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

[Marton Balint]

On Wed, 22 May 2019, Alexander Strasser wrote:

> 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.
>

Paul, let me know what you prefer.

Thanks,
Marton