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ResNet基本介绍

ResNet（Residual Network）是一种深度神经网络架构，由微软研究院的Kaiming He等人在2015年提出，并且在ILSVRC 2015竞赛中取得了很好的成绩。ResNet主要解决了随着网络深度增加而出现的退化问题，即当网络变得非常深时，训练误差和验证误差可能会开始上升，这并不是因为过拟合，而是由于深层网络难以优化。

ResNet的核心思想是引入了残差学习框架来简化许多层网络的训练。通过构建“跳跃连接”或称“捷径连接”，允许一层直接与更深层相连接。这种设计让网络可以学习到一个残差函数，相对于传统的学习未参考输入x的目标映射H(x)，残差块学习的是F(x) = H(x) - x。这样的结构使得信息能够跨过多层流动，从而缓解了梯度消失/爆炸的问题，也使得训练更深的网络成为可能。

残差块

残差块（Residual Block）是构成ResNet（Residual Network）的核心组件，它通过引入所谓的“跳跃连接”或“捷径连接”来解决深层网络训练中的梯度消失/爆炸问题。这种设计允许信息在多层之间直接传递，从而帮助优化非常深的神经网络。

残差块的核心思想是让网络去学习输入和输出之间的差异，也就是所谓的“残差”，而不是直接学习原始的输入到输出的映射。用数学语言来说，就是：

• 传统方法：网络尝试学习H(x)，即从输入x直接得到输出H(x)。
• 残差方法：网络学习的是F(x) = H(x) - x，即输入x与期望输出H(x)之间的差值。然后，实际的输出是由原输入加上这个差值得到，即H(x) = F(x) + x。

为什么这样做有效？

1. 简化学习任务：相比于学习复杂的非线性映射H(x)，学习残差F(x)可能要简单得多。特别是当H(x)接近于x时（例如，对于一些层来说不需要对输入进行太多改变），此时F(x)可以趋近于0，表示这些层只需学会恒等映射即可，这比学习复杂的变换要容易很多。

2. 缓解梯度问题：由于信息可以通过跳跃连接直接传递给后面的层，因此即使在深层网络中，早期层的信息也不会因为经过多层而完全丢失，从而减轻了梯度消失/爆炸的影响。

3. 提高模型性能：通过允许网络更深，同时保持良好的可训练性，ResNet能够实现更高的准确率，并且在多个视觉任务上取得了当时最佳的结果。

一个基本的残差块结构如图所示：

(图源:7.6. 残差网络（ResNet） — 动手学深度学习 2.0.0 documentation)

一个典型的残差块包含以下几个部分：

1. 卷积层：通常是一个或多个3x3的卷积层。这些层执行常规的特征提取任务。
2. 批量归一化层（Batch Normalization, BN）：每个卷积层后紧跟着一个批量归一化层，以加速收敛过程并提高模型的泛化能力。
3. ReLU激活函数：除了最后一个卷积层之后，其余卷积层之后都会应用ReLU激活函数来引入非线性。
4. 跳跃连接：这是残差块最独特的部分。它将输入直接加到经过上述处理后的输出上。如果输入和输出具有相同的尺寸（即相同数量的通道数和空间维度），则可以直接相加；如果它们不同，则需要使用额外的1x1卷积层对输入进行调整，以便能够正确地与输出相加。

当残差块的输入和输出特征图的通道数不同时，直接相加是不可行的。为了在这种情况下实现跳跃连接，需要对输入进行适当的变换，使得它与输出具有相同的维度。通常的做法是使用1x1卷积层来调整输入的通道数，将channels_in转换成channels_out。这个1x1卷积层不会改变输入的高度和宽度，只改变通道数。如右图所示。 

(图源:7.6. 残差网络（ResNet） — 动手学深度学习 2.0.0 documentation)

BuildingBlock和Bottleneck

Building Block和Bottleneck是两种不同类型的残差块结构，它们的主要区别在于内部的卷积层配置以及计算效率。

BuildingBlock

Building Block是最简单的残差块形式，通常用于较浅的ResNet模型中，比如ResNet-18和ResNet-34。它的基本结构包括：

• 两个3x3卷积层。
• 每个卷积层之后接一个批量归一化（Batch Normalization, BN）层。
• 第一个卷积层之后有一个ReLU激活函数。
• 跳跃连接（skip connection），将输入直接加到第二个卷积层的输出上。
• 最后一个ReLU激活函数位于跳跃连接之后。

