目录
1. 前言
什么是注意力机制？
注意力机制（Attention Mechanism）是机器学习中的一种数据处理方法，广泛应用在自然语言处理、图像识别及语音识别等各种不同类型的机器学习任务中。
通俗来讲：注意力机制就是希望网络能够自动学出来图片或者文字序列中的需要注意的地方。比如人眼在看一幅画的时候，不会将注意力平等地分配给画中的所有像素，而是将更多注意力分配给人们关注的地方。
从实现的角度来讲：注意力机制通过神经网络的操作生成一个掩码mask, mask上的值一个打分，评价当前需要关注的点的评分。
注意力机制可以分为：
通道注意力机制：对通道生成掩码mask，进行打分，代表是senet, Channel Attention Module
空间注意力机制：对空间进行掩码的生成，进行打分，代表是Spatial Attention Module
混合域注意力机制：同时对通道注意力和空间注意力进行评价打分，代表的有BAM, CBAM
2.论文摘要
ECCV2018论文地址： https://arxiv.org/abs/1807.06521
文章提出了卷积注意力模块（CBAM -- Convolutional Block Attention Module ），这是一种用于前馈卷积神经网络的简单而有效的注意力模块。 给定一个中间特征图，CBAM模块会沿着两个独立的维度（通道和空间）依次推断注意力图，然后将注意力图与输入特征图相乘以进行自适应特征优化。 由于CBAM是轻量级的通用模块，因此可以忽略的该模块的开销而将其无缝集成到任何CNN架构中，并且可以与基础CNN一起进行端到端训练。 本文通过在ImageNet-1K，MS COCO检测和VOC 2007检测数据集上进行的广泛实验来验证CBAM。 实验表明，使用该模块在各种模型上，并在分类和检测性能方面的持续改进，证明了CBAM的广泛适用性。
在该论文中，作者研究了网络架构中的注意力，注意力不仅要告诉我们重点关注哪里，还要提高关注点的表示。 目标是通过使用注意机制来增加表现力，关注重要特征并抑制不必要的特征。为了强调空间和通道这两个维度上的有意义特征，作者依次应用通道和空间注意模块，来分别在通道和空间维度上学习关注什么、在哪里关注。此外，通过了解要强调或抑制的信息也有助于网络内的信息流动。
主要网络架构也很简单，一个是通道注意力模块，另一个是空间注意力模块，CBAM就是先后集成了通道注意力模块和空间注意力模块。
Convolutional Block Attention Module (CBAM) 表示卷积模块的注意力机制模块，是一种结合了空间（spatial）和通道（channel）的注意力机制模块。相比于senet只关注通道（channel）的注意力机制可以取得更好的效果。
3.通道注意力机制（Channel Attention Module）
通道注意力机制是将特征图在空间维度上进行压缩，得到一个一维矢量后再进行操作。在空间维度上进行压缩时，不仅考虑到了平均值池化（Average Pooling）还考虑了最大值池化（Max Pooling）。平均池化和最大池化可用来聚合特征映射的空间信息，送到一个共享网络，压缩输入特征图的空间维数，逐元素求和合并，以产生通道注意力图。单就一张图来说，通道注意力，关注的是这张图上哪些内容是有重要作用的。平均值池化对特征图上的每一个像素点都有反馈，而最大值池化在进行梯度反向传播计算时，只有特征图中响应最大的地方有梯度的反馈。通道注意力机制可以表达为：
代码实现：
class ChannelAttention(nn.Module):
def __init__(self, in_planes, ratio=16):
super(ChannelAttention, self).__init__()
self.avg_pool = nn.AdaptiveAvgPool2d(1)
self.max_pool = nn.AdaptiveMaxPool2d(1)
self.fc = nn.Sequential(nn.Conv2d(in_planes, in_planes // 16, 1, bias=False),
nn.ReLU(),
nn.Conv2d(in_planes // 16, in_planes, 1, bias=False))
self.sigmoid = nn.Sigmoid()
def forward(self, x):
avg_out = self.fc(self.avg_pool(x))
max_out = self.fc(self.max_pool(x))
out = avg_out + max_out
return self.sigmoid(out)
应用代码：
out = self.conv2(out)
out = self.bn2(out) #[1,64,56,56]
ca_tmp = self.ca(out) #[1,64,1,1]
out = self.ca(out) * out #[1,64,56,56]
4.空间注意力机制（Spatial Attention Module）
空间注意力机制是对通道进行压缩，在通道维度分别进行了平均值池化和最大值池化。MaxPool的操作就是在通道上提取最大值，提取的次数是高乘以宽；AvgPool的操作就是在通道上提取平均值，提取的次数也是是高乘以宽；接着将前面所提取到的特征图（通道数都为1）合并得到一个2通道的特征图。
其中， 
为sigmoid操作，77表示卷积核的大小，77的卷积核比3*3的卷积核效果更好。
代码实现：
class SpatialAttention(nn.Module):
def __init__(self, kernel_size=7):
super(SpatialAttention, self).__init__()
self.conv1 = nn.Conv2d(2, 1, kernel_size, padding=kernel_size//2, bias=False)
self.sigmoid = nn.Sigmoid()
def forward(self, x):
avg_out = torch.mean(x, dim=1, keepdim=True)
max_out, _ = torch.max(x, dim=1, keepdim=True)
x = torch.cat([avg_out, max_out], dim=1)
x = self.conv1(x)
return self.sigmoid(x)
应用代码：
out = self.conv2(out)
out = self.bn2(out) #[1,64,56,56]
