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import torch.nn as nn
from mmcv.cnn import ConvModule
from mmcv.runner import auto_fp16, force_fp32
from mmdet.models.builder import HEADS
from mmdet.models.utils import ResLayer, SimplifiedBasicBlock
@HEADS.register_module()
class GlobalContextHead(nn.Module):
"""Global context head used in `SCNet <https://arxiv.org/abs/2012.10150>`_.
Args:
num_convs (int, optional): number of convolutional layer in GlbCtxHead.
Default: 4.
in_channels (int, optional): number of input channels. Default: 256.
conv_out_channels (int, optional): number of output channels before
classification layer. Default: 256.
num_classes (int, optional): number of classes. Default: 80.
loss_weight (float, optional): global context loss weight. Default: 1.
conv_cfg (dict, optional): config to init conv layer. Default: None.
norm_cfg (dict, optional): config to init norm layer. Default: None.
conv_to_res (bool, optional): if True, 2 convs will be grouped into
1 `SimplifiedBasicBlock` using a skip connection. Default: False.
"""
def __init__(self,
num_convs=4,
in_channels=256,
conv_out_channels=256,
num_classes=80,
loss_weight=1.0,
conv_cfg=None,
norm_cfg=None,
conv_to_res=False):
super(GlobalContextHead, self).__init__()
self.num_convs = num_convs
self.in_channels = in_channels
self.conv_out_channels = conv_out_channels
self.num_classes = num_classes
self.loss_weight = loss_weight
self.conv_cfg = conv_cfg
self.norm_cfg = norm_cfg
self.conv_to_res = conv_to_res
self.fp16_enabled = False
if self.conv_to_res:
num_res_blocks = num_convs // 2
self.convs = ResLayer(
SimplifiedBasicBlock,
in_channels,
self.conv_out_channels,
num_res_blocks,
conv_cfg=self.conv_cfg,
norm_cfg=self.norm_cfg)
self.num_convs = num_res_blocks
else:
self.convs = nn.ModuleList()
for i in range(self.num_convs):
in_channels = self.in_channels if i == 0 else conv_out_channels
self.convs.append(
ConvModule(
in_channels,
conv_out_channels,
3,
padding=1,
conv_cfg=self.conv_cfg,
norm_cfg=self.norm_cfg))
self.pool = nn.AdaptiveAvgPool2d(1)
self.fc = nn.Linear(conv_out_channels, num_classes)
self.criterion = nn.BCEWithLogitsLoss()
def init_weights(self):
"""Init weights for the head."""
nn.init.normal_(self.fc.weight, 0, 0.01)
nn.init.constant_(self.fc.bias, 0)
@auto_fp16()
def forward(self, feats):
"""Forward function."""
x = feats[-1]
for i in range(self.num_convs):
x = self.convs[i](x)
x = self.pool(x)
# multi-class prediction
mc_pred = x.reshape(x.size(0), -1)
mc_pred = self.fc(mc_pred)
return mc_pred, x
@force_fp32(apply_to=('pred', ))
def loss(self, pred, labels):
"""Loss function."""
labels = [lbl.unique() for lbl in labels]
targets = pred.new_zeros(pred.size())
for i, label in enumerate(labels):
targets[i, label] = 1.0
loss = self.loss_weight * self.criterion(pred, targets)
return loss
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