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. On Thu, Sep 16, 2021 at 7:55 PM Nicolas George <geo...@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 > _______________________________________________ > ffmpeg-devel mailing list > ffmpeg-devel@ffmpeg.org > https://ffmpeg.org/mailman/listinfo/ffmpeg-devel > > To unsubscribe, visit link above, or email > ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe". > _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org https://ffmpeg.org/mailman/listinfo/ffmpeg-devel To unsubscribe, visit link above, or email ffmpeg-devel-requ...@ffmpeg.org with subject "unsubscribe".
