  In this paper, we study hyperkahler metric and practice GMN's construction of
hyperkahler metric on focus-focus fibrations. We explicitly compute the
action-angel coordinates on the local model of focus-focus fibration, and show
its semi-global invariant should be harmonic to admit a compatible holomorphic
2-form. Then we study the canonical semi-flat metric on it. After the instanton
correction, finally we get a re-construction of the generalized Ooguri-Vafa
metric.
