# Neural style transfer

This tutorial uses deep learning to compose one image in the style of another image (ever wish you could paint like Picasso or Van Gogh?). This is known as neural style transfer and the technique is outlined in A Neural Algorithm of Artistic Style (Gatys et al.).

Neural style transfer is an optimization technique used to take two images—a content image and a style reference image (such as an artwork by a famous painter)—and blend them together so the output image looks like the content image, but “painted” in the style of the style reference image.

This is implemented by optimizing the output image to match the content statistics of the content image and the style statistics of the style reference image. These statistics are extracted from the images using a convolutional network.

For example, let’s take an image of this dog and Wassily Kandinsky's Composition 7:

Yellow Labrador Looking, from Wikimedia Commons

Now how would it look like if Kandinsky decided to paint the picture of this Dog exclusively with this style? Something like this?

## Setup

### Import and configure modules

import tensorflow as tf
import IPython.display as display

import matplotlib.pyplot as plt
import matplotlib as mpl
mpl.rcParams['figure.figsize'] = (12,12)
mpl.rcParams['axes.grid'] = False

import numpy as np
import PIL.Image
import time
import functools
def tensor_to_image(tensor):
tensor = tensor*255
tensor = np.array(tensor, dtype=np.uint8)
if np.ndim(tensor)>3:
assert tensor.shape[0] == 1
tensor = tensor[0]
return PIL.Image.fromarray(tensor)

# https://commons.wikimedia.org/wiki/File:Vassily_Kandinsky,_1913_-_Composition_7.jpg
90112/83281 [================================] - 0s 0us/step
196608/195196 [==============================] - 0s 0us/step

## Visualize the input

Define a function to load an image and limit its maximum dimension to 512 pixels.

max_dim = 512
img = tf.image.decode_image(img, channels=3)
img = tf.image.convert_image_dtype(img, tf.float32)

shape = tf.cast(tf.shape(img)[:-1], tf.float32)
long_dim = max(shape)
scale = max_dim / long_dim

new_shape = tf.cast(shape * scale, tf.int32)

img = tf.image.resize(img, new_shape)
img = img[tf.newaxis, :]
return img

Create a simple function to display an image:

def imshow(image, title=None):
if len(image.shape) > 3:
image = tf.squeeze(image, axis=0)

plt.imshow(image)
if title:
plt.title(title)

plt.subplot(1, 2, 1)
imshow(content_image, 'Content Image')

plt.subplot(1, 2, 2)
imshow(style_image, 'Style Image')

## Fast Style Transfer using TF-Hub

This tutorial demonstrates the original style-transfer algorithm, which optimizes the image content to a particular style. Before getting into the details, let's see how the TensorFlow Hub module does:

import tensorflow_hub as hub
stylized_image = hub_module(tf.constant(content_image), tf.constant(style_image))[0]
tensor_to_image(stylized_image)

## Define content and style representations

Use the intermediate layers of the model to get the content and style representations of the image. Starting from the network's input layer, the first few layer activations represent low-level features like edges and textures. As you step through the network, the final few layers represent higher-level features—object parts like wheels or eyes. In this case, you are using the VGG19 network architecture, a pretrained image classification network. These intermediate layers are necessary to define the representation of content and style from the images. For an input image, try to match the corresponding style and content target representations at these intermediate layers.

Load a VGG19 and test run it on our image to ensure it's used correctly:

x = tf.keras.applications.vgg19.preprocess_input(content_image*255)
x = tf.image.resize(x, (224, 224))
vgg = tf.keras.applications.VGG19(include_top=True, weights='imagenet')
prediction_probabilities = vgg(x)
prediction_probabilities.shape
574717952/574710816 [==============================] - 35s 0us/step

TensorShape([1, 1000])
predicted_top_5 = tf.keras.applications.vgg19.decode_predictions(prediction_probabilities.numpy())[0]
[(class_name, prob) for (number, class_name, prob) in predicted_top_5]
40960/35363 [==================================] - 0s 0us/step

