On Fri, 11 Nov 2016 13:45:54 +0100 Nicolas George <[email protected]> wrote:
> Le primidi 21 brumaire, an CCXXV, wm4 a écrit : > > No, but I'm confident that you can find one after having thought about > > the problem for so long. > > Well, I have thought about it for long, and not only is this the > solution I have found, but I do not think it is a "bad idea" at all. OK, let's think about alternative approaches. How about not trying to let libavfilter do synchronizing subtitles and other streams in all cases? Why not just send nothing to the subtitle buffer sources if the subtitles are sparse and there is no new packet? If it's sparse, make it sparse. I assume the whole point of this exercise is to prevent excessive buffering (e.g. not trying to read the next subtitle packet, which might read most of the file, and necessitate buffering it in memory). E.g. if you overlay video on subtitles, you'd normally require new frames for both subtitles and video. If you'd treat subtitles like video and audio, you'd have to try to read the next subtitle packet to know, well, whether there's a new subtitle or whether to use the previous one. If I understood this correctly, you want to send empty frames every 100ms if the duration of a subtitle is unknown. Why is it 100ms? Does it make a difference if it's 100ms or a few seconds (or until EOF) until the subtitle is naturally terminated? Why not 1ms? This seems like a very "approximate" solution - sure, it works, but it's akin to using polling and sleep calls in I/O or multithreadeded code. Maybe a heartbeat on every video frame? What if there's no video stream? Video can change timestamps and frame count, video could go sparser un-sparse at certain points. This related idea of slaving sbuffersrc to a master vbuffersrc. This approach works on video output, while sparseness is really a problem at the source (i.e. demuxer). It's questionable how this would work with subtitles demuxed from separate files (which also might have A/V streams). It also works on the video output, while the issue of subtitles with unknown duration is mostly a demuxing issue. What happens if there's a video->sub filter, how would it send heartbeats? Would it require a new libavfilter graph syntax for filters that generate subtitles within the graph, and would it require users to explicitly specify a "companion" video source? The whole problem is that it's hard to do determine whether a new subtitle frame should be available at a certain point inside libavfilter, which in turn is hard because with how generic libavfilter wants to be. It seems to me that not libavfilter should handle this, but the one which feeds libavfilter. If it feeds a new video frame to it, but no subtitle frame, it means there is no new subtitle yet due to sparseness. There is actually no need for weird heartbeat frames. The libavfilter API user is in the position to know whether the subtitle demuxer/decoder can produce a new packet/frame. It would be crazy if the API user had to send heartbeat frames in these situations, and had to care about how many heartbeats are sent when. In complex cases (where audio/video/subs are connected in a non-trivial way, possibly converting to each other at certain points), the user would have to be careful which buffersinks to read in order not to trigger excessive readahead. Also the user would possibly have to disable "automagic" synchronization mechanisms in other parts of libavfilter. Even then, you would need to "filter" sparse frames (update their timestamps, produce new ones, etc). This sounds very complex. What about filtering subtitles alone? This should be possible? Why would libavfilter in general be responsible to sync subtitles and video anyway? It should do that only on filters which have both subtitle and video inputs or so. Why does this need decoder enhancements anyway? How about it just uses the API in its current extent, which applications have handled for years? Again, special snowflake libavfilter/ffmpeg.c. Btw. video frames can also be sparse (think of mp4s that contain slide shows). Are we going to get video heartbeat frames? How are all the filters going to handle it? Even for not-sparse video, there seem to be cases (possibly fixed now) where libavfilter just excessively buffers when using ffmpeg.c. I'm still fighting such cases with my own libavfilter API use. (Interestingly, this often involves sparse video.) (Oh, and I don't claim to have understood the problem in its whole extent. But I do have a lot of experience with subtitles and unfortunately also with using libavfilter in a "generic" way.) > And I would really appreciate if in the future you refrained from that > kind of useless empty remark. You can raise practical concerns, ask for > explanations or rationales, of course. But a purely negative reply that > took you all of three minutes in answer to the result of years of design > is just incredibly rude. I find your conduct incredibly rude as well. It's not nice to take every reply as an offense, instead of e.g. starting a discussion. It's also not nice to call my remarks "useless". No, pointing out that a solution is sub-optimal is not rudeness. Why are you asking me for a better solution? You're the one who wants subtitles in libavfilter, not me. Thus it's your responsibility to come up with a good design. If there's no good design, then it's a sure sign that it's not a good idea to have subtitles in libavfilter. Indeed, subtitles are incredibly complex, and there are many cases that somehow need to be handled in a generic way, and it's not necessarily a good idea to add this complexity to libavfilter, which designed to handle audio and video data (and even after years of work, isn't that good at handling audio and video at the same time). It's like trying to push a square peg through a round hole. I haven't come to the conclusion yet that this is the case, so hold that thought. Please try not to reply to every paragraph separately or so. This makes it hard to follow discussions. In fact, I won't take part in a discussion of that kind. It wastes so much time because you can get lost in meaningless details. _______________________________________________ ffmpeg-devel mailing list [email protected] http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
