Most visualization techniques have been designed on the assumption that the data to be represented are free from uncertainty. Yet this is rarely the case. Recently the visualization community has risen to the challenge of incorporating an indication of uncertainty into visual representations, and in this article we review their work. We place the work in the context of a reference model for data visualization, that sees data pass through a pipeline of processes. This allows us to distinguish the visualization of uncertainty—which considers how we depict uncertainty specified with the data—and the uncertainty of visualization—which considers how much inaccuracy occurs as we process data through the pipeline. It has taken some time for uncertain visualization methods to be developed, and we explore why uncertainty visualization is hard—one explanation is that we typically need to find another display dimension and we may have used these up already! To organize the material we return to a typology developed by one of us in the early days of visualization, and make use of this to present a catalog of visualization techniques describing the research that has been done to extend each method to handle uncertainty. Finally we note the responsibility on us all to incorporate any known uncertainty into a visualization, so that integrity of the discipline is maintained.

An important issue in the design of visualization systems is to allow flexibility in providing a range of interfaces to a single body of algorithmic software. In this paper we describe how the ADVISE architecture provides exactly this flexibility. The architecture is cleanly separated into three layers: user interface, web service middleware and visualization components. This gives us the flexibility to provide a range of different delivery options, but all making use of the same basic set of visualization components. These delivery options comprise a range of user interfaces (visual pipeline editor, tailored application, web page), coupled with installation choice between a stand-alone desktop application, or a distributed client-server application.

An important issue in the design of visualization systems is to allow flexibility in providing a range of interfaces to a single body of algorithmic software. In this paper we describe how the ADVISE architecture provides exactly this flexibility. The architecture is cleanly separated into three layers: user interface, web service middleware and visualization components. This gives us the flexibility to provide a range of different delivery options, but all making use of the same basic set of visualization components. These delivery options comprise a range of user interfaces (visual pipeline editor, tailored application, web page), coupled with installation choice between a stand-alone desktop application, or a distributed client-server application.

The ability to perform fast, accurate, high-resolution visualization is fundamental to improving our understanding of anatomical data. As the volumes of data increase from improvements in scanning technology, the methods applied to visualization must evolve. In this paper, we address the interactive display of data from high-resolution magnetic resonance imaging scanning of a rabbit heart and subsequent histological imaging. We describe a visualization environment involving a tiled liquid crystal display panel display wall and associated software, which provides an interactive and intuitive user interface. The oView software is an OpenGL application that is written for the VR Juggler environment. This environment abstracts displays and devices away from the application itself, aiding portability between different systems, from desktop PCs to multi-tiled display walls. Portability between display walls has been demonstrated through its use on walls at the universities of both Leeds and Oxford. We discuss important factors to be considered for interactive two-dimensional display of large three-dimensional datasets, including the use of intuitive input devices and level of detail aspects.

This paper presents a novel liver segmentation algorithm. This is a model-driven approach; however, unlike previous techniques which use a statistical model obtained from a training set, we initialize patient-specific models directly from their own pre-segmentation. As a result, the non-trivial problems such as landmark correspondences, model registration etc. can be avoided. Moreover, by dividing the liver region into three sub-regions, we convert the problem of building one complex shape model into constructing three much simpler models, which can be fitted independently, greatly improving the computation efficiency. A robust graph-based narrow band optimal surface fitting scheme is also presented. The proposed approach is evaluated on 35 CT images. Compared to contemporary approaches, our approach has no training requirement and requires significantly less processing time, with an RMS error of 2.44±0.53mm against manual segmentation.

In this paper we look at the Line Integral Convolution method for flow visualiz ation and ways in which this can be applied to the visualization of two dimensional, steady flow fields in the presenc e of uncertainty. To achieve this, we start by studying the method and reviewing the history of modifications oth er authors have made to it in order to improve its efficiency or capabilities, and using these as a base for the visu alization of uncertain flow fields. Finally, we apply our methodology to a case study from the field of oceanogr aphy.

Recent years have seen a significant increase in the use of Interventional Radiology (IR) as an alternative to open surgery. A large number of IR procedures commences with needle puncture of a vessel to insert guidewires and catheters: these clinical skills are acquired by all radiologists during training on patients, associated with some discomfort and occasionally, complications. While some visual skills can be acquired using models such as the ones used in surgery, these have limitations for I R which relies heavily on a sense of touch. Both patients and trainees would benefit from a virtual environment (VE) conveying touch sensation to realistically mimic procedures. The authors are developing a high fidelity VE providing a validated alternative to the traditional apprenticeship model used for teaching the core skills. The current version of the CRaIVE simulator combines home made software, haptic devices and commercial equipments.

As stated by Johnson [Joh04], the visualization of uncertainty remains one of the major challenges for the visualization community. To achieve this, we need to understand and develop methods that allow us not only to consider uncertainty as an extra variable within the visualization process, but to treat it as an integral part. In this paper, we take contouring, one of the most widely used visualization techniques for two dimensional data, and focus on extending the concept of contouring to uncertainty. We develop special techniques for the visualization of uncertain contours. We illustrate the work through application to a case study in oceanography.

There is an increasing demand for the development of virtual environments for training in vascular interventional radiological procedures. This requires fast and precise segmentation of varied abdominal structures from a wide range of image modalities. This paper presents an efficient semi-automatic segmentation system which combines image processing techniques and mathematical morphology operations to obtain an initial segmentation close to the target structure shape. This initial segmentation is then embedded into a level set function to obtain a refined segmentation result. Minimal intervention is required in comparison to other level set based approaches. The approach also dramatically decreases processing time and reduces the risks of leaking at weak boundaries, without compromising the accuracy of the segmentation.

