VG10: Eurographics/IEEE VGTC on Volume Graphics

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A New Sampling Scheme for Slice Based Volume Rendering
(The Eurographics Association, 2010) Krüger, Jens; Ruediger Westermann and Gordon Kindlmann
In this paper we present a novel approach to generate proxy geometry for slice based volume rendering. The basic idea is derived from the behavior of a ray-caster and is a simple extension of the well known 2D object-aligned texture stack based technique. From this our novel scheme inherits the advantage that it enables hardware-based volume rendering for devices that do not support 3D textures. On these devices previous object-aligned 2D texture based approaches suffered from disturbing view angle dependent stack-switching artifacts which are avoided by our novel method. Our approach also shows benefits compared to the widely used view aligned slicing algorithm as it avoids jagged boundary artifacts and increases performance.
A Survey of Transfer Functions Suitable for Volume Rendering
(The Eurographics Association, 2010) Arens, Stephan; Domik, Gitta; Ruediger Westermann and Gordon Kindlmann
There are many transfer functions (TF) that emphasize or hide special features of volume data. Their potential to cleverly generate a color and opacity value for direct volume rendering is primarily determined by the used metrics besides the input data value. Despite this variety of TFs, for the most part simple one dimensional data value only based TFs are used in practice. This survey will therefore examine the differences in representative TF types (defined by their used metrics) to provide a basis for selecting the right TF type for the boundary conditions that describe an individual field of application and task. Besides fundamental properties like metrics or memory consumption, we will also give an assessment about user interaction and quality of feature emphasis.
A GPU-Supported Lossless Compression Scheme for Rendering Time-Varying Volume Data
(The Eurographics Association, 2010) Mensmann, Jörg; Ropinski, Timo; Hinrichs, Klaus; Ruediger Westermann and Gordon Kindlmann
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.
Efficient Acquisition and Clustering of Local Histograms for Representing Voxel Neighborhoods
(The Eurographics Association, 2010) Meß, Christian; Ropinski, Timo; Ruediger Westermann and Gordon Kindlmann
In the past years many interactive volume rendering techniques have been proposed, which exploit the neighboring environment of a voxel during rendering. In general on-the-fly acquisition of this environment is infeasible due to the high amount of data to be taken into account. To bypass this problem we propose a GPU preprocessing pipeline which allows to acquire and compress the neighborhood information for each voxel. Therefore, we represent the environment around each voxel by generating a local histogram (LH) of the surrounding voxel densities. By performing a vector quantization (VQ), the high number of LHs is than reduced to a few hundred cluster centroids, which are accessed through an index volume. To accelerate the required computational expensive processing steps, we take advantage of the highly parallel nature of this task and realize it using CUDA. For the LH compression we use an optimized hybrid CPU/GPU implementation of the k-means VQ algorithm. While the assignment of each LH to its nearest centroid is done on the GPU using CUDA, centroid recalculation after each iteration is done on the CPU. Our results demonstrate the applicability of the precomputed data, while the performance is increased by a factor of about 10 compared to previous approaches.
Multi-dimensional Reduction and Transfer Function Design using Parallel Coordinates
(The Eurographics Association, 2010) Zhao, Xin; Kaufman, Arie; Ruediger Westermann and Gordon Kindlmann
Multi-dimensional transfer functions are widely used to provide appropriate data classification for direct volume rendering. Nevertheless, the design of a multi-dimensional transfer function is a complicated task. In this paper, we propose to use parallel coordinates, a powerful tool to visualize high-dimensional geometry and analyze multivariate data, for multi-dimensional transfer function design. This approach has two major advantages: (1) Combining the information of spatial space (voxel position) and parameter space; (2) Selecting appropriate highdimensional parameters to obtain sophisticated data classification. Although parallel coordinates offers simple interface for the user to design the high-dimensional transfer function, some extra work such as sorting the coordinates is inevitable. Therefore, we use a local linear embedding technique for dimension reduction to reduce the burdensome calculations in the high dimensional parameter space and to represent the transfer function concisely. With the aid of parallel coordinates, we propose some novel high-dimensional transfer function widgets for better visualization results. We demonstrate the capability of our parallel coordinates based transfer function (PCbTF) design method for direct volume rendering using CT and MRI datasets.
