color: optimise RGB to HSV conversion routine using my latest findings,

and add a simple unit test function.

src/lol/image/color.h

@@ -84,20 +84,17 @@ public:
      */
     static vec3 RGBToHSV(vec3 src)
     {
-        float offset = 0.f;
-
-        if (src.r < src.g)
-            src = src.grb, offset = -2.f / 6.f;
+        float K = 0.f;
 
         if (src.g < src.b)
-            src = src.rbg, offset = -1.f - 4.f * offset;
+            src = src.rbg, K = -1.f;
 
         if (src.r < src.g)
-            src = src.grb, offset = -2.f / 6.f - offset;
+            src = src.grb, K = -2.f / 6.f - K;
 
-        float chroma = src.r - src.b;
-        return vec3(abs((src.g - src.b) / (6.f * chroma + 1e-40f) + offset),
-                    chroma / (src.r + 1e-40f),
+        float chroma = src.r - min(src.g, src.b);
+        return vec3(abs(K + (src.g - src.b) / (6.f * chroma + 1e-20f)),
+                    chroma / (src.r + 1e-20f),
                     src.r);
     }
 

test/unit/color.cpp

@@ -86,6 +86,21 @@ LOLUNIT_FIXTURE(ColorTest)
             LOLUNIT_ASSERT_DOUBLES_EQUAL(d2, d3, 0.0001);
         }
     }
+
+    LOLUNIT_TEST(RGBToHSV)
+    {
+        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::HSVToRGB(Color::RGBToHSV(v1));
+
+            LOLUNIT_ASSERT_DOUBLES_EQUAL(v1.r, v2.r, 0.0001);
+            LOLUNIT_ASSERT_DOUBLES_EQUAL(v1.g, v2.g, 0.0001);
+            LOLUNIT_ASSERT_DOUBLES_EQUAL(v1.b, v2.b, 0.0001);
+        }
+    }
 };
 
 } /* namespace lol */