PNG: from colour to black-and-white.

colors.rb

require 'cairo'

#input and output come as arguments
input, output = ARGV

surface = Cairo::ImageSurface.from_png input

goal_data = surface.data

HEIGHT, WIDTH = surface.height, surface.width

o_surface = Cairo::ImageSurface.new(WIDTH, HEIGHT) 

cr = Cairo::Context.new(o_surface)

(0..WIDTH-1).each do |x|
  (0..HEIGHT-1).each do |y|
    this_pixel = y * WIDTH * 4 + x * 4
    blue =  goal_data[this_pixel].ord
    red  =  goal_data[this_pixel+2].ord
    green=  goal_data[this_pixel+1].ord
    avg  =  (blue+red+green)/ 765.0
    cr.rectangle(x,y,1,1)
    cr.set_source_rgb(avg,avg,avg)
    cr.fill
  end
end

o_surface.write_to_png output