在用ffmpeg转换视频到flv过程中，需要设置关键帧的间隔，以便在播放过程中实现精确定位。在网上查找了不少，最后发现这个指令有效：
-g 1 -keyint_min 2 。
http://blog.csdn.net/windflow/article/details/6228088
’-g 0’ to disable motion estimation completely (you have only I-frames, which means it is about as good as JPEG compression).
‘keyint_min (min-keyint)’
Minimum GOP size.