User Interface

The UI to the warping algorithm has to depict the source and target volumes, in conjunction with the feature elements. Hardware-assisted volume rendering [4] makes possible a UI solely based on direct visualization of the volumes, with the embedded elements interactively scan-converted. Using a low-end rendering pipeline, however, the UI has to resort to geometric representations of the models embedded in the volumes. These geometric representations can be obtained in either of two ways:

• Pre-existing volumes are visualized by isosurface extraction via marching cubes [14]. Several different isosurfaces can be extracted to visualize all prominent features of the volume, a volume rendering guiding the extraction process.
• For volumes that were obtained by scan converting geometric models, the original model can be used.

Once geometric representations of the models are available, the animator can use the commercial modeler of his/her choice to specify the elements. Our system, shown in figure 6d, is based on Inventor, the Silicon Graphics (SGI) 3D programming environment. Models are drawn in user-defined materials, usually translucent, in order to distinguish them from the feature elements. These, in turn, are drawn in such a way that their attributes --- local coordinate system, scaling factors, and dimensionality --- are graphically depicted and altered using a minimal set of widgets.