('golden_retriever', 0.2366525),
('kuvasz', 0.03635752),
('Chesapeake_Bay_retriever', 0.02418279),
('Greater_Swiss_Mountain_dog', 0.018646086)]

Now load a VGG19 without the classification head, and list the layer names

vgg = tf.keras.applications.VGG19(include_top=False, weights='imagenet')

print()
for layer in vgg.layers:
print(layer.name)
80142336/80134624 [==============================] - 6s 0us/step

input_2
block1_conv1
block1_conv2
block1_pool
block2_conv1
block2_conv2
block2_pool
block3_conv1
block3_conv2
block3_conv3
block3_conv4
block3_pool
block4_conv1
block4_conv2
block4_conv3
block4_conv4
block4_pool
block5_conv1
block5_conv2
block5_conv3
block5_conv4
block5_pool

Choose intermediate layers from the network to represent the style and content of the image:

content_layers = ['block5_conv2']

style_layers = ['block1_conv1',
'block2_conv1',
'block3_conv1',
'block4_conv1',
'block5_conv1']

num_content_layers = len(content_layers)
num_style_layers = len(style_layers)

#### Intermediate layers for style and content

So why do these intermediate outputs within our pretrained image classification network allow us to define style and content representations?

At a high level, in order for a network to perform image classification (which this network has been trained to do), it must understand the image. This requires taking the raw image as input pixels and building an internal representation that converts the raw image pixels into a complex understanding of the features present within the image.

This is also a reason why convolutional neural networks are able to generalize well: they’re able to capture the invariances and defining features within classes (e.g. cats vs. dogs) that are agnostic to background noise and other nuisances. Thus, somewhere between where the raw image is fed into the model and the output classification label, the model serves as a complex feature extractor. By accessing intermediate layers of the model, you're able to describe the content and style of input images.

## Build the model

The networks in tf.keras.applications are designed so you can easily extract the intermediate layer values using the Keras functional API.

To define a model using the functional API, specify the inputs and outputs:

model = Model(inputs, outputs)

This following function builds a VGG19 model that returns a list of intermediate layer outputs:

def vgg_layers(layer_names):
""" Creates a vgg model that returns a list of intermediate output values."""
vgg = tf.keras.applications.VGG19(include_top=False, weights='imagenet')
vgg.trainable = False

outputs = [vgg.get_layer(name).output for name in layer_names]

model = tf.keras.Model([vgg.input], outputs)
return model

And to create the model:

style_extractor = vgg_layers(style_layers)
style_outputs = style_extractor(style_image*255)

#Look at the statistics of each layer's output
for name, output in zip(style_layers, style_outputs):
print(name)
print("  shape: ", output.numpy().shape)
print("  min: ", output.numpy().min())
print("  max: ", output.numpy().max())
print("  mean: ", output.numpy().mean())
print()
block1_conv1
shape:  (1, 336, 512, 64)
min:  0.0
max:  835.5256
mean:  33.97525

block2_conv1
shape:  (1, 168, 256, 128)
min:  0.0
max:  4625.8857
mean:  199.82687

block3_conv1
shape:  (1, 84, 128, 256)
min:  0.0
max:  8789.239
mean:  230.78099

block4_conv1
shape:  (1, 42, 64, 512)
min:  0.0
max:  21566.135
mean:  791.24005

block5_conv1
shape:  (1, 21, 32, 512)
min:  0.0
max:  3189.2542
mean:  59.179478

## Calculate style

The content of an image is represented by the values of the intermediate feature maps.

