Core Layers

Dense

keras.layers.core.Dense(units, activation=None, use_bias=True, kernel_initializer='glorot_uniform', bias_initializer='zeros', kernel_regularizer=None, bias_regularizer=None, activity_regularizer=None, kernel_constraint=None, bias_constraint=None)

Just your regular densely-connected NN layer.

Dense implements the operation: output = activation(dot(input, kernel) + bias) where activation is the element-wise activation function passed as the activation argument, kernel is a weights matrix created by the layer, and bias is a bias vector created by the layer (only applicable if use_bias is True).

Note: if the input to the layer has a rank greater than 2, then it is flattened prior to the initial dot product with kernel.

Example

# as first layer in a sequential model:
model = Sequential()
model.add(Dense(32, input_shape=(16,)))
# now the model will take as input arrays of shape (*, 16)
# and output arrays of shape (*, 32)

# after the first layer, you don't need to specify
# the size of the input anymore:
model.add(Dense(32))

Arguments

units: Positive integer, dimensionality of the output space.
activation: Activation function to use (see activations). If you don't specify anything, no activation is applied (ie. "linear" activation: a(x) = x).
use_bias: Boolean, whether the layer uses a bias vector.
kernel_initializer: Initializer for the kernel weights matrix (see initializers).
bias_initializer: Initializer for the bias vector (see initializers).
kernel_regularizer: Regularizer function applied to the kernel weights matrix (see regularizer).
bias_regularizer: Regularizer function applied to the bias vector (see regularizer).
activity_regularizer: Regularizer function applied to the output of the layer (its "activation"). (see regularizer).
kernel_constraint: Constraint function applied to the kernel weights matrix (see constraints).
bias_constraint: Constraint function applied to the bias vector (see constraints).

Input shape

nD tensor with shape: (batch_size, ..., input_dim). The most common situation would be a 2D input with shape (batch_size, input_dim).

Output shape

nD tensor with shape: (batch_size, ..., units). For instance, for a 2D input with shape (batch_size, input_dim), the output would have shape (batch_size, units).

[source]

Activation

keras.layers.core.Activation(activation)

Applies an activation function to an output.

Arguments

activation: name of activation function to use (see: activations), or alternatively, a Theano or TensorFlow operation.

Input shape

Arbitrary. Use the keyword argument input_shape (tuple of integers, does not include the samples axis) when using this layer as the first layer in a model.

Output shape

Same shape as input.

[source]

Dropout

keras.layers.core.Dropout(rate, noise_shape=None, seed=None)

Applies Dropout to the input.

Dropout consists in randomly setting a fraction rate of input units to 0 at each update during training time, which helps prevent overfitting.

Arguments

rate: float between 0 and 1. Fraction of the input units to drop.
noise_shape: 1D integer tensor representing the shape of the binary dropout mask that will be multiplied with the input. For instance, if your inputs have shape (batch_size, timesteps, features) and you want the dropout mask to be the same for all timesteps, you can use noise_shape=(batch_size, 1, features).
seed: A Python integer to use as random seed.

References

Dropout: A Simple Way to Prevent Neural Networks from Overfitting

[source]

Flatten

keras.layers.core.Flatten()

Flattens the input. Does not affect the batch size.

Example

model = Sequential()
model.add(Conv2D(64, 3, 3,
            border_mode='same',
            input_shape=(3, 32, 32)))
# now: model.output_shape == (None, 64, 32, 32)

model.add(Flatten())
# now: model.output_shape == (None, 65536)

[source]

Reshape

keras.layers.core.Reshape(target_shape)

Reshapes an output to a certain shape.

Arguments

target_shape: target shape. Tuple of integers. Does not include the batch axis.

Input shape

Arbitrary, although all dimensions in the input shaped must be fixed. Use the keyword argument input_shape (tuple of integers, does not include the batch axis) when using this layer as the first layer in a model.

Output shape

(batch_size,) + target_shape

Example

# as first layer in a Sequential model
model = Sequential()
model.add(Reshape((3, 4), input_shape=(12,)))
# now: model.output_shape == (None, 3, 4)
# note: `None` is the batch dimension

# as intermediate layer in a Sequential model
model.add(Reshape((6, 2)))
# now: model.output_shape == (None, 6, 2)

# also supports shape inference using `-1` as dimension
model.add(Reshape((-1, 2, 2)))
# now: model.output_shape == (None, 3, 2, 2)

[source]

Permute

keras.layers.core.Permute(dims)

Permutes the dimensions of the input according to a given pattern.

