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

[Karsten Suehring]

Hi Shevach,
if you have a closer look at eq. 7-34 you will see that a wrap-around is 
used for calculating QPY (%(52+...))
You can actually reach every QP value in the next macroblock, e.g. by 
using a negative mb_qp_delta in your example.
Best regards,
Karsten
Shevach Riabtsev wrote:
>  
> 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 + 
> QpBdOffset//Y / 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 
>  
>  
>  
> 
> 
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