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Francisco Alvaro, Richard Zanibbi. A shape-based layout descriptor for classifying spatial relationships in handwritten math. ACM Symposium on Document Engineering (DocEng), 2013. pp. 123-126.

We consider the difficult problem of classifying spatial relationships between symbols and subexpressions in handwritten mathematical expressions. We first improve existing geometric features based on bounding boxes and center points, normalizing them using the distance between the centers of the two symbols or subexpressions in question. We then propose a novel feature set for layout classification, using polar histograms computed over points in handwritten strokes. A series of experiments are presented in which a Support Vector Machine is used with these new features to classify spatial relationships of five types in the MathBrush corpus (horizontal, superscript, subscript, below, and inside (e.g. in a square root)). The normalized geometric features provide an improvement over previously published results, while the shape-based features provide a natural representation with results comparable to those for the geometric features. Combining the features produced a very small improvement in accuracy.