Introduction to Deep Learning with Keras

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Keras is a high-level neural networks API, capable of running on top of Tensorflow, Theano, and CNTK. It enables fast experimentation through a high level, user-friendly, modular and extensible API. Keras can also be run on both CPU and GPU.

Keras was developed and is maintained by Francois Chollet and is part of the Tensorflow core, which makes it Tensorflows preferred high-level API.

This article is the first of a little series explaining how to use Keras for deep learning.

In this article, we will go over the basics of Keras including the two most used Keras models (Sequential and Functional), the core layers as well as some preprocessing functionalities.

Installing Keras

I assume you already have a working installation of Tensorflow or Theano or CNTK. If you don’t check out the links above.

Keras can be installed using pip or conda:

pip install keras
or
conda install keras

Loading in a dataset

Keras provides seven different datasets, which can be loaded in using Keras directly. These include image datasets as well as a house price and a movie review datasets.

In this article, we will use the MNIST dataset, which contains 70000 28x28 grayscale images with 10 different classes. Keras splits it in a training set with 60000 instances and a testing set with 10000 instances.

from keras.datasets import mnist

(x_train, y_train), (x_test, y_test) = mnist.load_data()

To feed the images to a convolutional neural network we transform the dataframe to four dimensions. This can be done using numpys reshape method. We will also transform the data into floats and normalize it.

x_train = x_train.astype('float32')
x_test = x_test.astype('float32')
x_train /= 255
x_test /= 255

x_train = X_train.reshape(X_train.shape[0], 28, 28, 1)
x_test = X_test.reshape(X_test.shape[0], 28, 28, 1)

We will also transform our labels into a one-hot encoding using the to_categorical method from Keras.

from keras.utils import to_categorical

y_train = to_categorical(y_train, 10)
y_test = to_categorical(y_test, 10)

Creating a model with the sequential API

The easiest way of creating a model in Keras is by using the sequential API, which lets you stack one layer after the other. The problem with the sequential API is that it doesn’t allow models to have multiple inputs or outputs, which are needed for some problems.

Nevertheless, the sequential API is a perfect choice for most problems.

To create a convolutional neural network we only need to create a Sequential object and use the add function to add layers.

from keras.models import Sequential
from keras.layers import Conv2D, MaxPool2D, Dense, Flatten, Dropout

model = Sequential()
model.add(Conv2D(filters=32, kernel_size=(5,5), activation='relu', input_shape=x_train.shape[1:]))
model.add(Conv2D(filters=32, kernel_size=(5,5), activation='relu'))
model.add(MaxPool2D(pool_size=(2, 2)))
model.add(Dropout(rate=0.25))
model.add(Conv2D(filters=64, kernel_size=(3, 3), activation='relu'))
model.add(Conv2D(filters=64, kernel_size=(3, 3), activation='relu'))
model.add(MaxPool2D(pool_size=(2, 2)))
model.add(Dropout(rate=0.25))
model.add(Flatten())
model.add(Dense(256, activation='relu'))
model.add(Dropout(rate=0.5))
model.add(Dense(10, activation='softmax'))

The code above first of creates a Sequential object and adds a few convolutional, maxpooling and dropout layers. It then flattens the output and passes it two a last dense and dropout layer before passing it to our output layer. If you aren’t confident build a convolutional neural network(CNN) check out this great tutorial.

The sequential API also supports another syntax where the layers are passed to the constructor directly.

from keras.models import Sequential
from keras.layers import Conv2D, MaxPool2D, Dense, Flatten, Dropout

model = Sequential([
    Conv2D(filters=32, kernel_size=(5,5), activation='relu', input_shape=x_train.shape[1:]),
    Conv2D(filters=32, kernel_size=(5,5), activation='relu'),
    MaxPool2D(pool_size=(2, 2)),
    Dropout(rate=0.25),
    Conv2D(filters=64, kernel_size=(3,3), activation='relu'),
    Conv2D(filters=64, kernel_size=(3,3), activation='relu'),
    MaxPool2D(pool_size=(2, 2)),
    Dropout(rate=0.25),
    Flatten(),
    Dense(256, activation='relu'),
    Dropout(rate=0.5),
    Dense(10, activation='softmax')
])

Creating a model with the Functional API

Alternatively, the functional API allows you to create the same models but offers you more flexibility at the cost of simplicity and readability.

It can be used with multiple input and output layers as well as shared layers, which enables you to build really complex network structures.

