feat: implement new "small-steps" solver + joint improvements

examples3d/debug_chain_high_mass_ratio3.rs

@@ -0,0 +1,73 @@
+use rapier3d::prelude::*;
+use rapier_testbed3d::Testbed;
+
+pub fn init_world(testbed: &mut Testbed) {
+    /*
+     * World
+     */
+    let mut bodies = RigidBodySet::new();
+    let mut colliders = ColliderSet::new();
+    let mut impulse_joints = ImpulseJointSet::new();
+    let mut multibody_joints = MultibodyJointSet::new();
+    let use_articulations = false;
+
+    /*
+     * Create a chain with a very heavy ball at the end.
+     */
+    let num = 17;
+    let rad = 0.2;
+
+    let mut body_handles = Vec::new();
+
+    for i in 0..num {
+        let fi = i as f32;
+
+        let status = if i == 0 {
+            RigidBodyType::Fixed
+        } else {
+            RigidBodyType::Dynamic
+        };
+
+        let ball_rad = if i == num - 1 { rad * 10.0 } else { rad };
+        let shift1 = rad * 1.1;
+        let shift2 = ball_rad + rad * 0.1;
+        let z = if i == 0 {
+            0.0
+        } else {
+            (fi - 1.0) * 2.0 * shift1 + shift1 + shift2
+        };
+
+        let rigid_body = RigidBodyBuilder::new(status)
+            .translation(vector![0.0, 0.0, z])
+            .additional_solver_iterations(16);
+        let child_handle = bodies.insert(rigid_body);
+        let collider = ColliderBuilder::ball(ball_rad);
+        colliders.insert_with_parent(collider, child_handle, &mut bodies);
+
+        // Vertical joint.
+        if i > 0 {
+            let parent_handle = *body_handles.last().unwrap();
+            let joint = if i == 1 {
+                SphericalJointBuilder::new().local_anchor2(point![0.0, 0.0, -shift1 * 2.0])
+            } else {
+                SphericalJointBuilder::new()
+                    .local_anchor1(point![0.0, 0.0, shift1])
+                    .local_anchor2(point![0.0, 0.0, -shift2])
+            };
+
+            if use_articulations {
+                multibody_joints.insert(parent_handle, child_handle, joint, true);
+            } else {
+                impulse_joints.insert(parent_handle, child_handle, joint, true);
+            }
+        }
+
+        body_handles.push(child_handle);
+    }
+
+    /*
+     * Set up the testbed.
+     */
+    testbed.set_world(bodies, colliders, impulse_joints, multibody_joints);
+    testbed.look_at(point![10.0, 10.0, 10.0], Point::origin());
+}