继前文Unet和Unet++之后，本文将介绍Attention Unet。
Attention Unet地址，《Attention U-Net: Learning Where to Look for the Pancreas》。
AttentionUnet
Attention Unet发布于2018年，主要应用于医学领域的图像分割，全文中主要以肝脏的分割论证。
论文中心
Attention Unet主要的中心思想就是提出来Attention gate模块，使用soft-attention替代hard-attention，将attention集成到Unet的跳跃连接和上采样模块中，实现空间上的注意力机制。通过attention机制来抑制图像中的无关信息，突出局部的重要特征。
网络架构
Attention Unet的模型结构和Unet十分相像，只是增加了Attention Gate模块来对skip connection和upsampling层做attention机制（图2）。
在Attention Gate模块中，g和xl分别为skip connection的输出和下一层的输出，如图3。
需要注意的是，在计算Wg和Wx后，对两者进行相加。但是，此时g的维度和xl的维度并不相等，则需要对g做下采样或对xl做上采样。（我倾向于对xl做上采样，因为在原本的Unet中，在Decoder就需要对下一层做上采样，所以，直接使用这个上采样结果可以减少网络计算）。
Wg和Wx经过相加，ReLU激活，1x1x1卷积，Sigmoid激活，生成一个权重信息，将这个权重与原始输入xl相乘，得到了对xl的attention激活。这就是Attenton Gate的思想。
Attenton Gate还有一个比较重要的特点是：这个权重可以经由网络学习！因为soft-attention是可微的，可以微分的attention就可以通过神经网络算出梯度并且前向传播和后向反馈来学习得到attention的权重。以此来学习更重要的特征。
模型复现
Attention Unet代码
import torchimport torch.nn as nnimport torch.nn.functional as Ffrom torch.nn import initdef init_weights(net, init_type='normal', gain=0.02):def init_func(m):classname = m.__class__.__name__if hasattr(m, 'weight') and (classname.find('Conv') != -1 or classname.find('Linear') != -1):if init_type == 'normal':init.normal_(m.weight.data, 0.0, gain)elif init_type == 'xavier':init.xavier_normal_(m.weight.data, gain=gain)elif init_type == 'kaiming':init.kaiming_normal_(m.weight.data, a=0, mode='fan_in')elif init_type == 'orthogonal':init.orthogonal_(m.weight.data, gain=gain)else:raise NotImplementedError('initialization method [%s] is not implemented' %init_type)if hasattr(m, 'bias') and m.bias is not None:init.constant_(m.bias.data, 0.0)elif classname.find('BatchNorm2d') != -1:init.normal_(m.weight.data, 1.0, gain)init.constant_(m.bias.data, 0.0)print('initialize network with %s' % init_type)net.apply(init_func)class conv_block(nn.Module):def __init__(self, ch_in, ch_out):super(conv_block, self).__init__()self.conv = nn.Sequential(nn.Conv2d(ch_in,ch_out,kernel_size=3,stride=1,padding=1,bias=True), nn.BatchNorm2d(ch_out),nn.ReLU(inplace=True),nn.Conv2d(ch_out,ch_out,kernel_size=3,stride=1,padding=1,bias=True), nn.BatchNorm2d(ch_out),nn.ReLU(inplace=True))def forward(self, x):x = self.conv(x)return xclass up_conv(nn.Module):def __init__(self, ch_in, ch_out, convTranspose=True):super(up_conv, self).__init__()if convTranspose:self.up = nn.ConvTranspose2d(in_channels=ch_in, out_channels=ch_in,kernel_size=4,stride=2, padding=1)else:self.up = nn.Upsample(scale_factor=2)self.Conv = nn.Sequential(nn.Conv2d(ch_in,ch_out,kernel_size=3,stride=1,padding=1,bias=True), nn.BatchNorm2d(ch_out),nn.ReLU(inplace=True))def forward(self, x):x = self.up(x)x = self.Conv(x)return xclass single_conv(nn.Module):def __init__(self, ch_in, ch_out):super(single_conv, self).__init__()self.conv = nn.Sequential(nn.Conv2d(ch_in,ch_out,kernel_size=3,stride=1,padding=1,bias=True), nn.BatchNorm2d(ch_out),nn.ReLU(inplace=True))def forward(self, x):x = self.conv(x)return xclass Attention_block(nn.Module):def __init__(self, F_g, F_l, F_int):super(Attention_block, self).__init__()self.W_g = nn.Sequential(nn.Conv2d(F_g,F_int,kernel_size=1,stride=1,padding=0,bias=True), nn.BatchNorm2d(F_int))self.W_x = nn.Sequential(nn.Conv2d(F_l,F_int,kernel_size=1,stride=1,padding=0,bias=True), nn.BatchNorm2d(F_int))self.psi = nn.Sequential(nn.Conv2d(F_int, 1, kernel_size=1, stride=1, padding=0, bias=True),nn.BatchNorm2d(1), nn.Sigmoid())self.relu = nn.ReLU(inplace=True)def forward(self, g, x):g1 = self.W_g(g)x1 = self.W_x(x)psi = self.relu(g1 + x1)psi = self.psi(psi)return x * psiclass AttU_Net(nn.Module):"""in_channel: input image channelsnum_classes: output class number channel_list: a channel list for adjust the model sizecheckpoint: 是否有checkpointif False： call normal initconvTranspose: 是否使用反卷积上采样。