diff -r 000000000000 -r 6474c204b198 gfx/2d/ScaleFactor.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gfx/2d/ScaleFactor.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,91 @@
+/* -*- Mode: C++; tab-width: 20; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef MOZILLA_GFX_SCALEFACTOR_H_
+#define MOZILLA_GFX_SCALEFACTOR_H_
+
+#include "mozilla/Attributes.h"
+
+#include "gfxPoint.h"
+
+namespace mozilla {
+namespace gfx {
+
+/*
+ * This class represents a scaling factor between two different pixel unit
+ * systems. This is effectively a type-safe float, intended to be used in
+ * combination with the known-type instances of gfx::Point, gfx::Rect, etc.
+ *
+ * Note that some parts of the code that pre-date this class used separate
+ * scaling factors for the x and y axes. However, at runtime these values
+ * were always expected to be the same, so this class uses only one scale
+ * factor for both axes. The two constructors that take two-axis scaling
+ * factors check to ensure that this assertion holds.
+ */
+template<class src, class dst>
+struct ScaleFactor {
+  float scale;
+
+  MOZ_CONSTEXPR ScaleFactor() : scale(1.0) {}
+  MOZ_CONSTEXPR ScaleFactor(const ScaleFactor<src, dst>& aCopy) : scale(aCopy.scale) {}
+  explicit MOZ_CONSTEXPR ScaleFactor(float aScale) : scale(aScale) {}
+
+  explicit ScaleFactor(float aX, float aY) : scale(aX) {
+    MOZ_ASSERT(fabs(aX - aY) < 1e-6);
+  }
+
+  ScaleFactor<dst, src> Inverse() {
+    return ScaleFactor<dst, src>(1 / scale);
+  }
+
+  bool operator==(const ScaleFactor<src, dst>& aOther) const {
+    return scale == aOther.scale;
+  }
+
+  bool operator!=(const ScaleFactor<src, dst>& aOther) const {
+    return !(*this == aOther);
+  }
+
+  bool operator<(const ScaleFactor<src, dst>& aOther) const {
+    return scale < aOther.scale;
+  }
+
+  bool operator<=(const ScaleFactor<src, dst>& aOther) const {
+    return scale <= aOther.scale;
+  }
+
+  bool operator>(const ScaleFactor<src, dst>& aOther) const {
+    return scale > aOther.scale;
+  }
+
+  bool operator>=(const ScaleFactor<src, dst>& aOther) const {
+    return scale >= aOther.scale;
+  }
+
+  template<class other>
+  ScaleFactor<other, dst> operator/(const ScaleFactor<src, other>& aOther) const {
+    return ScaleFactor<other, dst>(scale / aOther.scale);
+  }
+
+  template<class other>
+  ScaleFactor<src, other> operator/(const ScaleFactor<other, dst>& aOther) const {
+    return ScaleFactor<src, other>(scale / aOther.scale);
+  }
+
+  template<class other>
+  ScaleFactor<src, other> operator*(const ScaleFactor<dst, other>& aOther) const {
+    return ScaleFactor<src, other>(scale * aOther.scale);
+  }
+
+  template<class other>
+  ScaleFactor<other, dst> operator*(const ScaleFactor<other, src>& aOther) const {
+    return ScaleFactor<other, dst>(scale * aOther.scale);
+  }
+};
+
+}
+}
+
+#endif /* MOZILLA_GFX_SCALEFACTOR_H_ */