[x264-devel] Need help to understand a piece of code

[Lingjiang Fang]

On Mon, 1 Feb 2021 22:39:36 +0300
BugMaster <BugMaster at narod.ru> wrote:

> On Mon, 1 Feb 2021 23:04:12 +0800, Lingjiang Fang wrote:
> 
> 
> > The code below puzzled me for a long time, can any give an
> > explanation for it ?  
> 
> > ...  
> 
> > I think this code want to solve the vbv overflow after
> > the current miniGOP, space is the predicted overflow size if there
> > is no consumption. 
> > But if we discard the cost of bbits, it means we tend
> > to decrease q and leak more bits from vbv buffer, won't it leads to
> > vbv underflow?  
> 
> Hi. By discard of the bbits do you mean this part?
> 
>             if( bbits * nb > bframe_cpb_duration * rcc->vbv_max_rate )
>                 nb = 0;
> 
> Yes, this part looks incorrect and was broken in:
> https://code.videolan.org/videolan/x264/-/commit/bb9b16b4722a1273885367f13f448516efe47ed1#2774abc5b23616ba88d9b08fa263902575a84d75_1714_1865
> It zeros B-frame bits in pbbits but doesn't zeros their duration in
> minigop_cpb_duration. Here is patch to fix it:
> https://code.videolan.org/BugMaster/x264/-/commit/a7e2c6ab8ff884e90f47a03258756fb0ad9f2e06

By zero bframe_cpb_duration, do you mean we don't need to consider
bframes if their consumptions larger than max input bits, and just
guarantee p frame won't cause overflow? 

> 
> 
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Regards,
Lingjiang Fang