Volumetric, or three-dimensional, digital imaging now plays a vital role in many areas of research such as medicine and geology. Medical images acquired by tomographic scanners for instance are often given as a stack of cross-sectional image slices. Such images are called ?volumetric? because they depict objects in their entire three-dimensional extent rather than just as a projection onto a two-dimensional image plane. Since huge amounts of volumetric data are continually being produced in many places around the world, techniques for their automatic analysis become ever more important....