Slimming Brick Cache Strategies for Seismic Horizon Propagation Algorithms
(The Eurographics Association, 2010) Gallon, Jonathan; Guillon, Sébastien; Jobard, Bruno; Barucq, Hélène; Keskes, Noomane; Ruediger Westermann and Gordon Kindlmann
In this paper, we propose a new bricked cache system suitable for a particular surface propagation algorithm : seismic horizon reconstruction. The application domain of this algorithm is the interpretation of seismic volumes used, for instance, by petroleum companies for oil prospecting. To ensure the optimality of such surface extraction, the algorithm must access randomly into the data volume. This lack of data locality imposes that the volume resides entirely in the main memory to reach decent performances. In case of volumes larger than the memory, we show that using a classical brick cache strategy can also produce good performances until a certain size. As the size of these volumes increases very quickly, and can now reach more than 200GB, we demonstrate that the performances of the classical algorithm are dramatically reduced when processed on standard workstation with a limited size of memory (currently 8GB to 16GB). In order to handle such large volumes, we introduce a new slimming brick cache strategy where bricks size evolves according to processed data : at each step of the algorithm, processed data could be removed from the cache. This new brick format allows to have a larger number of brick loaded in memory. We further improve the releasing mechanism by filling in priority the holes that appear in the surface during the propagation process. With this new cache strategy, horizons can be extracted into volumes that are up to 75 times the size of the available cache memory. We discuss the performances and results of this new approach applied on both synthetic and real data.
Depth-Enhanced Maximum Intensity Projection
(The Eurographics Association, 2010) Díaz, José; Vázquez, Pere-Pau; Ruediger Westermann and Gordon Kindlmann
The two most common methods for the visualization of volumetric data are Direct Volume Rendering (DVR) and Maximum Intensity Projection (MIP). Direct Volume Rendering is superior to MIP in providing a larger amount of properly shaded details, because it employs a more complex shading model together with the use of user-defined transfer functions. However, the generation of adequate transfer functions is a laborious and time costly task, even for expert users. As a consequence, medical doctors often use MIP because it does not require the definition of complex transfer functions and because it gives good results on contrasted images. Unfortunately, MIP does not allow to perceive depth ordering and therefore spatial context is lost. In this paper we present a new approach to MIP rendering that uses depth and simple color blending to disambiguate the ordering of internal structures, while maintaining most of the details visible through MIP. It is usually faster than DVR and only requires the transfer function used by MIP rendering.
Split-Voxel: A Simple Discontinuity-Preserving Voxel Representation for Volume Rendering
(The Eurographics Association, 2010) Agus, Marco; Gobbetti, Enrico; Guitián, José Antonio Iglesias; Marton, Fabio; Ruediger Westermann and Gordon Kindlmann
The most common representation of volumetric models is a regular grid of cubical voxels with one value each, from which a smooth scalar field is reconstructed. However, common real-world situations include cases in which volumes represent physical objects with well defined boundaries separating different materials, giving rise to models with quasi-impulsive gradient fields. In our split-voxel representation, we replace blocks of N 3 voxels by one single voxel that is split by a feature plane into two regions with constant values. This representation has little overhead over storing precomputed gradients, and has the advantage that feature planes provide minimal geometric information about the underlying volume regions that can be effectively exploited for volume rendering. We show how to convert a standard mono-resolution representation into a out-of-core multiresolution structure, both for labeled and continuous scalar volumes. We also show how to interactively explore the models using a multiresolution GPU raycasting framework. The technique supports real-time transfer function manipulation and proves particularly useful for fast multiresolution rendering, since accurate silhouettes are preserved even at very coarse levels of detail.
