when i use flannBaseMathcer to match 2 sets of orb descriptor, it occured an exception : unsigned long exception.
http://blog.csdn.net/holybin/article/details/40926315
this blog says that orb descriptior has a type of uchar, but flannMatcher doesn‘t support a uchar matcher like orb or brief.
so I used the BFMatcher and the exception vanished!