Getting frame number in display ordered

Hello,

I am trying to implement libxvidcore.a into my project (via  
libavcodec.a interface).
With self-patched libavcodec/libxvidff.c, I have got all of frames  
even using B-Frame.

Now I have a problem. In I-P-B pattern, decode order is not same as  
display order.
I want to know a pair of frame numbers in display and decode order for  
encoded frame.

Looking into xvid.h, it does not seems to keep original frame number  
(in display order)
inside xvid_enc_frame_t struct.

Would you please tell me how to get "display ordered frame number" for  
encoded frame?

Regards,
Takashi Mochizuki