使用光子映射渲染参与介质

光子映射是近年发展起来的一种新的全局光照算法。本文依据光子映射对实体物体的渲染，将其扩展到对包含参与介质的场景的渲染，为此提出了一个两路的渲染算法。在第一路中，光子从光源发射，并使用光子追踪来构造体光子图；第二路从视点出发向场景中发射光线，使用光线追踪来进行渲染，其中，根据构造好的光子图，用光线步进进行
行递归的辐射估计，得出最终光强。     

视点重要度驱动的自适应光子追踪

针对随机光子追踪方法绘制光照条件困难的场景的效果差、收敛慢问题,提出一种基于视点重要度的自适应光子追踪方法.首先根据场景的视点重要度构造视点重要度图,并基于视点重要度和光子路径可见性来构造新的重要性函数;然后结合自适应马尔科夫链蒙特卡洛方法和分布交换技术采样该函数,以产生新的光子路径;最后设计了一种新的采样分布选择策略,先计算执行分布交换的概率,再根据概率决定生成新路径的采样分布.实验结果表明,该方法能高效地绘制困难光照场景,绘制结果具有较高的质量.     

基于光子分布概率调整的高效的非匀质半透明材质绘制

针对非匀质半透明材质绘制代价高昂的问题,提出一种基于光子分布概率调整的高效非匀质半透明物体绘制算法.首先利用绘制图像所显现的区域生成场景中的关注区域;然后依据体积光子映射算法从光源处随机生成光子并进行跟踪;若光子与关注区域相交,则对该光子的分布概率进行调整,通过降低其能量来增加其落在关注区域内的概率;最后在概率调整时对高能量光子在方向上和在距离上进行切分,以保证关注区域内光子能量一致.实验结果表明,该算法能显著增加关注区域内的光子密度,并有效地减少非匀质半透明材质绘制的时间.     

基于栅格的全局光子图重建

基于光子图的光子映射算法能产生高质量的照片级图像。对于光照复杂的 场景,光子图需要存储大量光子以提高生成图像的质量,这不仅占用大量的内存空间,而且 光照估计的时间长。论文提出基于栅格的全局光子图重建的算法,即在光子包围盒被栅格化 后,其非空栅格中一定比例的光子被用来重建新的光子图,并保证重建前后栅格内光子能量 和守恒,这使得重建前后光子图的光照估计的效果相近。通过增加特定栅格中的重建光子数 目,能有效减少由几何偏差引起的光照估计误差,增强直接聚焦（焦散）和间接聚焦光照的 绘制效果;并使用简单方法检测生成图像中少量噪声,增加少量采样即可有效减少相应的噪 声。全局光子图重建算法的计算成本低,并保持生成图像的视觉独立性。     

基于光子映射的辐照度缓存算法

提出了一种全局光照计算方法,结合了两个知名的技术,光子映射和辐照度缓存.光子映射具有视点无关的优势,辐照度缓存可以快速计算间接光照,但后者是视点相关的,为了使光照缓存记录覆盖整个场景,辐照度缓存算法需要手动设置很多相机.利用这两种技术的各自优势,通过光子图来计算改进后的视点无关的辐照度缓存算法,实现了快速而准确的全局光...     

光子映射在CUDA中的研究与实现

通过修改光子映射算法的实现过程,使得该算法能够通过CUDA完全运行在最新的GPU上,从而能够充分利用GPU强大的并行计算能力,加速光子映射的实现。光子映射在CUDA中的实现主要通过两个方面来完成：构建光子图和估计辐射能。同时为了提高对光子图中的光子信息的查找速度,采用了kd-tree结构来存储光子信息,使得可以通过KNN（K-Nearest Neighbor）快速搜索光子图。在所测试环境中,渲染速度是CPU中的近1O倍。     

基于GPU的光子映射并行化算法

针对串行情况下光子映射算法速度慢的问题,对光子映射算法并行化进行可行性分析,充分利用图像处理器(GPU)的统一设备计算架构(CUDA)的并行和计算能力,实现光子映射算法的并行化。同时针对算法中光子发射追踪阶段生成GPU线程数与光子数相同的方法的不足以及平均分配方法所造成的资源浪费等,提出线程之间协同工作的方法并采用动态平衡处理,使光子渲染速度提升了将近一倍。实验结果证明了多线程间协同工作及动态平衡相结合方法的有效性。     

快速虚顶点光子映射的焦散绘制

目的 模拟绘制焦散效果是真实感图形绘制的重要组成部分。利用可编程GPU硬件,基于图像的光子映射方法绘制速度快,但由于使用了近似采样和计算,会产生失真。为了克服这一现象,针对理想的镜面反射、折射体,提出一种快速绘制焦散效果的方法（VOBPBT）。方法 该方法首次定义了光子路径映射图的概念,并提出通过基于虚顶点光子映射的光子束跟踪来准确构建光子路径映射图的方法;此外,方法也创新性地提出利用光子路径映射图来查找焦散三角形,创建焦散映射图的方法。结果 实验结果表明,本文绘制结果真实,可以处理多次递归反射、折射,能够绘制连贯的高频焦散效果,同时可以达到交互的绘制性能。结论 本文VOBPBT方法在可交互计算机仿真、计算机游戏、虚拟漫游等应用领域具有一定的实用价值。     

重要度驱动的自适应Projection Map在光子图中的应用

在全局光照算法中,光子图算法是一种与视点无关的物空间辐射度近似计算方法,提出了一种基于重要度驱动采样的自适应Projection map算法,利用它可以在光子图算法中提高光子发射的有效性和准确性,加快渲染速度并取得更好的图像质量.实验表明,该算法能够有效地减少光源发射光子的次数,提高光子的命中率,具有相当的应用价值.     

