quick and dirty wrapper around ffmpeg for encoding VP9 WebM files

vp9-encode.rb

#! /usr/bin/env ruby
# frozen_string_literal: true

# author: Nick Pelone <nick.pelone@gmail.com>

# Convert an input video file into VP9/WebM via a two-pass encoding process.

# The null file descriptor, platform-dependent.
# (This is where we send the first pass to, see: https://trac.ffmpeg.org/wiki/Encode/VP9)
NULL_FILEPTR = Gem.win_platform? ? 'NUL' : '/dev/null'

# Prints a message then a countdown timer of specified `length`
# @param [String] msg
# @param [Integer] length
# @return [void]
def countdown(msg, length = 3)
  puts msg
  length.times.reverse_each do |i|
    puts "#{i + 1}..."
    sleep(1)
  end
end

# Forms a ffmpeg invocation given input and output filenames,
# video and audio bitrates (in shortform, eg. 5M, 320k, etc),
# the pass number (for 2-pass encodes), and an `overwrite` option
# (defaults to true), which overwrites the target output file without
# interactive confirmation.
#
# @param [String] input
# @param [String] output
# @param [String] video_br
# @param [String] audio_br
# @param [Integer] pass
# @param [Boolean] overwrite
# @return [String]
def ffmpeg_cmd(input:, output:, video_br:, audio_br:, pass: 1, overwrite: true)
  overwrite_arg_pass2 = overwrite ? '-y' : ''

  case pass
  when 1
    "ffmpeg -i #{File.expand_path(input)} -c:v libvpx-vp9 -b:v #{video_br} -pass 1 -row-mt 1 -an -f webm -y #{NULL_FILEPTR}"
  when 2
    "ffmpeg -i #{File.expand_path(input)} -c:v libvpx-vp9 -b:v #{video_br} -pass 2 -row-mt 1 -c:a libopus -b:a #{audio_br} #{overwrite_arg_pass2} #{File.expand_path(output)}"
  end
end

#
# 'main'
#

abort("Usage: #{File.basename($PROGRAM_NAME)} [input] [video bitrate] [audio bitrate] [output]") if ARGV.empty?

system(
  ffmpeg_cmd(
    input: ARGV[0],
    video_br: ARGV[1],
    audio_br: ARGV[2],
    output: ARGV[3],
    pass: 1
  )
)

countdown('Starting second-pass endode in:')

system(
  ffmpeg_cmd(
    input: ARGV[0],
    video_br: ARGV[1],
    audio_br: ARGV[2],
    output: ARGV[3],
    pass: 2
  )
)