Recent Advances in Volume Visualization

In the past few years, there have been key advances in the three main approaches to the visualization of volumetric data: isosurfacing, slicing and volume rendering, which together make up the field of volume visualization.
In this survey paper we set the scene by describing the fundamental techniques for each of these approaches, using this to motivate the range of advances which have evolved over the past few years.
In isosurfacing, we see how the original marching cubes algorithm has matured, with improvements in robustness, topological consistency, accuracy and performance. In the performance area, we look in detail at pre-processing steps which help identify data which contributes to the particular isosurface required. In slicing too, there are performance gains from identifying active cells quickly.
In volume rendering, we describe the two main approaches of ray casting and projection. Both approaches have evolved technically over the past decade, and the holy grail of real-time volume rendering has arguably been reached.
The aim of this review paper is to pull these developments together in a coherent review of recent advances in volume visualization.     

Rendering of forest scenery using 3D textures

Visual simulation of forest scenery is a challenging problem which includes the following tough sub-problems: generation of vegetation, representation of trees, simulation of colour change of leaves, and rendering of numerous trees. Among those sub-problems, this paper treats mainly the last one. A conventional polygon-based rendering algorithm often produces troublesome aliasing effects when it is applied to the objects having complex fine surfaces, such as forest scenery. In this paper, we show that an extended volume rendering technique applied to 3D textures, i.e. volume data in this paper, of trees is effective in the concerned problem. Kajiya left, as further work, the problem of rendering forest scenery by applying his 3D texture called texel. Our rendering method consists of the following three steps: we first generate 3D textures of trees from their polygon-based geometric models, we next arrange the 3D textures, allowing their possible mutual intersections, on the surface of a given polygon-based terrain model according to a simulated vegetation, and we finally produce an image of forest scenery by applying the ray-tracing algorithm including our slightly extended volume rendering technique. © 1997 John Wiley & Sons, Ltd.     

Real‐time Indirect Illumination of Emissive Inhomogeneous Volumes using Layered Polygonal Area Lights

Indirect illumination involving with visually rich participating media such as turbulent smoke and loud explosions contributes significantly to the appearances of other objects in a rendering scene. However, previous real‐time techniques have focused only on the appearances of the media directly visible from the viewer. Specifically, appearances that can be indirectly seen over reflective surfaces have not attracted much attention. In this paper, we present a real‐time rendering technique for such indirect views that involves the participating media. To achieve real‐time performance for computing indirect views, we leverage layered polygonal area lights (LPALs) that can be obtained by slicing the media into multiple flat layers. Using this representation, radiance entering each surface point from each slice of the volume is analytically evaluated to achieve instant calculation. The analytic solution can be derived for standard bidirectional reflectance distribution functions (BRDFs) based on the microfacet theory. Accordingly, our method is sufficiently robust to work on surfaces with arbitrary shapes and roughness values. In addition, we propose a quadrature method for more accurate rendering of scenes with dense volumes, and a transformation of the domain of volumes to simplify the calculation and implementation of the proposed method. By taking advantage of these computation techniques, the proposed method achieves real‐time rendering of indirect illumination for emissive volumes.     

GPU加速的台风可视化方法

自然现象的可视化是计算机图形学和虚拟现实领域的重要研究内容。对传统光线投射算法分析的基础上进行改进,提出基于球壳体的光线投射算法。将GPU运用于球壳体数据场的体绘制,设计了基于球壳体数据场的顶点着色程序和像素着色程序。同时,对台风源数据格式进行解析,生成了用于台风可视化的体数据,采用提出的算法实现了台风云层和因子的可视化。实验结果表明,本文基于GPU的球壳体光线投射算法在球体表面较好地实现了实时台风可视化效果。     

A high-speed integrated renderer for interpreting multiple 3D volume data

This paper proposes an integrated rendering algorithm for visualizing 3D volumetric and geometric data, such as surfaces, lines and points, simultaneously with depth information, and other algorithms for improving the performance of the rendering process of the first. The first algorithm extends a volume rendering algorithm based on ray-tracing so that it can handle both 3D volumetric and geometric data. It processes these data in accordance with their original representation formats to eliminate conversion artefacts such as spurious or missing surfaces, and also gives special treatment to volume segments so as to avoid errors in visibility at the intersections between volume segments and geometric data. The other algorithms employ adaptive termination of ray-tracing, elimination of rays that do not intersect the volume, and adaptive undersampling over an image plane. These improve the performance by three to seven times over the brute-force approach. The cost and versatility of the algorithm are evaluated by using data from the results of 3D computational fluid dynamics.     

