t004-screencasts.html - adamsgaard.dk - my academic webpage

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---
t004-screencasts.html (4532B)
---
     1 
On Monday 2020-03-16 the buildings of the danish public
     2 sector were closed as an emergency response to COVID-19.  This
     3 includes Aarhus University where I teach two undergraduate courses.
     4 The university asks lecturers to move their teaching to digital
     5 platforms.  As many times before, this requires creative thinking
     6 for those of us who do not use Microsoft and Apple products.
     7 
     8 
I needed a way to record my pdf slideshows while talking over
     9 the presentation.  Ideally, I also wanted the ability to show the
    10 video 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, ffmpeg(1) makes
    14 it easy to record the screen and laptop audio.  I want to keep the
    15 fan noise low during recording by applying minimal compression and
    16 encoding.  The following shell script serves the purpose of starting
    17 and stopping recording:
    18 
    19 
#!/bin/sh
    20 lockfile=/tmp/screenrecord.pid
    21 
    22 startrecording() {
    23     out="$HOME/screenrecord-$(date '+%Y-%m-%d_%H:%M:%S').mkv"
    24     ffmpeg -y \
    25         -f x11grab \
    26         -framerate 60 \
    27         -s "$(xdpyinfo | grep dimensions | awk '{print $2}')" \
    28         -i $DISPLAY \
    29         -f sndio -i default \
    30         -r 30 \
    31         -c:v libx264rgb -crf 0 -preset ultrafast -c:a flac \
    32         "$out" >/dev/null 2>&1 &
    33     printf '%s' "$!" > "$lockfile"
    34 
    35     sleep 1
    36     if [ ! -f "$out" ]; then
    37         echo 'error: ffmpeg recording did not start' >&2
    38         notify-send -u CRITICAL "${0##*/}" 'ffmpeg recording did not start'
    39         rm -f "$lockfile"
    40         exit 1
    41     fi
    42 }
    43 
    44 stoprecording() {
    45     kill "$(cat "$lockfile")"
    46     rm -f "$lockfile"
    47     notify-send "${0##*/}" 'recording ended'
    48 }
    49 
    50 if [ -f "$lockfile" ]; then
    51     stoprecording
    52 else
    53     startrecording
    54 fi
    55 
    56 
    57 
I have bound the above script to the key binding Alt+r which
    58 makes it easy to start and stop recording in my X session.
    59 
    60 
On Linux systems, the sound driver sndio should be replaced
    61 by alsa in the above ffmpeg(1) command.  Audio recording is
    62 disabled by default on OpenBSD, but can be permanently enabled with
    63 the following commands:
    64 
    65 
# sysctl kern.audio.record=1
    66 # echo kern.audio.record=1 >> /etc/sysctl.conf
    67 
    68 
    69 
On OpenBSD I can show the webcam video feed with the video(1) command.
    71 The following script toggles the video feed:

    72 
    73 
#!/bin/sh
    74 # remember to `chown $USER /dev/video0`
    75 if pgrep video >/dev/null 2>&1; then
    76     pkill video
    77 else
    78     nohup video -s 320 >/dev/null 2>&1 &
    79 fi
    80 
    81 
    82 
On Linux, the command mpv /dev/video0 can take
    83 place of the video(1) command above.  I have the above script bound
    84 to the keybinding Alt+v so I can quickly show and hide my face while
    85 recording.
    86 
    87 
I set dwm(1), my window
    88 manager, to open the video feed as a floating window on the bottom
    89 right of the screen.  The full dwm configuration can be found here.
    91 
    92 
When I am done recording a lecture, I encode and compress the
    93 video file to save bandwidth during upload.  The following script
    94 encodes all input files and reduces file size to roughly 15% without
    95 concievable loss in quality:
    96 
    97 
#!/bin/sh
    98 
    99 encode() {
   100     ffmpeg -y -i "$1" \
   101         -c:v libx264 -threads 0 -preset faster -pix_fmt yuv420p \
   102         -c:a aac -crf 10 \
   103         "${1%.*}_out.mp4"
   104 }
   105 
   106 for f in "$@"; do
   107     encode "$f"
   108 done
   109 
   110 
   111 
If there is a delay between video and audio, this can also be
   112 adjusted using ffmpeg(1).  I correct for a 0.3 s delay that I
   113 encounter when recording on my laptop:
   114 
   115 
#!/bin/sh
   116 
   117 synchronize() {
   118     ffmpeg -y -i "$1" \
   119         -itsoffset 0.300 \
   120         -i "$1" \
   121         -map 0:v -map 1:a \
   122         -c copy \
   123         "${1%.*}_out.${1##*.}"
   124 }
   125 
   126 for f in "$@"; do
   127     synchronize "$f"
   128 done
   129 
   130 
   131 

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