sa_tmp = self.sa(out) #[1,1,56,56]
out = self.sa(out) * out #[1,64,56,56]
通道注意力和空间注意力这两个模块能够以并行或者顺序的方式组合在一块儿，可是做者发现顺序组合而且将通道注意力放在前面能够取得更好的效果。
5.CBAM与ResNet网络结构组合
6.可视化效果图
最后，是使用Grad-cam进行了可视化，以来证明CBAM是真正地提取出了积极有效的特征。
7.代码resnet_cbam.py
最关键的部分：
class BasicBlock(nn.Module):
expansion = 1
def __init__(self, inplanes, planes, stride=1, downsample=None):
super(BasicBlock, self).__init__()
self.conv1 = conv3x3(inplanes, planes, stride)
self.bn1 = nn.BatchNorm2d(planes)
self.relu = nn.ReLU(inplace=True)
self.conv2 = conv3x3(planes, planes)
self.bn2 = nn.BatchNorm2d(planes)
self.ca = ChannelAttention(planes)
self.sa = SpatialAttention()
self.downsample = downsample
self.stride = stride
def forward(self, x):
residual = x
out = self.conv1(x)
out = self.bn1(out)
out = self.relu(out)
out = self.conv2(out)
out = self.bn2(out) #[1,64,56,56]
ca_tmp = self.ca(out) #[1,64,1,1]
sa_tmp = self.sa(out) #[1,1,56,56]
out = self.ca(out) * out #[1,64,56,56]
out = self.sa(out) * out #[1,64,56,56]
if self.downsample is not None:
residual = self.downsample(x)
out += residual
out = self.relu(out)
return out
resnet_cbam.py
import torch
import torch.nn as nn
import math
import torch.utils.model_zoo as model_zoo
__all__ = ['ResNet', 'resnet18_cbam', 'resnet34_cbam', 'resnet50_cbam', 'resnet101_cbam',
'resnet152_cbam']
model_urls = {
'resnet18': 'https://download.pytorch.org/models/resnet18-5c106cde.pth',
'resnet34': 'https://download.pytorch.org/models/resnet34-333f7ec4.pth',
'resnet50': 'https://download.pytorch.org/models/resnet50-19c8e357.pth',
'resnet101': 'https://download.pytorch.org/models/resnet101-5d3b4d8f.pth',
'resnet152': 'https://download.pytorch.org/models/resnet152-b121ed2d.pth',
}
def conv3x3(in_planes, out_planes, stride=1):
"3x3 convolution with padding"
return nn.Conv2d(in_planes, out_planes, kernel_size=3, stride=stride,
padding=1, bias=False)
class ChannelAttention(nn.Module):
def __init__(self, in_planes, ratio=16):
super(ChannelAttention, self).__init__()
self.avg_pool = nn.AdaptiveAvgPool2d(1)
self.max_pool = nn.AdaptiveMaxPool2d(1)
self.fc = nn.Sequential(nn.Conv2d(in_planes, in_planes // 16, 1, bias=False),
nn.ReLU(),
nn.Conv2d(in_planes // 16, in_planes, 1, bias=False))
self.sigmoid = nn.Sigmoid()
def forward(self, x):
avg_out = self.fc(self.avg_pool(x))
max_out = self.fc(self.max_pool(x))
out = avg_out + max_out
return self.sigmoid(out)
class SpatialAttention(nn.Module):
def __init__(self, kernel_size=7):
super(SpatialAttention, self).__init__()
self.conv1 = nn.Conv2d(2, 1, kernel_size, padding=kernel_size//2, bias=False)
self.sigmoid = nn.Sigmoid()
def forward(self, x):
avg_out = torch.mean(x, dim=1, keepdim=True)
max_out, _ = torch.max(x, dim=1, keepdim=True)
x = torch.cat([avg_out, max_out], dim=1)
x = self.conv1(x)
return self.sigmoid(x)
class BasicBlock(nn.Module):
expansion = 1
