[Mp4-tech] Why MB-skip is not allowed in H.264 for I picture

[Shevach Riabtsev]

Thanks John
This is an interesting idea to hide the actual I picture in P-picture
envelope.  
But I see two difficulties:
1 . How to provide IDR pictures and hence random access for a decoder.
In other words an encoder has to produce IDR picture at the pre-defined
interval for random access. 
2. As a decoder sees a skip MB, it derives its MV (in the case when all
MBs are Intra, MV=0) and copies the corresponding 16x16 region from the
reference picture. In this case a spatial redundancy is not exploited.  
In some cases spatial redundancy gives more profit than temporal one,
even for Inter pictures. Unfortunately in H.264 standard only Inter SKIP
is defined. 
Intra Skip MB could have been defined as Intra16x16 MB with CBP=0 and
with a prediction mode equal the prediction mode of top or left MB.  
Regards
________________________________
From: John Smith [mailto:jsmith911 gmail.com] 
Sent: Tuesday, May 15, 2024 9:06 PM
To: Shevach Riabtsev
Subject: RE: [Mp4-tech] Why MB-skip is not allowed in H.264 for I
picture
You may encode picture as P, using intra-only modes. There will be some
overhead for mb_type but mb_skip will be available for use.
________________________________
From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Shevach Riabtsev
Sent: Tuesday, May 15, 2024 3:50 PM
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] Why MB-skip is not allowed in H.264 for I picture
Dear experts 
Unlike other standards (H.261, Mpeg2 and Mpeg4), spatial redundancy in
H.264 is much more exploited. Indeed, there are 9 intra prediction modes
for INTRA4x4, 4 modes for 16x16 versus DC/AC prediction of Mpeg4 or DC
only prediction in Mpeg2. Consequently for smooth I-pictures zero
coded_block_pattern is frequently observed.  
I wonder why mb-skip is forbidden in AVC for I-pictures. Is there any
reason? 
Regards, Shevach
Broadcom
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