Abstract: Compound surfaces are approximated by Zmap model to demarcate cutting areas against the remaining untouched zone,resulting in a binary image of each cutting layer with black pixels representing the cutting areas and white pixels showing the remaining zone.Run-length coding is under taken to describe the configuration of black image in each cutting layer.A boundary-tracing algorithm extracts boundaries by sequentially linking the terminal nodes according to the connectivity information of runs.The formed loops are sorted and arranged into a tree structure to represent the topological correlation of the cutting areas in a cutting layer. The presented method is simple and effective. It can recognize contours and islands. Experimental machining results demonstrate the feasibility of this algorithm.