UBC Theses and Dissertations
Indexing without invariants in model-based object recognition Beis, Jeffrey S.
This thesis presents a method to efficiently recognize 3D objects from single, 2D images by the use of a novel, probabilistic indexing technique. Indexing is a two-stage process that includes an offline training stage and a runtime lookup stage. During training, feature vectors representing object appearance are acquired from several points of view about each object and stored in the index. At runtime, for each image feature vector detected, a small set of the closest model vectors is recovered from the index and used to form match hypotheses. This set of nearest neighbours provides interpolation between the nearby training views of the objects, and is used to compute probability estimates that proposed matches are correct. The overall recognition process becomes extremely efficient when hypotheses are verified in order of their probabilities. Contributions of this thesis include the use of an indexing data structure (the /cd-tree) and search algorithm (Best-Bin First search) which, unlike the standard hash table methods, remain efficient to higher index space dimensionalities. This behavior is critical to provide discrimination between models in large databases. In addition, the repertoire of 3D objects that can be recognized has been significantly expanded from that in most previous indexing work, by explicitly avoiding the requirement for special-case invariant features. Finally, an incremental learning procedure has been introduced which extracts model grouping information from real images as the system performs recognition, and adds it into the index to improve indexing accuracy. A new clustering algorithm (Weighted Vector Quantization) is used to limit the memory requirements of this continual learning process. The indexing algorithm has been embedded within a fully functional automatic recognition system that typically requires only a few seconds to recognize objects in standard sized images. Experiments with real and synthetic images are presented, using indexing features derived from groupings of line segments. Indexing accuracy is shown to be high, as indicated by the rankings assigned to correct hypotheses. Experiments with the Best-Bin First search algorithm show that, if it is acceptable to miss a small fraction of the exact closest neighbours, the regime in which Кd-tree search remains efficient can be extended, roughly from 5-dimensional to 20-dimensional spaces, and that this efficiency holds for very large numbers of stored points. Finally, experiments with the Weighted Vector Quantization algorithm show that it is possible to incorporate real image data into the index via incremental learning so that indexing performance is improved without increasing the memory requirements of the system.
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