Useful for e.g. connecting RNNs and convnets together.

Example

model = Sequential()
model.add(Permute((2, 1), input_shape=(10, 64)))
# now: model.output_shape == (None, 64, 10)
# note: `None` is the batch dimension

Arguments

dims: Tuple of integers. Permutation pattern, does not include the samples dimension. Indexing starts at 1. For instance, (2, 1) permutes the first and second dimension of the input.

Input shape

Arbitrary. Use the keyword argument input_shape (tuple of integers, does not include the samples axis) when using this layer as the first layer in a model.

Output shape

Same as the input shape, but with the dimensions re-ordered according to the specified pattern.

[source]

RepeatVector

keras.layers.core.RepeatVector(n)

Repeats the input n times.

Example

model = Sequential()
model.add(Dense(32, input_dim=32))
# now: model.output_shape == (None, 32)
# note: `None` is the batch dimension

model.add(RepeatVector(3))
# now: model.output_shape == (None, 3, 32)

Arguments

n: integer, repetition factor.

Input shape

2D tensor of shape (num_samples, features).

Output shape

3D tensor of shape (num_samples, n, features).

[source]

Lambda

keras.layers.core.Lambda(function, output_shape=None, mask=None, arguments=None)

Wraps arbitrary expression as a Layer object.

Examples

# add a x -> x^2 layer
model.add(Lambda(lambda x: x ** 2))

# add a layer that returns the concatenation
# of the positive part of the input and
# the opposite of the negative part

def antirectifier(x):
    x -= K.mean(x, axis=1, keepdims=True)
    x = K.l2_normalize(x, axis=1)
    pos = K.relu(x)
    neg = K.relu(-x)
    return K.concatenate([pos, neg], axis=1)

def antirectifier_output_shape(input_shape):
    shape = list(input_shape)
    assert len(shape) == 2  # only valid for 2D tensors
    shape[-1] *= 2
    return tuple(shape)

model.add(Lambda(antirectifier,
         output_shape=antirectifier_output_shape))

Arguments

function: The function to be evaluated. Takes input tensor as first argument.
output_shape: Expected output shape from function. Only relevant when using Theano. Can be a tuple or function. If a tuple, it only specifies the first dimension onward; sample dimension is assumed either the same as the input: output_shape = (input_shape[0], ) + output_shape or, the input is None and the sample dimension is also None: output_shape = (None, ) + output_shape If a function, it specifies the entire shape as a function of the input shape: output_shape = f(input_shape)
arguments: optional dictionary of keyword arguments to be passed to the function.

Input shape

Arbitrary. Use the keyword argument input_shape (tuple of integers, does not include the samples axis) when using this layer as the first layer in a model.

Output shape

Specified by output_shape argument (or auto-inferred when using TensorFlow).

[source]

ActivityRegularization

keras.layers.core.ActivityRegularization(l1=0.0, l2=0.0)

Layer that applies an update to the cost function based input activity.

Arguments

l1: L1 regularization factor (positive float).
l2: L2 regularization factor (positive float).

Input shape

Arbitrary. Use the keyword argument input_shape (tuple of integers, does not include the samples axis) when using this layer as the first layer in a model.

Output shape

Same shape as input.

[source]

Masking

keras.layers.core.Masking(mask_value=0.0)

Masks a sequence by using a mask value to skip timesteps.

For each timestep in the input tensor (dimension #1 in the tensor), if all values in the input tensor at that timestep are equal to mask_value, then the timestep will be masked (skipped) in all downstream layers (as long as they support masking).

If any downstream layer does not support masking yet receives such an input mask, an exception will be raised.

Example

Consider a Numpy data array x of shape (samples, timesteps, features), to be fed to a LSTM layer. You want to mask timestep #3 and #5 because you lack data for these timesteps. You can:

set x[:, 3, :] = 0. and x[:, 5, :] = 0.
insert a Masking layer with mask_value=0. before the LSTM layer:

model = Sequential()
model.add(Masking(mask_value=0., input_shape=(timesteps, features)))
model.add(LSTM(32))