While much work has been done in Structural and Syntactical Pattern Recognition applied to drawings, most approaches are non-interactive. However, the recent emergence of viable pen-computers makes it desirable to handle pen-input such as sketches and drawings interactively. This paper presents a syntax-directed approach to parse sketches based on Relational Adjacency Grammars, which describe spatial and topological relations among parts of a sketch. Our approach uses a 2D grid to avoid re-scanning all the previous input whenever new strokes entered into the system, thus speeding up parsing considerably. To evaluate the performance of our approach we have tested the system using non-trivial inputs analyzed with two different grammars, one to design user interfaces and the other to describe floor-plans. The results clearly show the effectiveness of our approach and demonstrate good scalability to larger drawings.
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