Service-oriented architectures are increasingly being used as the architectural style for creating large distributed computer applications. This paper examines the provision of visualization as a service that can be made available to application designers in order to combine with other services. We develop a three-layer architecture: a client layer which provides the user interface; a stateful Web service middleware layer which provides a published interface to the visualization system; and finally, a visualization component layer which provides the core functionality of visualization techniques. This separation of middleware from the visualization components is crucial: it allows us to exploit the strengths of Web service technologies in providing standardized access to the system, and in maintaining state information throughout a session, but also gives us the freedom to build our visualization layer in an efficient and flexible way without the constraints of Web service protocols. We describe the design of a visualization service based on this architecture, and illustrate one aspect of the work by re-visiting an early example of Web-based visualization.

In many volume visualization applications there is some region of specific interest where we wish to see fine detail - yet we do not want to lose an impression of the overall picture. In this research we apply the notion of focus and context to texture-based volume rendering. A framework has been developed that enables users to achieve fast volumetric distortion and other effects of practical use. The framework has been implemented through direct programming of the graphics processor and integrated into a volume rendering system. Our driving application is the effective visualization of aneurysms, an important issue in neurosurgery. We have developed and evaluated an easy-to-use system that allows a neurosurgical team to explore the nature of cerebral aneurysms, visualizing the aneurysm itself in fine detail while still retaining a view of the surrounding vasculature.

The effective visualization of aneurysms is a very important issue in neurosurgery. However it is difficult to display both the aneurysm with sufficient detail, and the vessel network of the brain at the same time. This work offers a solution to both of these problems, applying the concept of focus and context to texture-based volume rendering. A flexible application has been developed, allowing different focus and context techniques to be used. This paper concentrates on the evaluation of the system by a group of neurosurgeons.

This paper traces the evolution of computational steering within visualization dataflow environments. In computational steering we integrate simulation and visualization into a single environment, in which the scientist can control the simulation on the basis of the visualization of the current results. This paper is essentially a review paper, in which we describe a number of projects with which we have been involved over the last twenty years. We begin with a historical look-back to the early development of dataflow visualization systems. These are also known as modular visualization environments. They typically consist of a library of elementary visualization components that can be wired together in a visual editor in order to compose a pipeline forming the visualization application. From an early stage these have been used for computational steering, because it is possible to include user code, for example a simulation, as a component in the pipeline. Moreover modules are able to run on a distributed set of resources, allowing simulation code to run on a remote resource. Indeed, if the simulation is producing large volumes of data, then the visualization modules can be co-located with the simulation thus reducing the amount of data returned to the desktop for visualization. This very useful facility has been surprisingly little used. Our own involvement in computational steering began in 1993 with the GRASPARC project, and interest was re-kindled in 2000 with the emergence of Grid computing and the UK e-science programme. Our first task was to re-work the distributed computing model of IRIS Explorer (the dataflow system we use at Leeds) so as to provide the security that is expected in modern computing environments. This was followed by a re-working of the architecture for computational steering, in the gViz project, where we separated the simulation code from the visualization dataflow, and allowed the simulation to run autonomously. This gives the advantage of disconnecting simulation lifetime from visualization system execution time. In doing this it allows simulations to run over greater time scales than the time period a user may wish to actively interact with them in any one session. Previously, shutting down the visualization would have shut down the simulation component. This was achieved using the gViz computational steering library. Our more recent work in the e-viz project has extended the architecture further, basing the work around an abstract description of the visualization pipeline from which a user interface for steering can be automatically generated. Likewise the pipeline description can be interpreted in terms of different visualization systems, providing an extra level of abstraction. As interest in service-oriented architectures develops, we are seeing a reworking of visualization systems as a pipeline of services, rather than modules. We can expect this trend to carry over to computational steering in future research.

Visualization taxonomies are an important means of imposing some structure on a rather diverse fi eld. We review some earlier work in this area, particularly work based on the use of a notation to label classes of visualization techniques that are appropriate to particular entities. We propose a new notation introducing it in the context of a new visualization reference model, one we hope will lead eventually to a means of describing visualizations in a clear and unambiguous way.

This paper describes a flexible and extensible user interface tool for interacting with simulations and visualization applications running on the Grid. It uses an XML description of the visualization pipeline to provide parameter and location information with which to configure the user interface and connect it to remote applications using the gViz library. Messages in this XML format allow for the dynamic alteration of the interface in response to user and system behaviour. A framework is provided into which user cre- ated widgets can function alongside or replace the system widgets. This framework also allows alternate communication mechanisms to be added to allow the encapsulation of a broad set of applications within a single pipeline.

A difficulty until recently has been the speed at which volume rendered images can be created. Ray casting using CPU technology is slow and does not allow real-time interaction by changing the viewing direction for example. This has dramatically changed in the last few years with the advent of GPU technology. The data can be stored as a 3D texture and fast GPU-based texture rendering now allows interactive volume rendering on commodity hardware. An excellent overview of this area is provided by the IEEE Visualization tutorial [1].

Advances in research and clinical techniques are providing increasing quantities of data at improved spatio-temporal resolution. It is therefore imperative to develop matching approaches for efficient analysis and intuitive presentation of this data. Using the example of advanced magnetic resonance imaging, this article will illustrate the challenges involved in computational reconstruction and interactive visualization of the three-dimensional cardiac anatomy, based on magnetic resonance imaging data with para-cellular resolution.