In-Game Style Transfer Tutorial Leveraging Unity* (Part 2)

Published: 05/18/2021

By Christian Mills, with Introduction by Peter Cross

Introduction

As part of this ongoing series focused on style transfer technology, we feel privileged that Graphics Innovator, Christian Mills, allowed us to repurpose much of his training in the Machine Learning and Style Transfer world, and share it with the game developer community.

Overview

In Part 1 of the tutorial, we installed Unity* and selected an image for style transfer, and optionally used Unity* Recorder to record in-game footage. In this part, we will use the free tier of Google Colab to train a style transfer model. Google Colab provides a virtual environment that allows anyone to write and execute arbitrary Python* code in their browser. It removes the need to set up a Python* environment on your local machine. It also provides free access to cloud GPUs for model training.

Important: Google Colab restricts GPU allocation for free users to 12 hours at a time. You will get disconnected from the server if you leave a notebook running past that. You need to wait for a while (probably 12 hours) for the time limit to reset.

Open Google Colab Notebook

First, get your copy of the Colab Notebook. You can open my copy of the notebook by clicking the link below.

Copy to Google Drive.

Save the notebook to your Google Drive since you can’t make changes to my copy. Click the Copy to Drive button.

It will reopen the notebook in a new tab where any changes made can be saved to your Google Drive. Close the original tab. The notebook will Autosave progress, or you can manually save by pressing CTRL-S.

Colab Notebooks Folder

Open your Google Drive, you will see a new folder named Colab Notebooks. This is where any notebooks you worked on in Google Colab will be saved.

Inside Colab Notebooks Folder

Open the new folder to see your copy of the notebook. Double-click on the notebook file, a pop up window with the option to open it in a Google Colab environment will appear. You can use this method to re-open the notebook.

Using a Colab Notebook

Colab Notebooks are primarily made up of code cells and text cells. Code cells can be executed in multiple ways. If you hover over or click on a code cell, a play option will appear on the left side of the cell. Click the play button to execute the code cell.

The other ways are to either press CTRL-Enter or Shift-Enter. CTRL-Enter executes the code cell in place while Shift-Enter executes the code cell and moves to the next cell.

To add more cells hover over either at the top or bottom of an existing cell. An option to create either a code or text cell will appear.

Connect to a Runtime Environment

Connect to a runtime environment before using the notebook. Click the Connect tab highlighted in the below screenshot.

Once the notebook is connected to a runtime environment, Go to the RAM/Disk readout and make sure the notebook is using a GPU backend.

If GPU backend is not displayed, below are the steps to set it manually.

  • Click on Edit tab and Select Notebook Settings

  • Select GPU from the Hardware Accelerator dropdown and click Save.

Continue in the Notebook

It is recommended to continue this post in the Colab notebook.
However, the notebook contents are demonstrated below if you are only reading through the tutorial.

Install the fastai Library

To install the fastai library which is built on top of PyTorch* use pure PyTorch* for training the model. The fastai library includes some convenience functions which can be used to download the training dataset.

%%capture
!pip install fastai==2.2.5

Import Dependencies

Import the required Python* modules and packages.

# Miscellaneous operating system interfaces
# https://docs.python.org/3/library/os.html
import os
# Time access and conversions
# https://docs.python.org/3/library/time.html
import time
# Object-oriented filesystem paths
# https://docs.python.org/3/library/pathlib.html#pathlib.Path
from pathlib import Path
# Tuple-like objects that have named fields
# https://docs.python.org/3/library/collections.html#collections.namedtuple
from collections import namedtuple
# A convenience function for downloading files from a url to a destination folder
# https://docs.fast.ai/data.external.html#untar_data
from fastai.data.external import untar_data
# Provides image processing capabilities
# https://pillow.readthedocs.io/en/stable/reference/Image.html
from PIL import Image
# The main PyTorch package
# https://pytorch.org/docs/stable/torch.html
import torch
# Used to iterate over the dataset during training
# https://pytorch.org/docs/stable/data.html#torch.utils.data.DataLoader
from torch.utils.data import DataLoader
# Contains definitions of models. We'll be downloading a pretrained VGG-19 model
# to judge the performance of our style transfer model.
# https://pytorch.org/vision/stable/models.html#torchvision.models.vgg19
from torchvision.models import vgg19
# Common image transforms that we'll use to process images before feeding them to the models
# https://pytorch.org/vision/stable/transforms.html
from torchvision import transforms
# Loads images from a directory and applies the specified transforms
# https://pytorch.org/vision/stable/datasets.html#imagefolder
from torchvision.datasets import ImageFolder

