For blind and visually impaired people, tactile reliefs offer many benefits over the more classic raised line drawings or tactile diagrams, as depth, 3D shape and surface textures are directly perceivable. However, without proper guidance some reliefs are still difficult to explore autonomously.

In this paper we propose a novel approach to hybrid visual steering of simulation ensembles. A simulation ensemble is a collection of simulation runs of the same simulation model using different sets of control parameters. Complex engineering systems have very large parameter spaces so a naïve sampling can result in prohibitively large simulation ensembles. Interactive steering of simulation ensembles provides the means to select relevant points in a multi-dimensional parameter space (design of experiment). Interactive steering efficiently reduces the number of simulation runs needed by coupling simulation and visualization and allowing a user to request new simulations on the fly. As system complexity grows, a pure interactive solution is not always sufficient. The new approach of hybrid steering combines interactive visual steering with automatic optimization. Hybrid steering allows a domain expert to interactively (in a visualization) select data points in an iterative manner, approximate the values in a continuous region of the simulation space (by regression) and automatically find the "best" points in this continuous region based on the specified constraints and objectives (by optimization). We argue that with the full spectrum of optimization options, the steering process can be improved substantially. We describe an integrated system consisting of a simulation, a visualization, and an optimization component. We also describe typical tasks and propose an interactive analysis workflow for complex engineering systems. We demonstrate our approach on a case study from automotive industry, the optimization of a hydraulic circuit in a high pressure common rail Diesel injection system.

Focus+context techniques provide visual guidance in visualizations by giving strong visual prominence to elements of interest while the context is suppressed. However, finding a visual feature to enhance for the focus to pop out from its context in a large dynamic scene, while leading to minimal visual deformation and subjective disturbance, is challenging. This paper proposes Attractive Flicker, a novel technique for visual guidance in dynamic narrative visualizations. We first show that flicker is a strong visual attractor in the entire visual field, without distorting, suppressing, or adding any scene elements. The novel aspect of our Attractive Flicker technique is that it consists of two signal stages: The first "orientation stage" is a short but intensive flicker stimulus to attract the attention to elements of interest. Subsequently, the intensive flicker is reduced to a minimally disturbing luminance oscillation ("engagement stage") as visual support to keep track of the focus elements. To find a good trade-off between attraction effectiveness and subjective annoyance caused by flicker, we conducted two perceptual studies to find suitable signal parameters. We showcase Attractive Flicker with the parameters obtained from the perceptual statistics in a study of molecular interactions. With Attractive Flicker, users were able to easily follow the narrative of the visualization on a large display, while the flickering of focus elements was not disturbing when observing the context.

Abstract This paper provides a comprehensive overview of urban reconstruction. While there exists a considerable body of literature, this topic is still under active research. The work reviewed in this survey stems from the following three research communities: computer graphics, computer vision and photogrammetry and remote sensing. Our goal is to provide a survey that will help researchers to better position their own work in the context of existing solutions, and to help newcomers and practitioners in computer graphics to quickly gain an overview of this vast field. Further, we would like to bring the mentioned research communities to even more interdisciplinary work, since the reconstruction problem itself is by far not solved.

We present a novel adaptive color mapping method for virtual objects in mixed-reality environments. In several mixed-reality applications, added virtual objects should be visually indistinguishable from real objects. Recent mixed-reality methods use global-illumination algorithms to approach this goal. However, simulating the light distribution is not enough for visually plausible images. Since the observing camera has its very own transfer function from real-world radiance values to RGB colors, virtual objects look artificial just because their rendered colors do not match with those of the camera. Our approach combines an on-line camera characterization method with a heuristic to map colors of virtual objects to colors as they would be seen by the observing camera. Previous tone-mapping functions were not designed for use in mixed-reality systems and thus did not take the camera-specific behavior into account. In contrast, our method takes the camera into account and thus can also handle changes of its parameters during runtime. The results show that virtual objects look visually more plausible than by just applying tone-mapping operators.

