Unexpected AabbCast2d aabb_collision_at with small max distance value

[bread-mountain4]

Bevy version

0.15.1

[Optional] Relevant system information

• Arch Linux
• rustc 1.84.1 (e71f9a9a9 2025-01-27)

What you did

I have some code that uses raycast2d over short distances that I expect to be similar to if I translated the aabb2 then did an intersection check.

What went wrong

I am getting some unexpected results with these raycasts. I get an unexpected value of a collision at distance 0.0 for two Aabb2ds that should be sufficiently far apart.

Here's a small unit test I wrote to do the raycast some other ways that do pass, then the last call I expected to be a None but it's a Some(0.0). The numbers were pulled directly from a live run of the program when the raycast2d returned an unexpected answer.

#[test]
fn test_debug() {
    let movement_amount = 0.974444;

    let window = Aabb2d {
        min: Vec2::new(-26.197449, -21.058212),
        max: Vec2::new(323.80255, 278.94177),
    };
    let side = Aabb2d {
        min: Vec2::new(472.0, -320.0),
        max: Vec2::new(473.0, 320.0),
    };

    assert_eq!(false, window.intersects(&side));
    assert_eq!(false, side.intersects(&window));

    let mut window_new = window.clone();
    window_new.translate_by(Vec2::new(movement_amount, 0.0));
    assert_eq!(false, window_new.intersects(&side));
    assert_eq!(false, side.intersects(&window_new));

    let window_new_new = Aabb2d {
        min: window.min + Vec2::new(movement_amount, 0.0),
        max: window.max + Vec2::new(movement_amount, 0.0),
    };

    assert_eq!(false, window_new_new.intersects(&side));
    assert_eq!(false, side.intersects(&window_new_new));

    let ray_direction = Dir2::new(Vec2::new(1.0, 0.0)).unwrap();
    let ray_towards_boundary_intersections = Ray2d::new(window.center(), ray_direction);
    let bounds_check_ray_cast =
        AabbCast2d::from_ray(window, ray_towards_boundary_intersections, movement_amount);

    let res = bounds_check_ray_cast.aabb_collision_at(side);
    assert_eq!(None, res);
}

---- collisions::tests::test_debug stdout ----
thread 'collisions::tests::test_debug' panicked at scroll_2d/src/collisions/tests.rs:234:5:
assertion `left == right` failed
  left: None
 right: Some(0.0)

[jfto23]

I think the failure is happening because the window passed to AabbCast2d::from_ray(window, ray_towards_boundary_intersections, movement_amount); is relative to the ray. If you want the Aabb2d to move along the ray with no offset, then its center should be zero when building the AabbCast2d.

[bread-mountain4]

Hmm, that did work although I find it a bit surprising. Mentally it seemed like I'd want to center the ray in the aabb2d and move it a specific distance along the ray. What would be the case for a Ray2d not centered at zero being used in an aabbcast2d? Would there be one at all?

[jfto23]

I'm not too sure what a use case for that would be, but there is a test that uses this. Either way, some more documentation would be good imo as it's currently not part of the docs nor part of the bounding volumes example.