利用球谐方法分散计算场景的全局光照

作为一种特殊的光照模型,辐射度在图形学中有着重要的应用.然而,由于其计算的复杂性,辐射度方法一直得不到广泛的应用,特别是在动态场景中.提出一种基于辐射度算法的全局光照实时加速算法,在原有基于点采样进行场景的全局光照近似的基础上,提出了不同的点采样计算方法,以提高辐射度方法对动画场景的适应性.算法对场景中的物体建立六面体包围盒,并在包围盒各顶点设置采样点,这样就可以在环境变化对其光照影响不大的情况下采用原有的计算结果,实现辐射度算法在动态场景中的计算加速.同时,对场景中的每个物体的光照,利用立方体映射计算直接光照;而对物体间的能量交换采用点采样方法进行近似计算并利用附近采样点的光照插值作为物体的光照.实验结果表明,该算法可以减少大量的间接光照计算,提高了辐射度算法的效率.     

聚类立即辐射度及在增强现实中的应用

提出一种聚类立即辐射度方法,以实现增强现实等领域需要高度真实感的全局光照算法来实现实时交互的绘制效果要求。为此,改进了传统的立即辐射度方法,将大量的用于表达间接光照的虚拟点光源聚类到少量的虚拟面光源中,并使用实时软阴影算法快速计算可见性。同时,借助图形硬件GPU加速场景绘制。实验结果表明,算法在增强现实环境等领域中支持完全动态场景,且在保证良好视觉效果的前提下获得了实时绘制帧率。     

Incremental Updates for Rapid Glossy Global Illumination

We present an integrated global illumination algorithm including non-diffuse light transport which can handle complex scenes and enables rapid incremental updates. We build on a unified algorithm which uses hierarchical radiosity with clustering and particle tracing for diffuse and non-diffuse transport respectively. We present a new algorithm which chooses between reconstructing specular effects such as caustics on the diffuse radiosity mesh, or special purpose caustic textures, when high frequencies are present. Algorithms are presented to choose the resolution of these textures and to reconstruct the high-frequency non-diffuse lighting effects. We use a dynamic spatial data structure to restrict the number of particles re-emitted during the local modifications of the scene. By combining this incremental particle trace with a line-space hierarchy for incremental update of diffuse illumination, we can locally modify complex scenes rapidly. We also develop an algorithm which, by permitting slight quality degradation during motion, achieves quasi-interactive updates. We present an implementation of our new method and its application to indoors and outdoors scenes.     

Radiosity in Flatland

The radiosity method for the simulation of interreflection of light between diffuse surfaces is such a common image synthesis technique that its derivation is worthy of study. We here examine the radiosity method in a two dimensional, flatland world. It is shown that the radiosity method is a simple finite element method for the solution of the integral equation governing global illumination. These two-dimensional studies help explain the radiosity method in general and suggest a number of improvements to existing algorithms. In particular, radiosity solutions can be improved using a priori discontinuity meshing, placing mesh boundaries on discontinuities such as shadow edges. When discontinuity meshing is used along with piecewise-linear approximations instead of the current piecewise-constant approximations, the accuracy of radiosity simulations can be greatly increased.     

Getting Rid of Links in Hierarchical Radiosity

Hierarchical radiosity with clustering has positioned itself as one of the most efficient algorithms for computing global illumination in non-trivial environments. However, using hierarchical radiosity for complex scenes is still problematic due to the necessity of storing a large number of transport coefficients between surfaces in the form of links. In this paper, we eliminate the need for storage of links through the use of a modified shooting method for solving the radiosity equation. By distributing only unshot radiosity in each step of the iteration, the number of links decreases exponentially. Recomputing these links instead of storing them increases computation time, but reduces memory consumption dramatically. Caching may be used to reduce the time overhead. We analyze the error behavior of the new algorithm in comparison with the normal gathering approach for hierarchical radiosity. In particular, we consider the relation between the global error of a hierarchical radiosity solution and the local error threshold for each link.     

Space-Time Hierarchical Radiosity with Clustering and Higher-Order Wavelets

We address in this paper the issue of computing diffuse global illumination solutions for animation sequences. The principal difficulties lie in the computational complexity of global illumination, emphasized by the movement of objects and the large number of frames to compute, as well as the potential for creating temporal discontinuities in the illumination, a particularly noticeable artifact. We demonstrate how space‐time hierarchical radiosity, i.e. the application to the time dimension of a hierarchical decomposition algorithm, can be effectively used to obtain smooth animations: first by proposing the integration of spatial clustering in a space‐time hierarchy; second, by using a higher‐order wavelet basis adapted for the temporal dimension. The resulting algorithm is capable of creating time‐dependent radiosity solutions efficiently.     

