determine_font_color.rb

Color = Struct.new(:red, :green, :blue)
FONT_COLOR_ALPHA = 0.9
COLOR_COMPONENT_MAX_VALUE = 255
WHITE_COLOR = Color.new(COLOR_COMPONENT_MAX_VALUE, COLOR_COMPONENT_MAX_VALUE, COLOR_COMPONENT_MAX_VALUE).freeze
BLACK_COLOR = Color.new(0, 0, 0).freeze

def blend_color_components background_value, overlay_value, alpha
  # https://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending
  (overlay_value * alpha + background_value * (1 - alpha)).round
end

def blend_colors background_color, overlay_color, alpha
  Color.new \
    blend_color_components(background_color.red, overlay_color.red, alpha),
    blend_color_components(background_color.green, overlay_color.green, alpha),
    blend_color_components(background_color.blue, overlay_color.blue, alpha)
end

def color_component_value_for_relative_luminance value, max_value
  relative_value = value.to_f / max_value

  if relative_value <= 0.03928
    relative_value / 12.92
  else
    ((relative_value + 0.055) / 1.055) ** 2.4
  end
end

def calculate_relative_luminance color, color_component_max_value = COLOR_COMPONENT_MAX_VALUE
  # https://www.w3.org/TR/WCAG21/#dfn-relative-luminance
  0.2126 * color_component_value_for_relative_luminance(color.red, color_component_max_value) +
  0.7152 * color_component_value_for_relative_luminance(color.green, color_component_max_value) +
  0.0722 * color_component_value_for_relative_luminance(color.blue, color_component_max_value)
end

def calculate_contrast_ratio luminance_1, luminance_2
  # https://www.w3.org/TR/WCAG21/#dfn-contrast-ratio
  darker_luminance, lighter_luminance = [luminance_1, luminance_2].sort
  (lighter_luminance + 0.05) / (darker_luminance + 0.05)
end

def determine_font_color background_color
  light_color = blend_colors background_color, WHITE_COLOR, FONT_COLOR_ALPHA
  dark_color = blend_colors background_color, BLACK_COLOR, FONT_COLOR_ALPHA

  background_luminance = calculate_relative_luminance background_color
  light_color_luminance = calculate_relative_luminance light_color
  dark_color_luminance = calculate_relative_luminance dark_color

  light_color_contrast_ratio = calculate_contrast_ratio background_luminance, light_color_luminance
  dark_color_contrast_ratio = calculate_contrast_ratio background_luminance, dark_color_luminance

  light_color_contrast_ratio > dark_color_contrast_ratio ? light_color : dark_color
end



# Usage:
determine_font_color WHITE_COLOR
# => #<struct Color red=25, green=25, blue=25>

determine_font_color BLACK_COLOR
# => #<struct Color red=230, green=230, blue=230>

determine_font_color Color.new(COLOR_COMPONENT_MAX_VALUE, 0, 0)
# => #<struct Color red=25, green=0, blue=0>

determine_font_color Color.new(0, COLOR_COMPONENT_MAX_VALUE, 0)
# => #<struct Color red=0, green=25, blue=0>

determine_font_color Color.new(0, 0, COLOR_COMPONENT_MAX_VALUE)
# => #<struct Color red=230, green=230, blue=255>