Deferred interpretation of gray-scale saddle features for recognition of touching and broken characters

Interpretation of gaps and touching characters continues to challenge current OCR designs. We approach this and other difficult problems in character recognition by deferring decisions to a stage where a character-specific knowledge base can be applied to the problem. We show how to extract and interpret saddle ridge features, at locations where there is either a narrow gap or a thin stroke. Since the color of the ideal image at these points cannot be locally deduced reliably from the features, special treatment is needed. Mathematically a saddle ridge is one where, roughly, there are strong eigenvalues of the Hessian of the gray scale surface that are of opposite sign. The recognition module is based on the matching of subgraphs homomorphic to previously defined prototypes. It generates candidate matchings of groups of input features with each part of the prototype. In this context, each saddle ridge is decided to be a piece of a stroke or a separation between strokes. The quality of each grouping is measured by the cost of transformations carrying the candidate features into the prototype.