Convert cursor to world coordinates

Click here for the full example code.

Bevy does not yet provide built-in functions to help with finding out what the cursor is pointing at.

3D games

There is a good (unofficial) plugin: bevy_mod_picking.

2D games

Simple hacky solution, for a game using the default Bevy 2d orthographic camera:

/// Used to help identify our main camera
struct MainCamera;

fn setup(mut commands: Commands) {
    commands.spawn()
        .insert_bundle(OrthographicCameraBundle::new_2d())
        .insert(MainCamera);
}

fn my_cursor_system(
    // need to get window dimensions
    wnds: Res<Windows>,
    // query to get camera transform
    q_camera: Query<&Transform, With<MainCamera>>
) {
    // get the primary window
    let wnd = wnds.get_primary().unwrap();

    // check if the cursor is in the primary window
    if let Some(pos) = wnd.cursor_position() {
        // get the size of the window
        let size = Vec2::new(wnd.width() as f32, wnd.height() as f32);

        // the default orthographic projection is in pixels from the center;
        // just undo the translation
        let p = pos - size / 2.0;

        // assuming there is exactly one main camera entity, so this is OK
        let camera_transform = q_camera.single().unwrap();

        // apply the camera transform
        let pos_wld = camera_transform.compute_matrix() * p.extend(0.0).extend(1.0);
        eprintln!("World coords: {}/{}", pos_wld.x, pos_wld.y);
    }
}