Interactive volume ray tracing

Die Visualisierung von volumetrischen Daten ist eine der interessantesten, aber sicherlich auch schwierigsten Anwendungsgebiete innerhalb der wissenschaftlichen Visualisierung. Im Gegensatz zu Oberflächenmodellen, repräsentieren solche Daten ein semi-transparentes Medium in einem 3D-Feld. Anwendungen reichen von medizinischen Untersuchungen, Simulation physikalischer Prozesse bis hin zur visuellen Kunst. Viele dieser Anwendungen verlangen Interaktivität hinsichtlich Darstellungs- und Visualisierungsparameter. Der Ray-Tracing- (Stahlverfolgungs-) Algorithmus wurde dabei, obwohl er inhärent die Interaktion mit einem solchen Medium simulieren kann, immer als zu langsam angesehen. Die meisten Forscher konzentrierten sich vielmehr auf Rasterisierungsansätze, da diese besser für Grafikkarten geeignet sind. Dabei leiden diese Ansätze entweder unter einer ungenügenden Qualität respektive Flexibilität. Die andere Alternative besteht darin, den Ray-Tracing-Algorithmus so zu beschleunigen, dass er sinnvoll für Visualisierungsanwendungen benutzt werden kann. Seit der Verfügbarkeit moderner Grafikkarten hat die Forschung auf diesem Gebiet nachgelassen, obwohl selbst moderne GPUs immer noch Limitierungen, wie beispielsweise der begrenzte Grafikkartenspeicher oder das umständliche Programmiermodell, enthalten. Die beiden in dieser Arbeit vorgestellten Methoden sind deshalb vollständig softwarebasiert, da es sinnvoller erscheint, möglichst viele Optimierungen in Software zu realisieren, bevor eine Portierung auf Hardware erfolgt. Die erste Methode wird impliziter Kd-Baum genannt, eine hierarchische und räumliche Beschleunigungstruktur, die ursprünglich für die Generierung von Isoflächen reguläre Gitterdatensätze entwickelt wurde. In der Zwischenzeit unterstützt sie auch die semi-transparente Darstellung, die Darstellung von zeitabhängigen Datensätzen und wurde erfolgreich für andere Anwendungen eingesetzt. Der zweite Algorithmus benutzt so genannte Plücker-Koordinaten, welche die Implementierung eines schnellen inkrementellen Traversierers für Datensätze erlauben, deren Primitive Tetraeder beziehungsweise Hexaeder sind. Beide Algorithmen wurden wesentlich optimiert, um eine interaktive Bildgenerierung volumetrischer Daten zu ermöglichen und stellen deshalb einen wichtigen Beitrag hin zu einem flexiblen und interaktiven Volumen-Ray-Tracing-System dar.Volume rendering is one of the most demanding and interesting topics among scientific visualization. Applications include medical examinations, simulation of physical processes, and visual art. Most of these applications demand interactivity with respect to the viewing and visualization parameters. The ray tracing algorithm, although inherently simulating light interaction with participating media, was always considered too slow. Instead, most researchers followed object-order algorithms better suited for graphics adapters, although such approaches often suffer either from low quality or lack of flexibility. Another alternative is to speed up the ray tracing algorithm to make it competitive for volumetric visualization tasks. Since the advent of modern graphic adapters, research in this area had somehow ceased, although some limitations of GPUs, e.g. limited graphics board memory and tedious programming model, are still a problem. The two methods discussed in this thesis are therefore purely software-based since it is believed that software implementations allow for a far better optimization process before porting algorithms to hardware. The first method is called implicit kd-tree, which is a hierarchical spatial acceleration structure originally developed for iso-surface rendering of regular data sets that now supports semi-transparent rendering, time-dependent data visualization, and is even used in non volume-rendering applications. The second algorithm uses so-called Plücker coordinates, providing a fast incremental traversal for data sets consisting of tetrahedral or hexahedral primitives. Both algorithms are highly optimized to support interactive rendering of volumetric data sets and are therefore major contributions towards a flexible and interactive volume ray tracing framework

Technical Report Fast Ray Traversal of Unstructured Volume Data using Plucker Tests

