A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Perceptual Impact <s> The human visual system has a remarkable ability to interpret smooth patterns of light on a surface in terms of 3-D surface geometry. Classical studies of shape-from-shading perception have assumed that surface irradiance varies with the angle between the local surface normal and a collimated light source. This model holds, for example, on a sunny day. One common situation in which this model fails to hold, however, is under diffuse lighting such as on a cloudy day. Here we report on the first psychophysical experiments that address shape-from-shading under a uniform diffuse-lighting condition. Our hypothesis was that shape perception can be explained with a perceptual model that “dark means deep”. We tested this hypothesis by comparing performance in a depth-discrimination task to performance in a brightness-discrimination task, using identical stimuli. We found a significant correlation between responses in the two tasks, supporting a dark-means-deep model. However, overall performance in the depth-disc... <s> BIB001 </s> A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Perceptual Impact <s> LightKit is a system for lighting three-dimensional synthetic scenes. LightKit simplifies the task of producing visually pleasing, easily interpretable images for visualization while making it harder to produce results where the scene illumination distracts from the visualization process. LightKit is based on lighting designs developed by artists and photographers and shown in previous studies to enhance shape perception. A key light provides natural overhead illumination of the scene, augmented by fill, head, and back lights. By default, lights are attached to a normalized, subject-centric, camera-relative coordinate frame to ensure consistent lighting independent of camera location or orientation. This system allows all lights to be positioned by specifying just six parameters. The intensity of each light is specified as a ratio to the key light intensity, allowing the scene's brightness to be adjusted using a single parameter. The color of each light is specified by a single normalized color parameter called warmth that is based on color temperature of natural sources. LightKit's default values for light position, intensity, and color are chosen to produce good results for a variety of scenes. LightKit is designed to work with both hardware graphics systems and, potentially, higher quality off-line rendering systems. We provide examples of images created using a LightKit implementation within the VTK visualization toolkit software framework. <s> BIB002 </s> A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Perceptual Impact <s> In many domains it is very important that observers form an accurate percept of 3-dimensional structure from 2-dimensional images of scenes or objects. This is particularly relevant for designers who need to make decisions concerning the refinement of novel objects that haven't been physically built yet. This study presents the results of two experiments whose goal was to test the effect of lighting direction on the shape perception of smooth surfaces using shading and lighting techniques commonly used in modeling and design software. The first experiment consisted of a 2 alternate forced choice task which compared the effect of the amount of shape difference between smooth surfaces lit by a single point light with whether the position of the light sources were the same or different for each surface. Results show that, as the difference between the shapes decreased, participants were more and more biased towards choosing the match shape lit by the same source as the test shape. In the second experiment, participants had to report the orientation at equivalent probe locations on pairs of smooth surfaces presented simultaneously, using gauge figures. The surfaces could either be the same or slightly different and the light source of each shape could either be at the same relative location or offset by 90° horizontally. Participants reported large differences in surface orientation when the lighting condition was different, even when the shapes were the same, confirming the first results. Our findings show that lighting conditions can have a strong effect on 3-dimensional perception, and suggest that great care should be taken when projection systems are used for 3D visualisation where an accurate representation is required, either by carefully choosing lighting conditions or by using more realistic rendering techniques. <s> BIB003 </s> A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Perceptual Impact <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> BIB004 </s> A Survey of Volumetric Illumination Techniques for Interactive Volume Rendering <s> Perceptual Impact <s> The process of surface perception is complex and based on several influencing factors, e.g., shading, silhouettes, occluding contours, and top down cognition. The accuracy of surface perception can be measured and the influencing factors can be modified in order to decrease the error in perception. This paper presents a novel concept of how a perceptual evaluation of a visualization technique can contribute to its redesign with the aim of improving the match between the distal and the proximal stimulus. During analysis of data from previous perceptual studies, we observed that the slant of 3D surfaces visualized on 2D screens is systematically underestimated. The visible trends in the error allowed us to create a statistical model of the perceived surface slant. Based on this statistical model we obtained from user experiments, we derived a new shading model that uses adjusted surface normals and aims to reduce the error in slant perception. The result is a shape-enhancement of visualization which is driven by an experimentally-founded statistical model. To assess the efficiency of the statistical shading model, we repeated the evaluation experiment and confirmed that the error in perception was decreased. Results of both user experiments are publicly-available datasets. <s> BIB005
In addition to the technical insights and the achievable illumination effects described above, the perceptual impact of the current techniques is also of relevance. Although, the perceptual qualities of advanced illumination techniques are frequently pointed out, remarkably little research has been done in order to investigate them in a systematic fashion. One study showing the perceptual benefits of advanced volumetric illumination models has been conducted with the goal to analyze the impact of the shadow volume propagation technique [RDRS10b] . The users participating in this study had to perform several tasks depending on the depth perception of static images, where one set was generated using gradient-based shading and the other one using shadow volume propagation. The results of the study indicate that depth perception is more accurate and performed faster when using shadow volume propagation. Šoltészová et al. evaluated the perceptual impact of chromatic soft shadows in the context of interactive volume rendering  . Inspired by illustrators, they replace the luminance decrease, usually present in shadowed areas, by a chromaticity blending. In their user study they showed the perceptual benefit of these chromatic shadows with respect to depth and surface perception. However, their results also indicate that brighter shadows might have a negative impact on the perceptual qualities of an image and should therefore be used with consideration. Šoltészová et al. also evaluated the perception of surface slant for surfaces with Lambertian shading and found that there is a systematic distortion BIB005 . They create a prediction model that is used to alter the normal or gradient in the shading of the surface slant such that it is perceived as the ground truth. This improves the perception of the surface slant when the angle is between 40-60 • . The difference before and after applying their corrective model can be seen in Figure 24 . Lindemann and Ropinski presented the results of a user study in which they have investigated the perceptual impact of seven volumetric illumination techniques BIB004 . Within their study depth and volume related tasks had to be performed. They showed that advanced volumetric illumination models make a difference when it is necessary to assess depth or size of volumetric objects in images. However, they could not find a relation between the time used to perform a certain task and the used illumination model. The results of this study indicate that the directional occlusion shading model has the best perceptual capabilities. In terms of light source placement Langer et al. BIB001 showed that local shape perception increase if a directional light source is angled slightly to the left or right above the viewing direction (11 • ) compared to placing it below the viewing direction. They further showed that diffuse light produced as good shape perception as the light coming from upper right or left. Caniard et al. BIB003 showed that shape perception is heavily dependent on light source position when using a single point light source under local illumination. Creating more complex light setups can increase the shape perception BIB002 . As noted before, Wanger et al.  showed that the sharpness of shadows increase the accuracy of matching shapes. Interreflections (multiple scattering) was shown by Madison et al. [MTK * 01] to be an important cue, in addition to shadows, for determining the contact of objects. Interreflections were also used by Weigle and Banks  , which showed that physically-based illumination can in some cases increase the perception of 3D flow-field geometry.