second_fpn.py

import torch
from mmcv.cnn import (build_norm_layer, build_upsample_layer, constant_init,
                      is_norm, kaiming_init)
from torch import nn as nn

from mmdet.models import NECKS


@NECKS.register_module()
class SECONDFPN(nn.Module):
    """FPN used in SECOND/PointPillars/PartA2/MVXNet.

    Args:
        in_channels (list[int]): Input channels of multi-scale feature maps
        out_channels (list[int]): Output channels of feature maps
        upsample_strides (list[int]): Strides used to upsample the feature maps
        norm_cfg (dict): Config dict of normalization layers
        upsample_cfg (dict): Config dict of upsample layers
    """

    def __init__(self,
                 in_channels=[128, 128, 256],
                 out_channels=[256, 256, 256],
                 upsample_strides=[1, 2, 4],
                 norm_cfg=dict(type='BN', eps=1e-3, momentum=0.01),
                 upsample_cfg=dict(type='deconv', bias=False)):
        # if for GroupNorm,
        # cfg is dict(type='GN', num_groups=num_groups, eps=1e-3, affine=True)
        super(SECONDFPN, self).__init__()
        assert len(out_channels) == len(upsample_strides) == len(in_channels)
        self.in_channels = in_channels
        self.out_channels = out_channels

        deblocks = []
        for i, out_channel in enumerate(out_channels):
            upsample_layer = build_upsample_layer(
                upsample_cfg,
                in_channels=in_channels[i],
                out_channels=out_channel,
                kernel_size=upsample_strides[i],
                stride=upsample_strides[i])
            deblock = nn.Sequential(upsample_layer,
                                    build_norm_layer(norm_cfg, out_channel)[1],
                                    nn.ReLU(inplace=True))
            deblocks.append(deblock)
        self.deblocks = nn.ModuleList(deblocks)

    def init_weights(self):
        """Initialize weights of FPN."""
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                kaiming_init(m)
            elif is_norm(m):
                constant_init(m, 1)

    def forward(self, x):
        """Forward function.

        Args:
            x (torch.Tensor): 4D Tensor in (N, C, H, W) shape.

        Returns:
            list[torch.Tensor]: Multi-level feature maps.
        """
        assert len(x) == len(self.in_channels)
        ups = [deblock(x[i]) for i, deblock in enumerate(self.deblocks)]

        if len(ups) > 1:
            out = torch.cat(ups, dim=1)
        else:
            out = ups[0]
        return [out]