Temporal radiance caching

Curved planar reformation (CPR) has proven to be a practical and widely used tool for the visualization of curved tubular structures within the human body. It has been useful in medical procedures involving the examination of blood vessels and the spine. However, it is more difficult to use it for large tubular structures such as the trachea and the colon because abnormalities may be smaller relative to the size of the structure and may not have such distinct density and shape characteristics. Our new approach improves on this situation by using volume rendering for hollow regions and standard CPR for the surrounding tissue. This effectively combines gray-scale contextual information with detailed color information from the area of interest. The approach is successfully used with each of the standard CPR types, and the resulting images are promising as an alternative to virtual endoscopy. Because CPR and volume rendering are tightly coupled, the projection method used has a significant effect on the properties of the volume renderer, such as distortion and isometry. We describe and compare the different CPR projection methods and how they affect the volume rendering process. A version of the algorithm is also presented which makes use of importance-driven techniques; this ensures the users' attention is always focused on the area of interest and also improves the speed of the algorithm.     

Real-time illustration of vascular structures

We present real-time vascular visualization methods, which extend on illustrative rendering techniques to particularly accentuate spatial depth and to improve the perceptive separation of important vascular properties such as branching level and supply area. The resulting visualization can and has already been used for direct projection on a patient's organ in the operation theater where the varying absorption and reflection characteristics of the surface limit the use of color. The important contributions of our work are a GPU-based hatching algorithm for complex tubular structures that emphasizes shape and depth as well as GPU-accelerated shadow-like depth indicators, which enable reliable comparisons of depth distances in a static monoscopic 3D visualization. In addition, we verify the expressiveness of our illustration methods in a large, quantitative study with 160 subjects.     

三维有限元数据场体绘制算法的研究

三维有限元数据场包含了庞大的信息量,不易于人们深刻理解和分析。可视化技术将数据场以图形、图像的形式显示出来,揭示出三维有限元数据场中蕴藏的丰富内涵。讨论了三维数据场可视化体绘制中射线跟踪法和直接投影法的优点及不足,提出了将射线跟踪法及直接投影法各自优点结合起来的新算法,应用于三维有限元数据场的体绘制。新算法一方面充分利用场在投影区域上的二维连贯性,每次推进的是一个面片而不是一个孤立的像素点,另一方面针对每个视线段子段,充分利用场在深度方向的连贯性,用分析积分法完成累积光强和透明度计算。算法效率高,统一性强。     

Ray-cast volume rendering accelerated by incremental trilinear interpolation and cell templates

Two related ideas for improving the speed of ray-cast volume rendering are studied in this paper. The first is an incremental algorithm for trilinear interpolation, a method commonly used in ray-cast volume rendering to calculate sample values. The incremental algorithm can expedite trilinear interpolation when many samples along a ray are located in one cell. The second is an efficient hybrid volume rendering restricted to parallel projection. In the preprocessing stage, acell template is created to store the information used by the incremental trilinear interpolation. When a cell is parallel projected, the information is retrieved from the template to compute the cell contribution. Because the algorithm with only one template may cause aliasing, an antialiasing technique exploiting multiple cell templates is proposed. With our method, ray-cast volume rendering can be accelerated considerably.     

Projection Mapping on Arbitrary Cubic Cell Complexes

This work presents a new representation used as a rendering primitive of surfaces. Our representation is defined by an arbitrary cubic cell complex: a projection‐based parameterization domain for surfaces where geometry and appearance information are stored as tile textures. This representation is used by our ray casting rendering algorithm called projection mapping, which can be used for rendering geometry and appearance details of surfaces from arbitrary viewpoints. The projection mapping algorithm uses a fragment shader based on linear and binary searches of the relief mapping algorithm. Instead of traditionally rendering the surface, only front faces of our rendering primitive (our arbitrary cubic cell complex) are drawn, and geometry and appearance details of the surface are rendered back by using projection mapping. Alternatively, another method is proposed for mapping appearance information on complex surfaces using our arbitrary cubic cell complexes. In this case, instead of reconstructing the geometry as in projection mapping, the original mesh of a surface is directly passed to the rendering algorithm. This algorithm is applied in the texture mapping of cultural heritage sculptures.     

