簡介

使用TensorFlow實現快速圖像風格遷移（Fast Neural Style Transfer）

原理

在之前介紹的圖像風格遷移中，我們根據內容圖片和風格圖片優化輸入圖片，使得內容損失函數和風格損失函數盡可能小

和DeepDream一樣，屬于網絡參數不變，根據損失函數調整輸入數據，因此每生成一張圖片都相當于訓練一個模型，需要很長時間

訓練模型需要很長時間，而使用訓練好的模型進行推斷則很快

使用快速圖像風格遷移可大大縮短生成一張遷移圖片所需的時間，其模型結構如下，包括轉換網絡和損失網絡

風格圖片是固定的，而內容圖片是可變的輸入，因此以上模型用于將任意圖片快速轉換為指定風格的圖片

• 轉換網絡：參數需要訓練，將內容圖片轉換成遷移圖片
• 損失網絡：計算遷移圖片和風格圖片之間的風格損失，以及遷移圖片和原始內容圖片之間的內容損失

經過訓練后，轉換網絡所生成的遷移圖片，在內容上和輸入的內容圖片相似，在風格上和指定的風格圖片相似

進行推斷時，僅使用轉換網絡，輸入內容圖片，即可得到對應的遷移圖片

如果有多個風格圖片，對每個風格分別訓練一個模型即可

實現

基于以下兩個項目進行修改，github.com/lengstrom/f…、github.com/hzy46/fast-…

依然通過之前用過的imagenet-vgg-verydeep-19.mat計算內容損失函數和風格損失函數

需要一些圖片作為輸入的內容圖片，對圖片具體內容沒有任何要求，也不需要任何標注，這里選擇使用MSCOCO數據集的train2014部分，cocodataset.org/#download，共82612張圖片

加載庫

# -*- coding: utf-8 -*-import tensorflow as tf import numpy as np import cv2 from imageio import imread, imsave import scipy.io import os import glob from tqdm import tqdm import matplotlib.pyplot as plt %matplotlib inline 復制代碼

查看風格圖片，共10張

style_images = glob.glob('styles/*.jpg') print(style_images) 復制代碼

加載內容圖片，去掉黑白圖片，處理成指定大小，暫時不進行歸一化，像素值范圍為0至255之間

def resize_and_crop(image, image_size):h = image.shape[0]w = image.shape[1]if h > w:image = image[h // 2 - w // 2: h // 2 + w // 2, :, :]else:image = image[:, w // 2 - h // 2: w // 2 + h // 2, :] image = cv2.resize(image, (image_size, image_size))return imageX_data = [] image_size = 256 paths = glob.glob('train2014/*.jpg') for i in tqdm(range(len(paths))):path = paths[i]image = imread(path)if len(image.shape) < 3:continueX_data.append(resize_and_crop(image, image_size)) X_data = np.array(X_data) print(X_data.shape) 復制代碼

