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import torch
import torch.nn as nn
import torch.utils.checkpoint as cp
from mmcv.cnn import (ConvModule, build_conv_layer, build_norm_layer,
constant_init, kaiming_init)
from mmcv.runner import load_checkpoint
from mmcv.utils.parrots_wrapper import _BatchNorm
from mmseg.utils import get_root_logger
from ..builder import BACKBONES
class GlobalContextExtractor(nn.Module):
"""Global Context Extractor for CGNet.
This class is employed to refine the joFint feature of both local feature
and surrounding context.
Args:
channel (int): Number of input feature channels.
reduction (int): Reductions for global context extractor. Default: 16.
with_cp (bool): Use checkpoint or not. Using checkpoint will save some
memory while slowing down the training speed. Default: False.
"""
def __init__(self, channel, reduction=16, with_cp=False):
super(GlobalContextExtractor, self).__init__()
self.channel = channel
self.reduction = reduction
assert reduction >= 1 and channel >= reduction
self.with_cp = with_cp
self.avg_pool = nn.AdaptiveAvgPool2d(1)
self.fc = nn.Sequential(
nn.Linear(channel, channel // reduction), nn.ReLU(inplace=True),
nn.Linear(channel // reduction, channel), nn.Sigmoid())
def forward(self, x):
def _inner_forward(x):
num_batch, num_channel = x.size()[:2]
y = self.avg_pool(x).view(num_batch, num_channel)
y = self.fc(y).view(num_batch, num_channel, 1, 1)
return x * y
if self.with_cp and x.requires_grad:
out = cp.checkpoint(_inner_forward, x)
else:
out = _inner_forward(x)
return out
class ContextGuidedBlock(nn.Module):
"""Context Guided Block for CGNet.
This class consists of four components: local feature extractor,
surrounding feature extractor, joint feature extractor and global
context extractor.
Args:
in_channels (int): Number of input feature channels.
out_channels (int): Number of output feature channels.
dilation (int): Dilation rate for surrounding context extractor.
Default: 2.
reduction (int): Reduction for global context extractor. Default: 16.
skip_connect (bool): Add input to output or not. Default: True.
downsample (bool): Downsample the input to 1/2 or not. Default: False.
conv_cfg (dict): Config dict for convolution layer.
Default: None, which means using conv2d.
norm_cfg (dict): Config dict for normalization layer.
Default: dict(type='BN', requires_grad=True).
act_cfg (dict): Config dict for activation layer.
Default: dict(type='PReLU').
with_cp (bool): Use checkpoint or not. Using checkpoint will save some
memory while slowing down the training speed. Default: False.
"""
def __init__(self,
in_channels,
out_channels,
dilation=2,
reduction=16,
skip_connect=True,
downsample=False,
conv_cfg=None,
norm_cfg=dict(type='BN', requires_grad=True),
act_cfg=dict(type='PReLU'),
with_cp=False):
super(ContextGuidedBlock, self).__init__()
self.with_cp = with_cp
self.downsample = downsample
channels = out_channels if downsample else out_channels // 2
if 'type' in act_cfg and act_cfg['type'] == 'PReLU':
act_cfg['num_parameters'] = channels
kernel_size = 3 if downsample else 1
stride = 2 if downsample else 1
padding = (kernel_size - 1) // 2
self.conv1x1 = ConvModule(
in_channels,
channels,
kernel_size,
stride,
padding,
conv_cfg=conv_cfg,
norm_cfg=norm_cfg,
act_cfg=act_cfg)
self.f_loc = build_conv_layer(
conv_cfg,
channels,
channels,
kernel_size=3,
padding=1,
groups=channels,
bias=False)
self.f_sur = build_conv_layer(
conv_cfg,
channels,
channels,
kernel_size=3,
padding=dilation,
groups=channels,
dilation=dilation,
bias=False)
self.bn = build_norm_layer(norm_cfg, 2 * channels)[1]
self.activate = nn.PReLU(2 * channels)
if downsample:
self.bottleneck = build_conv_layer(
conv_cfg,
2 * channels,
out_channels,
kernel_size=1,
bias=False)
self.skip_connect = skip_connect and not downsample
self.f_glo = GlobalContextExtractor(out_channels, reduction, with_cp)
def forward(self, x):
def _inner_forward(x):
out = self.conv1x1(x)
loc = self.f_loc(out)
sur = self.f_sur(out)
joi_feat = torch.cat([loc, sur], 1) # the joint feature
joi_feat = self.bn(joi_feat)
joi_feat = self.activate(joi_feat)
if self.downsample:
joi_feat = self.bottleneck(joi_feat) # channel = out_channels
# f_glo is employed to refine the joint feature
out = self.f_glo(joi_feat)
if self.skip_connect:
return x + out
else:
return out
if self.with_cp and x.requires_grad:
out = cp.checkpoint(_inner_forward, x)
else:
out = _inner_forward(x)
return out
class InputInjection(nn.Module):
"""Downsampling module for CGNet."""
def __init__(self, num_downsampling):
super(InputInjection, self).__init__()
self.pool = nn.ModuleList()
for i in range(num_downsampling):
self.pool.append(nn.AvgPool2d(3, stride=2, padding=1))
def forward(self, x):
for pool in self.pool:
x = pool(x)
return x
@BACKBONES.register_module()
class CGNet(nn.Module):
"""CGNet backbone.
A Light-weight Context Guided Network for Semantic Segmentation
arXiv: https://arxiv.org/abs/1811.08201
Args:
in_channels (int): Number of input image channels. Normally 3.
num_channels (tuple[int]): Numbers of feature channels at each stages.