Utility Functions

Define utility functions for making new directories, loading and saving images, and stylizing images using model checkpoints.

def make_dir(dir_name: str):
"""Create the specified directory if it doesn't already exist"""
dir_path = Path(dir_name)
try:
dir_path.mkdir()
except:
print("Directory already exists.")
def load_image(filename: str, size: int=None, scale: float=None):
"""Load the specified image and return it as a PIL Image"""
img = Image.open(filename)
if size is not None:
img = img.resize((size, size), Image.ANTIALIAS)
elif scale is not None:
img = img.resize((int(img.size[0] / scale), int(img.size[1] / scale)), Image.ANTIALIAS)
return img
def save_image(filename: str, data: torch.Tensor):
"""Save the Tensor data to an image file"""
img = data.clone().clamp(0, 255).numpy()
img = img.transpose(1, 2, 0).astype("uint8")
img = Image.fromarray(img)
img.save(filename)
def load_checkpoint(model_path):
state_dict = torch.load(model_path)
keys = [k for k in state_dict.keys()]
filters = set()
filters_list = [state_dict[k].shape[0] for k in keys if not (state_dict[k].shape[0] in filters or filters.add(state_dict[k].shape[0]))]
res_blocks = len(set(k.split('.')[1] for k in state_dict.keys() if 'resnets' in k))
model = TransformerNet(filters=filters_list[:-1], res_blocks=res_blocks)
model.load_state_dict(state_dict, strict=False)
return model
def stylize(model_path: str, input_image: str, output_image: str, content_scale: float=None,
device: str="cpu", export_onnx: bool=None):
"""Load a TransformerNet checkpoint, stylize an image and save the output"""
device = torch.device(device)
content_image = load_image(input_image, scale=content_scale)
content_transform = transforms.Compose([
transforms.ToTensor(),
transforms.Lambda(lambda x: x.mul(255))
])
content_image = content_transform(content_image)
content_image = content_image.unsqueeze(0).to(device)
with torch.no_grad():
style_model = load_checkpoint(model_path)
style_model.to(device)
if export_onnx:
assert export_onnx.endswith(".onnx"), "Export model file should end with .onnx"
output = torch.onnx._export(style_model, content_image, export_onnx, opset_version=9).cpu()
else:
output = style_model(content_image).cpu()
save_image(output_image, output[0])

Define the Style Transfer Model

Next, define the style transfer model itself. The model takes in an RGB image and generates a new image with the same dimensions. The features in the output image (e.g. color and texture) are then compared with the features of the style image and content image. The results of these comparisons are then used to update the parameters of the model so that it generates better images.

