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import torch.nn as nn
import torch.nn.functional as F
def conv1x1(in_planes, out_planes, stride=1):
"""1x1 convolution without padding"""
return nn.Conv2d(
in_planes, out_planes, kernel_size=1, stride=stride, padding=0, bias=False
)
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 ConvBlock(nn.Module):
def __init__(self, in_planes, planes, stride=1, bn=True):
super().__init__()
self.conv = conv3x3(in_planes, planes, stride)
self.bn = nn.BatchNorm2d(planes) if bn is True else None
self.act = nn.GELU()
def forward(self, x):
y = self.conv(x)
if self.bn:
y = self.bn(y) # F.layer_norm(y, y.shape[1:])
y = self.act(y)
return y
class FPN(nn.Module):
"""
ResNet+FPN, output resolution are 1/8 and 1/2.
Each block has 2 layers.
"""
def __init__(self, config):
super().__init__()
# Config
block = ConvBlock
initial_dim = config["initial_dim"]
block_dims = config["block_dims"]
# Class Variable
self.in_planes = initial_dim
# Networks
self.conv1 = nn.Conv2d(
1, initial_dim, kernel_size=7, stride=2, padding=3, bias=False
)
self.bn1 = nn.BatchNorm2d(initial_dim)
self.relu = nn.ReLU(inplace=True)
self.layer1 = self._make_layer(block, block_dims[0], stride=1) # 1/2
self.layer2 = self._make_layer(block, block_dims[1], stride=2) # 1/4
self.layer3 = self._make_layer(block, block_dims[2], stride=2) # 1/8
self.layer4 = self._make_layer(block, block_dims[3], stride=2) # 1/16
# 3. FPN upsample
self.layer3_outconv = conv1x1(block_dims[2], block_dims[3])
self.layer3_outconv2 = nn.Sequential(
ConvBlock(block_dims[3], block_dims[2]),
conv3x3(block_dims[2], block_dims[2]),
)
self.layer2_outconv = conv1x1(block_dims[1], block_dims[2])
self.layer2_outconv2 = nn.Sequential(
ConvBlock(block_dims[2], block_dims[1]),
conv3x3(block_dims[1], block_dims[1]),
)
self.layer1_outconv = conv1x1(block_dims[0], block_dims[1])
self.layer1_outconv2 = nn.Sequential(
ConvBlock(block_dims[1], block_dims[0]),
conv3x3(block_dims[0], block_dims[0]),
)
for m in self.modules():
if isinstance(m, nn.Conv2d):
nn.init.kaiming_normal_(m.weight, mode="fan_out", nonlinearity="relu")
elif isinstance(m, (nn.BatchNorm2d, nn.GroupNorm)):
nn.init.constant_(m.weight, 1)
nn.init.constant_(m.bias, 0)
def _make_layer(self, block, dim, stride=1):
layer1 = block(self.in_planes, dim, stride=stride)
layer2 = block(dim, dim, stride=1)
layers = (layer1, layer2)
self.in_planes = dim
return nn.Sequential(*layers)
def forward(self, x):
# ResNet Backbone
x0 = self.relu(self.bn1(self.conv1(x)))
x1 = self.layer1(x0) # 1/2
x2 = self.layer2(x1) # 1/4
x3 = self.layer3(x2) # 1/8
x4 = self.layer4(x3) # 1/16
# FPN
x4_out_2x = F.interpolate(
x4, scale_factor=2.0, mode="bilinear", align_corners=True
)
x3_out = self.layer3_outconv(x3)
x3_out = self.layer3_outconv2(x3_out + x4_out_2x)
x3_out_2x = F.interpolate(
x3_out, scale_factor=2.0, mode="bilinear", align_corners=True
)
x2_out = self.layer2_outconv(x2)
x2_out = self.layer2_outconv2(x2_out + x3_out_2x)
x2_out_2x = F.interpolate(
x2_out, scale_factor=2.0, mode="bilinear", align_corners=True
)
x1_out = self.layer1_outconv(x1)
x1_out = self.layer1_outconv2(x1_out + x2_out_2x)
return [x3_out, x1_out]
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