3.3.2 TWO FEATURE ABSTRACTION METHODS
A technique for making a problem more tractable is a change of dimension [Glinert & Gonczarowski 87]. Ideally, the NP-complete enumeration of all possible paths required of graph matching is replaced by an algorithm operating on a simpler model. The approach of Differential Depth Perception Filter (DDPF) used in this work is essentially such a reduction [Gadh et al 89].
Feature recognition via the Differential Depth Perception Filter is analogous to the manner in which humans view and discern individual features from a part. A human being best recognizes a part's features by viewing it from several directions. Using information of depth (obtained through stereo vision), outline or silhouette, color, shading, texture, etc., features can be discriminated from the part as a whole [Gadh90A].
The differential depth perception filter primarily uses "silhouette" information to discern protrusions and depressions on a given part. Each line of sight provides a different silhouette and each outline is incorporated into the filter model. As such, the filter is direction dependent.
Because silhouettes contain no faces, the filter model contains no faces, only the edges of the outlines. Because they are not included in the silhouettes, a substantial number of edges from the original model are not included in the filter model. This filtered model has a substantially smaller number of topological entities than the original model. This is a strategic reduction of the search space in which features, as characterized by the silhouettes, must be recognized.
The DDPF gathers 2D silhouettes of a 3D solid model taken from different perspectives. It then compounds these silhouettes to identify protrusions and depressions. The DDPF approach allows for a "fuzzier" recognition of an object. Non-essential variations in topology do not come into play in the identification of features. As such, all protrusions and depressions of any variety of shapes may be identified. The protrusions are further categorized into ribs and bosses.