Math: improve and clean up angle() tests.

src/Magnum/Math/Test/ComplexTest.cpp

@@ -425,12 +425,21 @@ void ComplexTest::invertedNormalizedNotNormalized() {
 }
 
 void ComplexTest::angle() {
+    auto a = Complex{ 1.5f, -2.0f}.normalized();
+    auto b = Complex{-4.0f,  3.5f}.normalized();
+
     /* Verify also that the angle is the same as angle between 2D vectors */
-    Rad angle = Math::angle(Complex( 1.5f, -2.0f).normalized(),
-                            Complex(-4.0f,  3.5f).normalized());
-    CORRADE_COMPARE(angle, Math::angle(Vector2( 1.5f, -2.0f).normalized(),
-                                       Vector2(-4.0f,  3.5f).normalized()));
-    CORRADE_COMPARE(angle, Rad(2.933128f));
+    CORRADE_COMPARE(Math::angle(a, b), Math::angle(
+        Vector2{ 1.5f, -2.0f}.normalized(),
+        Vector2{-4.0f,  3.5f}.normalized()));
+    CORRADE_COMPARE(Math::angle(a, b), 2.933128_radf);
+    CORRADE_COMPARE(Math::angle(-a, -b), 2.933128_radf);
+    CORRADE_COMPARE(Math::angle(-a, b), Rad(180.0_degf) - 2.933128_radf);
+    CORRADE_COMPARE(Math::angle(a, -b), Rad(180.0_degf) - 2.933128_radf);
+
+    /* Same / opposite */
+    CORRADE_COMPARE(Math::angle(a, a), 0.0_radf);
+    CORRADE_COMPARE(Math::angle(a, -a), 180.0_degf);
 }
 
 void ComplexTest::angleNotNormalized() {

src/Magnum/Math/Test/QuaternionTest.cpp

@@ -491,12 +491,24 @@ void QuaternionTest::rotationNotNormalized() {
 }
 
 void QuaternionTest::angle() {
+    auto a = Quaternion({1.0f, 2.0f, -3.0f}, -4.0f).normalized();
+    auto b = Quaternion({4.0f, -3.0f, 2.0f}, -1.0f).normalized();
+
     /* Verify also that the angle is the same as angle between 4D vectors */
-    Rad angle = Math::angle(Quaternion({1.0f, 2.0f, -3.0f}, -4.0f).normalized(),
-                            Quaternion({4.0f, -3.0f, 2.0f}, -1.0f).normalized());
-    CORRADE_COMPARE(angle, Math::angle(Vector4(1.0f, 2.0f, -3.0f, -4.0f).normalized(),
-                                       Vector4(4.0f, -3.0f, 2.0f, -1.0f).normalized()));
-    CORRADE_COMPARE(angle, Rad(1.704528f));
+    CORRADE_COMPARE(Math::angle(a, b), Math::angle(
+        Vector4{1.0f, 2.0f, -3.0f, -4.0f}.normalized(),
+        Vector4{4.0f, -3.0f, 2.0f, -1.0f}.normalized()));
+    CORRADE_COMPARE(Math::angle(a, b), 1.704528_radf);
+    CORRADE_COMPARE(Math::angle(-a, -b), 1.704528_radf);
+    CORRADE_COMPARE(Math::angle(-a, b), Rad(180.0_degf) - 1.704528_radf);
+    CORRADE_COMPARE(Math::angle(a, -b), Rad(180.0_degf) - 1.704528_radf);
+
+    /* Same / opposite. Well, almost. It's interesting how imprecise
+       normalization can get. */
+    CORRADE_COMPARE_WITH(Math::angle(a, a), 0.0_radf,
+        Corrade::TestSuite::Compare::around(0.0005_radf));
+    CORRADE_COMPARE_WITH(Math::angle(a, -a), 180.0_degf,
+        Corrade::TestSuite::Compare::around(0.0005_radf));
 }
 
 void QuaternionTest::angleNotNormalized() {

src/Magnum/Math/Test/VectorTest.cpp

@@ -25,6 +25,7 @@
 
 #include <sstream>
 #include <Corrade/TestSuite/Tester.h>
+#include <Corrade/TestSuite/Compare/Numeric.h>
 #include <Corrade/Utility/DebugStl.h>
 
 #include "Magnum/Math/Half.h"
@@ -126,6 +127,8 @@ typedef Vector<4, Float> Vector4;
 typedef Vector<4, Half> Vector4h;
 typedef Vector<4, Int> Vector4i;
 
+using namespace Literals;
+
 VectorTest::VectorTest() {
     addTests({&VectorTest::construct,
               &VectorTest::constructFromData,
@@ -565,9 +568,19 @@ void VectorTest::flipped() {
 }
 
 void VectorTest::angle() {
-    CORRADE_COMPARE(Math::angle(Vector3(2.0f,  3.0f, 4.0f).normalized(),
-                                Vector3(1.0f, -2.0f, 3.0f).normalized()),
-                    Rad(1.162514f));
+    auto a = Vector3{2.0f,  3.0f, 4.0f}.normalized();
+    auto b = Vector3{1.0f, -2.0f, 3.0f}.normalized();
+    CORRADE_COMPARE(Math::angle(a, b), 1.162514_radf);
+    CORRADE_COMPARE(Math::angle(-a, -b), 1.162514_radf);
+    CORRADE_COMPARE(Math::angle(-a, b), Rad(180.0_degf) - 1.162514_radf);
+    CORRADE_COMPARE(Math::angle(a, -b), Rad(180.0_degf) - 1.162514_radf);
+
+    /* Same / opposite. Well, almost. It's interesting how imprecise
+       normalization can get. */
+    CORRADE_COMPARE_WITH(Math::angle(a, a), 0.0_radf,
+        Corrade::TestSuite::Compare::around(0.0005_radf));
+    CORRADE_COMPARE_WITH(Math::angle(a, -a), 180.0_degf,
+        Corrade::TestSuite::Compare::around(0.0005_radf));
 }
 
 void VectorTest::angleNotNormalized() {