class TransformerNet(torch.nn.Module):
"""TransformerNet
https://github.com/pytorch/examples/blob/36441a83b6595524a538e342594ee6482754f374/fast_neural_style/neural_style/transformer_net.py#L4
"""
def __init__(self, filters=(32, 64, 128), res_blocks=5):
super(TransformerNet, self).__init__()
self.filters = filters
self.res_blocks = res_blocks if res_blocks > 0 else 1
# Initial convolution layers
self.conv1 = ConvLayer(3, filters[0], kernel_size=9, stride=1)
self.in1 = torch.nn.InstanceNorm2d(filters[0], affine=True)
self.conv2 = ConvLayer(filters[0], filters[1], kernel_size=3, stride=2)
self.in2 = torch.nn.InstanceNorm2d(filters[1], affine=True)
self.conv3 = ConvLayer(filters[1], filters[2], kernel_size=3, stride=2)
self.in3 = torch.nn.InstanceNorm2d(filters[2], affine=True)
# Residual layers
self.resnets = torch.nn.ModuleList()
for i in range(self.res_blocks):
self.resnets.append(ResidualBlock(filters[2]))
# Upsampling Layers
self.deconv1 = UpsampleConvLayer(filters[2], filters[1], kernel_size=3, stride=1, upsample=2)
self.in4 = torch.nn.InstanceNorm2d(filters[1], affine=True)
self.deconv2 = UpsampleConvLayer(filters[1], filters[0], kernel_size=3, stride=1, upsample=2)
self.in5 = torch.nn.InstanceNorm2d(filters[0], affine=True)
self.deconv3 = ConvLayer(filters[0], 3, kernel_size=9, stride=1)
# Non-linearities
self.relu = torch.nn.ReLU()
def forward(self, X):
conv1_y = self.relu(self.in1(self.conv1(X)))
conv2_y = self.relu(self.in2(self.conv2(conv1_y)))
conv3_y = self.relu(self.in3(self.conv3(conv2_y)))
y = self.resnets[0](conv3_y) + conv3_y
for i in range(1, self.res_blocks):
y = self.resnets[i](y) + y
y = self.relu(self.in4(self.deconv1(conv3_y + y)))
y = self.relu(self.in5(self.deconv2(conv2_y + y)))
y = self.deconv3(conv1_y + y)
return y
class ConvLayer(torch.nn.Module):
"""ConvLayer
https://github.com/pytorch/examples/blob/36441a83b6595524a538e342594ee6482754f374/fast_neural_style/neural_style/transformer_net.py#L44
"""
def __init__(self, in_channels, out_channels, kernel_size, stride):
super(ConvLayer, self).__init__()
reflection_padding = kernel_size // 2
self.reflection_pad = torch.nn.ReflectionPad2d(reflection_padding)
self.conv2d = torch.nn.Conv2d(in_channels, out_channels, kernel_size, stride)
def forward(self, x):
out = self.reflection_pad(x)
ut = self.conv2d(out)
return out
class ResidualBlock(torch.nn.Module):
"""ResidualBlock
introduced in: https://arxiv.org/abs/1512.03385
recommended architecture: http://torch.ch/blog/2016/02/04/resnets.html
https://github.com/pytorch/examples/blob/36441a83b6595524a538e342594ee6482754f374/fast_neural_style/neural_style/transformer_net.py#L57
"""
def __init__(self, channels):
super(ResidualBlock, self).__init__()
self.conv1 = ConvLayer(channels, channels, kernel_size=3, stride=1)
self.in1 = torch.nn.InstanceNorm2d(channels, affine=True)
self.conv2 = ConvLayer(channels, channels, kernel_size=3, stride=1)
self.in2 = torch.nn.InstanceNorm2d(channels, affine=True)
self.relu = torch.nn.ReLU()
def forward(self, x):
residual = x
out = self.relu(self.in1(self.conv1(x)))
out = self.in2(self.conv2(out))
out = out + residual
return out
class UpsampleConvLayer(torch.nn.Module):
"""UpsampleConvLayer
Upsamples the input and then does a convolution. This method gives better results
compared to ConvTranspose2d.
ref: http://distill.pub/2016/deconv-checkerboard/
https://github.com/pytorch/examples/blob/36441a83b6595524a538e342594ee6482754f374/fast_neural_style/neural_style/transformer_net.py#L79
"""
def __init__(self, in_channels, out_channels, kernel_size, stride, upsample=None):
super(UpsampleConvLayer, self).__init__()
self.upsample = upsample
reflection_padding = kernel_size // 2
self.reflection_pad = torch.nn.ReflectionPad2d(reflection_padding)
self.conv2d = torch.nn.Conv2d(in_channels, out_channels, kernel_size, stride)
def forward(self, x):
x_in = x
if self.upsample:
x_in = torch.nn.functional.interpolate(x_in, mode='nearest', scale_factor=self.upsample)
out = self.reflection_pad(x_in)
out = self.conv2d(out)
return out

Define the VGG-19 Model

Next, define the VGG -19 model which is used to judge the quality of the output images from the style transfer model. This model has been pretrained on a large image dataset. This means it has already learned to recognize a wide variety of features in images. Use this model to extract the features of the content image, style image, and stylized images.

