FCN网络简介

全卷积网络（Fully Convolutional Networks，FCN）是Jonathan Long等人于2015年在Fully Convolutional Networks for Semantic Segmentation一文中提出的用于图像语义分割的一种框架，是首个端对端的针对像素级预测的全卷积网络。FCN将传统CNN后面的全连接层换成了卷积层，这样网络的输出将是热力图而非类别；同时，为解决卷积和池化导致图像尺寸的变小，使用上采样方式对图像尺寸进行恢复。

网络结构

FCN网络结构主要分为两个部分：全卷积部分和反卷积部分。其中全卷积部分为一些经典的CNN网络（如VGG，ResNet等），用于提取特征；反卷积部分则是通过上采样得到原尺寸的语义分割图像。FCN的输入可以为任意尺寸的彩色图像，输出与输入尺寸相同，通道数为n（目标类别数）+1（背景），（原始FCN是在PASCAL数据集上训练的所以一共有20+1类）。FCN-8s网络结构如下：

21张概率图中每个像素处是一个概率，表明当前像素属于哪一种类别

这里为什么会产生568*568大小的图片呢，是因为原论文的源码中在第一个卷积层处将padding设置为100，这样做的目的是防止图片下采样32倍后尺寸小于7x7（因为下采样32倍后会经过7x7大小的卷积层），之后上采样32倍后会产生与原图不一样大小的图片，需要进行裁剪才能得到原图大小的输出。

PS：卷积向下取整，池化向上取整

官方模型是采用了VGG16作为backbone

VGG16网络结果如下图所示：

其中最大池化层为2x2 步长为2

论文中提出了三个模型分别是FCN-32s、FCN-16s、FCN-8s。

FCN-32s

pool5的输出直接上采样32倍恢复到原图大小，将损失了原图很多细节信息的特征图直接上采样，效果较差

现在的FCN的源码中FC6的卷积层的padding为3，这样可以使输出的图片高宽不变，防止输入图片过小导致该卷积层报错，例如若没有该padding，那么输入192x192的图片FC6的输入会是6x6大小的图片，FC6就报错了。
论文源码中的转置卷积的参数是冻结的，因为作者发现冻结和不冻结的结果相差不大，为了提高效率，所以就冻结了。此时转置卷积层相当于是双线性插值。这里效果不明显的原因是上采样倍数太大了

FCN-16s

pool5的输出上采样2倍（采样后大小与pool4的输出相同）然后与pool4输出相加然后再直接上采样16倍恢复到原图大小

FCN-8s

pool5的输出上采样2倍（采样后大小与pool4的输出相同）然后与pool4输出相加然后再上采样2倍（采样后大小与pool3的输出相同），然后与pool3输出相加然后再直接上采样8倍恢复到原图大小。

转置卷积计算公式：

o'为卷积输出大小，i‘为卷积输入大小，s为卷积核stride，k为卷积核大小，p为填充

实现FCN-8s时的参数如下

参数名称	参数值
f6.stride	1
f6.padding	3
f7.stride	1
f7.padding	1
转置卷积1.padding	1
转置卷积1.stride	2
转置卷积2.padding	1
转置卷积2.stride	2
转置卷积3.padding	4
转置卷积3.stride	8

规律：设倍率为x，当转置卷积的2*padding -x = k.size、 s为上采样倍率x时恰好可以上采样

原论文中FCN-32s、16s、8s中效果比较

Pytorch实现FCN-8s

网络结构

数据处理

数据集

数据集采用的是PASCAL VOC2012数据集

root样例 root = 'F:\VOCtrainval_11-May-2012\VOCdevkit\VOC2012' 到VOC2012

class VOC_Segmentation(Dataset):

    def __init__(self,root,text_name='train.txt',trans=None):
        super(VOC_Segmentation, self).__init__()
        #数据划分信息路径
        txt_path = os.path.join(root,'ImageSets','Segmentation',text_name)
        #图片路径
        image_path = os.path.join(root,'JPEGImages')
        #mask(label)路径
        mask_path = os.path.join(root,'SegmentationClass')

