Global illumination models for volume rendering

The increasing demand for realistic images has led to the development of several global illumination models and rendering techniques. Great effort has been taken to extend these illumination models and optimize the rendering techniques to produce more realistic images in less time. Despite this effort, most of these methods are designed for scenes consisting of geometric surface descriptions, and cannot directly render volumetric data. Volumetric data sets can be rendered using volume rendering techniques that, in order to decrease rendering times and therefore increase interactivity, typically employ only a local illumination model. The development of global illumination models and rendering techniques for volumetric data is the focus of this work. These illumination methods can be used to generate realistic images of scenes containing volumetric as well as geometric data. The volumetric global illumination methods can be employed by a visualization system in order to add intuitive cues to an image for a greater three-dimensional understanding of a scene. Also, these methods allow for amorphous effects that can not be captured using geometric illumination models, including clouds, fog, and smoke. A volumetric ray tracing method is presented that encompasses classical ray tracing while allowing for volume rendering effects. The definition of an intersection is extended to include intersection points for surface contributions to the intensity equation, and intersection segments for volumetric contributions. A method for accelerating primary and shadow rays is developed. A volumetric radiosity method is presented that encompasses classical radiosity while also including isotropic and diffuse volumetric interactions. Geometric surfaces and volumetric isosurfaces are approximated using standard surface patches, and voxels are used to approximate participating volumes. Diffuse volumetric interactions make it possible to give the appearance of shaded surfaces to scenes consisting of only volumetric data. Hierarchical techniques are applied to decrease the number of interactions required for a solution. Multipass methods are developed to increase realism by combining illumination methods. A multipass method that combines volumetric ray tracing and volumetric radiosity is presented. Also, a multipass method that extends volumetric ray tracing to account for indirect specular lighting is presented.