(图源:基于MindSpore实现ResNet50中药分拣_哔哩哔哩_bilibili ) 

这样的结构简单且有效，但随着网络深度的增加，计算成本也会显著上升，因为每个3x3卷积层都会对大量的特征图进行操作。 

Bottleneck

Bottleneck结构被设计用于更深的ResNet模型，例如ResNet-50、ResNet-101和ResNet-152。它通过引入瓶颈（bottleneck）机制来减少计算量，同时保持或增强模型性能。Bottleneck的基本结构如下：

• 一个1x1卷积层，用于减少通道数（降维），这被称为瓶颈。
• 一个3x3卷积层，这个卷积层在较少的通道上执行卷积运算。
• 另一个1x1卷积层，用于恢复原来的通道数（升维）。
• 每个卷积层后面都跟着BN层。
• 在第一个1x1卷积层和3x3卷积层之间有一个ReLU激活函数。
• 跳跃连接，将原始输入直接加到最后一个1x1卷积层的输出上。
• 最后一个ReLU激活函数位于跳跃连接之后。

(图源:基于MindSpore实现ResNet50中药分拣_哔哩哔哩_bilibili ) 

Bottleneck使得可以在不大幅增加计算负担的情况下堆叠更多的层，从而构建更深的网络。 

各类ResNet模型架构如下所示：

(图源:基于MindSpore实现ResNet50中药分拣_哔哩哔哩_bilibili )

数据集

本案例使用“中药炮制饮片”数据集，该数据集由程度中医药大学提供，共包含中药炮制饮片的3个品种，分为：蒲黄、山楂、王不留行，每个品种有4个炮制状态：生品、不及、适中、太过。其中每类包含500张图片共12类，图片尺寸为4K，图片格式为jpg。

数据集可视化如下：

(图源:基于MindSpore实现ResNet50中药分拣_哔哩哔哩_bilibili )

基于MindSpore实现ResNet50中药分拣

数据下载及预处理

from download import download
import osurl = "https://mindspore-courses.obs.cn-north-4.myhuaweicloud.com/deep%20learning/AI%2BX/data/zhongyiyao.zip"
# 创建的是调试任务，url修改为数据集上传生成的url链接
if not os.path.exists("dataset"):download(url, "dataset", kind="zip")

因图片原尺寸为4K过大，因此需要将其Resize至指定尺寸

from PIL import Image
import numpy as np
data_dir = "dataset/zhongyiyao/zhongyiyao"
new_data_path = "dataset1/zhongyiyao"
if not os.path.exists(new_data_path):for path in ['train','test']:data_path = data_dir + "/" + pathclasses = os.listdir(data_path)for (i,class_name) in enumerate(classes):floder_path =  data_path+"/"+class_nameprint(f"正在处理{floder_path}...")for image_name in os.listdir(floder_path):try:image = Image.open(floder_path + "/" + image_name)image = image.resize((1000,1000))target_dir = new_data_path+"/"+path+"/"+class_nameif not os.path.exists(target_dir):os.makedirs(target_dir)if not os.path.exists(target_dir+"/"+image_name):image.save(target_dir+"/"+image_name)except:pass     