Default: (32, 64, 128).
num_blocks (tuple[int]): Numbers of CG blocks at stage 1 and stage 2.
Default: (3, 21).
dilations (tuple[int]): Dilation rate for surrounding context
extractors at stage 1 and stage 2. Default: (2, 4).
reductions (tuple[int]): Reductions for global context extractors at
stage 1 and stage 2. Default: (8, 16).
conv_cfg (dict): Config dict for convolution layer.
Default: None, which means using conv2d.
norm_cfg (dict): Config dict for normalization layer.
Default: dict(type='BN', requires_grad=True).
act_cfg (dict): Config dict for activation layer.
Default: dict(type='PReLU').
norm_eval (bool): Whether to set norm layers to eval mode, namely,
freeze running stats (mean and var). Note: Effect on Batch Norm
and its variants only. Default: False.
with_cp (bool): Use checkpoint or not. Using checkpoint will save some
memory while slowing down the training speed. Default: False.
"""
def __init__(self,
in_channels=3,
num_channels=(32, 64, 128),
num_blocks=(3, 21),
dilations=(2, 4),
reductions=(8, 16),
conv_cfg=None,
norm_cfg=dict(type='BN', requires_grad=True),
act_cfg=dict(type='PReLU'),
norm_eval=False,
with_cp=False):
super(CGNet, self).__init__()
self.in_channels = in_channels
self.num_channels = num_channels
assert isinstance(self.num_channels, tuple) and len(
self.num_channels) == 3
self.num_blocks = num_blocks
assert isinstance(self.num_blocks, tuple) and len(self.num_blocks) == 2
self.dilations = dilations
assert isinstance(self.dilations, tuple) and len(self.dilations) == 2
self.reductions = reductions
assert isinstance(self.reductions, tuple) and len(self.reductions) == 2
self.conv_cfg = conv_cfg
self.norm_cfg = norm_cfg
self.act_cfg = act_cfg
if 'type' in self.act_cfg and self.act_cfg['type'] == 'PReLU':
self.act_cfg['num_parameters'] = num_channels[0]
self.norm_eval = norm_eval
self.with_cp = with_cp
cur_channels = in_channels
self.stem = nn.ModuleList()
for i in range(3):
self.stem.append(
ConvModule(
cur_channels,
num_channels[0],
3,
2 if i == 0 else 1,
padding=1,
conv_cfg=conv_cfg,
norm_cfg=norm_cfg,
act_cfg=act_cfg))
cur_channels = num_channels[0]
self.inject_2x = InputInjection(1) # down-sample for Input, factor=2
self.inject_4x = InputInjection(2) # down-sample for Input, factor=4
cur_channels += in_channels
self.norm_prelu_0 = nn.Sequential(
build_norm_layer(norm_cfg, cur_channels)[1],
nn.PReLU(cur_channels))
# stage 1
self.level1 = nn.ModuleList()
for i in range(num_blocks[0]):
self.level1.append(
ContextGuidedBlock(
cur_channels if i == 0 else num_channels[1],
num_channels[1],
dilations[0],
reductions[0],
downsample=(i == 0),
conv_cfg=conv_cfg,
norm_cfg=norm_cfg,
act_cfg=act_cfg,
with_cp=with_cp)) # CG block
cur_channels = 2 * num_channels[1] + in_channels
self.norm_prelu_1 = nn.Sequential(
build_norm_layer(norm_cfg, cur_channels)[1],
nn.PReLU(cur_channels))
# stage 2
self.level2 = nn.ModuleList()
for i in range(num_blocks[1]):
self.level2.append(
ContextGuidedBlock(
cur_channels if i == 0 else num_channels[2],
num_channels[2],
dilations[1],
reductions[1],
downsample=(i == 0),
conv_cfg=conv_cfg,
norm_cfg=norm_cfg,
act_cfg=act_cfg,
with_cp=with_cp)) # CG block
cur_channels = 2 * num_channels[2]
self.norm_prelu_2 = nn.Sequential(
build_norm_layer(norm_cfg, cur_channels)[1],
nn.PReLU(cur_channels))
def forward(self, x):
output = []
# stage 0
inp_2x = self.inject_2x(x)
inp_4x = self.inject_4x(x)
for layer in self.stem:
x = layer(x)
x = self.norm_prelu_0(torch.cat([x, inp_2x], 1))
output.append(x)
# stage 1
for i, layer in enumerate(self.level1):
x = layer(x)
if i == 0:
down1 = x
x = self.norm_prelu_1(torch.cat([x, down1, inp_4x], 1))
output.append(x)
# stage 2
for i, layer in enumerate(self.level2):
x = layer(x)
if i == 0:
down2 = x
x = self.norm_prelu_2(torch.cat([down2, x], 1))
output.append(x)
return output
def init_weights(self, pretrained=None):
"""Initialize the weights in backbone.
Args:
pretrained (str, optional): Path to pre-trained weights.
Defaults to None.
"""
if isinstance(pretrained, str):
logger = get_root_logger()
load_checkpoint(self, pretrained, strict=False, logger=logger)
elif pretrained is None:
for m in self.modules():
if isinstance(m, (nn.Conv2d, nn.Linear)):
kaiming_init(m)
elif isinstance(m, (_BatchNorm, nn.GroupNorm)):
constant_init(m, 1)
elif isinstance(m, nn.PReLU):
constant_init(m, 0)
else:
raise TypeError('pretrained must be a str or None')
def train(self, mode=True):
"""Convert the model into training mode whill keeping the normalization
layer freezed."""
super(CGNet, self).train(mode)
if mode and self.norm_eval:
for m in self.modules():
# trick: eval have effect on BatchNorm only
if isinstance(m, _BatchNorm):
m.eval()
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