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7 3D Shape Matching for Retrieval and Recognition

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locality-sensitive hashing for matching. Likewise, Li and Guskov [72] used spin images and normal based signatures to describe selected points over range scans. A combination of pyramid matching and support vector machines (SVMs) were applied for object recognition giving good results on CAD models and faces.

More recently, Chen and Bhanu [32] proposed an approach to recognize highly similar 3D objects in range images. As the authors claimed, several techniques have been proposed to recognize objects in dissimilar classes, however the task of recognizing objects with high similarity is challenging. Given an object, the authors extracted local surface patches on interest points found using curvature information. Due to the high dimensionality of the descriptors, these were embedded in a low dimensional space using FastMap [40]. Then, the low dimensional descriptors were organized in a kd-tree where efficient nearest neighbor algorithms can be applied. Using the kd-tree, it is possible to find correspondences between two objects. A SVM classifier ranks the correspondences according to geometric constraints returning the most promising correspondences which were verified with the iterative closest point algorithm. The object with the least mean square error is selected.

7.3 3D Shape Retrieval Techniques

The aim of this section is to present both mature and promising recent material concerning 3D shape retrieval and recognition. We provide detailed descriptions of four techniques: the depth-buffer descriptor, spin images, salient spectral features for shape matching, and heat kernel signatures.

The aforementioned methods address different aspects of shape matching. Firstly, the depth-buffer descriptor is a technique suitable for global matching. Secondly, the spin image is a pioneering representation and approach in 3D object recognition. Finally, salient spectral features and heat kernel signatures methods are recent proposals to tackle the problems of non-rigid and partial 3D shape matching.

An important issue to be considered before describing the approaches is shape representation. Although there are many ways to represent a 3D object, boundary representations have mostly been used where objects are represented by a limit surface that distinguishes the inside from the outside of the object. Moreover, the surface can be approximated in a piece-wise manner, reducing the amount of information needed to represent it at the expense of losing detail. The most common way is depicting the surface by a set of points (vertices) and polygons (faces) and, in fact, it is preferable to take triangular faces for efficiency and effectiveness in computation. Surprisingly, this representation allows one to conceive almost any object with the desired level of detail.

All the techniques presented in this section use triangular meshes for representing shapes.