contrast-color for SASS

contrast-color.gemspec

Gem::Specification.new do |spec|
  spec.name = 'contrast-color'
  spec.version = '0.1.1'
  spec.platform = Gem::Platform::RUBY
  spec.author = 'Tobias H. Michaelsen'
  spec.email = 'tobias.michaelsen@gmail.com'
  spec.summary = 'Generate contrast colors in SASS'
  spec.description = 'A simple extension to SASS that alows you to calculate colors that conforms to WAI and WCAG20 contrast requirements.'
  spec.homepage = 'https://gist.github.com/243103'
  spec.license = 'MIT'
  spec.files = ['contrast-color.rb']
  spec.test_file = 'test.rb'
  spec.require_path = '.'
  spec.add_dependency('sass')
end

contrast-color.rb

require 'sass'

module ContrastColor
  BRIGHTNESS_COEFS = [0.299, 0.587, 0.114]
  LUMINANCE_COEFS = [0.2126, 0.7152, 0.0722]

  module Color
    def diff(other)
      # W3C
      Utils.sum(Utils.abs(rgb, other.rgb))
    end

    def diff_alt(other)
      # 3D - Sqrt(dr^2+dg^2+db^2)
      Math.sqrt(Utils.sum(Utils.sq(Utils.abs(rgb, other.rgb))))
    end

    def brightness
      # W3C; Rec. 601 luma
      Utils.sum(Utils.mul(rgb, BRIGHTNESS_COEFS))
    end

    def brightness_alt
      # http://alienryderflex.com/hsp.html
      Math.sqrt(Utils.sum(Utils.mul(Utils.sq(rgb), [0.241, 0.691, 0.068])))
    end

    def luminance
      # http://www.w3.org/TR/WCAG20/#relativeluminancedef
      norm_rgb = rgb.map { |value| value.to_f / 255 }
      Utils.sum(Utils.mul(norm_rgb.map { |v| v <= 0.03928 ? v/12.92 : ((v+0.055)/1.055) ** 2.4 }, LUMINANCE_COEFS))
    end
  end

  module Functions
    def contrast_color(color, seed_color = nil)
      seed_color ||= color
      assert_type color, :Color
      assert_type seed_color, :Color
      direction = color.brightness > 127 ? darken_method : lighten_method
      new_color = seed_color
      percentage = 0.0
      until conform(new_color, color) or percentage > 100.0 do
        amount = Sass::Script::Number.new percentage, ['%']
        new_color = self.send direction, seed_color, amount
        percentage += 0.1
      end
      new_color
    end

    # http://www.w3.org/WAI/ER/WD-AERT/#color-contrast
    MIN_BRIGHT_DIFF = 125
    MIN_COLOR_DIFF = 500

    def conform(color1, color2, wcag20_level = :aa)
      wcag20_level = :aa unless /^aaa?(?:_large)?$/ === wcag20_level.to_s
      bright_diff = (color1.brightness - color2.brightness).abs
      color_diff = color1.diff(color2)
      bright_diff >= MIN_BRIGHT_DIFF && color_diff >= MIN_COLOR_DIFF && send("wcag20_conform_#{wcag20_level}".to_sym, color1, color2)
    end

    # http://www.w3.org/TR/WCAG20/#visual-audio-contrast-contrast
    MIN_CONTRAST_RATE_AA = 4.5
    MIN_CONTRAST_RATE_AA_LARGE_TEXT = 3
    MIN_CONTRAST_RATE_AAA = 7
    MIN_CONTRAST_RATE_AAA_LARGE_TEXT = 4.5

    def wcag20_conform_aa(color1, color2)
      contrast_ratio(color1, color2) >= MIN_CONTRAST_RATE_AA
    end
    alias :wcag20_conform :wcag20_conform_aa

    def wcag20_conform_aa_large(color1, color2)
      contrast_ratio(color1, color2) >= MIN_CONTRAST_RATE_AA_LARGE_TEXT
    end

    def wcag20_conform_aaa(color1, color2)
      contrast_ratio(color1, color2) >= MIN_CONTRAST_RATE_AAA
    end

    def wcag20_conform_aaa_large(color1, color2)
      contrast_ratio(color1, color2) >= MIN_CONTRAST_RATE_AAA_LARGE_TEXT
    end

    def contrast_ratio(color1, color2)
      assert_type color1, :Color
      assert_type color2, :Color
      l1, l2 = color1.luminance, color2.luminance
      l2, l1 = l1, l2 if l2 > l1
      (l1 + 0.05) / (l2 + 0.05)
    end

    private

    def lighten_method
      respond_to?(:add_brightness) ? :add_brightness : :lighten
    end

    def darken_method
      respond_to?(:detract_brightness) ? :detract_brightness : :darken
    end
  end

  module Utils
    def self.abs(array, other)
      array.zip(other).map { |x, y| (x.to_f - y.to_f).abs }
    end

    def self.mul(array, other)
      array.zip(other).map { |x, y| x.to_f * y.to_f }
    end

    def self.sum(array)
      array.inject(0) { |sum, value| sum + value.to_f }
    end

    def self.sq(array)
      array.map { |e| e ** 2 }
    end
  end
end

module Sass::Script
  class Color
    include ContrastColor::Color
  end

  module Functions
    include ContrastColor::Functions
  end
end

test.rb

require 'minitest/autorun'
require './contrast-color'

describe ContrastColor do
  it "calculates brightness" do
    white = Sass::Script::Color.new [255, 255, 255]
    white.brightness.must_be_within_delta 255.0
    grey = Sass::Script::Color.new [0xe5, 0xe5, 0xe5]
    grey.brightness.must_be_within_delta 0.9*255, 1.0
  end

  it 'should generate colors with enough contrast' do
    data = File.read(__FILE__).split('__END__').last
    css = Sass::Engine.new(data, :syntax => :scss).to_css
    rules = css.gsub(/\s+/, ' ').gsub(/\} a/, '}\na').split('\n')
    puts rules
    rules.each { |rule| rule.must_match(/a \{ color: (#?\w+); background: \1; \}/) }
  end
end

__END__
a {
  color: contrast_color(white, #ff0000);
  background: #ee0000;
  }

a {
  color: contrast_color(rgb(127,127,127));
  background: white;
  }

a {
  color: contrast_color(rgb(128,128,128));
  background: black;
  }

a {
  color: contrast_color(white);
  background: #585858;
  }

a {
  color: contrast_color(black);
  background: #a7a7a7;
  }

a {
  color: contrast_color(red);
  background: white;
  }

a {
  color: contrast_color(green);
  background: #c3ffc3;
  }

a {
  color: contrast_color(blue);
  background: #fafaff;
  }

a {
  color: contrast_color(#123456);
  background: #c3dbf2;
  }

This is now part of the sass-extras gem https://github.com/tobiashm/sass-extras