Fix move_and_collide causing sliding on slopes

Make sure the direction of the motion is preserved, unless the depth is
higher than the margin, which means the body needs depenetration in any
direction.

Also changed move_and_slide to avoid sliding on the first motion, in
order to avoid issues with unstable position on ground when jumping.

Co-authored-by: fabriceci <fabricecipolla@gmail.com>

10 files changed, 138 insertions, 25 deletions

diff --git a/doc/classes/PhysicsTestMotionResult2D.xml b/doc/classes/PhysicsTestMotionResult2D.xml
index da04ffa86a..bf3497386e 100644
--- a/doc/classes/PhysicsTestMotionResult2D.xml
+++ b/doc/classes/PhysicsTestMotionResult2D.xml
@@ -19,10 +19,16 @@
 		</member>
 		<member name="collider_velocity" type="Vector2" setter="" getter="get_collider_velocity" default="Vector2(0, 0)">
 		</member>
+		<member name="collision_depth" type="float" setter="" getter="get_collision_depth" default="0.0">
+		</member>
 		<member name="collision_normal" type="Vector2" setter="" getter="get_collision_normal" default="Vector2(0, 0)">
 		</member>
 		<member name="collision_point" type="Vector2" setter="" getter="get_collision_point" default="Vector2(0, 0)">
 		</member>
+		<member name="collision_safe_fraction" type="float" setter="" getter="get_collision_safe_fraction" default="0.0">
+		</member>
+		<member name="collision_unsafe_fraction" type="float" setter="" getter="get_collision_unsafe_fraction" default="0.0">
+		</member>
 		<member name="motion" type="Vector2" setter="" getter="get_motion" default="Vector2(0, 0)">
 		</member>
 		<member name="motion_remainder" type="Vector2" setter="" getter="get_motion_remainder" default="Vector2(0, 0)">
diff --git a/scene/2d/physics_body_2d.cpp b/scene/2d/physics_body_2d.cpp
index ce63b25ce0..baf092c530 100644
--- a/scene/2d/physics_body_2d.cpp
+++ b/scene/2d/physics_body_2d.cpp
@@ -76,13 +76,46 @@ Ref<KinematicCollision2D> PhysicsBody2D::_move(const Vector2 &p_motion, bool p_i
 	return Ref<KinematicCollision2D>();
 }
 
-bool PhysicsBody2D::move_and_collide(const Vector2 &p_motion, bool p_infinite_inertia, PhysicsServer2D::MotionResult &r_result, real_t p_margin, bool p_exclude_raycast_shapes, bool p_test_only) {
+bool PhysicsBody2D::move_and_collide(const Vector2 &p_motion, bool p_infinite_inertia, PhysicsServer2D::MotionResult &r_result, real_t p_margin, bool p_exclude_raycast_shapes, bool p_test_only, bool p_cancel_sliding) {
 	if (is_only_update_transform_changes_enabled()) {
 		ERR_PRINT("Move functions do not work together with 'sync to physics' option. Please read the documentation.");
 	}
 	Transform2D gt = get_global_transform();
 	bool colliding = PhysicsServer2D::get_singleton()->body_test_motion(get_rid(), gt, p_motion, p_infinite_inertia, p_margin, &r_result, p_exclude_raycast_shapes);
 
+	// Restore direction of motion to be along original motion,
+	// in order to avoid sliding due to recovery,
+	// but only if collision depth is low enough to avoid tunneling.
+	real_t motion_length = p_motion.length();
+	if (motion_length > CMP_EPSILON) {
+		real_t precision = 0.001;
+
+		if (colliding && p_cancel_sliding) {
+			// Can't just use margin as a threshold because collision depth is calculated on unsafe motion,
+			// so even in normal resting cases the depth can be a bit more than the margin.
+			precision += motion_length * (r_result.collision_unsafe_fraction - r_result.collision_safe_fraction);
+
+			if (r_result.collision_depth > (real_t)p_margin + precision) {
+				p_cancel_sliding = false;
+			}
+		}
+
+		if (p_cancel_sliding) {
+			// Check depth of recovery.
+			Vector2 motion_normal = p_motion / motion_length;
+			real_t dot = r_result.motion.dot(motion_normal);
+			Vector2 recovery = r_result.motion - motion_normal * dot;
+			real_t recovery_length = recovery.length();
+			// Fixes cases where canceling slide causes the motion to go too deep into the ground,
+			// Becauses we're only taking rest information into account and not general recovery.
+			if (recovery_length < (real_t)p_margin + precision) {
+				// Apply adjustment to motion.
+				r_result.motion = motion_normal * dot;
+				r_result.remainder = p_motion - r_result.motion;
+			}
+		}
+	}
+
 	if (!p_test_only) {
 		gt.elements[2] += r_result.motion;
 		set_global_transform(gt);
@@ -942,15 +975,16 @@ void CharacterBody2D::move_and_slide() {
 	floor_normal = Vector2();
 	floor_velocity = Vector2();
 
