Gradient descent method
Search line : gradient direction
The minimization in the search line must be  tangent to the contour of  f(x).
Problem: convergence is slow in most situations

So, using conjugate direction, for 2-dimension space,
we can find the minimization in 2 steps:
Find an initial direction, then search the minimization in this direction.
Find the conjugate direction, then search the minimization in the conjugate direction.

相关下载详见 “视觉计算研究论坛”「SIGVC BBS」：http://www.sigvc.org/bbs/thread-34-1-2.html