incorrect tangent_impulse reported by contact graph

[bcolloran]

Contact graph reports NaN or 0 for tangent_impulse when a frictional contact occurs between a capsule and a rectangle or 2 rects. Have not investigated other combinations of shapes, but I imagine this is not isolated to those cases?

thank you for Rapier!

repro code:

A test example:

use std::f32::consts::PI;

use rapier2d::prelude::*;
use rapier_testbed2d::Testbed;

pub fn init_world(testbed: &mut Testbed) {
    /*
     * World
     */
    let mut bodies = RigidBodySet::new();
    let mut colliders = ColliderSet::new();
    let impulse_joints = ImpulseJointSet::new();
    let multibody_joints = MultibodyJointSet::new();

    /*
     * The ground
     */
    let ground_size = 20.0;
    let ground_height = 1.0;

    let rigid_body = RigidBodyBuilder::fixed().translation(vector![0.0, -3.0]);
    let handle = bodies.insert(rigid_body);
    let collider = ColliderBuilder::cuboid(ground_size, ground_height)
        .rotation(0.25 * PI)
        .friction(0.9);
    colliders.insert_with_parent(collider, handle, &mut bodies);

    /*
     * A tilted capsule that cannot rotate.
     */
    let rigid_body = RigidBodyBuilder::dynamic()
        .translation(vector![0.0, 4.0])
        .lock_rotations();
    let handle = bodies.insert(rigid_body);
    let collider = ColliderBuilder::capsule_y(2.0, 1.0).friction(0.9);
    colliders.insert_with_parent(collider, handle, &mut bodies);

    /*
     * Set up the testbed.
     */
    testbed.set_world(bodies, colliders, impulse_joints, multibody_joints);
    testbed.look_at(point![0.0, 0.0], 10.0);
}

change to enable printing in src_testbed/testbed.rs, line 1430

               if harness.state.timestep_id % 100 == 0 {
                    let pairs: Vec<_> = harness.physics.narrow_phase.contact_pairs().collect();
                    println!("contact pairs: {:?}", pairs);
                }

quick debug impl for ContactPair in src/geometry/contact_pair.rs, line 133

impl std::fmt::Debug for ContactPair {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("ContactPair")
            .field("collider1", &self.collider1)
            .field("collider2", &self.collider2)
            .field("manifolds", &self.manifolds)
            .field("has_any_active_contact", &self.has_any_active_contact)
            .finish()
    }
}

full print output (note tangent_impulse values)

[
  ContactPair {
    collider1: ColliderHandle(Index { index: 0, generation: 0 }),
    collider2: ColliderHandle(Index { index: 1, generation: 0 }),
    manifolds: [
      ContactManifold { 
        points: [
          TrackedContact { 
            local_p1: [4.0150623, 1.0], 
            local_p2: [0.70710677, 1.2928929], 
            dist: 2.827232, 
            fid1: PackedFeatureId(3221225524), 
            fid2: PackedFeatureId(1073741826), 
            data: ContactData { 
              impulse: 0.0, 
              tangent_impulse: [[0.0]],
              warmstart_impulse: 0.0,
              warmstart_tangent_impulse: [[0.0]]
            } 
          },
          TrackedContact { 
            local_p1: [1.186636, 1.0], 
            local_p2: [0.70710677, -2.7071066], 
            dist: -0.0011950731, 
            fid1: PackedFeatureId(3221225524), 
            fid2: PackedFeatureId(1073741824), 
            data: ContactData { 
              impulse: 1.610124, 
              tangent_impulse: [[NaN]], 
              warmstart_impulse: 0.3220248, 
              warmstart_tangent_impulse: [[-0.2898223]] 
            } 
          }
        ],
        local_n1: [[-0.0, 1.0]],
        local_n2: [[0.70710677, -0.70710677]],
        subshape1: 0,
        subshape2: 0,
        subshape_pos1: None,
        subshape_pos2: None,
        data: ContactManifoldData { 
          rigid_body1: Some(RigidBodyHandle(Index { index: 0, generation: 0 })),
          rigid_body2: Some(RigidBodyHandle(Index { index: 1, generation: 0 })), 
          solver_flags: SolverFlags(COMPUTE_IMPULSES), 
          normal: [[-0.70710677, 0.70710677]], 
          solver_contacts: [SolverContact { 
            contact_id: 1,
            point: [0.13239408, -1.4542375],
            dist: -0.0011950731,
            friction: 0.9,
            restitution: 0.0,
            tangent_velocity: [[0.0, 0.0]],
            is_new: false,
            warmstart_impulse: 0.3220248,
            warmstart_tangent_impulse: [[-0.2898223]]
          }], 
          relative_dominance: 128, 
          user_data: 0
          }
        }
      ],
    has_any_active_contact: true 
  }
]