Use an hash for default options and merge to define params for RMagick

lib/really_simple_captcha/captcha/plain_captcha.rb

@@ -3,6 +3,17 @@ require 'base64'
 
 module ReallySimpleCaptcha::Captcha
   module PlainCaptcha
+    DEFAULT_OPTIONS = {
+      width: 120,
+      height: 40,
+      wave_amplitude: 4.0,
+      wave_length: 60.0,
+      implode_amount: 0.2,
+      pointsize: 22,
+      text_fill: 'darkblue',
+      background_color: 'white'
+    }
+
     include ActiveSupport::Configurable
     config_accessor :field_name
 
@@ -28,14 +39,8 @@ module ReallySimpleCaptcha::Captcha
       def plain_captcha_tag
         session[PlainCaptcha.field_name] = ReallySimpleCaptcha::Util.random_string(PlainCaptcha.text_length)
 
-        image = PlainCaptcha.generate_image(session[:plain_captcha], {
-          implode_amount: PlainCaptcha.implode_amount,
-          wave_amplitude: PlainCaptcha.wave_amplitude,
-          wave_length: PlainCaptcha.wave_length,
-          pointsize: PlainCaptcha.pointsize,
-          fill: PlainCaptcha.fill,
-          background_color: PlainCaptcha.background_color,
-        })
+        image = PlainCaptcha.generate_image(session[:plain_captcha],
+                                            PlainCaptcha.config.select { |k,v| DEFAULT_OPTIONS.has_key?(k) })
 
         content_tag :div, class: 'plain_captcha' do
           html = image_tag "data:image/gif;base64,#{image}", alt: "Captcha"
@@ -56,31 +61,21 @@ module ReallySimpleCaptcha::Captcha
     end
 
     def self.generate_image(captcha_text, args={})
-      width = args[:width] || 120
-      height = args[:height] || 40
+      options = DEFAULT_OPTIONS.merge(args)
 
-      wave_amplitude = args[:wave_amplitude] || 4.0
-      wave_length = args[:wave_length] || 60.0
-      implode_amount = args[:implode_amount] || 0.2
-
-      pointsize = args[:pointsize] || 22
-      text_fill = args[:fill] || 'darkblue'
-
-      background_color = args[:background_color] || 'white'
-
-      image = ::Magick::Image.new(width, height) do
-        self.background_color = background_color
+      image = ::Magick::Image.new(options[:width], options[:height]) do
+        self.background_color = options[:background_color]
       end
 
       text = ::Magick::Draw.new do
-        self.pointsize = pointsize
+        self.pointsize = options[:pointsize]
         self.gravity = ::Magick::CenterGravity
-        self.fill = text_fill
+        self.fill = options[:text_fill]
       end
 
       text.annotate(image, 0, 0, 0, 0, captcha_text)
 
-      image = image.wave(wave_amplitude, wave_length).implode(implode_amount)
+      image = image.wave(options[:wave_amplitude], options[:wave_length]).implode(options[:implode_amount])
 
       Base64.strict_encode64(image.to_blob { self.format = 'GIF' })
     end