[FFmpeg-devel] [PATCH] avfilter/f_loop: do not loop if loop size is 0

[Alexander Strasser]

Hi!

On 2019-05-20 20:51 +0200, Marton Balint wrote:
>
> On Mon, 20 May 2019, Gyan wrote:
>
> > On 20-05-2019 02:18 AM, Marton Balint wrote:
> > >
> > > On Sun, 19 May 2019, Paul B Mahol wrote:
> > >
> > > > On 5/19/19, Marton Balint <cus at passwd.hu> wrote:
> > > > >
> > > > > On Sun, 19 May 2019, Paul B Mahol wrote:
> > > > >
> > > > > > On 5/19/19, Marton Balint <cus at passwd.hu> wrote:
> > > > > > > Fixes infinte loop with -vf loop=loop=1.
> > > > > > >
> > > > > > > Possible regression since ef1aadffc785b48ed62c45d954289e754f43ef46.
> > > > > > >
> > > > > > > Signed-off-by: Marton Balint <cus at passwd.hu>
> > > > > > > ---
> > > > > > >  libavfilter/f_loop.c | 2 +-
> > > > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > > >
> > > > > > > diff --git a/libavfilter/f_loop.c b/libavfilter/f_loop.c
> > > > > > > index d9d55f9837..3da753dd1e 100644
> > > > > > > --- a/libavfilter/f_loop.c
> > > > > > > +++ b/libavfilter/f_loop.c
> > > > > > > @@ -343,7 +343,7 @@ static int activate(AVFilterContext *ctx)
> > > > > > >
> > > > > > >      FF_FILTER_FORWARD_STATUS_BACK(outlink, inlink);
> > > > > > >
> > > > > > > -    if (!s->eof && (s->nb_frames < s->size || !s->loop)) {
> > > > > > > +    if (!s->eof && (s->nb_frames < s->size ||
> > > > > > > !s->loop || !s->size)) {
> > > > > > >          ret = ff_inlink_consume_frame(inlink, &frame);
> > > > > > >          if (ret < 0)
> > > > > > >              return ret;
> > > > > > > --
> > > > > >
> > > > > > I think better fix is to change default and minimal
> > > > > > allowed loop size to
> > > > > > 1.
> > > > > > Does that sounds ok to you?
> > > > >
> > > > > Well, looping the whole length of the input would be more
> > > > > intuitive to me
> > > > > as the default.
> > > >
> > > > That would require infinite memory.
> > >
> > > So as the reverse filter. As long as it is properly documented that
> > > the looped stuff is kept in memory so the user should not use this
> > > for long clips, then I think it is fine.
> >
> > I disagree. Yes, for loop with only loop specified, it would be
> > intuitive to loop the whole stream, but relying on users to exercise due
> > diligence can't be counted upon. We're talking about a scenario where
> > the user hasn't bothered to specify the size variable because they don't
> > know or don't care or are sloppy. They won't take heed of the
> > documentation until the command fails. The defaults should be robust
> > against lax use.
>
> Fair enough, although I never liked the idea that we make the tool less
> handy because we target unexperienced users.

FWIW, I guess the default behaviour of looping the complete input is much
better from a user perspective.

The typical users that have a need to loop a small clip will probably not
want to spefify a size in frames and will probably not really understand
why they need to specify one.

The typical users that want to loop a particular number of frames,
potentially at given offset into the specified input will probably read
the manual and in turn quickly find and use the size and/or start
options.


> Anyway, I don't have strong feelings about this, maybe my patch has the
> benefit of keeping existing behaviour (which is similar to how aloop works)
> in contrast to what paul suggested, but I don't really mind Paul's or Bela's
> solution either.

I have no strong feelings either, but it seems the behaviour
implemented by your patch seems ato fit more into the overall
situation too.


  Alexander