[Mp4-tech] Video --H.264

[Gary Sullivan]

You're right that the spirit of this is to send no bits for a P_Skip
macroblock -- all that is sent is the indication that the macroblock is
skipped.  However, there is no assumption that the macroblock cannot be
moving.  Instead, the macroblock is predicted by inferring motion based
on what is happening in neighbouring macroblocks.
This can be very helpful, for example, in dealing with a moving camera
(global motion).  You can basically take care of a camera pan by coding
one motion vector for the upper-left macroblock and just letting the
P_Skip and "Direct" motion vector inference take care of predicting the
whole picture.
By defining the skipped macroblock in this way, we avoided the need to
have a separate feature called "global motion compensation".  As a
result, some people say that this standard does not have global motion
compensation, but they are basically wrong.  We just built that
capability into the basic motion representation process instead of
making it a separate switchable "tool".
Best Regards,
Gary Sullivan
________________________________
	From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Bule
	Sent: Thursday, July 08, 2023 11:07 AM
	To: stefano battista; mp4-tech lists.mpegif.org
	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 <mailto:bautz tin.it>  
		To: Bule <mailto:bulegato libero.it>  ;
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 <mailto:bulegato libero.it>  
			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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