Godot near clip plane calculation

func _sign(inp: float) -> float:
	if inp >= 0.0:
		return 1.0
	else:
		return -1.0

func _multiply_point(m: Transform, point: Vector3) -> Vector3:
	return m.xform(point)

func _multiply_vector(m: Basis, pos: Vector3) -> Vector3:
	return m * pos;

func _process(delta):
	var clip_plane: Transform = portal.transform
	var clip_plane_forward: Vector3 = -clip_plane.basis.z
	var dot: int = _sign(clip_plane_forward.dot(portal.transform.origin - transform.origin))

	var cam_space_pos: = _multiply_point(get_camera_transform(), clip_plane.origin)
	var cam_space_normal: = _multiply_vector(get_camera_transform().basis, clip_plane_forward) * dot
	var cam_space_dst = - cam_space_pos.dot(cam_space_normal) + near_clip_offset;

	if abs(cam_space_dst) > near_clip_limit:
		projection = PROJECTION_OBLIQUE_PERSPECTIVE
		oblique_near_plane = Quat (cam_space_normal.x, cam_space_normal.y, cam_space_normal.z, cam_space_dst)
	else:
		projection = PROJECTION_PERSPECTIVE