[XviD-devel] Confusion about Interlacing Implemantation

[mayfig]

Hey,

   Confused about some implementation of Interlacing argumant in MotionVector
of MacroBlock ,

   in ISO/IEC_14496_2_2001_PDF_Version, I noticed that in 6.2.6
MacroBlock---Page 54, in the 10th Item of the Table:

If the condition
   
    if((derived_mb_type == 0|| derived_mb_type ==1) && (vop_coding_type == "P" 
|| vop_coding_type == "S" && !mcsel)))

satisfied ,then there will be some steps below:
    motion_vector('forward");
    if(interlaced && field_prediction)
      motion_vector("forward");

My confusion is :

1. Does it mean ,when the condtion " if(interlaced && field_prediction)"
satisfied, the first " motion_vector("forward")" will be neglected,and only
completes the second operation of " motion_vector("forward")".
   Am I right?
2.What the difference of the first and the second  motion_vector('forward")?

3.I noticed that in the function of decoder_pframe (in the decoder.c of Xvid
1.0.2),there are

if (dec->interlacing && mb->field_pred) {
		get_motion_vector(dec, bs, x, y, 0, &mb->mvs[0], fcode, bound);
		get_motion_vector(dec, bs, x, y, 0, &mb->mvs[1], fcode, bound);
}
   Is it complete? why only twice not fourth times ? how to deal with
mb->mvs[2]
mb->mvs[3]?

   Sorry to take your time , Could anyone give me an explanation?

   Thanks.
    


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