非经典参与介质的渲染方法研究综述

参与介质在自然界中广泛存在,也是包括影视特效、电子游戏、仿真系统等大多数图形应用中的主要场景元素之一,对其外观的模拟和再现,可以极大地提升场景的真实感和沉浸感.但是由于参与介质本身结构以及光线在参与介质中的传播过程都非常复杂,所以迄今为止,对参与介质渲染的研究都一直是图形领域的热点和难点.为了处理的方便和计算的高效,传统的参与介质渲染方法都基于两点假设:独立散射假设和局部连续假设.这两点假设也是经典的辐射传输方程成立的关键.但实际上,自然界中的很多参与介质都不满足这两点假设,因此导致现有的参与介质渲染方法生成的图片效果和真实世界的效果存在一定的差异.近年来,研究者们提出了各种非经典参与介质渲染方法,试图打破上述的两点假设,用更符合物理客观规律的方式来处理参与介质,从而进一步提升参与介质渲染的物理真实感.从相干介质渲染技术和离散介质渲染技术两方面展开对现有的面向非经典参与介质的渲染方法进行分析和讨论,重点阐述经典和非经典参与介质渲染方法的区别,以及现有非经典参与介质渲染方法的原理、优势和不足.最后,展望一些开放性问题并进行总结.本综述希望能启发相关领域的研究人员进一步攻克非经典参与介质渲...     

A Closed‐Form Solution to Single Scattering for General Phase Functions and Light Distributions

Due to the intricate nature of the equation governing light transport in participating media, accurately and efficiently simulating radiative energy transfer remains very challenging in spite of its broad range of applications. As an alternative to traditional numerical estimation methods such as ray‐marching and volume‐slicing, a few analytical approaches to solving single scattering have been proposed but current techniques are limited to the assumption of isotropy, rely on simplifying approximations and/or require substantial numerical precomputation and storage. In this paper, we present the very first closed‐form solution to the air‐light integral in homogeneous media for general 1‐D anisotropic phase functions and punctual light sources. By addressing an open problem in the overall light transport literature, this novel theoretical result enables the analytical computation of exact solutions to complex scattering phenomena while achieving semi‐interactive performance on graphics hardware for several common scattering modes.     

Using real-time SAR simulation to assist pattern recognition applications in urban areas

In this paper, the impact of current developments in urban modeling and graphics programming to the potential of SAR simulation and interpretation in urban areas is presented. As it will be demonstrated, SAR simulation is feasible in real-time even for complex urban environments by applying modern graphics cards. For this purpose, the SAR geometry is implemented using programmable graphics processing units (GPUs), which are available as standard graphics hardware for 3D acceleration. Originally, the geometric models of urban areas, which provide the database for SAR simulation, were captured to generate realistic visualizations of virtual scenes. As will be demonstrated, such area covering databases can be modified to meet specific requirements of SAR simulations at different scales. Finally, the applicability of our SAR simulation as made available from off-the-shelf hardware components for the analysis and interpretation of SAR scenes by pattern recognition will be demonstrated.     

Multiple Scattering Approximation in Heterogeneous Media by Narrow Beam Distributions

Fast realistic rendering of objects in scattering media is still a challenging topic in computer graphics. In presence of participating media, a light beam is repeatedly scattered by media particles, changing direction and getting spread out. Explicitly evaluating this beam distribution would enable efficient simulation of multiple scattering events without involving costly stochastic methods. Narrow beam theory provides explicit equations that approximate light propagation in a narrow incident beam. Based on this theory, we propose a closed‐form distribution function for scattered beams. We successfully apply it to the image synthesis of scenes in which scattering occurs, and show that our proposed estimation method is more accurate than those based on the Wentzel‐Kramers‐Brillouin (WKB) theory.     

