motion vector

Hi
I am trying to extract motion vectors of all P - frames, from 3gp
video. I am trying to use mplayer for this purpose.

mplayer -lavdopts debug=32 xx.3gp
should give the motion vectors, if I what I understood from the man of
mplayer is right.

But when i do this, I do not get any motion vectors.

I am able to visualize them using -lavdopts vismv=1. But I need the
data.
thanks for your time
mark