pixanim.rb

require 'zip' # install with "gem install rubyzip"
require 'mechanize'
require 'json'

# usage: ruby pixanim.rb ID FORMAT, where ID is the pixiv id number and FORMAT is gif, webm, or apng
# test posts:
# http://www.pixiv.net/member_illust.php?mode=medium&illust_id=44305721
# http://www.pixiv.net/member_illust.php?mode=medium&illust_id=44303110
# http://www.pixiv.net/member_illust.php?mode=medium&illust_id=44353714
# http://www.pixiv.net/member_illust.php?mode=medium&illust_id=44349996
# http://www.pixiv.net/member_illust.php?mode=medium&illust_id=44315772
# http://www.pixiv.net/member_illust.php?mode=medium&illust_id=44303325
# http://www.pixiv.net/member_illust.php?mode=medium&illust_id=44343783

if ARGV[0].nil?
  raise "Must specify pixiv illustration id number"
end
pixiv_id = ARGV[0]

if ARGV[1].nil?
  raise "Must specify output format"
end
format = ARGV[1].downcase

mech = Mechanize.new

source_url = "http://www.pixiv.net/member_illust.php?mode=medium&illust_id=#{pixiv_id}"
zip_url = ""
frame_data = []
mech.get(source_url) do |page|
  # Get the zip url and frame delay by parsing javascript contained in a <script> tag on the page.
  # Not an neat solution, but I haven't found any other location that has the frame delays listed.
  scripts = page.search("body script").find_all do |node|
    node.text =~ /_ugoira600x600\.zip/
  end
  if scripts.any?
    javascript = scripts.first.text
    json = javascript.match(/;pixiv\.context\.ugokuIllustData\s+=\s+(\{.+\});(?:$|pixiv\.context)/)[1]
    data = JSON.parse(json)
    zip_url = data["src"].sub("_ugoira600x600.zip", "_ugoira1920x1080.zip")
    frame_data = data["frames"]
  else
    raise "Can't find javascript with frame data"
  end
end

zip_uri = URI(zip_url)
zip_blob = ""

Net::HTTP.start(zip_uri.host) do |http|
  resp = http.get(zip_uri.path, {"Referer" => "http://pixiv.net"})
  zip_blob = resp.body
end

folder = Zip::CentralDirectory.new
folder.read_from_stream(StringIO.new(zip_blob))

case format
when "gif"
  require 'rmagick'
  
  anim = Magick::ImageList.new
  delay_sum = 0
  folder.each_with_index do |file, i|
    # puts file.name
    image_blob = file.get_input_stream.read
    image = Magick::Image.from_blob(image_blob).first
    image.ticks_per_second = 1000
    delay = frame_data[i]["delay"]
    rounded_delay = (delay_sum + delay).round(-1) - delay_sum.round(-1)
    image.delay = rounded_delay
    delay_sum += delay
    anim << image
  end
  
  anim = anim.optimize_layers(Magick::OptimizeTransLayer)
  anim.write("./pixiv_anim_#{pixiv_id}.gif")

  puts "Animation successfully created as pixiv_anim_#{pixiv_id}.gif"
when "webm"
  FileUtils.mkdir_p("pixiv_anim_#{pixiv_id}")
  folder.each_with_index do |file, i|
    path = File.join("pixiv_anim_#{pixiv_id}", file.name)
    image_blob = file.get_input_stream.read
    File.open(path, "wb") do |f|
      f.write(image_blob)
    end
  end
  
  # Duplicate last frame to avoid it being displayed only for a very short amount of time.
  folder.to_a.last.name =~ /\A(\d{6})(\..+)\Z/
  new_last_index = $1.to_i + 1
  file_ext = $2
  new_last_filename = ("%06d" % new_last_index) + file_ext
  path_from = File.join("pixiv_anim_#{pixiv_id}", folder.to_a.last.name)
  path_to = File.join("pixiv_anim_#{pixiv_id}", new_last_filename)
  FileUtils.cp(path_from, path_to)
  
  delay_sum = 0
  File.open("pixiv_anim_#{pixiv_id}-timecodes.tc", "w+") do |f|
    f.write("# timecode format v2\n")
    frame_data.each do |img|
      f.write("#{delay_sum}\n")
      delay_sum += img["delay"]
    end
    f.write("#{delay_sum}\n")
    f.write("#{delay_sum}\n")
  end

  ext = folder.first.name.match(/\.(.+)$/)[1]
  system("ffmpeg -i pixiv_anim_#{pixiv_id}/%06d.#{ext} -codec:v libvpx -crf 4 -b:v 5000k -an pixiv_anim_#{pixiv_id}-tmp.webm")
  system("mkvmerge -o pixiv_anim_#{pixiv_id}.webm --timecodes 0:pixiv_anim_#{pixiv_id}-timecodes.tc pixiv_anim_#{pixiv_id}-tmp.webm")
  
  FileUtils.rm_rf("pixiv_anim_#{pixiv_id}")
  File.delete("pixiv_anim_#{pixiv_id}-timecodes.tc")
  File.delete("pixiv_anim_#{pixiv_id}-tmp.webm")
  
  puts "Animation successfully created as pixiv_anim_#{pixiv_id}.webm"
when "apng"
  require 'rmagick'
  
  FileUtils.mkdir_p("pixiv_anim_#{pixiv_id}")
  folder.each_with_index do |file, i|
    frame_path = File.join("pixiv_anim_#{pixiv_id}", "frame#{"%03d" % i}.png")
    delay_path = File.join("pixiv_anim_#{pixiv_id}", "frame#{"%03d" % i}.txt")
    image_blob = file.get_input_stream.read
    delay = frame_data[i]["delay"]
    image = Magick::Image.from_blob(image_blob).first
    image.format = "PNG"
    image.write(frame_path)
    File.open(delay_path, "wb") do |f|
      f.write("delay=#{delay}/1000")
    end
  end
  system("apngasm pixiv_anim_#{pixiv_id}.png pixiv_anim_#{pixiv_id}/frame*.png")
  FileUtils.rm_rf("pixiv_anim_#{pixiv_id}")
  
  puts "Animation successfully created as pixiv_anim_#{pixiv_id}.png"
else
  raise "Invalid format"
end

A few notes if someone decides to run it standalone under windows, like I did:

1. Requires rubyzip gem to work
2. Installing rmagick under windows is tricky - see top answer at http://stackoverflow.com/questions/4451213/ruby-1-9-2-how-to-install-rmagick-on-windows , also you'll need to use imagemagick version 6.7.9-9 according to https://github.com/rmagick/rmagick/wiki . The steps and parameters past step 2 described in the wiki weren't necessary.