Reworked PathFollow2D behaviour, based on such in version 2.1.

When rotation is enabled, the follower's rotation will be set to that of
the tangent to the path at it's current offset.
For closed looping paths the lookahead will now wrap around at the end of
the path.

fixes #13434

scene/2d/path_2d.cpp

@@ -107,34 +107,56 @@ void PathFollow2D::_update_transform() {
 	if (!c.is_valid())
 		return;
 
-	if (delta_offset == 0) {
-		return;
-	}
-
-	float o = offset;
+	float path_length = c->get_baked_length();
+	float bounded_offset = offset;
 	if (loop)
-		o = Math::fposmod(o, c->get_baked_length());
-
-	Vector2 pos = c->interpolate_baked(o, cubic);
+		bounded_offset = Math::fposmod(bounded_offset, path_length);
+	else
+		bounded_offset = CLAMP(bounded_offset, 0, path_length);
 
-	Vector2 displacement_offset = Vector2(h_offset, v_offset);
+	Vector2 pos = c->interpolate_baked(bounded_offset, cubic);
 
 	if (rotate) {
+		float ahead = bounded_offset + lookahead;
+
+		if (loop && ahead >= path_length) {
+			// If our lookahead will loop, we need to check if the path is closed.
+			int point_count = c->get_point_count();
+			if (point_count > 0) {
+				Vector2 start_point = c->get_point_position(0);
+				Vector2 end_point = c->get_point_position(point_count - 1);
+				if (start_point == end_point) {
+					// Since the path is closed we want to 'smooth off'
+					// the corner at the start/end.
+					// So we wrap the lookahead back round.
+					ahead = Math::fmod(ahead, path_length);
+				}
+			}
+		}
+
+		Vector2 ahead_pos = c->interpolate_baked(ahead, cubic);
 
-		Vector2 t_prev = (pos - c->interpolate_baked(o - delta_offset, cubic)).normalized();
-		Vector2 t_next = (c->interpolate_baked(o + delta_offset, cubic) - pos).normalized();
+		Vector2 tangent_to_curve;
+		if (ahead_pos == pos) {
+			// This will happen at the end of non-looping or non-closed paths.
+			// We'll try a look behind instead, in order to get a meaningful angle.
+			tangent_to_curve =
+					(pos - c->interpolate_baked(bounded_offset - lookahead, cubic)).normalized();
+		} else {
+			tangent_to_curve = (ahead_pos - pos).normalized();
+		}
 
-		float angle = t_prev.angle_to(t_next);
+		Vector2 normal_of_curve = -tangent_to_curve.tangent();
 
-		set_rotation(get_rotation() + angle);
+		pos += tangent_to_curve * h_offset;
+		pos += normal_of_curve * v_offset;
 
-		Vector2 n = t_next;
-		Vector2 t = -n.tangent();
-		pos += n * h_offset + t * v_offset;
+		set_rotation(tangent_to_curve.angle());
 
 	} else {
 
-		pos += displacement_offset;
+		pos.x += h_offset;
+		pos.y += v_offset;
 	}
 
 	set_position(pos);
@@ -185,6 +207,8 @@ bool PathFollow2D::_set(const StringName &p_name, const Variant &p_value) {
 		set_cubic_interpolation(p_value);
 	} else if (String(p_name) == "loop") {
 		set_loop(p_value);
+	} else if (String(p_name) == "lookahead") {
+		set_lookahead(p_value);
 	} else
 		return false;
 
@@ -207,6 +231,8 @@ bool PathFollow2D::_get(const StringName &p_name, Variant &r_ret) const {
 		r_ret = cubic;
 	} else if (String(p_name) == "loop") {
 		r_ret = loop;
+	} else if (String(p_name) == "lookahead") {
+		r_ret = lookahead;
 	} else
 		return false;
 
@@ -224,6 +250,7 @@ void PathFollow2D::_get_property_list(List<PropertyInfo> *p_list) const {
 	p_list->push_back(PropertyInfo(Variant::BOOL, "rotate"));
 	p_list->push_back(PropertyInfo(Variant::BOOL, "cubic_interp"));
 	p_list->push_back(PropertyInfo(Variant::BOOL, "loop"));
+	p_list->push_back(PropertyInfo(Variant::REAL, "lookahead", PROPERTY_HINT_RANGE, "0.001,1024.0,0.001"));
 }
 
 String PathFollow2D::get_configuration_warning() const {
@@ -263,7 +290,7 @@ void PathFollow2D::_bind_methods() {
 }
 
 void PathFollow2D::set_offset(float p_offset) {
-	delta_offset = p_offset - offset;
+
 	offset = p_offset;
 	if (path)
 		_update_transform();
@@ -314,6 +341,16 @@ float PathFollow2D::get_unit_offset() const {
 		return 0;
 }
 
+void PathFollow2D::set_lookahead(float p_lookahead) {
+
+	lookahead = p_lookahead;
+}
+
+float PathFollow2D::get_lookahead() const {
+
+	return lookahead;
+}
+
 void PathFollow2D::set_rotate(bool p_rotate) {
 
 	rotate = p_rotate;
@@ -338,11 +375,11 @@ bool PathFollow2D::has_loop() const {
 PathFollow2D::PathFollow2D() {
 
 	offset = 0;
-	delta_offset = 0;
 	h_offset = 0;
 	v_offset = 0;
 	path = NULL;
 	rotate = true;
 	cubic = true;
 	loop = true;
+	lookahead = 4;
 }

scene/2d/path_2d.h

@@ -60,9 +60,9 @@ public:
 private:
 	Path2D *path;
 	real_t offset;
-	real_t delta_offset; // change in offset since last _update_transform
 	real_t h_offset;
 	real_t v_offset;
+	real_t lookahead;
 	bool cubic;
 	bool loop;
 	bool rotate;
@@ -90,6 +90,9 @@ public:
 	void set_unit_offset(float p_unit_offset);
 	float get_unit_offset() const;
 
+	void set_lookahead(float p_lookahead);
+	float get_lookahead() const;
+
 	void set_loop(bool p_loop);
 	bool has_loop() const;