基于光流场动态参与介质绘制算法

为了解决动态参与介质场景连续帧的实时绘制问题,提出了一种基于光流场动态参与介质场景的绘制算法。首先,采用区域匹配的方法计算关键帧之间的光流场;然后,通过插值的方法计算中间帧之间的光流场,采用帧间光流连贯性函数表示帧与帧之间光流一致性的度量,保证帧与帧之间介质运动不会发生突变;最后,按照所产生的光流场,绘制连续动态变化的参与介质场景。在连续5帧动态参与介质场景绘制中,所提算法比基于径向基函数模型(RBF)参与介质的光子映射算法效率提高了近3倍,能够达到连续帧的实时绘制,且绘制质量比较高。     

A survey of photon mapping state-of-the-art research and future challenges

Global illumination is the core part of photo-realistic rendering. The photon mapping algorithm is an effective method for computing global illumination with its obvious advantage of caustic and color bleeding rendering. It is an active research field that has been developed over the past two decades. The deficiency of precise details and efficient rendering are still the main challenges of photon mapping. This report reviews recent work and classifies it into a set of categories including radiance estimation, photon relaxation, photon tracing, progressive photon mapping, and parallel methods. The goals of our report are giving readers an overall introduction to photon mapping and motivating further research to address the limitations of existing methods.     

Improved Stochastic Progressive Photon Mapping with Metropolis Sampling

This paper presents an improvement to the stochastic progressive photon mapping (SPPM), a method for robustly simulating complex global illumination with distributed ray tracing effects. Normally, similar to photon mapping and other particle tracing algorithms, SPPM would become inefficient when the photons are poorly distributed. An inordinate amount of photons are required to reduce the error caused by noise and bias to acceptable levels. In order to optimize the distribution of photons, we propose an extension of SPPM with a Metropolis‐Hastings algorithm, effectively exploiting local coherence among the light paths that contribute to the rendered image. A well‐designed scalar contribution function is introduced as our Metropolis sampling strategy, targeting at specific parts of image areas with large error to improve the efficiency of the radiance estimator. Experimental results demonstrate that the new Metropolis sampling based approach maintains the robustness of the standard SPPM method, while significantly improving the rendering efficiency for a wide range of scenes with complex lighting.     

Anisotropic density estimation for photon mapping

Computational Visual Media - Photon mapping is a widely used technique for global illumination rendering. In the density estimation step of photon mapping, the indirect radiance at a shading point...     

The Beam Radiance Estimate for Volumetric Photon Mapping

We present a new method for efficiently simulating the scattering of light within participating media. Using a theoretical reformulation of volumetric photon mapping, we develop a novel photon gathering technique for participating media. Traditional volumetric photon mapping samples the in‐scattered radiance at numerous points along the length of a single ray by performing costly range queries within the photon map. Our technique replaces these multiple point‐queries with a single beam‐query, which explicitly gathers all photons along the length of an entire ray. These photons are used to estimate the accumulated in‐scattered radiance arriving from a particular direction and need to be gathered only once per ray. Our method handles both fixed and adaptive kernels, is faster than regular volumetric photon mapping, and produces images with less noise.     

Mixing Monte Carlo and progressive rendering for improved global illumination

In this paper, we seek to eliminate the noise caused by caustic paths during progressive Monte Carlo path tracing. We employ a filtering strategy over path space, handling each subspace using specialized derivations of path tracing and progressive photon mapping. Evaluating diffuse paths with path tracing allows the use of sample stratification over both pixels and the image as a whole, whilst sharp detailed caustics are produced using progressive photon mapping. This is an efficient, low noise progressive algorithm with vanishing bias combining the advantages of both Monte Carlo methods, and particle tracing.     

Photon‐driven Irradiance Cache

We describe a global illumination method combining two well known techniques: photon mapping and irradiance caching. The photon mapping method has the advantage of being view independent but requires a costly additional rendering pass, called final gathering. As for irradiance caching, it is view‐dependent, irradiance is only computed and cached on surfaces of the scene as viewed by a single camera. To compute records covering the entire scene, the irradiance caching method has to be run for many cameras, which takes a long time and is a tedious task since the user has to place the needed cameras manually. Our method exploits the advantages of these two methods and avoids any intervention of the user. It computes a refined, view‐independent irradiance cache from a photon map. The global illumination solution is then rendered interactively using radiance cache splatting.     

The photon pipeline revisited

With the development of real-time ray tracing in recent years, it is now very interesting to ask if real-time performance can be achieved for high-quality rendering algorithms based on ray tracing. In this paper, we propose a pipelined architecture to implement reverse photon mapping. Our architecture can use real-time ray tracing to generate photon points and camera points, so the main challenge is how to implement the gathering phase that computes the final image. Traditionally, the gathering phase of photon mapping has only allowed coarse-grain parallelism, and this situation has been a source of inefficiency, cache thrashing, and limited throughput. To avail fine-grain pipelining and data parallelism, we arrange computations so that photons can be processed independently, similar to the way that triangles are efficiently processed in traditional real-time graphics hardware. We employ several techniques to improve cache behavior and to reduce communication overhead. Simulations show that the bandwidth requirements of this architecture are within the capacity of current and future hardware, and this suggests that photon mapping may be a good choice for real-time performance in the future.     

基于硬件的实时地平映射

地平映射是用于计算凹凸映射表面阴影的一项技术.在可编程图形硬件上运用并扩展地平映射技术,力求更准确地表达凹凸表面的层次细节.采取在表面切空间对光照向量进行插值的方法,在多遍渲染算法的基础上,使用三维体纹理代替二维地平贴图,一遍渲染即得到较好的实时自阴影效果.