Spectral volume rendering

Volume renderers for interactive analysis must be sufficiently versatile to render a broad range of volume images: unsegmented 'raw' images as recorded by a 3D scanner, labeled segmented images, multimodality images, or any combination of these. The usual strategy is to assign to each voxel a three component RGB color and an opacity value α. This so-called RGBα approach offers the possibility of distinguishing volume objects by color. However, these colors are connected to the objects themselves, thereby bypassing the idea that in reality the color of an object is also determined by the light source and light detectors c.q. human eyes. The physically realistic approach presented here models light interacting with the materials inside a voxel causing spectral changes in the light. The radiated spectrum falls upon a set of RGB detectors. The spectral approach is investigated to see whether it could enhance the visualization of volume data and interactive tools. For that purpose, a material is split into an absorbing part (the medium) and a scattering part (small particles). The medium is considered to be either achromatic or chromatic, while the particles are considered to scatter the light achromatically, elastically, or inelastically. It appears that inelastic scattering particles combined with an achromatic absorbing medium offer additional visual features: Objects are made visible through the surface structure of a surrounding volume object and volume and surface structures can be made visible at the same time. With one or two materials the method is faster than the RGBα approach, with three materials the performance is equal. The spectral approach can be considered as an extension of the RGBα approach with a greater visual flexibility and a better balance between quality and speed.