Asynchronous stdin

[qgustavor]

With the use of Web Streams videos could be downloaded then converted to other format without too expensive memory usage. I don't know if Emscripten supports that, as seems it blocks its thread, but if it's possible would be interesting.

[Kagami]

That's interesting question. There were discussions regarding live streams in #11 and #12 but using stdin for that haven't been considered.

Theoretically it might work. post-worker.js code need to be changed in order to be able to accept stdin events from main process. Then you start new ffmpeg command which should look like ffmpeg -f <input container> -i - -c <codec> -f <output container> -, pass input data and read processed data back with stdout events. There may be issues with binary data handling or some other unforseen pitfails but for the first glance it looks doable.

Do you want to try to implement this?

[Kagami]

Related:
emscripten-core/emscripten#4124
emscripten-core/emscripten#23 (comment)

Seems like Module.stdin is synchronous and we can't wait for onmessage call. Maybe if it's possible to periodically interrupt Emscripten and get opportunity to process events to worker's process code, then we can collect stdin data in some accumulator and synchronously send it to ffmpeg in Module.stdin handler later. Better to ask in Emscripten's bugtracker.

[qgustavor]

Well... some months ago I tried it, but based in videoconverter.js code: I managed to change the command. As it's synchronous exactly this problem happened: it don't receive message calls until the ffmpeg process stops.

I tried to find some way to make ffmpeg async, like Asyncify and Emterpreter, without success. After some time I gave up. As you're working actively on this I thought you already knew a way to fix this problem. Seems it's not that simple. 😕

[Kagami]

What exactly did you try with Asyncify and Emterpreter?
Maybe if you add emscripten_sleep to ffmpeg's stdin handler (need to patch it for that) then worker process will have a chance to receive messages from main process. I haven't tried that and don't know much about Emscripten internals, just some wild guesses.

[qgustavor]

I wanted do this, but in fact I don't even managed Emscripten to compile videoconverter.js: I use Windows, so shell scripts don't work. I tried to set up it in a Ubuntu machine but it also failed. Then I tried to set it using ffmpeg source directly, so I could use it on Windows.

After that, IIRC, I abandoned the idea, mostly because I was trying to implement it in Direct MEGA and after some problems I stopped working on this project.

[Kagami]

I can only suggest you to try to build ffmpeg.js in Ubuntu VM. You need to have emsdk installed (see here) and some basic dev packages. Then clone this project, checkout all submodules and type make ffmpeg-worker-mp4.js command (this target should be fastest to build).

[qgustavor]

Understood. I will set up a VM and try it again.

[qgustavor]

I tried in one of those lightweight Ubuntu distros, then it failed. I tried again with Ubuntu Server 16.04.1 i386 and I found that I was missing some steps and doing other wrong.

But it still failed. This was the last error that I couldn't find a solution: https://i.imgur.com/JZLWvWP.png I only found bug reports for this problem. From some of those maybe the problem is the distro, so I will try to run the amd64 version Edit: my computer doesn't support virtualization, so I can't run it. If you know the solution it will help a lot.

[Kagami]

I haven't encountered issues with lame build. I'm using 64bit distro/compilers though, so yes, that might be the case.
You can also try without lame, it's not required for your tests: delete --enable-libmp3lame and build/lame/dist/lib/libmp3lame.so \ lines and try again.

[qgustavor]

I tried without lame and in a 64bit distro (Cloud9). What I'm doing:

• clonning the repository
• git submodule init && git submodule
• sudo apt-get install emscripten
• installing auto-conf, automake and libtool.
• make ffmpeg-worker-mp4.js

Is some step wrong?

[bbf]

@qgustavor install pkg-config as well:
sudo apt-get install pkg-config

[qgustavor]

I retried in a fresh Cloud9 Ubuntu x64 instance and it still failed. There are the log files for make ffmpeg-worker-mp4.js and make ffmpeg-worker-webm.js.

[Kagami]

You're still building lame, see my previous comment how to disable it.
Also, I realized what's wrong with lame. Its configure script checks for xmmintr (SSE intrinsics) and on your machine it returns true while for me it's false and so build system doesn't try to compile vectorized code which apparently has some issues on Emscripten. I think I'll need to patch lame's configure to fix that (or maybe find some other hack with configure/Emscripten options).