Convert a gzipped LBM file into a simple GIF without animation

lbm2gif_simple.rb

#!/usr/bin/env ruby
#
# Offset | Length | Description
# ========================================
# 0      | 1      | minor width offset
# 1      | 1      | major width offset
# 2      | 1      | minor height offset
# 3      | 1      | major height offset
# 4      | 768    | 256-color pallet
# 772    | 1      | number of cycles (N)
# 773    | 6N     | cycle information
# 773+6N | W*H    | image information
#
require 'zlib'
require 'rmagick'

include Magick

exit(-1) if ARGV.length != 1


# Load raw drawing information
drawing = {
  width:   0,
  height:  0,
  ncycles: 0,
  cycles: [],
  pallet: [],
  data:   []
}

File.open(ARGV.first, 'rb') do |f|
  img = Zlib::GzipReader.new(f)

  drawing[:width]  = img.getbyte + 256 * img.getbyte
  drawing[:height] = img.getbyte + 256 * img.getbyte

  256.times do
    drawing[:pallet] << {
      red:   img.getbyte,
      green: img.getbyte,
      blue:  img.getbyte
    }
  end

  drawing[:ncycles] = img.getbyte
  drawing[:ncycles].times do
    drawing[:cycles] << {
      reversed: img.getbyte + 256 * img.getbyte,
      rate:     img.getbyte + 256 * img.getbyte,
      low:      img.getbyte,
      high:     img.getbyte
    }
  end

  (drawing[:width] * drawing[:height]).times do
    drawing[:data] << img.getbyte
  end
end


# Create a blank image for the drawing
img = Image.new(drawing[:width], drawing[:height]) do
  self.depth = 8
end

# Recreate the drawing pixel-by-pixel
drawing[:height].times do |y|
  drawing[:width].times do |x|
    # Look up the pixel color in our pallet
    off   = y * drawing[:width] + x
    pidx  = drawing[:data][off]
    color = drawing[:pallet][pidx]

    # Convert the 8-bit colors to 16-bit
    red   = 257 * color[:red]
    green = 257 * color[:green]
    blue  = 257 * color[:blue]

    img.store_pixels(x, y, 1, 1, [Pixel.new(red, green, blue, QuantumRange)])
  end
end

img.write('out.gif')