Two-stage Resampling for Bidirectional Path Tracing with Multiple Light Sub-paths

Recent advances in bidirectional path tracing (BPT) reveal that the use of multiple light sub-paths and the resampling of a small number of these can improve the efficiency of BPT. By increasing the number of pre-sampled light sub-paths, the possibility of generating light paths that provide large contributions can be better explored and this can alleviate the correlation of light paths due to the reuse of pre-sampled light sub-paths by all eye sub-paths. The increased number of pre-sampled light subpaths, however, also incurs a high computational cost. In this paper, we propose a two-stage resampling method for BPT to efficiently handle a large number of pre-sampled light sub-paths. We also derive a weighting function that can treat the changes in path probability due to the two-stage resampling. Our method can handle a two orders of magnitude larger number of presampled light sub-paths than previous methods in equal-time rendering, resulting in stable and better noise reduction than state-of-the-art methods.     

Tracing a ray through uniformly subdividedn-dimensional space

Finding the cells intersected by a ray in a uniformly subdivided space is a technique used in accelerated ray tracing and also other computer graphics applications. A related problem is that of finding the sequence of grid points in a 3D voxel space, which best approximates a given line. This paper examines earch of these problems, introduces a unifying approach and provides a generalisation of previous algorithms.Written while on sabbatical leave from Department of Computer Science, QMW University of London, Mile End Road, London E1 4NS, UK     

裁剪后二次曲面的光线跟踪

提出了一个对裁剪后二次曲面进行光线跟踪的新方法。在该方法中引进了一个易于由世界坐标求参数的辅助参数空间，通过预处理，事先将原参数域下的边界变换到新的参数空间。这样，光线与曲面的求交仍可按二次曲面的几何参数进行，求出交点后，再将交点变换到辅助参数空间，在新的参数域中判断交点是否在裁剪过的曲面上。     

Weighted random sampling with a reservoir

In this work, a new algorithm for drawing a weighted random sample of size m from a population of n weighted items, where m?n, is presented. The algorithm can generate a weighted random sample in one-pass over unknown populations.     

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.     

分子可视化中的光线追踪棋盘渲染

在分子可视化中使用光线追踪渲染图像能够极大地促进研究人员对分子结构的观察和感知,但现有的光线追踪方法存在实时性能不足以及渲染质量不佳的问题.文中提出了一种光线追踪棋盘渲染方法.该方法利用棋盘渲染技术对光线追踪方法进行优化,其流程主要划分为重投影、渲染、重建和孔填充4个阶段,在各阶段中,提出了针对棋盘渲染的改进,包括光线...     

Parallel randomized sampling for support vector machine (SVM) and support vector regression (SVR)

A parallel randomized support vector machine (PRSVM) and a parallel randomized support vector regression (PRSVR) algorithm based on a randomized sampling technique are proposed in this paper. The proposed PRSVM and PRSVR have four major advantages over previous methods. (1) We prove that the proposed algorithms achieve an average convergence rate that is so far the fastest bounded convergence rate, among all SVM decomposition training algorithms to the best of our knowledge. The fast average convergence bound is achieved by a unique priority based sampling mechanism. (2) Unlike previous work (Provably fast training algorithm for support vector machines, 2001) the proposed algorithms work for general linear-nonseparable SVM and general non-linear SVR problems. This improvement is achieved by modeling new LP-type problems based on Karush–Kuhn–Tucker optimality conditions. (3) The proposed algorithms are the first parallel version of randomized sampling algorithms for SVM and SVR. Both the analytical convergence bound and the numerical results in a real application show that the proposed algorithm has good scalability. (4) We present demonstrations of the algorithms based on both synthetic data and data obtained from a real word application. Performance comparisons with SVM^light show that the proposed algorithms may be efficiently implemented.     

Spectral Gradient Sampling for Path Tracing

Spectral Monte‐Carlo methods are currently the most powerful techniques for simulating light transport with wavelength‐dependent phenomena (e.g., dispersion, colored particle scattering, or diffraction gratings). Compared to trichromatic rendering, sampling the spectral domain requires significantly more samples for noise‐free images. Inspired by gradient‐domain rendering, which estimates image gradients, we propose spectral gradient sampling to estimate the gradients of the spectral distribution inside a pixel. These gradients can be sampled with a significantly lower variance by carefully correlating the path samples of a pixel in the spectral domain, and we introduce a mapping function that shifts paths with wavelength‐dependent interactions. We compute the result of each pixel by integrating the estimated gradients over the spectral domain using a one‐dimensional screened Poisson reconstruction. Our method improves convergence and reduces chromatic noise from spectral sampling, as demonstrated by our implementation within a conventional path tracer.     

