Capturing a screen sub section with ffmpeg

I tooted me scrolling through the browser tabs I opened from the Black Producers bandcamp list . I used ffmpeg to to grab a subsection of the screen with this command:

ffmpeg -f x11grab -show_region 1 -framerate 10 -video_size 720x480+100,200 -i $DISPLAY scroll.webm

• video_size sets the size and offset in the window (needed if grabbing a subsection)
• show_region paints a box around the active part, this helps dial in what is captured.

It took me a while to figure out what the input (-i) should be, in the end I figured out that it needs to be the X display, the easiest way to grab this is from the environment variable $DISPLAY .

I first made mp4's from the capture, but they were coming out all garbled, firefox refused to play and vlc was pretty sad. I move to webm and the capture is smaller and works in firefox.