Faster isosurface ray tracing using implicit KD-trees

The visualization of high-quality isosurfaces at interactive rates is an important tool in many simulation and visualization applications. Today, isosurfaces are most often visualized by extracting a polygonal approximation that is then rendered via graphics hardware or by using a special variant of preintegrated volume rendering. However, these approaches have a number of limitations in terms of the quality of the isosurface, lack of performance for complex data sets, or supported shading models. An alternative isosurface rendering method that does not suffer from these limitations is to directly ray trace the isosurface. However, this approach has been much too slow for interactive applications unless massively parallel shared-memory supercomputers have been used. In this paper, we implement interactive isosurface ray tracing on commodity desktop PCs by building on recent advances in real-time ray tracing of polygonal scenes and using those to improve isosurface ray tracing performance as well. The high performance and scalability of our approach will be demonstrated with several practical examples, including the visualization of highly complex isosurface data sets, the interactive rendering of hybrid polygonal/isosurface scenes, including high-quality ray traced shading effects, and even interactive global illumination on isosurfaces.     

Explanatory and illustrative visualization of special and general relativity

This paper describes methods for explanatory and illustrative visualizations used to communicate aspects of Einstein's theories of special and general relativity, their geometric structure, and of the related fields of cosmology and astrophysics. Our illustrations target a general audience of laypersons interested in relativity. We discuss visualization strategies, motivated by physics education and the didactics of mathematics, and describe what kind of visualization methods have proven to be useful for different types of media, such as still images in popular science magazines, film contributions to TV shows, oral presentations, or interactive museum installations. Our primary approach is to adopt an egocentric point of view: the recipients of a visualization participate in a visually enriched thought experiment that allows them to experience or explore a relativistic scenario. In addition, we often combine egocentric visualizations with more abstract illustrations based on an outside view in order to provide several presentations of the same phenomenon. Although our visualization tools often build upon existing methods and implementations, the underlying techniques have been improved by several novel technical contributions like image-based special relativistic rendering on GPUs, special relativistic 4D ray tracing for accelerating scene objects, an extension of general relativistic ray tracing to manifolds described by multiple charts, GPU-based interactive visualization of gravitational light deflection, as well as planetary terrain rendering. The usefulness and effectiveness of our visualizations are demonstrated by reporting on experiences with, and feedback from, recipients of visualizations and collaborators.     

Adaptive undersampling for efficient mobile ray tracing

Aiming to develop an efficient ray tracer for a mobile platform, we present an adaptive undersampling method that enhances the rendering speed by effectively replacing expensive ray-tracing operations with cheap interpolation whenever possible. Our method explores both object- and image-space information gathered during ray tracing to detect possibly problematic pixels. Rays are fired only for these pixels. We also present a postcorrection algorithm that minimizes annoying artifacts inevitably caused by undersampling. Our implementation on a mobile GPU demonstrates that this method can speed up the rendering computation significantly, while retaining almost the same visual quality of the rendering.     

Perceptually Guided Corrective Splatting

One of the basic difficulties with interactive walkthroughs is the high quality rendering of object surfaces with non-diffuse light scattering characteristics. Since full ray tracing at interactive rates is usually impossible, we render a precomputed global illumination solution using graphics hardware and use remaining computational power to correct the appearance of non-diffuse objects on-the-fly. The question arises, how to obtain the best image quality as perceived by a human observer within a limited amount of time for each frame. We address this problem by enforcing corrective computation for those non-diffuse objects that are selected using a computational model of visual attention. We consider both the saliency- and task-driven selection of those objects and benefit from the fact that shading artifacts of "unattended" objects are likely to remain unnoticed. We use a hierarchical image-space sampling scheme to control ray tracing and splat the generated point samples. The resulting image converges progressively to a ray traced solution if the viewing parameters remain unchanged. Moreover, we use a sample cache to enhance visual appearance if the time budget for correction has been too low for some frame. We check the validity of the cached samples using a novel criterion suited for non-diffuse surfaces and reproject valid samples into the current view.     

Relativistic Effects for Time‐Resolved Light Transport

We present a real‐time framework which allows interactive visualization of relativistic effects for time‐resolved light transport. We leverage data from two different sources: real‐world data acquired with an effective exposure time of less than 2 picoseconds, using an ultra‐fast imaging technique termed femto‐photography, and a transient renderer based on ray‐tracing. We explore the effects of time dilation, light aberration, frequency shift and radiance accumulation by modifying existing models of these relativistic effects to take into account the time‐resolved nature of light propagation. Unlike previous works, we do not impose limiting constraints in the visualization, allowing the virtual camera to explore freely a reconstructed 3D scene depicting dynamic illumination. Moreover, we consider not only linear motion, but also acceleration and rotation of the camera. We further introduce, for the first time, a pinhole camera model into our relativistic rendering framework, and account for subsequent changes in focal length and field of view as the camera moves through the scene.     