        #读入数据集文件名称
        with open(txt_path,'r') as f:
            file_names = [name.strip() for name in f.readlines() if len(name.strip()) > 0]
        #文件名拼接拼接路径
        self.images = [os.path.join(image_path,name+'.jpg') for name in file_names]
        self.mask = [os.path.join(mask_path,name+'.png') for name in file_names]
        self.trans = trans

    def __len__(self):
        return len(self.images)
    def __getitem__(self, index):
        '''
        albumentations图像增强库是基于cv2库的,
        cv2.imread()读入后图片的类型是numpy类型
        所以需要保证与cv2读入类型一致
        '''
        img = np.asarray(Image.open(self.images[index]))
        mask = np.asarray(Image.open(self.mask[index]),dtype=np.int32)

        if self.trans is not None:
            img,mask = self.trans(img,mask)

        return img,mask

为什么不直接使用cv2呢？

cv2.imread(path, flags)

path: 该参数制定图片的路径，可以使用相对路径，也可以使用绝对路径；
flags:指定以何种方式加载图片，有三个取值：
cv2.IMREAD_COLOR:读取一副彩色图片，图片的透明度会被忽略，默认为该值，实际取值为1；
cv2.IMREAD_GRAYSCALE:以灰度模式读取一张图片，实际取值为0
cv2.IMREAD_UNCHANGED:加载一副彩色图像，透明度不会被忽略

起初也想着是直接使用cv2.imread，但是PASCAL VOC2012中mask是调色板模式（单通道，像素取值为[0,255]）存储的，cv2.IMREAD_UNCHANGED和cv2.IMREAD_COLOR读入后会创建三通道的数据，使用cv2.IMREAD_GRAYSCALE读入后原有像素值会发生改变(255变成了220).

而PIL的image.open可以直接读入调色板模式的图片，只需要对数据类型进行改变即可。

Transforms

使用的是albumentations图像处理库，这个库中有很多高度封装的transform函数，可以实现傻瓜式同时对image和mask进行转换（pytorch自带的transform无法傻瓜式同时转换）

训练集

数据增强操作

随机变化原图，使用双线插值的模式，随机将原图放大到[0.5,2]倍的大小
模型训练默认的输入大小为480x480图片，数据集中的有些图片随机变化后无法满足要求，所以要进行填充，这里要注意image填充0也就是背景，mask填充255（后续计算损失时可以忽略掉像素值为255的像素，就不会产生影响了）
随机剪切处480x480大小的图片
随机进行水平翻转

还要注意image需要调用ToTensor函数，但mask不能直接调用ToTensor函数，因为ToTensor操作会进行归一化，而mask是调色板模式的图片，其中的像素值是处于[0,255]之间的，转化后会出错，只需要手动将他转化为Tensor即可

这里还要注意 mask不需要加通道！！！最后保证mask是(batch,height,width)格式就行，否则会影响到后续nn.CrossEntropyLoss的计算

class Train_transforms():
    def __init__(self,output_size=480,scale_prob=0.5,flip_prob=0.5,mean=(0.485, 0.456, 0.406), std=(0.229, 0.224, 0.225)):
        self.aug1 = A.Compose([
            #随机变换原图
            A.RandomScale(scale_limit=[0.5,1.5],interpolation=cv2.INTER_NEAREST,p=scale_prob),
            #小于480x480的img和mask进行填充
            A.PadIfNeeded(min_height=output_size,min_width=output_size,value=0,mask_value=255),
            #剪切
            A.RandomCrop(height=output_size,width=output_size,p=1),
            #翻转
            A.HorizontalFlip(p=flip_prob)
        ])
        self.aug2 = transforms.Compose([
            transforms.ToTensor(),
            transforms.Normalize(mean=mean,std=std)
        ])
    def __call__(self,image,mask):
        augmented = self.aug1(image=image,mask=mask)
        image,mask = augmented['image'],augmented['mask']
        image = self.aug2(image)
        mask = torch.as_tensor(np.array(mask),dtype=torch.int64)
        return image,mask

测试集

转换了图片大小到480x480

class Validate_trans():
    def __init__(self,output_size=480,mean=(0.485, 0.456, 0.406), std=(0.229, 0.224, 0.225)):
        self.aug1 = A.Resize(output_size,output_size,interpolation=cv2.INTER_NEAREST)
        self.aug = transforms.Compose([
            transforms.ToTensor(),
            transforms.Normalize(mean=mean,std=std)
        ])
    def __call__(self,image,mask):
        augmented = self.aug1(image=image,mask=mask)
        image,mask = augmented['image'],augmented['mask']
        image = self.aug(image)
        mask = torch.as_tensor(np.array(mask),dtype=torch.int64)
        return image,mask