均匀参与介质的渲染方法研究

参与介质在现实世界中广泛存在,光线在参与介质中的传播过程比在表面上的传播过程更加复杂,比如在高度散射参与介质中会发生成千上万次反射、在低散射参与介质中由于表面聚集出现体焦散效果,从而使得光线的模拟过程非常耗时。目前常用的方法包括点、光束和路径统一模型法（unifying points,beams and paths,UPBP）以及流型探索梅特罗波利斯光线传递方法（manifold exploration Metropolis light transport,MEMLT）等,这些方法在一定程度上改进了原有方法,但是在一些特殊情况下仍然需要很长时间才能收敛。本文介绍几种针对均匀参与介质的高效渲染方法。1）基于点的参与介质渲染方法,主要通过在参与介质内分布一些点来分别加速单次、二次和多次散射的计算,在GPU （graphics processing unit）实现的基础上,最终达到可交互的效率,并且支持对任意的均匀参与介质的编辑。2）基于多次反射的预计算模型,预计算出无限参与介质中的多次散射分布,通过分析光照分布的对称性,将该分布的维度从4维减低为3维,并且将该分布应用到多种蒙特卡洛渲染方法中,比如MEMLT、UPBP等,从而提高效率。3）参与介质中的路径指导方法,通过学习光线在参与介质中的分布,该分布用SD-tree （spatial-directional tree）来表示,与相位函数进行重采样来产生出射方向。以上3种方法分别从不同角度加快了参与介质的渲染效率。     

Efficient and Accurate Sound Propagation Using Adaptive Rectangular Decomposition

Accurate sound rendering can add significant realism to complement visual display in interactive applications, as well as facilitate acoustic predictions for many engineering applications, like accurate acoustic analysis for architectural design (Monks et al., 2000). Numerical simulation can provide this realism most naturally by modeling the underlying physics of wave propagation. However, wave simulation has traditionally posed a tough computational challenge. In this paper, we present a technique which relies on an adaptive rectangular decomposition of 3D scenes to enable efficient and accurate simulation of sound propagation in complex virtual environments. It exploits the known analytical solution of the wave equation in rectangular domains, and utilizes an efficient implementation of the discrete cosine transform on graphics processors (GPU) to achieve at least a 100-fold performance gain compared to a standard finite-difference time-domain (FDTD) implementation with comparable accuracy, while also being 10-fold more memory efficient. Consequently, we are able to perform accurate numerical acoustic simulation on large, complex scenes in the kilohertz range. To the best of our knowledge, it was not previously possible to perform such simulations on a desktop computer. Our work thus enables acoustic analysis on large scenes and auditory display for complex virtual environments on commodity hardware.     

Octree-based fusion for realtime 3D reconstruction

This paper proposes an octree-based surface representation for KinectFusion, a realtime reconstruction technique of in-door scenes using a low-cost moving depth camera and a commodity graphics hardware. In KinectFusion, the scene is represented as a signed distance function (SDF) and stored as an uniform grid of voxels. Though the grid-based SDF is suitable for parallel computation in graphics hardware, most of the storage are wasted, because the geometry is very sparse in the scene volume. In order to reduce the memory cost and save the computation time, we represent the SDF in an octree, and developed several octree-based algorithms for reconstruction update and surface prediction that are suitable for parallel computation in graphics hardware. In the reconstruction update step, the octree nodes are adaptively split in breath-first order. To handle scenes with moving objects, the corresponding nodes are automatically detected and removed to avoid storage overflow. In the surface prediction step, an octree-based ray tracing method is adopted and parallelized for graphic hardware. To further reduce the computation time, the octree is organized into four layers, called top layer, branch layer, middle layer and data layer. The experiments showed that, the proposed method consumes only less than 10% memory of original KinectFusion method, and achieves faster performance. Consequently, it can reconstruct scenes with more than 10 times larger size than the original KinectFusion on the same hardware setup.     