Error-controlled real-time cut updates for multi-resolution volume rendering

Multi-resolution techniques are required for rendering large volumetric datasets exceeding the size of the graphics card's memory or even the main memory. The cut through the multi-resolution volume representation is defined by selection criteria based on error metrics. For GPU-based volume rendering, this cut has to fit into the graphics card's memory and needs to be continuously updated due to the interaction with the volume such as changing the area of interest, the transfer function or the viewpoint. We introduce a greedy cut update algorithm based on split-and-collapse operations for updating the cut on a frame-to-frame basis. This approach is guided by a global data-based metric based on the distortion of classified voxel data, and it takes into account a limited download budget for transferring data from main memory into the graphics card to avoid large frame rate variations. Our out-of-core support for handling very large volumes also makes use of split-and-collapse operations to generate an extended cut in the main memory. Finally, we introduce an optimal polynomial-time cut update algorithm, which maximizes the error reduction between consecutive frames. This algorithm is used to verify how close to the optimum our greedy split-and-collapse algorithm performs.     

通过区域块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内显示三维数据场各种信息的巨大潜力。然而，生成这样的图形是极其昂贵的，而且高质量图形的绘制远远达不到交互实现的水平。体元投方法之所以能引起人们的极大举是因为在处理过程中充分地利用了体元的空间连贯性。本文提出了一个更有效的体绘制方法：区域块投影方法（Ｂｌｏｃｋ Ｐｒｏｊｅｃｔｉｏｎ Ｍｅｔｈｏｄ）。这一算法不仅利用了体元的空间连贯性而且还充分地利用了数据场函数值分布的     

Sinus endoscopy--application of advanced GPU volume rendering for virtual endoscopy

For difficult cases in endoscopic sinus surgery, a careful planning of the intervention is necessary. Due to the reduced field of view during the intervention, the surgeons have less information about the surrounding structures in the working area compared to open surgery. Virtual endoscopy enables the visualization of the operating field and additional information, such as risk structures (e.g., optical nerve and skull base) and target structures to be removed (e.g., mucosal swelling). The Sinus Endoscopy system provides the functional range of a virtual endoscopic system with special focus on a realistic representation. Furthermore, by using direct volume rendering, we avoid time-consuming segmentation steps for the use of individual patient datasets. However, the image quality of the endoscopic view can be adjusted in a way that a standard computer with a modern standard graphics card achieves interactive frame rates with low CPU utilization. Thereby, characteristics of the endoscopic view are systematically used for the optimization of the volume rendering speed. The system design was based on a careful analysis of the endoscopic sinus surgery and the resulting needs for computer support. As a small standalone application it can be instantly used for surgical planning and patient education. First results of a clinical evaluation with ENT surgeons were employed to fine-tune the user interface, in particular to reduce the number of controls by using appropriate default values wherever possible. The system was used for preoperative planning in 102 cases, provides useful information for intervention planning (e.g., anatomic variations of the Rec. Frontalis), and closely resembles the intraoperative situation.     

Shape-enhanced maximum intensity projection

Maximum intensity projection (MIP) displays the voxel with the maximum intensity along the viewing ray, and this offers simplicity in usage, as it does not require a complex transfer function, the specification of which is a highly challenging and time-consuming process in direct volume rendering (DVR). However, MIP also has its inherent limitation, the loss of spatial context and shape information. This paper proposes a novel technique, shape-enhanced maximum intensity projection (SEMIP), to resolve this limitation. Inspired by lighting in DVR to emphasize surface structures, SEMIP searches a valid gradient for the maximum intensity of each viewing ray, and applies gradient-based shading to improve shape and depth perception of structures. As SEMIP may result in the pixel values over the maximum intensity of the display device, a tone reduction technique is introduced to compress the intensity range of the rendered image while preserving the original local contrast. In addition, depth-based color cues are employed to enhance the visual perception of internal structures, and a focus and context interaction is used to highlight structures of interest. We demonstrate the effectiveness of the proposed SEMIP with several volume data sets, especially from the medical field.     