Abstract Because of its versatility, speed and robustness, shadow mapping has always been a popular algorithm for fast hard shadow generation since its introduction in 1978, first for offline film productions and later increasingly so in real‐time graphics. So it is not surprising that recent years have seen an explosion in the number of shadow map related publications. Because of the abundance of articles on the topic, it has become very hard for practitioners and researchers to select a suitable shadow algorithm, and therefore many applications miss out on the latest high‐quality shadow generation approaches. The goal of this survey is to rectify this situation by providing a detailed overview of this field. We show a detailed analysis of shadow mapping errors and derive a comprehensive classification of the existing methods. We discuss the most influential algorithms, consider their benefits and shortcomings and thereby provide the readers with the means to choose the shadow algorithm best suited to their needs.

In this paper we present a novel plausible realistic rendering method for mixed reality systems, which is useful for many real life application scenarios, like architecture, product visualization or edutainment. To allow virtual objects to seamlessly blend into the real environment, the real lighting conditions and the mutual illumination effects between real and virtual objects must be considered, while maintaining interactive frame rates (20-30fps). The most important such effects are indirect illumination and shadows cast between real and virtual objects. Our approach combines Instant Radiosity and Differential Rendering. In contrast to some previous solutions, we only need to render the scene once in order to find the mutual effects of virtual and real scenes. The dynamic real illumination is derived from the image stream of a fish-eye lens camera. We describe a new method to assign virtual point lights to multiple primary light sources, which can be real or virtual. We use imperfect shadow maps for calculating illumination from virtual point lights and have significantly improved their accuracy by taking the surface normal of a shadow caster into account. Temporal coherence is exploited to reduce flickering artifacts. Our results show that the presented method highly improves the illusion in mixed reality applications and significantly diminishes the artificial look of virtual objects superimposed onto real scenes.

The three papers in this special section are extended versions of papers originally presented at the ACM Symposium on Virtual Reality Software and Technology (VRST) 2007.

Abstract In this paper we propose a speed‐up technique for a local automatic colour equalization operator derived from a model of the human vision system. This method is characterized by local and global filtering effects, that simultaneously achieve different equalization tasks e.g. performing colour and lightness constancy, realizing dynamic image data driven stretching, controlling the contrast. We describe a way to quickly create a filtering mapping function to perform the global component of the mapping. This method is based on singular value decomposition (SVD) applied to sampled and filtered points in the input image. Then, the local information is added computing the basic algorithm on a neighbourhood of each input pixel. A slight quality loss is the price that we have to pay for a speed‐up of more than two orders of magnitude of the basic algorithm. We present the results on several images and discuss the efficiency and the drawbacks of the speed‐up technique.

Abstract Within computer graphics, the field of predictive rendering is concerned with those methods of image synthesis that yield results that do not only look real, but are also radiometrically correct renditions of nature, i.e. which are accurate predictions of what a real scene would look like under given lighting conditions. In order to guarantee the correctness of the results obtained by such techniques, three stages of such a rendering system have to be verified with particular care: the light reflection models, the light transport simulation and the perceptually based calculations used at display time. In this report, we will concentrate on the state of the art with respect to the second step in this chain. Various approaches for experimental verification of the implementation of a physically based rendering system have been proposed so far. However, the problem of proving that the results are correct is not fully solved yet, and no standardized methodology is available. We give an overview of existing literature, discuss the strengths and weaknesses of the described methods and illustrate the unsolved problems. We also briefly discuss the related issue of image quality metrics.

Impostors are image-based primitives commonly used to replace complex geometry in order to reduce the rendering time needed for displaying complex scenes. However, a big problem is the huge amount of memory required for impostors. This paper presents an algorithm that automatically places impostors into a scene so that a desired frame rate and image quality is always met, while at the same time not requiring enormous amounts of impostor memory. The low memory requirements are provided by a new placement method and through the simultaneous use of other acceleration techniques like visibility culling and geometric levels of detail.

