[FFmpeg-devel] [PATCH 4/7] avformat/avformat: add a flag to signal muxers that support storing cropping values

[Anton Khirnov]

Quoting James Almer (2023-10-10 13:21:42)
> On 10/10/2023 8:16 AM, Anton Khirnov wrote:
> > Quoting James Almer (2023-10-10 13:13:46)
> >> On 10/10/2023 8:09 AM, Anton Khirnov wrote:
> >>> Quoting James Almer (2023-10-07 18:25:00)
> >>>> Signed-off-by: James Almer <jamrial at gmail.com>
> >>>> ---
> >>>>    libavformat/avformat.h | 1 +
> >>>>    libavformat/mux.c      | 8 ++++++++
> >>>>    2 files changed, 9 insertions(+)
> >>>>
> >>>> diff --git a/libavformat/avformat.h b/libavformat/avformat.h
> >>>> index 9e7eca007e..c099ca8a01 100644
> >>>> --- a/libavformat/avformat.h
> >>>> +++ b/libavformat/avformat.h
> >>>> @@ -500,6 +500,7 @@ typedef struct AVProbeData {
> >>>>                                            The user or muxer can override this through
> >>>>                                            AVFormatContext.avoid_negative_ts
> >>>>                                            */
> >>>> +#define AVFMT_CROPPING     0x80000 /**< Format supports storing cropping values */
> >>>
> >>> I have mixed feeelings about this patch, for a bunch of reasons:
> >>> * It is quite ad-hoc - we don't do this for other side data types, and
> >>>     this approach would not scale if we did.
> >>> * If we do want to signal this, we probably want to distinguish between
> >>>     support for global and per-packet values.
> >>
> >> This patch came to be after some discussion from the first iteration of
> >> the set, where concerns about the cropping information being silently
> >> lost if apply_cropping was disabled during a transcoding or codec copy
> >> scenario where the output format didn't support storing said values.
> >>
> >>> * How do you expect this to be useful to the callers? I don't see this
> >>>     flag actually being used in the ffmpeg CLI patch.
> >>
> >> It's a format flag. Muxers use it, and the generic mux.c code will print
> >> a warning if needed.
> > 
> > Why is it public then?
> 
> So the library user can know beforehand if the cropping information will 
> be lost or not, and choose accordingly. The warning is there for the 
> cases where it was ignored.
> 
> I can add a check to the CLI for it, but other than to abort or outright 
> ignore the user request to not apply cropping i don't see what it could 
> do, as mux.c already prints a warning.

As I said above - there are MANY similar bits of information that the
muxer may or may not write. You cannot handle all of them with this
approach, and singling out cropping seems horribly ad-hoc to me.

-- 
Anton Khirnov