1 files changed, 103 insertions, 0 deletions
diff --git a/devtools/server/actors/accessibility/worker.js b/devtools/server/actors/accessibility/worker.js
new file mode 100644
index 0000000000..75dc78e5b2
--- /dev/null
+++ b/devtools/server/actors/accessibility/worker.js
@@ -0,0 +1,103 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+"use strict";
+
+/**
+ * Import `createTask` to communicate with `devtools/shared/worker`.
+ */
+importScripts("resource://gre/modules/workers/require.js");
+const { createTask } = require("resource://devtools/shared/worker/helper.js");
+
+/**
+ * @see LineGraphWidget.prototype.setDataFromTimestamps in Graphs.js
+ * @param number id
+ * @param array timestamps
+ * @param number interval
+ * @param number duration
+ */
+createTask(self, "getBgRGBA", ({ dataTextBuf, dataBackgroundBuf }) =>
+  getBgRGBA(dataTextBuf, dataBackgroundBuf)
+);
+
+/**
+ * Calculates the luminance of a rgba tuple based on the formula given in
+ * https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
+ *
+ * @param {Array} rgba An array with [r,g,b,a] values.
+ * @return {Number} The calculated luminance.
+ */
+function calculateLuminance(rgba) {
+  for (let i = 0; i < 3; i++) {
+    rgba[i] /= 255;
+    rgba[i] =
+      rgba[i] < 0.03928
+        ? rgba[i] / 12.92
+        : Math.pow((rgba[i] + 0.055) / 1.055, 2.4);
+  }
+  return 0.2126 * rgba[0] + 0.7152 * rgba[1] + 0.0722 * rgba[2];
+}
+
+/**
+ * Get RGBA or a range of RGBAs for the background pixels under the text. If luminance is
+ * uniform, only return one value of RGBA, otherwise return values that correspond to the
+ * min and max luminances.
+ * @param  {ImageData} dataTextBuf
+ *         pixel data for the accessible object with text visible.
+ * @param  {ImageData} dataBackgroundBuf
+ *         pixel data for the accessible object with transparent text.
+ * @return {Object}
+ *         RGBA or a range of RGBAs with min and max values.
+ */
+function getBgRGBA(dataTextBuf, dataBackgroundBuf) {
+  let min = [0, 0, 0, 1];
+  let max = [255, 255, 255, 1];
+  let minLuminance = 1;
+  let maxLuminance = 0;
+  const luminances = {};
+  const dataText = new Uint8ClampedArray(dataTextBuf);
+  const dataBackground = new Uint8ClampedArray(dataBackgroundBuf);
+
+  let foundDistinctColor = false;
+  for (let i = 0; i < dataText.length; i = i + 4) {
+    const tR = dataText[i];
+    const bgR = dataBackground[i];
+    const tG = dataText[i + 1];
+    const bgG = dataBackground[i + 1];
+    const tB = dataText[i + 2];
+    const bgB = dataBackground[i + 2];
+
+    // Ignore pixels that are the same where pixels that are different between the two
+    // images are assumed to belong to the text within the node.
+    if (tR === bgR && tG === bgG && tB === bgB) {
+      continue;
+    }
+
+    foundDistinctColor = true;
+
+    const bgColor = `rgb(${bgR}, ${bgG}, ${bgB})`;
+    let luminance = luminances[bgColor];
+
+    if (!luminance) {
+      // Calculate luminance for the RGB value and store it to only measure once.
+      luminance = calculateLuminance([bgR, bgG, bgB]);
+      luminances[bgColor] = luminance;
+    }
+
+    if (minLuminance >= luminance) {
+      minLuminance = luminance;
+      min = [bgR, bgG, bgB, 1];
+    }
+
+    if (maxLuminance <= luminance) {
+      maxLuminance = luminance;
+      max = [bgR, bgG, bgB, 1];
+    }
+  }
+
+  if (!foundDistinctColor) {
+    return null;
+  }
+
+  return minLuminance === maxLuminance ? { value: max } : { min, max };
+}