http://www.songho.ca/math/homogeneous/homogeneous.html
http://blog.csdn.net/zigzagfist/archive/2008/09/10/2910812.aspx
http://blog.csdn.net/Wu_Xiangwei/archive/2010/04/23/5519298.aspx
http://en.wikipedia.org/wiki/Homogeneous_coordinates#cite_note-0
http://en.wikipedia.org/wiki/Projective_geometry