Use a sass-based function instead of a ruby-based function for contrast calculations

frameworks/compass/stylesheets/compass/utilities/color/_contrast.scss

@@ -1,8 +1,28 @@
-$contrasted-default-dark:   #000;
-$contrasted-default-light:  #fff;
+$contrasted-dark-default:   #000 !default;
+$contrasted-light-default:  #fff !default;
+$contrasted-lightness-threshold: 30% !default;
+
+// Returns the `$light` color when the `$color` is dark
+// and the `$dark` color when the `$color` is light.
+// The `$threshold` is a percent between `0%` and `100%` and it determines
+// when the lightness of `$color` changes from "dark" to "light".
+@function contrast-color(
+  $color,
+  $dark: $contrasted-dark-default,
+  $light: $contrasted-light-default,
+  $threshold: $contrasted-lightness-threshold
+) {
+  @return if(lightness($color) < $threshold, $light, $dark)
+}
 
 // Sets the specified background color and calculates a dark or light contrasted text color.
-@mixin contrasted($bg, $dark:$contrasted-default-dark, $light:$contrasted-default-light){
-  background-color: $bg;
-  color: get_contrast_yiq($bg, $dark, $light);
+// The arguments are passed through to the [contrast-color function](#function-contrast-color).
+@mixin contrasted(
+  $background-color,
+  $dark: $contrasted-dark-default,
+  $light: $contrasted-light-default,
+  $threshold: $contrasted-lightness-threshold
+) {
+  background-color: $background-color;
+  color: contrast-color($background-color, $dark, $light, $threshold);
 }

lib/compass/sass_extensions/functions/colors.rb

@@ -41,13 +41,6 @@ module Compass::SassExtensions::Functions::Colors
     Sass::Script::String.new("##{alphastr}#{color.send(:hex_str)[1..-1]}".upcase)
   end
   
-  # Calculates the contrast of a color using the YIQ algorithm
-  # Returns one of either a dark or light color based on the YIQ
-  def get_contrast_yiq(color, dark = Sass::Script::Color.new([0,0,0]), light = Sass::Script::Color.new([255,255,255]))
-    yiq = ( (color.red*299) + (color.green*587) + (color.blue*114) ) / 1000;
-    yiq >= 128 ? dark : light
-  end
-
   private
   def scale_color_value(value, amount)
     if amount > 0

test/fixtures/stylesheets/compass/css/utilities.css

@@ -11,3 +11,16 @@
     clear: both;
     overflow: hidden;
     visibility: hidden; }
+
+p.light {
+  background-color: #b0201e;
+  color: black; }
+p.dark {
+  background-color: #5f1210;
+  color: white; }
+p.light-with-args {
+  background-color: #b0201e;
+  color: green; }
+p.dark-with-args {
+  background-color: #5f1210;
+  color: blue; }

test/fixtures/stylesheets/compass/sass/utilities.scss

@@ -7,3 +7,8 @@
 .pie-clearfix {
     @include pie-clearfix;
 }
+
+p.light { @include contrasted(#B0201E); }
+p.dark  { @include contrasted(#5F1210); }
+p.light-with-args { @include contrasted(#B0201E, green, blue); }
+p.dark-with-args  { @include contrasted(#5F1210, green, blue); }

test/sass_extensions_test.rb

@@ -88,11 +88,6 @@ class SassExtensionsTest < Test::Unit::TestCase
     assert_equal "true", evaluate("blank(' ')")
     assert_equal "true", evaluate("blank(-compass-space-list(' '))")
   end
-  
-  def test_get_contrast
-    assert_equal "white", evaluate("get_contrast_yiq(#000)")
-    assert_equal "black", evaluate("get_contrast_yiq(#fff)")
-  end
 
 protected
   def evaluate(value)