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 rendering techniques for different subsets of a 3D data set. Different structures 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 renderings are combined globally in a merging step (usually compositing in our case). This allows us to selectively choose the most suitable technique for depicting each object within the data while keeping the amount of information contained in the image at a reasonable level. This is especially useful when inner structures should be visualized together with semitransparent outer parts, similar to the focus-plus-context approach known from information visualization. We also present an implementation of our approach which allows us to explore volumetric data using two-level rendering at interactive frame rates.