1 files changed, 61 insertions, 9 deletions

diff --git a/src/services/color_convert/color_convert.js b/src/services/color_convert/color_convert.js
index 31ee3a6b..0acc7e7c 100644
--- a/src/services/color_convert/color_convert.js
+++ b/src/services/color_convert/color_convert.js
@@ -19,15 +19,21 @@ const rgb2hex = (r, g, b) => {
   return `#${((1 << 24) + (r << 16) + (g << 8) + b).toString(16).slice(1)}`
 }
 
-// https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
-// https://www.w3.org/TR/2008/REC-WCAG20-20081211/relative-luminance.xml
-// https://en.wikipedia.org/wiki/SRGB#The_reverse_transformation
-const c2linear = (b) => {
+/**
+ * Converts 8-bit RGB component into linear component
+ * https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
+ * https://www.w3.org/TR/2008/REC-WCAG20-20081211/relative-luminance.xml
+ * https://en.wikipedia.org/wiki/SRGB#The_reverse_transformation
+ *
+ * @param {Number} bit - color component [0..255]
+ * @returns {Number} linear component [0..1]
+ */
+const c2linear = (bit) => {
   // W3C gives 0.03928 while wikipedia states 0.04045
   // what those magical numbers mean - I don't know.
   // something about gamma-correction, i suppose.
   // Sticking with W3C example.
-  const c = b / 255
+  const c = bit / 255
   if (c < 0.03928) {
     return c / 12.92
   } else {
@@ -35,18 +41,36 @@ const c2linear = (b) => {
   }
 }
 
+/**
+ * Converts sRGB into linear RGB
+ * @param {Object} srgb - sRGB color
+ * @returns {Object} linear rgb color
+ */
 const srgbToLinear = (srgb) => {
   return 'rgb'.split('').reduce((acc, c) => { acc[c] = c2linear(srgb[c]); return acc }, {})
 }
 
-// https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
-// https://www.w3.org/TR/2008/REC-WCAG20-20081211/relative-luminance.xml
+/**
+ * Calculates relative luminance for given color
+ * https://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
+ * https://www.w3.org/TR/2008/REC-WCAG20-20081211/relative-luminance.xml
+ *
+ * @param {Object} srgb - sRGB color
+ * @returns {Number} relative luminance
+ */
 const relativeLuminance = (srgb) => {
   const {r, g, b} = srgbToLinear(srgb)
   return 0.2126 * r + 0.7152 * g + 0.0722 * b
 }
 
-// https://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
+/**
+ * Generates color ratio between two colors. Order is unimporant
+ * https://www.w3.org/TR/2008/REC-WCAG20-20081211/#contrast-ratiodef
+ *
+ * @param {Object} a - sRGB color
+ * @param {Object} b - sRGB color
+ * @returns {Number} color ratio
+ */
 const getContrastRatio = (a, b) => {
   const la = relativeLuminance(a)
   const lb = relativeLuminance(b)
@@ -55,6 +79,33 @@ const getContrastRatio = (a, b) => {
   return (l1 + 0.05) / (l2 + 0.05)
 }
 
+/**
+ * This generates what "worst case" color would look like for transparent
+ * segments. I.e. black with .2 alpha and pure-white background image
+ * could make white text unreadable
+ *
+ * @param {Object} srgb - transparent color
+ * @param {Number} alpha - color's opacity/alpha channel
+ * @param {Boolean} white - use white "background" if true, black otherwise
+ * @returns {Object} sRGB of resulting color
+ */
+const transparentWorstCase = (srgb, alpha, white = false) => {
+  const bg = 'rgb'.split('').reduce((acc, c) => { acc[c] = Number(white) * 255; return acc }, {})
+  return 'rgb'.split('').reduce((acc, c) => {
+    // Simplified https://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending
+    // for opaque bg and transparent fg
+    acc[c] = (srgb[c] * alpha + bg[c] * (1 - alpha))
+    return acc
+  }, {})
+}
+
+const worstCase = (bg, bga, text) => {
+  if (bga === 1 || typeof bga === 'undefined') return bg
+  // taken from https://github.com/toish/chromatism/blob/master/src/operations/contrastRatio.js
+  const blackWorse = ((text.r * 299) + (text.g * 587) + (text.b * 114)) / 1000 <= 128
+  return transparentWorstCase(bg, bga, !blackWorse)
+}
+
 const hex2rgb = (hex) => {
   const result = /^#?([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(hex)
   return result ? {
@@ -84,5 +135,6 @@ export {
   hex2rgb,
   mixrgb,
   rgbstr2hex,
-  getContrastRatio
+  getContrastRatio,
+  worstCase
 }