Two-level volume rendering

In this paper, we present a two-level approach for volume rendering, i.e., two-level volume rendering, which allows for selectively using different re ndering techniques for different subsets of a 3D data set. Different struct ures within the data set are rendered locally on an object-by-object basis by either DVR, MIP, surface rendering, value integration (x-ray-like images ), or nonphotorealistic rendering. All the results of subsequent object ren derings are combined globally in a merging step (usually compositing in our case). This allows us to selectively choose the most suitable technique fo r depicting each object within the data while keeping the amount of informa tion contained in the image at a reasonable level. This is especially usefu l when inner structures should be visualized together with semitransparent outer parts, similar to the focus-plus-context approach known from informat ion visualization. We also present an implementation of our approach which allows us to explore volumetric data using two-level rendering at interacti ve frame rates.