Facebook Video
setup and support

#!/bin/zsh
# Facebook Video droplet - Automator "Run Shell Script" body.
# Shell: /bin/zsh   Pass input: as arguments
#
# Needs ffmpeg at ~/bin/ffmpeg (one-time setup).
# Makes an H.264 MP4 aimed at about 90 MB, so it stays under
# Facebook's 100 MB limit. Long edge 1920 px (1280 px for long clips),
# never scaled up. iPhone HDR footage is converted to normal colour.
# Writes "<name>_facebook.mp4" beside the original, never overwrites.

FFMPEG="${FFMPEG_PATH:-$HOME/bin/ffmpeg}"
TARGET_MB=90        # aim for this size
LIMIT_MB=100        # warn if a result is bigger than this
MAX_KBPS=6000       # no need to go higher than this for Facebook
AUDIO_KBPS=128

alert() {
  osascript - "$1" <<'EOF' >/dev/null 2>&1
on run argv
  display dialog (item 1 of argv) buttons {"OK"} default button 1 with title "Facebook Video"
end run
EOF
}

if [ ! -x "$FFMPEG" ]; then
  alert "ffmpeg was not found at:

$FFMPEG

Do the one time ffmpeg setup first."
  exit 1
fi

lastdir=""
count=0

for f in "$@"; do
  dir=$(dirname "$f")
  base=$(basename "$f")
  name="${base%.*}"
  out="$dir/${name}_facebook.mp4"
  n=1
  while [ -e "$out" ]; do
    out="$dir/${name}_facebook_$n.mp4"
    n=$((n+1))
  done

  info=$("$FFMPEG" -hide_banner -i "$f" 2>&1)
  secs=$(printf '%s\n' "$info" | awk '/Duration:/ { split($2, t, /[:,]/); print t[1]*3600 + t[2]*60 + t[3]; exit }')
  if [ -z "$secs" ] || ! awk -v s="$secs" 'BEGIN { exit !(s > 0) }'; then
    alert "Could not read the length of this video:

$f"
    continue
  fi

  # Video bitrate that lands the whole file near TARGET_MB.
  vkbps=$(awk -v s="$secs" -v t="$TARGET_MB" -v a="$AUDIO_KBPS" -v m="$MAX_KBPS" \
    'BEGIN { v = int(t * 8000 / s - a); if (v > m) v = m; print v }')
  if [ "$vkbps" -lt 300 ]; then
    alert "This video is too long to fit under ${LIMIT_MB} MB at a watchable quality:

$f"
    continue
  fi

  # Long edge 1920, or 1280 when the bitrate is low (looks better than starved 1080p).
  if [ "$vkbps" -lt 2500 ]; then edge=1280; else edge=1920; fi
  scale="scale='if(gte(iw,ih),min($edge,iw),-2)':'if(gte(iw,ih),-2,min($edge,ih))'"

  # iPhone HDR (HLG / Dolby Vision / PQ) -> normal colour, so it does not look washed out.
  if printf '%s\n' "$info" | grep -Eq 'arib-std-b67|smpte2084'; then
    vf="zscale=t=linear:npl=100,format=gbrpf32le,zscale=p=bt709,tonemap=hable:desat=0,zscale=t=bt709:m=bt709:r=tv,$scale,format=yuv420p"
  else
    vf="$scale,format=yuv420p"
  fi

  passlog=$(mktemp /tmp/fbvideo_pass.XXXXXX)
  errfile=$(mktemp /tmp/fbvideo_err.XXXXXX)

  if "$FFMPEG" -hide_banner -nostdin -y -i "$f" -map 0:v:0 -vf "$vf" \
       -c:v libx264 -preset medium -b:v "${vkbps}k" -pass 1 -passlogfile "$passlog" \
       -an -f mp4 /dev/null > "$errfile" 2>&1 \
     && "$FFMPEG" -hide_banner -nostdin -y -i "$f" -map 0:v:0 -map '0:a:0?' -vf "$vf" \
       -c:v libx264 -preset medium -b:v "${vkbps}k" -pass 2 -passlogfile "$passlog" \
       -profile:v high -colorspace bt709 -color_primaries bt709 -color_trc bt709 \
       -c:a aac -b:a "${AUDIO_KBPS}k" -ac 2 -movflags +faststart "$out" >> "$errfile" 2>&1
  then
    lastdir="$dir"
    count=$((count+1))
    size_mb=$(( $(wc -c < "$out" | tr -d ' ') / 1000000 ))
    if [ "$size_mb" -ge "$LIMIT_MB" ]; then
      alert "Finished, but it came out at ${size_mb} MB, over the ${LIMIT_MB} MB limit:

$out"
    fi
  else
    msg=$(tail -n 5 "$errfile")
    rm -f "$out"
    alert "Could not convert:

$f

$msg"
  fi
  rm -f "$errfile" "$passlog"*
done

if [ -n "$lastdir" ]; then
  osascript -e "display notification \"$count video(s) ready for Facebook\" with title \"Facebook Video\"" >/dev/null 2>&1
  open "$lastdir" 2>/dev/null
fi