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。     

频域体绘制中的颜色合成

和传统体绘制算法相比，频域体绘制算法具有较低的算法复杂度，能够更快地完成对三维数据场的显示．然而这种方法却不能简捷有效地表示和处理数据场的深度信息，因此其生成的图象缺少深度的遮挡效果．有鉴于此，Ｌｅｖｏｙ等曾通过诸如线性深度补偿的技术来弥补这一缺憾．尽管如此，运用这种技术得到的结果和传统空域方法仍有明显的差别．本文首先着眼于对空域体绘制方法中广泛采用的离散颜色合成技术的分析，然后引出了透明度密度分布函数的概念，进而推导出连续形式颜色合成过程的解析描述．根据这个结果，只要对三维数据场的透明度分布加以适当限制，就能够通过频域体绘制方法来实现颜色合成，增加绘制的深度效果．作者最后指出，基本频域体绘制方法和Ｌｅｖｏｙ的线性深度补偿方法的实质可以看成是这种颜色合成过程的近似．文中对透明度均匀分布的情况做了详细讨论，并且给出了具体算法，同时用生成的图象实例相应地进行了比较．     

通过子区域投影方法直接绘制三维数据场

直接的体绘制技术提供了在一幅图形显示三维数据场各种信息的巨大替力，然而，生成这样的图形是极其昂贵的，而且高质量图的绘制远远达不到交互实现的水平。与光线投射方法每次独立地处理一条光线相比，体绘制的体元投影方法按一定的顺序逐一处理数据场内每一个体元，图形绘制的复杂性是Ｏ（ｎ）（ｎ是数据场体元个数，即三个方向长度的乘积）。体元投影方法引起人们极大的兴趣是因为在处理过程中充分地利用了体元的空间连贯性。本文     

旋转体的图案绘制和消隐技术

本文通过数据输入形式、旋转体的表示方法、图案绘制原理、坐标变换、投影方法、隐线处理等六方面的研究,开发了旋转体的三维表示及图案绘制方法,并研究成功消除隐线的新方法。     

基于力反馈技术的三维模型表面绘制技术研究

本文将力反馈技术引入虚拟绘制过程中,提出一种基于力反馈技术的三维模型表面绘制方法。首先分析毛笔在三维绘制过程中的受力和变形,采用弹簧-振子模型建立三维毛笔模型;然后由毛笔碰撞变形后的最小包围球找出碰撞相关点,计算碰撞相关点的平均法矢确定投影平面,通过计算毛笔的变形得出在投影平面上的笔触,将该笔触投影到三维模型表面,同时控制毛笔的受力和运笔路径叠加笔触获得具有特定书法效果的三维笔道;最后介绍了仿真系统,使用Phantom Desktop力反馈设备在该系统中实现了三维模型表面的绘制。仿真结果表明,该方法再现了三维模型表面绘制的力觉控制过程,增强了绘制过程中的真实感。     

虚拟内窥镜技术的研究与实现

虚拟内窥镜是用计算机处理CT或MRI获取的三维医学数据,以获得类似用标准内窥镜观察病人内脏过程观察效果的一种诊断的新方法。在广泛研究虚拟内窥镜技术的基础上,我们开发了一个小型的基于面绘制的虚拟内窥镜系统,实现了漫游过程的交互功能和动画回放,在虚拟内窥镜技术的实现上做了有益的尝试。     

基于OpenGL驱动的三维场景重构

基于OpenGL环境,提出利用场景渲染结果即帧缓存数据(主要包括颜色缓存及深度缓存)进行局部三维场景重构的方法.根据投影变换原理和深度缓存特点研究了逆投影变换,得到了三维点元的重构方法.在此基础上,利用帧缓存数据重构每像素对应的三维点元,从而构成相机坐标系下的三维点云模型,即原始三维场景在当前相机视角下的离散采样,被定义为虚拟视模型.根据透视投影和帧缓存的特点,分析了虚拟视模型的点位精度,实验表明虚拟视模型具有较高的相对精度.提出的场景重构方法具有天然的多分辨特性,支持场景简化与流式传输以及独立于场景的具体表示形式,可采用拦截方式从一般三维软件中实时重构虚拟视模型,从而提供了一种获取三维数据的新方法.     