class Vgg19(torch.nn.Module):
"""
https://github.com/pytorch/examples/blob/36441a83b6595524a538e342594ee6482754f374/fast_neural_style/neural_style/vgg.py#L7
"""
def __init__(self, requires_grad=False):
super(Vgg19, self).__init__()
self.feature_layers = [0, 3, 5]
self.vgg_pretrained_features = vgg19(pretrained=True).features
self.slice1 = torch.nn.Sequential()
self.slice2 = torch.nn.Sequential()
self.slice3 = torch.nn.Sequential()
self.slice4 = torch.nn.Sequential()
self.slice5 = torch.nn.Sequential()
for x in range(4):
self.slice1.add_module(str(x), self.vgg_pretrained_features[x])
for x in range(4, 9):
self.slice2.add_module(str(x), self.vgg_pretrained_features[x])
for x in range(9, 18):
self.slice3.add_module(str(x), self.vgg_pretrained_features[x])
for x in range(18, 27):
self.slice4.add_module(str(x), self.vgg_pretrained_features[x])
for x in range(27, 36):
self.slice5.add_module(str(x), self.vgg_pretrained_features[x])
if not requires_grad:
for param in self.parameters():
param.requires_grad = False
def forward(self, X):
h = self.slice1(X)
h_relu1_2 = h
h = self.slice2(h)
h_relu2_2 = h
h = self.slice3(h)
h_relu3_3 = h
h = self.slice4(h)
h_relu4_3 = h
h = self.slice5(h)
h_relu5_3 = h
vgg_outputs = namedtuple("VggOutputs", ['relu1_2', 'relu2_2', 'relu3_3', 'relu4_3', 'relu5_3'])
out = vgg_outputs(h_relu1_2, h_relu2_2, h_relu3_3, h_relu4_3, h_relu5_3)
return out

Define the Model Trainer

Define a new class to make the training style transfer model a bit easier. Along with training the model, this class saves the model’s current progress at set intervals. It also generates sample images to see how the model is performing. This allows to determine if the model is actually improving or whether it is already good enough to stop the training process early.

class Trainer(object):
def __init__(self, train_loader, style_transform, generator, opt_generator, style_criterion, perception_model, device):
self.train_loader = train_loader
self.style_transform = style_transform
self.generator = generator
self.opt_generator = opt_generator
self.style_criterion = style_criterion
self.perception_model = perception_model
self.device = device
self.generator.to(self.device)
def gram_matrix(self, y: torch.Tensor):
"""Compute the gram matrix a PyTorch Tensor"""
(b, ch, h, w) = y.size()
features = y.view(b, ch, w * h)
features_t = features.transpose(1, 2)
gram = features.bmm(features_t) / (ch * h * w)
return gram
def normalize_batch(self, batch: torch.Tensor):
"""Normalize a batch of Tensors using the imagenet mean and std """
mean = batch.new_tensor([0.485, 0.456, 0.406]).view(-1, 1, 1)
std = batch.new_tensor([0.229, 0.224, 0.225]).view(-1, 1, 1)
batch = batch.div_(255.0)
return (batch - mean) / std
def get_gram_style(self, style_image: str, style_size: int):
"""Get the Gram Matrices for the style image"""
style = load_image(style_image, size=style_size)
style = self.style_transform(style)
style = style.repeat(self.train_loader.batch_size, 1, 1, 1).to(self.device)
features_style = self.perception_model(self.normalize_batch(style))
gram_style = [self.gram_matrix(y) for y in features_style]
return gram_style
def save_checkpoint(self, path: str):
"""Save the current model weights at the specified path"""
self.generator.eval().cpu()
torch.save(self.generator.state_dict(), path)
print(f"Checkpoint saved at {path}")
def train(self, style_image, test_image, checkpoint_model_dir, epochs=5, content_weight=1e5, style_weight=1e10,
content_scale=None, style_size=None, log_interval=500, checkpoint_interval=500):
"""Train the style transfer model on the provided style image."""
gram_style = self.get_gram_style(style_image, style_size)
for e in range(epochs):
self.generator.train()
agg_content_loss = 0.
agg_style_loss = 0.
count = 0
for batch_id, (x, _) in enumerate(self.train_loader):
n_batch = len(x)
count += n_batch
self.opt_generator.zero_grad()
x = x.to(self.device)
y = self.generator(x)
y = self.normalize_batch(y.clone())
x = self.normalize_batch(x.clone())
features_y = self.perception_model(y)
features_x = self.perception_model(x)
content_loss = content_weight * self.style_criterion(features_y.relu2_2, features_x.relu2_2)
style_loss = 0.
for ft_y, gm_s in zip(features_y, gram_style):
gm_y = self.gram_matrix(ft_y)
style_loss += self.style_criterion(gm_y, gm_s[:n_batch, :, :])
style_loss = style_loss * style_weight
total_loss = content_loss + style_loss
total_loss.backward()
self.opt_generator.step()
agg_content_loss += content_loss.item()
agg_style_loss += style_loss.item()
if (batch_id + 1) % log_interval == 0:
mesg = f"{' '.join(time.ctime().replace(' ', ' ').split(' ')[1:-1])} "
mesg += f"Epoch {e + 1}: [{count}/{len(self.train_loader.dataset)}] "
mesg += f"content: {(agg_content_loss / (batch_id + 1)):.4f} "
mesg += f"style: {(agg_style_loss / (batch_id + 1)):.4f} "
mesg += f"total: {((agg_content_loss + agg_style_loss) / (batch_id + 1)):.4f}"
print(mesg)
if checkpoint_model_dir is not None and (batch_id + 1) % checkpoint_interval == 0:
ckpt_base = f"ckpt_epoch_{e}_batch_id_{batch_id + 1}"
ckpt_model_filename = ckpt_base + ".pth"
ckpt_model_path = os.path.join(checkpoint_model_dir, ckpt_model_filename)
self.save_checkpoint(ckpt_model_path)
output_image = ckpt_base + ".png"
output_image_path = os.path.join(checkpoint_model_dir, output_image)
stylize(ckpt_model_path, test_image, output_image_path)
self.generator.to(self.device).train()
print("Finished Training")
ckpt_model_path = os.path.join(checkpoint_model_dir, 'final.pth')
self.save_checkpoint(ckpt_model_path)
output_image_path = os.path.join(checkpoint_model_dir, 'final.png')
stylize(ckpt_model_path, test_image, output_image_path)