加載vgg19模型，并定義一個函數，對于給定的輸入，返回vgg19各個層的輸出值，就像在GAN中那樣，通過variable_scope重用實現網絡的重用

vgg = scipy.io.loadmat('imagenet-vgg-verydeep-19.mat') vgg_layers = vgg['layers']def vgg_endpoints(inputs, reuse=None):with tf.variable_scope('endpoints', reuse=reuse):def _weights(layer, expected_layer_name):W = vgg_layers[0][layer][0][0][2][0][0]b = vgg_layers[0][layer][0][0][2][0][1]layer_name = vgg_layers[0][layer][0][0][0][0]assert layer_name == expected_layer_namereturn W, bdef _conv2d_relu(prev_layer, layer, layer_name):W, b = _weights(layer, layer_name)W = tf.constant(W)b = tf.constant(np.reshape(b, (b.size)))return tf.nn.relu(tf.nn.conv2d(prev_layer, filter=W, strides=[1, 1, 1, 1], padding='SAME') + b)def _avgpool(prev_layer):return tf.nn.avg_pool(prev_layer, ksize=[1, 2, 2, 1], strides=[1, 2, 2, 1], padding='SAME')graph = {}graph['conv1_1'] = _conv2d_relu(inputs, 0, 'conv1_1')graph['conv1_2'] = _conv2d_relu(graph['conv1_1'], 2, 'conv1_2')graph['avgpool1'] = _avgpool(graph['conv1_2'])graph['conv2_1'] = _conv2d_relu(graph['avgpool1'], 5, 'conv2_1')graph['conv2_2'] = _conv2d_relu(graph['conv2_1'], 7, 'conv2_2')graph['avgpool2'] = _avgpool(graph['conv2_2'])graph['conv3_1'] = _conv2d_relu(graph['avgpool2'], 10, 'conv3_1')graph['conv3_2'] = _conv2d_relu(graph['conv3_1'], 12, 'conv3_2')graph['conv3_3'] = _conv2d_relu(graph['conv3_2'], 14, 'conv3_3')graph['conv3_4'] = _conv2d_relu(graph['conv3_3'], 16, 'conv3_4')graph['avgpool3'] = _avgpool(graph['conv3_4'])graph['conv4_1'] = _conv2d_relu(graph['avgpool3'], 19, 'conv4_1')graph['conv4_2'] = _conv2d_relu(graph['conv4_1'], 21, 'conv4_2')graph['conv4_3'] = _conv2d_relu(graph['conv4_2'], 23, 'conv4_3')graph['conv4_4'] = _conv2d_relu(graph['conv4_3'], 25, 'conv4_4')graph['avgpool4'] = _avgpool(graph['conv4_4'])graph['conv5_1'] = _conv2d_relu(graph['avgpool4'], 28, 'conv5_1')graph['conv5_2'] = _conv2d_relu(graph['conv5_1'], 30, 'conv5_2')graph['conv5_3'] = _conv2d_relu(graph['conv5_2'], 32, 'conv5_3')graph['conv5_4'] = _conv2d_relu(graph['conv5_3'], 34, 'conv5_4')graph['avgpool5'] = _avgpool(graph['conv5_4'])return graph 復制代碼

選擇一張風格圖，減去通道顏色均值后，得到風格圖片在vgg19各個層的輸出值，計算四個風格層對應的Gram矩陣

style_index = 1 X_style_data = resize_and_crop(imread(style_images[style_index]), image_size) X_style_data = np.expand_dims(X_style_data, 0) print(X_style_data.shape)MEAN_VALUES = np.array([123.68, 116.779, 103.939]).reshape((1, 1, 1, 3))X_style = tf.placeholder(dtype=tf.float32, shape=X_style_data.shape, name='X_style') style_endpoints = vgg_endpoints(X_style - MEAN_VALUES) STYLE_LAYERS = ['conv1_2', 'conv2_2', 'conv3_3', 'conv4_3'] style_features = {}sess = tf.Session() for layer_name in STYLE_LAYERS:features = sess.run(style_endpoints[layer_name], feed_dict={X_style: X_style_data})features = np.reshape(features, (-1, features.shape[3]))gram = np.matmul(features.T, features) / features.sizestyle_features[layer_name] = gram 復制代碼

