[Mp4-tech] Why in AVC standard mb_qp_delta is restricted to be in the range [-26,25]

[Shevach Riabtsev]

Dear experts 
According to 7.4.5 of the AVC standard:
The decoded value of mb_qp_delta shall be in the  range of -( 26 +
QpBdOffsetY / 2) to +( 25 + QpBdOffsetY / 2 ), inclusive. 
 If BitDepth is 8 then QpBdOffsetY=0 hence mb_qp_delta shall be in the
range [-26,25].  What reasons are beoynd the decision to constraint
mb_qp_delta value. 
I understand that it is unlikely that a reasonable Rate Control would
decide to change QP from 2 to 30 between two successive MBs, otherwise
serious visual artefacts (e.g. blockiness at MB edges) might be noticed.
Perhaps this is a reason for the restriction? 
On the other hand, we can imagine a scenario when say, 30 consecutive
MBs have CBP=0 (in this case qp_delta is not signalled, by the way this
is very frequent case in cartoon movies) and the Rate Control increments
QP by 1 per MB. Hence the first MB with non-zero CBP after the run of
CBP-zero MBs should carry mb_qp_delta=30. 
Regards,  Shevach 
Broadcom  Israel 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: /pipermail/mp4-tech/attachments/20080529/52ed3fc0/attachment-0001.html