@article{xie2007effective,
  title={An effective illustrative visualization framework based on photic extremum lines (PELs)},
  author={Xie, Xuexiang and He, Ying and Tian, Feng and Seah, Hock-Soon and Gu, Xianfeng and Qin, Hong},
  journal={IEEE Transactions on Visualization and Computer Graphics},
  volume={13},
  number={6},
  pages={1328--1335},
  year={2007},
  publisher={IEEE}
}