a godot 4 function to get MeshInstances under the cursor

pick_meshinstance3d.gd

func pick(in: Node3D) -> Array:
	var camera = get_viewport().get_camera_3d()
	var center_pos = get_viewport().size / 2.0
	var mouse_pos = get_viewport().get_mouse_position()
	var from = camera.project_ray_origin(center_pos)
	var to = from + camera.project_ray_normal(mouse_pos) * 50.0

	var pick_results: Array[MeshInstance3D] = []
	for m in in.get_children():
		m = m as MeshInstance3D

		var aabb = m.get_aabb()

		# avoid using VisualInstance3D.get_transformed_aabb() as it will be
		# removed in beta 4: https://github.com/godotengine/godot/pull/66940
		# probably the most random removal i've seen, lol
		
		# also note the order of operation is important:
		# aabb * m.global_transform produces wrong results
		# (seems to be inverted)
		var global_aabb = m.global_transform * aabb

		# it seems some click-through is still apparent,
		# maybe using AABB.grow() is appropriate.

		if global_aabb.intersects_ray(from, to):
			pick_results.append(m)

	if pick_results.is_empty():
		return []

	# sort by distance to the camera, in ascending order
	pick_results.sort_custom(
		func(a: MeshInstance3D, b: MeshInstance3D):
			var cam_pos = camera.global_transform.origin
			var adist = a.global_transform.origin.distance_squared_to(cam_pos)
			var bdist = b.global_transform.origin.distance_squared_to(cam_pos)
			return adist < bdist
	)
  
  # the actual occluded selection is pick_results[0]
  
  return pick_results