[FFmpeg-devel] Plans for libavfilter
Paul B Mahol
onemda at gmail.com
Fri Sep 17 19:02:56 EEST 2021
On Thu, Sep 16, 2021 at 8:31 PM Xiang Xiao <xiaoxiang781216 at gmail.com>
wrote:
> We are facing similar issues after working with libavfilter for more than
> one year. We are implementing a private solution for:
>
> - Partial graph configuration
> - Partial graph re-configuration
> - Global running API
>
> It's glad to see other people have the same feeling of the shortcoming
> design in avfilter and improve it in a general way.
>
Please refrain from off-topic and hijacking threads, thanks.
>
> On Thu, Sep 16, 2021 at 7:55 PM Nicolas George <george at nsup.org> wrote:
>
> > Since it is not healthy to keep everything for myself, here is a summary
> > of the projects I have in mind for the enhancement of libavfilter.
> >
> > The thing is, there are a lot of dependencies between these projects.
> > Working on them in a proper order would make achieving the goals easier.
> > On the other hand, if task B depends on task A, then working B without
> > addressing A involves a little more work for B, but also a lot more work
> > for A, and a lot more work for everything else that depends on A.
> >
> > This is one reason I am writing this mail: to make clear what needs to
> > be done, and how tasks depend on each-other and fit together.
> >
> > The earliest tasks are not very sexy. They are about making the code
> > clearer or more robust and generic rather than adding features. It is
> > always boring. But they are necessary, because parts of the process is
> > quite shaky, and we cannot afford to build on shaky code.
> >
> > If you want to help, please consider this and you will be welcome.
> >
> > Now that is established the specifics:
> >
> > - Multi-stage negotiation.
> >
> > That means negotiating properties that depend on other properties. I
> > think the colorspace stuff is in that case, it cannot be negotiated
> > until the pixel format is known.
> >
> > - Media type negotiation.
> >
> > That means that filters that do not care about the contents of what
> > they are filtering, filters that only act on the frame information
> > fields, can also not care if they are filtering audio or video or
> > other. There are a lot of these filters: pts manipulation, concat,
> > split, selection, etc.
> >
> > We cannot afford to have to update every one of them if we want to add
> > a new media type, so this is a prerequisite. And it is an aspect of
> > multi-stage negotiation, since pixel format or sample format can only
> > be negotiated once we know the media type.
> >
> > - Partial graph configuration.
> >
> > Right now, the negotiation happens in steps, first query_formats then
> > merge, then pick. But pick can be done on some links as soon as the
> > formats are merged to a singleton while other filters are still
> > delaying their query_formats, and once pick is done, the graph can
> > start running. That would allow filters with constraints more complex
> > than what AVFilterFormats can express. (We have a mechanism for that,
> > used by amerge and one or two other filters; I introduced it early and
> > it was a mistake, it is much too fragile and only converges in the
> > simplest cases.)
> >
> > - Partial graph re-configuration.
> >
> > If we can run a graph when not all filters are configured, then we can
> > de-configure and re-configure part of a graph. That would bring us the
> > support for changes in pixel format.
> >
> > - Global running API.
> >
> > Right now, we make libavfilter activate filters by pumping
> > request_frame() on one of the buffersinks, more or less randomly, and
> > we hope it will result in something useful. We need a better API, one
> > where we say libavfilter "start running these graphs", we push frames
> > on input as we have them, we are notified when frames arrive on
> > output.
> >
> > - Inter-filter threading.
> >
> > This is a big one, and quite self-explanatory. It goes hand to hand
> > with a global running API, because "start running these graphs" also
> > means "start as many threads as you need and keep them ready".
> >
> > Note that the network protocols in libavformat requires the same
> > thing. And I really do not want yet two other frigging threading
> > subsystems. This is why I have started slowly working on an unique
> > system suited for all needs, starting with both libavformat and
> > libavfilter but keeping also libavcodec in mind.
> >
> > - Out-of-band side-data.
> >
> > This is a mechanism to notify filters that the work they are currently
> > doing may become irrelevant, or other kinds of urgent information.
> > Filters can ignore it, and just do the work for nothing. But smarter
> > filters can take it into account and just skip until it becomes
> > relevant again.
> >
> > - Seeking.
> >
> > This is when applications notify the outputs they want to jump at a
> > specified time, and all the filters in the graph make sure the
> > instruction reach the inputs.
> >
> > This is probably rather easy and does not depend on many other things.
> > Out-of-band side-data is meant for that, among other things: notify a
> > filter "you can skip processing the frames you have queued, because we
> > are seeking anyway"; and if seeking fails, notify the user as fast as
> > possible "we tried seeking, it will not happen".
> >
> > A few things need ironing out, though. Do we want to handle it in
> > activate() or do we want a separate callback? Do we want a specific
> > mechanism for seeking or something more generic for all the messages
> > that go backwards in the graph? Do we want a queue for messages that
> > go backwards or would a single message be enough?
> >
> > - Subtitles.
> >
> > This one has been under the spotlight recently. What it means is
> > rather obvious. But it if far from trivial.
> >
> > For starters, it requires the negotiation of media type, because we
> > cannot afford to update all the utility filters for subtitles, they
> > have to work transparently.
> >
> > Then we have the issue that subtitles streams are sparse. A filter to
> > render a subtitle on a video frame needs to know if the next subtitle
> > is before or after the current frame, but if the subtitles are muxed
> > with the video, then the next subtitle can arrive only in several
> > minutes. The solution I have for that is heartbeat frames: subtitles
> > frames that contain no information except for their timestamp and mean
> > that no change has occurred. To generate them, link the buffersink for
> > the subtitles to the buffersink for the video.
> >
> > There are other issues to consider when designing subtitles: pixel
> > format and automatic conversions, overlapping, global styles, etc.
> >
> > - Data packets.
> >
> > This is about having packets of binary data inside libavfilter. That
> > means allowing to use bitstream filters like real filters in a graph
> > instead of the second, more limited, API just for them. It also means
> > codecs can be data → audio/video or audio/video → data filters.
> >
> > It requires the negotiation of media type.
> >
> > It also requires thinking how to fit the information present in
> > AVPacket into AVFrame. Some timestamps are not available, for example.
> >
> > I think that is all I have in mind for now.
> >
> > Regards,
> >
> > --
> > Nicolas George
> > _______________________________________________
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> > ffmpeg-devel at ffmpeg.org
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> >
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> >
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