Gaussian transfer functions for multi-field volume visualization

Journal ArticleVolume rendering is a flexible technique for visualizing dense 3D volumetric datasets. A central element of volume rendering is the conversion between data values and observable quantities such as color and opacity. This process is usually realized through the use of transfer functions that are precomputed and stored in lookup tables. For multidimensional transfer functions applied to multivariate data, these lookup tables become prohibitively large. We propose the direct evaluation of a particular type of transfer functions based on a sum of Gaussians. Because of their simple form (in terms of number of parameters), these functions and their analytic integrals along line segments can be evaluated efficiently on current graphics hardware, obviating the need for precomputed lookup tables. We have adopted these transfer functions because they are well suited for classification based on a unique combination of multiple data values that localize features in the transfer function domain. We apply this technique to the visualization of several multivariate datasets (CT, cryosection) that are difficult to classify and render accurately at interactive rates using traditional approaches

HAPTIC AND VISUAL SIMULATION OF BONE DISSECTION

Marco AgusIn bone dissection virtual simulation, force restitution represents the key to realistically mimicking a patient– specific operating environment. The force is rendered using haptic devices controlled by parametrized mathematical models that represent the bone–burr contact. This dissertation presents and discusses a haptic simulation of a bone cutting burr, that it is being developed as a component of a training system for temporal bone surgery. A physically based model was used to describe the burr– bone interaction, including haptic forces evaluation, bone erosion process and resulting debris. The model was experimentally validated and calibrated by employing a custom experimental set–up consisting of a force–controlled robot arm holding a high–speed rotating tool and a contact force measuring apparatus. Psychophysical testing was also carried out to assess individual reaction to the haptic environment. The results suggest that the simulator is capable of rendering the basic material differences required for bone burring tasks. The current implementation, directly operating on a voxel discretization of patientspecific 3D CT and MR imaging data, is efficient enough to provide real–time haptic and visual feedback on a low–end multi–processing PC platform.

Opacity-weighted color interpolation for volume sampling

volume rendering, compositing, ray tracing Volume rendering creates images from sampled volumetric data. The compute intensive nature of volume rendering has driven research in algorithm optimization. An important speed optimization is the use of preclassification and preshading. We demonstrate an artifact that results when interpolating from preclassified or preshaded colors and opacity values separately. This method is flawed, leading to visible artifacts. We present an improved technique, opacity-weighted color interpolation, evaluate the RMS error improvement, hardware and algorithm efficiency, and demonstrated improvements. We show analytically that opacity-weighted color interpolation exactly reproduces material based interpolation results for certain volume classifiers, with the efficiencies of preclassification. Our proposed technique may also have broad impact on opacitytexture-mapped polygon rendering

A Data-Virtualization System for Large Model Visualization

Interactive scientific visualizations are widely used for the visual exploration and examination of physical data resulting from measurements or simulations. Driven by technical advancements of data acquisition and simulation technologies, especially in the geo-scientific domain, large amounts of highly detailed subsurface data are generated. The oil and gas industry is particularly pushing such developments as hydrocarbon reservoirs are increasingly difficult to discover and exploit. Suitable visualization techniques are vital for the discovery of the reservoirs as well as their development and production. However, the ever-growing scale and complexity of geo-scientific data sets result in an expanding disparity between the size of the data and the capabilities of current computer systems with regard to limited memory and computing resources. In this thesis we present a unified out-of-core data-virtualization system supporting geo-scientific data sets consisting of multiple large seismic volumes and height-field surfaces, wherein each data set may exceed the size of the graphics memory or possibly even the main memory. Current data sets fall within the range of hundreds of gigabytes up to terabytes in size. Through the mutual utilization of memory and bandwidth resources by multiple data sets, our data-management system is able to share and balance limited system resources among different data sets. We employ multi-resolution methods based on hierarchical octree and quadtree data structures to generate level-of-detail working sets of the data stored in main memory and graphics memory for rendering. The working set generation in our system is based on a common feedback mechanism with inherent support for translucent geometric and volumetric data sets. This feedback mechanism collects information about required levels of detail during the rendering process and is capable of directly resolving data visibility without the application of any costly occlusion culling approaches. A central goal of the proposed out-of-core data management system is an effective virtualization of large data sets. Through an abstraction of the level-of-detail working sets, our system allows developers to work with extremely large data sets independent of their complex internal data representations and physical memory layouts. Based on this out-of-core data virtualization infrastructure, we present distinct rendering approaches for specific visualization problems of large geo-scientific data sets. We demonstrate the application of our data virtualization system and show how multi-resolution data can be treated exactly the same way as regular data sets during the rendering process. An efficient volume ray casting system is presented for the rendering of multiple arbitrarily overlapping multi-resolution volume data sets. Binary space-partitioning volume decomposition of the bounding boxes of the cube-shaped volumes is used to identify the overlapping and non-overlapping volume regions in order to optimize the rendering process. We further propose a ray casting-based rendering system for the visualization of geological subsurface models consisting of multiple very detailed height fields. The rendering of an entire stack of height-field surfaces is accomplished in a single rendering pass using a two-level acceleration structure, which combines a minimum-maximum quadtree for empty-space skipping and sorted lists of depth intervals to restrict ray intersection searches to relevant height fields and depth ranges. Ultimately, we present a unified rendering system for the visualization of entire geological models consisting of highly detailed stacked horizon surfaces and massive volume data. We demonstrate a single-pass ray casting approach facilitating correct visual interaction between distinct translucent model components, while increasing the rendering efficiency by reducing processing overhead of potentially invisible parts of the model. The combination of image-order rendering approaches and the level-of-detail feedback mechanism used by our out-of-core data-management system inherently accounts for occlusions of different data types without the application of costly culling techniques. The unified out-of-core data-management and virtualization infrastructure considerably facilitates the implementation of complex visualization systems. We demonstrate its applicability for the visualization of large geo-scientific data sets using output-sensitive rendering techniques. As a result, the magnitude and multitude of data sets that can be interactively visualized is significantly increased compared to existing approaches

Hardware accelerated volume texturing.

The emergence of volume graphics, a sub field in computer graphics, has been evident for the last 15 years. Growing from scientific visualization problems, volume graphics has established itself as an important field in general computer graphics. However, the general graphics fraternity still favour the established surface graphics techniques. This is due to well founded and established techniques and a complete pipeline through software onto display hardware. This enables real-time applications to be constructed with ease and used by a wide range of end users due to the readily available graphics hardware adopted by many computer manufacturers. Volume graphics has traditionally been restricted to high-end systems due to the complexity involved with rendering volume datasets. Either specialised graphics hardware or powerful computers were required to generate images, many of these not in real-time. Although there have been specialised hardware solutions to the volume rendering problem, the adoption of the volume dataset as a primitive relies on end-users with commodity hardware being able to display images at interactive rates. The recent emergence of programmable consumer level graphics hardware is now allowing these platforms to compute volume rendering at interactive rates. Most of the work in this field is directed towards scientific visualisation. The work in this thesis addresses the issues in providing real-time volume graphics techniques to the general graphics community using commodity graphics hardware. Real-time texturing of volumetric data is explored as an important set of techniques in delivering volume datasets as a general graphics primitive. The main contributions of this work are; The introduction of efficient acceleration techniques; Interactive display of amorphous phenomena modelled outside an object defined in a volume dataset; Interactive procedural texture synthesis for volume data; 2D texturing techniques and extensions for volume data in real-time; A flexible surface detail mapping algorithm that removes many previous restrictions Parts of this work have been presented at the 4th International Workshop on Volume Graphics and also published in Volume Graphics 2005