A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Introduction <s> This tutorial survey paper reviews several different models for light interaction with volume densities of absorbing, glowing, reflecting, and/or scattering material. They are, in order of increasing realism, absorption only, emission only, emission and absorption combined, single scattering of external illumination without shadows, single scattering with shadows, and multiple scattering. For each model the paper provides the physical assumptions, describes the applications for which it is appropriate, derives the differential or integral equations for light transport, presents calculation methods for solving them, and shows output images for a data set representing a cloud. Special attention is given to calculation methods for the multiple scattering model. > <s> BIB001 </s> A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Introduction <s> Particle-based simulation methods are used to model a wide range of complex phenomena and to solve time-dependent problems of various scales. Effective visualization of the resulting state should communicate subtle changes in the three-dimensional structure, spatial organization, and qualitative trends within a simulation as it evolves. We take steps toward understanding and using advanced shading models in the context of interactive particle visualization. Specifically, the impact of ambient occlusion and physically based diffuse interreflection is investigated using a formal user study. We find that these shading models provide additional visual cues that enable viewers to better understand subtle features within particle datasets. We also describe a visualization process that enables interactive navigation and exploration of large particle datasets, rendered with illumination effects from advanced shading models. Informal feedback from application scientists indicates that the results of this process enhance the data analysis tasks necessary for understanding complex particle datasets. <s> BIB002 </s> A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Introduction <s> In this paper, we present a user study in which we have investigated the influence of seven state-of-the-art volumetric illumination models on the spatial perception of volume rendered images. Within the study, we have compared gradient-based shading with half angle slicing, directional occlusion shading, multidirectional occlusion shading, shadow volume propagation, spherical harmonic lighting as well as dynamic ambient occlusion. To evaluate these models, users had to solve three tasks relying on correct depth as well as size perception. Our motivation for these three tasks was to find relations between the used illumination model, user accuracy and the elapsed time. In an additional task, users had to subjectively judge the output of the tested models. After first reviewing the models and their features, we will introduce the individual tasks and discuss their results. We discovered statistically significant differences in the testing performance of the techniques. Based on these findings, we have analyzed the models and extracted those features which are possibly relevant for the improved spatial comprehension in a relational task. We believe that a combination of these distinctive features could pave the way for a novel illumination model, which would be optimized based on our findings. <s> BIB003
Interactive volume rendering as a subdomain of scientific visualization has become mature in the last decade. Several approaches exist, which allow a domain expert to visualize and explore volumetric datasets at interactive frame rates on standard graphics hardware. While the varying rendering paradigms underlying these approaches directly influence image quality [SHC * 09], in most real-world use cases the standard emission absorption model is used to incorporate illumination effects BIB001 . However, the emission absorption model is only capable of simulating local lighting effects, where light is emitted and absorbed locally at each sample point processed during rendering. This result in volume rendered images, which convey the basic structures represented in a volumetric dataset (see Figure 1 (left)). Though, all structures are clearly visible in these images, information regarding the arrangement and size of structures is sometimes hard to convey. In the computer graphics community, more precisely the real-time rendering community, the quest for realistic interactive image synthesis is one of the major goals addressed since its early days  . Originally, mainly motivated by the goals to be able to produce more realistic imagery for computer games and animations, the perceptual benefits of more realistic representations could also be shown [WFG92,  BIB002 . Consequently, in the past years the scientific visualization community started to develop interactive volume rendering techniques, which do not only allow simulation of local lighting effects, but also more realistic global effects [RSHRL09] . The existing approaches span a wide range in terms of underlying rendering paradigms, consumed hardware resources as well as lighting capabilities, and thus have different perceptual impacts BIB003 . Due to the wide range of techniques and tradeoffs it can be hard for both application developers and new researchers to know which technique to use in a certain scenario, what has been done, and what the challenges are within the field. Therefore, we provide a survey of techniques dealing with volumetric illumination, which allows interactive data exploration, i. e., rendering parameters can be changed interactively. When developing such techniques, several challenges need to be addressed: • A volumetric optical model must be applied, which usually results in more complex computations due to the global nature of the illumination. • Interactive transfer function updates must be supported. This requires, that the resulting changes to the 3D structure of the data must be incorporated during illumination. • Graphics processing unit (GPU) algorithms need to be developed to allow interactivity. When successfully addressing these challenges, increasingly realistic volume rendered images can be generated interactively. As can be seen in Figure 1 (right), these images not only increase the degree of realism, but also support improved perceptual comprehension. While Figure 1 (left) shows the overall structure of the rendered computed tomography (CT) dataset of a human heart, the shadows added in Figure 1 (right) provide additional depth information. We will review and compare the existing techniques, seen in Table 1 , for advanced volumetric illumination with respect to their applicability, which we derive from the illumination capabilities, the performance behavior as well as their technical realization. Since this report addresses interactive advanced volumetric illumination techniques, potentially applicable in scientific visualization, we do not consider methods requiring precomputations that do not permit change of the transfer function during rendering. The goal is to provide the reader with an overview of the field and to support design decisions when exploiting current concepts. We have decided to fulfill this by adopting a classification which takes into account the technical realization, performance behavior as well as the supported illumination effects. Properties considered in the classification are for instance, the usage of preprocessing, because precomputation based techniques are usually not applicable to large volumetric datasets as the precomputed illumination volume would consume too much additional graphics memory. Other approaches might be bound to a certain rendering paradigm, which serves as another property, since it might hamper usage of a technique in a general manner. We have selected the covered properties, such that the classification allows an applicability-driven grouping of the existing advanced volumetric illumination techniques. To provide the reader with a mechanism to choose which advanced volumetric illumination model to be used, we will describe these and other properties as well as their implications along with our classification. Furthermore, we will point out the shortcomings of the current approaches to demonstrate possibilities for future research. The remainder of this article is structured as follows. In the next section we will discuss the models used for simulating volumetric illumination. We will have our starting point in the seminal work presented by Max BIB001 , with the emission absorption model which we successively enrich to later on support global volumetric illumination effects. The reader interested mainly in the concepts underlying the implementations of the approaches covered in this article may skip Section 2 and directly proceed to Section 3, where we classify the covered techniques. We then propose usage guidelines in Section 4, which motivates the selected criteria and can help the reader in comparing the different techniques. These guidelines have been formulated with the goal to support an application expert choosing the right algorithm for a specific use case scenario. Based on the classification, Section 5 discusses all covered techniques in greater detail. Since one of the main motivations for developing volumetric illumination models in the scientific visualization community is improved visual perception, we will discuss recent findings in this area with respect to interactive volume rendering in Section 6. Furthermore, based on the observations made in Section 6 and the limitations identified throughout the article, we will present future challenges in Section 7. Finally, the article concludes in Section 8.