neural_style_tutorial.py

"""
Neural Transfer Using PyTorch
=============================


**Author**: `Alexis Jacq <https://alexis-jacq.github.io>`_
 
**Edited by**: `Winston Herring <https://github.com/winston6>`_

Introduction
------------

This tutorial explains how to implement the `Neural-Style algorithm <https://arxiv.org/abs/1508.06576>`__
developed by Leon A. Gatys, Alexander S. Ecker and Matthias Bethge.
Neural-Style, or Neural-Transfer, allows you to take an image and
reproduce it with a new artistic style. The algorithm takes three images,
an input image, a content-image, and a style-image, and changes the input 
to resemble the content of the content-image and the artistic style of the style-image.

 
.. figure:: /_static/img/neural-style/neuralstyle.png
   :alt: content1
"""

######################################################################
# Underlying Principle
# --------------------
# 
# The principle is simple: we define two distances, one for the content
# (:math:`D_C`) and one for the style (:math:`D_S`). :math:`D_C` measures how different the content
# is between two images while :math:`D_S` measures how different the style is
# between two images. Then, we take a third image, the input, and
# transform it to minimize both its content-distance with the
# content-image and its style-distance with the style-image. Now we can
# import the necessary packages and begin the neural transfer.
# 
# Importing Packages and Selecting a Device
# -----------------------------------------
# Below is a  list of the packages needed to implement the neural transfer.
#
# -  ``torch``, ``torch.nn``, ``numpy`` (indispensables packages for
#    neural networks with PyTorch)
# -  ``torch.optim`` (efficient gradient descents)
# -  ``PIL``, ``PIL.Image``, ``matplotlib.pyplot`` (load and display
#    images)
# -  ``torchvision.transforms`` (transform PIL images into tensors)
# -  ``torchvision.models`` (train or load pretrained models)
# -  ``copy`` (to deep copy the models; system package)

from __future__ import print_function

import torch
import torch.nn as nn
import torch.nn.functional as F
import torch.optim as optim

from PIL import Image
import matplotlib.pyplot as plt

import torchvision.transforms as transforms
import torchvision.models as models

import copy


######################################################################
# Next, we need to choose which device to run the network on and import the
# content and style images. Running the neural transfer algorithm on large
# images takes longer and will go much faster when running on a GPU. We can
# use ``torch.cuda.is_available()`` to detect if there is a GPU available.
# Next, we set the ``torch.device`` for use throughout the tutorial. Also the ``.to(device)``
# method is used to move tensors or modules to a desired device. 

device = torch.device("cuda" if torch.cuda.is_available() else "cpu")

######################################################################
# Loading the Images
# ------------------
#
# Now we will import the style and content images. The original PIL images have values between 0 and 255, but when
# transformed into torch tensors, their values are converted to be between
# 0 and 1. The images also need to be resized to have the same dimensions.
# An important detail to note is that neural networks from the
# torch library are trained with tensor values ranging from 0 to 1. If you
# try to feed the networks with 0 to 255 tensor images, then the activated
# feature maps will be unable to sense the intended content and style.
# However, pretrained networks from the Caffe library are trained with 0
# to 255 tensor images. 
#
#
# .. Note::
#     Here are links to download the images required to run the tutorial:
#     `picasso.jpg <https://pytorch.org/tutorials/_static/img/neural-style/picasso.jpg>`__ and
#     `dancing.jpg <https://pytorch.org/tutorials/_static/img/neural-style/dancing.jpg>`__.
#     Download these two images and add them to a directory
#     with name ``images`` in your current working directory.

# desired size of the output image
imsize = 512 if torch.cuda.is_available() else 128  # use small size if no GPU

loader = transforms.Compose([
    transforms.Resize(imsize),  # scale imported image
    transforms.ToTensor()])  # transform it into a torch tensor


def image_loader(image_name):
    image = Image.open(image_name)
    # fake batch dimension required to fit network's input dimensions
    image = loader(image).unsqueeze(0)
    return image.to(device, torch.float)


style_img = image_loader("./data/images/neural-style/picasso.jpg")
content_img = image_loader("./data/images/neural-style/dancing.jpg")

assert style_img.size() == content_img.size(), \
    "we need to import style and content images of the same size"


