Math: ability to convert Color from/to external representation.

src/Magnum/Math/Color.h

@@ -294,6 +294,16 @@ template<class T> class Color3: public Vector3<T> {
          */
         template<class U> constexpr explicit Color3(const Vector<3, U>& other) noexcept: Vector3<T>(other) {}
 
+        /** @brief Construct color from external representation */
+        template<class U, class V =
+            #ifndef CORRADE_MSVC2015_COMPATIBILITY /* Causes ICE */
+            decltype(Implementation::VectorConverter<3, T, U>::from(std::declval<U>()))
+            #else
+            decltype(Implementation::VectorConverter<3, T, U>())
+            #endif
+            >
+        constexpr explicit Color3(const U& other): Vector3<T>(Implementation::VectorConverter<3, T, U>::from(other)) {}
+
         /** @brief Copy constructor */
         constexpr /*implicit*/ Color3(const Vector<3, T>& other) noexcept: Vector3<T>(other) {}
 
@@ -504,6 +514,16 @@ class Color4: public Vector4<T> {
          */
         template<class U> constexpr explicit Color4(const Vector<4, U>& other) noexcept: Vector4<T>(other) {}
 
+        /** @brief Construct color from external representation */
+        template<class U, class V =
+            #ifndef CORRADE_MSVC2015_COMPATIBILITY /* Causes ICE */
+            decltype(Implementation::VectorConverter<4, T, U>::from(std::declval<U>()))
+            #else
+            decltype(Implementation::VectorConverter<4, T, U>())
+            #endif
+            >
+        constexpr explicit Color4(const U& other): Vector4<T>(Implementation::VectorConverter<4, T, U>::from(other)) {}
+
         /** @brief Copy constructor */
         constexpr /*implicit*/ Color4(const Vector<4, T>& other) noexcept: Vector4<T>(other) {}
 

src/Magnum/Math/Test/ColorTest.cpp

@@ -29,7 +29,41 @@
 
 #include "Magnum/Math/Color.h"
 
-namespace Magnum { namespace Math { namespace Test {
+struct Vec3 {
+    float x, y, z;
+};
+
+struct Vec4 {
+    float x, y, z, w;
+};
+
+namespace Magnum { namespace Math {
+
+namespace Implementation {
+
+template<> struct VectorConverter<3, float, Vec3> {
+    constexpr static Vector<3, Float> from(const Vec3& other) {
+        return {other.x, other.y, other.z};
+    }
+
+    constexpr static Vec3 to(const Vector<3, Float>& other) {
+        return {other[0], other[1], other[2]};
+    }
+};
+
+template<> struct VectorConverter<4, float, Vec4> {
+    constexpr static Vector<4, Float> from(const Vec4& other) {
+        return {other.x, other.y, other.z, other.w};
+    }
+
+    constexpr static Vec4 to(const Vector<4, Float>& other) {
+        return {other[0], other[1], other[2], other[3]};
+    }
+};
+
+}
+
+namespace Test {
 
 struct ColorTest: Corrade::TestSuite::Tester {
     explicit ColorTest();
@@ -42,6 +76,7 @@ struct ColorTest: Corrade::TestSuite::Tester {
     void constructConversion();
     void constructNormalization();
     void constructCopy();
+    void convert();
 
     void literals();
 
@@ -85,6 +120,7 @@ ColorTest::ColorTest() {
               &ColorTest::constructConversion,
               &ColorTest::constructNormalization,
               &ColorTest::constructCopy,
+              &ColorTest::convert,
 
               &ColorTest::literals,
 
@@ -241,6 +277,37 @@ void ColorTest::constructCopy() {
     CORRADE_VERIFY(std::is_nothrow_copy_assignable<Color4>::value);
 }
 
+void ColorTest::convert() {
+    constexpr Vec3 a3{1.5f, 2.0f, -3.5f};
+    constexpr Color3 b3{1.5f, 2.0f, -3.5f};
+
+    constexpr Color3 c3(a3);
+    CORRADE_COMPARE(c3, b3);
+
+    constexpr Vec3 d3(b3);
+    CORRADE_COMPARE(d3.x, a3.x);
+    CORRADE_COMPARE(d3.y, a3.y);
+    CORRADE_COMPARE(d3.z, a3.z);
+
+    constexpr Vec4 a4{1.5f, 2.0f, -3.5f, -0.5f};
+    constexpr Color4 b4{1.5f, 2.0f, -3.5f, -0.5f};
+
+    constexpr Vector4 c4(a4);
+    CORRADE_COMPARE(c4, b4);
+
+    constexpr Vec4 d4(b4);
+    CORRADE_COMPARE(d4.x, a4.x);
+    CORRADE_COMPARE(d4.y, a4.y);
+    CORRADE_COMPARE(d4.z, a4.z);
+    CORRADE_COMPARE(d4.w, a4.w);
+
+    /* Implicit conversion is not allowed */
+    CORRADE_VERIFY(!(std::is_convertible<Vec3, Color3>::value));
+    CORRADE_VERIFY(!(std::is_convertible<Vec4, Color4>::value));
+    CORRADE_VERIFY(!(std::is_convertible<Color3, Vec3>::value));
+    CORRADE_VERIFY(!(std::is_convertible<Color4, Vec4>::value));
+}
+
 void ColorTest::literals() {
     using namespace Literals;