定義轉換網絡，典型的卷積、殘差、逆卷積結構，內容圖片輸入之前也需要減去通道顏色均值

batch_size = 4 X = tf.placeholder(dtype=tf.float32, shape=[None, None, None, 3], name='X') k_initializer = tf.truncated_normal_initializer(0, 0.1)def relu(x):return tf.nn.relu(x)def conv2d(inputs, filters, kernel_size, strides):p = int(kernel_size / 2)h0 = tf.pad(inputs, [[0, 0], [p, p], [p, p], [0, 0]], mode='reflect')return tf.layers.conv2d(inputs=h0, filters=filters, kernel_size=kernel_size, strides=strides, padding='valid', kernel_initializer=k_initializer)def deconv2d(inputs, filters, kernel_size, strides):shape = tf.shape(inputs)height, width = shape[1], shape[2]h0 = tf.image.resize_images(inputs, [height * strides * 2, width * strides * 2], tf.image.ResizeMethod.NEAREST_NEIGHBOR)return conv2d(h0, filters, kernel_size, strides)def instance_norm(inputs):return tf.contrib.layers.instance_norm(inputs)def residual(inputs, filters, kernel_size):h0 = relu(conv2d(inputs, filters, kernel_size, 1))h0 = conv2d(h0, filters, kernel_size, 1)return tf.add(inputs, h0)with tf.variable_scope('transformer', reuse=None):h0 = tf.pad(X - MEAN_VALUES, [[0, 0], [10, 10], [10, 10], [0, 0]], mode='reflect')h0 = relu(instance_norm(conv2d(h0, 32, 9, 1)))h0 = relu(instance_norm(conv2d(h0, 64, 3, 2)))h0 = relu(instance_norm(conv2d(h0, 128, 3, 2)))for i in range(5):h0 = residual(h0, 128, 3)h0 = relu(instance_norm(deconv2d(h0, 64, 3, 2)))h0 = relu(instance_norm(deconv2d(h0, 32, 3, 2)))h0 = tf.nn.tanh(instance_norm(conv2d(h0, 3, 9, 1)))h0 = (h0 + 1) / 2 * 255.shape = tf.shape(h0)g = tf.slice(h0, [0, 10, 10, 0], [-1, shape[1] - 20, shape[2] - 20, -1], name='g') 復制代碼

將轉換網絡的輸出即遷移圖片，以及原始內容圖片都輸入到vgg19，得到各自對應層的輸出，計算內容損失函數

CONTENT_LAYER = 'conv3_3' content_endpoints = vgg_endpoints(X - MEAN_VALUES, True) g_endpoints = vgg_endpoints(g - MEAN_VALUES, True)def get_content_loss(endpoints_x, endpoints_y, layer_name):x = endpoints_x[layer_name]y = endpoints_y[layer_name]return 2 * tf.nn.l2_loss(x - y) / tf.to_float(tf.size(x))content_loss = get_content_loss(content_endpoints, g_endpoints, CONTENT_LAYER) 復制代碼

根據遷移圖片和風格圖片在指定風格層的輸出，計算風格損失函數

style_loss = [] for layer_name in STYLE_LAYERS:layer = g_endpoints[layer_name]shape = tf.shape(layer)bs, height, width, channel = shape[0], shape[1], shape[2], shape[3]features = tf.reshape(layer, (bs, height * width, channel))gram = tf.matmul(tf.transpose(features, (0, 2, 1)), features) / tf.to_float(height * width * channel)style_gram = style_features[layer_name]style_loss.append(2 * tf.nn.l2_loss(gram - style_gram) / tf.to_float(tf.size(layer)))style_loss = tf.reduce_sum(style_loss) 復制代碼

計算全變差正則，得到總的損失函數

def get_total_variation_loss(inputs):h = inputs[:, :-1, :, :] - inputs[:, 1:, :, :]w = inputs[:, :, :-1, :] - inputs[:, :, 1:, :]return tf.nn.l2_loss(h) / tf.to_float(tf.size(h)) + tf.nn.l2_loss(w) / tf.to_float(tf.size(w)) total_variation_loss = get_total_variation_loss(g)content_weight = 1 style_weight = 250 total_variation_weight = 0.01loss = content_weight * content_loss + style_weight * style_loss + total_variation_weight * total_variation_loss 復制代碼

定義優化器，通過調整轉換網絡中的參數降低總損失

vars_t = [var for var in tf.trainable_variables() if var.name.startswith('transformer')] optimizer = tf.train.AdamOptimizer(learning_rate=0.001).minimize(loss, var_list=vars_t) 復制代碼

