#!/bin/sh

# convert video files into the WebM format in batches using ffmpeg (and preferrably GNU parallel)

# exit on error or use of undeclared variable or pipe error:
set -o errexit -o nounset -o pipefail

# -crf 10 : Constant Rate Factor (crf) gives video quality from 0 (lossless) to 63 (worst); default is 23.
# -c:a/v ... = transform the audio/video data using ...
# -b:a/v ... = audio/video bitrate is ...
# -cpu-used : from 0 (default) to 5. Use 1 or 2 for faster but worse encoding
# -row-mt 1 = better multi-threading supported since libvpx 1.6
# see https://trac.ffmpeg.org/wiki/Encode/VP9
# https://github.com/Kagami/webm.py/wiki/Notes-on-encoding-settings
# https://blog.programster.org/VP9-encoding
# https://developers.google.com/media/vp9/settings/vod/
# and https://notepad.patheticcockroach.com/4263/a-brief-tutorial-to-encode-in-x265-and-opus-using-ffmpeg/

convert() {
cpu_cores="$(getconf _NPROCESSORS_ONL)"
# Single pass for faster conversion:

# ffmpeg \
#   -y -loglevel error \
#   -i "$1" \
#   -threads "${cpu_cores:-1}" -cpu-used 1 -quality good \
#   -tile-columns 2 -frame-parallel 0 -auto-alt-ref 1 -lag-in-frames 25 \
#   -c:v libvpx-vp9 -b:v 750k -minrate 375k -maxrate 1088k -crf 33 \
#   -c:a libopus -ac 2 -b:a 48k -vbr on -compression_level 10 -frame_duration 40 -application audio \
#   -movflags faststart \
#   "${1%.[^.]*}".webm

# Double pass for better quality:
echo "Started first pass processing $1 ..."
ffmpeg \
  -y -loglevel error \
  -i "$1" \
  -pass 1 -speed 4 \
  -threads "${cpu_cores:-1}" -cpu-used 1 -quality good \
  -tile-columns 4 -frame-parallel 0 -auto-alt-ref 1 -lag-in-frames 25 \
  -c:v libvpx-vp9 -b:v 750k -minrate 375k -maxrate 1088k -crf 33 \
  -c:a libopus -ac 2 -b:a 48k -vbr on -compression_level 10 -frame_duration 40 -application audio \
  -movflags faststart \
  -f webm \
  /dev/null &&
  echo "... finished first and started second pass processing $1 ..."
ffmpeg \
  -y -loglevel error \
  -i "$1" \
  -pass 2 -speed 1 \
  -threads "${cpu_cores:-1}" -cpu-used 1 -quality good \
  -tile-columns 4 -frame-parallel 0 -auto-alt-ref 1 -lag-in-frames 25 \
  -c:v libvpx-vp9 -b:v 750k -minrate 375k -maxrate 1088k -crf 33 \
  -c:a libopus -ac 2 -b:a 48k -vbr on -compression_level 10 -frame_duration 40 -application audio \
  -movflags faststart \
  "${1%.[^.]*}".webm &&
  echo "... finished second and last pass processing $1."
} && export -f convert

if command -v parallel >/dev/null 2>&1; then
    parallel convert {} ::: "$@"
else
  for f in "$@"; do
    convert "$f"
  done
fi