[Bug?] Contact of circle and polyline at its vertex produces 0 normal

[ghost]

Not sure if this is something fixable as I'm not familiar with the math, but (seemingly) when a circle touches a line just at its vertex, a normal of 0 is produced.

In the case of a one-way platform, this can cause spurious collisions on the very ends of a platform when pass-through is supposed to happen.

Edit: I did some further testing and it seems that the circle wasn't even touching the line when this happens. See below ASCII art.

______o Zero normal contact is reported when the circle sits right beside the line, aligned but not touching it.

Here's a minimal test case:

use rapier2d::pipeline::*;
use rapier2d::dynamics::*;
use rapier2d::geometry::*;
use rapier2d::na::*;

fn main() {
    let mut pipeline = PhysicsPipeline::new();
    let gravity = Vector2::new(0.0, 0.0);
    let integration_parameters = IntegrationParameters::default();
    let mut broad_phase = BroadPhase::new();
    let mut narrow_phase = NarrowPhase::new();
    let mut bodies = RigidBodySet::new();
    let mut colliders = ColliderSet::new();
    let mut joints = JointSet::new();
    let mut ccd_solver = CCDSolver::new();

    // Circle
    let body = RigidBodyBuilder::new_dynamic()
        .translation(11.0, 0.0).build(); // Sits beyond the edge of the ground defined below.
    let handle = bodies.insert(body);
    let collider = ColliderBuilder::ball(1.0) 
        .translation(0.0, 1.0)
        .modify_solver_contacts(true)
        .build();
    colliders.insert(collider, handle, &mut bodies);

    // Ground
    let body = RigidBodyBuilder::new_static().build();
    let handle = bodies.insert(body);
    let collider = ColliderBuilder::polyline(vec![
        Point2::new(-10.0, 0.0),
        Point2::new(10.0, 0.0),
    ], None).build();
    colliders.insert(collider, handle, &mut bodies);

    loop {
        pipeline.step(
            &gravity,
            &integration_parameters,
            &mut broad_phase,
            &mut narrow_phase,
            &mut bodies,
            &mut colliders,
            &mut joints,
            &mut ccd_solver,
            &MyHook,
            &(),
        );
    }
}

// Convenience to detect contacts
struct MyHook;
impl PhysicsHooks for MyHook {
    fn active_hooks(&self) -> PhysicsHooksFlags {
        PhysicsHooksFlags::MODIFY_SOLVER_CONTACTS
    }

    fn modify_solver_contacts(&self, context: &mut ContactModificationContext) {
        if context.manifold.local_n1.x == 0.0 && context.manifold.local_n1.y == 0.0 {
            println!("Zero normal!");
        }
    }
}

[sebcrozet]

Hi! This is a bug. A zero normal may be returned in the case where the center of the circle is located on the line. But it should not be zero in other cases.

[ghost]

Thanks for the reply! It looks like I'll wait for the fix rather than try to terribly hack my code then :)