前言

• 深度卷积网络极大地推进深度学习各领域的发展，ILSVRC作为最具影响力的竞赛功不可没，促使了许多经典工作。我梳理了ILSVRC分类任务的各届冠军和亚军网络，简单介绍了它们的核心思想、网络架构及其实现。

  代码主要来自：https://github.com/weiaicunzai/pytorch-cifar100

• ImageNet和ILSVRC

• • ImageNet是一个超过15 million的图像数据集，大约有22,000类。

  • ILSVRC全称ImageNet Large-Scale Visual Recognition Challenge，从2010年开始举办到2017年最后一届，使用ImageNet数据集的一个子集，总共有1000类。

• 历届结果

年	网络/队名	val top-1	val top-5	test top-5	备注
2012	AlexNet	38.1%	16.4%	16.42%	5 CNNs
2012	AlexNet	36.7%	15.4%	15.32%	7CNNs。用了2011年的数据
2013	OverFeat			14.18%	7 fast models
2013	OverFeat			13.6%	赛后。7 big models
2013	ZFNet			13.51%	ZFNet论文上的结果是14.8
2013	Clarifai			11.74%
2013	Clarifai			11.20%	用了2011年的数据
2014	VGG			7.32%	7 nets, dense eval
2014	VGG（亚军）	23.7%	6.8%	6.8%	赛后。2 nets
2014	GoogleNet v1			6.67%	7 nets, 144 crops
	GoogleNet v2	20.1%	4.9%	4.82%	赛后。6 nets, 144 crops
	GoogleNet v3	17.2%	3.58%		赛后。4 nets, 144 crops
	GoogleNet v4	16.5%	3.1%	3.08%	赛后。v4+Inception-Res-v2
2015	ResNet			3.57%	6 models
2016	Trimps-Soushen			2.99%	公安三所
2016	ResNeXt（亚军）			3.03%	加州大学圣地亚哥分校
2017	SENet			2.25%	Momenta 与牛津大学

• 评价标准

  top1是指概率向量中最大的作为预测结果，若分类正确，则为正确；top5则只要概率向量中最大的前五名里有分类正确的，则为正确。

LeNet

Gradient-Based Learning Applied to Document Recognition

网络架构

import torch.nn as nn
import torch.nn.functional as func
class LeNet(nn.Module):def __init__(self):super(LeNet, self).__init__()self.conv1 = nn.Conv2d(1, 6, kernel_size=5)self.conv2 = nn.Conv2d(6, 16, kernel_size=5)self.fc1 = nn.Linear(16*16, 120)self.fc2 = nn.Linear(120, 84)self.fc3 = nn.Linear(84, 10)def forward(self, x):x = func.relu(self.conv1(x))x = func.max_pool2d(x, 2)x = func.relu(self.conv2(x))x = func.max_pool2d(x, 2)x = x.view(x.size(0), -1)x = func.relu(self.fc1(x))x = func.relu(self.fc2(x))x = self.fc3(x)return x

AlexNet

ImageNet Classification with Deep Convolutional Neural Networks

核心思想

• AlexNet相比前人有以下改进：

• 1.采用ReLU激活函数

  2.局部响应归一化LRN

  

  3.Overlapping Pooling

  4.引入Drop out

  5.数据增强

  6.多GPU并行

网络架构

• 代码实现

class AlexNet(nn.Module):def __init__(self, num_classes=NUM_CLASSES):super(AlexNet, self).__init__()self.features = nn.Sequential(nn.Conv2d(1, 96, kernel_size=11,padding=1),nn.ReLU(inplace=True),nn.MaxPool2d(kernel_size=2),nn.Conv2d(96, 256, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.MaxPool2d(kernel_size=2),nn.Conv2d(256, 384, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.Conv2d(384, 384, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.Conv2d(384, 256, kernel_size=3, padding=1),nn.ReLU(inplace=True),nn.MaxPool2d(kernel_size=2),)self.classifier = nn.Sequential(nn.Dropout(),nn.Linear(256 * 2 * 2, 4096),nn.ReLU(inplace=True),nn.Dropout(),nn.Linear(4096, 4096),nn.ReLU(inplace=True),nn.Linear(4096, 10),)def forward(self, x):x = self.features(x)x = x.view(x.size(0), 256 * 2 * 2)x = self.classifier(x)return x