######################################################################
# Now, let's create a function that displays an image by reconverting a 
# copy of it to PIL format and displaying the copy using 
# ``plt.imshow``. We will try displaying the content and style images 
# to ensure they were imported correctly.

unloader = transforms.ToPILImage()  # reconvert into PIL image

plt.ion()

def imshow(tensor, title=None):
    image = tensor.cpu().clone()  # we clone the tensor to not do changes on it
    image = image.squeeze(0)      # remove the fake batch dimension
    image = unloader(image)
    plt.imshow(image)
    if title is not None:
        plt.title(title)
    plt.pause(0.001) # pause a bit so that plots are updated


plt.figure()
imshow(style_img, title='Style Image')

plt.figure()
imshow(content_img, title='Content Image')

######################################################################
# Loss Functions
# --------------
# Content Loss
# ~~~~~~~~~~~~
# 
# The content loss is a function that represents a weighted version of the
# content distance for an individual layer. The function takes the feature
# maps :math:`F_{XL}` of a layer :math:`L` in a network processing input :math:`X` and returns the
# weighted content distance :math:`w_{CL}.D_C^L(X,C)` between the image :math:`X` and the
# content image :math:`C`. The feature maps of the content image(:math:`F_{CL}`) must be
# known by the function in order to calculate the content distance. We
# implement this function as a torch module with a constructor that takes
# :math:`F_{CL}` as an input. The distance :math:`\|F_{XL} - F_{CL}\|^2` is the mean square error
# between the two sets of feature maps, and can be computed using ``nn.MSELoss``.
# 
# We will add this content loss module directly after the convolution
# layer(s) that are being used to compute the content distance. This way
# each time the network is fed an input image the content losses will be
# computed at the desired layers and because of auto grad, all the
# gradients will be computed. Now, in order to make the content loss layer
# transparent we must define a ``forward`` method that computes the content
# loss and then returns the layer’s input. The computed loss is saved as a
# parameter of the module.
# 

class ContentLoss(nn.Module):

    def __init__(self, target,):
        super(ContentLoss, self).__init__()
        # we 'detach' the target content from the tree used
        # to dynamically compute the gradient: this is a stated value,
        # not a variable. Otherwise the forward method of the criterion
        # will throw an error.
        self.target = target.detach()

    def forward(self, input):
        self.loss = F.mse_loss(input, self.target)
        return input

######################################################################
# .. Note::
#    **Important detail**: although this module is named ``ContentLoss``, it
#    is not a true PyTorch Loss function. If you want to define your content
#    loss as a PyTorch Loss function, you have to create a PyTorch autograd function 
#    to recompute/implement the gradient manually in the ``backward``
#    method.

######################################################################
# Style Loss
# ~~~~~~~~~~
# 
# The style loss module is implemented similarly to the content loss
# module. It will act as a transparent layer in a
# network that computes the style loss of that layer. In order to
# calculate the style loss, we need to compute the gram matrix :math:`G_{XL}`. A gram
# matrix is the result of multiplying a given matrix by its transposed
# matrix. In this application the given matrix is a reshaped version of
# the feature maps :math:`F_{XL}` of a layer :math:`L`. :math:`F_{XL}` is reshaped to form :math:`\hat{F}_{XL}`, a :math:`K`\ x\ :math:`N`
# matrix, where :math:`K` is the number of feature maps at layer :math:`L` and :math:`N` is the
# length of any vectorized feature map :math:`F_{XL}^k`. For example, the first line
# of :math:`\hat{F}_{XL}` corresponds to the first vectorized feature map :math:`F_{XL}^1`.
# 
# Finally, the gram matrix must be normalized by dividing each element by
# the total number of elements in the matrix. This normalization is to
# counteract the fact that :math:`\hat{F}_{XL}` matrices with a large :math:`N` dimension yield
# larger values in the Gram matrix. These larger values will cause the
# first layers (before pooling layers) to have a larger impact during the
# gradient descent. Style features tend to be in the deeper layers of the
# network so this normalization step is crucial.
# 

def gram_matrix(input):
    a, b, c, d = input.size()  # a=batch size(=1)
    # b=number of feature maps
    # (c,d)=dimensions of a f. map (N=c*d)

    features = input.view(a * b, c * d)  # resize F_XL into \hat F_XL

    G = torch.mm(features, features.t())  # compute the gram product

    # we 'normalize' the values of the gram matrix
    # by dividing by the number of element in each feature maps.
    return G.div(a * b * c * d)


