Modified dendrogram of attribute space for multidimensional transfer function design

We introduce a modified dendrogram (MD) (with subtrees to represent clusters) and display it in 2D for multidimensional transfer function design. Such a transfer function for direct volume rendering employs a multidimensional space, termed attribute space. The MD reveals the hierarchical structure information of the attribute space. The user can design a transfer function in an intuitive and informative manner using the MD user interface in 2D instead of multidimensional space, where it is hard to ascertain the relationship of the space. In addition, we provide the capability to interactively modify the granularity of the MD. The coarse-grained MD primarily shows the global information of the attribute space while the fine-grained MD reveals the finer details, and the separation ability of the attribute space is completely preserved in the finest granularity. With this so called multigrained method, the user can efficiently create a transfer function using the coarse-grained MD, and then fine tune it with the fine-grained MDs. Our method is independent on the type of the attributes and supports arbitrary-dimension attribute space.