实验结果

ZFNet

Visualizing and Understanding Convolutional Networks

核心思想

• 利用反卷积可视化CNN学到的特征。

• 1.Unpooling：池化操作不可逆，但通过记录池化最大值的位置可实现逆操作。

  2.Rectification：ReLU

  3.Filtering：使用原卷积核的转置版本。

网络架构

实验结果

• 特征可视化：Layer2响应角落和边缘、颜色连接；Layer3有更复杂的不变性，捕获相似纹理；Layer4展示了明显的变化，跟类别更相关；Layer5看到整个物体。

• 训练过程特征演化：低层特征较快收敛，高层到后面才开始变化。

• 特征不变性：小变换在模型第一层变化明显，但在顶层影响较小。网络输出对翻转和缩放是稳定的，但除了旋转对称性的物体，输出对旋转并不是不变的。

• 遮挡敏感性：当对象被遮挡，准确性会明显下降。

• ImageNet结果

VGG

Very Deep Convolutional Networks for Large-Scale Image Recognition

核心思想

• 重复使用3x3卷积和2x2池化增加网络深度。

网络架构

• VGG19表示有19层conv或fc，参数量较大。

• 代码实现

cfg = {'A' : [64,     'M', 128,      'M', 256, 256,           'M', 512, 512,           'M', 512, 512,           'M'],'B' : [64, 64, 'M', 128, 128, 'M', 256, 256,           'M', 512, 512,           'M', 512, 512,           'M'],'D' : [64, 64, 'M', 128, 128, 'M', 256, 256, 256,      'M', 512, 512, 512,      'M', 512, 512, 512,      'M'],'E' : [64, 64, 'M', 128, 128, 'M', 256, 256, 256, 256, 'M', 512, 512, 512, 512, 'M', 512, 512, 512, 512, 'M']
}def vgg19_bn():return VGG(make_layers(cfg['E'], batch_norm=True))class VGG(nn.Module):def __init__(self, features, num_class=100):super().__init__()self.features = featuresself.classifier = nn.Sequential(nn.Linear(512, 4096),nn.ReLU(inplace=True),nn.Dropout(),nn.Linear(4096, 4096),nn.ReLU(inplace=True),nn.Dropout(),nn.Linear(4096, num_class))def forward(self, x):output = self.features(x)output = output.view(output.size()[0], -1)output = self.classifier(output)return outputdef make_layers(cfg, batch_norm=False):layers = []input_channel = 3for l in cfg:if l == 'M':layers += [nn.MaxPool2d(kernel_size=2, stride=2)]continuelayers += [nn.Conv2d(input_channel, l, kernel_size=3, padding=1)]if batch_norm:layers += [nn.BatchNorm2d(l)]layers += [nn.ReLU(inplace=True)]input_channel = lreturn nn.Sequential(*layers)

实验结果

GoogLeNet(v1)

Going Deeper with Convolutions

核心思想

• 提出Inception模块，可在保持计算成本的同时增加网络的深度和宽度。

• 代码实现

class Inception(nn.Module):def __init__(self, input_channels, n1x1, n3x3_reduce, n3x3, n5x5_reduce, n5x5, pool_proj):super().__init__()#1x1conv branchself.b1 = nn.Sequential(nn.Conv2d(input_channels, n1x1, kernel_size=1),nn.BatchNorm2d(n1x1),nn.ReLU(inplace=True))#1x1conv -> 3x3conv branchself.b2 = nn.Sequential(nn.Conv2d(input_channels, n3x3_reduce, kernel_size=1),nn.BatchNorm2d(n3x3_reduce),nn.ReLU(inplace=True),nn.Conv2d(n3x3_reduce, n3x3, kernel_size=3, padding=1),nn.BatchNorm2d(n3x3),nn.ReLU(inplace=True))#1x1conv -> 5x5conv branch#we use 2 3x3 conv filters stacked instead#of 1 5x5 filters to obtain the same receptive#field with fewer parametersself.b3 = nn.Sequential(nn.Conv2d(input_channels, n5x5_reduce, kernel_size=1),nn.BatchNorm2d(n5x5_reduce),nn.ReLU(inplace=True),nn.Conv2d(n5x5_reduce, n5x5, kernel_size=3, padding=1),nn.BatchNorm2d(n5x5, n5x5),nn.ReLU(inplace=True),nn.Conv2d(n5x5, n5x5, kernel_size=3, padding=1),nn.BatchNorm2d(n5x5),nn.ReLU(inplace=True))#3x3pooling -> 1x1conv#same convself.b4 = nn.Sequential(nn.MaxPool2d(3, stride=1, padding=1),nn.Conv2d(input_channels, pool_proj, kernel_size=1),nn.BatchNorm2d(pool_proj),nn.ReLU(inplace=True))def forward(self, x):return torch.cat([self.b1(x), self.b2(x), self.b3(x), self.b4(x)], dim=1)

