CPUParticles2D - Add ability to follow physics interpolated target

Allows a non-interpolated particle system to closely follow an interpolated target without tracking ahead of the target, by performing fixed timestep interpolation on the particle system global transform, and using this for emission.

Co-authored-by: lawnjelly <lawnjelly@gmail.com>

servers/rendering/renderer_canvas_cull.cpp

@@ -70,6 +70,11 @@ void RendererCanvasCull::_dependency_deleted(const RID &p_dependency, Dependency
 void RendererCanvasCull::_render_canvas_item_tree(RID p_to_render_target, Canvas::ChildItem *p_child_items, int p_child_item_count, const Transform2D &p_transform, const Rect2 &p_clip_rect, const Color &p_modulate, RendererCanvasRender::Light *p_lights, RendererCanvasRender::Light *p_directional_lights, RenderingServer::CanvasItemTextureFilter p_default_filter, RenderingServer::CanvasItemTextureRepeat p_default_repeat, bool p_snap_2d_vertices_to_pixel, uint32_t p_canvas_cull_mask, RenderingMethod::RenderInfo *r_render_info) {
 	RENDER_TIMESTAMP("Cull CanvasItem Tree");
 
+	// This is used to avoid passing the camera transform down the rendering
+	// function calls, as it won't be used in 99% of cases, because the camera
+	// transform is normally concatenated with the item global transform.
+	_current_camera_transform = p_transform;
+
 	memset(z_list, 0, z_range * sizeof(RendererCanvasRender::Item *));
 	memset(z_last_list, 0, z_range * sizeof(RendererCanvasRender::Item *));
 
@@ -242,14 +247,14 @@ void RendererCanvasCull::_attach_canvas_item_for_draw(RendererCanvasCull::Item *
 	}
 
 	if (((ci->commands != nullptr || ci->visibility_notifier) && p_clip_rect.intersects(p_global_rect, true)) || ci->vp_render || ci->copy_back_buffer) {
-		//something to draw?
+		// Something to draw?
 
 		if (ci->update_when_visible) {
 			RenderingServerDefault::redraw_request();
 		}
 
 		if (ci->commands != nullptr || ci->copy_back_buffer) {
-			ci->final_transform = p_transform;
+			ci->final_transform = !ci->use_identity_transform ? p_transform : _current_camera_transform;
 			ci->final_modulate = p_modulate * ci->self_modulate;
 			ci->global_rect_cache = p_global_rect;
 			ci->global_rect_cache.position -= p_clip_rect.position;
@@ -322,6 +327,10 @@ void RendererCanvasCull::_cull_canvas_item(Item *p_canvas_item, const Transform2
 		}
 	}
 
+	// Always calculate final transform as if not using identity xform.
+	// This is so the expected transform is passed to children.
+	// However, if use_identity_xform is set,
+	// we can override the transform for rendering purposes for this item only.
 	Transform2D self_xform;
 	Transform2D final_xform;
 	if (p_is_already_y_sorted) {
@@ -360,7 +369,12 @@ void RendererCanvasCull::_cull_canvas_item(Item *p_canvas_item, const Transform2
 		ci->repeat_source_item = repeat_source_item;
 	}
 
-	Rect2 global_rect = final_xform.xform(rect);
+	Rect2 global_rect;
+	if (!p_canvas_item->use_identity_transform) {
+		global_rect = final_xform.xform(rect);
+	} else {
+		global_rect = _current_camera_transform.xform(rect);
+	}
 	if (repeat_source_item && (repeat_size.x || repeat_size.y)) {
 		// Top-left repeated rect.
 		Rect2 corner_rect = global_rect;
@@ -686,6 +700,13 @@ void RendererCanvasCull::canvas_item_set_draw_behind_parent(RID p_item, bool p_e
 	canvas_item->behind = p_enable;
 }
 
+void RendererCanvasCull::canvas_item_set_use_identity_transform(RID p_item, bool p_enable) {
+	Item *canvas_item = canvas_item_owner.get_or_null(p_item);
+	ERR_FAIL_NULL(canvas_item);
+
+	canvas_item->use_identity_transform = p_enable;
+}
+
 void RendererCanvasCull::canvas_item_set_update_when_visible(RID p_item, bool p_update) {
 	Item *canvas_item = canvas_item_owner.get_or_null(p_item);
 	ERR_FAIL_NULL(canvas_item);

servers/rendering/renderer_canvas_cull.h

@@ -217,6 +217,8 @@ private:
 	RendererCanvasRender::Item **z_list;
 	RendererCanvasRender::Item **z_last_list;
 
+	Transform2D _current_camera_transform;
+
 public:
 	void render_canvas(RID p_render_target, Canvas *p_canvas, const Transform2D &p_transform, RendererCanvasRender::Light *p_lights, RendererCanvasRender::Light *p_directional_lights, const Rect2 &p_clip_rect, RS::CanvasItemTextureFilter p_default_filter, RS::CanvasItemTextureRepeat p_default_repeat, bool p_snap_2d_transforms_to_pixel, bool p_snap_2d_vertices_to_pixel, uint32_t p_canvas_cull_mask, RenderingMethod::RenderInfo *r_render_info = nullptr);
 
@@ -250,6 +252,7 @@ public:
 	void canvas_item_set_self_modulate(RID p_item, const Color &p_color);
 
 	void canvas_item_set_draw_behind_parent(RID p_item, bool p_enable);
+	void canvas_item_set_use_identity_transform(RID p_item, bool p_enable);
 
 	void canvas_item_set_update_when_visible(RID p_item, bool p_update);
 

servers/rendering/renderer_canvas_render.h

@@ -321,6 +321,7 @@ public:
 		bool update_when_visible : 1;
 		bool on_interpolate_transform_list : 1;
 		bool interpolated : 1;
+		bool use_identity_transform : 1;
 
 		struct CanvasGroup {
 			RS::CanvasGroupMode mode;
@@ -486,6 +487,7 @@ public:
 			repeat_source = false;
 			on_interpolate_transform_list = false;
 			interpolated = true;
+			use_identity_transform = false;
 		}
 		virtual ~Item() {
 			clear();

servers/rendering/rendering_server_default.h

@@ -962,6 +962,7 @@ public:
 	FUNC2(canvas_item_set_self_modulate, RID, const Color &)
 
 	FUNC2(canvas_item_set_draw_behind_parent, RID, bool)
+	FUNC2(canvas_item_set_use_identity_transform, RID, bool)
 
 	FUNC6(canvas_item_add_line, RID, const Point2 &, const Point2 &, const Color &, float, bool)
 	FUNC5(canvas_item_add_polyline, RID, const Vector<Point2> &, const Vector<Color> &, float, bool)

servers/rendering_server.h

@@ -1545,6 +1545,7 @@ public:
 	virtual void canvas_item_set_visibility_layer(RID p_item, uint32_t p_visibility_layer) = 0;
 
 	virtual void canvas_item_set_draw_behind_parent(RID p_item, bool p_enable) = 0;
+	virtual void canvas_item_set_use_identity_transform(RID p_item, bool p_enabled) = 0;
 
 	enum NinePatchAxisMode {
 		NINE_PATCH_STRETCH,