From 9c8bd2f178963420dac09110fb6e7560f6acaeac Mon Sep 17 00:00:00 2001
From: James Jackson-South <james_south@hotmail.com>
Date: Wed, 23 Nov 2016 00:41:38 +1100
Subject: [PATCH] Darken Lighten

---
 src/ImageSharp/Colors/ColorTransforms.cs      | 36 +++++++++++++++++++
 .../Colors/ColorTransformTests.cs             | 14 ++++++++
 2 files changed, 50 insertions(+)

diff --git a/src/ImageSharp/Colors/ColorTransforms.cs b/src/ImageSharp/Colors/ColorTransforms.cs
index 11d0139be..5993ac699 100644
--- a/src/ImageSharp/Colors/ColorTransforms.cs
+++ b/src/ImageSharp/Colors/ColorTransforms.cs
@@ -150,6 +150,42 @@ namespace ImageSharp
             return new Color(Pack(ref result));
         }
 
+        /// <summary>
+        /// Selects the darker of the backdrop and source colors. 
+        /// The backdrop is replaced with the source where the source is darker; otherwise, it is left unchanged.
+        /// </summary>
+        /// <param name="backdrop">The backdrop color.</param>
+        /// <param name="source">The source color.</param>
+        /// <returns>
+        /// The <see cref="Color"/>.
+        /// </returns>
+        public static Color Darken(Color backdrop, Color source)
+        {
+            Vector4 vb = backdrop.ToVector4();
+            Vector4 vs = source.ToVector4();
+            Vector4 result = Vector4.Min(vb, vs);
+            result.W = vb.W;
+            return new Color(Pack(ref result));
+        }
+
+        /// <summary>
+        /// Selects the lighter of the backdrop and source colors. 
+        /// The backdrop is replaced with the source where the source is lighter; otherwise, it is left unchanged.
+        /// </summary>
+        /// <param name="backdrop">The backdrop color.</param>
+        /// <param name="source">The source color.</param>
+        /// <returns>
+        /// The <see cref="Color"/>.
+        /// </returns>
+        public static Color Lighten(Color backdrop, Color source)
+        {
+            Vector4 vb = backdrop.ToVector4();
+            Vector4 vs = source.ToVector4();
+            Vector4 result = Vector4.Max(vb, vs);
+            result.W = vb.W;
+            return new Color(Pack(ref result));
+        }
+
         /// <summary>
         /// Linearly interpolates from one color to another based on the given weighting.
         /// </summary>
diff --git a/tests/ImageSharp.Tests/Colors/ColorTransformTests.cs b/tests/ImageSharp.Tests/Colors/ColorTransformTests.cs
index dc710549b..41835e7c6 100644
--- a/tests/ImageSharp.Tests/Colors/ColorTransformTests.cs
+++ b/tests/ImageSharp.Tests/Colors/ColorTransformTests.cs
@@ -56,5 +56,19 @@ namespace ImageSharp.Tests.Colors
             Color overlay = Color.Overlay(Backdrop, Source);
             Assert.True(overlay == new Color(153, 82, 0));
         }
+
+        [Fact]
+        public void Darken()
+        {
+            Color darken = Color.Darken(Backdrop, Source);
+            Assert.True(darken == new Color(0, 102, 0));
+        }
+
+        [Fact]
+        public void Lighten()
+        {
+            Color lighten = Color.Lighten(Backdrop, Source);
+            Assert.True(lighten == new Color(204, 102, 153));
+        }
     }
 }