[Mp4-tech] H264 VIDEO: Max. Number of Slices Per Picture

[Ahmed, Syed Mahmood]

Gary, all:
I was searching the archives and found an interesting exchange between Gary and Tomo san.
In his reply to Tomo san, Gary seems to suggest that the SliceRate (as provided in Table A-4, page 265 of the spec) provides an upper limit (max value) for the long-term average of the number of slices per picture.  
However, the spec in Section A.3.3, on page 263 says: 
"...shall satisfy  the constraint that the number of slices in picture n is less than or equal to MaxMBPS * ( tr( n )   tr( n   1 ) ) ÷ SliceRate ...".  
For Main Profile, Level 3, for PAL, the above calculates to 40500 * 0.04 / 22 = 73.6. 
It looks to me like the SliceRate is more like a constraint on the minimum number of macroblocks per slice 
Thanks
		[Mp4-tech] [H.264, VIDEO] maximum number of slices
		Gary Sullivan garysull windows.microsoft.com  <mailto:mp4-tech%40lists.mpegif.org?Subject=%5BMp4-tech%5D%20%5BH.264%2C%20VIDEO%5D%20%20maximum%20number%20of%20slices&In-Reply-To=785708.49318.qm%40web50904.mail.re2.yahoo.com> 
		Wed Mar 28 19:27:56 EDT 2007 
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		Tomo,
		That is almost true, but not quite true.  The the number of allowed slices increases if the decoder is given more time to decode a picture (as measured by the difference between decoding times, i.e., between CPB removal times).
		If fixed_frame_rate_flag is 1 (so the sequence is fixed at the maximum frame rate which in this case is 25 fps at 720x576 picture size), then it is true that the *average* number of slices per picture over a (very) long period of time cannot exceed 22.  But there's a difference between talking about the average over time and talking about the limit on any particular picture.
		The constraint is imposed on a per picture basis, based on the amount of time between decoding timestamps (CPB removal times).  For that picture size (720x756), the decoding times of frames can't be closer together than 1/25th of a second (for level 3).  But if the decoding times are farther apart, the number of allowed slices for the picture increases.
		The limit is really more time-based than picture-based.
		Best Regards,
		Gary Sullivan 
		+> -----Original Message-----
		+> From: mp4-tech-bounces lists.mpegif.org <http://lists.mpegif.org/mailman/listinfo/mp4-tech>  
		+> [mailto:mp4-tech-bounces lists.mpegif.org <http://lists.mpegif.org/mailman/listinfo/mp4-tech> ] On Behalf Of Tomo
		+> Sent: Wednesday, March 28, 2024 1:38 PM
		+> To: mp4-tech lists.mpegif.org <http://lists.mpegif.org/mailman/listinfo/mp4-tech> 
		+> Subject: [Mp4-tech] [H.264, VIDEO] maximum number of slices
		+> 
		+> Hi
		+> 
		+> I was told that (Main profile, Level 3) bitstream can have
		+> 22 slices (maximum) per frame.
		+> Is that correct?
		+> 
		+> Standard limits "slice rate" to 22 for this level and profile.
		+> But I do not think the slice rate == the number of slices per frame.
		+> 
		+> So, in case PAL/NTSC (D1) rate sources, what will be the maximum
		+> number of slices per frame?  Or is it indeed 22?
		+> 
		+> Also, somewhat relating question. Are there any NAL size limitation?
		+> Can a NAL be 600Kbits?  [600K is arbitrary I picked). 
		+> 
		+> Thanks
		+> 
		+> 
		+> Tomo
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