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。     

Multi‐Image Based Photon Tracing for Interactive Global Illumination of Dynamic Scenes

Image space photon mapping has the advantage of simple implementation on GPU without pre‐computation of complex acceleration structures. However, existing approaches use only a single image for tracing caustic photons, so they are limited to computing only a part of the global illumination effects for very simple scenes. In this paper we fully extend the image space approach by using multiple environment maps for photon mapping computation to achieve interactive global illumination of dynamic complex scenes. The two key problems due to the introduction of multiple images are 1) selecting the images to ensure adequate scene coverage; and 2) reliably computing ray‐geometry intersections with multiple images. We present effective solutions to these problems and show that, with multiple environment maps, the image‐space photon mapping approach can achieve interactive global illumination of dynamic complex scenes. The advantages of the method are demonstrated by comparison with other existing interactive global illumination methods.     

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。     

Caustic Forecasting: Unbiased Estimation of Caustic Lighting for Global Illumination

We present an unbiased method for generating caustic lighting using importance sampled Path Tracing with Caustic Forecasting. Our technique is part of a straightforward rendering scheme which extends the Illumination by Weak Singularities method to allow for fully unbiased global illumination with rapid convergence. A photon shooting preprocess, similar to that used in Photon Mapping, generates photons that interact with specular geometry. These photons are then clustered, effectively dividing the scene into regions which will contribute similar amounts of caustic lighting to the image. Finally, the photons are stored into spatial data structures associated with each cluster, and the clusters themselves are organized into a spatial data structure for fast searching. During rendering we use clusters to decide the caustic energy importance of a region, and use the local photons to aid in importance sampling, effectively reducing the number of samples required to capture caustic lighting.     

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。     

Adaptive lattice‐based light rendering of participating media

The visual world around us displays a rich set of light effects because of translucent and participating media. It is hard and time consuming to render these effects with scattering, caustic, and shaft because of the complex interaction between light and different media. This paper presents a new rendering method based on adaptive lattice for lighting participating media of translucent materials such as marble, wax, and shaft light. Firstly, on the basis of the lattice‐based photon tracing model, multi‐scale hierarchical lattice was constructed by mixed lattice types sampling combined cubic Cartesian and face‐centered cubic with view‐dependent adaptive resolution. Then, an adaptive method to trace diffuse photons and marked specular photons with different phase functions was suggested. Multiple lights and heterogeneous materials were also considered here. Further, the mixed rendering method and GPU accelerate technology were introduced to render different light effects under different participating media. Copyright © 2011 John Wiley & Sons, Ltd.     

GPU-Assisted High Quality Particle Rendering

Visualizing dynamic participating media in particle form by fully solving equations from the light transport theory is a computationally very expensive process. In this paper, we present a computational pipeline for particle volume rendering that is easily accelerated by the current GPU. To fully harness its massively parallel computing power, we transform input particles into a volumetric density field using a GPU-assisted, adaptive density estimation technique that iteratively adapts the smoothing length for local grid cells. Then, the volume data is visualized efficiently based on the volume photon mapping method where our GPU techniques further improve the rendering quality offered by previous implementations while performing rendering computation in acceptable time. It is demonstrated that high quality volume renderings can be easily produced from large particle datasets in time frames of a few seconds to less than a minute.     

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。     

Photon Differential Splatting for Rendering Caustics

We present a photon splatting technique which reduces noise and blur in the rendering of caustics. Blurring of illumination edges is an inherent problem in photon splatting, as each photon is unaware of its neighbours when being splatted. This means that the splat size is usually based on heuristics rather than knowledge of the local flux density. We use photon differentials to determine the size and shape of the splats such that we achieve adaptive anisotropic flux density estimation in photon splatting. As compared to previous work that uses photon differentials, we present the first method where no photons or beams or differentials need to be stored in a map. We also present improvements in the theory of photon differentials, which give more accurate results and a faster implementation. Our technique has good potential for GPU acceleration, and we limit the number of parameters requiring user adjustment to an overall smoothing parameter and the number of photons to be traced.     