网络架构

• 代码实现

def googlenet():return GoogleNet()class GoogleNet(nn.Module):def __init__(self, num_class=100):super().__init__()self.prelayer = nn.Sequential(nn.Conv2d(3, 192, kernel_size=3, padding=1),nn.BatchNorm2d(192),nn.ReLU(inplace=True))#although we only use 1 conv layer as prelayer,#we still use name a3, b3.......self.a3 = Inception(192, 64, 96, 128, 16, 32, 32)self.b3 = Inception(256, 128, 128, 192, 32, 96, 64)#"""In general, an Inception network is a network consisting of#modules of the above type stacked upon each other, with occasional#max-pooling layers with stride 2 to halve the resolution of the#grid"""self.maxpool = nn.MaxPool2d(3, stride=2, padding=1)self.a4 = Inception(480, 192, 96, 208, 16, 48, 64)self.b4 = Inception(512, 160, 112, 224, 24, 64, 64)self.c4 = Inception(512, 128, 128, 256, 24, 64, 64)self.d4 = Inception(512, 112, 144, 288, 32, 64, 64)self.e4 = Inception(528, 256, 160, 320, 32, 128, 128)self.a5 = Inception(832, 256, 160, 320, 32, 128, 128)self.b5 = Inception(832, 384, 192, 384, 48, 128, 128)#input feature size: 8*8*1024self.avgpool = nn.AdaptiveAvgPool2d((1, 1))self.dropout = nn.Dropout2d(p=0.4)self.linear = nn.Linear(1024, num_class)def forward(self, x):output = self.prelayer(x)output = self.a3(output)output = self.b3(output)output = self.maxpool(output)output = self.a4(output)output = self.b4(output)output = self.c4(output)output = self.d4(output)output = self.e4(output)output = self.maxpool(output)output = self.a5(output)output = self.b5(output)#"""It was found that a move from fully connected layers to#average pooling improved the top-1 accuracy by about 0.6%,#however the use of dropout remained essential even after#removing the fully connected layers."""output = self.avgpool(output)output = self.dropout(output)output = output.view(output.size()[0], -1)output = self.linear(output)return output

实验结果

ResNet

Deep Residual Learning for Image Recognition

核心思想

• 为了解决深层网络难以训练的问题，提出了残差模块和深度残差网络

• 1.假设网络输入是x，经学习的输出是F(x)，最终拟合目标是H(x)。

  2.深层网络相比浅层网络有一些层是多余的，若让多余层学习恒等变换H(x)=x，那么网络性能不该比浅层网络要差。

  3.传统网络训练目标H(x)=F(x)，残差网络训练目标H(x)=F(x)+x。

  4.为了学习恒等变换，传统网络要求网络学习F(x)=H(x)=x，残差网络只需学习   F(x)=H(x)−x=x−x=0。残差学习之所以有效是因为让网络学习F(x)=0比学习F(x)=x要容易。