Approximate Bias Compensation for Rendering Scenes with Heterogeneous Participating Media

In this paper we present a novel method for high‐quality rendering of scenes with participating media. Our technique is based on instant radiosity, which is used to approximate indirect illumination between surfaces by gathering light from a set of virtual point lights (VPLs). It has been shown that this principle can be applied to participating media as well, so that the combined single scattering contribution of VPLs within the medium yields full multiple scattering. As in the surface case, VPL methods for participating media are prone to singularities, which appear as bright “splotches” in the image. These artifacts are usually countered by clamping the VPLs' contribution, but this leads to energy loss within the short‐distance light transport. Bias compensation recovers the missing energy, but previous approaches are prohibitively costly. We investigate VPL‐based methods for rendering scenes with participating media, and propose a novel and efficient approximate bias compensation technique. We evaluate our technique using various test scenes, showing it to be visually indistinguishable from ground truth.     

R4: Realistic rain rendering in realtime

Realistic rain simulation is a challenging problem due to the variety of different phenomena to consider. In this paper we propose a new rain rendering algorithm that extends present state of the art in the field, achieving real-time rendering of rain streaks and splashes with complex illumination effects, along with fog, halos and light glows as hints of the participating media. Our algorithm creates particles in the scene using an artist-defined storm distribution (e.g., provided as a 2D cloud distribution). Unlike previous algorithms, no restrictions are imposed on the rain area dimension or shape. Our technique adaptively samples the storm area to simulate rain particles only in the relevant regions and only around the observer. Particle simulation is executed entirely in the graphics hardware, by placing the particles at their updated coordinates at each time-step, also checking for collisions with the scene. To render the rain streaks, we use precomputed images and combine them to achieve complex illumination effects. Several optimizations are introduced to render realistic rain with virtually millions of falling rain droplets.     

大规模复杂场景交互绘制技术综述

大规模复杂场景的快速绘制是虚拟现实、实时仿真和三维交互设计等许多重要应用的底层支撑技术,也是诸多研究领域面临的一个基本问题.随着近几年三维扫描和建模技术的飞速发展,三维场景的规模和复杂度不断增大,大规模复杂场景的交互绘制受到了国内外研究者越来越多的重视并取得了一系列研究成果.首先简要回顾了大规模复杂场景交互绘制的研究进展情况;然后通过对其中涉及的主要关键技术进行总结分析,并对国内外典型的绘制系统进行比较和分类,阐述了大规模复杂场景交互绘制的主要研究内容,给出了大规模复杂场景交互绘制系统所应包含的基本组成部分和一般框架;最后对今后的发展方向做出了展望.     

Irradiance Gradients in the Presence of Participating Media and Occlusions

In this paper we present a technique for computing translational gradients of indirect surface reflectance in scenes containing participating media and significant occlusions. These gradients describe how the incident radiance field changes with respect to translation on surfaces. Previous techniques for computing gradients ignore the effects of volume scattering and attenuation and assume that radiance is constant along rays connecting surfaces. We present a novel gradient formulation that correctly captures the influence of participating media. Our formulation accurately accounts for changes of occlusion, including the effect of surfaces occluding scattering media. We show how the proposed gradients can be used within an irradiance caching framework to more accurately handle scenes with participating media, providing significant improvements in interpolation quality.     

Higher Order Ray Marching

Rendering participating media is still a challenging and time consuming task. In such media light interacts at every differential point of its path. Several rendering algorithms are based on ray marching: dividing the path of light into segments and calculating interactions at each of them. In this work, we revisit and analyze ray marching both as a quadrature integrator and as an initial value problem solver, and apply higher order adaptive solvers that ensure several interesting properties, such as faster convergence, adaptiveness to the mathematical definition of light transport and robustness to singularities. We compare several numerical methods, including standard ray marching and Monte Carlo integration, and illustrate the benefits of different solvers for a variety of scenes. Any participating media rendering algorithm that is based on ray marching may benefit from the application of our approach by reducing the number of needed samples (and therefore, rendering time) and increasing accuracy.     