将数据集划分为训练集、测试集、验证集

from sklearn.model_selection import train_test_split
import shutildef split_data(X, y, test_size=0.2, val_size=0.2, random_state=42):"""This function splits the data into training, validation, and test sets."""# Split the data into training and test setsX_train, X_test, y_train, y_test = train_test_split(X, y, test_size=test_size, random_state=random_state)# Split the training data into training and validation setsX_train, X_val, y_train, y_val = train_test_split(X_train, y_train, test_size=val_size/(1-test_size), random_state=random_state)return X_train, X_val, X_test, y_train, y_val, y_testdata_dir = "dataset1/zhongyiyao"
floders = os.listdir(data_dir)
target = ['train','test','valid']
if set(floders) == set(target):# 如果已经划分则跳过pass
elif 'train' in floders:# 如果已经划分了train，test，那么只需要从train里边划分出validfloders = os.listdir(data_dir)new_data_dir = os.path.join(data_dir,'train')classes = os.listdir(new_data_dir)if '.ipynb_checkpoints' in classes:classes.remove('.ipynb_checkpoints')imgs = []labels = []for (i,class_name) in enumerate(classes):new_path =  new_data_dir+"/"+class_namefor image_name in os.listdir(new_path):imgs.append(image_name)labels.append(class_name)imgs_train,imgs_val,labels_train,labels_val = X_train, X_test, y_train, y_test = train_test_split(imgs, labels, test_size=0.2, random_state=42)print("划分训练集图片数：",len(imgs_train))print("划分验证集图片数：",len(imgs_val))target_data_dir = os.path.join(data_dir,'valid')if not os.path.exists(target_data_dir):os.mkdir(target_data_dir)for (img,label) in zip(imgs_val,labels_val):source_path = os.path.join(data_dir,'train',label)target_path = os.path.join(data_dir,'valid',label)if not os.path.exists(target_path):os.mkdir(target_path)source_img = os.path.join(source_path,img)target_img = os.path.join(target_path,img)shutil.move(source_img,target_img)
else:phones = os.listdir(data_dir)imgs = []labels = []for phone in phones:phone_data_dir = os.path.join(data_dir,phone)yaowu_list = os.listdir(phone_data_dir)for yaowu in yaowu_list:yaowu_data_dir = os.path.join(phone_data_dir,yaowu)chengdu_list = os.listdir(yaowu_data_dir)for chengdu in chengdu_list:chengdu_data_dir = os.path.join(yaowu_data_dir,chengdu)for img in os.listdir(chengdu_data_dir):imgs.append(img)label = ' '.join([phone,yaowu,chengdu])labels.append(label)imgs_train, imgs_val, imgs_test, labels_train, labels_val, labels_test = split_data(imgs, labels, test_size=0.2, val_size=0.2, random_state=42)img_label_tuple_list = [(imgs_train,labels_train),(imgs_val,labels_val),(imgs_test,labels_test)]for (i,split) in enumerate(spilits):target_data_dir = os.path.join(data_dir,split)if not os.path.exists(target_data_dir):os.mkdir(target_data_dir)imgs_list,labels_list = img_label_tuple_list[i]for (img,label) in zip(imgs_list,labels_list):label_split = label.split(' ')source_img = os.path.join(data_dir,label_split[0],label_split[1],label_split[2],img)target_img_dir = os.path.join(target_data_dir,label_split[1]+"_"+label_split[2])if not os.path.exists(target_img_dir):os.mkdir(target_img_dir)target_img = os.path.join(target_img_dir,img)shutil.move(source_img,target_img)

定义数据加载方式

from mindspore.dataset import GeneratorDataset
import mindspore.dataset.vision as vision
import mindspore.dataset.transforms as transforms
from mindspore import dtype as mstype
# 注意没有使用Mindspore提供的ImageFloder进行加载，原因是调试任务中'.ipynb_checkpoints'缓存文件夹会被当作类文件夹进行识别，导致数据集加载错误
class Iterable:def __init__(self,data_path):self._data = []self._label = []self._error_list = []if data_path.endswith(('JPG','jpg','png','PNG')):# 用作推理，所以没有labelimage = Image.open(data_path)self._data.append(image)self._label.append(0)else:classes = os.listdir(data_path)if '.ipynb_checkpoints' in classes:classes.remove('.ipynb_checkpoints')for (i,class_name) in enumerate(classes):new_path =  data_path+"/"+class_namefor image_name in os.listdir(new_path):try:image = Image.open(new_path + "/" + image_name)self._data.append(image)self._label.append(i)except:passdef __getitem__(self, index):return self._data[index], self._label[index]def __len__(self):return len(self._data)def get_error_list(self,):return self._error_listdef create_dataset_zhongyao(dataset_dir,usage,resize,batch_size,workers):data = Iterable(dataset_dir)data_set = GeneratorDataset(data,column_names=['image','label'])trans = []if usage == "train":trans += [vision.RandomCrop(700, (4, 4, 4, 4)),# 这里随机裁剪尺度可以设置vision.RandomHorizontalFlip(prob=0.5)]trans += [vision.Resize((resize,resize)),vision.Rescale(1.0 / 255.0, 0.0),vision.Normalize([0.4914, 0.4822, 0.4465], [0.2023, 0.1994, 0.2010]),vision.HWC2CHW()]target_trans = transforms.TypeCast(mstype.int32)# 数据映射操作data_set = data_set.map(operations=trans,input_columns='image',num_parallel_workers=workers)data_set = data_set.map(operations=target_trans,input_columns='label',num_parallel_workers=workers)# 批量操作data_set = data_set.batch(batch_size,drop_remainder=True)return data_set

