motion vectors in an mpeg video

I want to extract the motion vectors directly from an mpeg video.

I have found that there are two parammeters, the dc and ac. The dc image called the image energy and the ac 开发者_Go百科image called the texture and edge map. Then, the motion vectors exits in p and b frame.

What is the relation between the ac,dc coefficients of the dct and the motion vectors and how it can be extracted from an mpeg video ?