######################################################################
# Now the style loss module looks almost exactly like the content loss
# module. The style distance is also computed using the mean square
# error between :math:`G_{XL}` and :math:`G_{SL}`.
# 

class StyleLoss(nn.Module):

    def __init__(self, target_feature):
        super(StyleLoss, self).__init__()
        self.target = gram_matrix(target_feature).detach()

    def forward(self, input):
        G = gram_matrix(input)
        self.loss = F.mse_loss(G, self.target)
        return input


######################################################################
# Importing the Model
# -------------------
# 
# Now we need to import a pretrained neural network. We will use a 19
# layer VGG network like the one used in the paper.
# 
# PyTorch’s implementation of VGG is a module divided into two child
# ``Sequential`` modules: ``features`` (containing convolution and pooling layers),
# and ``classifier`` (containing fully connected layers). We will use the
# ``features`` module because we need the output of the individual
# convolution layers to measure content and style loss. Some layers have
# different behavior during training than evaluation, so we must set the
# network to evaluation mode using ``.eval()``.
# 

cnn = models.vgg19(pretrained=True).features.to(device).eval()



######################################################################
# Additionally, VGG networks are trained on images with each channel
# normalized by mean=[0.485, 0.456, 0.406] and std=[0.229, 0.224, 0.225].
# We will use them to normalize the image before sending it into the network.
# 

cnn_normalization_mean = torch.tensor([0.485, 0.456, 0.406]).to(device)
cnn_normalization_std = torch.tensor([0.229, 0.224, 0.225]).to(device)

# create a module to normalize input image so we can easily put it in a
# ``nn.Sequential``
class Normalization(nn.Module):
    def __init__(self, mean, std):
        super(Normalization, self).__init__()
        # .view the mean and std to make them [C x 1 x 1] so that they can
        # directly work with image Tensor of shape [B x C x H x W].
        # B is batch size. C is number of channels. H is height and W is width.
        self.mean = torch.tensor(mean).view(-1, 1, 1)
        self.std = torch.tensor(std).view(-1, 1, 1)

    def forward(self, img):
        # normalize ``img``
        return (img - self.mean) / self.std


######################################################################
# A ``Sequential`` module contains an ordered list of child modules. For
# instance, ``vgg19.features`` contains a sequence (``Conv2d``, ``ReLU``, ``MaxPool2d``,
# ``Conv2d``, ``ReLU``…) aligned in the right order of depth. We need to add our
# content loss and style loss layers immediately after the convolution
# layer they are detecting. To do this we must create a new ``Sequential``
# module that has content loss and style loss modules correctly inserted.
# 

# desired depth layers to compute style/content losses :
content_layers_default = ['conv_4']
style_layers_default = ['conv_1', 'conv_2', 'conv_3', 'conv_4', 'conv_5']

def get_style_model_and_losses(cnn, normalization_mean, normalization_std,
                               style_img, content_img,
                               content_layers=content_layers_default,
                               style_layers=style_layers_default):
    # normalization module
    normalization = Normalization(normalization_mean, normalization_std).to(device)

    # just in order to have an iterable access to or list of content/style
    # losses
    content_losses = []
    style_losses = []

    # assuming that ``cnn`` is a ``nn.Sequential``, so we make a new ``nn.Sequential``
    # to put in modules that are supposed to be activated sequentially
    model = nn.Sequential(normalization)

    i = 0  # increment every time we see a conv
    for layer in cnn.children():
        if isinstance(layer, nn.Conv2d):
            i += 1
            name = 'conv_{}'.format(i)
        elif isinstance(layer, nn.ReLU):
            name = 'relu_{}'.format(i)
            # The in-place version doesn't play very nicely with the ``ContentLoss``
            # and ``StyleLoss`` we insert below. So we replace with out-of-place
            # ones here.
            layer = nn.ReLU(inplace=False)
        elif isinstance(layer, nn.MaxPool2d):
            name = 'pool_{}'.format(i)
        elif isinstance(layer, nn.BatchNorm2d):
            name = 'bn_{}'.format(i)
        else:
            raise RuntimeError('Unrecognized layer: {}'.format(layer.__class__.__name__))

        model.add_module(name, layer)

        if name in content_layers:
            # add content loss:
            target = model(content_img).detach()
            content_loss = ContentLoss(target)
            model.add_module("content_loss_{}".format(i), content_loss)
            content_losses.append(content_loss)

        if name in style_layers:
            # add style loss:
            target_feature = model(style_img).detach()
            style_loss = StyleLoss(target_feature)
            model.add_module("style_loss_{}".format(i), style_loss)
            style_losses.append(style_loss)