Real-time Surface Analysis and Tagged Material Cleansing for Virtual Colonoscopy
(The Eurographics Association, 2010) Russ, Christoph; Kubisch, Christoph; Qiu, Feng; Hong, Wei; Ljung, Patric; Ruediger Westermann and Gordon Kindlmann
The most common representation of volumetric models is a regular grid of cubical voxels with one value each, from which a smooth scalar field is reconstructed. However, common real-world situations include cases in which volumes represent physical objects with well defined boundaries separating different materials, giving rise to models with quasi-impulsive gradient fields. In our split-voxel representation, we replace blocks of N 3 voxels by one single voxel that is split by a feature plane into two regions with constant values. This representation has little overhead over storing precomputed gradients, and has the advantage that feature planes provide minimal geometric information about the underlying volume regions that can be effectively exploited for volume rendering. We show how to convert a standard mono-resolution representation into a out-of-core multiresolution structure, both for labeled and continuous scalar volumes. We also show how to interactively explore the models using a multiresolution GPU raycasting framework. The technique supports real-time transfer function manipulation and proves particularly useful for fast multiresolution rendering, since accurate silhouettes are preserved even at very coarse levels of detail.
Multi-layer Volume Ray Casting on GPU
(The Eurographics Association, 2010) Li, Wei; Ruediger Westermann and Gordon Kindlmann
We present multi-layer volume ray casting that integrates various volume rendering enhancements into a unified framework, including empty-space skipping, volume clipping, embedding opaque and semi-transparent objects, volume editing through constructional solid geometry (CSG) operations, etc. The central idea is to consider all these objects as volume-of-interests(VOIs). Each VOI is assigned a priority number to determine how the overlapped regions are handled. All the information of the VOIs are encoded into a ray-layer buffer through depth peeling combined with layer simplification. Each pixel of a ray-layer buffer contains the information of a set of ray segments, including the starting locations and the material IDs. The multi-layer ray caster then renders (or skips) each ray segment with the proper viewing parameters depending on the material IDs. The ray-layer buffer is also used to estimate the gradients of surfaces between the layers to improve shading.
Fast Volumetric Data Exploration with Importance-Based Accumulated Transparency Modulation
(The Eurographics Association, 2010) Wan, Yong; Hansen, Chuck; Ruediger Westermann and Gordon Kindlmann
Direct volume rendering techniques have been successfully applied to visualizing volumetric datasets across many application domains. Due to the sensitivity of transfer functions and the complexity of fine-tuning transfer functions, direct volume rendering is still not widely used in practice. For fast volumetric data exploration, we propose Importance-Based Accumulated Transparency Modulation which does not rely on transfer function manipulation. This novel rendering algorithm is a generalization and extension of the Maximum Intensity Difference Accumulation technique. By only modifying the accumulated transparency, the resulted volume renderings are essentially high dynamic range. We show that by using several common importance measures, different features of the volumetric datasets can be highlighted. The results can be easily extended to a high-dimensional importance difference space, by mixing the results from an arbitrary number of importance measures with weighting factors, which all control the final output with a monotonic behavior. With Importance-Based Accumulated Transparency Modulation, the end-user can explore a wide variety of volumetric datasets quickly without the burden of manually setting and adjusting a transfer function.
Wavelet-based Multiresolution Isosurface Rendering
(The Eurographics Association, 2010) Steinberger, Markus; Grabner, Markus; Ruediger Westermann and Gordon Kindlmann
We present an interactive rendering method for isosurfaces in a voxel grid. The underlying trivariate function is represented as a spline wavelet hierarchy, which allows for adaptive (view-dependent) selection of the desired level-of-detail by superimposing appropriately weighted basis functions. Different root finding techniques are compared with respect to their precision and efficiency. Both wavelet reconstruction and root finding are implemented in CUDA to utilize the high computational performance of Nvidia's hardware and to obtain high quality results. We tested our methods with datasets of up to 5123 voxels and demonstrate interactive frame rates for a viewport size of up to 1024x768 pixels.