A Two-Pass Hardware-Based Method for Hierarchical Radiosity

Finite elements methods for radiosity are aimed at computing global illumination solutions efficiently. However these methods are not suitable for obtaining high quality images due to the lack of error control. Two-pass methods allow to achieve that level of quality computing illumination at each pixel and thus introducing a high computing overhead. We present a two-pass method for radiosity that allows to produce high quality images avoiding most of the per-pixel computations. The method computes a coarse hierarchical radiosity solution and then performs a second pass using current graphics hardware accelerators to generate illumination as high definition textures.     

A Median Cut Algorithm for Efficient Sampling of Radiosity Functions

This paper presents an efficient method for sampling the illumination functions in higher order radiosity algorithms. In such algorithms, the illumination function is not assumed to be constant across each patch, but it is approximated by a function which is at least C¹ continuous. Our median cut sampling algorithm is inspired by the observation that many form factors are computed at higher precision than is necessary. While a high sampling rate is necessary in regions of high illumination, dark areas can be sampled at a much lower rate to compute the received radiosity within a given precision. We adaptively subdivide the emitter into regions of approximately equal influence on the result. Form factors are evaluated by the disk approximation and a ray tracing based test for occlusion detection. The implementation of a higher order radiosity system using B-splines as radiosity function is described. The median cut algorithm can also be used for radiosity algorithms based on the constant radiosity assumption.     

Bounded Radiosity – Illumination on General Surfaces and Clusters

Traditionally, Radiosity algorithms have been restricted to scenes made from planar patches. Most algorithms for computing form factors and the subdivision criterion for hierarchical methods implicitly assume planar patches. In this paper, we present a new radiosity algorithm that is solely based on simple geometric information about surface elements, namely their bounding boxes and cone of normals. Using this information allows to compute efficient error bounds that can be used for the subdivision oracle and for computing the energy transfer. Due to the simple interface to geometric objects, our algorithm not only allows for computing illumination on general curved surfaces, but it can also be directly applied to a hieararchy of clusters. Several examples demonstrate the advantages of the new approach.     

基于内容的网格生成算法

在基于模型的编码技术中,选择合适的网络模型对提高模糊的运动估计精度、编码效率和得到高质量的解码图像都是至关重要的。本文提出的基于图像内容的自适应网络模型生成算法,首先利用数学形态学中的水线算法把编码图像分割成许多纹理一致的区域,所分割的区域反映了图像的结构、轮廓和边界;再对这些区域的边界进行多边形拟合,得到多边形各个边的端点作为网格模型的节点,以这些节点为基础就能生成一个Ｄｅｌａｕｎａｙ三角形网格     

一种辐射度全局光照网格模型的简化方法*

全局光照模型计算通常将环境中的表面分解得足够细,以精确地捕捉由于物体间相互遮挡所引起的阴影效果及其他一些光照效果.因而,一个复杂场景经全局光照计算后,其模型复杂度远远超出了当今图形工作站的实时绘制能力.给出了一种辐射度全局光照网格模型的简化方法.算法首先根据辐射度计算的特点以及人眼的视觉特点,提出以辐射度最大相对变化值为准则,以面片合并法实现全局光照网格模型的第1步简化,将原辐射度全局光照网格模型简化为能量相对变化在用户定义误差范围内的一些超面区域.然后利用顶点删除法实现超面区域边界的简化,进一步加大原网格模型的简化程度.试验表明,这种算法不仅能有效地简化辐射度全局光照网格模型,而且能较好地保持原光照网格模型的特征.     

An Exhaustive Error-Bounding Algorithm for Hierarchical Radiosity

This paper presents a complete algorithm for the evaluation and control of error in radiosity calculations. Providing such control is both extremely important for industrial applications andd one of the most challenging issues remaining in global illumination research. In order to control the error, we need to estimate the accuracy of the calculation while computing the energy exchanged between two objects. Having this information for each radiosity interaction allows to allocate more resources to refine interactions with greater potential error, and to avoid spending more time to refine interactions already represented with sufficient accuracy. Until now, the accuracy of the computed energy exchange could only be approximated using heuristic algorithms. This paper presents the first exhaustive algorithm to compute fully reliable upper and lower bounds on the energy being exchanged in each interaction. This is accomplished by computing first and second derivatives of the radiosity function where appropriate, and making use of two concavity conjectures. These bounds are then used in a refinement criterion for hierarchical radiosity, resulting in a global illumination algorithm with complete control of the error incurred. Results are presented, demonstrating the possibility to create radiosity solutions with guaranteed precision. We then extend our algorithm to consider linear bounding functions instead of constant functions, thus creating simpler meshes in regions where the function is concave, without loss of precision. Our experiments show that the computation of radiosity derivatives along with the radiosity values only requires a modest extra cost, with the advantage of a much greater precision.     