基于GPU的近似软影实时绘制

通过对阴影图算法进行扩展,提出一种完全基于GPU的近似软影实时绘制算法,它是一种3遍算法:第一遍从光源中心计算场景的深度图;第二遍采用几何着色器提取物体的轮廓边,同时在轮廓边上生成新的几何图元,利用硬件自动插值功能向外绘制线性近似半影图,并根据第一遍得到的深度图在像素着色器中对背面轮廓形成的半影区进行剔除;对于重叠的半影区设定片元的伪深度值,利用硬件进行自动融合.第三遍分别查询深度图和半影图,确定场景的本影区以及半影区中像素的亮度,从而得到面光源照射下场景的近似软影效果.     

On‐the‐fly generation and rendering of infinite cities on the GPU

In this paper, we present a new approach for shape‐grammar‐based generation and rendering of huge cities in real‐time on the graphics processing unit (GPU). Traditional approaches rely on evaluating a shape grammar and storing the geometry produced as a preprocessing step. During rendering, the pregenerated data is then streamed to the GPU. By interweaving generation and rendering, we overcome the problems and limitations of streaming pregenerated data. Using our methods of visibility pruning and adaptive level of detail, we are able to dynamically generate only the geometry needed to render the current view in real‐time directly on the GPU. We also present a robust and efficient way to dynamically update a scene's derivation tree and geometry, enabling us to exploit frame‐to‐frame coherence. Our combined generation and rendering is significantly faster than all previous work. For detailed scenes, we are capable of generating geometry more rapidly than even just copying pregenerated data from main memory, enabling us to render cities with thousands of buildings at up to 100 frames per second, even with the camera moving at supersonic speed.     

基于自适应光子发射的渐进式光子映射

通过对渐进式光子映射算法进行扩展,提出了一种基于自适应光子发射的渐进式光子映射算法.渐进式光子映射是一个多遍的全局光照算法,通过不断发射光子并渐进更新场景各点的光能估计能使其最终能收敛到无偏差的结果.由于渐进式光子映射完全使用密度估计来计算各点的光能,因此其收敛速度受光子分布影响较大.利用渐进式光子映射算法中固有的场景统计信息以及其多遍的特点,设计了一个自适应的光子发射策略,使得发射的光子能更多的分布在对最终绘制有效的区域,提高了原算法的绘制效率.     

Grouped photon mapping

This paper proposes a novel architecture called Grouped Photon Mapping, which combines standard photon mapping with the light-beam concept to improve the nearest-neighbor density estimation method. Based on spatial coherence, we cluster all of photons, which are deposited in the photon map, into different beam-like groups. Each group of photons is individually stored in an isolated photon map. By the distribution of the photons in each photon map, we construct a polygonal boundary to represent a beam-like illuminated area. These boundaries offer a more accurate and flexible sampling area to filter neighbor photons around the query point. In addition, by a level of detail technique, we can control the photon-count in each group to obtain a balance between biases and noise. The results of our experiments prove that our method can successfully reduce bias errors and light leakage. Especially, for complicated caustic effects through a gemstone-like object, we can render a smoother result than standard photon mapping.     

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.     

Accelerating Volume Raycasting using Proxy Spheres

In this paper, we propose an efficient solution that addresses the performance problems of current single-pass GPU raycasting algorithms. Our paper provides more control over the rendering process by introducing tighter ray segments for raycasting, while at the same time avoiding the introduction of any new rendering artefacts. We achieve this by dynamically generating, on the GPU, a coarsely fitted proxy geometry, composed of spheres, for the active blocks. The spheres are then rasterised into two z-buffers by a single rendering pass. The resulting two z-buffers are used as the first-hit and last-hit points for the subsequent raycaster. With this approach, only the valid ray segments between the two z-buffers need to be sampled during raycasting. This also provides more coherent parallelism on the GPU due to more consistent ray length and avoidance of the overheads and dynamic branching of performing checks on a per-sample basis during the raycasting pass.
Our technique is ideal for dynamic data exploration in which both the transfer function and view parameters need to be changed frequently at runtime. The rendering results of our algorithm are identical to the general cube-based proxy geometry algorithm, but the performance can be up to 15.7 times faster. Furthermore, the approach can be adopted by any existing raycasting system in a straightforward way.     