基于有效裁剪被遮挡单元的非规则数据场体绘制算法

提出了一种被遮挡单元的裁剪算法,以加速非规则数据场的体绘制过程。在基于一组平行切割平面的体绘制方法上,新算法通过对图像不透明度缓冲区中的值进行求平均操作,并将所计算的结果存储在一个与不透明度缓冲区间同样大小的平均不透明度缓冲区中,使得只需根据每一数据单元重心投影点在平均不透明度缓冲区中的值,就可得到此数据单元的可见性,从而有效裁剪掉被遮挡单元,降低需处理的数据量,加速体绘制过程。     

Projected slabs: approximation of perspective projection and error analysis

Virtual endoscopy is a promising medical application for volume‐rendering techniques where perspective projection is mandatory. Most of the acceleration techniques for direct volume rendering use parallel projection. This paper presents an algorithm to approximate perspective volume rendering using parallel projected slabs. The introduced error due to the approximation is investigated. An analytical study of the maximum and average error is made. This method is applied to VolumePro 500. Based on the error analysis, the basic algorithm is improved. This improvement increases the frame rate, keeping the global maximum error bounded. The usability of the algorithm is shown through the virtual endoscopic investigation of various types of medical data sets. Copyright © 2002 John Wiley & Sons, Ltd.     

基于改进光线投射算法的体数据显示

光线投射算法属于直接体绘制（DVR）中应用比较广泛的算法,其优点是绘制质量高,但是存在采样点计算量大,绘制速度慢的问题.针对这一问题,本文利用投射光线在物空间的传递性质,提出了一种改进的计算采样点位置的算法,加快采样点的获取速度,提高图像三维重建的效率.该算法在PC机平台上得到了实现,不仅在图像质量上得到保证而且绘制速度又有很大提高,为图像的三维重建提供了有效的手段.     

交互式动态体绘制及其加速算法

体绘制三维成象法是一门新兴的3D采样数据场可视化技术，在医学成象和科学可视化领域有着极为广泛的应用，但由于3D数据量大，其使用往往受到巨大计算开销的限制，因此很多研究人员致力于静态体绘制加速算法的研究，并解决医学图象三维可视化中三维体数据显示速度与成象质量问题，因而提出了一种交互式动态体绘制算法，即从任意的视点距离和视线方向进行动态编制，并在分析其算法复杂度的基础上，提出一种新的加速算法，同时使得动态体绘制过程几乎达到实时的效果，经验证，这种算法比标准算法快4～5倍。     

Interactive High-Quality Maximum Intensity Projection

Maximum Intensity Projection (MIP) is a volume rendering technique which is used to visualize high-intensity structures within volumetric data. At each pixel the highest data value, which is encountered along a corresponding viewing ray is depicted. MIP is, for example, commonly used to extract vascular structures from medical data sets (angiography). Due to lack of depth information in MIP images, animation or interactive variation of viewing parameters is frequently used for investigation. Up to now no MIP algorithms exist which are of both interactive speed and high quality. In this paper we present a high-quality MIP algorithm (trilinear interpolation within cells), which is up to 50 times faster than brute-force MIP and at least 20 times faster than comparable optimized techniques. This speed-up is accomplished by using an alternative storage scheme for volume cells (sorted by value) and by removing cells which do not contribute to any MIP projection (regardless of the viewing direction) in a preprocessing step. Also, a fast maximum estimation within cells is used to further speed up the algorithm.     

基于梯度模分布的视点选择

在三维体数据的可视化中,信息的获取不仅与传输函数的设置有关,而且与用户观察的视点有关.提出一种新的应用直接体绘制来可视化体数据时最佳视点的选择算法,通过计算梯度模在图像上的分布,寻找梯度模分布最为均匀且投影面积最大的视点方向,同时,在视域球上离散采样得到初始视点墒分布后,使用自适应步长的梯度下降法来优化全局最佳视点方向.实验结果证实了所提出方法能够有效地找到最佳视点.     