基于图象的交互绘制和数据压缩

体数据的绘制速度是交互可视化的瓶颈。将基于图象的绘制方法和传统的体绘制方法相结合,提出了一个新的基于图象的快速图形图象绘制系统,大大提高了体数据交互可视化的速度。该方法首先用传统的体绘制方法得到一列采样图象,然后根据图象间象素的冗合性,通过Ｗａｒｐｉｎｇ变换压缩数据,由压缩后的数据Ｗａｒｐｉｎｇ生成任意视点的视图。为了进一步提高计算速度,给出了递推Ｗａｒｐｉｎｇ变换计算流程。     

Nonlinear ray tracing: Visualizing strange worlds

Nonlinear ray tracing is investigated in this work. Sources of nonlinearity such as gravity centers, gravity lines, chaotic dynamical systems, and parametric curved rays are discussed. Curved rays are represented either iteratively or hierarchically. Algorithms for testing whether a curved ray and a 3D object intersect are presented. Sample images of a test implementation show the feasibility of the approach. Applications of nonlinear ray tracing are the visualization of relativistic effects, visualization of the geometric behavior of nonlinear dynamical systems, visualization of the movement of charged particles in a force field (e.g., electron movement), virtual reality, and arts.     

基于八叉树邻域分析光线跟踪的云三维模拟

针对目前三维云模拟绘制效率低、计算资源消耗大、绘制效果差等问题,提出基于八叉树邻域分析的光线跟踪算法,并用于WRF模式云数据的三维模拟。使用八叉树结构优化传统光线跟踪算法的数据存储结构,通过存储节点编码和划分层次改进邻域分析算法,通过简化光线的折射公式优化Whitted光照模型,借助OpenGL和Vapor工具实现云数据的三维可视化。实验结果表明,该方法降低了绘制时间,提高了渲染效率,更好体现了云的真实物理特征。     

Accelerating Refractive Rendering of Transparent Objects

In this paper, a technique is proposed for the rendering of transparent objects interactively using the method of refractive rendering. In the proposed technique, the refractive rendering algorithm is performed in two stages, namely the pre‐computation stage and the shading stage. In the pre‐computation stage, ray‐traced information, including directions and positions of rays, are generated by a ray tracing process and are stored in a set of ray lists. In the shading stage, these data are retrieved from the ray lists for computing the shading of an object. Using the proposed technique, photorealistic image of transparent objects and gemstones with various cuttings, material properties, lighting and background can be rendered interactively. By combining the refractive rendering technique with conventional shading techniques, jewelry and crystal designs can be rendered at a much higher speed comparing with conventional ray tracing techniques.     

结合K-D树和Shell的快速动态等值面光线跟踪法

目的 K-D树和Shell常被用于加速等值面光线跟踪法。如果Isovalue保持不变时,Shell方法效率更高,否则Shell方法需要重构Shell,而K-D树方法速度更快。提出一种结合K-D树和Shell的动态等值面光线跟踪方法,其关键是如何实现两者的平稳切换。方法首先改进基于K-D树的等值面光线跟踪算法,使得该方法在绘制过程中渐进地构建Shell数据结构。在Isovalue发生变化后,首先使用改进的基于K-D树的等值面光线跟踪算法进行绘制,并在绘制过程中渐进地构建新的Shell数据结构,从而平滑地过渡到绘制效率更高的基于Shell的等值面光线跟踪算法。结果实验中,在Isovalue动态变化时本文方法的效率接近K-D树方法;但用户在进行缩放、旋转等操作时,本文方法能达到Shell方法的速度。实验结果表明本文方法结合了两者的优点。结论提出了一种快速Isovalue光线跟踪算法,综合运用K-D树和Shell两种数据结构,在Isovalue保持不变和动态变化两者场合都实现了较高的绘制速度。     

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 tracing CSG trees using the Sticks representation scheme

A fast ray tracing algorithm of CSG tree is presented. The algorithm uses an adaptive space subdivisions approach, based on the conversion of the objects of the scene into the volumetric Sticks representation scheme. This conversion scheme, which requires O(kn²) memory space to represent data in a n³ grid, makes it possible either to obtain very fast low-quality frontal orthographic projections, or to produce a high-quality rendering of the scene in time less than that needed by classical ray tracing and nearly independent of the number of objects in the scene. Furthermore, the characteristics of the Sticks scheme can be exploited to compute geometric or topological properties of the represented objects. Comparative analyses between the Sticks representation scheme and classical space subdivision schemes and between our Sticksbased ray tracing and classical algorithms are presented.     

GPU上的水彩画风格实时渲染及动画绘制

论文提出了一种基于GPU 的对三维场景进行实时水彩画效果渲染的方法。 该方法的大部分过程使用图像空间的技术实现。算法将画面分为细节层、环境层、笔触层分 别渲染,再进行合成。在过程中使用环境遮挡、shadow mapping 等技术进行快速的阴影计算, 并使用图像滤镜的方法模拟水彩的多种主要特征。由于该方法以图像空间的技术为主,因此 可以利用GPU 并行处理的特点对计算过程进行加速,进而达到实时的渲染速度。最后建立 动画脚本分析系统,进行实时动画渲染,表明该方法在计算机动画、游戏等数字娱乐产业领 域有较大的应用潜力。     

