color: write the RGB to HSL routine and its associated unit tests.

src/lol/image/color.h

@@ -103,6 +103,31 @@ public:
         return vec4(RGBToHSV(src.rgb), src.a);
     }
 
+    /*
+     * Convert RGB to HSL
+     */
+    static vec3 RGBToHSL(vec3 src)
+    {
+        float K = 0.f;
+
+        if (src.g < src.b)
+            src = src.rbg, K = -1.f;
+
+        if (src.r < src.g)
+            src = src.grb, K = -2.f / 6.f - K;
+
+        float chroma = src.r - min(src.g, src.b);
+        float luma = src.r + min(src.g, src.b);
+        return vec3(abs(K + (src.g - src.b) / (6.f * chroma + 1e-20f)),
+                    chroma / (min(luma, 2.f - luma) + 1e-20f),
+                    0.5f * luma);
+    }
+
+    static vec4 RGBToHSL(vec4 src)
+    {
+        return vec4(RGBToHSL(src.rgb), src.a);
+    }
+
     /*
      * Convert linear HSV to linear HSL
      */

test/unit/color.cpp

@@ -101,6 +101,22 @@ LOLUNIT_FIXTURE(ColorTest)
             LOLUNIT_ASSERT_DOUBLES_EQUAL(v1.b, v2.b, 0.0001);
         }
     }
+
+    LOLUNIT_TEST(RGBToHSL)
+    {
+        for (int r = 0; r < 20; r++)
+        for (int g = 0; g < 20; g++)
+        for (int b = 0; b < 20; b++)
+        {
+            vec3 v1 = vec3(r / 20.f, g / 20.f, b / 20.f);
+            vec3 v2 = Color::RGBToHSL(v1);
+            vec3 v3 = Color::HSVToHSL(Color::RGBToHSV(v1));
+
+            LOLUNIT_ASSERT_DOUBLES_EQUAL(v2.x, v3.x, 0.0001);
+            LOLUNIT_ASSERT_DOUBLES_EQUAL(v2.y, v3.y, 0.0001);
+            LOLUNIT_ASSERT_DOUBLES_EQUAL(v2.z, v3.z, 0.0001);
+        }
+    }
 };
 
 } /* namespace lol */