Nonrigid motion analysis: Articulated and elastic motion

Motion of physical objects in the world is, in general, nonrigid. In robotics and computer vision, the motion of nonrigid objects is of growing interest to researchers from a wide spectrum of disciplines. The nonrigid objects being studied may be generally categorized into three groups according to the degree of deformation of body parts: articulated, elastic, and fluid. In articulated motion, individual rigid parts of an object move independently of one another and the motion of the whole object is nonrigid in nature. Elastic motion is nonrigid motion that conforms to a certain degree of continuity or smoothness. Fluid motion violates even the continuity assumption and may involve topological variations and turbulent deformations, This paper presents an overview of existing work on articulated and elastic motion, motivated by problems relating to the motion of the human body and of an animal heart, respectively. We study various approaches for recovering the 3D structure and motion of objects through a sequence of images in a bottom-up fashion, a strategy widely employed by various investigators. These approaches are classified as (1) motion recovery without shape models, and (2) model-based analysis. In the discussion of each algorithm, we also include a description of the complexity of feature and motion constraints, which are highly related to each other. (C) 1998 Academic Press.