医学图像三维可视化中任意平面切片的研究与实现

对医学图像的三维重建技术进行研究,利用介于面绘制和体绘制之间的切片法来实现三维体数据可视化,并针对断层图像序列所生成三维体数据的切片方法进行了深入讨论。提出一种可操作性很强的切片方法,能较好地实现三维体数据中冠状面、矢状面、特别是任意平面的切片。同时,该方法已经用VC6．0结合VTK（Visualization Toolkit）编程进行实现,效果符合医学诊断要求,并得到了有关应用。     

Concentric Spherical Representation for Omnidirectional Soft Shadow

Soft shadows play an important role in photo‐realistic rendering. Although there are many efficient soft shadow algorithms, most of them focus on the one‐side light source situation, where a planar light source is on the outside of the scene. In fact, in many situations, such as games, light sources are omnidirectional. They may be surrounded by a number of 3D objects. This paper proposes a soft shadow algorithm for the omnidirectional situation. We develop a concentric spherical representation to model the behaviour of omnidirectional light sources. To provide better rendering results, a novel summed area table based filtering scheme for spherical functions is proposed. In addition, we utilize unicube mapping, which samples the spherical space more uniformly, to further improve the filtering quality.     

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。     

大坝地震反应数据场快速体绘制算法

为了弥补传统体绘制静止画面时，不能明确判断场值集中部位的不足，提出了一种快速体绘制算法，针对大坝地震反应有限元计算数据场，根据每个单元法线与视线夹角最小的表面来选择剖切方向，将单元剖切成多个四边形面后进行颜色融合和叠加，大大提高了体绘制速度，实现了动态观察体绘制图，从而清楚地分析场值集中部位。     

Distributed shared memory for roaming large volumes

We present a cluster-based volume rendering system for roaming very large volumes. This system allows to move a gigabyte-sized probe inside a total volume of several tens or hundreds of gigabytes in real-time. While the size of the probe is limited by the total amount of texture memory on the cluster, the size of the total data set has no theoretical limit. The cluster is used as a distributed graphics processing unit that both aggregates graphics power and graphics memory. A hardware-accelerated volume renderer runs in parallel on the cluster nodes and the final image compositing is implemented using a pipelined sort-last rendering algorithm. Meanwhile, volume bricking and volume paging allow efficient data caching. On each rendering node, a distributed hierarchical cache system implements a global software-based distributed shared memory on the cluster. In case of a cache miss, this system first checks page residency on the other cluster nodes instead of directly accessing local disks. Using two Gigabit Ethernet network interfaces per node, we accelerate data fetching by a factor of 4 compared to directly accessing local disks. The system also implements asynchronous disk access and texture loading, which makes it possible to overlap data loading, volume slicing and rendering for optimal volume roaming.     

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.     

针对全空子数据体的GPU体绘制

目的 体绘制是3维数据可视化的主要方法之一。用于体绘制的数据体中包含有大量的空体素,导致光线投射算法进行没有意义的重采样计算,必然降低绘制算法效率。针对全空子数据体体绘制低效问题,提出基于GPU体高效绘制方法。方法 利用八叉树数据结构组织数据,有效管理包含许多空体素的子数据体。通过绘制八叉树非全空叶子节点子数据体表面,使光线投射算法中起始和终止重采样位置更接近数据体中的可视部分,同时根据八叉树全空节点子数据体判定纹理查询结果,计算合适的跳跃步长,快速跳过八叉树中全空节点子数据体。结果 当数据体中空体素较多时,确定合适的八叉树深度,有效地跳过数据体中的空体素,减少体绘制运算量,实现对原基于体包围盒表面绘制的GPU光线投射算法的加速。结论 设计不透明度函数,凸显数据体中层位面,并将算法成功应用于地震数据可视化,取得很好应用效果。     

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.     

