使用 keras 的 L2 归一化输出

Question

I would like to build a neural net with Keras with Tensorflow backend which outputs an L2 normalized vector. I have tried the following but for some reason it does not normalize the output:

import keras.backend as K
input = Input(shape=input_shape)
...
dense7 = Dense(output_dim=3)(flatten6)
l2_norm = Lambda(lambda  x: K.l2_normalize(x,axis=1))(dense7)
return Model(input=input, output=l2_norm)

Answer 1

I found the problem!

So I am using tensorflow as a backed and K.l2_normalize(x, axis) calls the tf.nn.l2_normalize(x, dim, epsilon=1e-12, name=None). Notice that this method has one extra parameter epsilon. And this method looks as follows:

with ops.name_scope(name, "l2_normalize", [x]) as name:
   x = ops.convert_to_tensor(x, name="x")
   square_sum = math_ops.reduce_sum(math_ops.square(x), dim, keep_dims=True)
   x_inv_norm = math_ops.rsqrt(math_ops.maximum(square_sum, epsilon))
return math_ops.mul(x, x_inv_norm, name=name)

So if the output of the net contains numbers lower then epsilon (which is set to 1e-12 by default) then it is not normalized correctly which is what happens in my case.

Answer 2

You can use the function, which is called by tensorflow.keras.backend.l2_normalize to set the epsilon value:

from tensorflow.python.ops import nn
nn.l2_normalize(x, axis=None, epsilon=1e-12) 

@thebeancounter You can define your own L2 Layer. For example to support masking: if there follow layers after the L2 Normalization, which depend on masking you should use the following:

class L2Layer(tf.keras.layers.Layer):
    def __init__(self):
        super(L2Layer, self).__init__()
        self.supports_masking = True

    def call(self, inputs, mask=None):
        return K.l2_normalize(inputs, axis=2)

Answer 3

我认为您可以使用像这样的最后一层：

tf.keras.layers.Lambda(lambda x: tf.math.l2_normalize(x, axis=1))