A Radiosity Solution for Curved Surface Environments

Radiosity has been a popular method for photorealistic image generation.But the determination of form factors between curved patches is the most difficult and time consuming procedure,and also the errors caused by approximating source patch‘s radiosity with average values are obvious.In this paper,a radiosity algorithm for rendering curved surfaces represented by parameters is described.The contributed radiosity from differential areas on four vertices of the source patch to a receiving point is calculated firstly,then the contribution from the inner area of the source patch is evaluated by interpolating the values on four corners.Both the difficult problem of determining form-factors between curved surfaces and errors mentioned above have been avoided.Comparison of the experimental results using the new algorithm has been made with the ones obtained by traditional method.Some associated techniques such as the visibility test and the adaptive subdivision are also described.     

Radiosity for dynamic scenes in flatland with the visibility complex

The radiosity method is particularly suitable for global illumination calculations in static environments. Nonetheless, recent applications of image synthesis such as architectural simulation or lighting design require the ability to modify environments. Previous methods have attempted to deal with dynamic environments (environments where the geometry, the material properties, etc., can change)but still suffer some limitations in the case of moving objects. One of the main problems remaining is the efficient and accurate detection of which form factors must really be recomputed, since their calculation is the most time-consuming part of the radiosity method. To correctly understand and solve this problem, we start with a method in 2D for polygonal scenes using the visibility complex. It is a powerful data structure representing the visibility relationships between objects in the plane. We have developed and implemented an algorithm which uses this structure to efficiently compute the discontinuity mesh and the form factors for static scenes. We also propose an extension to our algorithm to efficiently update only the modified form factors when an object is moving. This approach enhances our understanding and will hopefully lead to efficient solutions in 3D.     

极大熵条件下的动态搜索优化图象重建方法

提出一种满足多种准则的动态搜索优化图象重建方法。从图象场的本质出发，设立了３个准则函数。熵函数的导数是非线性的，本文将其变换为近似的线性公式以获得迭代公路。动态搜索则尽可能避免很多算法在迭代过程中对图象校正过量或不足的问题。     

Monte Carlo辐射度聚光灯模型

辐射度算法能绘制出更为真实的场景，而近几年发展起来的Monte Carlo辐射度算法可对复杂场景全局光照进行快速、有效的求解，基于Monte Carlo辐射度算法提出了一个精确的方向光源模型，通过直接设置光源角度即可快速生成舞台的聚光灯效果。     

A unified hierarchical algorithm for global illumination withscattering volumes and object clusters

The paper presents a new radiosity algorithm that allows the simultaneous computation of energy exchanges between surface elements, scattering volume distributions, and groups of surfaces, or object clusters. The new technique is based on a hierarchical formulation of the zonal method, and efficiently integrates volumes and surfaces. In particular no initial linking stage is needed, even for inhomogeneous volumes, thanks to the construction of a global spatial hierarchy. An analogy between object clusters and scattering volumes results in a powerful clustering radiosity algorithm, with no initial linking between surfaces and fast computation of average visibility information through a cluster. We show that the accurate distribution of the energy emitted or received at the cluster level can produce even better results than isotropic clustering at a marginal cost. The resulting algorithm is fast and, more importantly, truly progressive as it allows the quick calculation of approximate solutions with a smooth convergence towards very accurate simulations     

Using Subdivision on Hierarchical Data to Reconstruct Radiosity Distribution

Computing global illumination by finite element techniques usually generates a piecewise constant approximation of the radiosity distribution on surfaces. Directly displaying such scenes generates artefacts due to discretization errors. We propose to remedy this drawback by considering the piecewise constant output to be samples of a (piecewise) smooth function in object space and reconstruct this function by applying a binary subdivision scheme. We design custom taylored subdivision schemes with quadratic precision for the efficient refinement of cell- or pixel-type data. The technique naturally allows to reconstruct functions from non-uniform samples which result from adaptive binary splitting of the original domain (quadtree). This type of output is produced, e.g., by hierarchical radiosity algorithms. The result of the subdivision process can be mapped as a texture on the respective surface patch which allows to exploit graphics hardware for considerably accelerating the display.     

Radiosity with Well Distributed Ray Sets

In this paper we present a new radiosity algorithm, based on the notion of a well distributed ray set (WDRS). A WDRS is a set of rays, connecting mutually visible points and patches, that forms an approximate representation of the radiosity operator and the radiosity distribution. We propose an algorithm that constructs an optimal WDRS for a given accuracy and mesh. The construction is based on discrete importance sampling as in previously proposed stochastic radiosity algorithms, and on quasi Monte Carlo sampling. Quasi Monte Carlo sampling leads to faster convergence rates and the fact that the sampling is deterministic makes it possible to represent the well distributed ray set very efficiently in computer memory. Like previously proposed stochastic radiosity algorithms, the new algorithm is well suited for computing the radiance distribution in very complex diffuse scenes, when it is not feasible to explicitly compute and store form factors as in classical radiosity algorithms. Experiments show that the new algorithm is often more efficient than previously proposed Monte Carlo radiosity algorithms by half an order of magnitude and more.