[FFmpeg-devel] [PATCH 07/12] ratecontrol: fix stuffing bits for 30000/1001 fps one frame vbv

[Baptiste Coudurier]

Hi Michael

On Thu, Jul 5, 2018 at 3:43 AM, Michael Niedermayer <michael at niedermayer.cc>
wrote:

> On Wed, Jul 04, 2018 at 11:35:09AM -0700, Baptiste Coudurier wrote:
> > ---
> >  libavcodec/ratecontrol.c | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/libavcodec/ratecontrol.c b/libavcodec/ratecontrol.c
> > index 49d169ba25..28bdddbad1 100644
> > --- a/libavcodec/ratecontrol.c
> > +++ b/libavcodec/ratecontrol.c
> > @@ -705,16 +705,16 @@ int ff_vbv_update(MpegEncContext *s, int
> frame_size)
> >          rcc->buffer_index += av_clip(left, min_rate, max_rate);
> >
> >          if (rcc->buffer_index > buffer_size) {
> > -            int stuffing = ceil((rcc->buffer_index - buffer_size) / 8);
> > +            int stuffing = rcc->buffer_index - buffer_size;
> >
> > -            if (stuffing < 4 && s->codec_id == AV_CODEC_ID_MPEG4)
> > -                stuffing = 4;
> > -            rcc->buffer_index -= 8 * stuffing;
> > +            if (stuffing < 32 && s->codec_id == AV_CODEC_ID_MPEG4)
> > +                stuffing = 32;
> > +            rcc->buffer_index -= stuffing;
> >
> >              if (s->avctx->debug & FF_DEBUG_RC)
> > -                av_log(s->avctx, AV_LOG_DEBUG, "stuffing %d bytes\n",
> stuffing);
> > +                av_log(s->avctx, AV_LOG_DEBUG, "stuffing %d bytes\n",
> stuffing>>3);
> >
> > -            return stuffing;
> > +            return stuffing>>3;
> >          }
>
> This would break VBV for most codecs
> buffer_index is a double, the if() condition is triggered if the current
> frame
> is too small. All the rounding has to be up as in the ceil() that was
> there.
> The stuffing that is added, is minimal, so a decrease would be expected to
> break cases occasionally
>
> Can you explain how this code fails, in what exact case
> (max/min rate/ buffer_size/fps) ?
>

Basically It doesn't work for IMX50, requiring exact frame size of 208541
bytes at 30000/1001 fps
and vbv delay of 3003

-- 
Baptiste Coudurier