captcha_with_mini_magick.rb

image = MiniMagick::Image.open('source.jpg')
image.colorspace 'gray'

pixels = image.get_pixels

0.upto(image.height - 1) do |y|
  image.combine_options do |c|
    0.upto(image.width - 1) do |x|
      color = pixels[y][x][0]
      c.draw "fill #{black_color} color #{x},#{y} point" if color < 250
    end
  end
end

image.write('destination.jpg')