A survey of photon mapping state-of-the-art research and future challenges <s> Combination with bidirectional path tracing <s> In this paper we present a new Monte Carlo rendering algorithm that seamlessly integrates the ideas of shooting and gathering power to create photorealistic images. The algorithm can be explained as a generalisation of the well-known path tracing algorithm. Test results show that it performs signicantly better for typical indoor scenes where indirect lighting is important. <s> BIB001 </s> A survey of photon mapping state-of-the-art research and future challenges <s> Combination with bidirectional path tracing <s> This paper presents a new importance sampling strategy for Monte Carlo ray tracing in which a rough estimate of the irradiance based on the photon map is combined with the local reflection model to construct more efficient probability density functions that can be used in an importance sampling scheme. <s> BIB002 </s> A survey of photon mapping state-of-the-art research and future challenges <s> Combination with bidirectional path tracing <s> We present a new sampling space for light transport paths that makes it possible to describe Monte Carlo path integration and photon density estimation in the same framework. A key contribution of our paper is the introduction of vertex perturbations, which extends the space of paths with loosely coupled connections. The new framework enables the computation of path probabilities in the same space under the same measure, which allows us to use multiple importance sampling to combine Monte Carlo path integration and photon density estimation. The resulting algorithm, unified path sampling, can robustly render complex combinations and glossy surfaces and caustics that are problematic for existing light transport simulation methods. <s> BIB003 </s> A survey of photon mapping state-of-the-art research and future challenges <s> Combination with bidirectional path tracing <s> Developing robust light transport simulation algorithms that are capable of dealing with arbitrary input scenes remains an elusive challenge. Although efficient global illumination algorithms exist, an acceptable approximation error in a reasonable amount of time is usually only achieved for specific types of input scenes. To address this problem, we present a reformulation of photon mapping as a bidirectional path sampling technique for Monte Carlo light transport simulation. The benefit of our new formulation is twofold. First, it makes it possible, for the first time, to explain in a formal manner the relative efficiency of photon mapping and bidirectional path tracing, which have so far been considered conceptually incompatible solutions to the light transport problem. Second, it allows for a seamless integration of the two methods into a more robust combined rendering algorithm via multiple importance sampling. A progressive version of this algorithm is consistent and efficiently handles a wide variety of lighting conditions, ranging from direct illumination, diffuse and glossy inter-reflections, to specular-diffuse-specular light transport. Our analysis shows that this algorithm inherits the high asymptotic performance from bidirectional path tracing for most light path types, while benefiting from the efficiency of photon mapping for specular-diffuse-specular lighting effects. <s> BIB004 </s> A survey of photon mapping state-of-the-art research and future challenges <s> Combination with bidirectional path tracing <s> Graphics Processing Units (GPUs) recently became general enough to enable implementation of a variety of light transport algorithms. However, the efficiency of these GPU implementations has received relatively little attention in the research literature and no systematic study on the topic exists to date. The goal of our work is to fill this gap. Our main contribution is a comprehensive and in-depth investigation of the efficiency of the GPU implementation of a number of classic as well as more recent progressive light transport simulation algorithms. We present several improvements over the state-of-the-art. In particular, our light vertex cache, a new approach to mapping connections of subpath vertices in bidirectional path tracing on the GPU, outperforms the existing implementations by 30--60p. We also describe a first GPU implementation of the recently introduced vertex connection and merging algorithm [Georgiev et al. 2012], showing that even relatively complex light transport algorithms can be efficiently mapped on the GPU. With the implementation of many of the state-of-the-art algorithms within a single system at our disposal, we present a unique direct comparison and analysis of their relative performance. <s> BIB005 </s> A survey of photon mapping state-of-the-art research and future challenges <s> Combination with bidirectional path tracing <s> Efficiently computing light transport in participating media in a manner that is robust to variations in media density, scattering albedo, and anisotropy is a difficult and important problem in realistic image synthesis. While many specialized rendering techniques can efficiently resolve subsets of transport in specific media, no single approach can robustly handle all types of effects. To address this problem we unify volumetric density estimation, using point and beam estimators, and Monte Carlo solutions to the path integral formulation of the rendering and radiative transport equations. We extend multiple importance sampling to correctly handle combinations of these fundamentally different classes of estimators. This, in turn, allows us to develop a single rendering algorithm that correctly combines the benefits and mediates the limitations of these powerful volume rendering techniques. <s> BIB006 </s> A survey of photon mapping state-of-the-art research and future challenges <s> Combination with bidirectional path tracing <s> As path tracing allows for more realistic and faster lighting, an increasing number of movies are created the physically based way. With examples from recent movies, the architectures and novel workflows of the next generation of production renderers are introduced to a wide audience including technical directors, artists, and researchers. <s> BIB007
Path tracing is a very general simulation of global illumination (Kajiya, 1986) , and bidirectional path tracing (BPT) BIB001 performs well on a wide range of illumination and scene configurations. However, there is no global illumination method which can simulate all types of light transport efficiently. BIB002 simply used the photon mapping method in improved path tracing with a new importance sampling strategy through a rough estimate of photon irradiance. Until SPPM was proposed to combine photon density estimation and distributed ray tracing, a new idea about combination had been the research focus. Two efficient combinations between BPT and PM were presented by BIB003 and BIB004 , and the two methods combine the best of both algorithms. BIB004 proposed a vertex connection and merging (VCM) algorithm using different derivations concurrently. They considered the last step in photon mapping as establishing a regular vertex connection between the eye path and the light path. This technique is called vertex merging, as it can be intuitively thought to weld the endpoints of the two sub-paths if they lie close to each other. The first GPU implementation of vertex connection and merging algorithm was presented by BIB005 . In the implementation, a full hash grid was used to store the light path vertices, and each eye path point was connected to a predetermined number of light path vertices. BIB003 considered an extension of BPT that samples paths in the higher-dimensional space of PM, under the name unified path sampling. This was done by considering a random perturbation of the eye path vertex in an r-neighborhood. The combinations were even extended to the participating media in BIB006 . The recent development of combining photon mapping with bidirectional path tracing is significant, as this kind of method can handle the speculardiffuse-specular (SDS) light transport efficiently. What is more, the path tracing method is going back to the mainstream of the way that movies are rendered BIB007 , which can handle gigantic amounts of geometry, textures, and light sources. Combining with bidirectional path tracing allows the photon mapping technique to make a contribution to larger-scale industrial rendering.