A Hardware-Assisted Visibility-Ordering Algorithm With Applications To Volume Rendering

We propose a hardware-assisted visibility ordering algorithm. From a given viewpoint, a (back-to-front) visibility ordering of a set of objects is a partial order on the objects such that if object A obstructs object B, then A precedes A in the ordering. Such orderings are useful because they are the building blocks of other rendering algorithms such as direct volume rendering of unstructured grids. The traditional way to compute the visibility order is to build a set of visibility relations (e.g. A), and then run a topological sort on the set of relations to actually get the partial ordering. Our technique instead works by assigning a layer number to each primitive, which directly determines the visibility ordering. Objects that have the same layer number are independent, and have no obstruction between each other. We use a simple technique which exploits a combination of the z- and stencil buffers to compute the layer number of each primitive. One application of our technique is to obtain a fast unstructured volume rendering algorithm. In this paper, we present our technique and its implementation in OpenGL. We also discuss its performance and some optimizations on some recent graphics hardware architectures.