Avoid duplicate math when computing VR scene hierarchy.

chrome/browser/android/vr_shell/ui_elements.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_ANDROID_VR_SHELL_UI_ELEMENTS_H_
 #define CHROME_BROWSER_ANDROID_VR_SHELL_UI_ELEMENTS_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 41 matching lines @@
   void Rotate(float ax, float ay, float az, float rad);
   void Translate(float tx, float ty, float tz);
   void Scale(float sx, float sy, float sz);
 
   gvr::Mat4f to_world;
 };
 
 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;
 
   // Computes the distance from |ray_origin| to this rectangles's plane, along
   // |ray_vector|. Returns true and populates |distance| if the calculation is
   // possible, and false if the ray is parallel to the plane.
   bool GetRayDistance(const gvr::Vec3f& ray_origin,
                       const gvr::Vec3f& ray_vector,
                       float* distance) const;
 
   // Projects a 3D world point onto the X and Y axes of the transformed
   // rectangle, returning 2D coordinates relative to the un-transformed unit
   // rectangle. This allows beam intersection points to be mapped to sprite
   // pixel coordinates. Points that fall onto the rectangle will generate X and
   // Y values on the interval [-0.5, 0.5].
   gvr::Vec2f GetUnitRectangleCoordinates(const gvr::Vec3f& world_point);
 
  private:
   Transform transform_;
 };
 
 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.
   bool IsVisible() const;
 
   // Indicates whether the element should be tested for cursor input.
   bool IsHitTestable() const;
 
   // Valid IDs are non-negative.
   int id = -1;
@@ skipping 48 matching lines @@
 
   Fill fill = Fill::NONE;
 
   Colorf edge_color = {1.0f, 1.0f, 1.0f, 1.0f};
   Colorf center_color = {1.0f, 1.0f, 1.0f, 1.0f};
 
   int gridline_count = 1;
 
   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);
 };
 
 }  // namespace vr_shell
 
 #endif  // CHROME_BROWSER_ANDROID_VR_SHELL_UI_ELEMENTS_H_