Tetsuo Asano Naoki Katoh Koji Obokata Takeshi Tokuyama Combinatorial and Geometric Problems Related to Digital Halftoning. 58-71 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2616&spage=58 2002 conf/tfcv/2002 Theoretical Foundations of Computer Vision db/conf/tfcv/tfcv2002.html#AsanoKOT02