A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Ray-Tracing Based Techniques ( ) <s> We present an efficient framework for the high-quality rendering of discretely sampled surface-based objects with refractive effects. This requires an accurate estimation of the refraction coefficients, paired with efficient and accurate surface detection, space traversal, and backdrop image sampling. Our framework achieves these goals, by employing a high-quality spline-based filter in conjunction with a novel filtered octree space decomposition with pre-classified cells that is carefully matched to the filter and voxel neighborhood characteristics. Here, we benefit greatly from the non-negativity of the B-spline kernel. Finally, we describe an innovative scheme that achieves the high quality of pixel super-sampling on a flat backdrop plane without the overhead of tracing the actual rays across the refractive object. <s> BIB001 </s> A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Ray-Tracing Based Techniques ( ) <s> The field of volume visualization has undergone rapid development during the past years, both due to advances in suitable computing hardware and due to the increasing availability of large volume datasets. Recent work has focused on increasing the visual realism in Direct Volume Rendering (DVR) by integrating a number of visually plausible but often effect-specific rendering techniques, for instance modeling of light occlusion and depth of field. Besides yielding more attractive renderings, especially the more realistic lighting has a positive effect on perceptual tasks. Although these new rendering techniques yield impressive results, they exhibit limitations in terms of their exibility and their performance. Monte Carlo ray tracing (MCRT), coupled with physically based light transport, is the de-facto standard for synthesizing highly realistic images in the graphics domain, although usually not from volumetric data. Due to the stochastic sampling of MCRT algorithms, numerous effects can be achieved in a relatively straight-forward fashion. For this reason, we have developed a practical framework that applies MCRT techniques also to direct volume rendering (DVR). With this work, we demonstrate that a host of realistic effects, including physically based lighting, can be simulated in a generic and flexible fashion, leading to interactive DVR with improved realism. In the hope that this improved approach to DVR will see more use in practice, we have made available our framework under a permissive open source license. <s> BIB002
The work discussed above focuses mainly on increasing the lighting realism in interactive volume rendering applications. This is often achieved by making assumptions regarding the lighting setup or the illumination propagation, e. g., assuming that only directional scattering occurs. As ray-tracing has been widely used in polygonal rendering to support physically correct and thus realistic images, a few authors have also investigated this direction in the area of volume rendering. Gradients, as used for the gradient-based shading approaches, can also be exploited to realize a volumetric ray-tracer supporting more physically correct illumination. One such approach, considering only first order rays to be traversed, has been proposed by Stegmaier et al. [SSKE05] . With their approach they are able to simulate mirror-like reflections and refraction, by casting a ray and computing its deflection. To simulate mirror-like reflections, they perform an environment texture lookup. More sophisticated refraction approaches have been presented by others BIB001  . Monte Carlo ray-tracing methods have also been applied in the area of interactive volume rendering in order to produce realistic images. The benefit of these approaches is, that they are directly coupled with physically based light transport. The first Monte Carlo based approach has been presented by Rezk-Salama [RS07]. It supports single and multiple scattering, whereby interactivity is achieved by constraining the expensive scattering computations to selected surfaces only. However, due to this restriction it is not possible to render translucent materials such as clouds. More recently, Kroes et al. presented an interactive Monte Carlo ray-tracer, which does not limit the scattering to selected boundary surfaces BIB002 . Their approach simulates several real-world factors influencing volumetric illumination, such as multiple arbitrarily shaped and textured area light sources, a real-world camera with lens and aperture, as well as complex material properties. This combination allows images showing a high degree of realism to be generated (see Figure 23 ). To adequately represent both, volumetric and surface-like materials, their approach uses stochastic sampling with the gradient magnitude as threshold to determine if a BRDF or a phase function should be used for material representation. It is restricted to single scattering and uses a progressive approach to allow interactivity. It is shown that the image converges after roughly a half to one second when parameters such as clip plane or transfer function is changed. A minor amount of additional data is required in order to perform filtering and noise removal, but the method is independent of the data set size in terms of memory.