It turns out, the style of an image can be described by the means and correlations across the different feature maps. Calculate a Gram matrix that includes this information by taking the outer product of the feature vector with itself at each location, and averaging that outer product over all locations. This Gram matrix can be calculated for a particular layer as:

$$G^l_{cd} = \frac{\sum_{ij} F^l_{ijc}(x)F^l_{ijd}(x)}{IJ}$$

This can be implemented concisely using the tf.linalg.einsum function:

def gram_matrix(input_tensor):
result = tf.linalg.einsum('bijc,bijd->bcd', input_tensor, input_tensor)
input_shape = tf.shape(input_tensor)
num_locations = tf.cast(input_shape[1]*input_shape[2], tf.float32)
return result/(num_locations)

## Extract style and content

Build a model that returns the style and content tensors.

class StyleContentModel(tf.keras.models.Model):
def __init__(self, style_layers, content_layers):
super(StyleContentModel, self).__init__()
self.vgg =  vgg_layers(style_layers + content_layers)
self.style_layers = style_layers
self.content_layers = content_layers
self.num_style_layers = len(style_layers)
self.vgg.trainable = False

def call(self, inputs):
"Expects float input in [0,1]"
inputs = inputs*255.0
preprocessed_input = tf.keras.applications.vgg19.preprocess_input(inputs)
outputs = self.vgg(preprocessed_input)
style_outputs, content_outputs = (outputs[:self.num_style_layers],
outputs[self.num_style_layers:])

style_outputs = [gram_matrix(style_output)
for style_output in style_outputs]

content_dict = {content_name:value
for content_name, value
in zip(self.content_layers, content_outputs)}

style_dict = {style_name:value
for style_name, value
in zip(self.style_layers, style_outputs)}

return {'content':content_dict, 'style':style_dict}

When called on an image, this model returns the gram matrix (style) of the style_layers and content of the content_layers:

extractor = StyleContentModel(style_layers, content_layers)

results = extractor(tf.constant(content_image))

print('Styles:')
for name, output in sorted(results['style'].items()):
print("  ", name)
print("    shape: ", output.numpy().shape)
print("    min: ", output.numpy().min())
print("    max: ", output.numpy().max())
print("    mean: ", output.numpy().mean())
print()

print("Contents:")
for name, output in sorted(results['content'].items()):
print("  ", name)
print("    shape: ", output.numpy().shape)
print("    min: ", output.numpy().min())
print("    max: ", output.numpy().max())
print("    mean: ", output.numpy().mean())

Styles:
block1_conv1
shape:  (1, 64, 64)
min:  0.0055228462
max:  28014.566
mean:  263.79025

block2_conv1
shape:  (1, 128, 128)
min:  0.0
max:  61479.49
mean:  9100.949

block3_conv1
shape:  (1, 256, 256)
min:  0.0
max:  545623.4
mean:  7660.976

block4_conv1
shape:  (1, 512, 512)
min:  0.0
max:  4320502.0
mean:  134288.84

block5_conv1
shape:  (1, 512, 512)
min:  0.0
max:  110005.37
mean:  1487.0381

Contents:
block5_conv2
shape:  (1, 26, 32, 512)
min:  0.0
max:  2410.8796
mean:  13.764149

With this style and content extractor, you can now implement the style transfer algorithm. Do this by calculating the mean square error for your image's output relative to each target, then take the weighted sum of these losses.

Set your style and content target values:

style_targets = extractor(style_image)['style']
content_targets = extractor(content_image)['content']

Define a tf.Variable to contain the image to optimize. To make this quick, initialize it with the content image (the tf.Variable must be the same shape as the content image):

image = tf.Variable(content_image)

Since this is a float image, define a function to keep the pixel values between 0 and 1:

def clip_0_1(image):
return tf.clip_by_value(image, clip_value_min=0.0, clip_value_max=1.0)

Create an optimizer. The paper recommends LBFGS, but Adam works okay, too:

To optimize this, use a weighted combination of the two losses to get the total loss:

style_weight=1e-2
content_weight=1e4
def style_content_loss(outputs):
style_outputs = outputs['style']
content_outputs = outputs['content']
for name in style_outputs.keys()])
style_loss *= style_weight / num_style_layers

for name in content_outputs.keys()])
content_loss *= content_weight / num_content_layers
loss = style_loss + content_loss
return loss

Use tf.GradientTape to update the image.

