[Mp4-tech] On restriction on the total number of bits of data per MB

[Shevach Riabtsev]

Dear experts
According to A.3.1 (item "n") and A.3.2 (item "j") sections, there is a
restriction on the total number of bits of data per MB for both Main and
High profiles. 
Indeed, for Main Profile the following restriction is present:
    Number of bits of macroblock_layer( ) data for any macroblock is not
greater than 3200.
For High Profile the analogous statement is present:
     Number of bits of macroblock_layer( ) data for any macroblock is
not greater than 128 + RawMbBits.
e.g. for 4:2:0 case,  RawMbBits = 3072. 
How RT encoder's designer can gurantee that the size of a MB will not
exceed 3200 bits?  
Indeed, the only available mechanism effectively impacting on MB length
is the QP adjustment. For instance, one can estimate bit-size (or
bin-size for CABAC case) of a given MB basing on the variance and the
mean of the given MB. This estimation is true with some (may be very
high) significance level, but in my opinion this method can't gurantee
that the size would not exceed 3200. So, if the actual size of an "evil"
MB exceeds 3200 limit, what to do  - adjust QP and start again with the
same MB!!!
 Regards,  Shevach
Broadcom
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