Tripod operators for recognizing objects in range images: rapid rejection of library objects

The tripod operator, a class of feature extraction operators for range images which facilitate the recognition and localization of objects is described. It consists of three points in 3-space fixed at the vertices of an equilateral triangle and a procedure for making several scalar measurements in the coordinate frame of the triangle. The triangle is moved as a rigid body until the three vertices lie on the surface of some range image or modeled object. The resulting measurements are local shape features which are invariant under rigid motions. These features contain all the shape information in the surface points involved, and no other information. Tripod operators are applicable to all 3-D shapes. A cloud of points in feature space was generated for each object by random placement of the operator. Then new feature measurements were made by operator placements in a range image containing one of those objects. Using a simple nearest-neighbor approach, the authors determined which objects were rejected and which remained as recognition candidates. Experiments showed that tripod operators had excellent discriminating power