• bottleneck

• 代码实现

class BottleNeck(nn.Module):"""Residual block for resnet over 50 layers"""expansion = 4def __init__(self, in_channels, out_channels, stride=1):super().__init__()self.residual_function = nn.Sequential(nn.Conv2d(in_channels, out_channels, kernel_size=1, bias=False),nn.BatchNorm2d(out_channels),nn.ReLU(inplace=True),nn.Conv2d(out_channels, out_channels, stride=stride, kernel_size=3, padding=1, bias=False),nn.BatchNorm2d(out_channels),nn.ReLU(inplace=True),nn.Conv2d(out_channels, out_channels * BottleNeck.expansion, kernel_size=1, bias=False),nn.BatchNorm2d(out_channels * BottleNeck.expansion),)self.shortcut = nn.Sequential()if stride != 1 or in_channels != out_channels * BottleNeck.expansion:self.shortcut = nn.Sequential(nn.Conv2d(in_channels, out_channels * BottleNeck.expansion, stride=stride, kernel_size=1, bias=False),nn.BatchNorm2d(out_channels * BottleNeck.expansion))def forward(self, x):return nn.ReLU(inplace=True)(self.residual_function(x) + self.shortcut(x))

网络架构

• 代码实现

def resnet152():""" return a ResNet 152 object"""return ResNet(BottleNeck, [3, 8, 36, 3])class ResNet(nn.Module):def __init__(self, block, num_block, num_classes=100):super().__init__()self.in_channels = 64self.conv1 = nn.Sequential(nn.Conv2d(3, 64, kernel_size=3, padding=1, bias=False),nn.BatchNorm2d(64),nn.ReLU(inplace=True))#we use a different inputsize than the original paper#so conv2_x's stride is 1self.conv2_x = self._make_layer(block, 64, num_block[0], 1)self.conv3_x = self._make_layer(block, 128, num_block[1], 2)self.conv4_x = self._make_layer(block, 256, num_block[2], 2)self.conv5_x = self._make_layer(block, 512, num_block[3], 2)self.avg_pool = nn.AdaptiveAvgPool2d((1, 1))self.fc = nn.Linear(512 * block.expansion, num_classes)def _make_layer(self, block, out_channels, num_blocks, stride):"""make resnet layers(by layer i didnt mean this 'layer' was thesame as a neuron netowork layer, ex. conv layer), one layer maycontain more than one residual blockArgs:block: block type, basic block or bottle neck blockout_channels: output depth channel number of this layernum_blocks: how many blocks per layerstride: the stride of the first block of this layerReturn:return a resnet layer"""# we have num_block blocks per layer, the first block# could be 1 or 2, other blocks would always be 1strides = [stride] + [1] * (num_blocks - 1)layers = []for stride in strides:layers.append(block(self.in_channels, out_channels, stride))self.in_channels = out_channels * block.expansionreturn nn.Sequential(*layers)def forward(self, x):output = self.conv1(x)output = self.conv2_x(output)output = self.conv3_x(output)output = self.conv4_x(output)output = self.conv5_x(output)output = self.avg_pool(output)output = output.view(output.size(0), -1)output = self.fc(output)return output

实验结果

ResNeXt

Aggregated Residual Transformations for Deep Neural Networks

核心思想

• 通过重复构建block来聚合一组相同拓扑结构的特征，并提出一个新维度”cardinality“。

• ResNeXt结合了VGG、ResNet重复堆叠模块和Inception的split-transform-merge的思想。

以下三者等价，文章采用第三种实现，其使用了组卷积。

• 代码实现

CARDINALITY = 32
DEPTH = 4
BASEWIDTH = 64class ResNextBottleNeckC(nn.Module):def __init__(self, in_channels, out_channels, stride):super().__init__()C = CARDINALITY #How many groups a feature map was splitted into#"""We note that the input/output width of the template is fixed as#256-d (Fig. 3), We note that the input/output width of the template#is fixed as 256-d (Fig. 3), and all widths are dou- bled each time#when the feature map is subsampled (see Table 1)."""D = int(DEPTH * out_channels / BASEWIDTH) #number of channels per groupself.split_transforms = nn.Sequential(nn.Conv2d(in_channels, C * D, kernel_size=1, groups=C, bias=False),nn.BatchNorm2d(C * D),nn.ReLU(inplace=True),nn.Conv2d(C * D, C * D, kernel_size=3, stride=stride, groups=C, padding=1, bias=False),nn.BatchNorm2d(C * D),nn.ReLU(inplace=True),nn.Conv2d(C * D, out_channels * 4, kernel_size=1, bias=False),nn.BatchNorm2d(out_channels * 4),)self.shortcut = nn.Sequential()if stride != 1 or in_channels != out_channels * 4:self.shortcut = nn.Sequential(nn.Conv2d(in_channels, out_channels * 4, stride=stride, kernel_size=1, bias=False),nn.BatchNorm2d(out_channels * 4))def forward(self, x):return F.relu(self.split_transforms(x) + self.shortcut(x))