def __init__(self, inplanes, planes, stride=1, downsample=None):
super(BasicBlock, self).__init__()
self.conv1 = conv3x3(inplanes, planes, stride)
self.bn1 = nn.BatchNorm2d(planes)
self.relu = nn.ReLU(inplace=True)
self.conv2 = conv3x3(planes, planes)
self.bn2 = nn.BatchNorm2d(planes)
self.ca = ChannelAttention(planes)
self.sa = SpatialAttention()
self.downsample = downsample
self.stride = stride
def forward(self, x):
residual = x
out = self.conv1(x)
out = self.bn1(out)
out = self.relu(out)
out = self.conv2(out)
out = self.bn2(out) #[1,64,56,56]
ca_tmp = self.ca(out) #[1,64,1,1]
sa_tmp = self.sa(out) #[1,1,56,56]
out = self.ca(out) * out #[1,64,56,56]
out = self.sa(out) * out #[1,64,56,56]
if self.downsample is not None:
residual = self.downsample(x)
out += residual
out = self.relu(out)
return out
class Bottleneck(nn.Module):
expansion = 4
def __init__(self, inplanes, planes, stride=1, downsample=None):
super(Bottleneck, self).__init__()
self.conv1 = nn.Conv2d(inplanes, planes, kernel_size=1, bias=False)
self.bn1 = nn.BatchNorm2d(planes)
self.conv2 = nn.Conv2d(planes, planes, kernel_size=3, stride=stride,
padding=1, bias=False)
self.bn2 = nn.BatchNorm2d(planes)
self.conv3 = nn.Conv2d(planes, planes * 4, kernel_size=1, bias=False)
self.bn3 = nn.BatchNorm2d(planes * 4)
self.relu = nn.ReLU(inplace=True)
self.ca = ChannelAttention(planes * 4)
self.sa = SpatialAttention()
self.downsample = downsample
self.stride = stride
def forward(self, x):
residual = x
out = self.conv1(x)
out = self.bn1(out)
out = self.relu(out)
out = self.conv2(out)
out = self.bn2(out)
out = self.relu(out)
out = self.conv3(out)
out = self.bn3(out)
out = self.ca(out) * out
out = self.sa(out) * out
if self.downsample is not None:
residual = self.downsample(x)
out += residual
out = self.relu(out)
return out
class ResNet(nn.Module):
def __init__(self, block, layers, num_classes=1000):
self.inplanes = 64
super(ResNet, self).__init__()
self.conv1 = nn.Conv2d(3, 64, kernel_size=7, stride=2, padding=3,
bias=False)
self.bn1 = nn.BatchNorm2d(64)
self.relu = nn.ReLU(inplace=True)
self.maxpool = nn.MaxPool2d(kernel_size=3, stride=2, padding=1)
self.layer1 = self._make_layer(block, 64, layers[0])
self.layer2 = self._make_layer(block, 128, layers[1], stride=2)
self.layer3 = self._make_layer(block, 256, layers[2], stride=2)
self.layer4 = self._make_layer(block, 512, layers[3], stride=2)
self.avgpool = nn.AdaptiveAvgPool2d((1, 1))
self.fc = nn.Linear(512 * block.expansion, num_classes)
for m in self.modules():
if isinstance(m, nn.Conv2d):
n = m.kernel_size[0] * m.kernel_size[1] * m.out_channels
m.weight.data.normal_(0, math.sqrt(2. / n))
elif isinstance(m, nn.BatchNorm2d):
m.weight.data.fill_(1)
m.bias.data.zero_()
def _make_layer(self, block, planes, blocks, stride=1):
downsample = None
if stride != 1 or self.inplanes != planes * block.expansion:
downsample = nn.Sequential(