Abstract We present a simple but powerful algorithm for optimizing the usage of hardware occlusion queries in arbitrary complex scenes. Our method minimizes the number of issued queries and reduces the delays due to the latency of query results. We reuse the results of occlusion queries from the last frame in order to initiate and schedule the queries in the next frame. This is done by processing nodes of a spatial hierarchy in a front‐to‐back order and interleaving occlusion queries with rendering of certain previously visible nodes. The proposed scheduling of the queries makes use of spatial and temporal coherence of visibility. Despite its simplicity, the algorithm achieves good culling efficiency for scenes of various types. The implementation of the algorithm is straightforward and it can be easily integrated in existing real‐time rendering packages based on common hierarchical data structures. Categories and Subject Descriptors (according to ACM CCS): I.3.7 [Computer Graphics]: Three‐Dimensional Graphics and Realism

Abstract For more than two decades Computer graphics researchers have tried to achieve photo‐realism in their images as reliable as possible, mainly by simulating the physical laws of light and adding one effect after the other. The recent years have brought a change of efforts towards real‐time methods, easy‐to‐use systems, integration with vision, modelling tools and the like. The quality of images is mostly accepted as sufficient for real world applications, but where are we really? There are still numerous problems to be solved, and there is notable progress in these areas. No question, the plug‐in philosophy of some commercial products has enabled several of these new techniques to be distributed quite fast. But unfortunately, many other of these developments happen in isolated systems for the pure purpose of publication, and never make it into commercial software. This presentation wants to make people more aware of such activities, and evaluate the steps we still have to go towards perfect photo‐realism. The talk will start with an attempt to give a brief overview of the rendering history, highlighting the main research directions at different times. It will explain the driving forces of the developments, which are complexity, speed, and accuracy, and maybe also expression in recent years. Solved and unsolved areas are examined, and compared to practically solved but theoretically incomplete topics such as translucency, tone mapping, light source and BTF descriptions, and error metrics for image quality evaluation. The difference lies mainly in the difference between believable, correct, and predictive images. Also, for really realistic images modelling complexity is still an issue. Finally, some recent work on polarization and fluorescence is presented.

We report on using computed tomography (CT) as a model acquisition tool for complex objects in computer graphics. Unlike other modeling and scanning techniques the complexity of the object is irrelevant in CT, which naturally enables to model objects with, for example, concavities, holes, twists or fine surface details. Once the data is scanned, one can apply post-processing techniques for data enhancement, modification or presentation. For demonstration purposes we chose to scan a Christmas tree which exhibits high complexity which is difficult or even impossible to handle with other techniques. However, care has to be taken to achieve good scanning results with CT. Further, we illustrate post-processing by means of data segmentation and photorealistic as well as non-photorealistic surface and volume rendering techniques.

We present a new method to compute radiance in photon map based global illumination simulation for polygonal scenes with general bidirectional scattering distribution functions (BSDFs). Our new radiance estimation uses the actual geometry in the neighborhood of the illuminated point, and does not assume that the nearest neighbor photons lie in the same plane as the point, nor that they are distributed in a circular area around that point. This allows us to achieve accurate indirect illumination by direct visualization of the photon map — which is especially important for the simulation of caustics (LS+DS*E paths) — even in the vicinity of edges and corners of objects, and on surfaces with differently oriented small geometric details.

The client-server architecture is widely used in the construction of virtual environments because it allows specific services to be provided to the connected clients and management tasks to be performed, such as reducing the network load by performing message filtering. The majority of client-server systems are incompatible with each other, both in terms of hardware and software. In particular, the different communication protocols make the cooperation between the various systems difficult. This paper guides the reader through the construction and implementation of a server that consists of clearly distinct components, developed to reflect a reference model for a generic and open client-server system. This server, employed in the PAVR network, not only allows incompatible clients and applications to be linked into a common environment with little effort, but, in addition, the detailed description of its implementation can provide valuable help to whoever undertakes the task of coding such systems.

