Simple Autoencoder Example with Keras in Python

   Autoencoder is a neural network model that learns from the data to imitate the output based on the input data. It can only represent a data-specific and a lossy version of the trained data. Autoencoder is also a kind of compression and reconstructing method with a neural network. In this tutorial, we'll learn how to build a simple autoencoder with Keras in Python. The tutorial covers:
  1. Preparing the data
  2. Defining the autoencoder model
  3. Restoring the image
  4. Source code listing
   We'll start by loading the required Python libraries for this tutorial.

from keras.layers import Dense 
from keras.layers import Input, LeakyReLU
from keras.models import Model
from keras.datasets.mnist import load_data
from numpy import reshape
import matplotlib.pyplot as plt

Preparing the data

    We'll use MNIST handwritten digits dataset to train the autoencoder. First, we'll load it and prepare it by doing some changes. Autoencoder requires only input data so that we only focus on x part of the dataset. We'll scale it into the range of [0, 1].

(xtrain, _), (xtest, _) = load_data()

xtrain = xtrain.astype('float32') / 255
xtest = xtest.astype('float32') / 255
print(xtrain.shape, xtest.shape)
(60000, 28, 28) (10000, 28, 28) 

Next, we'll define input size and latent vector. We'll learn why we need the latent vector below. Then we'll reshape images data into the vector type.

input_size = xtrain.shape[1] * xtrain.shape[2]
latent_size = 16

x_train = xtrain.reshape((len(xtrain), input_size))
x_test = xtest.reshape((len(xtest), input_size))
print(x_train.shape)
(60000, 784)
print(x_test.shape)
(10000, 784) 


Defining the autoencoder model

   Next, we'll define encoder, decoder, and autoencoder models. The encoder compresses the data into the middle layer that is a latent vector. The decoder decompresses the data from the latent vector. The latent vector represents a compressed version of the original data that the decoder can restore new data from it. The autoencoder orchestrates to train both encoder and decoder models.

We'll define the encoder starting from the input layer. Encoder contains the Dense layer and ReLU leaky activations. The last output layer defines the latent vector size.

enc_input = Input(shape=(input_size,)) 
 
enc_dense1 = Dense(units=256, activation="relu")(enc_input)
enc_activ1 = LeakyReLU()(enc_dense1)
enc_dense2 = Dense(units=latent_size)(enc_activ1)
enc_output = LeakyReLU()(enc_dense2)
encoder = Model(enc_input, enc_output)

We'll define the decoder starting from the input layer. The decoder also contains a similar Dense layer and ReLU leaky activations. The last output layer defines the input size and Sigmoid activation.

dec_input = Input(shape=(latent_size,))
 
dec_dense1 = Dense(units=256, activation="relu")(dec_input)
dec_activ1 = LeakyReLU()(dec_dense1)
dec_dense2 = Dense(units=input_size, activation='sigmoid')(dec_activ1)
dec_output = LeakyReLU()(dec_dense2)
decoder = Model(dec_input, dec_output)
decoder.summary() 

Finally, we'll define the autoencoder. It is a combination of encoder and decoder with an additional input layer.

aen_input = Input(shape=(input_size,))
aen_enc_output = encoder(aen_input)
aen_dec_output = decoder(aen_enc_output)

aen = Model(aen_input, aen_dec_output)
aen.summary()
 
Layer (type)                 Output Shape              Param #   
=================================================================
input_5 (InputLayer)         (None, 784)               0         
_________________________________________________________________
model_5 (Model)              (None, 16)                205072    
_________________________________________________________________
model_6 (Model)              (None, 784)               205840    
=================================================================
Total params: 410,912
Trainable params: 410,912
Non-trainable params: 0
_________________________________________________________________  
 
Here you can see that the vector with the size of 784 is shrinking to the 16 and returning back to the 784 again. We'll add the Rmsprop optimizer to train the model and train it on x training data.
Now, we can compile and fit the model with train data.