Dynamic Sampling and Rendering of Algebraic Point Set Surfaces

Algebraic Point Set Surfaces (APSS) define a smooth surface from a set of points using local moving least‐squares (MLS) fitting of algebraic spheres. In this paper we first revisit the spherical fitting problem and provide a new, more generic solution that includes intuitive parameters for curvature control of the fitted spheres. As a second contribution we present a novel real‐time rendering system of such surfaces using a dynamic up‐sampling strategy combined with a conventional splatting algorithm for high quality rendering. Our approach also includes a new view dependent geometric error tailored to efficient and adaptive up‐sampling of the surface. One of the key features of our system is its high degree of flexibility that enables us to achieve high performance even for highly dynamic data or complex models by exploiting temporal coherence at the primitive level. We also address the issue of efficient spatial search data structures with respect to construction, access and GPU friendliness. Finally, we present an efficient parallel GPU implementation of the algorithms and search structures.     

GPU加速的八叉树体绘制算法

提出一种针对物体空间为序体绘制的空域跳过算法：采用双层次空间跳过,先以规则的数据分块作粗略地跳过,再以八叉树获得更高粒度的优化。该方法进一步解决了超过可用纹理内存容量的大规模体数据实时绘制问题,允许实时改变传递函数。针对该算法引入的CPU高负载瓶颈,提出一种新算法,在图形处理器(GPU)内快速计算采样面片,平衡了CPU与GPU间的运算负载。结合上述两种算法,实现高效的大规模体数据绘制并无损图像质量。     

基于GPU的交互式体数据切割

为实现实时高效的体数据切割操作,提出一种将人机交互和基于GPU的体绘制相结合的切割方法.先通过人机交互生成多个切割体的几何形状并三角化,将其作为基于GPU的光线投射算法的代理面,然后利用深度剥离算法获得光线在所有代理面上的出入射点坐标,并用多个通道完成体绘制.该方法在单个3D纹理上进行,可以实现多个任意形状的切割体切割,在节省GPU内存空间的同时,提高了切割后的绘制效率.     

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.     

基于顶点编程的三维纹理硬件体绘制算法

为克服图形硬件对传统纹理映射体绘制的限制,提出并分析讨论了采用顶点编程来有效地实现基于纹理的体绘制中的切片组与包围盒相交过程的方法。这种新颖的技术能保证顶点处理器、片段处理器与内存宽带间工作量的平衡。同时,通过对梯度的实时计算来减少在传统纹理映射体绘制中巨大的内存消耗。最后应用这种技术结合空区域跳跃技术,有效地去除了体数据中的空区域,降低了硬件的负载,加速了体绘制的过程。测试表明对较大规模的体数据,该算法能较大地提高性能。     

Rapid and Accurate Contact Determination between Spline Models using ShellTrees

In this paper, we present an efficient algorithm for contact determination between spline models. We make use of a new hierarchy, called ShellTree , that comprises of spherical shells and oriented bounding boxes. Each spherical shell corresponds to a portion of the volume between two concentric spheres. Given large spline models, our algorithm decomposes each surface into Bézier patches as part of pre-processing. At runtime it dynamically computes a tight fitting axis-aligned bounding box across each Bézier patch and efficiently checks all such boxes for overlap. Using off-line and on-line techniques for tree construction, our algorithm computes ShellTrees for Bézier patches and performs fast overlap tests between them to detect collisions. The overall approach can trade off runtime performance for reduced memory requirements. We have implemented the algorithm and tested it on large models, each composed of hundred of patches. Its performance varies with the configurations of the objects. For many complex models composed of hundreds of patches, it can accurately compute the contacts in a few milliseconds.     

从多幅参考图像合成目标图像的逆映射算法

提出了一种从多幅参考图像来合成新视点目标图像的逆映射算法.这种方法不但成功地填补了由于投影区域扩张而产生的第1类空洞,而且还成功地填补了由于空间非深度连续物体相互遮挡而产生的第2类空洞,从而方便地实现了虚拟环境中的漫游.基于物体表面深度的连续性,提出了一种位移预测方法.该方法可以从单幅参考图像获得逆映射过程中所需要的目标图像的位移信息,从而大大提高了算法的效率.与通常的正向映射算法相比,该算法克服了多幅参考图像所带来的计算量成倍增长等问题,而且误差较小.