Stroke-Based Transfer Function Design

In this paper we propose a user interface for the design of 1D transfer functions. The user can select a feature of interest by drawing one or more strokes directly onto the volume rendering near its silhouette. Based on the stroke(s), our algorithm performs a histogram analysis in order to identify the desired feature in histogram space. Once the feature of interest has been identified, we automatically generate a component transfer function, which associates optical properties with the previously determined intervals in the domain of the data values. By supporting direct interaction techniques, which are performed in the image domain, the transfer function design becomes more intuitive compared to the specification performed in the histogram domain. To be able to modify and combine the previously generated component transfer functions conveniently, we propose a user interface, which has been inspired by the layer mechanism commonly found in image processing software. With this user interface, the optical properties assigned through a component function can be altered, and the component functions to be combined into a final transfer function can be selected.