diff -r 000000000000 -r 6474c204b198 layout/base/UnitTransforms.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/layout/base/UnitTransforms.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,91 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 MOZ_UNIT_TRANSFORMS_H_
+#define MOZ_UNIT_TRANSFORMS_H_
+
+#include "Units.h"
+
+namespace mozilla {
+
+// Convenience functions for converting an entity from one strongly-typed
+// coordinate system to another without changing the values it stores (this
+// can be thought of as a cast).
+// To use these functions, you must provide a justification for each use!
+// Feel free to add more justifications to PixelCastJustification, along with
+// a comment that explains under what circumstances it is appropriate to use.
+
+MOZ_BEGIN_ENUM_CLASS(PixelCastJustification, uint8_t)
+  // For the root layer, Screen Pixel = Parent Layer Pixel.
+  ScreenToParentLayerForRoot,
+  // For the root composition size we want to view it as layer pixels in any layer
+  ParentLayerToLayerForRootComposition
+MOZ_END_ENUM_CLASS(PixelCastJustification)
+
+template <class TargetUnits, class SourceUnits>
+gfx::SizeTyped<TargetUnits> ViewAs(const gfx::SizeTyped<SourceUnits>& aSize, PixelCastJustification) {
+  return gfx::SizeTyped<TargetUnits>(aSize.width, aSize.height);
+}
+template <class TargetUnits, class SourceUnits>
+gfx::IntSizeTyped<TargetUnits> ViewAs(const gfx::IntSizeTyped<SourceUnits>& aSize, PixelCastJustification) {
+  return gfx::IntSizeTyped<TargetUnits>(aSize.width, aSize.height);
+}
+
+// Convenience functions for casting untyped entities to typed entities.
+// Using these functions does not require a justification, but once we convert
+// all code to use strongly typed units they should not be needed any longer.
+template <class TargetUnits>
+gfx::PointTyped<TargetUnits> ViewAs(const gfxPoint& aPoint) {
+  return gfx::PointTyped<TargetUnits>(aPoint.x, aPoint.y);
+}
+template <class TargetUnits>
+gfx::RectTyped<TargetUnits> ViewAs(const gfxRect& aRect) {
+  return gfx::RectTyped<TargetUnits>(aRect.x, aRect.y, aRect.width, aRect.height);
+}
+template <class TargetUnits>
+gfx::IntSizeTyped<TargetUnits> ViewAs(const nsIntSize& aSize) {
+  return gfx::IntSizeTyped<TargetUnits>(aSize.width, aSize.height);
+}
+template <class TargetUnits>
+gfx::IntPointTyped<TargetUnits> ViewAs(const nsIntPoint& aPoint) {
+  return gfx::IntPointTyped<TargetUnits>(aPoint.x, aPoint.y);
+}
+template <class TargetUnits>
+gfx::IntRectTyped<TargetUnits> ViewAs(const nsIntRect& aRect) {
+  return gfx::IntRectTyped<TargetUnits>(aRect.x, aRect.y, aRect.width, aRect.height);
+}
+
+// Convenience functions for casting typed entities to untyped entities.
+// Using these functions does not require a justification, but once we convert
+// all code to use strongly typed units they should not be needed any longer.
+template <class SourceUnits>
+gfxPoint ViewAsUntyped(const gfx::PointTyped<SourceUnits>& aPoint) {
+  return gfxPoint(aPoint.x, aPoint.y);
+}
+template <class SourceUnits>
+gfxRect ViewAsUntyped(const gfx::RectTyped<SourceUnits>& aRect) {
+  return gfxRect(aRect.x, aRect.y, aRect.width, aRect.height);
+}
+
+// Convenience functions for transforming an entity from one strongly-typed
+// coordinate system to another using the provided transformation matrix.
+template <typename TargetUnits, typename SourceUnits>
+static gfx::PointTyped<TargetUnits> TransformTo(const gfx3DMatrix& aTransform,
+                                                const gfx::PointTyped<SourceUnits>& aPoint)
+{
+  return ViewAs<TargetUnits>(aTransform.Transform(ViewAsUntyped(aPoint)));
+}
+template <typename TargetUnits, typename SourceUnits>
+static gfx::RectTyped<TargetUnits> TransformTo(const gfx3DMatrix& aTransform,
+                                               const gfx::RectTyped<SourceUnits>& aRect)
+{
+  return ViewAs<TargetUnits>(aTransform.TransformBounds(ViewAsUntyped(aRect)));
+}
+
+
+}
+
+#endif