Transfer Learning and Fine Tuning using Keras

Introduction#

This topic includes short, brief but comprehensive examples of loading pre-trained weights, inserting new layers on top or in the middle of pre-tained ones, and training a new network with partly pre-trained weights. An example for each of out-of-the-box pre-trained networks, available in Keras library (VGG, ResNet, Inception, Xception, MobileNet), is required.

Transfer Learning using Keras and VGG

In this example, three brief and comprehensive sub-examples are presented:

Loading weights from available pre-trained models, included with Keras library
Stacking another network for training on top of any layers of VGG
Inserting a layer in the middle of other layers
Tips and general rule-of-thumbs for Fine-Tuning and transfer learning with VGG

Loading pre-trained weights

Pre-trained on ImageNet models, including VGG-16 and VGG-19, are available in Keras. Here and after in this example, VGG-16 will be used. For more information, please visit Keras Applications documentation.

from keras import applications

# This will load the whole VGG16 network, including the top Dense layers.
# Note: by specifying the shape of top layers, input tensor shape is forced
# to be (224, 224, 3), therefore you can use it only on 224x224 images.
vgg_model = applications.VGG16(weights='imagenet', include_top=True)

# If you are only interested in convolution filters. Note that by not
# specifying the shape of top layers, the input tensor shape is (None, None, 3),
# so you can use them for any size of images.
vgg_model = applications.VGG16(weights='imagenet', include_top=False)

# If you want to specify input tensor
from keras.layers import Input
input_tensor = Input(shape=(160, 160, 3))
vgg_model = applications.VGG16(weights='imagenet',
                               include_top=False,
                               input_tensor=input_tensor)

# To see the models' architecture and layer names, run the following
vgg_model.summary()

Create a new network with bottom layers taken from VGG

Assume that for some specific task for images with the size (160, 160, 3), you want to use pre-trained bottom layers of VGG, up to layer with the name block2_pool.

vgg_model = applications.VGG16(weights='imagenet',
                               include_top=False,
                               input_shape=(160, 160, 3))

# Creating dictionary that maps layer names to the layers
layer_dict = dict([(layer.name, layer) for layer in vgg_model.layers])

# Getting output tensor of the last VGG layer that we want to include
x = layer_dict['block2_pool'].output

# Stacking a new simple convolutional network on top of it    
x = Conv2D(filters=64, kernel_size=(3, 3), activation='relu')(x)
x = MaxPooling2D(pool_size=(2, 2))(x)
x = Flatten()(x)
x = Dense(256, activation='relu')(x)
x = Dropout(0.5)(x)
x = Dense(10, activation='softmax')(x)

# Creating new model. Please note that this is NOT a Sequential() model.
from keras.models import Model
custom_model = Model(input=vgg_model.input, output=x)

# Make sure that the pre-trained bottom layers are not trainable
for layer in custom_model.layers[:7]:
    layer.trainable = False

# Do not forget to compile it
custom_model.compile(loss='categorical_crossentropy',
                     optimizer='rmsprop',
                     metrics=['accuracy'])

Remove multiple layers and insert a new one in the middle

Assume that you need to speed up VGG16 by replacing block1_conv1 and block2_conv2 with a single convolutional layer, in such a way that the pre-trained weights are saved. The idea is to disassemble the whole network to separate layers, then assemble it back. Here is the code specifically for your task:

vgg_model = applications.VGG16(include_top=True, weights='imagenet')

# Disassemble layers
layers = [l for l in vgg_model.layers]

# Defining new convolutional layer.
# Important: the number of filters should be the same!
# Note: the receiptive field of two 3x3 convolutions is 5x5.
new_conv = Conv2D(filters=64, 
                  kernel_size=(5, 5),
                  name='new_conv',
                  padding='same')(layers[0].output)

# Now stack everything back
# Note: If you are going to fine tune the model, do not forget to
#       mark other layers as un-trainable
x = new_conv
for i in range(3, len(layers)):
    layers[i].trainable = False
    x = layers[i](x)

# Final touch
result_model = Model(input=layer[0].input, output=x)