Avoid duplicate math when computing VR scene hierarchy.

chrome/browser/android/vr_shell/ui_elements.h

Index: chrome/browser/android/vr_shell/ui_elements.h
diff --git a/chrome/browser/android/vr_shell/ui_elements.h b/chrome/browser/android/vr_shell/ui_elements.h
index 69ed336d6ba8d3562ab6fe9b4f4b29752a659adb..13165341f72c66ab5d3a356e4adf7c9f0da1d7b8 100644
--- a/chrome/browser/android/vr_shell/ui_elements.h
+++ b/chrome/browser/android/vr_shell/ui_elements.h
@@ -59,7 +59,7 @@ struct Transform {
 class WorldRectangle {
  public:
   const gvr::Mat4f& TransformMatrix() const;
-  void SetTransform(const Transform& transform);
+  Transform* GetTransform() { return &transform_; }
 
   gvr::Vec3f GetCenter() const;
   gvr::Vec3f GetNormal() const;
@@ -86,6 +86,8 @@ struct ContentRectangle : public WorldRectangle {
   ContentRectangle();
   ~ContentRectangle();
 
+  Transform* GetInheritableTransform() { return &inheritable_transform_; }

[tiborg, 2017/03/24 21:58:37]

Do we need this? inheritable_transform_ is public anyways.

[cjgrant, 2017/03/27 14:07:52]

My motivation:  This object started as a struct because it basically just stored
some coordinates and a transform.  We've since added a bunch of methods on it,
and have started using the methods for rendering.  I'm trying to set this thing
up to be switched to a class, but it should probably be an all-or-nothing move. 
I'll switch back.

[tiborg, 2017/03/27 14:30:18]

I think we can also make a gradual switch (even though I also prefer the
all-or-nothing solution). I would then make the variable private though so that
it is clear to only use the getter and setter.

[cjgrant, 2017/03/27 15:20:16]

Done. Lets go all-or-nothing here.

+
   void Animate(int64_t time);
 
   // Indicates whether the element should be visually rendered.
@@ -154,6 +156,12 @@ struct ContentRectangle : public WorldRectangle {
 
   int draw_phase = 1;
 
+  // This transform can be used by children to derive position of its parent.
+  Transform inheritable_transform_;

[tiborg, 2017/03/24 21:58:37]

Remove trailing _?

[cjgrant, 2017/03/27 15:20:16]

Done.

+
+  // A flag usable during transformation calculates to avoid duplicate work.
+  bool dirty;
+
  private:
   DISALLOW_COPY_AND_ASSIGN(ContentRectangle);
 };