[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 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."

- linjie