Mount Google Drive

Before proceeding further, mount the Google Drive to access the project folder. There is a python library which is specifically developed for working in a Colab notebook which provides this functionality.

from google.colab import drive

Use the drive.mount() method to mount the whole Google Drive inside a new directory called drive.

Run the code cell below, after which you will be prompted to open a link to allow Google Colab to access your Drive.

Once you allow access you will be provided with an authorization code. Copy and paste the code into a text box that appears in the output of the code cell and press Enter.

drive.mount('/content/drive')

If you look in the new drive folder, you will see the main Drive folder is named MyDrive. All the folders and files in your Drive are accessible in MyDrive.
If you placed and named your project folder as shown in part 1 of this tutorial, it will be located at /content/drive/MyDrive/Style_Transfer_Project.

You will require that path to store in Python* variables in the next section.

Set the Directories

Create several variables to store the paths to various directories.

  • The dataset directory
  • The Google Drive style transfer project directory
  • The style images directory
  • The test image directory
  • The model checkpoint directory

The dataset directory will be on the Google Colab environment while the rest will be on your Google Drive. This will allow you to continue the progress while preventing the dataset from filling up your Drive storage.

It is recommended to create separate checkpoint directories for each training session. This makes it easier to compare results from experiments.

dataset_dir = "/content/dataset"
project_dir = '/content/drive/MyDrive/Style_Transfer_Project'
style_images_dir = f"{project_dir}/style_images"
test_images_dir = f"{project_dir}/test_images"
checkpoints_dir = f"{project_dir}/checkpoints"
make_dir(checkpoints_dir)

Download Training Dataset

Use the COCO train 2014 image dataset to train our model. It’s about 13.5 GB unzipped. It is just high enough to trigger the disk space warning without actually using up the available disk space. You are likely get a disk space warning while the dataset is being unzipped. Click ignore in the popup window. Delete the zip file once the folder is unzipped.

coco_url = 'http://images.cocodataset.org/zips/train2014.zip'
untar_data(coco_url, 'coco.zip', dataset_dir)
if os.path.exists('coco.zip'): os.remove('coco.zip')

Split Gameplay Video

In this section, split the gameplay video if you have made one. Store the frames in a new subdirectory called video_frames in the dataset_dir.

!mkdir ./dataset/video_frames/

Use the ffmpeg command-line tool to split the video file. Google Colab will already have the tool installed.

In the code cell below replace

/content/drive/MyDrive/Style_Transfer_Project/movie_001.mp4 with the path of your video file.