虚拟环境中的三维立体图生成算法

虚拟环境技术是一种高度逼真的模拟人在自然环境中视、听和动等行为的人机界面技术。其中三维立体图生成是一项核心技术。本文对三维立体图显示的原理进行介绍，提出一种立体图生成算法并给出立体图实例。最后对立体图的加速问题进行了讨论。     

An Adaptive Method for Area Light Sources and Daylight in Ray Tracing

This paper proposes an adaptive method for taking both (diffuse or not) planar area light sources and daylight into account in a ray tracing environment which separates the calculation of direct and indirect illumination. In a given point, direct illumination due to a light source or to natural light is represented by a vector, the direction and magnitude of which being computed through an adaptive area approach, which is driven by the solid angle according to which a part of the source is seen from the current point. In the case of unoccluded diffuse polygonal sources, an analytical formula is used which gives an exact value for this vector.     

A new parallel ray-tracing system based on object decomposition

We propose a new parallel ray-tracing hardware architecture in which processors are connected as a ring. Most parallel ray-tracing algorithms subdivide the whole object space into subregions; A processor handles only rays entering the subregion assigned to it. Here we assign each processor objects that are spread over the whole object space. The processors trace rays on their own objects. The respective partial results are combined to form the final image. This scheme is especially suitable for synthesizing animated sequences because objects need not be reallocated for every frame. Preliminary results show a speed-up factor almost linearly proportional to the number of processors.     

Load balancing strategies for a parallel ray-tracing system based on constant subdivision

Static and dynamic load balancing strategies for a multiprocessor system for a ray tracing algorithm based on constant subdivision are presented. An object space is divided into regular cubes (subspaces), whose boundary planes are perpendicular to the coordinate axes, and these are allocated to the processors in the system. Here, load balancing among the processors is the most important problem. Firstly, in a category of static load balancing, strategies for mapping the subspaces into the processors are evaluated by simulation. Moreover, we propose a hierarchical multiprocessor system in order to realize dynamic load balancing with the static one. Its architecture can overcome the limitation of the static load balancing in a large scale multiprocessor system.     

Gradient Outlier Removal for Gradient‐Domain Path Tracing

We present a new outlier removal technique for a gradient‐domain path tracing (G‐PT) that computes image gradients as well as colors. Our approach rejects gradient outliers whose estimated errors are much higher than those of the other gradients for improving reconstruction quality for the G‐PT. We formulate our outlier removal problem as a least trimmed squares optimization, which employs only a subset of gradients so that a final image can be reconstructed without including the gradient outliers. In addition, we design this outlier removal process so that the chosen subset of gradients maintains connectivity through gradients between pixels, preventing pixels from being isolated. Lastly, the optimal number of inlier gradients is estimated to minimize our reconstruction error. We have demonstrated that our reconstruction with robustly rejecting gradient outliers produces visually and numerically improved results, compared to the previous screened Poisson reconstruction that uses all the gradients.     

任意梯度折射率介质中光线追迹的仿真与分析

针对研究光线传输问题,光线在任意梯度折射率介质中的传输路径难以用解析式给出精确解,通常采用数值方法求解,而欧拉法、龙格库塔法和泰勒级数展开法,正是针对介质中光线传输进行追迹的数值方法。在对折射率离散分布介质中的光线追迹过程中,所需空阊点的折射率及其梯度采用距离加权插值和Barron梯度算子进行求解。通过对任意梯度折射率介质中的光线传输进行仿真,并将仿真结果与解析解进行比较和分析,结果表明龙格库塔法的追迹精度最高,泰勒级数展开法次之,而欧拉法的相对最低;此外,光线追迹精度还受到追迹步长和插值方法精度的影响。     

Volume rendering by adaptive refinement

Volume rendering is a technique for visualizing sampled scalar functions of three spatial dimensions by computing 2D projections of a colored semi-transparent gel. This paper presents a volume-rendering algorithm, in which image quality is adaptively refined over time. An initial image is generated by casting a small number of rays into the data, less than one ray per pixel, and interpolating between the resulting colors. Subsequent images are generated by alternately casting more rays and interpolating. The usefulness of these rays is maximized by distributing them according to measures of local image complexity. Examples from two applications are given: molecular graphics and medical imaging.     

Rendering gems with aterism or chatoyancy

In this paper we describe an algorithm to render the asterism or chatoyancy effect in gems. This effect is due to light dispersion caused by inclusions consisting of vast numbers of thin fibrous needles. We develop models for distribution of the microfacet directions of the inclusions and of the light dispersion by them. Based on these models we derive an algorithm to render gems with the asterism effect. By introducting a ray distribution map, a new algorithm for tracing dispersed light rays is developed.     

Ray Tracing with Extended Cameras

This paper discusses an extended camera model for ray tracing. As an alternative to standard camera modules an abstract camera machine is presented. It represents a framework of extended cameras which is based on standard mapping functions. They are integrated within the abstract camera machine to complete the camera function, which generates rays out of image locations (pixels). Modelling the camera function as an abstract camera machine in combination with standard mapping functions opens a wide field of applications and the specification of extended cameras is greatly simplified. By using extended cameras it is easily possible to produce special and artistic effects, e.g. a local non-linear zoom of especially interesting regions while still retaining an overview of the whole scene. Overviews of given scenes can be modelled and several views of the same object can be integrated into one picture. Several examples of extended cameras designed with the abstract camera machine are discussed and colour plates made with these cameras are presented.