A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Lattice Based Techniques ( ) <s> Texture based volume rendering is a technique to efficiently visualize volumetric data using texture mapping hardware. We present an algorithm that extends this approach to render shadows for the volume. The algorithm takes advantage of fast frame buffer operations which modern graphics hardware offers, but does not depend on any special purpose hardware. The visual impression of the final image is significantly improved by bringing more structure and three dimensional information into the often foggyish appearance of texture based volume renderings. Although the algorithm does not perform lighting calculations, the resulting image has a shaded appearance, which is a further visual cue to spatial understanding of the data and lets the images appear more realistic. As calculating the shadows is independent of the visualization process it can be applied to any form of volume visualization, though volume rendering based on two- or three-dimensional texture mapping hardware makes the most sense. Compared to unshadowed texture based volume rendering, performance decreases by less than 50%, which is still sufficient to guarantee interactive manipulation of the volume data. In the special case where only the camera is moving with the light position fixed to the scene there is no performance decrease at all, because recalculation has only to be done if the position of the light source with respect to the volume changes. <s> BIB001 </s> A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Lattice Based Techniques ( ) <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> Lattice Based Techniques ( ) <s> Volume models often show high depth complexity. This poses di±culties to the observer in judging the spatial relationships accurately. Illustrators usually use certain techniques such as halos or edge darkening in order to enhance depth perception of certain structures. Halos may be dark or light, and even colored. Halo construction on a volumetric basis impacts rendering performance due to the complexity of the construction process. ::: In this paper we present Vicinity Occlusion Maps: a simple and fast method to compute the light occlusion due to neighboring voxels. Vicinity Occlusion Maps may be ::: used to generate flexible halos around objects or selected structures in order to enhance depth perception or accentuate the presence of some structures in volumetric ::: models at a low cost. The user may freely select the structure that requires the halos to be generated, its color and size, and our proposed application generates those in real ::: time. They may also be used to perform vicinity shading in realtime, or even to combine both effects. <s> BIB003 </s> A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Lattice Based Techniques ( ) <s> Advanced volume visualization techniques are important for both visual arts and visualization. An interactive volumetric lighting model simulates scattering and shadowing to generate high-quality images. Compared to gradient-based shading, this model achieves superior image quality and higher frame rates. User study results indicate that this model can also significantly improve depth perception. <s> BIB004
We classify all those approaches that compute the illumination directly based on the underlying grid as lattice based techniques. While many of the previous approaches are computing the illumination per sample during rendering, the lattice based techniques perform the computations per voxel, usually by exploiting the nature of the lattice. In this survey we focus on techniques which allow interactive data exploration, though it should be mentioned that other techniques working on non-regular grids exist [QXF * 07]. Shadow Volume Propagation is a lattice-based approach, where illumination information is propagated in a preprocessing step through the regular grid defining the volumetric data set. The first algorithm realizing this principle was proposed by Behrens and Ratering in the context of slice based volume rendering BIB001 . Their technique simulates shadows caused by attenuation of one distant light source. These shadows, which are stored within a shadow volume, are computed by attenuating illumination slice by slice through the volume. More recently, the lattice-based propagation concept has been exploited also by others.  where t( x ) is the transport color, as assigned through the transfer function, l i ( x , ω l ) is the luminance of the attenuated direct light as defined in Equation 5, and c( x , ωo) the chromaticity of the indirect in-scattered light. The transport color t( x ) as well as the chromaticity c( x , ωo) are wave length dependent, while the scalar l i ( x , ω l ) describes the incident (achromatic) luminance. Multiplying all together results in the incident radiance. Chromaticity c( x , ωo) is computed from the in-scattered light where Ω is the unit sphere centered around x and L chrom ( x, ω i ) is the chromaticity of Equation 3 with multiple scattering. To allow unidirectional illumination propagation, s( x , ω i , ωo) is chosen to be a strongly forward peaked phase function: The cone angle θ is used to control the amount of scattering and depends on the intensity at position x , and the phase function is a Phong lobe whose extent is controlled by the exponent β, restricted to the cone angle θ. C is a constant, chosen with respect to β, which ensures that the function integrates to one over all angles. During rendering, the illumination volume is accessed to look up the luminance value and scattering color of the current voxel. The chromaticity c( x , ωo) and the luminance l i ( x , ω i ) are fetched from the illumination volume and used within Lms( x , ωo) as shown above. In cases where a high gradient magnitude is present specular reflections, and thus surface-based illumination, is also integrated into s( x , ω i , ωo). Shadow volume propagation can be combined with various rendering paradigms and does not require a noticeable amount of preprocessing since the light propagation is carried out on the GPU. It supports point and directional light sources, whereby a more recent extension also supports area light sources BIB004 . While the position of the light source is unconstrained, the number of light sources is limited to one, since only one illumination propagation direction is supported. By simulating the diffusion process, shadow volume propagation supports multiple scattering. Rendering times are quite low, since only one additional 3D texture fetch is required to obtain the illumination values. However, when the transfer function is changed, the illumination information needs to be updated and thus the propagation step needs to be recomputed. Nevertheless, this still supports interactive frame rates. Since the illumination volume is stored and processed on the GPU, a sufficient amount of graphics memory is required. Piecewise Integration [HLY08] focus on direct light, i. e., Equation 3, with a single light source. They propose to discretize Equation 5 by dividing rays into k segments and evaluating the incoming radiance using: Short rays are first cast towards the light source for each voxel and the piecewise segments are stored in an intermediate 3D texture using a multiresolution data structure BIB002 . Then, again for each voxel, global rays are sent towards the light source now taking steps of size equal to the short rays and sampling from the piecewise segment 3D texture. In addition to direct illumination, first order inscattering is approximated by treating scattering in the vicinity as emission. To preserve interactivity, the final radiance is progressively refined by sending one ray at a time and blending the result into a final 3D texture. The piecewise integration technique can be combined with other rendering paradigms, even though it has been developed for volume ray-casting. It supports a single directional or point light source, which can be dynamically moved, and it includes an approximation of first order inscattering. Rendering times are low, since only a single extra lookup is required to compute the incoming radiance at the sample position. The radiance must be recomputed as soon as the light source or transfer function changes, which can be performed interactively and progressively. Three additional 3D textures are required for the computations. However, a lower resolution grid with multiresolution structure is used to alleviate some of the additional memory requirements. Table techniques were first exploited by Diaz et al. BIB003 for computation of various illumination effects in interactive direct volume rendering. The 2D summed area table (SAT) is a data structure where each cell (x, y) stores the sum of every cell located between the initial position (0, 0) and (x, y), such that the 2D SAT for each pixel p can be computed as: