Interactive Rendering with Coherent Ray Tracing

For almost two decades researchers have argued that ray tracing will eventually become faster than the rasterization technique that completely dominates todays graphics hardware. However, this has not happened yet. Ray tracing is still exclusively being used for off-line rendering of photorealistic images and it is commonly believed that ray tracing is simply too costly to ever challenge rasterization-based algorithms for interactive use. However, there is hardly any scientific analysis that supports either point of view. In particular there is no evidence of where the crossover point might be, at which ray tracing would eventually become faster, or if such a point does exist at all.
This paper provides several contributions to this discussion: We first present a highly optimized implementation of a ray tracer that improves performance by more than an order of magnitude compared to currently available ray tracers. The new algorithm make better use of computational resources such as caches and SIMD instructions and better exploits image and object space coherence. Secondly, we show that this software implementation can challenge and even outperform high-end graphics hardware in interactive rendering performance for complex environments. We also provide an brief overview of the benefits of ray tracing over rasterization algorithms and point out the potential of interactive ray tracing both in hardware and software.     

PERIS: A programming environment for realistic image synthesis

A computer graphics system called PERIS is described. It offers two levels of user interfaces to serve both as a testbed to facilitate further research work and as a pragmatic system for CAD/CAM applications. Flexibility of the system is achieved by providing tools of solid modeling, surface modeling and procedural model representations for physical environment modeling and by including interfaces of immediate display, scan line rendering, ray tracing and two-way ray tracing for realistic image synthesis. A linear octree data structure is established to support the modeling and rendering processes, greatly reducing the computations involved. Meanwhile, we also introduce a new illumination model which unifies most of the existing models on a theoretical basis. An improved Cook-Torrance model is then derived. By examining the merits and limitations of the classic ray tracing methods, we propose a new rendering technique of two-way ray tracing that allows more accurate simulation of light propagation in the environment.     

Adaptive Ray‐bundle Tracing with Memory Usage Prediction: Efficient Global Illumination in Large Scenes

This paper proposes an adaptive rendering technique for ray‐bundle tracing. Ray‐bundle tracing can be done by per‐pixel linked‐list construction on a GPU rasterization pipeline. This rasterization based approach offers significant benefits for the efficient generation of light maps (e.g., hardware acceleration, tessellation, and recycling of shaders used in real‐time graphics). However, it is inapplicable to large and complex scenes due to the limited capacity of the GPU memory because it requires a high‐resolution frame buffer and high‐capacity node buffer for the linked‐lists. In addition, memory overflow can potentially occur on the per‐pixel linked‐list since the memory usage of the lists is usually unknown before the rendering process. We introduce an adaptive tiling technique with memory usage prediction. Our method uses an appropriately tiled frame buffer, thus eliminating almost all of the overflow risks thanks to our adaptive tile subdivision scheme. Using this technique, we are able to render high‐quality light maps of large and complex scenes which cannot be computed using previous ray‐bundle based methods.     

Special relativistic visualization by local ray tracing

Special relativistic visualization offers the possibility of experiencing the optical effects of traveling near the speed of light, including apparent geometric distortions as well as Doppler and searchlight effects. Early high-quality computer graphics images of relativistic scenes were created using offline, computationally expensive CPU-side 4D ray tracing. Alternate approaches such as image-based rendering and polygon-distortion methods are able to achieve interactivity, but exhibit inferior visual quality due to sampling artifacts. In this paper, we introduce a hybrid rendering technique based on polygon distortion and local ray tracing that facilitates interactive high-quality visualization of multiple objects moving at relativistic speeds in arbitrary directions. The method starts by calculating tight image-space footprints for the apparent triangles of the 3D scene objects. The final image is generated using a single image-space ray tracing step incorporating Doppler and searchlight effects. Our implementation uses GPU shader programming and hardware texture filtering to achieve high rendering speed.     

Parallel rendering of fractal surfaces

Fractal surfaces are a sueful modeling technique for terrain in computer graphics. Although an algorithm exists for ray tracing (Mandelbrot) fractal surfaces, the technique is computationally very expensive. The large degree of parallelism inherent in the problem suggests the use of parallel architectures for generating these images. We describe a parallel rendering algorithm for shared memory MIMD machines which takes advantage of image coherence to reduce computation. This algorithm has, on a Sequent Balance 2100 with 20 processors, demonstrated a near-linear speedup. We examine the possible synchronization bottlenecks by statically assigning different numbers of CPUs to sections of the screen.This work was supported in part by DARPA under contract MDA903-86-C-0182.     

基于分类及环境特征的树木真实感绘制

生成高度真实感的虚拟自然场景一直是图形学研究领域中的一个富有挑战性的难题.作为自然场景的重要组成部分,树木的真实感模拟也得到人们的广泛重视.树木种类繁多,形态各异,复杂的结构使其无论在造形、存储还是在绘制上都存在着相当的困难.针对不同的环境特征和不同类型的树木,根据它们的具体特点需采用不同的绘制手段.对于阔叶树,采用基于OpenGL的深度缓存阴影生成算法;对于针叶树,则采用结合光线跟踪和纹元(texel)绘制的技术;而对于距离视点比较远的树,采用的是体纹理映射(volumetrictexturemapping)的方法.实践证明,这几种绘制技术基本上可以满足各种不同类型场景的树木的绘制要求.     

