def subtract(self, val):
        """
        Subtract a constant value or an image from all images.

        Parameters
        ----------
        val : int, float, or ndarray
            Value to subtract.
        """
        if isinstance(val, ndarray):
            if val.shape != self.value_shape:
                raise Exception('Cannot subtract image with dimensions %s '
                                'from images with dimension %s' % (str(val.shape), str(self.value_shape)))

        return self.map(lambda x: x - val, value_shape=self.value_shape)
Subtract a constant value or an image from all images.

        Parameters
        ----------
        val : int, float, or ndarray
            Value to subtract.