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Item Sampling and Anti-Aliasing of Discrete 3-D Volume Density Textures(Eurographics Association, 1991) Sakas, Georgios; Gerth, MatthiasIn recent years, a number of techniques have been developed for rendering volume effects (haze, fog, smoke, clouds, etc.) in order to enhance reality in computer-generated imagery as well as to improve the performance of flying, ship, and driving optical simulators. For modeling such effects, volume 'density' objects are used, which are defined by their density distribution in 3-D space. For such a description a three-dimensional voxel field (solid texture) is usually used. Since we deal with 3-D textures, the methods used for sampling 2-D pixel fields cannot always be employed. In this paper, we propose two variants of a new technique for sampling and anti-aliasing 3-D density voxel fields. First, we point out the problems which occur when such 3-D textures are sampled, especially when the point sampling Monte-Carlo method is used. 'Distance sampling' and 'pyramidal-volume sampling' are then introduced. The first ,technique samples the texture along a straight line defined by the eye position and the pixel midpoint, whereas the pyramidalvolume technique approximately samples the volume of the pyramid defined by the eye and the four pixel comers. In comparison to other existing methods, both methods greatly reduce aliasing and calculation time. Especially the second one provides a constant-time filtering, whereby minimizing the number of texture evaluations. In the last paper section we demonstrate the applicability of the proposed methods for animation as well as for visualization purposes.Item Fast Rendering of Arbitrary Distributed Volume Densities(Eurographics Association, 1990) Sakas, GeorgiosIn recent years a number of techniques have been developed for rendering volume effects (haze, fog, smoke, clouds, etc.). These techniques are either time consuming (ray-tracing, radiosity) or do not account for arbitrary density distributions. In this paper we briefly analyze the physics of illuminations of volumes and we propose several simplifications suitable. for computer graphics practice. In particular, we present a method for rendering arbitrary distributions by means of projective polygonal rendering and solid texturing techniques in approximately the time needed for a usual polygonal object. The proposed method provides good results in a fraction of the computing time required for approaches like ray-tracing or radiosity. Solid texturing is used to define the density distribution and a point-sampling Monte-Carlo method with user-adjustable accuracy to evaluate the illumination model along the path through the volume. Thus, a trade-off between computing time and picture quality exists. With this technique one can move through or around the volume and to place objects and/or light sources in the volume. By means of rendering methods like shadowing polyhedra, objects can cast shadows on the volume and/or the volume can shadow the ground.Item Advanced Applications of Volume Visualization Methods in Medicine(Eurographics Association, 1997) Sakas, Georgios; Pommert, AndreasTomographicmedical imaging techniques have become more popular in recent years. The wide availability of CT,MRI and Ultrasound inmost large hospitals results in a rapidly increasing number of examinations with these devices. The State of The Art Report summarises the application of techniques developed over the recent years for visualising volumetric medical data common in modern medical imaging modalities such as CT, MRA, MRI, Nuclear Medicine, 3D-Ultrasound, Laser Confocal Microscopy etc. Although all of the modalities listed above provide ”slices of the body”, significant differences exist between the image content of each modality. The focus of the Report is be less in explaining algorithms and rendering techniques, but rather to point out their applicability, benefits, and potential in the medical environment. In the first part, methods for all steps of the volume visualization pipeline from data preprocessing to object display are reviewed, with special emphasis on data structures, segmentation, and surface- and volume-based rendering. Furthermore,multimodalitymatching, interventionrehearsal, and aspects of image quality are discussed. In the second part applications are illustrated fromthe areas of craniofacial surgery, traumatology, neurosurgery, radiotherapy, and medical education. Furtherly, some new applications of volumetricmethods are presented: 3D ultrasound, laser confocal datasets, and 3D-reconstruction of cardiological datasets, i.e. vessels as well as ventricles. These new volumetric methods are currently under development but due to their enormeous application potential they are expected to be clinically accepted within the next years.Item Pseudo-Satellitefilm Using Fractal Clouds to Enhance Animated Weather Forecasting(Blackwell Science Ltd and the Eurographics Association, 1993) Sakas, Georgios; Schroder, Florian; Koppert, Hans-JoachimWe have developed a system enabling the National German Meteorological Office to generate pseudo-satellite images and video sequences based on their weather forecasting simulation data. With our system meteorologists can visualize the past and the current weather situation, evaluate their simulation results, and produce animated weather forecast videos broadcasted by several television stations. Realistic images are generated by interpolating the extremely coarse weather simulation data grid and enhancing the result using fractal clouds. It also enables the meteorologists to interactively change the forecast data in order to compensate the lack of accuracy or the known errors in their simulation models. Our system TRITON enables the visualization of complex weather simulations in a more natural way by presenting an intuitively understandable forecast.Item A Functional Approach to the Visual Simulation of Gaseous Turbulence(Blackwell Science Ltd and the Eurographics Association, 1992) Sakas, Georgios; Westermann, RudigerThis paper presents a functional method for the visual simulation of 2-D or 3-D turbulent gaseous motion by using time-varying fractals. The used function incorporates results from the"spectral theory of turbulence", thereby providing a physics-based approach adapted to the needs of computer graphics. The involved turbulence function is band-limited, continuous, differentiable, anisotrop, and smooth, provides different fractal dimensions along each axis, may be evaluated locally with different parameters, and requires only minimal storage space, thus supporting an implementation on large parallel processing networks with small nodes. Inhomogeneity in the form of local disturbances of the turbulence field may also be easily considered. The parameters used to describe turbulent motion are rather intuitive, so that they may be utilized easily by users. Examples for modeling different types of clouds and fire are given.