@tf.function()
def train_step(image):
outputs = extractor(image)
loss = style_content_loss(outputs)

image.assign(clip_0_1(image))

Now run a few steps to test:

train_step(image)
train_step(image)
train_step(image)
tensor_to_image(image)

Since it's working, perform a longer optimization:

import time
start = time.time()

epochs = 10
steps_per_epoch = 100

step = 0
for n in range(epochs):
for m in range(steps_per_epoch):
step += 1
train_step(image)
print(".", end='')
display.clear_output(wait=True)
display.display(tensor_to_image(image))
print("Train step: {}".format(step))

end = time.time()
print("Total time: {:.1f}".format(end-start))

Train step: 1000
Total time: 22.3

## Total variation loss

One downside to this basic implementation is that it produces a lot of high frequency artifacts. Decrease these using an explicit regularization term on the high frequency components of the image. In style transfer, this is often called the total variation loss:

def high_pass_x_y(image):
x_var = image[:,:,1:,:] - image[:,:,:-1,:]
y_var = image[:,1:,:,:] - image[:,:-1,:,:]

return x_var, y_var
x_deltas, y_deltas = high_pass_x_y(content_image)

plt.figure(figsize=(14,10))
plt.subplot(2,2,1)
imshow(clip_0_1(2*y_deltas+0.5), "Horizontal Deltas: Original")

plt.subplot(2,2,2)
imshow(clip_0_1(2*x_deltas+0.5), "Vertical Deltas: Original")

x_deltas, y_deltas = high_pass_x_y(image)

plt.subplot(2,2,3)
imshow(clip_0_1(2*y_deltas+0.5), "Horizontal Deltas: Styled")

plt.subplot(2,2,4)
imshow(clip_0_1(2*x_deltas+0.5), "Vertical Deltas: Styled")

This shows how the high frequency components have increased.

Also, this high frequency component is basically an edge-detector. You can get similar output from the Sobel edge detector, for example:

plt.figure(figsize=(14,10))

sobel = tf.image.sobel_edges(content_image)
plt.subplot(1,2,1)
imshow(clip_0_1(sobel[...,0]/4+0.5), "Horizontal Sobel-edges")
plt.subplot(1,2,2)
imshow(clip_0_1(sobel[...,1]/4+0.5), "Vertical Sobel-edges")

The regularization loss associated with this is the sum of the squares of the values:

def total_variation_loss(image):
x_deltas, y_deltas = high_pass_x_y(image)
return tf.reduce_sum(tf.abs(x_deltas)) + tf.reduce_sum(tf.abs(y_deltas))
total_variation_loss(image).numpy()
149295.17

That demonstrated what it does. But there's no need to implement it yourself, TensorFlow includes a standard implementation:

tf.image.total_variation(image).numpy()
array([149295.17], dtype=float32)

## Re-run the optimization

Choose a weight for the total_variation_loss:

total_variation_weight=30

Now include it in the train_step function:

@tf.function()
def train_step(image):
outputs = extractor(image)
loss = style_content_loss(outputs)
loss += total_variation_weight*tf.image.total_variation(image)

image.assign(clip_0_1(image))

Reinitialize the optimization variable:

image = tf.Variable(content_image)

And run the optimization:

import time
start = time.time()

epochs = 10
steps_per_epoch = 100

step = 0
for n in range(epochs):
for m in range(steps_per_epoch):
step += 1
train_step(image)
print(".", end='')
display.clear_output(wait=True)
display.display(tensor_to_image(image))
print("Train step: {}".format(step))

end = time.time()
print("Total time: {:.1f}".format(end-start))

Train step: 1000
Total time: 23.2

Finally, save the result:

file_name = 'stylized-image.png'
tensor_to_image(image).save(file_name)

try:
except ImportError:
pass
else: