Direct volume rendering techniques are used to visualize and explore large scalar volumes. Transfer functions (TFs) that assign opacity and color to scalar values are very important to display volume features, but their specification is not trivial or intuitive. This work presents an interactive, semi-automatic tool to assist the user in the generation of opacity and color TFs. We use the histogram approach proposed by Kindlmann and Durkin [9] to reduce the scope of candidate TFs presented to the user following the Design Galleries method [18]. The combination of these two solutions leads to a single interactive tool that allows the user to deal with different aspects of TF specification.