How to use the itkwidgets.trait_types.ITKImage function in itkwidgets

# start the generator
            next(i)
        return inner
    return f


@widgets.register
class Viewer(ViewerParent):
    """Viewer widget class."""
    _view_name = Unicode('ViewerView').tag(sync=True)
    _model_name = Unicode('ViewerModel').tag(sync=True)
    _view_module = Unicode('itkwidgets').tag(sync=True)
    _model_module = Unicode('itkwidgets').tag(sync=True)
    _view_module_version = Unicode('^0.25.0').tag(sync=True)
    _model_module_version = Unicode('^0.25.0').tag(sync=True)
    image = ITKImage(
        default_value=None,
        allow_none=True,
        help="Image to visualize.").tag(
        sync=False,
        **itkimage_serialization)
    rendered_image = ITKImage(
        default_value=None,
        allow_none=True).tag(
        sync=True,
        **itkimage_serialization)
    _rendering_image = CBool(
        default_value=False,
        help="We are currently volume rendering the image.").tag(sync=True)
    interpolation = CBool(
        default_value=True,
        help="Use linear interpolation in slicing planes.").tag(sync=True)

@widgets.register
class Viewer(ViewerParent):
    """Viewer widget class."""
    _view_name = Unicode('ViewerView').tag(sync=True)
    _model_name = Unicode('ViewerModel').tag(sync=True)
    _view_module = Unicode('itkwidgets').tag(sync=True)
    _model_module = Unicode('itkwidgets').tag(sync=True)
    _view_module_version = Unicode('^0.25.0').tag(sync=True)
    _model_module_version = Unicode('^0.25.0').tag(sync=True)
    image = ITKImage(
        default_value=None,
        allow_none=True,
        help="Image to visualize.").tag(
        sync=False,
        **itkimage_serialization)
    rendered_image = ITKImage(
        default_value=None,
        allow_none=True).tag(
        sync=True,
        **itkimage_serialization)
    _rendering_image = CBool(
        default_value=False,
        help="We are currently volume rendering the image.").tag(sync=True)
    interpolation = CBool(
        default_value=True,
        help="Use linear interpolation in slicing planes.").tag(sync=True)
    cmap = Colormap('Viridis (matplotlib)').tag(sync=True)
    _custom_cmap = NDArray(dtype=np.float32, default_value=None, allow_none=True,
                           help="RGB triples from 0.0 to 1.0 that define a custom linear, sequential colormap")\
        .tag(sync=True, **array_serialization)\
        .valid(shape_constraints(None, 3))
    shadow = CBool(