模型搭建

backbone使用的是VGG模型，具体使用的是VGG16也就是下图中的D

#vgg块
def vgg_block(in_channels,out_channels,num):
    block = []
    for _ in range(num):
        block.append(nn.Conv2d(in_channels, out_channels, kernel_size=3, padding=1, stride=1))
        block.append(nn.ReLU())
        in_channels = out_channels
    block.append(nn.MaxPool2d(kernel_size=2,stride=2))
    return nn.Sequential(*block)

class VGG(nn.Module):

    def __init__(self,num_classes,struct,in_channel=3):
        super(VGG, self).__init__()
        blk = []
        out_channels = []
        conv_nums = []
        for num,out_channel in struct:
            out_channels.append(out_channel)
            conv_nums.append(num)
        #这里这样写便于后续取出某层的输出
        self.layer1 = vgg_block(in_channel,out_channels[0],conv_nums[0])
        self.layer2 = vgg_block(out_channels[0], out_channels[1], conv_nums[1])
        self.layer3 = vgg_block(out_channels[1], out_channels[2], conv_nums[2])
        self.layer4 = vgg_block(out_channels[2], out_channels[3], conv_nums[3])
        self.layer5 = vgg_block(out_channels[3], out_channels[4], conv_nums[4])
        blk=[nn.Flatten(),
                    nn.Linear(7*7*512,4096),
                    nn.Dropout(0.5),
                    nn.ReLU(),
                    nn.Linear(4096,4096),
                    nn.Dropout(0.5),
                    nn.ReLU(),
                    nn.Linear(4096,num_classes)]
        self.top = nn.Sequential(*blk)
        self.__init_net()
    def forward(self,x):
        x = self.layer1(x)
        x = self.layer2(x)
        x = self.layer3(x)
        x = self.layer4(x)
        x = self.layer5(x)
        x = self.top(x)
        return x

    def __init_net(self):
        for layer in self.modules():
            if type(layer) == nn.Conv2d:
                nn.init.kaiming_normal_(layer.weight,mode='fan_out',nonlinearity='relu')
            elif type(layer) == nn.Linear:
                nn.init.xavier_normal_(layer.weight)
            elif type(layer) == nn.BatchNorm2d:
                nn.init.constant_(layer.weight,1) #均值为0
                nn.init.constant_(layer.bias,0) #方差为1

FCN-Head

FCN-Head也就是网络结构中的FC6和FC7

class FCN_Head(nn.Module):

    def __init__(self,in_channel,out_channel):
        super(FCN_Head, self).__init__()
        self.fc6 = nn.Sequential(
            nn.Conv2d(in_channel,out_channel,kernel_size=7,stride=1,padding=3),
            nn.BatchNorm2d(out_channel),
            nn.ReLU(),
            nn.Dropout(0.1)
        )
        self.fc7 = nn.Sequential(
            nn.Conv2d(out_channel,out_channel,kernel_size=1),
            nn.BatchNorm2d(out_channel),
            nn.ReLU(),
            nn.Dropout(0.1)
        )

    def forward(self,x):
        x = self.fc6(x)
        x = self.fc7(x)
        return x

FCN-8s

class FCN(nn.Module):

    def __init__(self,backbone,head,num_classes,channel_nums):
        super(FCN, self).__init__()
        self.backbone = backbone
        self.head = head
        #调整通道数
        self.layer3_conv = nn.Conv2d(channel_nums[0],num_classes,kernel_size=1)    #256
        self.layer4_conv = nn.Conv2d(channel_nums[1],num_classes,kernel_size=1)    #512
        self.layer5_conv = nn.Conv2d(channel_nums[2],num_classes,kernel_size=1)    #4096
        
