A systolic algorithm for extracting regions from a planar graph: Zen-Chung Shih. Department and Institute of Information Science, National Chiao Tung University, Hsinchu 30050, Taiwan, Republic of China. R. C. T. Lee. Institute of Computer and Decision Sciences, National Tsing Hua University, Hsinchu 30043, Taiwan and Academia Sinica, Nankang, Taipei, Republic of China. S. N. Yang. Institute of Computer and Decision Sciences, National Tsing Hua University, Hsinchu 30043, Taiwan, Republic of China. Received January 6, 1988; accepted January 4, 1989