[FFmpeg-devel] [PATCH, v3] fftools/ffmpeg_filter: add -autoscale to disable/enable the default scale

[Fu, Linjie]

> -----Original Message-----
> From: ffmpeg-devel [mailto:ffmpeg-devel-bounces at ffmpeg.org] On Behalf
> Of Fu, Linjie
> Sent: Monday, July 22, 2019 16:11
> To: FFmpeg development discussions and patches <ffmpeg-
> devel at ffmpeg.org>
> Subject: Re: [FFmpeg-devel] [PATCH, v3] fftools/ffmpeg_filter: add -
> autoscale to disable/enable the default scale
> 
> > -----Original Message-----
> > From: ffmpeg-devel [mailto:ffmpeg-devel-bounces at ffmpeg.org] On
> Behalf
> > Of Gyan
> > Sent: Saturday, July 20, 2019 13:29
> > To: ffmpeg-devel at ffmpeg.org
> > Subject: Re: [FFmpeg-devel] [PATCH, v3] fftools/ffmpeg_filter: add -
> > autoscale to disable/enable the default scale
> 
> > > diff --git a/doc/ffmpeg.texi b/doc/ffmpeg.texi
> > > index cd35eb49c8..99121b6981 100644
> > > --- a/doc/ffmpeg.texi
> > > +++ b/doc/ffmpeg.texi
> > > + at item -autoscale
> > > +Automatically scale the video according to the resolution of first frame.
> > > +Enabled by default, use @option{-noautoscale} to disable it. When
> > autoscale is
> > > +disabled, all output frames might not be in the same resolution and may
> > require
> > > +some additional explicit processing according to your final
> > rendering/output
> > > +destination. Disabling autoscale may not work in all situations. Therefore,
> > it
> > > +is not recommended to disable it unless you really know what you are
> > doing.
> > > +Disable autoscale at your own risk.
> >
> > Since the auto scaling happens at the end of the graph, what may the
> > "additional explicit processing" be?
> 
> Vpp processing may not be influenced,  a warning in transcode is needed.
> The expression seems to be improper, how about:
> 
> "When autoscale is disabled, all output frames of filter graph might not be in
> the same
> resolution and may be inadequate for encoder/muxer."
> 
> or other suggestions?
> 
> > > @@ -3640,6 +3642,12 @@ const OptionDef options[] = {
> > >       { "autorotate",       HAS_ARG | OPT_BOOL | OPT_SPEC |
> > >                             OPT_EXPERT | OPT_INPUT,                                { .off =
> > OFFSET(autorotate) },
> > >           "automatically insert correct rotate filters" },
> > > +    { "autoscale",        HAS_ARG | OPT_BOOL | OPT_SPEC |
> > > +                          OPT_EXPERT | OPT_INPUT,                                { .off =
> > OFFSET(autoscale) },
> > > +        "automatically insert a scale filter at the end of the filter graph if a
> > resolution"
> > > +        "change is detected. This ensures all frames are the same resolution
> > as the first frame"
> > > +        "when they leave the filter chain (this option is enabled by
> default)."
> > > +        "If disabled, some encoders/muxers may not support this mode."},
> > Which muxers can detect or check for prop changes within coded
> > bitstreams? Which encoders are known to be able to handle
> > changing resolution?
> 
> It's not supported currently (even in libvpx-vp9, since vp9 supports dynamic
> resolution in spec).
> 
> I don't intend to be so absolutely in doc,  will it be better for you to modify
> like:
> "If disabled, encoders/muxers won't support this mode currently."
> 
Are these modifications acceptable?
If no against, will update a new version. Thanks.

- linjie