How do i obtain the reference frame?

"The reference frame has to be obtained first and it should be get subtracted with the current frame to acquire the frame with tracked ."

http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=06179193&tag=1

This is what i found out from IEEE... aside from acquiring the frame, how do I use image subtraction?

[Jan]

I get a Page-Not-Found when I click on your link.