In this paper, we develop concepts for presenting interactive content in form of a slideshow in a virtual environment, similar to conventional desktop presentation software. We demonstrate how traditional content like text and images can be integrated into 3D models and embedded applications to form a seamless presentation combining the advantages of traditional presentation methods with 3D interaction techniques and different 3D output devices. We demonstrate how different combinations of output devices can be used for presenter and audience, and discuss their various advantages.

We present a method that makes the use of photon tracing methods feasible for complex scenes when a totally accurate solution is not essential. This is accomplished by using orientation lightmaps, which average the illumination of complex objects depending on the surface normal. Through this averaging, they considerably reduce the variance of the stochastic solution. In order to use these specialised lightmaps, which consume comparatively small amounts of memory, no changes have to be made to the basic photon tracing algorithm. Also, they can be freely mixed with normal lightmaps. This gives the user good control over the amount of inaccuracy he introduces by their application. The area computations necessary for their insertion are performed using a stochastic sampling method that performs well for highly complex objects.

In virtual environments containing a very large number of objects, the limited amount of available resources often proves to be a bottleneck, causing a competition for those resources, for example network bandwidth, processing power or the rendering pipeline. This leads to a degradation of the system's performance, as only a small number of elements can be granted the resource required. We present a generic scheduling algorithm that allows us to achieve a graceful degradation: it is output sensitive, minimizes the risk of starvation and enforces priorities based on a freely definable error metric. Hence it can be employed in virtual environments of almost any size, to schedule elements which are competing for a determined resource because of a bottleneck.

Augmented Reality overlays computer generated images over the real world. These images have to be generated using transformations which correctly project a point in virtual space onto its corresponding point in the real world.

Distributed Open Inventor is an extension to the popular Open Inventor toolkit for interactive 3D graphics. The toolkit is extended with the concept of a distributed shared scene graph, similar to distributed shared memory. From the application programmer's perspective, multiple workstations share a common scene graph. The proposed system introduces a convenient mechanism for writing distributed graphical applications based on a popular tool in an almost transparent manner. Local variations in the scene graph allow for a wide range of possible applications, and local low latency interaction mechanisms called input streams enable high performance while saving the programmer from network peculiarities.

The available contrast of common display devices is much lower than image data often demand. Usually a variant of an average of the field of view is used to normalize the image. An alternative approach is introduced to define the mapping of rendered values to the displayable intensity range. The luminance range is chosen such that a minimum amount of information is lost thereby preserving the contrast ratio of all correctly displayed parts. The loss of information can be regulated by different error functions. If the loss is too large, the luminance range can be increased, but the original contrast is not presented any more. In this case the method represents an improvement of Schlick's (1994) mapping technique. The newly introduced method can be applied on color and gray scale images, rendered in absolute or fictitious units.

Every rendering process consists of two steps. The first is the computing of luminance values by methods like ray tracing or radiosity, and the second step is the mapping of the computed values to values appropriate for displaying. In the last years, as alternative to simple linear scaling which maps the average value to the medium luminance, some new ways of mapping were introduced. These new methods are based on photography analogies and on human vision models. All existing methods follow, implicitly or explicitly, the reflected light metering principle. The method introduced in this paper is the first that follows the incident light metering used in professional photography and in the movie industry. Actually the irradiances are measured using a set of diffusors, which are placed automatically in the scene, and a linear scale factor based on these measurements is used to map the computed radiances to the display device. The diffusors act as half space integrators, they collect the light energy from all half space directions. The light comes from the primary light sources, or it is the result of various interreflections. The newly introduced method reproduces original colors faithfully even for scenes with very low or very high average reflectivity.

A good image metric is often needed in digital image synthesis. It can be used to check the convergence behavior in progressive methods, to compare images rendered using various rendering methods etc. Since images are rendered to be observed by humans, an image metric should correspond to human perception as well. We propose here a new algorithm which operates in the original image space. There is no need for Fourier or wavelet transforms. Furthermore, the new metric is view distance dependent. The new method uses the contrast sensitivity function. The main idea is to place a number of various rectangles in images, and to compute the CIE LUV average color difference between corresponding rectangles. Errors are then weighted according to the rectangle size and the contrast sensitivity function.

