004-screencasts.txt (4139B)
1 On Monday 2020-03-16 the buildings of the danish public sector were 2 closed as an emergency response to COVID-19. This includes Aarhus 3 University where I teach two undergraduate courses. The university 4 asks lecturers to move their teaching to digital platforms. As many 5 times before, this requires creative thinking for those of us who 6 do not use Microsoft and Apple products. 7 8 I needed a way to record my pdf slideshows while talking over the 9 presentation. Ideally, I also wanted the ability to show the video 10 of my webcam as an overlay in an attempt to make the presentation 11 a bit more engaging when explaining complex parts. 12 13 Fortunately, [1]ffmpeg(1) makes it easy to record the screen and 14 laptop audio. I want to keep the fan noise low during recording 15 by applying minimal compression and encoding. The following shell 16 script serves the purpose of starting and stopping recording: 17 18 #!/bin/sh 19 lockfile=/tmp/screenrecord.pid 20 21 startrecording() { 22 out="$HOME/screenrecord-$(date '+%Y-%m-%d_%H:%M:%S').mkv" 23 ffmpeg -y \ 24 -f x11grab \ 25 -framerate 60 \ 26 -s "$(xdpyinfo | grep dimensions | awk '{print $2}')" \ 27 -i $DISPLAY \ 28 -f sndio -i default \ 29 -r 30 \ 30 -c:v libx264rgb -crf 0 -preset ultrafast -c:a flac \ 31 "$out" >/dev/null 2>&1 & 32 printf '%s' "$!" > "$lockfile" 33 34 sleep 1 35 if [ ! -f "$out" ]; then 36 echo 'error: ffmpeg recording did not start' >&2 37 notify-send -u CRITICAL "${0##*/}" 'ffmpeg recording did not start' 38 rm -f "$lockfile" 39 exit 1 40 fi 41 } 42 43 stoprecording() { 44 kill "$(cat "$lockfile")" 45 rm -f "$lockfile" 46 notify-send "${0##*/}" 'recording ended' 47 } 48 49 if [ -f "$lockfile" ]; then 50 stoprecording 51 else 52 startrecording 53 fi 54 55 I have bound the above script to the key binding Alt+r which makes 56 it easy to start and stop recording in my X session. 57 58 On Linux systems, the sound driver sndio should be replaced by alsa 59 in the above ffmpeg(1) command. Audio recording is disabled by 60 default on OpenBSD, but can be permanently enabled with the following 61 commands: 62 63 # sysctl kern.audio.record=1 64 # echo kern.audio.record=1 >> /etc/sysctl.conf 65 66 On OpenBSD I can show the webcam video feed with the [2]video(1) 67 command. The following script toggles the video feed: 68 69 #!/bin/sh 70 # remember to `chown $USER /dev/video0` 71 if pgrep video >/dev/null 2>&1; then 72 pkill video 73 else 74 nohup video -s 320 >/dev/null 2>&1 & 75 fi 76 77 On Linux, the command mpv /dev/video0 can take place of the video(1) 78 command above. I have the above script bound to the keybinding Alt+v 79 so I can quickly show and hide my face while recording. 80 81 I set [3]dwm(1), my window manager, to open the video feed as a 82 floating window on the bottom right of the screen. The full dwm 83 configuration can be found [4]here. 84 85 When I am done recording a lecture, I encode and compress the video 86 file to save bandwidth during upload. The following script encodes 87 all input files and reduces file size to roughly 15% without 88 concievable loss in quality: 89 90 #!/bin/sh 91 92 encode() { 93 ffmpeg -y -i "$1" \ 94 -c:v libx264 -threads 0 -preset faster -pix_fmt yuv420p \ 95 -c:a aac -crf 10 \ 96 "${1%.*}_out.mp4" 97 } 98 99 for f in "$@"; do 100 encode "$f" 101 done 102 103 If there is a delay between video and audio, this can also be 104 adjusted using ffmpeg(1). I correct for a 0.3 s delay that I 105 encounter when recording on my laptop: 106 107 #!/bin/sh 108 109 synchronize() { 110 ffmpeg -y -i "$1" \ 111 -itsoffset 0.300 \ 112 -i "$1" \ 113 -map 0:v -map 1:a \ 114 -c copy \ 115 "${1%.*}_out.${1##*.}" 116 } 117 118 for f in "$@"; do 119 synchronize "$f" 120 done 121 122 [5]Example screen recording using ffmpeg(1) and video(1) with the 123 above scripts. 124 125 References: 126 127 [1] https://ffmpeg.org/ 128 [2] https://man.openbsd.org/man1/video.1 129 [3] https://dwm.suckless.org/ 130 [4] https://src.adamsgaard.dk/dwm/ 131 [5] https://adamsgaard.dk/video/screencast.mp4