Description

Volume clipping plays a decisive role in understanding 3D volumetric data sets because it allows to cut away selected parts of the volume based on the position of voxels in the data set. Very often clipping is the only way to uncover important, otherwise hidden details of a data set. We think that this geometric approach can be regarded as complementary to the specification of transfer functions, which are based on the data values and their derivatives only. Therefore, we suggest to use a combination of data-driven transfer functions and geometry-guided clipping to achieve very effective volume visualizations.

The design of useful transfer functions has recently attracted some attention. In most applications of interactive volume clipping, however, only a straightforward approach is adopted - with one or multiple clip planes serving as clip geometry. Typical applications are medical imaging of radiological data or volume slicing of huge seismic 3D data sets in the oil and gas industry. Although some clip geometries can be approximated by multiple clip planes, many useful and important geometries are not supported. Cutting a cube-shaped opening into a volume is a prominent example.

Therefore, we have developed ways to efficiently implement complex clip geometries. These methods are tailored to texture-based volume rendering on graphics hardware, by exploiting advanced fragment operations. In our first approach, a clip object is represented by a tessellated boundary surface. The basic idea is to store the depth structure of the clip geometry in 2D textures whose texels have a one-to-one correspondence to pixels on the viewing plane. In the second approach, a clip object is voxelized and represented by an additional volume data set. Clip regions are specified by marking corresponding voxels in this volume.

In addition to the core clipping techniques for volume visualization, issues related to volume shading are of specific interest. Volume shading, in general, extends mere volume rendering by adding illumination terms to the volume rendering integral. Lighting introduces further information on the spatial structure and orientation of features in the volume data set and can thus facilitate a better understanding of the original data. The combination of volume shading and volume clipping, however, introduces issues of how illumination needs to be computed in the vicinity of the clipping object. On one hand, the orientation of the clipping surface should be represented. One the other hand, properties of the scalar field should still influence the appearance of the clipping surface. We have developed an optical model which takes into account a consistent shading of the clipping surface.

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References

[WEE02] D. Weiskopf, K. Engel, and T. Ertl. Volume Clipping via Per-Fragment Operations in Texture-Based Volume Visualization, IEEE Visualization 2002 Proceedings, R. Moorhead, M. Gross, K. I. Joy (eds.), ACM Press, October 2002, 93-100.

[WEE03] D. Weiskopf, K. Engel, and T. Ertl. Interactive Clipping Techniques for Texture-Based Volume Visualization and Volume Shading. IEEE Transactions on Visualization and Computer Graphics, to appear, 2003.