As pen-based devices become more popular, it has motivated several novel research issues in the area of pen computing. One interesting and practical problem is the storage and retrieval of hand-drawn sketches. A sketch can consist of handwritten notes, symbols, free-form hand-drawings, annotations on a document etc. It will be very useful to store the sketches in a database and then retrieve them later. In our prior work, we proposed a method for hand-drawn sketch retrieval by performing stroke-by-stroke matching and by considering the spatial relationship between them. In this paper, we propose a method to simplify a sketch and perform matching in a hierarchical manner. With this approach, a shaded region or a region of complex strokes can be detected automatically and represented as a single hyper-stroke. In the matching stage, the similarity is computed by combining the similarity scores across the feature spaces at each level and the similarity of the stroke hierarchies. A sketch may be better represented by exploiting the structural relationship between the strokes. In addition, as a general rule to the hierarchical approach, it has the advantage of reduced computation thus the retrieval process would speed up.
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