From 328866ee6a7d1378d42b741ca6b7219cd9d02593 Mon Sep 17 00:00:00 2001
From: lawnjelly <lawnjelly@gmail.com>
Date: Fri, 6 May 2022 12:09:06 +0100
Subject: [PATCH] Physics Interpolation - Reset on adding child to SceneTree

For convenience, branches added to the SceneTree now have physics interpolation reset after the first update of the transform to the VisualServer.
---
 scene/3d/visual_instance.cpp |  9 +++++++++
 scene/main/node.cpp          | 23 +++++++++++++++++++++++
 scene/main/node.h            |  6 ++++++
 3 files changed, 38 insertions(+)

diff --git a/scene/3d/visual_instance.cpp b/scene/3d/visual_instance.cpp
index d6d8be962cf..ce4fbca8d0d 100644
--- a/scene/3d/visual_instance.cpp
+++ b/scene/3d/visual_instance.cpp
@@ -101,6 +101,15 @@ void VisualInstance::_notification(int p_what) {
 				if (!_is_using_identity_transform()) {
 					Transform gt = get_global_transform();
 					VisualServer::get_singleton()->instance_set_transform(instance, gt);
+
+					// For instance when first adding to the tree, when the previous transform is
+					// unset, to prevent streaking from the origin.
+					if (_is_physics_interpolation_reset_requested()) {
+						if (_is_vi_visible()) {
+							_notification(NOTIFICATION_RESET_PHYSICS_INTERPOLATION);
+						}
+						_set_physics_interpolation_reset_requested(false);
+					}
 				}
 			}
 		} break;
diff --git a/scene/main/node.cpp b/scene/main/node.cpp
index 58c30564f07..f6f738d602a 100644
--- a/scene/main/node.cpp
+++ b/scene/main/node.cpp
@@ -221,6 +221,18 @@ void Node::_propagate_physics_interpolated(bool p_interpolated) {
 	data.blocked--;
 }
 
+void Node::_propagate_physics_interpolation_reset_requested() {
+	if (is_physics_interpolated()) {
+		data.physics_interpolation_reset_requested = true;
+	}
+
+	data.blocked++;
+	for (int i = 0; i < data.children.size(); i++) {
+		data.children[i]->_propagate_physics_interpolation_reset_requested();
+	}
+	data.blocked--;
+}
+
 void Node::_propagate_enter_tree() {
 	// this needs to happen to all children before any enter_tree
 
@@ -1014,6 +1026,10 @@ void Node::_set_physics_interpolated_client_side(bool p_enable) {
 	data.physics_interpolated_client_side = p_enable;
 }
 
+void Node::_set_physics_interpolation_reset_requested(bool p_enable) {
+	data.physics_interpolation_reset_requested = p_enable;
+}
+
 void Node::_set_use_identity_transform(bool p_enable) {
 	data.use_identity_transform = p_enable;
 }
@@ -1249,6 +1265,12 @@ void Node::_add_child_nocheck(Node *p_child, const StringName &p_name) {
 	//recognize children created in this node constructor
 	p_child->data.parent_owned = data.in_constructor;
 	add_child_notify(p_child);
+
+	// Allow physics interpolated nodes to automatically reset when added to the tree
+	// (this is to save the user doing this manually each time)
+	if (is_inside_tree() && get_tree()->is_physics_interpolation_enabled()) {
+		p_child->_propagate_physics_interpolation_reset_requested();
+	}
 }
 
 void Node::add_child(Node *p_child, bool p_legible_unique_name) {
@@ -3185,6 +3207,7 @@ Node::Node() {
 	data.inside_tree = false;
 	data.ready_notified = false;
 	data.physics_interpolated = true;
+	data.physics_interpolation_reset_requested = false;
 	data.physics_interpolated_client_side = false;
 	data.use_identity_transform = false;
 
diff --git a/scene/main/node.h b/scene/main/node.h
index 133f6e1ac5e..5d20f9e59f9 100644
--- a/scene/main/node.h
+++ b/scene/main/node.h
@@ -151,6 +151,9 @@ private:
 		// is switched on.
 		bool physics_interpolated : 1;
 
+		// We can auto-reset physics interpolation when e.g. adding a node for the first time
+		bool physics_interpolation_reset_requested : 1;
+
 		// Most nodes need not be interpolated in the scene tree, physics interpolation
 		// is normally only needed in the VisualServer. However if we need to read the
 		// interpolated transform of a node in the SceneTree, it is necessary to duplicate
@@ -197,6 +200,7 @@ private:
 	void _propagate_exit_tree();
 	void _propagate_after_exit_branch(bool p_exiting_tree);
 	void _propagate_physics_interpolated(bool p_interpolated);
+	void _propagate_physics_interpolation_reset_requested();
 	void _print_stray_nodes();
 	void _propagate_pause_owner(Node *p_owner);
 	Array _get_node_and_resource(const NodePath &p_path);
@@ -243,6 +247,8 @@ protected:
 	void _set_name_nocheck(const StringName &p_name);
 	void _set_physics_interpolated_client_side(bool p_enable);
 	bool _is_physics_interpolated_client_side() const { return data.physics_interpolated_client_side; }
+	void _set_physics_interpolation_reset_requested(bool p_enable);
+	bool _is_physics_interpolation_reset_requested() const { return data.physics_interpolation_reset_requested; }
 	void _set_use_identity_transform(bool p_enable);
 	bool _is_using_identity_transform() const { return data.use_identity_transform; }