A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Zhou et al. [ZRL <s> Pr e-computed radiance transfer (PRT) has been used to render volumetric data under distant low-frequency illumination at real-time rates, including natural illumination, soft shadows, attenuation from semi-transparent occluders and multiple scattering. PRT requires a lengthy pre-process, which is acceptable only for static volume data. However, in practical volume rendering, general transfer functions are used. Manipulating such a transfer function will result in a dynamic radiance transfer which has to be re-computed. This work proposes a fast way for this re-computation. While previous work has used CPU Monte Carlo ray-tracing for pre-computation and requires time in the order of many minutes, our GPU implementation uses a hierarchical visibility approximation implemented entirely on the GPU and requires only a few seconds for typical scenes. <s> BIB001 </s> A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Zhou et al. [ZRL <s> We present a direct interblock interpolation technique that enables direct volume rendering of blocked, multiresolution volumes. The proposed method smoothly interpolates between blocks of arbitrary block-wise level-of-detail (LOD) without sample replication or padding. This permits extreme changes in resolution across block boundaries and removes the interblock dependency for the LOD creation process. In addition the full data reduction from the LOD selection can be maintained throughout the rendering pipeline. Our rendering pipeline employs a flat block subdivision followed by a transfer function based adaptive LOD scheme. We demonstrate the effectiveness of our method by rendering volumes of the order of gigabytes using consumer graphics cards on desktop PC systems. <s> BIB002 </s> A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Zhou et al. [ZRL <s> A novel technique for efficient computation of global light propagation in interactive DVR is presented in this paper. The approach is based on a combination of local shadows from the vicinity of each voxel with global shadows calculated at high resolution but stored in a sparser grid. The resulting intensities are then used as the initial illumination for an additional pass that computes first order scattering effects. The method captures global shadowing effects with enhanced shadows of near structures. A GPU framework is used to evaluate the illumination updates at interactive frame rates, using incremental refinements of the in-scattered light. <s> BIB003 </s> A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Zhou et al. [ZRL <s> In this paper we present a heuristic approach for simulating advanced light material interactions in the context of interactive volume rendering. In contrast to previous work, we are able to incorporate complex material functions, which allow to simulate reflectance and scattering. We exploit a common representation of these material properties based on spherical harmonic basis functions, to combine the achieved reflectance and scattering effects with natural lighting conditions, i. e., incorporating colored area light sources. To achieve these goals, we introduce a modified SH projection technique, which is not just tailored at a single material category, but adapts to the present material. Thus, reflecting and scattering materials as assigned trough the transfer function can be captured in a unified approach. We will describe the required extensions to the standard volume rendering integral and present an approximation which allows to realize the material effects in order to achieve interactive frame rates. By exploiting a combination of CPU and GPU processing, we are able to modify material properties and can change the illumination conditions interactively. We will demonstrate the outcome of the proposed approach based on renderings of real-world data sets and report the achieved computation times. <s> BIB004
* 08] presented a method for rendering smoke under low-frequency illumination by decomposing the smoke animation into a spherical harmonics basis representation and a sequence of residual fields. The light computation is then performed using the spherical harmonics representation of the density field. The computed illumination is combined with the high-frequency residual fields during ray marching to get high details in the final rendering. In a more recent paper, spherical harmonics were used to support more advanced material properties BIB004 . Therefore, the authors exploit the approach proposed by Ritschel BIB001 to compute spherical harmonic coefficients. During ray-casting, the spherical integral over the product of an area light source and voxel occlusion is efficiently computed using the precomputed occlusion information. Furthermore, realistic light material interaction effects are integrated while still achieving interactive volume frame rates. By using a modified spherical harmonic projection approach, tailored specifically for volume rendering, it becomes possible to realize non cone-shaped phase functions in the area of interactive volume rendering. Thus, more realistic material representations become possible. One problem with using a basis function such as spherical harmonics is the increased storage requirement. Higher frequencies can be represented when using more coefficients, thus obtaining better results. However, each coefficient needs to be stored to be used during rendering. Kronander et al. [KJL * 12] address this storage requirement issue and also decrease the time it takes to update the visibility. In their method, both volume data and spherical harmonic coefficients exploit the multiresolution technique of Ljung et al. BIB002 . The sparse representation of volume data and visibility allows substantial reductions with respect to memory requirements. To also decrease the visibility update time, the two pass approach introduced by Hernell et al. BIB003 is used. First, local visibility is computed over a small neighborhood by sending rays uniformly distributed on the sphere. Then, piecewise integration of the local visibility is performed using the spherical harmonics representation of Dirac delta functions in the direction of integration in order to compute global visibility. Since both local and global visibility have been computed, the user can switch between them during rendering and thus reveal structures hidden by global features while still keeping spatial comprehension. The spherical harmonics based methods discussed here are not bound to any specific rendering paradigm, although only volume ray-casting has been used so far. An arbitrary number of dynamic directional light sources are supported, i. e., environment maps. Area and point light sources are also supported, each adding an overhead since the direction towards the sample along the ray changes and the spherical harmonics representation of the light source must be rotated. A comparison to diffuse gradient based shading can be seen in Figure 22 . Storage and computation time limits the accuracy of the spherical harmonics representation, thus restricting the illumination to low frequency effects. In general, light sources are constrained to be outside of the volume. However, when using local visibility, e. g., as in [KJL * 12], they may be positioned inside. The rendering time is interactive, only requiring some (in general 1-4) extra texture lookups and scalar products. The light source representation must be recomputed or rotated when the illumination changes but this is generally fast and can be performed in real-time. The visibility must be recomputed when the transfer function or clip plane changes, which is expensive but can be performed interactively. The memory requirements are high as each coefficient needs to be stored for the visibility in the volume. We refer to the work by Kronander et al. [KJL * 12] for an indepth analysis of how to tradeoff memory size and image quality.