If you recorded a lot of footage, you might want to keep an eye on the available disk space and manually stop the code cell from running. This will not be a problem if you only recorded several minutes of gameplay.

!ffmpeg -i /content/drive/MyDrive/Style_Transfer_Project/movie_001.mp4 ./dataset/video_frames/%05d.png -hide_banner

Create the Trainer Variables

Define the variables required to define a new Trainer.

Define the DataLoader

Define a DataLoader which is responsible for iterating through the dataset during training.

Specify the batch_size which indicates the number of images will be fed to the model at a time.

Note: Every image in a batch needs to be the same size.

Set the size using the image_size variable.

Images are required to be processed before being fed to the model. Define the pre-processing steps using the transforms.Compose() method. Below are the pre-processing steps:

  1. Resize the images in the current batch to the target image_size
  2. Crop the images so that they are all square
  3. Convert the images to PyTorch Tensors
  4. Multiply the color channel values by 255

Store the list of images in the dataset_dir along with the pre-processing steps in a new variable called train_dataset.
Finally, DataLoader is created using the train_dataset and specified batch_size

batch_size = 4
image_size = 256
transform = transforms.Compose([transforms.Resize(image_size),
transforms.CenterCrop(image_size),
transforms.ToTensor(),
transforms.Lambda(lambda x: x.mul(255))
])
train_dataset = ImageFolder(dataset_dir, transform)
train_loader = DataLoader(train_dataset, batch_size=batch_size)

Select Compute Device

Double check the id to see if cuda GPU is available using the torch.cuda.is_available() method.

use_cuda = True
device = "cuda" if (use_cuda and torch.cuda.is_available()) else "cpu"
print(f"Using: {device}")

Define Transforms for Style Image

Next, define the transforms used to process the style image before feeding it to the VGG-19 model. The processing steps are basically the same as for the training images except the style image will have already been resized.

  1. Convert the image to a PyTorch* Tensor
  2. Multiply the pixel values by 255
style_transform = transforms.Compose([transforms.ToTensor(),
transforms.Lambda(lambda x: x.mul(255))
])

Create the Style Transfer Model

Next, create a new instance of the style transfer model. Here you can experiment with tradeoffs between performance and quality.

Tuning Model Inference Speed:

The easiest way to make the style transfer model faster is to make it smaller. You can tune the size of the model by adjusting the size of the layers or by using fewer layers.

Resolution: 960x540
Filters: (16, 32, 64)

================================================================
Total params: 424,899
Trainable params: 424,899
Non-trainable params: 0
----------------------------------------------------------------
Input size (MB): 5.93
Forward/backward pass size (MB): 2210.61
Params size (MB): 1.62
Estimated Total Size (MB): 2218.17
----------------------------------------------------------------

Resolution: 960x540
Filters: (32, 64, 128)

================================================================
Total params: 1,679,235
Trainable params: 1,679,235
Non-trainable params: 0
----------------------------------------------------------------
Input size (MB): 5.93
Forward/backward pass size (MB): 4385.35
Params size (MB): 6.41
Estimated Total Size (MB): 4397.69
----------------------------------------------------------------

By default, the style transfer model uses the following values:

  • filters: (32, 64, 128)
  • res_blocks: 5

The filters variable determines the size of the layers in the model. The resnet_blocks variable determines the number of ResidualBlocks that form the core of the model.

Setting filters to (8, 16, 32) and keeping res_blocks at 5 significantly improves performance in Unity* with minimal impact on quality.

filters = (8, 16, 32)
res_blocks = 5
generator = TransformerNet(filters=filters, res_blocks=res_blocks).to(device)

Create the Optimizer for the Style Transfer Model

Next, define the optimizer for the model. The optimizer determines how the model gets updated during training. The optimizer takes in the model parameters and a learning rate. The learning rate determines how much the model gets updated after each batch of images.
Use a learning rate of 1e-3 which is equivalent to 0.001.

Notation Examples:

  • 1e-4 = 0.0001
  • 1e0 = 1.0
  • 1e5 = 100000.0
  • 5e10 = 50000000000.0
lr = 1e-3
opt_generator = torch.optim.Adam(generator.parameters(), lr)

Define How Model Performance Will Be Measured

Use Mean Squared Error (MSE) to compare the difference between the features of the content image and stylized image and also between the features of the stylized image and the target style image.

style_criterion = torch.nn.MSELoss()

Note: If you are not familiar with MSE, take a look at the toy example below.

