A confusing difference on resNet architecture between caffe prototxt and pytorch


(Fostor Hunt) #1

I noticed that there exists a difference on resNet architecture between caffe and pytorch. It’s about downsample operation:
in pytorch, downsample is executed at the end of each Bottleneck (if needed) as blow:

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.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)
    if self.downsample is not None:
        residual = self.downsample(x)
    out += residual
    out = self.relu(out)
    return out

But in caffe prototxt, the position of downsample is at the top of each Botteleneck.


I am curious about the reason, and what will happen to this difference?


(Ben) #2

It’s the same in both cases - the downsizing is not at the top or bottom of the main “trunk” path, but rather in the parallel shortcut path.

In the caffe case it’s the res5a_branch1 convolution that’s doing the downsizing. In the PyTorch case note that it’s downsample(x), not downsample(out) - this is the parallel shortcut branch, not an operation being added after bn3(out).

Ben


(Fostor Hunt) #3

Thanks for your reply! I do make a stupid mistake. XD