-	int slide_count = max_slides;
-	while (slide_count) {
+	// No sliding on first attempt to keep floor motion stable when possible.
+	bool sliding_enabled = false;
+	for (int iteration = 0; iteration < max_slides; ++iteration) {
 		PhysicsServer2D::MotionResult result;
 		bool found_collision = false;
 
 		for (int i = 0; i < 2; ++i) {
 			bool collided;
 			if (i == 0) { //collide
-				collided = move_and_collide(motion, infinite_inertia, result, margin);
+				collided = move_and_collide(motion, infinite_inertia, result, margin, true, false, !sliding_enabled);
 				if (!collided) {
 					motion = Vector2(); //clear because no collision happened and motion completed
 				}
@@ -966,7 +1000,6 @@ void CharacterBody2D::move_and_slide() {
 				found_collision = true;
 
 				motion_results.push_back(result);
-				motion = result.remainder;
 
 				if (up_direction == Vector2()) {
 					//all is a wall
@@ -980,7 +1013,7 @@ void CharacterBody2D::move_and_slide() {
 						floor_velocity = result.collider_velocity;
 
 						if (stop_on_slope) {
-							if ((body_velocity_normal + up_direction).length() < 0.01 && result.motion.length() < 1) {
+							if ((body_velocity_normal + up_direction).length() < 0.01) {
 								Transform2D gt = get_global_transform();
 								gt.elements[2] -= result.motion.slide(up_direction);
 								set_global_transform(gt);
@@ -995,16 +1028,20 @@ void CharacterBody2D::move_and_slide() {
 					}
 				}
 
-				motion = motion.slide(result.collision_normal);
-				linear_velocity = linear_velocity.slide(result.collision_normal);
+				if (sliding_enabled || !on_floor) {
+					motion = result.remainder.slide(result.collision_normal);
+					linear_velocity = linear_velocity.slide(result.collision_normal);
+				} else {
+					motion = result.remainder;
+				}
 			}
+
+			sliding_enabled = true;
 		}
 
 		if (!found_collision || motion == Vector2()) {
 			break;
 		}
-
-		--slide_count;
 	}
 
 	if (!was_on_floor || snap == Vector2()) {
diff --git a/scene/2d/physics_body_2d.h b/scene/2d/physics_body_2d.h
index f084a247aa..b1d48a2cc9 100644
--- a/scene/2d/physics_body_2d.h
+++ b/scene/2d/physics_body_2d.h
@@ -50,7 +50,7 @@ protected:
 	Ref<KinematicCollision2D> _move(const Vector2 &p_motion, bool p_infinite_inertia = true, bool p_exclude_raycast_shapes = true, bool p_test_only = false, real_t p_margin = 0.08);
 
 public:
-	bool move_and_collide(const Vector2 &p_motion, bool p_infinite_inertia, PhysicsServer2D::MotionResult &r_result, real_t p_margin, bool p_exclude_raycast_shapes = true, bool p_test_only = false);
+	bool move_and_collide(const Vector2 &p_motion, bool p_infinite_inertia, PhysicsServer2D::MotionResult &r_result, real_t p_margin, bool p_exclude_raycast_shapes = true, bool p_test_only = false, bool p_cancel_sliding = true);
 	bool test_move(const Transform2D &p_from, const Vector2 &p_motion, bool p_infinite_inertia = true, bool p_exclude_raycast_shapes = true, const Ref<KinematicCollision2D> &r_collision = Ref<KinematicCollision2D>(), real_t p_margin = 0.08);
 
 	TypedArray<PhysicsBody2D> get_collision_exceptions();
diff --git a/scene/3d/physics_body_3d.cpp b/scene/3d/physics_body_3d.cpp
index 28a0c72fe3..c9afb5f201 100644
--- a/scene/3d/physics_body_3d.cpp
+++ b/scene/3d/physics_body_3d.cpp
@@ -118,10 +118,43 @@ Ref<KinematicCollision3D> PhysicsBody3D::_move(const Vector3 &p_motion, bool p_i
 	return Ref<KinematicCollision3D>();
 }
 
