Add a Viewport method to get automatically computed 2D stretch transform

`Viewport.get_stretch_transform()` returns the automatically computed
2D stretch transform. Combined with `Transform2D.get_scale()`, this is
useful when using the `canvas_items` stretch mode in a project.

There are many situations where knowing this factor is useful:

- Divide Camera2D zoom to keep the size of the 2D game world identical
  regardless of the 2D scale factor (so that UI elements can still be scaled).
- Make certain controls always drawn at 1:1 scale
  (e.g. for the crosshair in a FPS). This is done by dividing the Control
  node's scale by the scale factor.

2 files changed, 6 insertions, 0 deletions

diff --git a/scene/main/viewport.cpp b/scene/main/viewport.cpp
index 54f66e8d4e..917da7d19e 100644
--- a/scene/main/viewport.cpp
+++ b/scene/main/viewport.cpp
@@ -1254,6 +1254,10 @@ Ref<World2D> Viewport::get_world_2d() const {
 	return world_2d;
 }
 
+Transform2D Viewport::get_stretch_transform() const {
+	return stretch_transform;
+}
+
 Transform2D Viewport::get_final_transform() const {
 	ERR_READ_THREAD_GUARD_V(Transform2D());
 	return stretch_transform * global_canvas_transform;
@@ -4553,6 +4557,7 @@ void Viewport::_bind_methods() {
 
 	ClassDB::bind_method(D_METHOD("set_global_canvas_transform", "xform"), &Viewport::set_global_canvas_transform);
 	ClassDB::bind_method(D_METHOD("get_global_canvas_transform"), &Viewport::get_global_canvas_transform);
+	ClassDB::bind_method(D_METHOD("get_stretch_transform"), &Viewport::get_stretch_transform);
 	ClassDB::bind_method(D_METHOD("get_final_transform"), &Viewport::get_final_transform);
 	ClassDB::bind_method(D_METHOD("get_screen_transform"), &Viewport::get_screen_transform);
 
diff --git a/scene/main/viewport.h b/scene/main/viewport.h
index a18dc1f6f0..5fb61e2220 100644
--- a/scene/main/viewport.h
+++ b/scene/main/viewport.h
@@ -514,6 +514,7 @@ public:
 	void set_global_canvas_transform(const Transform2D &p_transform);
 	Transform2D get_global_canvas_transform() const;
 
+	Transform2D get_stretch_transform() const;
 	virtual Transform2D get_final_transform() const;
 
 	void gui_set_root_order_dirty();