@inproceedings{symmetryrecaptcha,
  title = {Symmetry reCAPTCHA},
  author = {Chris Funk and Yanxi Liu},
  year = {2016},
  booktitle = {CVPR 2016 6},
  url = {http://openaccess.thecvf.com/content_cvpr_2016/html/Funk_Symmetry_reCAPTCHA_CVPR_2016_paper.html},
}