-bool PhysicsBody3D::move_and_collide(const Vector3 &p_motion, bool p_infinite_inertia, PhysicsServer3D::MotionResult &r_result, real_t p_margin, bool p_exclude_raycast_shapes, bool p_test_only) {
+bool PhysicsBody3D::move_and_collide(const Vector3 &p_motion, bool p_infinite_inertia, PhysicsServer3D::MotionResult &r_result, real_t p_margin, bool p_exclude_raycast_shapes, bool p_test_only, bool p_cancel_sliding) {
 	Transform3D gt = get_global_transform();
 	bool colliding = PhysicsServer3D::get_singleton()->body_test_motion(get_rid(), gt, p_motion, p_infinite_inertia, p_margin, &r_result, p_exclude_raycast_shapes);
 
+	// Restore direction of motion to be along original motion,
+	// in order to avoid sliding due to recovery,
+	// but only if collision depth is low enough to avoid tunneling.
+	real_t motion_length = p_motion.length();
+	if (motion_length > CMP_EPSILON) {
+		real_t precision = CMP_EPSILON;
+
+		if (colliding && p_cancel_sliding) {
+			// Can't just use margin as a threshold because collision depth is calculated on unsafe motion,
+			// so even in normal resting cases the depth can be a bit more than the margin.
+			precision += motion_length * (r_result.collision_unsafe_fraction - r_result.collision_safe_fraction);
+
+			if (r_result.collision_depth > (real_t)p_margin + precision) {
+				p_cancel_sliding = false;
+			}
+		}
+
+		if (p_cancel_sliding) {
+			// Check depth of recovery.
+			Vector3 motion_normal = p_motion / motion_length;
+			real_t dot = r_result.motion.dot(motion_normal);
+			Vector3 recovery = r_result.motion - motion_normal * dot;
+			real_t recovery_length = recovery.length();
+			// Fixes cases where canceling slide causes the motion to go too deep into the ground,
+			// Becauses we're only taking rest information into account and not general recovery.
+			if (recovery_length < (real_t)p_margin + precision) {
+				// Apply adjustment to motion.
+				r_result.motion = motion_normal * dot;
+				r_result.remainder = p_motion - r_result.motion;
+			}
+		}
+	}
+
 	for (int i = 0; i < 3; i++) {
 		if (locked_axis & (1 << i)) {
 			r_result.motion[i] = 0;
@@ -978,15 +1011,16 @@ void CharacterBody3D::move_and_slide() {
 	floor_normal = Vector3();
 	floor_velocity = Vector3();
 
-	int slide_count = max_slides;
-	while (slide_count) {
+	// No sliding on first attempt to keep motion stable when possible.
+	bool sliding_enabled = false;
+	for (int iteration = 0; iteration < max_slides; ++iteration) {
 		PhysicsServer3D::MotionResult result;
 		bool found_collision = false;
 
 		for (int i = 0; i < 2; ++i) {
 			bool collided;
 			if (i == 0) { //collide
-				collided = move_and_collide(motion, infinite_inertia, result, margin);
+				collided = move_and_collide(motion, infinite_inertia, result, margin, true, false, !sliding_enabled);
 				if (!collided) {
 					motion = Vector3(); //clear because no collision happened and motion completed
 				}
@@ -1002,7 +1036,6 @@ void CharacterBody3D::move_and_slide() {
 				found_collision = true;
 
 				motion_results.push_back(result);
-				motion = result.remainder;
 
 				if (up_direction == Vector3()) {
 					//all is a wall
@@ -1016,7 +1049,7 @@ void CharacterBody3D::move_and_slide() {
 						floor_velocity = result.collider_velocity;
 
 						if (stop_on_slope) {
-							if ((body_velocity_normal + up_direction).length() < 0.01 && result.motion.length() < 1) {
+							if ((body_velocity_normal + up_direction).length() < 0.01) {
 								Transform3D gt = get_global_transform();
 								gt.origin -= result.motion.slide(up_direction);
 								set_global_transform(gt);
@@ -1031,22 +1064,26 @@ void CharacterBody3D::move_and_slide() {
 					}
 				}
 
