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

[Gyan]

On 22-07-2019 01:40 PM, Fu, Linjie wrote:
>> -----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."

So other than    `-c:v rawvideo -f rawvideo`, there is no combination 
that supports changing frame sizes?

Gyan