Ekaitz's tech blog:
I make stuff at ElenQ Technology and I talk about it

When you battle using your arguments that’s called a discussion… That’s exactly what I’ve been doing for a couple of days with ffmpeg: I’ve been using arguments trying to reach an understanding.

I wanted to record my screen, and a couple of cameras, for reasons you’ll know about later, and I didn’t want to play around with new GUI programs and configuration so I decided to go for a simple ffmpeg setup.

You probably had played with ffmpeg in the past. It has tons of different input arguments and options. It’s crazy.

Most of my previous times using it were just video conversions and it’s as simple as choosing the right extensions for files, but when it comes to video and audio recording it gets complicated. I have no idea about video and audio encodings and I don’t really have the time to dig in such an exciting topic. I searched on the Internet for examples and I found some: cool.

I played around with multiple inputs and outputs, I changed arguments I can’t even remember now and it kinda worked until I decided to record my voice at the same time. Delay.

What to do then?

Just go to the internet an keep searching.

I found a project called vokoscreen that now is archived because it’s migrating from ffmpeg to gstreamer (I also struggled with gstreamer in the past but that’s another story) but it worked fine. It was in the repos of my distro, it only asked me to install one dependency, a couple of megs only… Great!

I tried to make a screencast and the audio worked like a charm. I went for the code, read it and realized the arguments it uses to call ffmpeg are easy to find.

Even better, in the program itself there’s a button to show a log of what it does and it dumps the exact call it does.

With that and some extra things I learned from the investigation in the deep abyss of the Internet, boom! There it goes:

ffmpeg
    -y -f x11grab -draw_mouse 1 -framerate 25 -video_size 1920x1080 -i :0+0,0 \
    -f alsa -ac 2 -i default \
    -pix_fmt yuv420p -c:v libx264 -preset veryfast \
    -c:a libmp3lame -q:v 1 -s 1920x1080 -f matroska \
    output.mkv

Today in half an hour I solved the thing I’ve been struggling with a couple of days. But I think I wouldn’t be able to solve it if I didn’t struggle with it last days… I don’t know.

The good thing is I learned a couple of things from this I’ll write down here to avoid forgetting them:

Multiple inputs

Like the command in the example, ffmpeg can get multiple inputs. In the case of the example, they are x11grab (my screen) and alsa‘s default input (the microphone). More inputs can be combined, like music playing in the background or whatever you want.

Multiple outputs

There’s also the chance to put multiple outputs there just like the multiple input thing does but in the output part of the command¹.

Pipe

You can even pipe the command to a different one, like:

ffmpeg [...] - | ffplay -i -

In this case you can use one of the outputs to record to a file and another one to ffplay which plays the video in screen.

This is useful if you want to record from a webcam and you want to see what you are recording.

Closing note

So yeah, I was an ignorant about ffmpeg and I still am.

But at least I learned a couple of the arguments and learned how to deal with all my cameras and screens at the same time.

Good enough.

I mean, it works, right? And that’s the most important thing².