McGlone - 8 
archies; a class-subclass graph, and a part-whole graph. The class-subclass tree 
incorporates the notion of a generic airplane as opposed to a specific type of air 
plane. By parameterizing the model variables, the same basic model serves as a 
representative of a class and for specific instances of the class. The part-whole tree 
models each part of the object and its attachment to the other parts. The part- 
whole relationship is modeled at differing levels of detail, allowing the recognition 
process to focus on larger, more important pieces first and then verify the object 
using the smaller parts. 
The recognition process proceeds by locating “ribbons 1 in the image, sets of parallel 
edges assumed to correspond to a generalized cone. By reasoning about constraints 
between image and object space and on object space measurements and relation 
ships, feasible interpretations of the image structure in terms of the object models 
are generated. 
The object modeling in ACRONYM suffers from the general shortcomings of gen 
eralized cone models—the inability to precisely model complex objects and the lim 
itations on representable objects. ACRONYM embodies only the recognition phase 
described above (Section 3.2), since it does not try to generate a 3D description of 
the world. 
4.2. ThingWorld—superquadrics 
Pentland describes an object modeling and recognition system, ThingWorld, which 
models objects as collections of rigid parts connected by hinges or joints. Each part 
of the object is represented as a superquadric. To avoid the computational problems 
of fitting superquadrics, modal dynamics are used to model the deformations of the 
elementary volumetric primitives. Describing the object’s deformations in terms of 
its natural strain and vibration modes allows the efficient modeling of deformations 
of the volumetric primitives while greatly reducing the size and complexity of the 
equations involved. 
This work generates models which capture the main structure of the object, with 
out rendering medium- or fine-level structure. The superquadrics yield only an 
approximation for objects with precisely defined geometries, such as a rectangular 
prism. While Pentland discusses the possibility of recognition, no specific tests are 
described. Superquadric models will be useful for recognition only if the descrip 
tions produced, i.e., the superquadric parameters, are repeatable enough to use in 
indexing into a database. 
4.3. Deformable nodal models based on superquadrics 
Terzopoulos and metaxas [Terzopoulos and Metaxas, 1991] describe an object mod 
eling system based on deformable superquadrics, the basic shape of the object is 
defined by the superquadric, while deviations from the superquadric are modeled by 
a finite element mesh. In this way the large number of degrees of freedom of finite 
element models are constrained by their attachment to the superquadric. The au 
thors mention using the parameters of the superquadric to index into a recognition 
database, although no experiments were performed. It is unclear how successful this