'''
FPN in PyTorch.
See the paper "Feature Pyramid Networks for Object Detection" for more details.
'''
import torch
import torch.nn as nn
import torch.nn.functional as F
from torch.autograd import Variable
# 4. block == Bottlenect (for Bottom-up)
class Bottleneck(nn.Module):
expansion = 4
# layer1 : (64, 256), (64, 64) , (1 , 1) // layer2 : (256, 512), (128, 128) , (2, 1) // layer3 : (512, 1024), (256, 256) , (2, 1)
def __init__(self, in_planes, planes, stride=1):
super(Bottleneck, self).__init__()
self.conv1 = nn.Conv2d(in_planes, 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, self.expansion*planes, kernel_size=1, bias=False)
self.bn3 = nn.BatchNorm2d(self.expansion*planes)
# 5. shortcut 정의 하기 = Resnet 을 한다.
self.shortcut = nn.Sequential()
if stride != 1 or in_planes != self.expansion*planes: # layer1,2,3,4의 for 2개에서 모두 shortcut이 이루어진다.
self.shortcut = nn.Sequential(
nn.Conv2d(in_planes, self.expansion*planes, kernel_size=1, stride=stride, bias=False),
nn.BatchNorm2d(self.expansion*planes)
)
def forward(self, x):
out = F.relu(self.bn1(self.conv1(x))) # x's channel-> layer1 : 64 // layer2 : 128 // layer3 : 256 // layer4 : 512
out = F.relu(self.bn2(self.conv2(out)))
out = self.bn3(self.conv3(out)) # out's channel-> layer1 : 256 // layer2 : 512 // layer3 : 1024 // layer4 : 2048
out += self.shortcut(x) # bottom-up에서 Resnet의 구조가 작동하는 부분. x와 out의 channel이 다른것은 어떻게 처리하는지는 위에 정의 되어 있다.
out = F.relu(out)
return out
class FPN(nn.Module):
def __init__(self, block, num_blocks):
super(FPN, self).__init__()
self.in_planes = 64
# 1. input channel 3 (RGB) // output channel 64 // kernel size 7*7 // stride가 1이면 같은 size output이지만, stride = 2여서 output == input//2 size
self.conv1 = nn.Conv2d(3, 64, kernel_size=7, stride=2, padding=3, bias=False)
self.bn1 = nn.BatchNorm2d(64)
# 2. Bottom-up layers. 4개의 layer를 통과하고 나오는 결과 : channel 2048, width와 high는 대략 /(4(#1)*8(layer2,3,4)) 가 된다.. 하나의 _make_layer에 의해서 2로 나눠지므로.
self.layer1 = self._make_layer(block, 64, num_blocks[0], stride=1)
self.layer2 = self._make_layer(block, 128, num_blocks[1], stride=2)
self.layer3 = self._make_layer(block, 256, num_blocks[2], stride=2)
self.layer4 = self._make_layer(block, 512, num_blocks[3], stride=2)
# Top layer
self.toplayer = nn.Conv2d(2048, 256, kernel_size=1, stride=1, padding=0) # Reduce channels
# Smooth layers
self.smooth1 = nn.Conv2d(256, 256, kernel_size=3, stride=1, padding=1)
self.smooth2 = nn.Conv2d(256, 256, kernel_size=3, stride=1, padding=1)
self.smooth3 = nn.Conv2d(256, 256, kernel_size=3, stride=1, padding=1)
# Lateral layers
self.latlayer1 = nn.Conv2d(1024, 256, kernel_size=1, stride=1, padding=0)
self.latlayer2 = nn.Conv2d( 512, 256, kernel_size=1, stride=1, padding=0)
self.latlayer3 = nn.Conv2d( 256, 256, kernel_size=1, stride=1, padding=0)
# 3. call block (for Bottom-up)
def _make_layer(self, block, planes, num_blocks, stride):
strides = [stride] + [1]*(num_blocks-1) # [1or2] + [1] = [1or2 , 1]
layers = []
for stride in strides:
# layer1 : (64, 256), (64, 64) , (1 , 1) // layer2 : (256, 512), (128, 128) , (2, 1) // layer3 : (512, 1024), (256, 256) , (2, 1) // in_plain, planes, sride
layers.append(block(self.in_planes, planes, stride))
self.in_planes = planes * block.expansion
return nn.Sequential(*layers)
def _upsample_add(self, x, y):
_,_,H,W = y.size()
return F.upsample(x, size=(H,W), mode='bilinear') + y
# upsampling mode 설명 : https://pytorch.org/docs/0.3.1/_modules/torch/nn/modules/upsampling.html#Upsample
def forward(self, x):
# Bottom-up
c1 = F.relu(self.bn1(self.conv1(x)))
c1 = F.max_pool2d(c1, kernel_size=3, stride=2, padding=1) # 1. 위의 7*7 conv와 max_pool에 의해서 w/h가 /4 처리가 된다.
c2 = self.layer1(c1) # c2 size = torch.Size([1, 256, 150, 225])
c3 = self.layer2(c2) # c3 size = torch.Size([1, 512, 75, 113])
c4 = self.layer3(c3) # c4 size = torch.Size([1, 1024, 38, 57])
c5 = self.layer4(c4) # c5 size = torch.Size([1, 2048, 19, 29])
# Top-down
p5 = self.toplayer(c5)
p4 = self._upsample_add(p5, self.latlayer1(c4))
p3 = self._upsample_add(p4, self.latlayer2(c3))
p2 = self._upsample_add(p3, self.latlayer3(c2))
# Smooth
p4 = self.smooth1(p4)
p3 = self.smooth2(p3)
p2 = self.smooth3(p2)
return p2, p3, p4, p5
def FPN101():
# return FPN(Bottleneck, [2,4,23,3])
return FPN(Bottleneck, [2,2,2,2])
def test():
net = FPN101()
fms = net(Variable( torch.randn(1,3,600,900) )) # from torch.autograd import Variable -> dL/dw 처럼 w에 관하여 미분한 값을 구할때 사용. 여기서 randn이 w이다.
for fm in fms:
print(fm.size())
test()
"""
input : torch.size([1, 3, 600, 900])
fms[0] = torch.Size([1, 256, 150, 225])
fms[1] = torch.Size([1, 256, 75, 113])
fms[2] = torch.Size([1, 256, 38, 57])
fms[3] = torch.Size([1, 256, 19, 29])
"""
——
그림2
