random_image_generator.exs

defmodule Image do
  defstruct [:grid, :pixel_map]
end

defmodule RandomImageGenerator do

  def create(width, bar_size, filename \\ "out.png") do
    %Image{}
    |> create_grid(width)
    |> build_pixel_map(width, bar_size)
    |> draw_image(width)
    |> save_image(filename)
  end

  defp create_grid(image, width) do
    grid =
      (0..(width - 1))
      |> Enum.to_list
      |> Enum.map(fn(_) -> 100 end)
      |> Enum.with_index

    %Image{image|grid: grid}
  end

  defp save_image(image, filename), do: File.write(filename, image)

  defp draw_image(%Image{pixel_map: pixel_map}, width) do
    image = :egd.create(width, width)
    Enum.each(pixel_map, &fill_rectangle(&1, image))

    :egd.render(image)
  end

  defp fill_rectangle({start, stop}, image) do
    egd_fill = :egd.color(generate_rgb_triple())
    :ok      = :egd.filledRectangle(image, start, stop, egd_fill)
  end

  defp build_pixel_map(%Image{grid: grid} = image, width, bar_size) do
    pixel_map = Enum.map(grid, fn({_code, index}) -> construct_map(index, width, bar_size) end)
    %Image{image|pixel_map: pixel_map}
  end

  defp construct_map(index, width, bar_size) do
    horizontal   = rem(index, width) * bar_size
    vertical     = div(index, width) * bar_size
    top_left     = {horizontal, vertical}
    bottom_right = {horizontal + bar_size, vertical + width}
    {top_left, bottom_right}
  end

  defp generate_rgb_triple, do: {Enum.random(0..255), Enum.random(0..255), Enum.random(0..255)}
end

RandomImageGenerator.create(6000, 2)
# check for out.png in the dir you ran from or the filename specified