Transfer learning toy example.

1 - Train a simple convnet on the MNIST dataset the first 5 digits [0..4]. 2 - Freeze convolutional layers and fine-tune dense layers for the classification of digits [5..9].

Get to 99.8% test accuracy after 5 epochs for the first five digits classifier and 99.2% for the last five digits after transfer + fine-tuning.

    from __future__ import print_function

    import datetime
    import keras
    from keras.datasets import mnist
    from keras.models import Sequential
    from keras.layers import Dense, Dropout, Activation, Flatten
    from keras.layers import Conv2D, MaxPooling2D
    from keras import backend as K

    now = datetime.datetime.now

    batch_size = 128
    num_classes = 5
    epochs = 5

    # input image dimensions
    img_rows, img_cols = 28, 28
    # number of convolutional filters to use
    filters = 32
    # size of pooling area for max pooling
    pool_size = 2
    # convolution kernel size
    kernel_size = 3

    if K.image_data_format() == 'channels_first':
        input_shape = (1, img_rows, img_cols)
    else:
        input_shape = (img_rows, img_cols, 1)

    def train_model(model, train, test, num_classes):
        x_train = train[0].reshape((train[0].shape[0],) + input_shape)
        x_test = test[0].reshape((test[0].shape[0],) + input_shape)
        x_train = x_train.astype('float32')
        x_test = x_test.astype('float32')
        x_train /= 255
        x_test /= 255
        print('x_train shape:', x_train.shape)
        print(x_train.shape[0], 'train samples')
        print(x_test.shape[0], 'test samples')

        # convert class vectors to binary class matrices
        y_train = keras.utils.to_categorical(train[1], num_classes)
        y_test = keras.utils.to_categorical(test[1], num_classes)

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

        t = now()
        model.fit(x_train, y_train,
                  batch_size=batch_size,
                  epochs=epochs,
                  verbose=1,
                  validation_data=(x_test, y_test))
        print('Training time: %s' % (now() - t))
        score = model.evaluate(x_test, y_test, verbose=0)
        print('Test score:', score[0])
        print('Test accuracy:', score[1])

    # the data, split between train and test sets
    (x_train, y_train), (x_test, y_test) = mnist.load_data()

    # create two datasets one with digits below 5 and one with 5 and above
    x_train_lt5 = x_train[y_train < 5]
    y_train_lt5 = y_train[y_train < 5]
    x_test_lt5 = x_test[y_test < 5]
    y_test_lt5 = y_test[y_test < 5]

    x_train_gte5 = x_train[y_train >= 5]
    y_train_gte5 = y_train[y_train >= 5] - 5
    x_test_gte5 = x_test[y_test >= 5]
    y_test_gte5 = y_test[y_test >= 5] - 5

    # define two groups of layers: feature (convolutions) and classification (dense)
    feature_layers = [
        Conv2D(filters, kernel_size,
               padding='valid',
               input_shape=input_shape),
        Activation('relu'),
        Conv2D(filters, kernel_size),
        Activation('relu'),
        MaxPooling2D(pool_size=pool_size),
        Dropout(0.25),
        Flatten(),
    ]

    classification_layers = [
        Dense(128),
        Activation('relu'),
        Dropout(0.5),
        Dense(num_classes),
        Activation('softmax')
    ]

    # create complete model
    model = Sequential(feature_layers + classification_layers)

    # train model for 5-digit classification [0..4]
    train_model(model,
                (x_train_lt5, y_train_lt5),
                (x_test_lt5, y_test_lt5), num_classes)

    # freeze feature layers and rebuild model
    for l in feature_layers:
        l.trainable = False

    # transfer: train dense layers for new classification task [5..9]
    train_model(model,
                (x_train_gte5, y_train_gte5),
                (x_test_gte5, y_test_gte5), num_classes)

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