Abstract: Direct volume rendering (DVR) is a flexible technique for visualizing and exploring scientific and biomedical volumetric data sets. Transfer functions associate field values with colors and opacities, however, for complex data are often not sufficient for encoding all relevant information. We introduce a novel visualization technique termed texture-enhanced DVR to visualize supplementary data such as material properties and additional data fields. Smooth transitions in the underlying data are represented by coherently synthesizing and morphing textures within user defined regions of interest. The framework seamlessly integrates into the conventional DVR process, can be executed on the GPU, is extremely powerful and flexible, and enables entirely novel visualizations. The novel algorithm for multi-dimensional texture synthesis and morphing is especially suitable for parallel architectures such as GPUs or direct volume rendering (DVR) hardware. We introduce several improvements to previous 2D synthesis algorithms, such as new appearance space attributes and an improved jitter function, and extend it to 3D. We then modify the synthesis algorithm to use it for texture morphing which can be applied to arbitrary many 2D input textures and can be spatially controlled using weight maps. Our results suggest that the algorithm produces higher quality textures than alternative algorithms with similar speed. Compared to higher quality texture synthesis algorithms, our solution is considerably faster and allows the synthesis of additional channels without affecting the running time of the synthesis at all. Overall the presented technique provides an excellent trade-off between speed and quality, is highly flexible, allows the use of arbitrary channels such as transparencies and displacement maps, can be extended to arbitrary dimensions, is suitable for a GPU-implementation, and allows entirely novel effects when used in the direct volume rendering process.