Re: [FFmpeg-devel] [RFC] fftools/ffmpeg and libavdevice/sdl issue

[Zhao Zhili <quinkblack@foxmail.com>]

> On Dec 13, 2023, at 17:08, Anton Khirnov <anton@khirnov.net> wrote:
> 
> Quoting Zhao Zhili (2023-12-12 18:27:39)
>> Now it's time to talk about the libavdevice/sdl issue.
>> 
>> SDL output is broken with ffmpeg multithread refactor. SDL 'muxer' write_header
>> and write_packet must be run in the same thread. And to make it work portable
>> and reliable, SDL 'muxer' must be run in main thread. It's a common requirement
>> for render to be run in main thread.
>> 
>> There are at least two trac tickets for the same issue: #10644 and #10649.
>> 
>> And there are two patches for the issue:
>> https://patchwork.ffmpeg.org/project/ffmpeg/patch/20230918063728.198377-1-haihao.xiang@intel.com/
>> https://patchwork.ffmpeg.org/project/ffmpeg/patch/20231101090115.10655-1-angus.chen@intel.com/
>> 
>> There patches might work on Linux, but not portable.
>> 
>> A simple workaround is pipe the output of ffmpeg to ffplay:
>> 
>> ./ffmpeg -re -i /e/video/cctv.mp4 -an -f yuv4mpegpipe - |ffplay -
>> 
>> To fix it, another thread can be used to drive transcode rather than main thread.
>> A main loop should be created on main thread, and prepared to handle any special
>> tasks like render. It sounds a lot of work. I'm not sure if it worth for a single use case.
>> However, maybe we can have a libavfilter/vsink_preview after that.
>> 
>> What should we do?
> 
> Honestly I don't see how this could be done in ffmpeg CLI without
> disgusting hacks, but before that the question is: why is there an SDL
> "muxer" and why would anyone want to use it in ffmpeg CLI? What actual
> use cases does it serve that cannot be better handled otherwise?

I pasted wrong tickets, they are #10625 and #10649.
https://trac.ffmpeg.org/ticket/10625

The use case is realtime preview. The function lavd/sdl2 provides is limited. A vsink_preview
filter is more appropriate. It has the same thread issue with libavfilter.

> 
> -- 
> Anton Khirnov
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