@staticmethod
def _imgnorm(img, low=None, high=None):
    """
        Normalize intensity (B&W) image. Values at
        low -> 0
        high -> 1

        """
    if low is None:
        low = img.min()
    if high is None:
        high = img.max()
    if high == low:
        return 0 * img
    else:
        return (img - low) / (high - low)