网络架构

• 代码实现

  以下部分跟ResNet基本一致，重点关注ResNextBottleNeckC的实现。

def resnext50():""" return a resnext50(c32x4d) network"""return ResNext(ResNextBottleNeckC, [3, 4, 6, 3])class ResNext(nn.Module):def __init__(self, block, num_blocks, class_names=100):super().__init__()self.in_channels = 64self.conv1 = nn.Sequential(nn.Conv2d(3, 64, 3, stride=1, padding=1, bias=False),nn.BatchNorm2d(64),nn.ReLU(inplace=True))self.conv2 = self._make_layer(block, num_blocks[0], 64, 1)self.conv3 = self._make_layer(block, num_blocks[1], 128, 2)self.conv4 = self._make_layer(block, num_blocks[2], 256, 2)self.conv5 = self._make_layer(block, num_blocks[3], 512, 2)self.avg = nn.AdaptiveAvgPool2d((1, 1))self.fc = nn.Linear(512 * 4, 100)def forward(self, x):x = self.conv1(x)x = self.conv2(x)x = self.conv3(x)x = self.conv4(x)x = self.conv5(x)x = self.avg(x)x = x.view(x.size(0), -1)x = self.fc(x)return xdef _make_layer(self, block, num_block, out_channels, stride):"""Building resnext blockArgs:block: block type(default resnext bottleneck c)num_block: number of blocks per layerout_channels: output channels per blockstride: block strideReturns:a resnext layer"""strides = [stride] + [1] * (num_block - 1)layers = []for stride in strides:layers.append(block(self.in_channels, out_channels, stride))self.in_channels = out_channels * 4return nn.Sequential(*layers)

实验结果

SENet

Squeeze-and-Excitation Networks

核心思想

• 卷积操作融合了空间和特征通道信息。大量工作研究了空间部分，而本文重点关注特征通道的关系，并提出了Squeeze-and-Excitation(SE)block，对通道间的依赖关系进行建模，自适应校准通道方面的特征响应。

• SE block

  表示transformation（一系列卷积操作）；表示squeeze，产生通道描述；表示excitation，通过参数WW来建模通道的重要性。表示reweight，将excitation输出的权重逐乘以先前特征，完成特征重标定。

• SE-ResNet Module

  

• 代码实现

class BottleneckResidualSEBlock(nn.Module):expansion = 4def __init__(self, in_channels, out_channels, stride, r=16):super().__init__()self.residual = nn.Sequential(nn.Conv2d(in_channels, out_channels, 1),nn.BatchNorm2d(out_channels),nn.ReLU(inplace=True),nn.Conv2d(out_channels, out_channels, 3, stride=stride, padding=1),nn.BatchNorm2d(out_channels),nn.ReLU(inplace=True),nn.Conv2d(out_channels, out_channels * self.expansion, 1),nn.BatchNorm2d(out_channels * self.expansion),nn.ReLU(inplace=True))self.squeeze = nn.AdaptiveAvgPool2d(1)self.excitation = nn.Sequential(nn.Linear(out_channels * self.expansion, out_channels * self.expansion // r),nn.ReLU(inplace=True),nn.Linear(out_channels * self.expansion // r, out_channels * self.expansion),nn.Sigmoid())self.shortcut = nn.Sequential()if stride != 1 or in_channels != out_channels * self.expansion:self.shortcut = nn.Sequential(nn.Conv2d(in_channels, out_channels * self.expansion, 1, stride=stride),nn.BatchNorm2d(out_channels * self.expansion))def forward(self, x):shortcut = self.shortcut(x)residual = self.residual(x)squeeze = self.squeeze(residual)squeeze = squeeze.view(squeeze.size(0), -1)excitation = self.excitation(squeeze)excitation = excitation.view(residual.size(0), residual.size(1), 1, 1)x = residual * excitation.expand_as(residual) + shortcutreturn F.relu(x)

