[FFmpeg-devel] [PATCH v3] avcodec: Add explicit capability flag for encoder flushing

[Josh Allmann]

On Mon, 13 Apr 2020 at 04:39, Anton Khirnov <anton at khirnov.net> wrote:
>
> Quoting Philip Langdale (2020-04-11 01:47:43)
> > We've been in this fuzzy situation where maybe you could call
> > avcodec_flush_buffers() on an encoder but there weren't any encoders
> > that supported it except maybe audiotoolboxenc. Then we added flush
> > support to nvenc very intentionally, and it worked, but that was more a
> > coincidence than anything else. And if you call avcodec_flush_buffers()
> > on an encoder that doesn't support it, it'll leave the encoder in an
> > undefined state, so that's not great.
> >
> > As part of cleaning this up, this change introduces a formal capability
> > flag for encoders that support flushing and ensures a flush call is a
> > no-op for any other encoder. This allows client code to check if it is
> > meaningful to call flush on an encoder before actually doing it.
> >
> > I have not attempted to separate the steps taken inside
> > avcodec_flush_buffers() because it's not doing anything that's wrong
> > for an encoder. But I did add a sanity check to reject attempts to
> > flush a frame threaded encoder because I couldn't wrap my head around
> > whether that code path was actually safe or not. As this combination
> > doesn't exist today, we'll deal with it if it ever comes up.
> >
> > Signed-off-by: Philip Langdale <philipl at overt.org>
> > ---
>
> I'm missing two things here:
> - motivation in the commit message (and possibly in the doxy too) - why
>   is this needed and how is it better than just closing the encoder and
>   creating a new one

One use case is segmented encoding: processing one segment at a time,
where closing and re-opening a new encoder is expensive (as is the
case with HW / nvenc). The original writeup for the motivation leading
up to "encoder flush" can be found here:
https://patchwork.ffmpeg.org/project/ffmpeg/patch/1574125994-7782-1-git-send-email-joshua.allmann@gmail.com/

Josh