expanding webm

piston.bash

#!/usr/local/bin/bash

dir="$(mktemp -d -t piston)"

cleanup() {
  rm -rf "$dir"
}
trap cleanup EXIT

times=()
i=0

add() {
  t=$1
  shift 1

  times[i]=$t
  ffmpeg -loglevel error "$@" -c:v libvpx-vp9 -t "$t" "$dir/$((i++)).webm"
}

addn() {
  # initial frame is 160x160
  add "$1" -i "frames/$2.png" -vf "pad=iw+40*$2:ih:color=#00000000"
}

addn 0.9 0
for j in {0..4}; do
  addn 0.02 "$j"
done

addn 0.9 4
for j in {4..0}; do
  addn 0.02 "$j"
done

addn 1 0

# create a list of files for the next step
for ((j=0; j < i; ++j)); do
  echo "file '$dir/$j.webm'" >> "$dir/paths.txt"
  echo duration "${times[j]}" >> "$dir/paths.txt"
done

# make audio
ffmpeg -loglevel error -i piston_in.ogg -i piston_out.ogg -lavfi "[0]adelay=900[a];[1]adelay=1900[b];[a][b]amix=2:normalize=0" -y "$dir/audio_out.ogg"

# merge everything
ffmpeg -loglevel error -f concat -safe 0 -i "$dir/paths.txt" -i "$dir/audio_out.ogg" -c copy -y SPOILER_piston.webm