Mean Squared Error in Python*

x = [1, 2, 3, 4]
y = [5, 6, 7, 8]
sum_of_squares = 0
for i in range(len(x)):
error = x[i] - y[i]
squared_error = error**2
sum_of_squares += squared_error
mse = sum_of_squares / len(x)
mse

Mean Squared Error in PyTorch*

x_t = torch.Tensor(x)
y_t = torch.Tensor(y)
mse_loss = torch.nn.MSELoss()
mse_loss(x_t, y_t)

Create a New VGG-19 Perception Model

Create a new VGG-19 model. The pretrained model will be downloaded the first time this cell is run.

perception_model = Vgg19(requires_grad=False).to(device)

Create a New Trainer

You can now create a new trainer instance using the variables defined above.

trainer = Trainer(train_loader=train_loader,
style_transform=style_transform,
generator=generator,
opt_generator=opt_generator,
style_criterion=style_criterion,
perception_model=perception_model,
device=device)

Tuning the Stylized Image

The stylized image will be influenced by the following:

  • Influence of the content image
  • Influence of the style image
  • Size of the style image

It is recommend keeping the content_weight at 1e5 and adjusting the style_weight between 5e8 and 1e11.

The ideal style_weight will vary depending on the style image. Starting out low, training for 5-10 checkpoint intervals, and increasing the style weight as required.

# The file path for the target style image
style_image = f"{style_images_dir}/Xe HPG Mesh shader_right_square.jpg"
# The file path for a sample input image for demonstrating the model's progress during training
test_image = f"{test_images_dir}/011.png"
# The number of times to iterate through the entire training dataset
epochs = 1
# The influence from the input image on the stylized image
# Default: 1e5
content_weight = 1e5
# The influence from the style image on the stylized image
# Default: 1e10
style_weight = 1e10
# (test_image resolution) / content_scale
# Default: 1.0
content_scale = 0.8
# Target size for style_image = (style_size, styl_size)
# Default: 256
style_size = 720
# The number of training batches to wait before printing the progress of the model
log_interval = 500
# The number of training to wait before saving the current model weights
checkpoint_interval = 500

Train the Model

Once the below code cell is executed, open the checkpoints folder in Google Drive in another tab. You can view the model progress by looking at the sample style images that get generated with each checkpoint. You can stop the training process early by clicking the stop button where the play button normally is on the left side of the code cell.

trainer.train(style_image=style_image,
test_image=test_image,
checkpoint_model_dir=checkpoints_dir,
epochs=epochs,
content_weight=content_weight,
style_weight=style_weight,
content_scale=content_scale,
style_size=style_size,
log_interval=log_interval,
checkpoint_interval=checkpoint_interval)

Export the Model to ONNX

You can export the model to ONNX format. PyTorch* exports model by feeding a sample input into the model and tracing what operators are used to compute the outputs.
Use (1, 3, 960, 540) Tensor with random values as sample input. This is equivalent to feeding a 960x540 RGB image to the model. The resolution doesn’t matter as you can feed images with arbitrary resolutions once the model is exported.
The ONNX file will be saved to the project folder in Google Drive.

Note: You will get a warning after running the code cell below recommending that you use ONNX opset 11 or above. Unity* has prioritized support for opset 9 for Barracuda and higher opsets are not fully supported.

checkpoint_path = f"{checkpoints_dir}/final.pth"
style_model = load_checkpoint(checkpoint_path)
x = torch.randn(1, 3, 960, 540).cpu()
torch.onnx.export(style_model.cpu(), # Model being run
x, # Sample input
f"{project_dir}/final.onnx", # Path to save ONNX file
export_params=True, # Store trained weights
opset_version=9, # Which ONNX version to use
do_constant_folding=True # Replace operations that have all constant inputs with pre-computed nodes
)

Conclusion

That is everything needed to train your own style transfer models. In the next section, we will add the code to use the trained ONNX file in Unity*.

Previous Tutorial Sections:

Part 1

Part 1.5

Next Tutorial Section:

Part 3

GitHub Repository

Product and Performance Information

1

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.