To make a spectral representation of colour practicable for volume rendering, a new lowdimension subspace method is used to act as the carrier of spectral information. With that model spectral light material interaction can be integrated into existing volume rendering methods at almost no penalty. In addition, slow rendering methods can profit from the new technique of post-illumination — generating spectral images in real-time for arbitrary light spectra under a fixed viewpoint. Thus, the capability of spectral rendering to create distinct impressions of a scene under different lighting conditions is established as a method of real-time interaction. Although we use an achromatic opacity in our rendering, we show how spectral rendering permits different dataset features to be emphasized or hidden as long as they have not been entirely obscured. The use of post-illumination is an order of magnitude faster than changing the transfer function and repeating the projection step. To put the user in control of the spectral visualization we devise a new widget, a “light-dial”, for interactively changing the illumination and include a usability study of this new light space exploration tool. Applied to spectral transfer functions, different lights bring out or hide specific qualities of the data. In conjunction with postillumination this provides a new means for preparing data for visualization and forms a new degree of freedom for guided exploration of volumetric data sets.

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