drawing an "empty" circle in dragonruby

circle.rb

  def draw_circle(args, diameter: 200)
    diameter.times do |i|
      r1 = diameter / 2
      h1 = i - r1
      l1 = Math::sqrt(r1 * r1 - h1 * h1)
      r2 = r1 - 4 # Modify this to adjust the size of the inner circle. the bigger this is, the more empty space, the smaller, the less empty space.
      h2 = (i - r2) - (r1 - r2)
      if h2.abs < r2
        l2 = Math::sqrt(r2 * r2 - h2 * h2)
        args.render_target(:open_circle).lines << [i, r1 - l2, i, r1 - l1]
        args.render_target(:open_circle).lines << [i, r1 + l2, i, r1 + l1]
      else
        args.render_target(:open_circle).lines << [i, r1 - l1, i, r1 + l1]
      end
    end

    circle = {
      source_x: 0, source_y: 0,
      source_w: diameter, source_h: diameter
    }

    args.outputs.sprites << circle.merge({
      x: 500,
      y: 500,
      w: 200, h: 200, path: :open_circle,
    })
  end

See also:

https://gist.github.com/xenobrain/bd1d5e8909686930e27f658e3d385d7a