Direct volume rendering plays an important role in the investigation of volumetric data. In order to display these data, their values must be associated to optical properties. This task is accomplished by transfer functions (TFs), which assign properties like color and opacity to voxel values in the volume. TFs are very important to produce informative images that reveal volume features, but their specification is a non-trivial and unintuitive problem. Without any help in the TF design process, the user goes through a frustrating and time-consuming trial-and-error process. This work proposes a two-level interactive framework combining a few useful semi-automatic design techniques for one-dimensional opacity and color transfer functions. We use the histogram approach proposed by Kindlmann and Durkin, manual design aided by dual domain interaction (space domain and TF domain), stochastic evolutive design and qualitative analysis of multiple TF possibilities using a Design Galleries-inspired method (thumbnails in the first level of interaction and refinement of zoomed-in images in the second level). The combination of these strategies offers precise control over the transfer function specification, plus some interactive guidance, as well as semi-automatic and evolutive search in the TF space.