Adding shadows to a texture-based volume renderer

Texture based volume rendering is a technique to efficiently visualize volumetric data using texture mapping hardware. We present an algorithm that extends this approach to render shadows for the volume. The algorithm takes advantage of fast frame buffer operations which modern graphics hardware offers, but does not depend on any special purpose hardware. The visual impression of the final image is significantly improved by bringing more structure and three dimensional information into the often foggyish appearance of texture based volume renderings. Although the algorithm does not perform lighting calculations, the resulting image has a shaded appearance, which is a further visual cue to spatial understanding of the data and lets the images appear more realistic. As calculating the shadows is independent of the visualization process it can be applied to any form of volume visualization, though volume rendering based on two- or three-dimensional texture mapping hardware makes the most sense. Compared to unshadowed texture based volume rendering, performance decreases by less than 50%, which is still sufficient to guarantee interactive manipulation of the volume data. In the special case where only the camera is moving with the light position fixed to the scene there is no performance decrease at all, because recalculation has only to be done if the position of the light source with respect to the volume changes.