加载数据

import mindspore as ms
import random
data_dir = "dataset1/zhongyiyao"
train_dir = data_dir+"/"+"train"
valid_dir = data_dir+"/"+"valid"
test_dir = data_dir+"/"+"test"
batch_size = 32 # 批量大小
image_size = 224 # 训练图像空间大小
workers = 4 # 并行线程个数
num_classes = 12 # 分类数量# 设置随机种子，使得模型结果复现
seed = 42
ms.set_seed(seed)
np.random.seed(seed)
random.seed(seed)dataset_train = create_dataset_zhongyao(dataset_dir=train_dir,usage="train",resize=image_size,batch_size=batch_size,workers=workers)
step_size_train = dataset_train.get_dataset_size()dataset_val = create_dataset_zhongyao(dataset_dir=valid_dir,usage="valid",resize=image_size,batch_size=batch_size,workers=workers)
dataset_test = create_dataset_zhongyao(dataset_dir=test_dir,usage="test",resize=image_size,batch_size=batch_size,workers=workers)
step_size_val = dataset_val.get_dataset_size()print(f'训练集数据：{dataset_train.get_dataset_size()*batch_size}\n')
print(f'验证集数据：{dataset_val.get_dataset_size()*batch_size}\n')
print(f'测试集数据：{dataset_test.get_dataset_size()*batch_size}\n')

实现标签映射

#index_label的映射
index_label_dict = {}
classes = os.listdir(train_dir)
if '.ipynb_checkpoints' in classes:classes.remove('.ipynb_checkpoints')
for i,label in enumerate(classes):index_label_dict[i] = label
label2chin = {'ph_sp':'蒲黄-生品',  'ph_bj':'蒲黄-不及', 'ph_sz':'蒲黄-适中', 'ph_tg':'蒲黄-太过', 'sz_sp':'山楂-生品','sz_bj':'山楂-不及', 'sz_sz':'山楂-适中', 'sz_tg':'山楂-太过', 'wblx_sp':'王不留行-生品', 'wblx_bj':'王不留行-不及','wblx_sz':'王不留行-适中', 'wblx_tg':'王不留行-太过'}
index_label_dict

定义ResNet50

残差块定义

class ResidualBlock(nn.Cell):expansion = 4  # 最后一个卷积核的数量是第一个卷积核数量的4倍def __init__(self, in_channel: int, out_channel: int,stride: int = 1, down_sample: Optional[nn.Cell] = None) -> None:super(ResidualBlock, self).__init__()self.conv1 = nn.Conv2d(in_channel, out_channel,kernel_size=1, weight_init=weight_init)self.norm1 = nn.BatchNorm2d(out_channel)self.conv2 = nn.Conv2d(out_channel, out_channel,kernel_size=3, stride=stride,weight_init=weight_init)self.norm2 = nn.BatchNorm2d(out_channel)self.conv3 = nn.Conv2d(out_channel, out_channel * self.expansion,kernel_size=1, weight_init=weight_init)self.norm3 = nn.BatchNorm2d(out_channel * self.expansion)self.relu = nn.ReLU()self.down_sample = down_sampledef construct(self, x):identity = x  # shortscuts分支out = self.conv1(x)  # 主分支第一层：1*1卷积层out = self.norm1(out)out = self.relu(out)out = self.conv2(out)  # 主分支第二层：3*3卷积层out = self.norm2(out)out = self.relu(out)out = self.conv3(out)  # 主分支第三层：1*1卷积层out = self.norm3(out)if self.down_sample is not None:identity = self.down_sample(x)out += identity  # 输出为主分支与shortcuts之和out = self.relu(out)return out

