中单纯等于
3、前端样点的值，取值更加合理。通过在 Matlab 中对算法进行实现，验证了该方法对图像质量的改进效果； 3. 提出一种基于各向异性体数据的错切变形算法。对于三个方向间距相差不大的体数据，可以利用该算法直接进行绘制，避免同性化过程造成的信息丢失。该算法对各向同性体数据也同样适用，更具通用性； 4. 完成了本文提出的算法的软件实现，并采用可变重采样率的方式，可以同时满足对交互速度和图像质量的要求。 关键词：关键词：三维可视化，体绘制，重采样，错切变形，图像合成 Abstract The method of volume rendering can generate images that refl
4、ect the internal details of objects, and is becoming the mainstream of 3D visualization algorithm. However, as the large amount of computation, it is difficult to meet the requirement of real-time rendering. To address this issue, the shear-warp algorithm reduces the most time-consuming re-sampling
5、process from 3D to 2D, and becomes the fastest volume rendering algorithm, having broad application prospects. But the algorithm still has some problems, such as the relatively large re-sampling step resulted in decreased image quality, and the volume data must be isotropic, but the isotropic proces
6、sing may lead to loss of data information. In this thesis, the shear-warp algorithm is primarily studied and corresponding solutions for the problems within this algorithm are proposed. Experiments have verified the effectiveness of the solutions. The primary contents of this thesis are as follows:
7、1. Researched and summarized the main volume rendering algorithms, especially the shear-warp algorithm. The most important concepts in this algorithm such as view matrix decomposition, coordinate system definition and conversion are further studied. Introduced a virtual sphere model which maps the 2
8、D screen coordinate into a 3D space coordinate, and then the space rotation matrix can be computed. 2. For the problem of declining image quality in shear-warp algorithm, proposed an improved method of image composition. In this method, the value of the light segment for compositing is decided by th
9、e two ends sampling points, not only the first point, and the value is more reasonable. Through simulation in Matlab, the methods effect on image quality improvement is verified. 3. Presented a shear-warp algorithm based on the anisotropic volume data. For the data that is anisotropic but the spacin
10、g of the adjacent points is not great different in three directions, this method can render directly and avoid the information loss during the isotropic processing. Of course, this method is also applicable to isotropic volume data. 4. Completed the software implementation of the proposed algorithms
11、, using a variable re-sampling rate, by which the requirements of rendering speed and image quality can be met at the same time. Keywords: 3D Visualization, Volume Rendering, Re-Sampling, Shear-Warp, Image Compositing 目 录 第一章 绪论. 1 1 1. .1 1课题背景. 1 1 1. .2 2医学图像处理的研究内容. 2 1 1. .3 3三维可视化技术概述. 3 1.3.1 面绘制. 4 1.3.2 体绘制. 5 1.3.3 点绘制. 6 1 1. .4 4DICOM 文件结构. 6 1 1. .5 5本文的主要内容. 9 第二章 基于体绘制的三维可视化技术. 10 2 2. .1 1体绘制中的几个重要概念. 10 2.1.1 三维体数据. 10 2.1.2 物质分类与颜色赋值.11 2.1.3 重采样. 12 2.1.4 光学模型. 12 2.1.5 图像合成. 15 2 2. .2 2体绘制算法流程.