Ray tracing-based interactive diffuse indirect illumination

Despite great efforts in recent years to accelerate global illumination computation, the real-time ray tracing of fully dynamic scenes to support photorealistic indirect illumination effects has yet to be achieved in computer graphics. In this paper, we propose an extended ray tracing model that can be readily implemented on a GPU to facilitate the interactive generation of diffuse indirect illumination, the quality of which is comparable to that generated by the traditional, time-consuming photon mapping method and final gathering. Our method employs three types of (multilevel) grids to represent the indirect light in a scene using a form that facilitates the efficient estimation of the reflected radiance caused by diffuse interreflection. This method includes the mathematical tool of spherical harmonics and a rendering scheme that performs the final gathering step with a minimal cost during ray tracing, which guarantees the interactive frame rates. We evaluated our technique using several dynamic scenes with nontrivial complexity, which demonstrated its effectiveness.     

The use of coherent ray tracing for physically accurate rendering

As the power of modern microprocessors increases, the coherent ray tracing becomes increasingly popular in computer graphics because the use of SIMD instructions considerably speeds up this operation. However, after speeding up ray tracing, it turns out that other algorithms for physically accurate rendering, such as the calculation of illumination or application of texture, etc., become a bottleneck in improving the performance. In this paper, a coherent physically accurate rendering algorithm is proposed that makes use of SIMD instructions of modern processors at each stage of the image generation. Coherent algorithms for the calculation of illumination and materials, for antialiasing, and for tone mapping are presented. The comparison of the execution time of coherent and incoherent algorithms using benchmark scenes showed that the former are considerably faster.     

CD漫游体光线追踪算法中反走样的研究

图形绘制在虚拟现实中占有很重要的地位，走样亦为图形绘制中的一关键技术。有序抖动算法是反走样技术的一种。介绍了有序抖动算法在CD漫游体光线追踪算法中的应用。     

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。     

并行计算在光线追踪中的应用

目前,计算机图形学的技术包括有光栅化,光线追踪,辐射度3种主流算法.而光线追踪又以其绘制的真实感性及实现的方便性,受到了广泛的应用.但光线追踪的最大问题在于性能.围绕光线追踪中的核心算法-射线与三角形相交算法展开讨论,引入基于Intel TBB并行编程工具的线程级并行技术以及基于SIMD的指令并行技术,提高其算法的速度.其加速技术可以使光线跟踪的性能在计算机多核的情况下,相对于串行跟踪方法有明显的改善.     

光线跟踪方法在体绘制中的应用与发展

针对光线跟踪方法存在求交计算量大、实时交互性差等缺点.从体光线跟踪算法、基于图形硬件加速的光线跟踪体绘制方法,并行光线跟踪体绘制技术这3个方面,对国内外光线跟踪方法在体绘制中的应用技术进行了分类描述与综述,重点介绍了体光照模型、光线跟踪混合绘制、基于可编程硬件加速、并行绘制算法及体系结构,并结合应用阐述了各自特点及其相互联系,最后对光线跟踪方法在体绘制的应用提出了研究建议.     

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     

Fast ray tracing and the potential effects on graphics and gaming courses

The modern graphics processing units (GPUs), found on almost every personal computer, use the z-buffer algorithm to compute visibility. Ray tracing, an alternative to the z-buffer algorithm, delivers higher visual quality than the z-buffer algorithm but has historically been too slow for interactive use. However, ray tracing has benefited from improvements in computer hardware, and many believe it will replace the z-buffer algorithm as the graphics engine on PCs. If this replacement happens, it will imply fundamental changes in both the API to and capabilities of 3D graphics engines. This paper overviews the backgrounds in z-buffer and ray tracing, presents our case that ray tracing will replace z-buffer in the near future, and discusses the implications for graphics oriented classes should this switch to ray tracing occur. Since computer gaming is one of the most important industry driving graphics hardware and the fact that recently there are many computer science courses related to games and games development, we also describe the potential impact on games related classes.     

A meshing scheme for efficient hardware implementation of butterfly subdivision using displacement mapping

Displacement mapping is an effective technique for encoding the high levels of detail of surface models using coarse triangle meshes and displacement maps. These maps are 2D representations containing the distances between the coarse mesh and the surface to represent. Displacement maps have been used in many applications such as ray tracing, image warping, and volume rendering. In this article, we propose modifications to our previous grouping strategy, a new subdivision strategy based on the Modified Butterfly algorithm and new heuristics for the adaptive subdivision procedure, and, finally, the corresponding modifications on our hardware proposal. A meshing scheme and an adaptive subdivision strategy based on displacement mapping reduce the bottleneck between the CPU and graphics pipeline common in high-performance graphics systems.     

An Adaptive Spectral Rendering with a Perceptual Control

In this paper, we present a spectral rendering method based on a ray tracing algorithm and guided by a perceptual control of the error made. An adaptive representation of spectral data for light sources and materials is used and induces, for each pixel, the evaluation of an algebraic expression. For each visible wavelength, the computations of complex spread in refractive and dispersive materials, based on several photon maps, are locally restricted by an adaptive evaluation of the expression. This method allows to simulate high quality physically-based pictures and, in particular, some specific phenomena like dispersion in transparent objects, scattered caustics,. . .     

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.     

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