An architecture for Java-based real-time distributed visualization

In this paper, we present a Java-based software architecture for real-time visualization that utilizes a cluster of conventional PCs to generate high-quality interactive graphics. Normally, a large multiprocessor computer would be needed for interactive visualization tasks requiring more processing power than a single PC can provide. By using clusters of PCs, enormous cost savings can be realized, and proprietary "high-end" hardware is no longer necessary for these tasks. Our architecture minimizes the amount of synchronization needed between PCs, resulting in excellent scalability. It provides a modular framework that can accommodate a wide variety of rendering algorithms and data formats, provided that the rendering algorithms can generate pixels individually and the data is duplicated on each PC. Demonstration modules that implement ray tracing, fractal rendering, and volume rendering algorithms were developed to evaluate the architecture. Results are encouraging-using 15 PCs connected to a standard 100 Megabit/s Ethernet network, the system can interactively render simple to moderately complex data sets at modest resolution. Excellent scalability is achieved; however, our tests were limited to a cluster of 15 PCs. Results also demonstrate that Java is a viable platform for real-time distributed visualization.     

Interactive isosurface ray tracing of time-varying tetrahedral volumes

We describe a system for interactively rendering isosurfaces of tetrahedral finite-element scalar fields using coherent ray tracing techniques on the CPU. By employing state-of-the art methods in polygonal ray tracing, namely aggressive packet/frustum traversal of a bounding volume hierarchy, we can accomodate large and time-varying unstructured data. In conjunction with this efficiency structure, we introduce a novel technique for intersecting ray packets with tetrahedral primitives. Ray tracing is flexible, allowing for dynamic changes in isovalue and time step, visualization of multiple isosurfaces, shadows, and depth-peeling transparency effects. The resulting system offers the intuitive simplicity of isosurfacing, guaranteed-correct visual results, and ultimately a scalable, dynamic and consistently interactive solution for visualizing unstructured volumes.     

Generating exact ray-traced animation frames by reprojection

Reprojection techniques can create approximate ray traced animation frames. Extending an existing algorithm yields exact frames and full ray tracing, with up to 92 percent savings in rendering time. We present an algorithm that exploits spatiotemporal coherence between frames to significantly decrease the rendering time of ray traced animations. This method produces inferred ray traced images of any scene that can be ray traced using a point sampled method. The images created by the algorithm are not approximated frames created from weighted averages of other frames, nor are they frames patched together from near frame pixel values. The algorithm guarantees that a color seen in a subpixel would be returned by a ray passing somewhere through that subpixel, but not necessarily though the center. This algorithm efficiently creates frames of any view that can be ray traced. While the savings increase with the complexity of the rendered objects and the preponderance of diffuse objects, significant savings occur with reflective and refractive objects. However, the technique requires the ray tracing method to be point sample oriented     

Hardware-assisted visibility sorting for unstructured volume rendering

Harvesting the power of modern graphics hardware to solve the complex problem of real-time rendering of large unstructured meshes is a major research goal in the volume visualization community. While, for regular grids, texture-based techniques are well-suited for current GPUs, the steps necessary for rendering unstructured meshes are not so easily mapped to current hardware. We propose a novel volume rendering technique that simplifies the CPU-based processing and shifts much of the sorting burden to the GPU, where it can be performed more efficiently. Our hardware-assisted visibility sorting algorithm is a hybrid technique that operates in both object-space and image-space. In object-space, the algorithm performs a partial sort of the 3D primitives in preparation for rasterization. The goal of the partial sort is to create a list of primitives that generate fragments in nearly sorted order. In image-space, the fragment stream is incrementally sorted using a fixed-depth sorting network. In our algorithm, the object-space work is performed by the CPU and the fragment-level sorting is done completely on the GPU. A prototype implementation of the algorithm demonstrates that the fragment-level sorting achieves rendering rates of between one and six million tetrahedral cells per second on an ATI Radeon 9800.     

Dynamic shader generation for GPU-based multi-volume ray casting

Real-time performance for rendering multiple intersecting volumetric objects requires the speed and flexibility of modern GPUs. This requirement has restricted programming of the necessary shaders to GPU experts only. A visualization system that dynamically generates GPU shaders for multi-volume ray casting from a user-definable abstract render graph overcomes this limitation.     

Fast ray-tracing of rectilinear volume data using distancetransforms

The paper discusses and experimentally compares distance based acceleration algorithms for ray tracing of volumetric data with an emphasis on the Chessboard Distance (CD) voxel traversal. The acceleration of this class of algorithms is achieved by skipping empty macro regions, which are defined for each background voxel of the volume. Background voxels are labeled in a preprocessing phase by a value, defining the macro region size, which is equal to the voxel distance to the nearest foreground voxel. The CD algorithm exploits the chessboard distance and defines the ray as a nonuniform sequence of samples positioned at voxel faces. This feature assures that no foreground voxels are missed during the scene traversal. Further, due to parallelepipedal shape of the macro region, it supports accelerated visualization of cubic, regular, and rectilinear grids. The CD algorithm is suitable for all modifications of the ray tracing/ray casting techniques being used in volume visualization and volume graphics. However, when used for rendering based on local surface interpolation, it also enables fast search of intersections between rays and the interpolated surface, further improving speed of the process