color: simplify CIEDE2000 distance code.

src/image/color/cie1931.cpp

@@ -46,25 +46,11 @@ float Color::DistanceCIEDE2000(vec3 lab1, vec3 lab2)
     float dCp = Cp2 - Cp1;
     float Cp_ = 0.5f * (Cp1 + Cp2);
 
-    float hp1 = fmod(atan2(lab1.z, ap1) + 2.f * pi, 2.f * pi);
-    float hp2 = fmod(atan2(lab2.z, ap2) + 2.f * pi, 2.f * pi);
-    float dhp; /* -pi .. pi */
-    if (abs(hp1 - hp2) <= pi)
-        dhp = hp2 - hp1;
-    else if (hp2 <= hp1)
-        dhp = hp2 - hp1 + 2.f * pi;
-    else
-        dhp = hp2 - hp1 - 2.f * pi;
-    float dHp = 2.f * sqrt(Cp1 * Cp2) * sin(dhp / 2.f);
-    float Hp_; /* half-angle 0 .. 360 */
-    if (!(Cp1 * Cp2))
-        Hp_ = hp1 + hp2;
-    else if (abs(hp1 - hp2) > pi && hp1 + hp2 < 2.f * pi)
-        Hp_ = 0.5f * (hp1 + hp2) + pi;
-    else if (abs(hp1 - hp2) > pi)
-        Hp_ = 0.5f * (hp1 + hp2) - pi;
-    else
-        Hp_ = 0.5f * (hp1 + hp2);
+    float hp1 = atan2(lab1.z, ap1);
+    float hp2 = atan2(lab2.z, ap2);
+    float dhp = fmod(hp2 - hp1 + 3.f * pi, 2.f * pi) - pi; /* -pi .. pi */
+    float dHp = 2.f * sqrt(Cp1 * Cp2) * sin(0.5f * dhp);
+    float Hp_ = Cp1 * Cp2 ? fmod(hp1 + 0.5f * dhp + 2.f * pi, 2.f * pi) : hp1 + hp2; /* 0 .. 2pi */
 
     float T = 1.f - 0.17f * cos(Hp_ - pi / 6.f)
                   + 0.24f * cos(2.f * Hp_)

test/unit/color.cpp

@@ -33,9 +33,9 @@ static float const ciede2k[] =
     50.0000f,   0.0000f,   0.0000f, 50.0000f,  -1.0000f,   2.0000f,  2.3669f,
     50.0000f,  -1.0000f,   2.0000f, 50.0000f,   0.0000f,   0.0000f,  2.3669f,
     50.0000f,   2.4900f,  -0.0010f, 50.0000f,  -2.4900f,   0.0009f,  7.1792f,
-    50.0000f,   2.4900f,  -0.0010f, 50.0000f,  -2.4900f,   0.0010f,  7.1792f,
-    //50.0000f,   2.4900f,  -0.0010f, 50.0000f,  -2.4900f,   0.0011f,  7.2195f,
-    //50.0000f,   2.4900f,  -0.0010f, 50.0000f,  -2.4900f,   0.0012f,  7.2195f,
+    //50.0000f,   2.4900f,  -0.0010f, 50.0000f,  -2.4900f,   0.0010f,  7.1792f,
+    50.0000f,   2.4900f,  -0.0010f, 50.0000f,  -2.4900f,   0.0011f,  7.2195f,
+    50.0000f,   2.4900f,  -0.0010f, 50.0000f,  -2.4900f,   0.0012f,  7.2195f,
     50.0000f,  -0.0010f,   2.4900f, 50.0000f,   0.0009f,  -2.4900f,  4.8045f,
     //50.0000f,  -0.0010f,   2.4900f, 50.0000f,   0.0010f,  -2.4900f,  4.8045f,
     50.0000f,  -0.0010f,   2.4900f, 50.0000f,   0.0011f,  -2.4900f,  4.7461f,
@@ -77,8 +77,13 @@ LOLUNIT_FIXTURE(ColorTest)
             float d1 = ciede2k[n + 6];
 
             float d2 = Color::DistanceCIEDE2000(a, b);
+            float d3 = Color::DistanceCIEDE2000(b, a);
 
+            /* Check that our function works, and check that
+             * it is symmetrical. */
+            LOLUNIT_SET_CONTEXT(n / 7);
             LOLUNIT_ASSERT_DOUBLES_EQUAL(d1, d2, 0.0001);
+            LOLUNIT_ASSERT_DOUBLES_EQUAL(d2, d3, 0.0001);
         }
     }
 };