True: use nn.convTransposeFlase: use nn.Upsample"""def __init__(self, in_channel=3, num_classes=1, channel_list=[64, 128, 256, 512, 1024], checkpoint=False, convTranspose=True):super(AttU_Net, self).__init__()self.Maxpool = nn.MaxPool2d(kernel_size=2, stride=2)self.Conv1 = conv_block(ch_in=in_channel, ch_out=channel_list[0])self.Conv2 = conv_block(ch_in=channel_list[0], ch_out=channel_list[1])self.Conv3 = conv_block(ch_in=channel_list[1], ch_out=channel_list[2])self.Conv4 = conv_block(ch_in=channel_list[2], ch_out=channel_list[3])self.Conv5 = conv_block(ch_in=channel_list[3], ch_out=channel_list[4])self.Up5 = up_conv(ch_in=channel_list[4], ch_out=channel_list[3], convTranspose=convTranspose)self.Att5 = Attention_block(F_g=channel_list[3],F_l=channel_list[3],F_int=channel_list[2])self.Up_conv5 = conv_block(ch_in=channel_list[4], ch_out=channel_list[3])self.Up4 = up_conv(ch_in=channel_list[3], ch_out=channel_list[2], convTranspose=convTranspose)self.Att4 = Attention_block(F_g=channel_list[2],F_l=channel_list[2],F_int=channel_list[1])self.Up_conv4 = conv_block(ch_in=channel_list[3], ch_out=channel_list[2])self.Up3 = up_conv(ch_in=channel_list[2], ch_out=channel_list[1], convTranspose=convTranspose)self.Att3 = Attention_block(F_g=channel_list[1],F_l=channel_list[1],F_int=64)self.Up_conv3 = conv_block(ch_in=channel_list[2], ch_out=channel_list[1])self.Up2 = up_conv(ch_in=channel_list[1], ch_out=channel_list[0], convTranspose=convTranspose)self.Att2 = Attention_block(F_g=channel_list[0],F_l=channel_list[0],F_int=channel_list[0] // 2)self.Up_conv2 = conv_block(ch_in=channel_list[1], ch_out=channel_list[0])self.Conv_1x1 = nn.Conv2d(channel_list[0],num_classes,kernel_size=1,stride=1,padding=0)if not checkpoint:init_weights(self)def forward(self, x):# encoderx1 = self.Conv1(x)x2 = self.Maxpool(x1)x2 = self.Conv2(x2)x3 = self.Maxpool(x2)x3 = self.Conv3(x3)x4 = self.Maxpool(x3)x4 = self.Conv4(x4)x5 = self.Maxpool(x4)x5 = self.Conv5(x5)# decoderd5 = self.Up5(x5)x4 = self.Att5(g=d5, x=x4)d5 = torch.cat((x4, d5), dim=1)d5 = self.Up_conv5(d5)d4 = self.Up4(d5)x3 = self.Att4(g=d4, x=x3)d4 = torch.cat((x3, d4), dim=1)d4 = self.Up_conv4(d4)d3 = self.Up3(d4)x2 = self.Att3(g=d3, x=x2)d3 = torch.cat((x2, d3), dim=1)d3 = self.Up_conv3(d3)d2 = self.Up2(d3)x1 = self.Att2(g=d2, x=x1)d2 = torch.cat((x1, d2), dim=1)d2 = self.Up_conv2(d2)d1 = self.Conv_1x1(d2)return d1数据集
数据集依旧使用Camvid数据集，见Camvid数据集的构建和使用。
# 导入库import osos.environ['CUDA_VISIBLE_DEVICES'] = '0'import torchimport torch.nn as nnimport torch.optim as optimimport torch.nn.functional as Ffrom torch import optimfrom torch.utils.data import Dataset, DataLoader, random_splitfrom tqdm import tqdmimport warningswarnings.filterwarnings("ignore")import os.path as ospimport matplotlib.pyplot as pltfrom PIL import Imageimport numpy as npimport albumentations as Afrom albumentations.pytorch.transforms import ToTensorV2torch.manual_seed(17)# 自定义数据集CamVidDatasetclass CamVidDataset(torch.utils.data.Dataset):"""CamVid Dataset. Read images, apply augmentation and preprocessing transformations.Args:images_dir (str): path to images foldermasks_dir (str): path to segmentation masks folderclass_values (list): values of classes to extract from segmentation maskaugmentation (albumentations.Compose): data transfromation pipeline (e.g. flip, scale, etc.)preprocessing (albumentations.Compose): data preprocessing (e.g. noralization, shape manipulation, etc.)"""def __init__(self, images_dir, masks_dir):self.transform = A.Compose([A.Resize(224, 224),A.HorizontalFlip(),A.VerticalFlip(),A.Normalize(),ToTensorV2(),]) self.ids = os.listdir(images_dir)self.images_fps = [os.path.join(images_dir, image_id) for image_id in self.ids]self.masks_fps = [os.path.join(masks_dir, image_id) for image_id in self.ids]def __getitem__(self, i):# read dataimage = np.array(Image.open(self.images_fps[i]).convert('RGB'))mask = np.array( Image.open(self.masks_fps[i]).convert('RGB'))image = self.transform(image=image,mask=mask)return image['image'], image['mask'][:,:,0]def __len__(self):return len(self.ids)# 设置数据集路径DATA_DIR = r'dataset\camvid' # 根据自己的路径来设置x_train_dir = os.path.join(DATA_DIR, 'train_images')y_train_dir = os.path.join(DATA_DIR, 'train_labels')x_valid_dir = os.path.join(DATA_DIR, 'valid_images')y_valid_dir = os.path.join(DATA_DIR, 'valid_labels')train_dataset = CamVidDataset(x_train_dir, y_train_dir, )val_dataset = CamVidDataset(x_valid_dir, y_valid_dir, )train_loader = DataLoader(train_dataset, batch_size=32, shuffle=True,drop_last=True)val_loader = DataLoader(val_dataset, batch_size=32, shuffle=True,drop_last=True)模型训练
model = AttentionUnet(num_classes=33).cuda()#model.load_state_dict(torch.load(r"checkpoints/Unet_100.pth"),strict=False)from d2l import torch as d2lfrom tqdm import tqdmimport pandas as pd#损失函数选用多分类交叉熵损失函数lossf = nn.CrossEntropyLoss(ignore_index=255)#选用adam优化器来训练optimizer = optim.SGD(model.parameters(),lr=0.1)scheduler = torch.optim.lr_scheduler.StepLR(optimizer, step_size=50, gamma=0.1, last_epoch=-1)#训练50轮epochs_num = 50def train_ch13(net, train_iter, test_iter, loss, trainer, num_epochs,scheduler, devices=d2l.try_all_gpus()):timer, num_batches = d2l.Timer(), len(train_iter)animator = d2l.Animator(xlabel='epoch', xlim=[1, num_epochs], ylim=[0, 1],legend=['train loss', 'train acc', 'test acc'])net = nn.DataParallel(net, device_ids=devices).to(devices[0])loss_list = []train_acc_list = []test_acc_list = []epochs_list = []time_list = []for epoch in range(num_epochs):# Sum of training loss, sum of training accuracy, no. of examples,# no. of predictionsmetric = d2l.Accumulator(4)for i, (features, labels) in enumerate(train_iter):timer.start()l, acc = d2l.train_batch_ch13(net, features, labels.long(), loss, trainer, devices)metric.add(l, acc, labels.shape[0], labels.numel())timer.stop()if (i + 1) % (num_batches // 5) == 0 or i == num_batches - 1:animator.add(epoch + (i + 1) / num_batches, (metric[0] / metric[2], metric[1] / metric[3],None))test_acc = d2l.evaluate_accuracy_gpu(net, test_iter)animator.add(epoch + 1, (None, None, test_acc))scheduler.step()# print(f'loss {metric[0] / metric[2]:.3f}, train acc '# f'{metric[1] / metric[3]:.3f}, test acc {test_acc:.3f}')# print(f'{metric[2] * num_epochs / timer.sum():.1f} examples/sec on '# f'{str(devices)}')print(f"epoch {epoch+1} --- loss {metric[0] / metric[2]:.3f} ---train acc {metric[1] / metric[3]:.3f} --- test acc {test_acc:.3f} --- cost time {timer.sum()}")#---------保存训练数据---------------df = pd.DataFrame()loss_list.append(metric[0] / metric[2])train_acc_list.append(metric[1] / metric[3])test_acc_list.append(test_acc)epochs_list.append(epoch+1)time_list.append(timer.sum())df['epoch'] = epochs_listdf['loss'] = loss_listdf['train_acc'] = train_acc_listdf['test_acc'] = test_acc_listdf['time'] = time_listdf.to_excel("savefile/AttentionUnet_camvid1.xlsx")#----------------保存模型-------------------if np.mod(epoch+1, 5) == 0:torch.save(model.state_dict(), f'checkpoints/AttentionUnet_{epoch+1}.pth')开始训练
train_ch13(model, train_loader, val_loader, lossf, optimizer, epochs_num,scheduler)训练结果
插在最后。
最近很多同学找我要代码，我有时候长时间不看就容易遗漏。我把代码和数据文件传到网盘上，供大家自行下载。
链接：https://pan.baidu.com/s/1taJlov4VvN-Nwp_xoUbgOA?pwd=yumi
提取码：yumi
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