The Lit Sphere: A Model for Capturing NPR Shading from Art
Peter-Pike Sloan, William Martin, Amy Gooch, Bruce Gooch
Graphics Interface 2001 

  @inproceedings{Sloan:2001:TLS,
    title = "The Lit Sphere: A Model for Capturing NPR Shading from Art",
    author = "Peter-Pike Sloan and William Martin and Amy Gooch and Bruce Gooch",
    booktitle = "Graphics Interface 2001",
    month = jun, 
    year = 2001,
    pages = "143--150",
  }