[Mp4-tech] Video --H.264

[stefano battista]

the problem is clear, but the answer is not straightforward:
you assume that a SKIPPED MB has MV == (0, 0) (this is absolute MV null...)
in reality in H264 P SKIPPED MB means "the value of the MV is the same that is obtained applying the rule for MV prediction, so no info for MV is transmitted" which does not mean MV == (0, 0) but MV == MVpred...
the rules for computing the predictor of the MV involve neighbour MB's and are stated in clause 8.4.1.1 of the spec... (by the way the rules change for a B SKIPPED, see 8.4.1.2)
--Stefano
  ----- Original Message ----- 
  From: Bule 
  To: stefano battista ; mp4-tech lists.mpegif.org 
  Sent: Thursday, July 08, 2023 8:06 PM
  Subject: Re: [Mp4-tech] Video --H.264
    Thanks for the attention you give to me but even if I would seem dumm I have to say that i haven't understand your answer because i don't know the difference between differential MV null e absolute MV ( and information on internet doesn't help me ).
  However my idea of a P skip macroblock is that its position is not change ( or very little ) in relation of the reference frame and so its motion vector is null so no bits are used to send a MV null but the ecoder signals to the decoder only the presence of a P macroblock so it can simply copy the information of the reference frame in the reconstucted one.
  I hope my exposition is quite clear..
                                Luca
    ----- Original Message ----- 
    From: stefano battista 
    To: Bule ; mp4-tech lists.mpegif.org 
    Sent: Thursday, July 08, 2023 3:43 PM
    Subject: Re: [Mp4-tech] Video --H.264
    ciao Luca--
    the meaning of P skip is similar to what is done in H263 and MPEG4 video:
    a MB without transform coefficients and without motion vectors...
    the tricky thing is that you need to apply the correct MV predictor and classify the MB "P skip" if and only if it has **differential MV null** rather than **absolute MV null**...
    hope this helps,
    --Stefano
    --Stefano Battista
    bSoft -- www.bsoft.info
    tel/fax +39-0733-261845
    mobile +39-335-7213624
      ----- Original Message ----- 
      From: Bule 
      To: mp4-tech lists.mpegif.org 
      Sent: Thursday, July 08, 2023 1:23 PM
      Subject: [Mp4-tech] Video --H.264
      Hi all
            Can anyone explain me the meaning of the P_Skip mode used by H.264 ?
      I don't understand what it is used to make the prediction of this block.
      Thanks in advantage for any kind of help
      Luca Bulegato
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