图像风格迁移（Pytorch）

图像风格迁移

最后要生成的图片是怎样的是难以想象的，所以朴素的监督学习方法可能不会生效，

Content Loss

根据输入图片和输出图片的像素差别可以比较损失

(l_{content} = frac{1}{2}sum (C_c-T_c)^2)

Style Loss

从中间提取多个特征层来衡量损失。

利用(Gram) (Matrix)（格拉姆矩阵）可以衡量风格的相关性，对于一个实矩阵(X)，矩阵(XX^T)是(X)的行向量的格拉姆矩阵

(l_{style}=sum wi(Ts-Ss)^2)

总的损失函数

(L_{total(S,C,T)}=alpha l_{content}(C,T)+eta L_{style}(S,T))

---

代码

from PIL import Image
import matplotlib.pyplot as plt
import numpy as np

import torch
import torch.optim as optim
from torchvision import transforms, models

vgg = models.vgg19(pretrained=True).features	#使用预训练的VGG19，features表示只提取不包括全连接层的部分

for i in vgg.parameters():
    i.requires_grad_(False)		#不要求训练VGG的参数

定义一个显示图片的函数

def load_img(path, max_size=400,shape=None):
    img = Image.open(path).convert('RGB')
    
    if(max(img.size)) > max_size:	#规定图像的最大尺寸
        size = max_size
    else:
        size = max(img.size)
    
    if shape is not None:
        size = shape
    transform = transforms.Compose([
        transforms.Resize(size),
        transforms.ToTensor(),
        transforms.Normalize((0.485, 0.456, 0.406),
                             (0.229, 0.224, 0.225))
    ])
    '''删除alpha通道(jpg)， 转为png，补足另一个维度-batch'''
    img = transform(img)[:3,:,:].unsqueeze(0)
    return img

载入图像

content  = load_img('./images/turtle.jpg')
style = load_img('./images/wave.jpg', shape=content.shape[-2:])		#让两张图尺寸一样

'''转换为plt可以画出来的形式'''
def im_convert(tensor):
    img = tensor.clone().detach()
    img = img.numpy().squeeze()
    img = img.transpose(1,2,0)
    img = img * np.array((0.229, 0.224, 0.225)) + np.array((0.485, 0.456, 0.406))
    img = img.clip(0,1)
    return img

使用的图像为（左边为Content Image，右边为Style Image）：

定义几个待会要用到的函数

def get_features(img, model, layers=None):
    '''获取特征层'''
    if layers is None:
        layers = {
            '0':'conv1_1',
            '5':'conv2_1',
            '10':'conv3_1',
            '19':'conv4_1',
            '21':'conv4_2',    #content层
            '28':'conv5_1'
        }
    
    features = {}
    x = img
    for name, layer in model._modules.items():
        x = layer(x)
        if name in layers:
            features[layers[name]] = x
    
    return features

def gram_matrix(tensor):
    '''计算Gram matrix'''
    _, d, h, w = tensor.size()  #第一个是batch_size
    
    tensor = tensor.view(d, h*w)
    
    gram = torch.mm(tensor, tensor.t())
    
    return gram    

content_features = get_features(content, vgg)
style_features = get_features(style, vgg)

style_grams = {layer:gram_matrix(style_features[layer]) for layer in style_features}

target = content.clone().requires_grad_(True)

'''定义不同层的权重'''
style_weights = {
    'conv1_1': 1,
    'conv2_1': 0.8,
    'conv3_1': 0.5,
    'conv4_1': 0.3,
    'conv5_1': 0.1,
}
'''定义2种损失对应的权重'''
content_weight = 1
style_weight = 1e6

训练过程

show_every = 400
optimizer = optim.Adam([target], lr=0.003)
steps = 2000

for ii in range(steps):
    target_features = get_features(target, vgg)
    
    content_loss = torch.mean((target_features['conv4_2'] - content_features['conv4_2'])**2)   
    style_loss = 0
    '''加上每一层的gram_matrix矩阵的损失'''
    for layer in style_weights:
        target_feature = target_features[layer]
        target_gram = gram_matrix(target_feature)
        _, d, h, w = target_feature.shape
        style_gram = style_grams[layer]
        layer_style_loss = style_weights[layer] * torch.mean((target_gram - style_gram)**2)
        style_loss += layer_style_loss/(d*h*w)     #加到总的style_loss里，除以大小
        
    total_loss = content_weight * content_loss + style_weight * style_loss
    
    optimizer.zero_grad()
    total_loss.backward()
    optimizer.step()
    
    if ii % show_every == 0 :
        print('Total Loss:',total_loss.item())
        plt.imshow(im_convert(target))
        plt.show()

将输入的图像和最后得到的混合图作比较：

没有达到最好的效果，还有可以优化的空间√

参考：

1. Image Style Transfer Using Convolutional Neural Networks论文
2. Udacity——PyTorch Scholarship Challenge