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

This code should work for 2D games with orthographic projections. It is "undoing" the projection and camera transformations.

(there will likely be slight inaccuracy from floating-point calculations)

This should work regardless of the scaling settings of your projection, and camera transform.

/// Used to help identify our main camera
#[derive(Component)]
struct MainCamera;

fn setup(mut commands: Commands) {
    commands.spawn((
        Camera2dBundle::default(),
        MainCamera,
    ));
}

fn my_cursor_system(
    // need to get window dimensions
    wnds: Res<Windows>,
    // query to get camera transform
    q_camera: Query<(&Camera, &GlobalTransform), With<MainCamera>>
) {
    // get the camera info and transform
    // assuming there is exactly one main camera entity, so query::single() is OK
    let (camera, camera_transform) = q_camera.single();

    // get the window that the camera is displaying to (or the primary window)
    let wnd = if let RenderTarget::Window(id) = camera.target {
        wnds.get(id).unwrap()
    } else {
        wnds.get_primary().unwrap()
    };

    // check if the cursor is inside the window and get its position
    if let Some(screen_pos) = wnd.cursor_position() {
        // get the size of the window
        let window_size = Vec2::new(wnd.width() as f32, wnd.height() as f32);

        // convert screen position [0..resolution] to ndc [-1..1] (gpu coordinates)
        let ndc = (screen_pos / window_size) * 2.0 - Vec2::ONE;

        // matrix for undoing the projection and camera transform
        let ndc_to_world = camera_transform.compute_matrix() * camera.projection_matrix().inverse();

        // use it to convert ndc to world-space coordinates
        let world_pos = ndc_to_world.project_point3(ndc.extend(-1.0));

        // reduce it to a 2D value
        let world_pos: Vec2 = world_pos.truncate();

        eprintln!("World coords: {}/{}", world_pos.x, world_pos.y);
    }
}