        #转置卷积层1
        self.transpose_conv1 =nn.Sequential(
            nn.ConvTranspose2d(num_classes, num_classes, kernel_size=4, stride=2, padding=1),
            nn.BatchNorm2d(num_classes),
            nn.ReLU()
        )
        # 转置卷积层2
        self.transpose_conv2 = nn.Sequential(
            nn.ConvTranspose2d(num_classes, num_classes, kernel_size=4, stride=2, padding=1),
            nn.BatchNorm2d(num_classes),
            nn.ReLU()
        )
        # 转置卷积层3
        self.transpose_conv3 = nn.Sequential(
            nn.ConvTranspose2d(num_classes,num_classes,kernel_size=16,stride=8,padding=4),
            nn.BatchNorm2d(num_classes),
            nn.ReLU()
        )

    def forward(self,x):
        #out = OrderedDict {layer4:{},layer5:{},layer3:{}}
        out = self.backbone(x)
        layer5_out, layer4_out,layer3_out = out['layer5'],out['layer4'],out['layer3']
        layer5_out = self.head(layer5_out)

        layer5_out = self.layer5_conv(layer5_out)
        layer4_out = self.layer4_conv(layer4_out)
        layer3_out = self.layer3_conv(layer3_out)

        x = self.transpose_conv1(layer5_out)
        x = self.transpose_conv2(x + layer4_out)
        x = self.transpose_conv3(x + layer3_out)
        return x

def fcn_vgg16(num_classes=20,pretrain_backbone=False):
    num_classes += 1
    struct = [(2, 64), (2, 128), (3, 256), (3, 512), (3, 512)]
    backbone = VGG(num_classes=num_classes,struct=struct)
    if pretrain_backbone is True:
        #https://download.pytorch.org/models/vgg16-397923af.pth
        load_weights(backbone,'../weights/vgg16.pth')

    return_layers = {'layer3':"layer3",'layer4':'layer4','layer5':"layer5"}
    backbone = torchvision.models._utils.IntermediateLayerGetter(backbone,return_layers)
    # x = torch.randn((1,3,224,224))
    # x = backbone(x)
    head = FCN_Head(in_channel=512,out_channel=4*512)
    model = FCN(backbone=backbone,head=head,num_classes=num_classes,channel_nums=[256,512,4096])

    return model

fcn_vgg16模型

def fcn_vgg16(num_classes=20,pretrain_backbone=False):
    num_classes += 1
    #vgg16结构
    struct = [(2, 64), (2, 128), (3, 256), (3, 512), (3, 512)]
    backbone = VGG(num_classes=num_classes,struct=struct)
    if pretrain_backbone is True:
        #https://download.pytorch.org/models/vgg16-397923af.pth
        load_weights(backbone,'../weights/vgg16.pth')

    return_layers = {'layer3':"layer3",'layer4':'layer4','layer5':"layer5"}
    backbone = torchvision.models._utils.IntermediateLayerGetter(backbone,return_layers)
    # x = torch.randn((1,3,224,224))
    # x = backbone(x)
    head = FCN_Head(in_channel=512,out_channel=4*512)
                                                                              #layer3 layer4 fcn_head输出通道数
    model = FCN(backbone=backbone,head=head,num_classes=num_classes,channel_nums=[256,512,4096])

    return model

训练

辅助函数

语义分割混淆矩阵：用于计算global acc、acc、IoU

class ConfusionMatrix():

    def __init__(self,num_classes):
        self.n = num_classes
        self.mat = None

    def update(self,a,b):
        if self.mat is None:
            self.mat = torch.zeros(self.n,self.n,device=a.device)

        #统计所有有效预测像素的索引
        k = (a>=0) & (a < self.n)

        #计数
        indexs = self.n * a[k].to(torch.int64) + b[k]

        self.mat += torch.bincount(indexs,minlength=self.n**2).reshape(self.n,self.n)

    def reset(self):
        self.mat.zero_()

    def compute(self):
        h = self.mat.float()
        global_acc = torch.diag(h).sum() / h.sum()
        acc = torch.diag(h)/h.sum(1)
        iou = torch.diag(h) / (h.sum(1)+h.sum(0) - torch.diag(h))

