feat(color-contrast): add support for CSS mix-blend-mode

lib/commons/color/flatten-colors.js

@@ -1,11 +1,66 @@
 import Color from './color';
 
+// clamp a value between two numbers (inclusive)
+function clamp(value, min, max) {
+  return Math.min(Math.max(min, value), max);
+}
+
 // how to combine background and foreground colors together when using
 // the CSS property `mix-blend-mode`. Defaults to `normal`
 // @see https://www.w3.org/TR/compositing-1/#blendingseparable
 const blendFunctions = {
   normal(Cb, Cs) {
     return Cs;
+  },
+  multiply(Cb, Cs) {
+    // @see https://www.w3.org/TR/compositing-1/#blendingmultiply
+    return Cs * Cb;
+  },
+  screen(Cb, Cs) {
+    // @see https://www.w3.org/TR/compositing-1/#blendingscreen
+    return Cb + Cs - Cb * Cs;
+  },
+  overlay(Cb, Cs) {
+    // @see https://www.w3.org/TR/compositing-1/#blendingoverlay
+    return this['hard-light'](Cs, Cb);
+  },
+  darken(Cb, Cs) {
+    // @see https://www.w3.org/TR/compositing-1/#blendingdarken
+    return Math.min(Cb, Cs);
+  },
+  lighten(Cb, Cs) {
+    // @see https://www.w3.org/TR/compositing-1/#blendinglighten
+    return Math.max(Cb, Cs);
+  },
+  'color-dodge'(Cb, Cs) {
+    // @see https://www.w3.org/TR/compositing-1/#blendingcolordodge
+    return Cb === 0 ? 0 : Cs === 1 ? 1 : Math.min(1, Cb / (1 - Cs));
+  },
+  'color-burn'(Cb, Cs) {
+    // @see https://www.w3.org/TR/compositing-1/#blendingcolorburn
+    return Cb === 1 ? 1 : Cs === 0 ? 0 : 1 - Math.min(1, (1 - Cb) / Cs);
+  },
+  'hard-light'(Cb, Cs) {
+    // @see https://www.w3.org/TR/compositing-1/#blendinghardlight
+
+    return Cs <= 0.5 ? this.multiply(Cb, 2 * Cs) : this.screen(Cb, 2 * Cs - 1);
+  },
+  'soft-light'(Cb, Cs) {
+    // @see https://www.w3.org/TR/compositing-1/#blendingsoftlight
+    if (Cs <= 0.5) {
+      return Cb - (1 - 2 * Cs) * Cb * (1 - Cb);
+    } else {
+      const D = Cb <= 0.25 ? ((16 * Cb - 12) * Cb + 4) * Cb : Math.sqrt(Cb);
+      return Cb + (2 * Cs - 1) * (D - Cb);
+    }
+  },
+  difference(Cb, Cs) {
+    // @see https://www.w3.org/TR/compositing-1/#blendingdifference
+    return Math.abs(Cb - Cs);
+  },
+  exclusion(Cb, Cs) {
+    // @see https://www.w3.org/TR/compositing-1/#blendingexclusion
+    return Cb + Cs - 2 * Cb * Cs;
   }
 };
 
@@ -19,12 +74,12 @@ const blendFunctions = {
 // @see https://www.w3.org/TR/compositing-1/#blending
 // @see https://ciechanow.ski/alpha-compositing/
 function simpleAlphaCompositing(Cs, αs, Cb, αb, blendMode) {
-  // RGB color space doesn't have decimal values so we will follow what browsers do and round
-  // e.g. rgb(255.2, 127.5, 127.8) === rgb(255, 128, 128)
-  return Math.round(
+  return (
     αs * (1 - αb) * Cs +
-      αs * αb * blendFunctions[blendMode](Cb, Cs) +
-      (1 - αs) * αb * Cb
+    // Note: Cs and Cb values need to be between 0 and 1 inclusive for the blend function
+    // @see https://www.w3.org/TR/compositing-1/#simplealphacompositing
+    αs * αb * blendFunctions[blendMode](Cb / 255, Cs / 255) * 255 +
+    (1 - αs) * αb * Cb
   );
 }
 
@@ -63,12 +118,22 @@ function flattenColors(fgColor, bgColor, blendMode = 'normal') {
 