訓練模型，每輪訓練結束后，用一張測試圖片進行測試，并且將一些tensor的值寫入events文件，便于使用tensorboard查看

style_name = style_images[style_index] style_name = style_name[style_name.find('/') + 1:].rstrip('.jpg') OUTPUT_DIR = 'samples_%s' % style_name if not os.path.exists(OUTPUT_DIR):os.mkdir(OUTPUT_DIR)tf.summary.scalar('losses/content_loss', content_loss) tf.summary.scalar('losses/style_loss', style_loss) tf.summary.scalar('losses/total_variation_loss', total_variation_loss) tf.summary.scalar('losses/loss', loss) tf.summary.scalar('weighted_losses/weighted_content_loss', content_weight * content_loss) tf.summary.scalar('weighted_losses/weighted_style_loss', style_weight * style_loss) tf.summary.scalar('weighted_losses/weighted_total_variation_loss', total_variation_weight * total_variation_loss) tf.summary.image('transformed', g) tf.summary.image('origin', X) summary = tf.summary.merge_all() writer = tf.summary.FileWriter(OUTPUT_DIR)sess.run(tf.global_variables_initializer()) losses = [] epochs = 2X_sample = imread('sjtu.jpg') h_sample = X_sample.shape[0] w_sample = X_sample.shape[1]for e in range(epochs):data_index = np.arange(X_data.shape[0])np.random.shuffle(data_index)X_data = X_data[data_index]for i in tqdm(range(X_data.shape[0] // batch_size)):X_batch = X_data[i * batch_size: i * batch_size + batch_size]ls_, _ = sess.run([loss, optimizer], feed_dict={X: X_batch})losses.append(ls_)if i > 0 and i % 20 == 0:writer.add_summary(sess.run(summary, feed_dict={X: X_batch}), e * X_data.shape[0] // batch_size + i)writer.flush()print('Epoch %d Loss %f' % (e, np.mean(losses)))losses = []gen_img = sess.run(g, feed_dict={X: [X_sample]})[0]gen_img = np.clip(gen_img, 0, 255)result = np.zeros((h_sample, w_sample * 2, 3))result[:, :w_sample, :] = X_sample / 255.result[:, w_sample:, :] = gen_img[:h_sample, :w_sample, :] / 255.plt.axis('off')plt.imshow(result)plt.show()imsave(os.path.join(OUTPUT_DIR, 'sample_%d.jpg' % e), result) 復制代碼

保存模型

saver = tf.train.Saver() saver.save(sess, os.path.join(OUTPUT_DIR, 'fast_style_transfer')) 復制代碼

測試圖片依舊是之前用過的交大廟門

風格遷移結果

訓練過程中可以使用tensorboard查看訓練過程

tensorboard --logdir=samples_starry 復制代碼

在單機上使用以下代碼即可快速完成風格遷移，在CPU上也只需要10秒左右

# -*- coding: utf-8 -*-import tensorflow as tf import numpy as np from imageio import imread, imsave import os import timedef the_current_time():print(time.strftime("%Y-%m-%d %H:%M:%S", time.localtime(int(time.time()))))style = 'wave' model = 'samples_%s' % style content_image = 'sjtu.jpg' result_image = 'sjtu_%s.jpg' % style X_image = imread(content_image)sess = tf.Session() sess.run(tf.global_variables_initializer())saver = tf.train.import_meta_graph(os.path.join(model, 'fast_style_transfer.meta')) saver.restore(sess, tf.train.latest_checkpoint(model))graph = tf.get_default_graph() X = graph.get_tensor_by_name('X:0') g = graph.get_tensor_by_name('transformer/g:0')the_current_time()gen_img = sess.run(g, feed_dict={X: [X_image]})[0] gen_img = np.clip(gen_img, 0, 255) / 255. imsave(result_image, gen_img)the_current_time() 復制代碼

對于其他風格圖片，用相同方法訓練對應模型即可

參考

• Perceptual Losses for Real-Time Style Transfer and Super-Resolution：arxiv.org/abs/1603.08…
• Fast Style Transfer in TensorFlow：github.com/lengstrom/f…
• A Tensorflow Implementation for Fast Neural Style：github.com/hzy46/fast-…

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