[Mp4-tech] [H.264, VIDEO] maximum number of slices

[Gary Sullivan]

Tomo-san,
Regarding the question about maximum NAL unit size --
I believe the answer is Yes, a single NAL unit can contain more than 600 kbits.  I think the maximum NAL unit size for a VCL NAL unit of Main profile Level 3 is about 1,250,000 bytes when using CABAC (assuming the presence of a maximal amount of cabac_zero_word syntax elements in the NAL unit, making the limit effectively the full CPB capacity), and about 650,000 bytes when using CAVLC (assuming hitting the limit of 400 bytes per macroblock for every macroblock, plus a maximal-size slice header).  A non-VCL NAL unit can be bigger -- about 1,500,000 bytes, I think.
In the extreme CABAC case, the excess cabac_zero_word syntax elements and their associated emulation prevention bytes can just be discarded (by the decoder) if they arrive, to bring the limit back down to be essentially the same as for CAVLC.
But there is a constraint based on time also.  If the quantity of data in the set of NAL units for a picture is bigger than 311,040 bytes (720x576*1.5 divided by MinCR, where MinCR = 2) for level 3, then the decoder must be given proportionally more than 1/25th of a second of time to decode the picture. 
This aspect is somewhat similar to slice rate.  If the decoder is given more time to decode the data, the quantity of bytes or slices can increase.
Best Regards,
Gary Sullivan
+> -----Original Message-----
+> From: mp4-tech-bounces lists.mpegif.org 
+> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Tomo
+> Sent: Wednesday, March 28, 2024 1:38 PM
+> To: mp4-tech lists.mpegif.org
+> 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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