[Using Sass to automatically pick text colors] #4 - step 2

functions.scss

/**
 * Calculate the luminance for a color.
 * See https://www.w3.org/TR/WCAG20-TECHS/G17.html#G17-tests
 */
@function luminance($color) {
  $red: nth($linear-channel-values, red($color) + 1);
  $green: nth($linear-channel-values, green($color) + 1);
  $blue: nth($linear-channel-values, blue($color) + 1);

  @return .2126 * $red + .7152 * $green + .0722 * $blue;
}

/**
 * Calculate the contrast ratio between two colors.
 * See https://www.w3.org/TR/WCAG20-TECHS/G17.html#G17-tests
 */
@function contrast($back, $front) {
  $backLum: luminance($back) + .05;
  $foreLum: luminance($front) + .05;

  @return max($backLum, $foreLum) / min($backLum, $foreLum);
}

/**
 * Determine whether to use dark or light text on top of given color.
 * Returns black for dark text and white for light text.
 */
@function choose-contrast-color($color) {
  $lightContrast: contrast($color, white);
  $darkContrast: contrast($color, black);

  @if ($lightContrast > $darkContrast) {
    @return white;
  }
  @else {
    @return black;
  }
}

Where do I get the $linear-channel-values ?

Found it:

https://github.com/material-components/material-components-web/blob/master/packages/mdc-theme/_constants.scss

https://github.com/material-components/material-components-web/blob/master/packages/mdc-theme/_constants.scss is currently 404. Frustratingly.

Sass now supports pow, which removes the need for a lookup table in the first place, making the article I wrote partially obsolete 🙂
This is a good thing!

If you're looking to implement something like choose-contrast-color nowadays, I would suggest looking at the current implementation of Material Components Web, specifically the contrast-tone function: https://github.com/material-components/material-components-web/blob/master/packages/mdc-theme/_theme-color.scss

The above link is valid as of May 2021, but if you're reading this in the distant future, I'm sure you'll be able to find the relevant code somewhere in the Material Components Web repo, or whatever succeeds it.

👍