虚拟样机模型转换及演示技术研究

由于三维模型数据格式不一致，导致复杂产品的虚拟样机无法导入DIVISION MockUp软件演示系统。本文以某轴承虚拟样机为例，从模型转换，以及产品演示的角度进行了研究。根据CAD系统和DIVISION MockUp系统之间的图形交换机制，分析了数据储存的核心系统，成功解决了非标准格式三维实体模型的数据传递问题。通过对比虚拟样机模型，找到了一种在DIVISION演示时保证模型精度和演示速度的新方法，为在虚拟现实环境下应用虚拟样机产品做了有益的探索。     

基于图象绘制的虚拟实现系统环境

虚拟实现的一个重要目标是使用计算机生成逼真的视觉世界，使用户可以对虚拟世界的客体进行交互考察，虚拟实现的实现有两种方法，传统上，用三信图形学的方法实现建模和绘制。此方法需要繁琐的建查和昂贵的专用绘制硬件，而且图形绘制的质量的场景的复杂性受到很大的限制。基于图象的绘制是实现虚拟实现系统的一种新方法。它克服了三维图形方法的缺点。本文在总结已有技术的基础上，提出一个实现基于图象绘制的虚拟现实系统模型，并     

基于色彩分布合成的交互式可视化方法

在科学计算可视化中,由于数据场的数据一般都比较庞大,使得成像速度较慢,阻碍了人对数据场进行高效的分析和理解.基于体绘制中色彩在虚拟面上分布合成的方法,本文提出一种交互式可视化方法.该方法通过虚拟面上保存的已有的图像信息,提供细化、强化和弱化三种手段在虚拟面上进行局部操作,使用户可以对感兴趣的部分进行快速方便的成像和分析,提高可视化的效率.     

一种通用的3D绘制图象合成方法

合成３Ｄ绘制图象是一个有趣的主题，为了使复杂的图象更真实，需要合成场景的光照效果，已有的几种方法在合成光照效果方面不太理想，尤其在阴影合成方面。本文提出了一种新的３Ｄ绘制图象的合成方法，它结合了Ｄｕｆｆ的ＲＧＢＡＺ方法和ＳｈａｄｏｗＢｕｆｆｅｒ技术。     

Two-level volume rendering

Presents a two-level approach for volume rendering, which allows for selectively using different rendering techniques for different subsets of a 3D data set. Different structures within the data set are rendered locally on an object-by-object basis by either direct volume rendering (DVR), maximum-intensity projection (MIP), surface rendering, value integration (X-ray-like images) or non-photorealistic rendering (NPR). All the results of subsequent object renderings are combined globally in a merging step (usually compositing in our case). This allows us to selectively choose the most suitable technique for depicting each object within the data while keeping the amount of information contained in the image at a reasonable level. This is especially useful when inner structures should be visualized together with semi-transparent outer parts, similar to the focus+context approach known from information visualization. We also present an implementation of our approach which allows us to explore volumetric data using two-level rendering at interactive frame rates     

Efficient Compositing Methods for the Sort-Last-Sparse Parallel Volume Rendering System on Distributed Memory Multicomputers

In the sort-last-sparse parallel volume rendering system on distributed memory multicomputers, one can achieve a very good performance improvement in the rendering phase by increasing the number of processors. This is because each processor can render images locally without communicating with other processors. However, in the compositing phase, a processor has to exchange local images with other processors. When the number of processors exceeds a threshold, the image compositing time becomes a bottleneck. In this paper, we propose three compositing methods to efficiently reduce the compositing time in parallel volume rendering. They are the binary-swap with bounding rectangle (BSBR) method, the binary-swap with run-length encoding and static load-balancing (BSLC) method, and the binary-swap with bounding rectangle and run-length encoding (BSBRC) method. The proposed methods were implemented on an SP2 parallel machine along with the binary-swap compositing method. The experimental results show that the BSBRC method has the best performance among these four methods.     

High-quality splatting on rectilinear grids with efficient cullingof occluded voxels

Splatting is a popular volume rendering algorithm that pairs good image quality with an efficient volume projection scheme. The current axis-aligned sheet-buffer approach, however, bears certain inaccuracies. The effect of these is less noticeable in still images, but clearly revealed in animated viewing, where disturbing popping of object brightness occurs at certain view angle transitions. In previous work, we presented a new variant of sheet-buffered splatting in which the compositing sheets are oriented parallel to the image plane. This scheme not only eliminates the condition for popping, but also produces images of higher quality. In this paper, we summarize this new paradigm and extend it in a number of ways. We devise a new solution to render rectilinear grids of equivalent cost to the traditional approach that treats the anisotropic volume as being warped into a cubic grid. This enables us to use the usual radially symmetric kernels, which can be projected without inaccuracies. Next, current splatting approaches necessitate the projection of all voxels in the iso-interval(s), although only a subset of these voxels may eventually be visible in the final image. To eliminate these wasteful computations we propose a novel front-to-back approach that employs an occlusion map to determine if a splat contributes to the image before it is projected, thus skipping occluded splats. Additional measures are presented for further speedups. In addition, we present an efficient list-based volume traversal scheme that facilitates the quick modification of transfer functions and iso-values     

