t004-screencasts.txt - adamsgaard.dk - my academic webpage
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t004-screencasts.txt (4139B)
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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