class Color::Palette::MonoContrast

Generates a monochromatic constrasting colour palette for background and foreground. What does this mean?

Monochromatic: A single colour is used to generate the base palette, and this colour is lightened five times and darkened five times to provide eleven distinct colours.

Contrasting: The foreground is also generated as a monochromatic colour palettte; however, all generated colours are tested to see that they are appropriately contrasting to ensure maximum readability of the foreground against the background.

Constants

DEFAULT_MINIMUM_BRIGHTNESS_DIFF
DEFAULT_MINIMUM_COLOR_DIFF

Attributes

background[R]

Hash of CSS background colour values.

This is always 11 values:

0

The starting colour.

+1..+5

Lighter colours.

-1..-5

Darker colours.

foreground[R]

Hash of CSS foreground colour values.

This is always 11 values:

0

The starting colour.

+1..+5

Lighter colours.

-1..-5

Darker colours.

minimum_brightness_diff[RW]

The minimum brightness difference between the background and the foreground, and must be between 0..1. Setting this value will regenerate the palette based on the base colours. The default value for this is 125 / 255.0. If this value is set to nil, it will be restored to the default.

minimum_color_diff[RW]

The minimum colour difference between the background and the foreground, and must be between 0..3. Setting this value will regenerate the palette based on the base colours. The default value for this is 500 / 255.0.

Public Class Methods

new(background, foreground = nil) click to toggle source

Generate the initial palette.

# File lib/color/palette/monocontrast.rb, line 88
def initialize(background, foreground = nil)
  @minimum_brightness_diff = DEFAULT_MINIMUM_BRIGHTNESS_DIFF
  @minimum_color_diff = DEFAULT_MINIMUM_COLOR_DIFF

  regenerate(background, foreground)
end

Public Instance Methods

brightness_diff(c1, c2) click to toggle source

Returns the absolute difference between the brightness levels of two colours. This will be a decimal value between 0 and 1. W3C accessibility guidelines for colour contrast suggest that this value be at least approximately 0.49 (125 / 255.0) for proper contrast.

# File lib/color/palette/monocontrast.rb, line 166
def brightness_diff(c1, c2)
  (c1.brightness - c2.brightness).abs
end
calculate_foreground(background, foreground) click to toggle source

Given a background colour and a foreground colour, modifies the foreground colour so that it will have enough contrast to be seen against the background colour.

Uses mininum_brightness_diff and minimum_color_diff.

# File lib/color/palette/monocontrast.rb, line 134
def calculate_foreground(background, foreground)
  nfg = nil
    # Loop through brighter and darker versions of the foreground color.
    # The numbers here represent the amount of foreground color to mix
    # with black and white.
  [100, 75, 50, 25, 0].each do |percent|
    dfg = foreground.darken_by(percent)
    lfg = foreground.lighten_by(percent)

    dbd = brightness_diff(background, dfg)
    lbd = brightness_diff(background, lfg)

    if lbd > dbd
      nfg = lfg
      nbd = lbd
    else
      nfg = dfg
      nbd = dbd
    end

    ncd = color_diff(background, nfg)

    break if nbd >= @minimum_brightness_diff and ncd >= @minimum_color_diff
  end
  nfg
end
color_diff(c1, c2) click to toggle source

Returns the contrast between to colours, a decimal value between 0 and

  1. W3C accessibility guidelines for colour

contrast suggest that this value be at least approximately 1.96 (500 / 255.0) for proper contrast.

# File lib/color/palette/monocontrast.rb, line 174
def color_diff(c1, c2)
  r = (c1.r - c2.r).abs
  g = (c1.g - c2.g).abs
  b = (c1.b - c2.b).abs
  r + g + b
end
minimum_color_diff=(cd) click to toggle source
# File lib/color/palette/monocontrast.rb, line 74
def minimum_color_diff=(cd) #:noco:
  if cd.nil?
    @minimum_color_diff = DEFAULT_MINIMUM_COLOR_DIFF
  elsif cd > 3.0
    @minimum_color_diff = 3.0
  elsif cd < 0.0
    @minimum_color_diff = 0.0
  else
    @minimum_color_diff = cd
  end
  regenerate(@background[0], @foreground[0])
end
regenerate(background, foreground = nil) click to toggle source

Generate the colour palettes.

# File lib/color/palette/monocontrast.rb, line 96
def regenerate(background, foreground = nil)
  foreground ||= background
  background = background.to_rgb
  foreground = foreground.to_rgb

  @background = {}
  @foreground = {}

  @background[-5] = background.darken_by(10)
  @background[-4] = background.darken_by(25)
  @background[-3] = background.darken_by(50)
  @background[-2] = background.darken_by(75)
  @background[-1] = background.darken_by(85)
  @background[ 0] = background
  @background[+1] = background.lighten_by(85)
  @background[+2] = background.lighten_by(75)
  @background[+3] = background.lighten_by(50)
  @background[+4] = background.lighten_by(25)
  @background[+5] = background.lighten_by(10)

  @foreground[-5] = calculate_foreground(@background[-5], foreground)
  @foreground[-4] = calculate_foreground(@background[-4], foreground)
  @foreground[-3] = calculate_foreground(@background[-3], foreground)
  @foreground[-2] = calculate_foreground(@background[-2], foreground)
  @foreground[-1] = calculate_foreground(@background[-1], foreground)
  @foreground[ 0] = calculate_foreground(@background[ 0], foreground)
  @foreground[+1] = calculate_foreground(@background[+1], foreground)
  @foreground[+2] = calculate_foreground(@background[+2], foreground)
  @foreground[+3] = calculate_foreground(@background[+3], foreground)
  @foreground[+4] = calculate_foreground(@background[+4], foreground)
  @foreground[+5] = calculate_foreground(@background[+5], foreground)
end