Advanced Light Material Interaction for Direct Volume Rendering
(The Eurographics Association, 2010) Lindemann, Florian; Ropinski, Timo; Ruediger Westermann and Gordon Kindlmann
In this paper we present a heuristic approach for simulating advanced light material interactions in the context of interactive volume rendering. In contrast to previous work, we are able to incorporate complex material functions, which allow to simulate reflectance and scattering. We exploit a common representation of these material properties based on spherical harmonic basis functions, to combine the achieved reflectance and scattering effects with natural lighting conditions, i. e., incorporating colored area light sources. To achieve these goals, we introduce a modified SH projection technique, which is not just tailored at a single material category, but adapts to the present material. Thus, reflecting and scattering materials as assigned trough the transfer function can be captured in a unified approach. We will describe the required extensions to the standard volume rendering integral and present an approximation which allows to realize the material effects in order to achieve interactive frame rates. By exploiting a combination of CPU and GPU processing, we are able to modify material properties and can change the illumination conditions interactively. We will demonstrate the outcome of the proposed approach based on renderings of real-world data sets and report the achieved computation times.
Concurrent Volume Visualization of Real-Time fMRI
(The Eurographics Association, 2010) Nguyen, Tan Khoa; Eklund, Anders; Ohlsson, Henrik; Hernell, Frida; Ljung, Patric; Forsell, Camilla; Andersson, Mats; Knutsson, Hans; Ynnerman, Anders; Ruediger Westermann and Gordon Kindlmann
We present a novel approach to interactive and concurrent volume visualization of functional Magnetic Resonance Imaging (fMRI). While the patient is in the scanner, data is extracted in real-time using state-of-the-art signal processing techniques. The fMRI signal is treated as light emission when rendering a patient-specific high resolution reference MRI volume, obtained at the beginning of the experiment. As a result, the brain glows and emits light from active regions. The low resolution fMRI signal is thus effectively fused with the reference brain with the current transfer function settings yielding an effective focus and context visualization. The delay from a change in the fMRI signal to the visualization is approximately 2 seconds. The advantage of our method over standard 2D slice based methods is shown in a user study. We demonstrate our technique through experiments providing interactive visualization to the fMRI operator and also to the test subject in the scanner through a head mounted display.
Feature-Driven Ambient Occlusion for Direct Volume Rendering
(The Eurographics Association, 2010) Ancel, Alexandre; Dischler, Jean-Michel; Mongenet, Catherine; Ruediger Westermann and Gordon Kindlmann
Ambient occlusion techniques were introduced to improve data comprehension by bringing soft fading shadows. They consist in attenuating light by considering the occlusion resulting from the presence of neighboring structures. Recently introduced in volume rendering, we show that the straightforward application of ambient occlusion in direct volume rendering has its limits as rendering a multi-layer volume results in overdarkening the internal layers of the volume. This paper proposes to address the overdarkening issue by computing ambient occlusion according to the features present in the dataset. This allows us to neglect inter-occlusions between features without losing the auto-occlusions that give cues on the shape of the considered features. We use a GPU-based approach with bricking to speed up the computations of our ambient occlusion method. Results show that our approach not only improves the visual quality of images compared to classical ambient occlusion, but it is also less parametersensitive, thus furthermore improving usability for every-day users.
Volumetric Evaluation of Meshless Data From Smoothed Particle Hydrodynamics Simulations
(The Eurographics Association, 2010) Jang, Yun; Fuchs, Raphael; Schindler, Benjamin; Peikert, Ronny; Ruediger Westermann and Gordon Kindlmann
As an alternative to conventional Eulerian methods in the field of computational fluid dynamics (CFD), smoothed particle hydrodynamics (SPH) has been developed. Its mesh-free method is useful in problems, especially, where a free surface is present. Although researchers are currently able to simulate up to hundreds of millions of particles in a volume, the analysis and visualization of the particle datasets are still limited especially, due to the lack of connectivity between particles and the number of particles. In this paper, we present a volumetric evaluation technique of SPH particle data using hierarchical particle data structures. Our approach does not resample or triangulate the meshless particle data on grid structures, instead, the evaluations are performed in a fragment program with the original SPH kernels used in the CFD simulations. The SPH particle information and the hierarchical data structures are stored in 2D and 3D textures respectively and the 3D texture stores the access pointers, such as texture coordinates, to the 2D textures. To achieve interactive frame rates during interaction we suggest to control the kernel radii while generating the hierarchical data structures. This approach allows us to visualize over a million of SPH particles interactively.

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