Volume visualization and exploration through flexible transfer function design

Direct volume rendering (DVR) is a well-known method for exploring volumetric data sets. Optical properties are assigned to the volume data and then a DVR algorithm produces visualizations by sampling volume elements and projecting them into the image plane. The mapping from voxel values to optical attribute values is known as transfer function (TF). Therefore, the quality of a visualization is highly dependent on the TF employed, but its specification is a non-trivial and unintuitive task. Without any help during the TF design process, the user goes through a frustrating and time-consuming trial-and-error cycle. This paper presents a useful combination of TF design techniques in an interactive workspace for volume visualization. Our strategy relies on semi-automatic TFs generation methods: boundary emphasis, stochastic evolutive search in TF space, and manual TF specification aided by dual domain interaction. A two-level user interface was also developed. In the first level, it provides multiple simultaneous interactive visualizations of the volume data using different TFs, while in the second one, a detailed visualization of a single TF and the respective rendered volume are displayed. Moreover, in the second level, the TF can be manually refined and the volume can be further inspected through geometric tools. The techniques combined in this work are complementary, allowing easy and fast TF design and data exploration.