网络架构

• 代码实现

def seresnet50():return SEResNet(BottleneckResidualSEBlock, [3, 4, 6, 3])class SEResNet(nn.Module):def __init__(self, block, block_num, class_num=100):super().__init__()self.in_channels = 64self.pre = nn.Sequential(nn.Conv2d(3, 64, 3, padding=1),nn.BatchNorm2d(64),nn.ReLU(inplace=True))self.stage1 = self._make_stage(block, block_num[0], 64, 1)self.stage2 = self._make_stage(block, block_num[1], 128, 2)self.stage3 = self._make_stage(block, block_num[2], 256, 2)self.stage4 = self._make_stage(block, block_num[3], 516, 2)self.linear = nn.Linear(self.in_channels, class_num)def forward(self, x):x = self.pre(x)x = self.stage1(x)x = self.stage2(x)x = self.stage3(x)x = self.stage4(x)x = F.adaptive_avg_pool2d(x, 1)x = x.view(x.size(0), -1)x = self.linear(x)return xdef _make_stage(self, block, num, out_channels, stride):layers = []layers.append(block(self.in_channels, out_channels, stride))self.in_channels = out_channels * block.expansionwhile num - 1:layers.append(block(self.in_channels, out_channels, 1))num -= 1return nn.Sequential(*layers)

实验结果

总结

一、小结

1.LeNet[1998]：CNN的鼻祖。

2.AlexNet[2012]：第一个深度CNN。

3.ZFNet[2012]：通过DeconvNet可视化CNN学习到的特征。

4.VGG[2014]：重复堆叠3x3卷积增加网络深度。

5.GoogLeNet[2014]：提出Inception模块，在控制参数和计算量的前提下，增加网络的深度与宽度。

6.ResNet[2015]：提出残差网络，解决了深层网络的优化问题。

7.ResNeXt[2016]：ResNet和Inception的结合体，Inception中每个分支结构相同，无需人为设计。

8.SENet[2017]：提出SE block，关注特征的通道关系。

二、经典模型中结构、参数对比

参考

• paper

[1]LeCun Y, Bottou L, Bengio Y, et al. Gradient-based learning applied to document recognition[J]. Proceedings of the IEEE, 1998, 86(11): 2278-2324.

[2]Krizhevsky A, Sutskever I, Hinton G E. Imagenet classification with deep convolutional neural networks[C]//Advances in neural information processing systems. 2012: 1097-1105.

[3]Zeiler M D, Fergus R. Visualizing and understanding convolutional networks[C]//European conference on computer vision. springer, Cham, 2014: 818-833.

[4]Simonyan K, Zisserman A. Very deep convolutional networks for large-scale image recognition[J]. arXiv preprint arXiv:1409.1556, 2014.

[5]Szegedy C, Liu W, Jia Y, et al. Going deeper with convolutions[C]//Proceedings of the IEEE conference on computer vision and pattern recognition. 2015: 1-9.

[6]He K, Zhang X, Ren S, et al. Deep residual learning for image recognition[C]//Proceedings of the IEEE conference on computer vision and pattern recognition. 2016: 770-778.

[7]Xie S, Girshick R, Dollár P, et al. Aggregated residual transformations for deep neural networks[C]//Proceedings of the IEEE conference on computer vision and pattern recognition. 2017: 1492-1500.

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ImageNet历年冠军和相关CNN模型

残差网络ResNet笔记

(二)计算机视觉四大基本任务(分类、定位、检测、分割)

论文笔记：CNN经典结构2（WideResNet，FractalNet，DenseNet，ResNeXt，DPN，SENet）

论文笔记：CNN经典结构1（AlexNet，ZFNet，OverFeat，VGG，GoogleNet，ResNet）

深度学习在计算机视觉领域（包括图像，视频，3-D点云，深度图）的应用一览

（本文出自平台合作作者Vincent，cs硕士在读。）

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