[FFmpeg-devel] [PATCH] yuv4mpeg: add rough duration estimate and seeking.

wm4 nfxjfg at googlemail.com
Mon Jun 22 21:00:01 CEST 2015


On Mon, 22 Jun 2015 07:52:21 -0400
"Ronald S. Bultje" <rsbultje at gmail.com> wrote:

> Hi,
> 
> On Mon, Jun 22, 2015 at 6:24 AM, wm4 <nfxjfg at googlemail.com> wrote:
> 
> > On Mon, 22 Jun 2015 11:23:55 +0200
> > Hendrik Leppkes <h.leppkes at gmail.com> wrote:
> >
> > > On Mon, Jun 22, 2015 at 11:02 AM, wm4 <nfxjfg at googlemail.com> wrote:
> > > > On Sun, 21 Jun 2015 18:34:33 -0400
> > > > "Ronald S. Bultje" <rsbultje at gmail.com> wrote:
> > > >
> > > >> Hi,
> > > >>
> > > >> On Sun, Jun 21, 2015 at 5:17 PM, Hendrik Leppkes <h.leppkes at gmail.com
> > >
> > > >> wrote:
> > > >>
> > > >> > On Sun, Jun 21, 2015 at 10:41 PM, Ronald S. Bultje <
> > rsbultje at gmail.com>
> > > >> > wrote:
> > > >> > > ---
> > > >> > >  libavformat/yuv4mpeg.h    |  1 +
> > > >> > >  libavformat/yuv4mpegdec.c | 26 +++++++++++++++++++-------
> > > >> > >  2 files changed, 20 insertions(+), 7 deletions(-)
> > > >> > >
> > > >> >
> > > >> > What happens if a seek does not end up on a perfect frame boundary?
> > > >> > Wouldn't that entirely screw over any further reading?
> > > >> >
> > > >> > So is it really that "rough", or does it actually work precise
> > enough
> > > >> > to do that?
> > > >>
> > > >>
> > > >> It works for all y4m files I see in the wild, but the problem is that
> > the
> > > >> y4m spec is a little confusing. The frame header magic is "FRAME",
> > followed
> > > >> by optional per-frame options, and then a "\n". This is always empty
> > (i.e.
> > > >> "FRAME\n"), but in theory it could be non-empty and then this doesn't
> > work.
> > > >>
> > > >> I don't know how much we care about such theoretical files to make a
> > more
> > > >> difficult dur/seek implementation.
> > > >
> > > > Why not use generic seeking mode? Add AVFMT_GENERIC_INDEX to the format
> > > > flags. The utils.c will do seeking by doing byte seeks and using the
> > > > byte position of previous packets. If you seek forward, it will read
> > > > and skip packets until the target is reached (for parts of the file
> > > > that have not been indexed yet). This should be very reliable, but of
> > > > course seek speed will depend on I/O bandwidth.
> > >
> > > Generic seeking is rather expensive, and if a format is already strict
> > > CBR, doing a seek based on that is probably better.
> > > I wonder how hard it would be to write a re-sync function to find the
> > > next FRAME header when a seek does actually end up on the wrong spot.
> >
> > OK, but I'd rather have slow seeking than possibly randomly inaccurate
> > seeking. Maybe the seek mode could be switched on AVFMT_FLAG_FAST_SEEK.
> 
> 
> Why don't we do that once we start seeing files with frame-headers other
> than empty "FRAME\n" in the wild? Right now they are just theoretical...

I don't mind. If this happens, it'd be extremely visible anyway,
because the pixels won't line up.


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