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import torch.nn as nn
import torch.nn.functional as F
from mmcv.cnn import ConvModule, xavier_init
from mmcv.cnn.bricks import NonLocal2d
from ..builder import NECKS
@NECKS.register_module()
class BFP(nn.Module):
"""BFP (Balanced Feature Pyramids)
BFP takes multi-level features as inputs and gather them into a single one,
then refine the gathered feature and scatter the refined results to
multi-level features. This module is used in Libra R-CNN (CVPR 2019), see
the paper `Libra R-CNN: Towards Balanced Learning for Object Detection
<https://arxiv.org/abs/1904.02701>`_ for details.
Args:
in_channels (int): Number of input channels (feature maps of all levels
should have the same channels).
num_levels (int): Number of input feature levels.
conv_cfg (dict): The config dict for convolution layers.
norm_cfg (dict): The config dict for normalization layers.
refine_level (int): Index of integration and refine level of BSF in
multi-level features from bottom to top.
refine_type (str): Type of the refine op, currently support
[None, 'conv', 'non_local'].
"""
def __init__(self,
in_channels,
num_levels,
refine_level=2,
refine_type=None,
conv_cfg=None,
norm_cfg=None):
super(BFP, self).__init__()
assert refine_type in [None, 'conv', 'non_local']
self.in_channels = in_channels
self.num_levels = num_levels
self.conv_cfg = conv_cfg
self.norm_cfg = norm_cfg
self.refine_level = refine_level
self.refine_type = refine_type
assert 0 <= self.refine_level < self.num_levels
if self.refine_type == 'conv':
self.refine = ConvModule(
self.in_channels,
self.in_channels,
3,
padding=1,
conv_cfg=self.conv_cfg,
norm_cfg=self.norm_cfg)
elif self.refine_type == 'non_local':
self.refine = NonLocal2d(
self.in_channels,
reduction=1,
use_scale=False,
conv_cfg=self.conv_cfg,
norm_cfg=self.norm_cfg)
def init_weights(self):
"""Initialize the weights of FPN module."""
for m in self.modules():
if isinstance(m, nn.Conv2d):
xavier_init(m, distribution='uniform')
def forward(self, inputs):
"""Forward function."""
assert len(inputs) == self.num_levels
# step 1: gather multi-level features by resize and average
feats = []
gather_size = inputs[self.refine_level].size()[2:]
for i in range(self.num_levels):
if i < self.refine_level:
gathered = F.adaptive_max_pool2d(
inputs[i], output_size=gather_size)
else:
gathered = F.interpolate(
inputs[i], size=gather_size, mode='nearest')
feats.append(gathered)
bsf = sum(feats) / len(feats)
# step 2: refine gathered features
if self.refine_type is not None:
bsf = self.refine(bsf)
# step 3: scatter refined features to multi-levels by a residual path
outs = []
for i in range(self.num_levels):
out_size = inputs[i].size()[2:]
if i < self.refine_level:
residual = F.interpolate(bsf, size=out_size, mode='nearest')
else:
residual = F.adaptive_max_pool2d(bsf, output_size=out_size)
outs.append(residual + inputs[i])
return tuple(outs)
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