def region_2d_to_vector_3d(region: 'bpy.types.Region', rv3d:
    'bpy.types.RegionView3D', coord) ->'mathutils.Vector':
    """Return a direction vector from the viewport at the specific 2d region coordinate. 

    :param region: region of the 3D viewport, typically bpy.context.region. 
    :type region: 'bpy.types.Region'
    :param rv3d: 3D region data, typically bpy.context.space_data.region_3d. 
    :type rv3d: 'bpy.types.RegionView3D'
    :param coord: 2d coordinates relative to the region: (event.mouse_region_x, event.mouse_region_y) for example. 
    :return:  normalized 3d vector. 
    """
    pass
