[FFmpeg-devel] Plans for libavfilter

[Paul B Mahol]

On Thu, Sep 16, 2021 at 1:55 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.
>

This is short sighted and irrelevant and also source  of confusion for
users.

I really see no way to tell you to stop insisting on this nonsense. So I
kindly ask you
yet again to stop further work you planned on libavfilter for everyone else
good outcome.



> - 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.
>
>
This is the only useful entry so far.


> - 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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