The hough transform in ruby. Only looks for straight black lines.

ruby-hough.rb

begin
  ['oily_png', 'pp'].each do |g|
    require g
  end
rescue LoadError => e
  puts "Could not load '#{e}'"; exit
end

class Hough
  Convert = "/usr/local/bin/convert"
  
  def initialize(image_path, options={})
    @verbose = options.delete(:verbose)
    @image = ChunkyPNG::Image.from_file(image_path)
    @image_path = image_path
  end

  def is_dark?(color)
    ChunkyPNG::Color.r(color) + ChunkyPNG::Color.g(color) + ChunkyPNG::Color.b(color) < 40
  end

  def is_light?(color)
    ChunkyPNG::Color.r(color) + ChunkyPNG::Color.g(color) + ChunkyPNG::Color.b(color) > 600
  end

  def angles(theta)
    @angles ||= {}
    @angles[theta] ||= {:cos => Math.cos(theta), :sin => Math.sin(theta)}
  end

  def get_hough_matrix
    hough = Hash.new(0)
    @image.height.times do |y|
      @image.width.times do |x|
        if is_dark?(@image[x,y]) && is_light?(@image[x,y + 1]) 
          (0..20).step(0.2).each do |theta|
            distance = (x * angles(theta)[:cos] + y * angles(theta)[:sin]).to_i
            hough[[theta, distance]] += 1 if distance >= 0
          end
        end
      end
    end
    return hough
  end

  def average_theta
     at = (get_hough_matrix.sort_by {|k,v| v }.take(20).inject(0.0) {|m,v| m + v[0][0] } / 20)
     pp "Average theta: #{at}"
     return at
  end

  def rotate(output_path)
   `#{Convert} #{@image_path} -rotate #{average_theta} #{output_path}`
  end

end

if __FILE__ == $0
  output_path = "path/to/output"
  image_path = "path/to/image"

  h = Hough.new(image_path, :verbose => true)
  h.rotate(output_path)

  `open #{output_path}`
end

I'd love to use parts of this gist for a (non-commercial) project of mine.

If you don't mind me using it, could you please specify under what license it's available for use (or if it's flat-out just public domain)?

(I know this request sounds dumb, but I'd prefer to avoid trouble if at all possible.)

Sure, would love if you'd use it, and thanks for asking. I would mention that the parameter for rotate that convert accepts is incorrect you need to calculate the arc-tangent to get the angle. (Check out Math.atan()) Use it however you like, I'm not going to license the gist. Attribution would be nice, but I don't really care. Wish you the best of luck, have fun.

Ooh nice. This is just what I was looking for!

There is however a small bug, it sometimes tries accessing a coordinate out of bounds on line 35. So I've changed line 33 to read (@image.height - 1).times do |y| which has fixed it.