体绘制中基于透明度标尺的色彩合成方法

科学可视化中，体绘制是非常重要的一类方法，它的基本操作是色彩合成．本文对色彩合成过程中透明度的变化序列进行了考察，提出了一个基于透明度基准区与视点无关的透明度标尺，并将各类物质的透明度转换为基于透明度标尺的步长，使成像时的色彩合成过程省略许多重复计算，从而提高可视化效率．     

基于Ray-Casting算法对医学图像进行三维体绘制重建

对医学图像三维体绘制的基本流程、光学模型及原理进行了分析,并深入研究了光线投射Ray-Casting算法的模型及原理,就其颜色赋值、重采样、图像合成等关键技术进行了深入讨论和研究,最后在MITK平台实现了医学图像的体绘制。该算法对体绘制的三维模型能够进行光照属性、表面属性和环境参数的交互操作;同时可进行法平面切割和动态平面切割显示;此外,还可以进行旋转、缩放、平移等交互操作。     

Fast Polyhedral Cell Sorting for Interactive Rendering of Unstructured Grids

Direct volume rendering based on projective methods works by projecting, in visibility order, the polyhedral cells of a mesh onto the image plane, and incrementally compositing the cell's color and opacity into the final image. Crucial to this method is the computation of a visibility ordering of the cells. If the mesh is "well-behaved" (acyclic and convex), then the MPVO method of Williams provides a very fast sorting algorithm; however, this method only computes an approximate ordering in general datasets, resulting in visual artifacts when rendered. A recent method of Silva et al. removed the assumption that the mesh is convex, by means of a sweep algorithm used in conjunction with the MPVO method; their algorithm is substantially faster than previous exact methods for general meshes.
In this paper we propose a new technique, which we call BSP-XMPVO, which is based on a fast and simple way of using binary space partitions on the boundary elements of the mesh to augment the ordering produced by MPVO. Our results are shown to be orders of magnitude better than previous exact methods of sorting cells.     

Scalable 3D video of dynamic scenes

In this paper we present a scalable 3D video framework for capturing and rendering dynamic scenes. The acquisition system is based on multiple sparsely placed 3D video bricks, each comprising a projector, two grayscale cameras, and a color camera. Relying on structured light with complementary patterns, texture images and pattern-augmented views of the scene are acquired simultaneously by time-multiplexed projections and synchronized camera exposures. Using space–time stereo on the acquired pattern images, high-quality depth maps are extracted, whose corresponding surface samples are merged into a view-independent, point-based 3D data structure. This representation allows for effective photo-consistency enforcement and outlier removal, leading to a significant decrease of visual artifacts and a high resulting rendering quality using EWA volume splatting. Our framework and its view-independent representation allow for simple and straightforward editing of 3D video. In order to demonstrate its flexibility, we show compositing techniques and spatiotemporal effects.     

Reflection and Refraction on Implicit Surfaces

Implicit surfaces are often used in computer graphics. They can be easily modeled and rendered, and many objects are composed of them in our daily life. In this paper, based on the concept of virtual objects, a novel method of real-time rendering is presented for reflection and refraction on implicit surface. The method is used to construct virtual objects from real objects quickly, and then render the virtual objects as if they were real objects except for one more step of merging their images with the real objects＇ images. Characteristics of implicit surfaces are used to compute virtual objects effectively and quickly. GPUs （Graphics Processing Units） are used to compute virtual vertices quickly and further accelerate the computing and rendering processes. As a result, realistic effects of reflections and refractions on implicit surfaces are rendered in real time.     

Fast Visualization of Object Contours by Non-Photorealistic Volume Rendering

In this paper we present a fast visualization technique for volumetric data, which is based on a recent non-photorealistic rendering technique. Our new approach enables alternative insights into 3D data sets (compared to traditional approaches such as direct volume rendering or iso-surface rendering). Object contours, which usually are characterized by locally high gradient values, are visualized regardless of their density values. Cumbersome tuning of transfer functions, as usually needed for setting up DVR views is avoided. Instead, a small number of parameters is available to adjust the non-photorealistic display. Based on the magnitude of local gradient information as well as on the angle between viewing direction and gradient vector, data values are mapped to visual properties (color, opacity), which then are combined to form the rendered image (MIP is proposed as the default compositing stragtegy here). Due to the fast implementation of this alternative rendering approach, it is possible to interactively investigate the 3D data, and quickly learn about internal structures. Several further extensions of our new approach, such as level lines are also presented in this paper.