et al. [@Solteszova2010] developed an illumination model based on the work of Schott et al. [@Schott2009] which is able to generate realistic shadows in volume rendering. Besides simulating realistic lighting effects, illustrative techniques have also been used to improve perception in medical visualization. Bruckner et al. [@Bruckner2007] proposed Style Transfer Functions, which realize non-photorealistic rendering in the context of volume visualization. Šoltészová et al. [@Solteszova2011] apply chromadepth to improve shadow perception in volume visualization, and Ebert et al. [@Ebert2000] applied different illustrative techniques such as halos, boundary enhancement, or silhouettes in volume visualization.

Maximum Intensity Projection (MIP) is a rendering technique often used to visualize contrast-enhanced data. However, with this technique, due to the lack of shading, is difficult to perceive the depth of the individual objects and their relative positions. Diaz et al. [@Diaz2010] presented a technique to improve depth perception on MIP images. Therefore, they compute a depth value while traversing the volume dataset, and use this value to modify the final color of the MIP image. Bruckner et al. [@Bruckner2009] incorporated shading into MIP, improving depth perception and allowing a smooth transition between MIP and direct volume rendering.

**Vessel visualization.** Vessel datasets are often represented as a 