        return global_acc,acc,iou

训练函数

def train(model,train_iter,validate_iter,loss,optimizer,epochs,num_classes=21):
    confusion_matrix = ConfusionMatrix(num_classes=num_classes)
    device = get_device()
    best_mean_iou = 0
    for epoch in range(1,epochs):
        train_loss = 0
        batch_id = 0
        train_num = 0
        model.train()
        for X,y in train_iter:
            batch_id += 1
            train_num += len(X)
            X,y = X.to(device),y.to(device)
            y_hat = model(X)
            l = loss(y_hat,y)
            train_loss += l.item()
            optimizer.zero_grad()
            l.backward()
            optimizer.step()
            if batch_id % 30 == 0:
                print(f"epoch{epoch}[{batch_id}/{len(train_iter)}]:loss:{train_loss / train_num}")
        model.eval()
        vaild_loss = 0
        valid_num = 0
        for X, y in validate_iter:
            with torch.no_grad():
                X, y = X.to(device), y.to(device)
                y_hat = model(X)
                l = loss(y_hat, y)
                vaild_loss += l.item()
                valid_num += len(X)
                confusion_matrix.update(y.flatten(), y_hat.argmax(1).flatten())
        global_acc, acc, IoU = confusion_matrix.compute()
        print(
            f"epoch:{epoch},train_loss:{train_loss / train_num},valid_loss:{vaild_loss / valid_num},global_acc:{global_acc},acc:{acc.mean()},mean_IoU:{IoU.mean()}")
        if best_mean_iou < IoU:
            weights = model.state_dict()
            torch.save(weights, './weights/fcn_vgg16.pth')
        confusion_matrix.reset()

预测

辅助函数

使用下述代码可以读入mask中的调色板模式，也就是每个像素值对应的RGB值，这样在得到预测后的图片时就可以使用调色板进行调色了。

# 读取mask标签
target = Image.open("2007_000039.png")
# 获取调色板
palette = target.getpalette()
palette = np.reshape(palette, (-1, 3)).tolist()
# 转换成字典子形式
pd = dict((i, color) for i, color in enumerate(palette))

json_str = json.dumps(pd)
with open("palette.json", "w") as f:
    f.write(json_str)

预测函数

def main():
    classes = 20
    weights_path = './weights/fcn_vgg16.pth'
    img_path = './test.jpg'
    pallette_path = "./utils/palette.json"
    with open(pallette_path,'rb') as f:
        pallette_dict = json.load(f)
        pallette = []
        for v in pallette_dict.values():
            pallette += v
    #获取设备
    device = torch.device("cuda:0" if torch.cuda.is_available() else "cpu")
    print("using {} deviece".format(device))

    #创建模型
    model = fcn_vgg16().to(device)
    weights_dict = torch.load(weights_path,map_location='cpu')

    # #去除辅助分类器
    # for k in list(weights_dict.keys()):
    #     if "aux" in k:
    #         del weights_dict[k]
    #加载权重
    model.load_state_dict(weights_dict)
    model.to(device)
    #加载图片
    original_img = Image.open(img_path)

    #转换
    data_transform = transforms.Compose([transforms.Resize((480,480)),
                                         transforms.ToTensor(),
                                         transforms.Normalize(mean=(0.485, 0.456, 0.406),
                                                              std=(0.229, 0.224, 0.225))])
    img = data_transform(original_img)

    #添加batch维度
    img = torch.unsqueeze(img,dim=0)

    model.eval()
    with torch.no_grad():
        output = model(img.to(device))
        prediction = output.argmax(1).squeeze(0) #消除batch维度
        prediction = prediction.to('cpu').numpy().astype(np.uint8)
        mask = Image.fromarray(prediction)
        mask.putpalette(pallette)
        mask.save("test_result.png")

#原图和分割图混在一起
def blend():
    image1 = Image.open("test.jpg")
    image2 = Image.open("test_result.png")

    image1 = image1.convert('RGBA')
    image2 = image2.convert('RGBA')
    #blended_img = img1 * (1 – alpha) + img2* alpha 进行
    image1 = image1.resize(image2.size)
    image = Image.blend(image1,image2,0.4)
    image.show()

参考链接:

FCN网络结构详解(语义分割)_哔哩哔哩_bilibiliFCN讲解：FCN网络结构详解(语义分割)_哔哩哔哩_bilibili

FCN源码实现：GitHub - WZMIAOMIAO/deep-learning-for-image-processing: deep learning for image processing including classification and object-detection etc.

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