CssColorContrast

From 2 colors, calculates the contrast ratio defined for WCAG 2.0.

Hexadecimal notation, RGB/HSL/HWB functions are supported as input formats.

Installation

Add this line to your application's Gemfile:

gem 'css_color_contrast'

And then execute:

$ bundle install

Or install it yourself as:

$ gem install css_color_contrast

Usage

Calculate the contrast ratio

For example, if you want to calculate the contrast ratio between yellow and black:

require 'css_color_contrast'

CssColorContrast.ratio('#ff0', [0, 0, 0])
# => 19.555999999999997

CssColorContrast.ratio('rgb(255, 255, 0)', 'black')
# => 19.555999999999997

CssColorContrast.ratio('hsl(60deg, 100%, 50%)', 'hwb(60deg 0% 100%)')
# => 19.555999999999997

The following formats are supported for the arguments of CssColorContrast.ratio().

  • RGB values as an Array of Integers: [255, 255, 0], [0, 0, 0], etc.
  • RGB values in hexadecimal notation: #ff0, #ffff00, #FF0, etc.
  • RGB values in functional notation: rgb(255, 255, 0), rgb(255 255 0), etc.
  • HSL colors in functional notation: hsl(60deg, 100%, 50%), hsl(60 100% 50%), etc.
  • [Experimental] HWB colors in functional notation: hwb(60deg 0% 0%), hwb(60 0% 0%), etc.
  • Extended color keywords: white, black, red, etc.

Calculate the relative luminance

You can also calculate the relative luminance of a given color as follows:

require 'css_color_contrast'

CssColorContrast.relative_luminance('#ff0')
# => 0.9278

Command-line interface

A command-line tool, css_color_contrast is provided for a demonstration purpose.

Invoke it as follows:

$ css_color_contrast
To calculate the contrast ratio between 2 colors, enter the following command:

  > ratio: color1 color2

For the values of color1 and color2, hexadecimal notation, RGB/HSL/HWB
functions and the extended color keywords are supported.

To finish the program, enter 'exit'.
>

Commands

ratio:

Calculate the contrast ratio between two colors as in the next case:

> ratio: rgb(255 255 0) black
19.555999999999997
>

adjust:

It takes 3 arguments, 2 colors and a contrast ratio to be satisfied.

In the example below, it tries to adjust the lightness of the second color so that the contrast ratio between the 2 colors is just above the target ratio.

> adjust: white red 4.5
#ee0000
>

info:

Print the properties of a given color:

> info: #ff0
---
name: yellow
hex: #ffff00
rgb: rgb(255,255,0)
hsl: hsl(60.00,100.00%,50.00%)
hwb: hwb(60.00,0.00%,0.00%)
>

When the 3rd argument is omitted, it defaults to 4.5.

Assignment to a variable

You can assign a color value to a varible. Variable names begin with @.

> @deeper_red: #e00
#e00
> ratio: white @deeper_red
4.530325445433122
> info: @deeper_red
---
name: #ee0000
hex: #ee0000
rgb: rgb(238,0,0)
hsl: hsl(0.00,100.00%,46.67%)
hwb: hwb(0.00,0.00%,6.67%)

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/nico-hn/css_color_contrast. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the code of conduct.

License

The gem is available as open source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the CssColorContrast project's codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.