It's a bit older but the paper meight still be interesting. They track at least 3 color-coded markers and calibrate a magnetic tracker with the result to make it more acurate. The magentic tracker helps to speed things up and make everything less error-prone with it's inaccurate results

• Homepage: http://www.cs.unc.edu/~us/hybrid.html
• Paper: http://www.cs.unc.edu/~us/chen/paper.ps.gz
• Hardware: a camery and a magnetic tracker from http://www.ascension-tech.com/products/flockofbirds/
• Sourcecode: http://www.cs.unc.edu/~us/MagTrack/data.html

-- MarcusWolschon - 27 Feb 2002