创建残差块的函数 

def make_layer(last_out_channel, block: Type[Union[ResidualBlockBase, ResidualBlock]],channel: int, block_nums: int, stride: int = 1):down_sample = None  # shortcuts分支if stride != 1 or last_out_channel != channel * block.expansion:down_sample = nn.SequentialCell([nn.Conv2d(last_out_channel, channel * block.expansion,kernel_size=1, stride=stride, weight_init=weight_init),nn.BatchNorm2d(channel * block.expansion, gamma_init=gamma_init)])layers = []layers.append(block(last_out_channel, channel, stride=stride, down_sample=down_sample))in_channel = channel * block.expansion# 堆叠残差网络for _ in range(1, block_nums):layers.append(block(in_channel, channel))return nn.SequentialCell(layers)

ResNet定义

from mindspore import load_checkpoint, load_param_into_net
from mindspore import opsclass ResNet(nn.Cell):def __init__(self, block: Type[Union[ResidualBlockBase, ResidualBlock]],layer_nums: List[int], num_classes: int, input_channel: int) -> None:super(ResNet, self).__init__()self.relu = nn.ReLU()# 第一个卷积层，输入channel为3（彩色图像），输出channel为64self.conv1 = nn.Conv2d(3, 64, kernel_size=7, stride=2, weight_init=weight_init)self.norm = nn.BatchNorm2d(64)# 最大池化层，缩小图片的尺寸self.max_pool = nn.MaxPool2d(kernel_size=3, stride=2, pad_mode='same')# 各个残差网络结构块定义self.layer1 = make_layer(64, block, 64, layer_nums[0])self.layer2 = make_layer(64 * block.expansion, block, 128, layer_nums[1], stride=2)self.layer3 = make_layer(128 * block.expansion, block, 256, layer_nums[2], stride=2)self.layer4 = make_layer(256 * block.expansion, block, 512, layer_nums[3], stride=2)# 平均池化层self.avg_pool = ops.ReduceMean(keep_dims=True)# self.avg_pool = nn.AvgPool2d()# flattern层self.flatten = nn.Flatten()# 全连接层self.fc = nn.Dense(in_channels=input_channel, out_channels=num_classes)def construct(self, x):x = self.conv1(x)x = self.norm(x)x = self.relu(x)x = self.max_pool(x)x = self.layer1(x)x = self.layer2(x)x = self.layer3(x)x = self.layer4(x)x = self.avg_pool(x,(2,3))x = self.flatten(x)x = self.fc(x)return x

使用预训练的ResNet微调进行预测

def _resnet(model_url: str, block: Type[Union[ResidualBlockBase, ResidualBlock]],layers: List[int], num_classes: int, pretrained: bool, pretrained_ckpt: str,input_channel: int):model = ResNet(block, layers, num_classes, input_channel)if pretrained:# 加载预训练模型download(url=model_url, path=pretrained_ckpt)param_dict = load_checkpoint(pretrained_ckpt)load_param_into_net(model, param_dict)return modeldef resnet50(num_classes: int = 1000, pretrained: bool = False):resnet50_url = "https://obs.dualstack.cn-north-4.myhuaweicloud.com/mindspore-website/notebook/models/application/resnet50_224_new.ckpt"resnet50_ckpt = "./LoadPretrainedModel/resnet50_224_new.ckpt"return _resnet(resnet50_url, ResidualBlock, [3, 4, 6, 3], num_classes,pretrained, resnet50_ckpt, 2048)

network = resnet50(pretrained=True)
num_class = 12
# 全连接层输入层的大小
in_channel = network.fc.in_channels
fc = nn.Dense(in_channels=in_channel, out_channels=num_class)
# 重置全连接层
network.fc = fcfor param in network.get_parameters():param.requires_grad = True

