From 5ac2d26329974ea928e72d8f816a6e53423f9807 Mon Sep 17 00:00:00 2001
From: Tobias Bengfort <tobias.bengfort@posteo.de>
Date: Tue, 26 Sep 2017 21:32:09 +0200
Subject: [PATCH] Improve gamma approximation

- Use `pow()` if available (e.g. from sass-planifolia or MathSass)
- Use an approximation that is optimized to produce good contrast values

See also https://github.com/oddbird/accoutrement-color/pull/6 for a
similar change.
---
 core/bourbon/utilities/_gamma.scss       | 9 ++++++---
 spec/bourbon/utilities/gamma_spec.rb     | 2 +-
 spec/bourbon/utilities/lightness_spec.rb | 2 +-
 3 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/core/bourbon/utilities/_gamma.scss b/core/bourbon/utilities/_gamma.scss
index 2314f5a9a..3e5145b12 100644
--- a/core/bourbon/utilities/_gamma.scss
+++ b/core/bourbon/utilities/_gamma.scss
@@ -2,8 +2,7 @@
 
 /// Performs gamma correction on a single color channel.
 ///
-/// Note that Sass does not have a `pow()` function, so the calculation
-/// is approximate.
+/// Note that the calculation is approximate if a `pow()` is not available.
 ///
 /// @argument {number (0-1)} $channel
 ///
@@ -16,6 +15,10 @@
     @return $channel / 12.92;
   } @else {
     $c: ($channel + 0.055) / 1.055;
-    @return (133 * $c * $c * $c + 155 * $c * $c) / 288;
+    @if function-exists("pow") {
+      @return pow($c, 2.4);
+    } @else {
+      @return 0.56 * $c * $c * $c + 0.44 * $c * $c;
+    }
   }
 }
diff --git a/spec/bourbon/utilities/gamma_spec.rb b/spec/bourbon/utilities/gamma_spec.rb
index cdfa8d3f5..2cb0beaaf 100644
--- a/spec/bourbon/utilities/gamma_spec.rb
+++ b/spec/bourbon/utilities/gamma_spec.rb
@@ -7,7 +7,7 @@
 
   context "called on a color channel" do
     it "outputs a gamma value between 0 and 1" do
-      rule = "content: 0.13185;"
+      rule = "content: 0.12168;"
 
       expect(".gamma").to have_ruleset(rule)
     end
diff --git a/spec/bourbon/utilities/lightness_spec.rb b/spec/bourbon/utilities/lightness_spec.rb
index 38c9aa9ea..834acbf39 100644
--- a/spec/bourbon/utilities/lightness_spec.rb
+++ b/spec/bourbon/utilities/lightness_spec.rb
@@ -23,7 +23,7 @@
 
   context "called on gray" do
     it "outputs a number between 0 and 1 to indicate lightness" do
-      rule = "content: 0.21795;"
+      rule = "content: 0.20503;"
 
       expect(".lightness-gray").to have_ruleset(rule)
     end