FFmpeg Window Capture

If you’ve read the FFmpeg website’s Capture/Desktop page or another similar tutorial about screen recording, you may have learned that the x11grab device can be used to capture a specific region of your screen, but did you know that you can also record a window by it’s ID just like OBS does?

First, you’ll need to find the ID of the window that you want to capture with xwininfo:

Now we can use the -window_id parameter of the x11grab device with FFmpeg to capture the window. Here’s an example of a “nearly lossless” MP4 capture at 60 frames per second with a constant quality of 10:

ffmpeg -f x11grab -thread_queue_size 4096 -framerate 60 -window_id 0x940000a \
  -c:v libx264 -crf 10 output.mp4

Recording With NVENC

If you’re using an NVIDIA card with proprietary drivers, you can encode with NVENC:

ffmpeg -f x11grab -thread_queue_size 4096 -framerate 60 -window_id 0x940000a \
  -c:v hevc_nvenc -preset slow -tune hq -tier high -cq 10 output.mp4

Recording Desktop Audio

If you’d like to record desktop audio with Pulse or PipeWire, you can find the name of your audio card with pactl (line-wrapped to 80 columns):

$ pactl list short | grep monitor
75      alsa_output.usb-Focusrite_Scarlett_2i2_USB_Y8QUZ0C9981932-00.analog-ster
eo.monitor        PipeWire        s32le 2ch 48000Hz       RUNNING
77      alsa_output.pci-0000_2d_00.1.hdmi-stereo-extra2.monitor PipeWi
e        s32le 2ch 48000Hz RUNNING
79      alsa_output.usb-C-Media_Electronics_Inc._USB_Multimedia_Audio_Device-00.
analog-stereo.monitor     PipeWire        s16le 2ch 48000Hz       RUNNING
81      alsa_output.pci-0000_2f_00.4.iec958-stereo.monitor      PipeWire        
s32le 2ch 48000Hz RUNNING
212     easyeffects_sink.monitor        PipeWire        float32le 2ch 48000Hz   
RUNNING
448     alsa_output.usb-SmartAction_FiiO_USB_Audio_Class_2.0_DAC_0007-00.analog-
stereo.monitor    PipeWire        s32le 2ch 48000Hz       RUNNING
858     soundux_sink.monitor    PipeWire        float32le 2ch 48000Hz   RUNNING

You can then record audio and video together with FFmpeg like so:

ffmpeg -f x11grab -thread_queue_size 4096 -framerate 60 -window_id 0x940000a \
  -f pulse -thread_queue_size 8192 -ac 2 \
  -i alsa_output.usb-SmartAction_FiiO_USB_Audio_Class_2.0_DAC_0007-00.analog-stereo.monitor \
  -c:a aac -b:a 256k \
  -c:v libx264 -crf 10 output.mp4

Putting It All Together in a Script

Below is a script that automates this somewhat. Note that you will need to edit the audio_device variable for the -a flag to work. You may also want to edit the recording_dir as well.

#!/usr/bin/env bash

recording_dir='.'
date=$(date +%Y-%m-%d_%s)
show_cursor=0
show_region=0
record_audio=0
use_nvenc=0
prefix=""
audio_device=''

read -d '' usage <<EOF
USAGE
  window-capture [OPTIONS]

OPTIONS
  -h  Show this help.
  -a  Capture audio.  Make sure you modify the "audio_device" in the script!
  -n  Encode with NVENC.
  -c  Capture X cursor (disabled by default).
  -s  Show capture region.
  -p  Filename prefix (override default from window title).
  -d  Capture directory (override "$recording_dir").
EOF

chomp(){ sed 's/^\s\+//' | sed 's/\s\+$//' ; }

while getopts ':hcsp:d:an' opt; do
  case ${opt} in
    h)
      cat <<<"$usage"
      exit 0
      ;;
    c)
      show_cursor=1
      ;;
    s)
      show_region=1
      ;;
    p)
      prefix="$OPTARG"
      ;;
    d)
      recording_dir="$OPTARG"
      ;;
    a)
      [ -z "$audio_device" ] \
        && echo 'ERROR: Set "audio_device" first!' \
        && exit 1
      record_audio=1
      ;;
    n)
      use_nvenc=1
      ;;
    *)
      cat <<<"$usage"
      exit 1
      ;;
  esac
done

window_info=$(xwininfo)
winid_line=$(grep '^xwininfo:\s\+[wW]indow\s\+[iI][dD]:\s\+' <<<"$window_info")
window_id=$(awk '{print $4}' <<<"$winid_line")
if [ -z "$prefix" ]; then
  prefix=$( \
    sed 's/^xwininfo:\s\+[wW]indow\s\+[iI][dD]:\s\+[0-9xXa-fA-F\]\+\s\+"\(.*\)"\s*$/\1/' \
      <<<"$winid_line" \
    | sed 's/\s/_/g' | sed 's/\~/home/g' | sed 's/\//-/g' | sed 's/@/_/g' | chomp)
fi

if [ $record_audio -eq 1 ]; then
  audio_opts="-f pulse -thread_queue_size 8192 -ac 2 -i $audio_device -c:a aac -b:a 256k"
else
  audio_opts="-an"
fi

if [ $use_nvenc -eq 1 ]; then
  enc_opts='-c:v hevc_nvenc -preset slow -tune hq -tier high -cq 10'
else
  enc_opts='-c:v libx265 -crf 10'
fi

ffmpeg \
  -f x11grab -thread_queue_size 8192 -framerate 60 -window_id $window_id \
  -show_region $show_region -draw_mouse $show_cursor -i :0.0 \
  $audio_opts \
  $enc_opts \
  "$recording_dir/$prefix"_"$date.mkv" \
;

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