def _call__(fieldname=None, direction='thumbnail', height=None, width=None,
    scale=None, **parameters):
    """Interface defining an actual scaling operation.

        Arguments are:

        ``context``
            some object with images on

        ``fieldname``
            name of the field to scale

        ``direction``
            is same as PIL direction on scale

        ``width`` and ``height``
            target size

        ``scale``
            name of the current scale, if there is one. Can be used to retrieve
             additional information such as cropping boxes.

        ``**parameters``
            is a dict with optional additional expected keyword arguments

        Expected to return a triple of ``value, format, dimensions``
        or ``None`` on failure.

        ``value``
            is expected to be an storeable value

        ``format``
            is the minor part of the ``image`` mimetype

        ``dimensions``
            is a tuple (width, height)
        """
