Abstract: In freehand sketches,a single primitive would often be fractured.To obtain standard sketches and turn fractured strokes into standard primitives,this paper presented a judging and fitting method for fractured freehand multi-stroke based on tolerance zone.The preprocessing of the sketch is to transform all strokes into approximate polylines and to obtain all strokes categorized by type.And the turning points of approximate polylines are stored for further use.This method uses the trend of freehand strokes to construct imaginary strokes.And tolerance zone is established based on the stored turning points of freehand strokes and imaginary ones.The number of the sampling points of the strokes with smaller minimum bounding rectangle,which fall into the tolerance zone of the other stroke and the imaginary one,is counted,and it’s used to judge whether two strokes are fractured multi-strokes or not.Multi-strokes are clustered together,and as a result,the sketch is divided into a set of stroke group. According to the stroke classification of each stroke group,stroke groups are fitted with geometric primitives .The examples shown in this paper validates the feasibility of this method.It lays the foundation for further sketch recognition.

Key words: Fractured multi-stroke,Tolerance zone,Imaginary stroke,Stroke clustering,Curve fitting