constraint.py

from tensorflow.python.keras.constraints import Constraint
from tensorflow.python.ops import math_ops, array_ops


class TightFrame(Constraint):
    """
    Parseval (tight) frame contstraint, as introduced in https://arxiv.org/abs/1704.08847

    Constraints the weight matrix to be a tight frame, so that the Lipschitz
    constant of the layer is <= 1. This increases the robustness of the network
    to adversarial noise.

    Warning: This constraint simply performs the update step on the weight matrix
    (or the unfolded weight matrix for convolutional layers). Thus, it does not
    handle the necessary scalings for convolutional layers.

    Args:
        scale (float):    Retraction parameter (length of retraction step).
        num_passes (int): Number of retraction steps.

    Returns:
        Weight matrix after applying regularizer.
    """

    def __init__(self, scale, num_passes=1):
        """[summary]

        Args:
            scale ([type]): [description]
            num_passes (int, optional): [description]. Defaults to 1.

        Raises:
            ValueError: [description]
        """
        self.scale = scale

        if num_passes < 1:
            raise ValueError(
                "Number of passes cannot be non-positive! (got {})".format(num_passes)
            )
        self.num_passes = num_passes

    def __call__(self, w):
        """[summary]

        Args:
            w ([type]): weight of conv or linear layers

        Returns:
            [type]: returns new weights
        """
        transpose_channels = len(w.shape) == 4

        # Move channels_num to the front in order to make the dimensions correct for matmul
        if transpose_channels:
            w_reordered = array_ops.reshape(w, (-1, w.shape[3]))

        else:
            w_reordered = w

        last = w_reordered
        for i in range(self.num_passes):
            temp1 = math_ops.matmul(last, last, transpose_a=True)
            temp2 = (1 + self.scale) * w_reordered - self.scale * math_ops.matmul(
                w_reordered, temp1
            )

            last = temp2

        # Move channels_num to the back again
        if transpose_channels:
            return array_ops.reshape(last, w.shape)
        else:
            return last

    def get_config(self):
        return {"scale": self.scale, "num_passes": self.num_passes}


# Alias
tight_frame = TightFrame