-				motion = motion.slide(result.collision_normal);
-				linear_velocity = linear_velocity.slide(result.collision_normal);
+				if (sliding_enabled || !on_floor) {
+					motion = result.remainder.slide(result.collision_normal);
+					linear_velocity = linear_velocity.slide(result.collision_normal);
 
-				for (int j = 0; j < 3; j++) {
-					if (locked_axis & (1 << j)) {
-						linear_velocity[j] = 0.0;
+					for (int j = 0; j < 3; j++) {
+						if (locked_axis & (1 << j)) {
+							linear_velocity[j] = 0.0;
+						}
 					}
+				} else {
+					motion = result.remainder;
 				}
 			}
+
+			sliding_enabled = true;
 		}
 
 		if (!found_collision || motion == Vector3()) {
 			break;
 		}
-
-		--slide_count;
 	}
 
 	if (!was_on_floor || snap == Vector3()) {
diff --git a/scene/3d/physics_body_3d.h b/scene/3d/physics_body_3d.h
index 7d7adf1624..bc4556494d 100644
--- a/scene/3d/physics_body_3d.h
+++ b/scene/3d/physics_body_3d.h
@@ -53,7 +53,7 @@ protected:
 	Ref<KinematicCollision3D> _move(const Vector3 &p_motion, bool p_infinite_inertia = true, bool p_exclude_raycast_shapes = true, bool p_test_only = false, real_t p_margin = 0.001);
 
 public:
-	bool move_and_collide(const Vector3 &p_motion, bool p_infinite_inertia, PhysicsServer3D::MotionResult &r_result, real_t p_margin, bool p_exclude_raycast_shapes = true, bool p_test_only = false);
+	bool move_and_collide(const Vector3 &p_motion, bool p_infinite_inertia, PhysicsServer3D::MotionResult &r_result, real_t p_margin, bool p_exclude_raycast_shapes = true, bool p_test_only = false, bool p_cancel_sliding = true);
 	bool test_move(const Transform3D &p_from, const Vector3 &p_motion, bool p_infinite_inertia = true, bool p_exclude_raycast_shapes = true, const Ref<KinematicCollision3D> &r_collision = Ref<KinematicCollision3D>(), real_t p_margin = 0.001);
 
 	void set_axis_lock(PhysicsServer3D::BodyAxis p_axis, bool p_lock);
diff --git a/servers/physics_2d/space_2d_sw.cpp b/servers/physics_2d/space_2d_sw.cpp
index 1380e57b57..905f08011b 100644
--- a/servers/physics_2d/space_2d_sw.cpp
+++ b/servers/physics_2d/space_2d_sw.cpp
@@ -1142,6 +1142,9 @@ bool Space2DSW::test_body_motion(Body2DSW *p_body, const Transform2D &p_from, co
 				r_result->collision_local_shape = rcd.best_local_shape;
 				r_result->collision_normal = rcd.best_normal;
 				r_result->collision_point = rcd.best_contact;
+				r_result->collision_depth = rcd.best_len;
+				r_result->collision_safe_fraction = safe;
+				r_result->collision_unsafe_fraction = unsafe;
 				r_result->collider_metadata = rcd.best_object->get_shape_metadata(rcd.best_shape);
 
 				const Body2DSW *body = static_cast<const Body2DSW *>(rcd.best_object);
diff --git a/servers/physics_3d/space_3d_sw.cpp b/servers/physics_3d/space_3d_sw.cpp
index c1e09c9a22..e411ff8266 100644
--- a/servers/physics_3d/space_3d_sw.cpp
+++ b/servers/physics_3d/space_3d_sw.cpp
@@ -1032,6 +1032,9 @@ bool Space3DSW::test_body_motion(Body3DSW *p_body, const Transform3D &p_from, co
 				r_result->collision_local_shape = rcd.best_local_shape;
 				r_result->collision_normal = rcd.best_normal;
 				r_result->collision_point = rcd.best_contact;
+				r_result->collision_depth = rcd.best_len;
+				r_result->collision_safe_fraction = safe;
+				r_result->collision_unsafe_fraction = unsafe;
 				//r_result->collider_metadata = rcd.best_object->get_shape_metadata(rcd.best_shape);
 
 				const Body3DSW *body = static_cast<const Body3DSW *>(rcd.best_object);
diff --git a/servers/physics_server_2d.cpp b/servers/physics_server_2d.cpp
index faab4f43b8..12200989fd 100644
--- a/servers/physics_server_2d.cpp
+++ b/servers/physics_server_2d.cpp
@@ -487,6 +487,18 @@ int PhysicsTestMotionResult2D::get_collider_shape() const {
 	return result.collider_shape;
 }
 