aen.compile(optimizer="rmsprop", loss="binary_crossentropy")
aen.fit(x_train, x_train, epochs=20, batch_size=256, shuffle=True)


Restoring the image

Finally, we'll restore the test data and visualize them in a plot. First, we'll encode the image, then predict with a decoder model. We'll check the result visualizing in a plot.

encoded_images = encoder.predict(x_test)
decoded_images = decoder.predict(encoded_images)
pred_images = reshape(decoded_images, newshape=(decoded_images.shape[0], 28, 28)) 
 
n = 10
plt.figure(figsize=(10, 2))
for i in range(n): 
 ax = plt.subplot(2, n, i + 1)
 ax.get_xaxis().set_visible(False)
 ax.get_yaxis().set_visible(False)
 plt.imshow(xtest[i].reshape(28, 28))
 plt.gray()
 
 ax = plt.subplot(2, n, i + 1 + n)
 ax.get_xaxis().set_visible(False)
 ax.get_yaxis().set_visible(False)
 plt.imshow(pred_images[i].reshape(28, 28))

plt.show()
The first row in a plot shows the original images in test data. The second row contains the restored data with the autoencoder model.

   In this tutorial, we've briefly learned how to build s simple autoencoder with Keras in Python. The full source code is listed below.


Source code listing

from keras.layers import Dense
from keras.layers import Input, LeakyReLU
from keras.models import Model
from keras.datasets.mnist import load_data
from numpy import reshape
import matplotlib.pyplot as plt

(xtrain, _), (xtest, _) = load_data()
xtrain = xtrain.astype('float32') / 255.
xtest = xtest.astype('float32') / 255.
print(xtrain.shape, xtest.shape)  

input_size = xtrain.shape[1] * xtrain.shape[2]
latent_size = 16
x_train = xtrain.reshape((len(xtrain), input_size))
x_test = xtest.reshape((len(xtest), input_size))
print(x_train.shape)
print(x_test.shape)

# Encoder
enc_input = Input(shape=(input_size,))
enc_dense1 = Dense(units=256, activation="relu")(enc_input)
enc_activ1 = LeakyReLU()(enc_dense1)
enc_dense2 = Dense(units=latent_size)(enc_activ1)
enc_output = LeakyReLU()(enc_dense2)
encoder = Model(enc_input, enc_output)
encoder.summary()

# Decoder
dec_input = Input(shape=(latent_size,))
dec_dense1 = Dense(units=256, activation="relu")(dec_input)
dec_activ1 = LeakyReLU()(dec_dense1)
dec_dense2 = Dense(units=input_size, activation='sigmoid')(dec_activ1)
dec_output = LeakyReLU()(dec_dense2)
decoder = Model(dec_input, dec_output)
decoder.summary()

# Autoencoder
aen_input = Input(shape=(input_size,))
aen_enc_output = encoder(aen_input)
aen_dec_output = decoder(aen_enc_output)
aen = Model(aen_input, aen_dec_output)
aen.summary()

aen.compile(optimizer="rmsprop", loss="binary_crossentropy")
aen.fit(x_train, x_train, epochs=20, batch_size=256, shuffle=True)

encoded_images = encoder.predict(x_test)
decoded_images = decoder.predict(encoded_images)
pred_images = reshape(decoded_images, newshape=(decoded_images.shape[0], 28, 28)) 
 
n = 10
plt.figure(figsize=(10, 2))
for i in range(n): 
 ax = plt.subplot(2, n, i + 1)
 ax.get_xaxis().set_visible(False)
 ax.get_yaxis().set_visible(False)
 plt.imshow(xtest[i].reshape(28, 28))
 plt.gray()
 
 ax = plt.subplot(2, n, i + 1 + n)
 ax.get_xaxis().set_visible(False)
 ax.get_yaxis().set_visible(False)
 plt.imshow(pred_images[i].reshape(28, 28))

plt.show()


Reference:
  1. Building Autoencoders in Keras

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