Summary and Results

In this paper, a flexible framework for volume tracing, which allows for the arbitrary combination of different density functions, transfer functions and phase functions in a participating material, has been presented. Participating media have absorption, emission and scattering properties and are rendered using a ray-marching algorithm. The framework has been implemented as an add-on to the Advanced Rendering Toolkit. Participating media can be integrated seamlessly into ``conventional'' scenes. No restrictions whatever apply. Density functions can be defined by means of procedural density functions (like in solid texturing), density emitters and volume data sets. Different kinds of filters may be applied to volume data sets (Mean, Gaussian, Laplace, ...) to improve data quality before rendering. Furthermore, isosurfaces and region boundaries of volume data sets may be rendered. Finally, different types of volume data interpolation can be used. Transfer functions can be designed by combining arbitrary numbers of transfer function ``primitives'' like constant-, colormap-, general- and mapped transfer functions. Constant transfer functions may be used to model homogenous-atmosphere-like participating media. Colormap transfer functions are used to map density values to arbitrary color maps. General- and mapped transfer functions can be used to combine arbitrary numbers of transfer functions. Different kinds of phase functions are used to define scattering characteristics of participating media. Isotropic, anisotropic, Lambert, Henyey-Greenstein and Mie scattering have been implemented. Although some very realistic images of natural phenomena and different types of volume data sets can be rendered, some ideas would deserve further investigation in future work: More sophisticated density functions may be examined, for example, physically based models to create more convincing images of natural phenomena. Wavelength dependent scattering functions should be implemented quite easily because ART itself already is able to perform wavelength-dependent rendering. Furthermore, right now, participating media do not act as light sources even if they possess an emission property. This shortfall could be circumvented by means of participating media as volume light sources, or by means of global illumination modells. Global illumination models also would be useful in rendering effects like volume caustics and indirect lighting by participating media.