Abstract:
Finding appropriate transfer functions for direct volume rendering is
a difficult problem because of the large amount of user
experimentation typically involved. Ideally, the dataset being
rendered should itself be able to suggest a transfer function which
makes the important structures visible. We demonstrate that this is
possible for a large class of scalar volume data, namely that where
the region of interest is the boundary between different materials. A
transfer function which makes boundaries readily visible can be
generated from the relationship between three quantities: the data
value and its first and second directional derivatives along the
gradient direction. A data structure we term the histogram
volume captures the relationship between these quantities throughout
the volume in a position independent, computationally efficient
fashion. We describe the theoretical importance of the quantities
measured by the histogram volume, the implementation issues in its
calculation, and a method for semiautomatic transfer function
generation through its analysis. The techniques presented here make
direct volume rendering easier to use, not only because there are much
fewer variables for the user to adjust to find an informative
rendering, but because using them is more intuitive then current
interfaces for transfer function specification. Furthermore, the
results are derived solely from the original dataset and its inherent
patterns of values, without the introduction of any artificial
structures or limitations. Examples with volume datasets from a
variety of disciplines illustrate the generality and strength of the
techniques.