nn.Conv2d(self.inplanes, planes * block.expansion,
kernel_size=1, stride=stride, bias=False),
nn.BatchNorm2d(planes * block.expansion),
)
layers = []
layers.append(block(self.inplanes, planes, stride, downsample))
self.inplanes = planes * block.expansion
for i in range(1, blocks):
layers.append(block(self.inplanes, planes))
return nn.Sequential(*layers)
def forward(self, x):
x = self.conv1(x)
x = self.bn1(x)
x = self.relu(x)
x = self.maxpool(x)
x = self.layer1(x)
x = self.layer2(x)
x = self.layer3(x)
x = self.layer4(x)
x = self.avgpool(x)
x = x.view(x.size(0), -1)
x = self.fc(x)
return x
def resnet18_cbam(pretrained=False, **kwargs):
"""Constructs a ResNet-18 model.
Args:
pretrained (bool): If True, returns a model pre-trained on ImageNet
"""
model = ResNet(BasicBlock, [2, 2, 2, 2], **kwargs)
if pretrained:
pretrained_state_dict = model_zoo.load_url(model_urls['resnet18'])
now_state_dict = model.state_dict()
now_state_dict.update(pretrained_state_dict)
model.load_state_dict(now_state_dict)
return model
def resnet34_cbam(pretrained=False, **kwargs):
"""Constructs a ResNet-34 model.
Args:
pretrained (bool): If True, returns a model pre-trained on ImageNet
"""
model = ResNet(BasicBlock, [3, 4, 6, 3], **kwargs)
if pretrained:
pretrained_state_dict = model_zoo.load_url(model_urls['resnet34'])
now_state_dict = model.state_dict()
now_state_dict.update(pretrained_state_dict)
model.load_state_dict(now_state_dict)
return model
def resnet50_cbam(pretrained=False, **kwargs):
"""Constructs a ResNet-50 model.
Args:
pretrained (bool): If True, returns a model pre-trained on ImageNet
"""
model = ResNet(Bottleneck, [3, 4, 6, 3], **kwargs)
if pretrained:
pretrained_state_dict = model_zoo.load_url(model_urls['resnet50'])
now_state_dict = model.state_dict()
now_state_dict.update(pretrained_state_dict)
model.load_state_dict(now_state_dict)
return model
def resnet101_cbam(pretrained=False, **kwargs):
"""Constructs a ResNet-101 model.
Args:
pretrained (bool): If True, returns a model pre-trained on ImageNet
"""
model = ResNet(Bottleneck, [3, 4, 23, 3], **kwargs)
if pretrained:
pretrained_state_dict = model_zoo.load_url(model_urls['resnet101'])
now_state_dict = model.state_dict()
now_state_dict.update(pretrained_state_dict)
model.load_state_dict(now_state_dict)
return model
def resnet152_cbam(pretrained=False, **kwargs):
"""Constructs a ResNet-152 model.
Args:
pretrained (bool): If True, returns a model pre-trained on ImageNet
"""
model = ResNet(Bottleneck, [3, 8, 36, 3], **kwargs)
if pretrained:
pretrained_state_dict = model_zoo.load_url(model_urls['resnet152'])
now_state_dict = model.state_dict()
now_state_dict.update(pretrained_state_dict)
model.load_state_dict(now_state_dict)
return model
if __name__ == '__main__':
net = resnet18_cbam()
input = torch.randn(1,3,224,224)
out = net(input)
print(out.shape)
a = 0