On-line segmentation of freehand sketches by knowledge-based nonlinear thresholding operations

This paper describes an on-line procedure for segmenting freehand sketches into 2D primitives and smooth curves by knowledge-based nonlinear thresholding operations. Drawing position, direction, speed and acceleration information is used for establishing fuzzy knowledge and inferring user's intention. The procedure can be divided into three stages: (1) obtaining corner points based on the fuzzy knowledge and adaptive thresholds, which includes detecting “obtuse” and “acute” corner points; (2) finding smooth join points, during which stage we use both curve linearity and curvature; and (3) classifying the sketches into the corresponding curves according to curve linearity and changes of curve convexity. The method is illustrated and tested with a number of sketches.