Jörg Mensmann

Interactive Volume Rendering with Dynamic Ambient Occlusion and Color Bleeding

We propose a method for rendering volumetric data sets at interactive frame rates while supporting dynamic ambient occlusion as well as an approximation to color bleeding. In contrast to ambient occlusion approaches for polygonal data, techniques for volumetric data sets have to face additional challenges, since by changing rendering parameters, such as the transfer function or the thresholding, the structure of the data set and thus the light interactions may vary drastically. Therefore, during a preprocessing step which is independent of the rendering parameters we capture light interactions for all combinations of structures extractable from a volumetric data set. In order to compute the light interactions between the different structures, we combine this preprocessed information during rendering based on the rendering parameters defined interactively by the user. Thus our method supports interactive exploration of a volumetric data set but still gives the user control over the most important rendering parameters. For instance, if the user alters the transfer function to extract different structures from a volumetric data set the light interactions between the extracted structures are captured in the rendering while still allowing interactive frame rates. Compared to known local illumination models for volume rendering our method does not introduce any substantial rendering overhead and can be integrated easily into existing volume rendering applications. In this paper we will explain our approach, discuss the implications for interactive volume rendering and present the achieved results.

Shape-based transfer functions for volume visualization

We present a novel classification technique for volume visualization that takes the shape of volumetric features into account. The presented technique enables the user to distinguish features based on their 3D shape and to assign individual optical properties to these. Based on a rough pre-segmentation that can be done by windowing, we exploit the curve-skeleton of each volumetric structure in order to derive a shape descriptor similar to those used in current shape recognition algorithms. The shape descriptor distinguishes three main shape classes: longitudinal, surface-like, and blobby shapes. In contrast to previous approaches, the classification is not performed on a per-voxel level but assigns a uniform shape descriptor to each feature and therefore allows a more intuitive user interface for the assignment of optical properties. By using the proposed technique, it becomes for instance possible to distinguish blobby heart structures filled with contrast agents from potentially occluding vessels and rib bones. After introducing the basic concepts, we show how the presented technique performs on real world data, and we discuss current limitations.

Accelerating volume raycasting using occlusion frustums

GPU-based volume raycasting allows to produce high quality renderings on current graphics hardware. The use of such raycasters is on the rise due to their inherent flexibility as well as the advances in hardware performance and functionality. Although recent raycasting systems achieve interactive frame rates on high-end graphics hardware, further improved performance would enable more complex rendering techniques, e. g., advanced illumination models. In this paper we introduce a novel approach to empty space leaping in order to reduce the number of costly volume texture fetches during ray traversal. We generate an optimized proxy geometry for raycasting which is based on occlusion frustums obtained from previous frames. Our technique does not rely on any preprocessing, introduces no image artifacts, and—in contrast to previous point-based methods—works also for non-continuous view changes. Besides the technical realization and the performance results, we also discuss the potential problems of ray coherence in relation to our approach and restrictions in current GPU architectures. The presented technique has been implemented using fragment and geometry shaders and can be integrated easily into existing raycasting systems.

A GPU-supported lossless compression scheme for rendering time-varying volume data

Since the size of time-varying volumetric data sets typically exceeds the amount of available GPU and main memory, out-of-core streaming techniques are required to support interactive rendering. To deal with the performance bottlenecks of hard-disk transfer rate and graphics bus bandwidth, we present a hybrid CPU/GPU scheme for lossless compression and data streaming that combines a temporal prediction model, which allows to exploit coherence between time steps, and variable-length coding with a fast block compression algorithm. This combination becomes possible by exploiting the CUDA computing architecture for unpacking and assembling data packets on the GPU. The system allows near-interactive performance even for rendering large real-world data sets with a low signal-to-noise-ratio, while not degrading image quality. It uses standard volume raycasting and can be easily combined with existing acceleration methods and advanced visualization techniques.

Slab-Based Raycasting: Exploiting GPU Computing for Volume Visualization

The ray traversal in GPU-based volume raycasting is usually implemented in a fragment shader, utilizing the hardware in a way that was not originally intended. New programming interfaces for GPU computing, such as CUDA or OpenCL, support a more general programming model and the use of additional device features, which are not accessible through traditional shader programming. In this paper we first compare fragment shader implementations of basic raycasting to implementations directly translated to CUDA kernels. Then we propose a new slab-based raycasting technique that is modeled specifically to use the additional device features to accelerate volume rendering. We conclude that new GPU computing approaches can only gain a small performance advantage when directly porting the basic raycasting algorithm. However, they can be beneficial through novel acceleration methods that use the hardware features not available to shader implementations.

Integrating Current Weather Effects into Urban Visualization

General interest in visualizations of digital 3D city models is growing rapidly, and several applications are available that display such models realistically. Many authors have emphasized the importance of realistic illumination for computer generated images, and this applies especially to 3D city visualization. However, current 3D city visualization applications rarely implement techniques for achieving realistic illumination, in particular the effects caused by current weather. At most, some geospatial visualization systems render artificial skies – sometimes with a georeferenced determination of the sun position – to give the user the notion of a real sky.