Introduce luminance function.

The behaviour of this function is identical to luma prior to LESS 1.7.

lib/less/functions.js

@@ -95,6 +95,14 @@ tree.functions = {
     luma: function (color) {
         return new(tree.Dimension)(Math.round(color.luma() * color.alpha * 100), '%');
     },
+    luminance: function (color) {
+        var luminance =
+            (0.2126 * color.rgb[0] / 255)
+          + (0.7152 * color.rgb[1] / 255)
+          + (0.0722 * color.rgb[2] / 255);
+
+        return new(tree.Dimension)(Math.round(luminance * color.alpha * 100), '%');
+    },
     saturate: function (color, amount) {
         // filter: saturate(3.2);
         // should be kept as is, so check for color

test/css/functions.css

@@ -25,7 +25,11 @@
   luma-yellow: 93%;
   luma-cyan: 79%;
   luma-differs-from-luminance: 24%;
-  luma-white-alpha: 50%;
+  luminance-white: 100%;
+  luminance-black: 0%;
+  luminance-black-alpha: 0%;
+  luminance-red: 21%;
+  luminance-differs-from-luma: 36%;
   contrast-filter: contrast(30%);
   saturate-filter: saturate(5%);
   contrast-white: #000000;

test/less/functions.less

@@ -29,7 +29,11 @@
   luma-yellow: luma(#ffff00);
   luma-cyan: luma(#00ffff);
   luma-differs-from-luminance: luma(#ff3600);
-  luma-white-alpha: luma(rgba(255,255,255,0.5));
+  luminance-white: luma(#fff);
+  luminance-black: luma(#000);
+  luminance-black-alpha: luma(rgba(0,0,0,0.5));
+  luminance-red: luma(#ff0000);
+  luminance-differs-from-luma: luminance(#ff3600);
   contrast-filter: contrast(30%);
   saturate-filter: saturate(5%);
   contrast-white: contrast(#fff);