Add property for constraining the distance between endpoints

demo/addons/ropesim/Rope.gd

@@ -23,6 +23,13 @@ signal on_point_count_changed()
 ## Overall rope length. Will be distributed uniformly among all segments.
 @export var rope_length: float = 100: set = _set_length
 
+## Maximum euclidean distance between rope endpoints. Zero or negative for no limitation.
+## This is an approximation and not 100% accurate.
+## It is intended as a simple way to constraint the rope length when both endpoints are fixed by a RopeHandle.
+## The actual length of the rope might differ depending on the number of constraint iterations.
+## Fixed points in between are not taken into account.
+@export var max_endpoint_distance: float = -1
+
 ## (Optional) Allows to distribute the length of rope segment in a non-uniform manner.
 ## Useful when certain parts of the rope should be more detailed than the rest.
 ## For example, if it is known that most movement happens at the beginning of the rope, a curve with

demo/rope_examples/handles.tscn

@@ -150,3 +150,40 @@ script = ExtResource("3_h0p3k")
 num_segments = 20
 rope_length = 200.0
 metadata/_edit_group_ = true
+
+[node name="Label6" type="Label" parent="."]
+offset_left = 304.0
+offset_top = 511.0
+offset_right = 823.0
+offset_bottom = 534.0
+text = "max_endpoint_distance limits the distance between both rope endpoints."
+
+[node name="Rope5" type="Node2D" parent="."]
+position = Vector2(356, 591)
+script = ExtResource("3_h0p3k")
+max_endpoint_distance = 150.0
+fixate_begin = false
+
+[node name="RopeHandleBegin" type="Marker2D" parent="Rope5"]
+position = Vector2(-10, -4)
+script = ExtResource("1_n2oah")
+rope_path = NodePath("..")
+rope_position = 0.0
+strength = 1.0
+
+[node name="RopeHandleEnd" type="Marker2D" parent="Rope5"]
+position = Vector2(204, 22)
+script = ExtResource("1_n2oah")
+rope_path = NodePath("..")
+strength = 1.0
+
+[node name="Rope6" type="Node2D" parent="."]
+position = Vector2(636, 564)
+script = ExtResource("3_h0p3k")
+max_endpoint_distance = 150.0
+
+[node name="RopeHandleEnd" type="Marker2D" parent="Rope6"]
+position = Vector2(160, 86)
+script = ExtResource("1_n2oah")
+rope_path = NodePath("..")
+strength = 1.0

src/NativeRopeContext.cpp

@@ -39,6 +39,7 @@ void NativeRopeContext::load_context(Node2D* rope)
     gravity_direction = rope->get("gravity_direction");
     damping = rope->get("damping");
     stiffness = rope->get("stiffness");
+    max_endpoint_distance = rope->get("max_endpoint_distance");
     num_constraint_iterations = rope->get("num_constraint_iterations");
     seg_lengths = rope->call("get_segment_lengths");
     simulation_weights = rope->get("_simulation_weights");
@@ -151,30 +152,51 @@ void NativeRopeContext::_simulate_stiffness(PackedVector2Array* velocities) cons
     }
 }
 
+static void constraint_segment(Vector2* point_a, Vector2* point_b, float weight_a, float weight_b, float seg_length)
+{
+    const Vector2 diff = *point_b - *point_a;
+    const float distance = diff.length();
+    const float error = (seg_length - distance) * 0.5f;
+    const Vector2 dir = error * (diff / distance);
+
+    // If one point has a weight < 1.0, the other point must compensate the difference in
+    // relation to its own weight.
+    // This is especially relevant with fixate_begin = true or with arbitrary weights = 0.0.
+    // In that case non-fixed point should be constrained by the whole error distance, not
+    // just half of it, because the other one can obviously not move.
+    // It actually works quite fine without this compensation, but this is more correct and
+    // produces better results.
+    *point_a -= (weight_a + weight_a * (1.0 - weight_b)) * dir;
+    *point_b += (weight_b + weight_b * (1.0 - weight_a)) * dir;
+}
+
 void NativeRopeContext::_constraint()
 {
+    const bool use_euclid_constraint = max_endpoint_distance > 0;
+    Vector2* first_point;
+    Vector2* last_point;
+    float euclid_constraint_first_weight;
+    float max_stretch_length_sqr;
+
+    if (use_euclid_constraint)
+    {
+        first_point = &points[0];
+        last_point = &points[(int)points.size() - 1];
+        euclid_constraint_first_weight = fixate_begin ? 0.0 : 1.0;
+        max_stretch_length_sqr = max_endpoint_distance * max_endpoint_distance;
+    }
+
     for (int _ = 0; _ < num_constraint_iterations; ++_)
     {
-        for (int i = 0; i < points.size() - 1; ++i)
+        if (use_euclid_constraint)
         {
-            const Vector2 diff = points[i + 1] - points[i];
-            const float distance = diff.length();
-            const float error = (seg_lengths[i] - distance) * 0.5f;
-            const Vector2 dir = error * (diff / distance);
-            const float weight = simulation_weights[i];
-            const float next_weight = simulation_weights[i + 1];
-
-            // If one point has a weight < 1.0, the other point must compensate the difference in
-            // relation to its own weight.
-            // This is especially relevant with fixate_begin = true or with arbitrary weights = 0.0.
-            // In that case non-fixed point should be constrained by the whole error distance, not
-            // just half of it, because the other one can obviously not move.
-            // It actually works quite fine without this compensation, but this is more correct and
-            // produces better results.
-            // points[i] -= weight * dir;
-            // points[i + 1] += next_weight * dir;
-            points[i] -= (weight + weight * (1.0 - next_weight)) * dir;
-            points[i + 1] += (next_weight + next_weight * (1.0 - weight)) * dir;
+            const float rope_length_sqr = first_point->distance_squared_to(*last_point);
+
+            if (rope_length_sqr > max_stretch_length_sqr)
+                constraint_segment(first_point, last_point, euclid_constraint_first_weight, 1.0, max_endpoint_distance);
         }
+
+        for (int i = 0; i < points.size() - 1; ++i)
+            constraint_segment(&points[i], &points[i + 1], simulation_weights[i], simulation_weights[i + 1], seg_lengths[i]);
     }
 }

src/NativeRopeContext.hpp

@@ -33,6 +33,7 @@ namespace godot
             Vector2 gravity_direction;
             float damping;
             float stiffness;
+            float max_endpoint_distance;
             int num_constraint_iterations;
             Ref<Curve> damping_curve;
             bool fixate_begin;