    # now we trim off the layers after the last content and style losses
    for i in range(len(model) - 1, -1, -1):
        if isinstance(model[i], ContentLoss) or isinstance(model[i], StyleLoss):
            break

    model = model[:(i + 1)]

    return model, style_losses, content_losses


######################################################################
# Next, we select the input image. You can use a copy of the content image
# or white noise.
# 

input_img = content_img.clone()
# if you want to use white noise by using the following code:
#
# ::
#
#    input_img = torch.randn(content_img.data.size(), device=device)

# add the original input image to the figure:
plt.figure()
imshow(input_img, title='Input Image')


######################################################################
# Gradient Descent
# ----------------
# 
# As Leon Gatys, the author of the algorithm, suggested `here <https://discuss.pytorch.org/t/pytorch-tutorial-for-neural-transfert-of-artistic-style/336/20?u=alexis-jacq>`__, we will use
# L-BFGS algorithm to run our gradient descent. Unlike training a network,
# we want to train the input image in order to minimize the content/style
# losses. We will create a PyTorch L-BFGS optimizer ``optim.LBFGS`` and pass
# our image to it as the tensor to optimize.
# 

def get_input_optimizer(input_img):
    # this line to show that input is a parameter that requires a gradient
    optimizer = optim.LBFGS([input_img])
    return optimizer


######################################################################
# Finally, we must define a function that performs the neural transfer. For
# each iteration of the networks, it is fed an updated input and computes
# new losses. We will run the ``backward`` methods of each loss module to
# dynamically compute their gradients. The optimizer requires a “closure”
# function, which reevaluates the module and returns the loss.
# 
# We still have one final constraint to address. The network may try to
# optimize the input with values that exceed the 0 to 1 tensor range for
# the image. We can address this by correcting the input values to be
# between 0 to 1 each time the network is run.
# 

def run_style_transfer(cnn, normalization_mean, normalization_std,
                       content_img, style_img, input_img, num_steps=300,
                       style_weight=1000000, content_weight=1):
    """Run the style transfer."""
    print('Building the style transfer model..')
    model, style_losses, content_losses = get_style_model_and_losses(cnn,
        normalization_mean, normalization_std, style_img, content_img)

    # We want to optimize the input and not the model parameters so we
    # update all the requires_grad fields accordingly
    input_img.requires_grad_(True)
    # We also put the model in evaluation mode, so that specific layers 
    # such as dropout or batch normalization layers behave correctly. 
    model.eval()
    model.requires_grad_(False)

    optimizer = get_input_optimizer(input_img)

    print('Optimizing..')
    run = [0]
    while run[0] <= num_steps:

        def closure():
            # correct the values of updated input image
            with torch.no_grad():
                input_img.clamp_(0, 1)

            optimizer.zero_grad()
            model(input_img)
            style_score = 0
            content_score = 0

            for sl in style_losses:
                style_score += sl.loss
            for cl in content_losses:
                content_score += cl.loss

            style_score *= style_weight
            content_score *= content_weight

            loss = style_score + content_score
            loss.backward()

            run[0] += 1
            if run[0] % 50 == 0:
                print("run {}:".format(run))
                print('Style Loss : {:4f} Content Loss: {:4f}'.format(
                    style_score.item(), content_score.item()))
                print()

            return style_score + content_score

        optimizer.step(closure)

    # a last correction...
    with torch.no_grad():
        input_img.clamp_(0, 1)

    return input_img


######################################################################
# Finally, we can run the algorithm.
# 

output = run_style_transfer(cnn, cnn_normalization_mean, cnn_normalization_std,
                            content_img, style_img, input_img)

plt.figure()
imshow(output, title='Output Image')

# sphinx_gallery_thumbnail_number = 4
plt.ioff()
plt.show()