上采样（upsampling）一般包括2种方式：
- Resize，如双线性插值直接缩放，类似于图像缩放，概念可见最邻近插值算法和双线性插值算法——图像缩放
- Deconvolution，也叫Transposed Convolution，可见逆卷积的详细解释ConvTranspose2d（fractionally-strided convolutions)
第二种方法如何用pytorch实现可见上面的链接
这里想要介绍的是如何使用pytorch实现第一种方法：
- 有两个模块都支持该上采样的实现，一个是torch.nn模块，详情可见：pytorch torch.nn 实现上采样——nn.Upsample （但是现在这种方法已经不推荐使用了，最好使用下面的方法）
- 一个是torch.nn.funtional模块，详情可见：pytorch torch.nn.functional实现插值和上采样
举例：
1)使用torch.nn模块实现一个生成器为：
import torch.nn as nn
import torch.nn.functional as F
class ConvLayer(nn.Module):
def __init__(self, in_channels, out_channels, kernel_size, stride):
super(ConvLayer, self).__init__()
padding = kernel_size // 2
self.reflection_pad = nn.ReflectionPad2d(padding)
self.conv = nn.Conv2d(in_channels, out_channels, kernel_size, stride)
def forward(self, x):
out = self.reflection_pad(x)
out = self.conv(out)
return out
class Generator(nn.Module):
def __init__(self, in_channels):
super(Generator, self).__init__()
self.in_channels = in_channels
self.encoder = nn.Sequential(
ConvLayer(self.in_channels, 32, 3, 2),
nn.BatchNorm2d(32),
nn.ReLU(),
ConvLayer(32, 64, 3, 2),
nn.BatchNorm2d(64),
nn.ReLU(),
ConvLayer(64, 128, 3, 2),
)
upsample = nn.Upsample(scale_factor=2, mode='bilinear', align_corners=True)
self.decoder = nn.Sequential(
upsample,
nn.Conv2d(128, 64, 1),
nn.BatchNorm2d(64),
nn.ReLU(),
upsample,
nn.Conv2d(64, 32, 1),
nn.BatchNorm2d(32),
nn.ReLU(),
upsample,
nn.Conv2d(32, 3, 1),
nn.Tanh()
)
def forward(self, x):
x = self.encoder(x)
out = self.decoder(x)
return out
def test():
net = Generator(3)
for module in net.children():
print(module)
x = Variable(torch.randn(2,3,224,224))
output = net(x)
print('output :', output.size())
print(type(output))
if __name__ == '__main__':
test()
返回：
model.py .Sequential(
(0): ConvLayer(
(reflection_pad): ReflectionPad2d((1, 1, 1, 1))
(conv): Conv2d(3, 32, kernel_size=(3, 3), stride=(2, 2))
)
(1): BatchNorm2d(32, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
(2): ReLU()
(3): ConvLayer(
(reflection_pad): ReflectionPad2d((1, 1, 1, 1))
(conv): Conv2d(32, 64, kernel_size=(3, 3), stride=(2, 2))
)
(4): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
(5): ReLU()
(6): ConvLayer(
(reflection_pad): ReflectionPad2d((1, 1, 1, 1))
(conv): Conv2d(64, 128, kernel_size=(3, 3), stride=(2, 2))
)
)
Sequential(
(0): Upsample(scale_factor=2, mode=bilinear)
(1): Conv2d(128, 64, kernel_size=(1, 1), stride=(1, 1))
(2): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
(3): ReLU()
(4): Upsample(scale_factor=2, mode=bilinear)
(5): Conv2d(64, 32, kernel_size=(1, 1), stride=(1, 1))
(6): BatchNorm2d(32, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
(7): ReLU()
(8): Upsample(scale_factor=2, mode=bilinear)
(9): Conv2d(32, 3, kernel_size=(1, 1), stride=(1, 1))
(10): Tanh()
)
output : torch.Size([2, 3, 224, 224])
<class 'torch.Tensor'>
但是这个会有警告：
UserWarning: nn.Upsample is deprecated. Use nn.functional.interpolate instead.
可使用torch.nn.functional模块替换为：
import torch.nn as nn
import torch.nn.functional as F
class ConvLayer(nn.Module):
def __init__(self, in_channels, out_channels, kernel_size, stride):
super(ConvLayer, self).__init__()
padding = kernel_size // 2
self.reflection_pad = nn.ReflectionPad2d(padding)
self.conv = nn.Conv2d(in_channels, out_channels, kernel_size, stride)
def forward(self, x):
out = self.reflection_pad(x)
out = self.conv(out)
return out
class Generator(nn.Module):
def __init__(self, in_channels):
super(Generator, self).__init__()
self.in_channels = in_channels
self.encoder = nn.Sequential(
ConvLayer(self.in_channels, 32, 3, 2),
nn.BatchNorm2d(32),
nn.ReLU(),
ConvLayer(32, 64, 3, 2),
nn.BatchNorm2d(64),
nn.ReLU(),
ConvLayer(64, 128, 3, 2),
)
self.decoder1 = nn.Sequential(
nn.Conv2d(128, 64, 1),
nn.BatchNorm2d(64),
nn.ReLU()
)
self.decoder2 = nn.Sequential(
nn.Conv2d(64, 32, 1),
nn.BatchNorm2d(32),
nn.ReLU()
)
self.decoder3 = nn.Sequential(
nn.Conv2d(32, 3, 1),
nn.Tanh()
)
def forward(self, x):
x = self.encoder(x)
x = F.interpolate(x, scale_factor=2, mode='bilinear', align_corners=True)
x = self.decoder1(x)
x = F.interpolate(x, scale_factor=2, mode='bilinear', align_corners=True)
x = self.decoder2(x)
x = F.interpolate(x, scale_factor=2, mode='bilinear', align_corners=True)
out = self.decoder3(x)
return out
def test():
net = Generator(3)
for module in net.children():
print(module)
x = Variable(torch.randn(2,3,224,224))
output = net(x)
print('output :', output.size())
print(type(output))
if __name__ == '__main__':
test()
返回：
model.py .Sequential(
(0): ConvLayer(
(reflection_pad): ReflectionPad2d((1, 1, 1, 1))
(conv): Conv2d(3, 32, kernel_size=(3, 3), stride=(2, 2))
)
(1): BatchNorm2d(32, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
(2): ReLU()
(3): ConvLayer(
(reflection_pad): ReflectionPad2d((1, 1, 1, 1))
(conv): Conv2d(32, 64, kernel_size=(3, 3), stride=(2, 2))
)
(4): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
(5): ReLU()
(6): ConvLayer(
(reflection_pad): ReflectionPad2d((1, 1, 1, 1))
(conv): Conv2d(64, 128, kernel_size=(3, 3), stride=(2, 2))
)
)
Sequential(
(0): Conv2d(128, 64, kernel_size=(1, 1), stride=(1, 1))
(1): BatchNorm2d(64, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
(2): ReLU()
)
Sequential(
(0): Conv2d(64, 32, kernel_size=(1, 1), stride=(1, 1))
(1): BatchNorm2d(32, eps=1e-05, momentum=0.1, affine=True, track_running_stats=True)
(2): ReLU()
)
Sequential(
(0): Conv2d(32, 3, kernel_size=(1, 1), stride=(1, 1))
(1): Tanh()
)
output : torch.Size([2, 3, 224, 224])
<class 'torch.Tensor'>