custom_layers.py

from keras.engine import Layer, InputSpec
try:
    from keras import initializations
except ImportError:
    from keras import initializers as initializations
import keras.backend as K

class Scale(Layer):
    '''Custom Layer for DenseNet used for BatchNormalization.
    
    Learns a set of weights and biases used for scaling the input data.
    the output consists simply in an element-wise multiplication of the input
    and a sum of a set of constants:

        out = in * gamma + beta,

    where 'gamma' and 'beta' are the weights and biases larned.

    # Arguments
        axis: integer, axis along which to normalize in mode 0. For instance,
            if your input tensor has shape (samples, channels, rows, cols),
            set axis to 1 to normalize per feature map (channels axis).
        momentum: momentum in the computation of the
            exponential average of the mean and standard deviation
            of the data, for feature-wise normalization.
        weights: Initialization weights.
            List of 2 Numpy arrays, with shapes:
            `[(input_shape,), (input_shape,)]`
        beta_init: name of initialization function for shift parameter
            (see [initializations](../initializations.md)), or alternatively,
            Theano/TensorFlow function to use for weights initialization.
            This parameter is only relevant if you don't pass a `weights` argument.
        gamma_init: name of initialization function for scale parameter (see
            [initializations](../initializations.md)), or alternatively,
            Theano/TensorFlow function to use for weights initialization.
            This parameter is only relevant if you don't pass a `weights` argument.
    '''
    def __init__(self, weights=None, axis=-1, momentum = 0.9, beta_init='zero', gamma_init='one', **kwargs):
        self.momentum = momentum
        self.axis = axis
        self.beta_init = initializations.get(beta_init)
        self.gamma_init = initializations.get(gamma_init)
        self.initial_weights = weights
        super(Scale, self).__init__(**kwargs)

    def build(self, input_shape):
        self.input_spec = [InputSpec(shape=input_shape)]
        shape = (int(input_shape[self.axis]),)

        # Tensorflow >= 1.0.0 compatibility
        self.gamma = K.variable(self.gamma_init(shape), name='{}_gamma'.format(self.name))
        self.beta = K.variable(self.beta_init(shape), name='{}_beta'.format(self.name))
        #self.gamma = self.gamma_init(shape, name='{}_gamma'.format(self.name))
        #self.beta = self.beta_init(shape, name='{}_beta'.format(self.name))
        self.trainable_weights = [self.gamma, self.beta]

        if self.initial_weights is not None:
            self.set_weights(self.initial_weights)
            del self.initial_weights

    def call(self, x, mask=None):
        input_shape = self.input_spec[0].shape
        broadcast_shape = [1] * len(input_shape)
        broadcast_shape[self.axis] = input_shape[self.axis]

        out = K.reshape(self.gamma, broadcast_shape) * x + K.reshape(self.beta, broadcast_shape)
        return out

    def get_config(self):
        config = {"momentum": self.momentum, "axis": self.axis}
        base_config = super(Scale, self).get_config()
        return dict(list(base_config.items()) + list(config.items()))