基于面向对象八叉树的飞机虚拟维修场景渲染

为了加速大规模虚拟场景的渲染速度,采用基于面向对象八叉树的方法对场景进行渲染。该方法将面向对象技术与传统八叉树技术相结合,采用面向对象八叉树剖分虚拟场景,对场景进行管理;将物体结构树的最小零部件作为最小存储单元,采用叶节点保存对象信息,减小树的存储量和处理时间,降低算法的计算负担;在面向对象八叉树的基础上,采用模型遮挡裁剪算法对位于视域范围内的模型进行遮挡裁剪,减小实际渲染的物体数量,提高渲染速率。通过对飞机虚拟维修场景进行渲染实验,证明了该方法的有效性。     

遮挡线索增强的最大密度投影算法

遮挡线索增强的最大密度投影算法（OEMIP）旨在解决最大密度投影法 （MIP）不能正确表达遮挡线索的问题,它包括两个步骤：首先使用K-Means 聚类算法从 MIP 结果图像中自动提取结构特征;然后根据结构特征自适应调节MIP 绘制过程以正确表 达遮挡线索。此外,引入绘制优先级以避免重要特征被次要特征严重遮挡,并给出绘制优先 级的自动设置算法。多个体数据的测试结果表明OEMIP 能显著增强遮挡线索,且能实现实 时交互。     

Incremental Volume Rendering Using Hierarchical Compression

We present a new algorithm here for efficient incremental rendering of volumetric datasets. The primary goal of this algorithm is to give average workstations the ability to efficiently render volume data received over relatively low bandwidth network links in such a way that rapid user feedback is maintained. Common limitations of workstation rendering of volume data include: large memory overheads, the requirement of expensive rendering hardware, and high speed processing ability. The rendering algorithm presented here overcomes these problems by making use of the efficient Shear-Warp Factorisation method which does not require specialised graphics hardware. However the original Shear-Warp algorithm suffers from a high memory overhead and does not provide for incremental rendering which is required should rapid user feedback be maintained. Our algorithm represents the volumetric data using a hierarchical data structure which provides for the incremental classification and rendering of volume data. This exploits the multiscale nature of the octree data structure. The algorithm reduces the memory footprint of the original Shear-Warp Factorisation algorithm by a factor of more than two, while maintaining good rendering performance. These factors make our octree algorithm more suitable for implementation on average desktop workstations for the purposes of interactive exploration of volume models over a network. Results from tests using typical volume datasets will be presented which demonstrate the ability of the algorithm to achieve high rendering rates for both incremental rendering and standard rendering while reducing the runtime memory requirements.     

A Parallel Algorithm for Volume Projections on SIMD Mesh-Connected Computers

Projections are widely used in machine vision, volume rendering, and computer graphics. For applications with 3D volume data, we design a parallel projection algorithm on SIMD mesh-connected computers and implement the algorithm on the Parallel Algebraic Logic (PAL) computer. The algorithm is a parallel ray casting algorithm for both orthographic and perspective projections. It decomposes a volume projection into two transformations that can be implemented in the SIMD fashion to solve the data distribution and redistribution problem caused by non-regular data access patterns in volume projections.     

Hardware‐Assisted Projected Tetrahedra

We present a flexible and highly efficient hardware‐assisted volume renderer grounded on the original Projected Tetrahedra (PT) algorithm. Unlike recent similar approaches, our method is exclusively based on the rasterization of simple geometric primitives and takes full advantage of graphics hardware. Both vertex and geometry shaders are used to compute the tetrahedral projection, while the volume ray integral is evaluated in a fragment shader; hence, volume rendering is performed entirely on the GPU within a single pass through the pipeline. We apply a CUDA‐based visibility ordering achieving rendering and sorting performance of over 6 M Tet/s for unstructured datasets. Furthermore, as each tetrahedron is processed independently, we employ a data‐parallel solution which is neither bound by GPU memory size nor does it rely on auxiliary volume information. In addition, iso‐surfaces can be readily extracted during the rendering process, and time‐varying data are handled without extra burden.