Body Modeling

In order to manipulate and animate a human figure with computer graphics, a suitable figure must be modeled. This entails constructing a satisfactory surface skin for the overall human body shape, defining a skeletal structure which admits proper joint motions, adding clothing to improve the verisimilitude of analyses (as well as providing an appropriate measure of modesty), sizing body dimensions according to some target individual or population, and providing visualization tools to show physically-relevant body attributes such as torque loads and strength. In computer graphics, the designer gets a wide choice of representations for the surfaces or volumes of objects. We will briefly review current geometric modeling schemes with an emphasis on their relevance to human figures. We classify geometric models into two broad categories: boundary schemes and volumetric schemes. In a boundary representation the surface of the object is approximated by or partitioned into (non-overlapping) 0-, 1-, or 2- dimensional primitives. We will examine in turn those representations relevant to human modeling: points and lines, polygons, and curved surface patches. In a volumetric representation the 3D volume of the object is decomposed into (possibly overlapping) primitive volumes. Under volumetric schemes we discuss voxels, constructive solid geometry, ellipsoids, cylinders, spheres, and potential functions. The simplest surface model is just a collection of 3D points or lines. Surfaces represented by points require a fairly dense distribution of points for accurate modeling. Clouds of points with depth shading were used until the early 1980’s for human models on vector graphics displays. They took advantage of the display’s speed and hierarchical transformations to produce the perceptual depth effect triggered by moving points [Joh76] (for example, [GM86]). A related technique to retain display speed while offering more shape information is to use parallel rings or strips of points. This technique is used in LifeForms™ [Lif91, Cal91]. Artistically positioned “sketch lines” were used in one of the earliest human figure models [Fet82] and subsequently in a Mick Jagger music video, “Hard Woman” from Digital Productions. Polygonal (polyhedral) models are one of the most commonly encountered representations in computer graphics.