Semi-automatic transfer function generation for non-domain specific direct volume rendering

The field of volume rendering is focused on the visualization of three-dimensional data sets. Although it is predominantly used in biomedical applications, volume rendering has proven useful in fields such as meteorology, physics, and fluid dynamics as a means of analyzing features of interest in three-dimensional scalar fields. The features visualized by volume rendering differ by application, though most applications focus on providing the user with a model for understanding the physical structure represented in the data such as materials or the boundaries between materials. One form of volume rendering, direct volume rendering (DVR), has proven to be a particularly powerful tool for visualizing material and boundary structures represented in volume data through the use of transfer functions which map each unit of the data to optical properties such as color and opacity. Specifying these transfer functions in a manner that yields an informative rendering is often done manually by trial and error and has become the topic of much research. While automated techniques for transfer function creation do exist, many rely on domain-specific knowledge and produce less informative renderings than those generated by manually constructed transfer functions. This thesis presents a novel extension to a successful semi-automated transfer function technique in an effort to minimize the time and effort required in creation of informative transfer functions. In particular, the method proposed provides a means for the semi-automatic generation of transfer functions which highlight and classify material boundaries in a non-domain specific manner

Lighting transfer functions using gradient aligned sampling

Figure 1: With the absence of lighting in the brown region in (a), very few of the inner structures are visible. With the addition of shading in (b) these structures become more visible, but erratic variations of lighting in near homogeneous regions make the larger boundaries of interest difficult to see. By increasing the opacity at these boundaries in (c) the boundary surface are clearly visible, however, the sense of material thickness from absorption in homogeneous regions is lost. With our technique shown in (d), lighting transfer functions are used to selectively enhance boundary surfaces of interest, allowing opacity to be used to illustrate material thickness and depth. Figures (a), (b), and (d) share the same color and opacity transfer function and have variations only in their lighting transfer function. An important task in volume rendering is the visualization of boundaries between materials. This is typically accomplished using transfer functions that increase opacity based on a voxel’s value and gradient. Lighting also plays a crucial role in illustrating surfaces. In this paper we present a multi-dimensional transfer function method for enhancing surfaces, not through the variation of opacity, but through the modification of surface shading. The technique uses a lighting transfer function that takes into account the distribution of values along a material boundary and features a novel interface for visualizing and specifying these transfer functions. With our method, the user is given a means of visualizing boundaries without modifying opacity, allowing opacity to be used for illustrating the thickness of homogeneous materials through the absorption of light