Line Integral Convolution (LIC) is a common approach for the visualisation of 2D vector fields. It is well suited for visualizing the direction of a flow field, but it gives no information about the orientation of the underlying vectors. We introduce Oriented Line Integral Convolution (OLIC), where direction as well as orientation are encoded within the resulting image. This is achieved by using a sparse input texture and a ramp like (anisotropic) convolution kernel. This method can be used for animations, whereby the computation of so called pixel traces fastens the calculation process. Various OLICs illustrating simple and real world vector fields are shown.

In the rendering process of radiosity scenes, the computed radiosity values have to be converted to color values that can be displayed on a CRT monitor. Usually radiosity calculations as well as various color calculations are performed in the RGB color space. Unfortunately the radiosity values can be outside a monitor gamut, which is a three-dimensional solid surrounding the set of all displayable colors for that device. The radiosity approach can be used either for rendering images with global illumination effects or for accurate physical simulation of light propagation. This paper introduces several techniques to transform radiosity values to monitor colors. These techniques use either clipping or compression, either scene dependent or independent algorithms and work either in the RGB, CIELUV or CIELAB color space. A simple, yet visually appealing, nonlinear color compression technique is introduced to generate smooth intensity gradients on a CRT monitor. The problems of accurate perceptual display of radiosity scenes are discussed as well as some new ideas from color theory are presented that might be helpful for a solution of this task.>

This paper examines the possibilities of improving halftoning techniques using dispersed dots. This corresponds to finding micro-dot distributions that approximate the intensity levels that have to be rendered. A widely used halftoning method is ordered dithering, which uses a threshold matrix to decide if a micro-dot should be set in the output image. A way to generate improved threshold matrices for ordered dithering will be introduced that avoids unwanted low-frequency portions without introducing too much random noise into the rendered image. Since the presented method produces images of high quality it is ideally suited for output generation in high-end image processing systems. >

Abstract This paper presents an efficient method for sampling the illumination functions in higher order radiosity algorithms. In such algorithms, the illumination function is not assumed to be constant across each patch, but it is approximated by a function which is at least C1 continuous. Our median cut sampling algorithm is inspired by the observation that many form factors are computed at higher precision than is necessary. While a high sampling rate is necessary in regions of high illumination, dark areas can be sampled at a much lower rate to compute the received radiosity within a given precision. We adaptively subdivide the emitter into regions of approximately equal influence on the result. Form factors are evaluated by the disk approximation and a ray tracing based test for occlusion detection. The implementation of a higher order radiosity system using B‐splines as radiosity function is described. The median cut algorithm can also be used for radiosity algorithms based on the constant radiosity assumption.

Ray tracing is a well known technique for generating realistic images. One of the major drawbacks of this approach are the extensive computational requirements for image calculation. When generating animation sequences frame by frame the computational cost might easily become intolerable. In the last years several methods have been devised for accelerating the computational speed by using spatial and temporal coherence. While these techniques work only under certain restrictions, a new approach is presented in this paper which leads to a considerable speed-up of the calculation process without putting any limitations on camera or object movement. In principle, the method is an extension of /ArKi87/, where rays are considered points in 5D space, by the time dimension. CSG is used for object description and has been modified correspondingly to allow easy use of coherence properties. The paper describes the theoretical background and the main concepts of a practical implementation.

Solid geometric models can be deformed to free-form solids by the use of trivariate B-splines. This paper describes the problems of implementing such transformations for shaded rendering. The surfaces are subdivided into triangles adaptively so that the error in image space is limited. This adaptive triangulation ensures a smooth appearance of the resulting pictures.

Stochastic sampling is a good alternative to pure oversampling in terms of image quality. A method for adaptively controlling the number of required samples to the complexity of the picture is presented. The quality of the obtained picture can be controlled by two well-understandable parameters, these parameters define an error interval sire and the probability that a pixel lies within it. The usefulness of the method is described by applying it to distributed ray-tracing.