Use WCAG contrast algo (twbs#30168)

scss/_functions.scss

@@ -88,14 +88,43 @@
 }
 
 // Color contrast
-@function color-yiq($color, $dark: $yiq-text-dark, $light: $yiq-text-light) {
-  $r: red($color);
-  $g: green($color);
-  $b: blue($color);
+// See https://github.com/twbs/bootstrap/pull/30168
 
-  $yiq: (($r * 299) + ($g * 587) + ($b * 114)) / 1000;
+// A list of pre-calculated numbers of pow(($value / 255 + .055) / 1.055, 2.4). (from 0 to 255)
+// stylelint-disable-next-line scss/dollar-variable-default, scss/dollar-variable-pattern
+$_luminance-list: .0008 .001 .0011 .0013 .0015 .0017 .002 .0022 .0025 .0027 .003 .0033 .0037 .004 .0044 .0048 .0052 .0056 .006 .0065 .007 .0075 .008 .0086 .0091 .0097 .0103 .011 .0116 .0123 .013 .0137 .0144 .0152 .016 .0168 .0176 .0185 .0194 .0203 .0212 .0222 .0232 .0242 .0252 .0262 .0273 .0284 .0296 .0307 .0319 .0331 .0343 .0356 .0369 .0382 .0395 .0409 .0423 .0437 .0452 .0467 .0482 .0497 .0513 .0529 .0545 .0561 .0578 .0595 .0612 .063 .0648 .0666 .0685 .0704 .0723 .0742 .0762 .0782 .0802 .0823 .0844 .0865 .0887 .0908 .0931 .0953 .0976 .0999 .1022 .1046 .107 .1095 .1119 .1144 .117 .1195 .1221 .1248 .1274 .1301 .1329 .1356 .1384 .1413 .1441 .147 .15 .1529 .1559 .159 .162 .1651 .1683 .1714 .1746 .1779 .1812 .1845 .1878 .1912 .1946 .1981 .2016 .2051 .2086 .2122 .2159 .2195 .2232 .227 .2307 .2346 .2384 .2423 .2462 .2502 .2542 .2582 .2623 .2664 .2705 .2747 .2789 .2831 .2874 .2918 .2961 .3005 .305 .3095 .314 .3185 .3231 .3278 .3325 .3372 .3419 .3467 .3515 .3564 .3613 .3663 .3712 .3763 .3813 .3864 .3916 .3968 .402 .4072 .4125 .4179 .4233 .4287 .4342 .4397 .4452 .4508 .4564 .4621 .4678 .4735 .4793 .4851 .491 .4969 .5029 .5089 .5149 .521 .5271 .5333 .5395 .5457 .552 .5583 .5647 .5711 .5776 .5841 .5906 .5972 .6038 .6105 .6172 .624 .6308 .6376 .6445 .6514 .6584 .6654 .6724 .6795 .6867 .6939 .7011 .7084 .7157 .7231 .7305 .7379 .7454 .7529 .7605 .7682 .7758 .7835 .7913 .7991 .807 .8148 .8228 .8308 .8388 .8469 .855 .8632 .8714 .8796 .8879 .8963 .9047 .9131 .9216 .9301 .9387 .9473 .956 .9647 .9734 .9823 .9911 1;
 
-  @return if($yiq >= $yiq-contrasted-threshold, $dark, $light);
+@function color-contrast($background, $color-contrast-dark: $color-contrast-dark, $color-contrast-light: $color-contrast-light) {
+  $l1: luminance($background);
+  $l2: luminance(opaque($background, $color-contrast-light));
+
+  $contrast: if($l1 > $l2, ($l1 + .05) / ($l2 + .05), ($l2 + .05) / ($l1 + .05));
+
+  @return if($contrast < $min-contrast-ratio, $color-contrast-dark, $color-contrast-light);
+}
+
+// Return WCAG2.0 relative luminance
+// See https://www.w3.org/WAI/GL/wiki/Relative_luminance
+// See https://www.w3.org/TR/WCAG20-TECHS/G17.html#G17-tests
+@function luminance($color) {
+  $rgb: (
+    "r": red($color),
+    "g": green($color),
+    "b": blue($color)
+  );
+
+  @each $name, $value in $rgb {
+    $value: if($value / 255 < .03928, $value / 255 / 12.92, nth($_luminance-list, $value + 1));
+    $rgb: map-merge($rgb, ($name: $value));
+  }
+
+  @return (map-get($rgb, "r") * .2126) + (map-get($rgb, "g") * .7152) + (map-get($rgb, "b") * .0722);
+}
+
+// Return opaque color
+// opaque(#fff, rgba(0, 0, 0, .5)) => #808080
+@function opaque($background, $foreground) {
+  @return mix(rgba($foreground, 1), $background, opacity($foreground) * 100);
 }
 
 // Request a color level

scss/_variables.scss

@@ -79,14 +79,15 @@ $theme-colors: (
 ) !default;
 
 // Set a specific jump point for requesting color jumps
-$theme-color-interval:      8% !default;
+$theme-color-interval: 8% !default;
 
-// The yiq lightness value that determines when the lightness of color changes from "dark" to "light". Acceptable values are between 0 and 255.
-$yiq-contrasted-threshold:  150 !default;
+// The contrast ratio to reach against white, to determine if color changes from "light" to "dark". Acceptable values for WCAG 2.0 are 3, 4.5 and 7.
+// See https://www.w3.org/TR/WCAG20/#visual-audio-contrast-contrast
+$min-contrast-ratio:   3 !default;
 
-// Customize the light and dark text colors for use in our YIQ color contrast function.
-$yiq-text-dark:             $gray-900 !default;
-$yiq-text-light:            $white !default;
+// Customize the light and dark text colors for use in our color contrast function.
+$color-contrast-dark:      $gray-900 !default;
+$color-contrast-light:     $white !default;
 
 // fusv-disable
 $blue-100: tint-color($blue, 8) !default;

scss/mixins/_buttons.scss

@@ -6,13 +6,13 @@
 @mixin button-variant(
   $background,
   $border,
-  $color: color-yiq($background),
+  $color: color-contrast($background),
   $hover-background: darken($background, 7.5%),
   $hover-border: darken($border, 10%),
-  $hover-color: color-yiq($hover-background),
+  $hover-color: color-contrast($hover-background),
   $active-background: darken($background, 10%),
   $active-border: darken($border, 12.5%),
-  $active-color: color-yiq($active-background)
+  $active-color: color-contrast($active-background)
 ) {
   color: $color;
   @include gradient-bg($background);
@@ -69,10 +69,10 @@
 
 @mixin button-outline-variant(
   $color,
-  $color-hover: color-yiq($color),
+  $color-hover: color-contrast($color),
   $active-background: $color,
   $active-border: $color,
-  $active-color: color-yiq($active-background)
+  $active-color: color-contrast($active-background)
 ) {
   color: $color;
   border-color: $color;

scss/mixins/_forms.scss

@@ -32,7 +32,7 @@
     margin-top: .1rem;
     @include font-size($form-feedback-tooltip-font-size);
     line-height: $form-feedback-tooltip-line-height;
-    color: color-yiq($color);
+    color: color-contrast($color);
     background-color: rgba($color, $form-feedback-tooltip-opacity);
     @include border-radius($form-feedback-tooltip-border-radius);
   }