@classmethod
def register(cls, name, transformation):
    """
        Register new transformation function.

        >>> brighter = lambda image, c: image * c
        >>> TransformImage.register('brighter', brighter)
        >>> transform = TransformImage(0).by('brighter', 1.5)

        :param string name: Name of transformation
        :param function transformation: Transformation function.
        """
    cls.transformations[name] = transformation
