VolVis was developed by The Center for Visual Computing at Stony Brook, under the direction of Dr. Arie Kaufman [1]. It is a comprehensive volume visualization software system that has served as the basis for many projects at Stony Brook and elsewhere. VolVis unites numerous visualization methods within one system, providing a flexible tool for the physician, scientist, and engineer as well as the visualization developer and researcher. The VolVis system has been designed to meet the following key objectives:

• Diversity: VolVis supplies a wide range of functionality with numerous methods provided within each functional component. For example, VolVis provides various projection methods, including ray casting, ray tracing and "isosurfaces."

• Ease of use: The VolVis user interface is organized into functional components, providing an easy-to-use visualization system. One advantage of this approach over data-flow systems is that the user does not have to learn how to link numerous modules in order to perform a task.

• Extensibility: The structure of the VolVis system allows a visualization programmer to add new representations and algorithms. For this purpose, an extensible and hierarchical abstract model was developed that contains definitions for all objects in the system.

• Portability: The VolVis system, written in C, is highly portable, running on most UNIX workstations sup-

FIGURE 22 VolVis main user interface and an example session. See also Plate 144.

porting X/Motif. Recently, VolVis has been ported to PC running Windows 95/NT, which can take advantage of the newly available VolumePro board from Mitsubishi and achieve interactive rendering. The system has been tested on Silicon Graphics, Sun, Hewlett-Packard, Digital Equipment Corporation, and IBM workstations and PCs.

• Free availability: The high cost of most visualization systems and difficulties in obtaining their source code often lead researchers to write their own tools for specific visualization tasks. VolVis is freely available as source code to nonprofit organizations.

Figure 22 shows an example session of the VolVis system. The long window on the left is the main VolVis interface window, with buttons for each of the major components of the system. Two of the basic input data classes of VolVis are volumetric data and 3D geometric data. The input data is processed by the Modeling and Filtering components of the system to produce either a 3D volume model or a 3D geometric surface model of the data.

The Measurement component can be used to obtain quantitative information from the data models. Surface area, volume, histogram, and distance information can be extracted from the data using one of several methods. Isosurface volume and surface area measurements can be taken either on an entire volume or on a surface-tracked section.

Most of the interaction in VolVis occurs within the Manipulation component of the system, which consists of three sections: the Object Control section, the Navigation section, and the Animation section.

The Object Control section of the system is extensive, allowing the user to manipulate parameters ofthe objects in the scene. This includes modifications to the color, texture, and shading parameters of each volume, as well as more complex operations such as positioning of cut planes and data segmentation. The color and position of all light sources can be interactively manipulated by the user. Also, viewing parameters, such as the final image size, and global parameters, such as ambient lighting and the background color, can be modified.

The Navigator allows the user to interactively manipulate objects within the system. The user can translate, scale, and rotate all volumes and light sources, as well as the view itself. The Navigator can also be used to interactively manipulate the view in a manner similar to a flight simulator. To provide interactive navigation speed, a fast rendering algorithm was developed that involves projecting reduced resolution representations of all objects in the scene.

The Animator also allows the user to specify transformations to be applied to objects within the scene, but as opposed to the Navigator, which is used to apply a single transformation at a time, the Animator can be used to specify a sequence of transformations to produce an animation. In addition to simple rotation, translation, and scaling animations, the Navigator can be used to interactively specify a "flight path," which can then be passed to the Animator and rendered to create an animation.

The VolVis system is input device independent. To achieve this, a device unified interface (DUI), developed by the VolVis

FIGURE 23 Head composite display in VolVis. See also Plate 145.
FIGURE 24 3D rendering of head with mirrors in VolVis display. See also Plate 146.

team, provides a generalized and easily expandable protocol for communication between applications and input devices. The key idea of the DUI is to convert raw data received from different input sources into unified format parameters of a "virtual input device".

The Rendering component encompasses several different rendering algorithms, including geometry-based techniques such as Marching Cubes, global illumination methods such as ray tracing, and direct volume rendering algorithms such as ray casting with compositing (see Fig. 23).

Rendering is one of the most essential components of the VolVis system. For the user, speed and accuracy are both important, yet often conflicting, aspects of the rendering process. For this reason, a variety of rendering techniques have been implemented within the VolVis system, ranging from the fast, rough approximation of the final image, to the comparatively slow, accurate rendering within a global illumination model. Also, each rendering algorithm by itself supports several levels of accuracy, giving the user an even greater amount of control.

One of the VolVis rendering techniques, volumetric ray tracing, is built upon a global illumination model. Global effects can often be desirable in scientific applications. For example, by placing mirrors in the scene, a single image can show several views of an object in a natural, intuitive manner, leading to a better understanding of the 3D nature of the scene (see Fig. 24).

VolVis has been used by scientists and researchers in many different areas, such as dendritic path visualization [2] and 3D virtual colonoscopy [3].

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