[Mp4-tech] [h.264]Why is there a constraint on max number ofbins...?

[Gary Sullivan]

In a sense, there was a fear that the compression performance could be too good.  We had to stop that from happening, for job security reasons.  :-)
Actually...
The issue (discovered by Frank Bossen of NTT DoCoMo) is that in certain highly-artificial circumstances (such as the deliberate creation of "evil" bitstreams), it might be possible for the intent of imposing practicality by placing a constraint on the maximum bit rate to be circumvented by a huge explosion in the number of decoded symbols produced per coded bit by the CABAC parsing process.  If not corrected, this situation would have allowed the creation of a bitstream that was technically conforming but that could not be decoded properly by ordinary practical decoder implementations.
So an explicit bound was placed (at the picture level) on the amount of compression that could take place between the bin representation (which is like an ordinary VLC representation) and the output of the arithmetic encoder.  If the bound is exceeded, the encoder must insert some "filler bits" to keep the total under the bound.
Best Regards,
Gary Sullivan
+> -----Original Message-----
+> From: mp4-tech-bounces lists.mpegif.org 
+> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of 
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+> Sent: Thursday, March 03, 2024 3:12 PM
+> To: mp4-tech lists.mpegif.org
+> Subject: [Mp4-tech] [h.264]Why is there a constraint on max 
+> number ofbins...?
+> 
+> Will anyone please explain the underlying reason for
+> this constrain?
+> 
+> [quote from the standard doc]
+> When entropy_coding_mode_flag is equal to 1, the
+> number of bins resulting from decoding the contents of
+> all VCL NAL units of a coded picture shall not exceed
+> ( 32 ¡Â 3 ) * NumBytesInVclNALunits + 96 *
+> PicSizeInMbs.
+> 
+> 
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