   // formula: αo = αs + αb x (1 - αs)
   // clamp alpha between 0 and 1
-  const a = Math.max(
-    0,
-    Math.min(fgColor.alpha + bgColor.alpha * (1 - fgColor.alpha), 1)
-  );
+  const αo = clamp(fgColor.alpha + bgColor.alpha * (1 - fgColor.alpha), 0, 1);
+
+  // simple alpha compositing gives premultiplied values, but our Color
+  // constructor takes unpremultiplied values. So we need to divide the
+  // final color values by the final alpha
+  // formula: Co = co / αo
+  // @see https://www.w3.org/TR/compositing-1/#simplealphacompositing
+  // @see https://github.com/w3c/fxtf-drafts/issues/440#issuecomment-956418953
+  //
+  // RGB color space doesn't have decimal values so we will follow what browsers do and round
+  // e.g. rgb(255.2, 127.5, 127.8) === rgb(255, 128, 128)
+  const Cr = Math.round(r / αo);
+  const Cg = Math.round(g / αo);
+  const Cb = Math.round(b / αo);
 
-  return new Color(r, g, b, a);
+  return new Color(Cr, Cg, Cb, αo);
 }
 
 export default flattenColors;

lib/commons/color/get-background-color.js

@@ -27,8 +27,9 @@ export default function getBackgroundColor(
 ) {
   let bgColors = getTextShadowColors(elm, { minRatio: shadowOutlineEmMax });
   if (bgColors.length) {
-    bgColors = [bgColors.reduce(flattenShadowColors)];
+    bgColors = [{ color: bgColors.reduce(flattenShadowColors) }];
   }
+
   const elmStack = getBackgroundStack(elm);
 
   // Search the stack until we have an alpha === 1 background
@@ -53,7 +54,11 @@ export default function getBackgroundColor(
     if (bgColor.alpha !== 0) {
       // store elements contributing to the br color.
       bgElms.push(bgElm);
-      bgColors.unshift(bgColor);
+      const blendMode = bgElmStyle.getPropertyValue('mix-blend-mode');
+      bgColors.unshift({
+        color: bgColor,
+        blendMode: normalizeBlendMode(blendMode)
+      });
 
       // Exit if the background is opaque
       return bgColor.alpha === 1;
@@ -72,14 +77,28 @@ export default function getBackgroundColor(
   );
   bgColors.unshift(...pageBgs);
 
+  // default to white if bgColors is empty
+  if (bgColors.length === 0) {
+    return new Color(255, 255, 255, 1);
+  }
   // Mix the colors together. Colors must be mixed in bottom up
   // order (background to foreground order) to produce the correct
   // result.
   // @see https://github.com/dequelabs/axe-core/issues/2924
-  var colors = bgColors.reduce((bgColor, fgColor) => {
-    return flattenColors(fgColor, bgColor);
+  const blendedColor = bgColors.reduce((bgColor, fgColor) => {
+    return flattenColors(
+      fgColor.color,
+      bgColor.color instanceof Color ? bgColor.color : bgColor,
+      fgColor.blendMode
+    );
   });
-  return colors;
+
+  // default page background is white which must be mixed last
+  // @see https://www.w3.org/TR/compositing-1/#pagebackdrop
+  return flattenColors(
+    blendedColor.color instanceof Color ? blendedColor.color : blendedColor,
+    new Color(255, 255, 255, 1)
+  );
 }
 
 /**
@@ -99,6 +118,9 @@ function elmPartiallyObscured(elm, bgElm, bgColor) {
   return obscured;
 }
 
+function normalizeBlendMode(blendmode) {
+  return !!blendmode ? blendmode : undefined;
+}
 /**
  * Get the page background color.
  * @private
@@ -131,24 +153,30 @@ function getPageBackgroundColors(elm, stackContainsBody) {
     const bodyBgColor = getOwnBackgroundColor(bodyStyle);
     const bodyBgColorApplies =
       bodyBgColor.alpha !== 0 && visuallyContains(elm, body);
-
     if (
       (bodyBgColor.alpha !== 0 && htmlBgColor.alpha === 0) ||
       (bodyBgColorApplies && bodyBgColor.alpha !== 1)
     ) {
-      pageColors.unshift(bodyBgColor);
+      pageColors.unshift({
+        color: bodyBgColor,
+        blendMode: normalizeBlendMode(
+          bodyStyle.getPropertyValue('mix-blend-mode')
+        )
+      });
     }
 
     if (
       htmlBgColor.alpha !== 0 &&
       (!bodyBgColorApplies || (bodyBgColorApplies && bodyBgColor.alpha !== 1))
     ) {
-      pageColors.unshift(htmlBgColor);
+      pageColors.unshift({
+        color: htmlBgColor,
+        blendMode: normalizeBlendMode(
+          htmlStyle.getPropertyValue('mix-blend-mode')
+        )
+      });
     }
   }
 
-  // default page background is white
-  pageColors.unshift(new Color(255, 255, 255, 1));
-
   return pageColors;
 }