There is a growing need for custom visualization applications to deal with the rising amounts of volume data to be analyzed in fields like medicine, seismology, and meteorology. Visual programming techniques have been used in visualization and other fields to analyze and visualize data in an intuitive manner. However, this additional step of abstraction often results in a performance penalty during the actual rendering. In order to prevent this impact, a careful modularization of the required processing steps is necessary, which provides flexibility and good performance at the same time. In this paper, we will describe the technical foundations as well as the possible applications of such a modularization for GPU-based volume raycasting, which can be considered the state-of-the-art technique for interactive volume rendering. Based on the proposed modularization on a functional level, we will show how to integrate GPU-based volume ray-casting in a visual programming environment in such a way that a high degree of flexibility is achieved without any performance impact.