Not sure why you need 3 points for 2D image registration. You need four variable...

hiroaki on Sept 13, 2007 | parent | context | favorite | on: Yahoo! MapMixer

Not sure why you need 3 points for 2D image registration. You need four variables here: two to determine the origin (x,y), one each for orientation and scale. You get these variables pinned with 2 points.