Add color premultiply/unpremultiply (flutter#57)

Author: bdero

impeller/geometry/color.h

@@ -49,6 +49,17 @@ struct Color {
            alpha == c.alpha;
   }
 
+  constexpr Color Premultiply() const {
+    return {red * alpha, green * alpha, blue * alpha, alpha};
+  }
+
+  constexpr Color Unpremultiply() const {
+    if (ScalarNearlyEqual(alpha, 0.0)) {
+      return Color::BlackTransparent();
+    }
+    return {red / alpha, green / alpha, blue / alpha, alpha};
+  }
+
   static constexpr Color White() { return {1.0, 1.0, 1.0, 1.0}; }
 
   static constexpr Color Black() { return {0.0, 0.0, 0.0, 1.0}; }

impeller/geometry/geometry_unittests.cc

@@ -2,7 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include <limits>
 #include "impeller/geometry/geometry_unittests.h"
 #include <limits>
 #include "flutter/testing/testing.h"
@@ -568,6 +567,29 @@ TEST(GeometryTest, PointReflect) {
   }
 }
 
+TEST(GeometryTest, ColorPremultiply) {
+  {
+    Color a(1.0, 0.5, 0.2, 0.5);
+    Color premultiplied = a.Premultiply();
+    Color expected = Color(0.5, 0.25, 0.1, 0.5);
+    ASSERT_COLOR_NEAR(premultiplied, expected);
+  }
+
+  {
+    Color a(0.5, 0.25, 0.1, 0.5);
+    Color unpremultiplied = a.Unpremultiply();
+    Color expected = Color(1.0, 0.5, 0.2, 0.5);
+    ASSERT_COLOR_NEAR(unpremultiplied, expected);
+  }
+
+  {
+    Color a(0.5, 0.25, 0.1, 0.0);
+    Color unpremultiplied = a.Unpremultiply();
+    Color expected = Color(0.0, 0.0, 0.0, 0.0);
+    ASSERT_COLOR_NEAR(unpremultiplied, expected);
+  }
+}
+
 TEST(GeometryTest, CanConvertBetweenDegressAndRadians) {
   {
     auto deg = Degrees{90.0};

impeller/geometry/geometry_unittests.h

@@ -58,6 +58,15 @@ inline ::testing::AssertionResult RectNear(impeller::Rect a, impeller::Rect b) {
                : ::testing::AssertionFailure() << "Rects are not equal.";
 }
 
+inline ::testing::AssertionResult ColorNear(impeller::Color a,
+                                            impeller::Color b) {
+  auto equal = NumberNear(a.red, b.red) && NumberNear(a.green, b.green) &&
+               NumberNear(a.blue, b.blue) && NumberNear(a.alpha, b.alpha);
+
+  return equal ? ::testing::AssertionSuccess()
+               : ::testing::AssertionFailure() << "Colors are not equal.";
+}
+
 inline ::testing::AssertionResult PointNear(impeller::Point a,
                                             impeller::Point b) {
   auto equal = NumberNear(a.x, b.x) && NumberNear(a.y, b.y);
@@ -76,5 +85,6 @@ inline ::testing::AssertionResult SizeNear(impeller::Size a, impeller::Size b) {
 #define ASSERT_MATRIX_NEAR(a, b) ASSERT_PRED2(&::MatrixNear, a, b)
 #define ASSERT_QUATERNION_NEAR(a, b) ASSERT_PRED2(&::QuaternionNear, a, b)
 #define ASSERT_RECT_NEAR(a, b) ASSERT_PRED2(&::RectNear, a, b)
+#define ASSERT_COLOR_NEAR(a, b) ASSERT_PRED2(&::ColorNear, a, b)
 #define ASSERT_POINT_NEAR(a, b) ASSERT_PRED2(&::PointNear, a, b)
 #define ASSERT_SIZE_NEAR(a, b) ASSERT_PRED2(&::SizeNear, a, b)