+real_t PhysicsTestMotionResult2D::get_collision_depth() const {
+	return result.collision_depth;
+}
+
+real_t PhysicsTestMotionResult2D::get_collision_safe_fraction() const {
+	return result.collision_safe_fraction;
+}
+
+real_t PhysicsTestMotionResult2D::get_collision_unsafe_fraction() const {
+	return result.collision_unsafe_fraction;
+}
+
 void PhysicsTestMotionResult2D::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("get_motion"), &PhysicsTestMotionResult2D::get_motion);
 	ClassDB::bind_method(D_METHOD("get_motion_remainder"), &PhysicsTestMotionResult2D::get_motion_remainder);
@@ -497,6 +509,9 @@ void PhysicsTestMotionResult2D::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("get_collider_rid"), &PhysicsTestMotionResult2D::get_collider_rid);
 	ClassDB::bind_method(D_METHOD("get_collider"), &PhysicsTestMotionResult2D::get_collider);
 	ClassDB::bind_method(D_METHOD("get_collider_shape"), &PhysicsTestMotionResult2D::get_collider_shape);
+	ClassDB::bind_method(D_METHOD("get_collision_depth"), &PhysicsTestMotionResult2D::get_collision_depth);
+	ClassDB::bind_method(D_METHOD("get_collision_safe_fraction"), &PhysicsTestMotionResult2D::get_collision_safe_fraction);
+	ClassDB::bind_method(D_METHOD("get_collision_unsafe_fraction"), &PhysicsTestMotionResult2D::get_collision_unsafe_fraction);
 
 	ADD_PROPERTY(PropertyInfo(Variant::VECTOR2, "motion"), "", "get_motion");
 	ADD_PROPERTY(PropertyInfo(Variant::VECTOR2, "motion_remainder"), "", "get_motion_remainder");
@@ -507,6 +522,9 @@ void PhysicsTestMotionResult2D::_bind_methods() {
 	ADD_PROPERTY(PropertyInfo(Variant::RID, "collider_rid"), "", "get_collider_rid");
 	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "collider"), "", "get_collider");
 	ADD_PROPERTY(PropertyInfo(Variant::INT, "collider_shape"), "", "get_collider_shape");
+	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "collision_depth"), "", "get_collision_depth");
+	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "collision_safe_fraction"), "", "get_collision_safe_fraction");
+	ADD_PROPERTY(PropertyInfo(Variant::FLOAT, "collision_unsafe_fraction"), "", "get_collision_unsafe_fraction");
 }
 
 ///////////////////////////////////////
diff --git a/servers/physics_server_2d.h b/servers/physics_server_2d.h
index df39d6ae50..90f50810f9 100644
--- a/servers/physics_server_2d.h
+++ b/servers/physics_server_2d.h
@@ -493,6 +493,9 @@ public:
 		Vector2 collision_point;
 		Vector2 collision_normal;
 		Vector2 collider_velocity;
+		real_t collision_depth = 0.0;
+		real_t collision_safe_fraction = 0.0;
+		real_t collision_unsafe_fraction = 0.0;
 		int collision_local_shape = 0;
 		ObjectID collider_id;
 		RID collider;
@@ -619,6 +622,9 @@ public:
 	RID get_collider_rid() const;
 	Object *get_collider() const;
 	int get_collider_shape() const;
+	real_t get_collision_depth() const;
+	real_t get_collision_safe_fraction() const;
+	real_t get_collision_unsafe_fraction() const;
 };
 
 typedef PhysicsServer2D *(*CreatePhysicsServer2DCallback)();
diff --git a/servers/physics_server_3d.h b/servers/physics_server_3d.h
index d9b805de87..fcdd207843 100644
--- a/servers/physics_server_3d.h
+++ b/servers/physics_server_3d.h
@@ -497,6 +497,9 @@ public:
 		Vector3 collision_point;
 		Vector3 collision_normal;
 		Vector3 collider_velocity;
+		real_t collision_depth = 0.0;
+		real_t collision_safe_fraction = 0.0;
+		real_t collision_unsafe_fraction = 0.0;
 		int collision_local_shape = 0;
 		ObjectID collider_id;
 		RID collider;