When using the functional API we always need to pass the previous layer to the current layer. It also requires the use of an input layer.

from keras.models import Model
from keras.layers import Conv2D, MaxPool2D, Dense, Flatten, Dropout, Input

inputs = Input(shape=x_train.shape[1:])

x = Conv2D(filters=32, kernel_size=(5,5), activation='relu')(inputs)
x = Conv2D(filters=32, kernel_size=(5,5), activation='relu')(x)
x = MaxPool2D(pool_size=(2, 2))(x)
x = Dropout(rate=0.25)(x)

x = Conv2D(filters=64, kernel_size=(3,3), activation='relu')(x)
x = Conv2D(filters=64, kernel_size=(3,3), activation='relu')(x)
x = MaxPool2D(pool_size=(2, 2))(x)
x = Dropout(rate=0.25)(x)

x = Flatten()(x)
x = Dense(256, activation='relu')(x)
x = Dropout(rate=0.5)(x)
predictions = Dense(10, activation='softmax')(x)

model = Model(inputs=inputs, outputs=predictions)

The functional API is also often used for transfer learning which we will look at in another article.

Compile a model

Before we can start training our model we need to configure the learning process. For this, we need to specify an optimizer, a loss function and optionally some metrics like accuracy.

The loss function is a measure on how good our model is at achieving the given objective.

An optimizer is used to minimize the loss(objective) function by updating the weights using the gradients.

model.compile(
    loss='categorical_crossentropy', 
    optimizer='adam', 
    metrics=['accuracy']
)

Augmenting Image data

Augmentation is a process of creating more data from existing once. For images you can to little transformations like rotating the image, zooming into the image, adding noise and many more.

This helps to make the model more robust and solves the problem of having not enough data. Keras has a method called ImageDataGenerator which can be used for augmenting images.

from keras.preprocessing.image import ImageDataGenerator

datagen = ImageDataGenerator(
  rotation_range=10,
  zoom_range=0.1,
  width_shift_range=0.1,
  height_shift_range=0.1
)

This ImageDataGenerator will create new images that have been rotated, zoomed in or out, and shifted in width and height.

Fit a model

Now that we defined and compiled our model it’s ready for training. To train a model we would normally use the fit method but because we are using a datagenerator we will use fit_generator and pass it our generator, X data, y data as well as the number of epochs and the batch size. We will also pass it a validation set so we can monitor the loss and accuracy on both sets as well as steps_per_epoch which is required when using a generator and is just set to the length of the training set divided by the batch_size.

epochs = 3
batch_size = 32
history = model.fit_generator(datagen.flow(x_train, y_train, batch_size=batch_size), epochs=epochs,
                              validation_data=(x_test, y_test), steps_per_epoch=x_train.shape[0]//batch_size)

This outputs:

Epoch 1/5
1875/1875 [==============================] - 22s 12ms/step - loss: 0.1037 - acc: 0.9741 - val_loss: 0.0445 - val_acc: 0.9908
Epoch 2/5
1875/1875 [==============================] - 22s 12ms/step - loss: 0.0879 - acc: 0.9781 - val_loss: 0.0259 - val_acc: 0.9937
Epoch 3/5
1875/1875 [==============================] - 22s 12ms/step - loss: 0.0835 - acc: 0.9788 - val_loss: 0.0321 - val_acc: 0.9926
Epoch 4/5
1875/1875 [==============================] - 22s 12ms/step - loss: 0.0819 - acc: 0.9792 - val_loss: 0.0264 - val_acc: 0.9936
Epoch 5/5
1875/1875 [==============================] - 22s 12ms/step - loss: 0.0790 - acc: 0.9790 - val_loss: 0.0220 - val_acc: 0.9938

Visualizing the training process

We can visualize our training and testing accuracy and loss for each epoch so we can get intuition about the performance of our model. The accuracy and loss over epochs are saved in the history variable we got whilst training and we will use Matplotlib to visualize this data.

import matplotlib.pyplot as plt

plt.plot(history.history['acc'], label='training accuracy')
plt.plot(history.history['val_acc'], label='testing accuracy')
plt.title('Accuracy')
plt.xlabel('epochs')
plt.ylabel('accuracy')
plt.legend()

Training/Testing accuracy over epochs

Figure 2: Training/Testing accuracy over epochs

plt.plot(history.history['loss'], label='training loss')
plt.plot(history.history['val_loss'], label='testing loss')
plt.title('Loss')
plt.xlabel('epochs')
plt.ylabel('loss')
plt.legend()

Training/Testting loss over epochs

Figure 3: Training/Testing loss over epochs

In the graphs above we can see that our model isn’t overfitting as well as that we could train more epochs because the validation loss is still decreasing.

Conclusion

Keras is a high-level neural networks API, capable of running on top of Tensorflow, Theano and CNTK. It enables fast experimentation through a high level, user-friendly, modular and extensible API and as well as running on the CPU and GPU.

This article was the first of a little series going over how Keras works. In it, we went over the installation as well as how to create a simple convolutional neural network.

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The code covered in this article is available as a Github Repository.

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