Transfer Functions in Direct Volume Rendering: Design, Interface, Interaction

A principle of direct volume rendering is that visualizations can be created without creating intermediate geometric structure, such as polygons comprising an isosurface, but simply by a “direct” mapping from volume data points to composited image elements. Together with traditional computer graphics elements such as camera, lighting, and shading, the central ingredient in that direct mapping is the assignment of optical properties (opacity, color, etc.) to the values comprising the volume dataset. This is the role of the transfer function [6, 26, 28]. As simple and direct as that mapping is, it is also extremely flexible, because of the immense variety of possible transfer functions. Because that flexibility is generally unconstrained, the most important parameter in producing a meaningful and intelligible volume rendering is also one of the hardest parameters to set appropriately. This motivates the study of transfer function design, the development of new interfaces for transfer function specification, and a consideration of how interaction techniques in visualization systems can simplify transfer function creation. These course notes attempt to describe the current state of the art of transfer functions in direct volume rendering. Existing approaches can be approximately located in a continuum between data-driven and image-driven methods, which determine transfer functions based on information extracted from the volume dataset, or from direct volume rendered images, respectively.