3.3.2 TWO FEATURE ABSTRACTION METHODS
The second approach to feature extraction in preliminary stages of investigation involves a Medial Axis Transformation (MAT) of the three dimensional part. This results in a reduction of a solid model to a representative set of surfaces from which shape information is readily elicited.
The medial axes of a two-dimensional object are the locus of the center of a set of maximal discs which can be fit into the object. Medial axes are determined by fitting maximal discs into a polygon.
The 2D MAT contains two elements: the medial axis and a radius function. The radius function is defined for each point on the axes. It gives the distance from the medial axes to the original object boundary for each given point and can be used to exactly reconstruct the original part from the medial axes. The medial axes which go into all convex corners are known as spurs.
The MAT definition is similarly extended to three-dimensional objects. The MAT then consists of a set of medial surfaces (instead of axes) and a radius function. The radius function is used with a set of maximal spheres (instead of discs) to reconstruct the original object. It is contended that the set of medial surfaces of an object serve as better descriptors of shape than a b-rep model, and further that the information usually needed to identify shape features is made available in a more useful format. Information about the conjunction of surfaces is speculated to be usable in recognizing and determining features.