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Diffstat (limited to '')
-rw-r--r-- | devtools/server/actors/accessibility/worker.js | 103 |
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 }; +} |