Figure 1: Three different tetrahedral data sets rendered interactively in software on a single PC. From left to right: isosurface of the bluntfin dataset (isovalue = 1.4), same dataset with direct volume rendering, isosurface of the Buckminster Fulleren (isovalue = 20,000,000), and direct rendering of the Ell dataset. The importance of high-performance rendering of unstructured data sets has increased significantly mainly due to its use in scientific simulations such as computational fluid dynamics and finite element computations. However, the unstructured nature of these data sets complicate their handling and lead to low performance even with hardware support through GPUs. In this paper, we present a new two-step interactive rendering approach for unstructured tetrahedral grids using ray-tracing. The first tetrahedron along a ray is found using common techniques from realtime ray tracing. We then continue traversing the volume using a fast Plücker test to select the next tetrahedra from the set of neighbors. This new traversal algorithm achieves interactive performance with simple shading on medium size data sets and scales linearly in a distributed setup. Since the volume is rendered directly, it is applicable for creating isosurfaces as well as rendering images based on emission-absorption models

Recent Advancements in Ray tracing-based Volume Rendering Techniques Abstract

Recent developments in ray tracing algorithms have shown that interactive high-quality volume rendering is no longer unreachable even for softwarebased implementations. Compared to graphics board and custom hardware implementations they offer additional benefits, e.g. support for large models and for integration with different types of data set topologies. This paper summarizes the state-of-the-art in ray tracing based volume rendering algorithms. Comparisons to current GPU implementations show strengths and drawbacks of different approaches.

Terrain Guided Multi-Level Instancing of Highly Complex Plant Populations

Figure 1: A panoramic view over a highly complex model of the Puget Sound Area. The ground terrain consists of 134 million triangles. It is covered with billions of plant instances, where each plant model is made up of several thousand polygons. In this paper we demonstrate how todays ray tracing techniques can be applied to photo-realistically render extremely huge landscapes covered with trees and forests, where a user can freely choose between highly detailed close-up views or flyover scenarios. This is made possible by mapping a number of square sub-scenes onto a huge polygonal terrain during run-time. The full plant population results from the combination of these tiles, which are iterated over the terrain. This will be demonstrated at the example of a highly complex, plant covered ecosystem containing trillions of triangles

EUROGRAPHICS 2006 STAR – State of The Art Report Abstract Interactive Volume Rendering with Ray Tracing

Recent research on high-performance ray tracing has achieved real-time performance even for highly complex surface models already on a single PC. In this report we provide an overview of techniques for extending real-time ray tracing also to interactive volume rendering. We review fast rendering techniques for different volume representations and rendering modes in a variety of computing environments. The physically-based rendering approach of ray tracing enables high image quality and allows for easily mixing surface, volume, and other primitives in a scene, while fully accounting for all of their optical interactions. We present optimized implementations and discuss the use of upcoming high-performance processors for volume ray tracing. Categories and Subject Descriptors (according to ACM CCS): I.3.3 [Computer Graphics]: Raytracing 1

(Guest Editors) Modeling Visual Attention in VR: Measuring the Accuracy of Predicted Scanpaths

Dynamic human vision is an important contributing factor to the design of perceptually-based Virtual Reality. A common strategy relies on either an implicit assumption or explicit measurement of gaze direction. Given the spatial location of foveal vision, computational resources are directed at enhancing the foveated region in realtime. To obtain an explicit gaze measurement, an eye tracker may be used. In the absence of an eye tracker, a computational model of visual attention may be substituted to predict visually salient features. The fidelity of the resultant real-time system hinges on the agreement between predicted and actual regions foveated by the human. The contributions of this paper are the development and evaluation of a novel method for the comparison of human and artificial scanpaths recorded in VR. The novelty of the present approach is the application of previous accuracy measures to scanpath comparison in VR where analysis is complicated by head movements and dynamic imagery. An attentional model previously used for view-dependent enhancement of Virtual Reality is evaluated. Analysis shows that the correlation between human and artificial scanpaths is much lower than expected. Recommendations are made for improvements to the model to foster closer correspondence to human attentional patterns in VR. 1