Include text contrast function?

[hadley]

It's not a perfect match for this package, but it seems like a reasonable home:

rel_l <- function(x) {
  scale <- function(x) {
    ifelse(x <= 0.03928, x / 12.92, ((x + 0.055) / 1.055)^2.4)
  }
  rgb <- farver::decode_colour(x) / 255
  0.2126 * scale(rgb[, 1]) + 0.7152 * scale(rgb[, 2]) + 0.0722 * scale(rgb[, 3])
}
contrast_ratio <- function(x, y) {
  x_l <- rel_l(x)
  y_l <- rel_l(y)

  (pmax(x_l, y_l) + 0.05) / (pmin(x_l, y_l) + 0.05)
}

This implements the WCAG 2.0 colour contrast computation (https://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html) which is useful for checking that colours have sufficient contrast to be easily readable.