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

[Paul B Mahol]

On 5/23/19, Marton Balint <cus at passwd.hu> wrote:
>
>
> 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.

Initial patch is fine, as additional patch you could print warning if size is 0.

>
> Thanks,
> Marton
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