N Volumetric Hatching Nonphotorealistic Rendering of Medical Volume Data 44

duces images in the manner of traditional styles such as painting or drawing, is proving to be a useful alternative to conventional volume or surface rendering in medical visualization. Typically, such illustrations use simple lines to demonstrate medical shapes and features, omitting a great deal of detail. They frequently highlight the most relevant information better than glossy, colored images. Medical illustration has long regarded pen and ink as an important medium. Thus, because medical professionals are familiar with pen-and-ink illustrations, they could readily accept them as an alternative to conventional rendering of medical images. Several authors, such as Treavett and Chen, have begun using NPR in medical visualization (see the “Related Work in Nonphotorealistic Rendering” sidebar for a discussion of other visualization techniques). The general approach has been to apply existing surface-hatching techniques to surface models created from the volume data by marching cubes or a similar method. Surface visualization, however, might not be well suited to portraying soft tissues, which are difficult to model with isosurfaces. This article introduces volumetric hatching, a novel technique that produces pen-and-ink-style images from medical volume data. Unlike previous approaches that generate full-surface models, our technique uses the characteristics of the volume near the stroke being produced to generate a local intermediate surface. Because global isosurfaces can’t exactly model many medical subjects, our volume-based method has considerable advantages. Our method is largely insensitive to surface artifacts. We focus on hatching with line strokes to portray muscles, intestines, brains, and so on. Hatching with line strokes requires determining not just the position of the line strokes, but also their orientation. Thus, the strokes not only illustrate the subject’s shape, but also describe its character in some way—for example, by displaying fiber orientations for muscles. Volumetric hatching can’t replace conventional medical visualization. Rather, it can supply an alternative description that can enhance medical understanding. It might be particularly useful with the vector-based images of functional anatomy, where it could both describe motion sequences more compactly and focus on specific medical features demonstrated in the animation.