模型训练与推理

num_epochs = 50
# early stopping
patience = 5
lr = nn.cosine_decay_lr(min_lr=0.00001, max_lr=0.001, total_step=step_size_train * num_epochs,step_per_epoch=step_size_train, decay_epoch=num_epochs)
opt = nn.Momentum(params=network.trainable_params(), learning_rate=lr, momentum=0.9)
loss_fn = nn.SoftmaxCrossEntropyWithLogits(sparse=True, reduction='mean')
model = ms.Model(network, loss_fn, opt, metrics={'acc'})# 最佳模型存储路径
best_acc = 0
best_ckpt_dir = "./BestCheckpoint"
best_ckpt_path = "./BestCheckpoint/resnet50-best.ckpt"def train_loop(model, dataset, loss_fn, optimizer):# Define forward functiondef forward_fn(data, label):logits = model(data)loss = loss_fn(logits, label)return loss, logits# Get gradient functiongrad_fn = ms.ops.value_and_grad(forward_fn, None, optimizer.parameters, has_aux=True)# Define function of one-step trainingdef train_step(data, label):(loss, _), grads = grad_fn(data, label)loss = ops.depend(loss, optimizer(grads))return losssize = dataset.get_dataset_size()model.set_train()for batch, (data, label) in enumerate(dataset.create_tuple_iterator()):loss = train_step(data, label)if batch % 100 == 0 or batch == step_size_train - 1:loss, current = loss.asnumpy(), batchprint(f"loss: {loss:>7f}  [{current:>3d}/{size:>3d}]")from sklearn.metrics import classification_reportdef test_loop(model, dataset, loss_fn):num_batches = dataset.get_dataset_size()model.set_train(False)total, test_loss, correct = 0, 0, 0y_true = []y_pred = []for data, label in dataset.create_tuple_iterator():y_true.extend(label.asnumpy().tolist())pred = model(data)total += len(data)test_loss += loss_fn(pred, label).asnumpy()y_pred.extend(pred.argmax(1).asnumpy().tolist())correct += (pred.argmax(1) == label).asnumpy().sum()test_loss /= num_batchescorrect /= totalprint(f"Test: \n Accuracy: {(100*correct):>0.1f}%, Avg loss: {test_loss:>8f} \n")print(classification_report(y_true,y_pred,target_names= list(index_label_dict.values()),digits=3))return correct,test_loss

开始训练 

no_improvement_count = 0
acc_list = []
loss_list = []
stop_epoch = num_epochs
for t in range(num_epochs):print(f"Epoch {t+1}\n-------------------------------")train_loop(network, dataset_train, loss_fn, opt)acc,loss = test_loop(network, dataset_val, loss_fn)acc_list.append(acc)loss_list.append(loss)if acc > best_acc:best_acc = accif not os.path.exists(best_ckpt_dir):os.mkdir(best_ckpt_dir)ms.save_checkpoint(network, best_ckpt_path)no_improvement_count = 0else:no_improvement_count += 1if no_improvement_count > patience:print('Early stopping triggered. Restoring best weights...')stop_epoch = tbreak 
print("Done!")

模型推理

import matplotlib.pyplot as pltnum_class = 12  # 
net = resnet50(num_class)
best_ckpt_path = 'BestCheckpoint/resnet50-best.ckpt'
# 加载模型参数
param_dict = ms.load_checkpoint(best_ckpt_path)
ms.load_param_into_net(net, param_dict)
model = ms.Model(net)
image_size = 224
workers = 1def visualize_model(dataset_test):# 加载验证集的数据进行验证data = next(dataset_test.create_tuple_iterator())# print(data)images = data[0].asnumpy()labels = data[1].asnumpy()# 预测图像类别output = model.predict(ms.Tensor(data[0]))pred = np.argmax(output.asnumpy(), axis=1)# 显示图像及图像的预测值plt.figure(figsize=(10, 6))for i in range(6):plt.subplot(2, 3, i+1)color = 'blue' if pred[i] == labels[i] else 'red'plt.title('predict:{}  actual:{}'.format(index_label_dict[pred[i]],index_label_dict[labels[i]]), color=color)picture_show = np.transpose(images[i], (1, 2, 0))mean = np.array([0.4914, 0.4822, 0.4465])std = np.array([0.2023, 0.1994, 0.2010])picture_show = std * picture_show + meanpicture_show = np.clip(picture_show, 0, 1)plt.imshow(picture_show)plt.axis('off')plt.show()visualize_model(dataset_val)

更多内容可参考MindSpore官方教程：

基于MindSpore实现ResNet50中药分拣_哔哩哔哩_bilibili