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.     

Ray Accelerator: Efficient and Flexible Ray Tracing on a Heterogeneous Architecture

We present a hybrid ray tracing system, where the work is divided between the CPU cores and the GPU in an integrated chip, and communication occurs via shared memory. Rays are organized in large packets that can be distributed among the two units as needed. Testing visibility between rays and the scene is mostly performed using an optimized kernel on the GPU, but the CPU can help as necessary. The CPU cores typically handle most or all shading, which makes it easy to support complex appearances. For efficiency, the CPU cores shade whole batches of rays by sorting them on material and shading each material using a vectorized kernel. In addition, we introduce a method to support light paths with arbitrary recursion, such as multiple recursive Whitted‐style ray tracing and adaptive sampling where the result of a ray is examined before sending the next, while still batching up rays for the benefit of GPU‐accelerated traversal and vectorized shading. This allows our system to achieve high rendering performance while maintaining the flexibility to accommodate different rendering algorithms.     

Simplified photon mapping for real-time caustics rendering

The objective of this paper is to adapt photon mapping for real-time simulation of caustics. High-performance algorithm adapted for the GPU and implemented on the basis of cross-platform OpenGL and OpenCL APIs is proposed. For effective rendering of caustics and generation of photon map OpenGL shaders are used. Voxel acceleration structure constructed directly on the GPU by means of OpenCL provides fast access to photon map. Performance estimation for different equipment is given.     

A Prism‐Free Method for Silhouette Rendering in Inverse Displacement Mapping

Silhouette is a key feature that distinguishes displacement mapping from normal mapping. However the silhouette rendering in the GPU implementation of displacement mapping (which is often called inversed displacement mapping) is tricky. Previous approaches rely mostly on construction of additional extruding prism‐like geometry, which slows down the rendering significantly. In this paper, we proposed a method for solving the silhouette rendering problem in inverse displace mapping without using any extruding prism‐like geometry. At each step of intersection finding, we continuously bends the viewing ray according to the current local tangent space associated with the surface. Thus, it allows mapping a displacement map onto an arbitrary curved surface with more accurate silhouette. While our method is simple, it offers surprisingly good results over Curved Relief Map (CRM) [ [OP05] ] in many difficult or degenerated cases.     

Perception-motivated multiresolution rendering on sole-cube maps

This paper presents a novel GPU-based multiresolution rendering on sole-cube maps (SCMs), which is a variant of geometry images built upon spherical parameterization. Given spherical parametrization of a manifold mesh, the sphere domain is gnomonically projected to a closed cube, which constitutes the 6-chart sole-cube maps. A quadtree structure of SCMs and normal map atlas are then constructed by using the regular re-sampling. Then, by packing the quadtree nodes into the SCMs texture atlas, a new parallel multiresolution rendering is processed on the latest GPU in two rendering passes: the multiresolution node selection in fragment shader; the triangulation in vertex shader followed by the node culling operation in geometry shader. The proposed approach generates adaptive mesh surfaces dynamically, and can be fully implemented in GPU parallelization. The proposed scheme alleviates the computing load of multiresolution mesh refinement on CPU, and our GPU-based multiresolution rendering is demonstrated with a variety of examples. Our user study confirmed that the visual quality of the SCMs multiresolution rendering, in comparison with the meshes/geometry images rendering, is also highly efficient especially for complex models in large-scale virtual environment.