Hardware support for adaptive tessellation of Bézier surfaces based on local tests

Bézier representations have been widely employed as a standard way of designing complex scenes with very good quality results. These surfaces are usually tessellated, in the software application, into triangle models to be rendered. Then, the final image is generated in the graphics card so that its triangle rendering capabilities are exploited.In this work we present an adaptive tessellation algorithm and the corresponding architecture to be implemented in hardware. The objective of the proposal is to avoid the potential bottleneck associated with the transmission of complex triangular models from CPU to graphics cards. The algorithm we propose is based on a layer strip representation method and a new data management that permits generation and efficient storage of the tessellated mesh. The corresponding architecture has to be included as an additional unit at the input of the graphics card. As a consequence, the transmission requirements from CPU to graphics card are greatly reduced as the tessellation is performed in the graphics card. On the other hand, the adaptive strategy employed permits selection of the number of triangles of the final mesh as a trade off between computational requirements and quality of the final mesh. The efficient data management proposed, together with the low storage requirements of the architecture, makes it a good candidate for its hardware implementation and inclusion in future graphics cards.     

Instant Caching for Interactive Global Illumination

The ability to interactively render dynamic scenes with global illumination is one of the main challenges in computer graphics. The improvement in performance of interactive ray tracing brought about by significant advances in hardware and careful exploitation of coherence has rendered the potential of interactive global illumination a reality. However, the simulation of complex light transport phenomena, such as diffuse interreflections, is still quite costly to compute in real time. In this paper we present a caching scheme, termed Instant Caching, based on a combination of irradiance caching and instant radiosity. By reutilising calculations from neighbouring computations this results in a speedup over previous instant radiosity‐based approaches. Additionally, temporal coherence is exploited by identifying which computations have been invalidated due to geometric transformations and updating only those paths. The exploitation of spatial and temporal coherence allows us to achieve superior frame rates for interactive global illumination within dynamic scenes, without any precomputation or quality loss when compared to previous methods; handling of lighting and material changes are also demonstrated.     

基于SOPC及图形加速引擎的座舱显示系统

提出一种基于可编程片上系统和图形加速引擎的飞机座舱综合显示系统设计方案。为避免图形加速引擎直接对帧存储器进行零碎操作导致的存储器操作瓶颈,引入图形缓存机制。根据图形像素的存储特点提出“远区域优先”图形缓存页面淘汰算法。对汉字及自定义位图等操作采取软硬件结合的方式达到系统性能和资源利用的平衡,利用硬件锁保证帧存储器一致性。通过对模块进行波形仿真实现系统级仿真结果的可视化验证。     

An Analytical Solution to Single Scattering in Homogeneous Participating Media

Despite their numerous applications, efficiently rendering participating media remains a challenging task due to the intricacy of the radiative transport equation. As they provide a generic means of solving a wide variety of problems, numerical methods are most often used to solve the air-light integral even under simplifying assumptions. In this paper, we present a novel analytical approach to single scattering from isotropic point light sources in homogeneous media. We derive the first closed-form solution to the air-light integral in isotropic media and extend this formulation to anisotropic phase functions. The technique relies neither on pre-computation nor on storage, and we provide a practical implementation allowing for an explicit control on the accuracy of the solutions. Finally, we demonstrate its quantitative and qualitative benefits over both previous numerical and analytical approaches.     

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.     

Compressed shadow maps

Shadow mapping has been subject to extensive investigation, but previous shadow map algorithms cannot usually generate high-quality shadows with a small memory footprint. In this paper, we present compressed shadow maps as a solution to this problem. A compressed shadow map reduces memory consumption by representing lit surfaces with endpoints of intermediate line segments as opposed to the conventional array-based pixel structures. Compressed shadow maps are only discretized in the vertical direction while the horizontal direction is represented by floating-point accuracy. The compression also helps with the shadow map self-shadowing problems. We compare our algorithm against all of the most popular shadow map algorithms and show, on average, order of magnitude improvements in storage requirements in our test scenes. The algorithm is simple to implement, can be added easily to existing software renderers, and lets us use graphics hardware for shadow visualization.