backbone_utils.py

from collections import OrderedDict
from torch import nn
from torchvision.ops.feature_pyramid_network import FeaturePyramidNetwork, LastLevelMaxPool

from torchvision.ops import misc as misc_nn_ops
from .._utils import IntermediateLayerGetter
from .. import resnet


class BackboneWithFPN(nn.Sequential):
    def __init__(self, backbone, return_layers, in_channels_list, out_channels):
        body = IntermediateLayerGetter(backbone, return_layers=return_layers)
        fpn = FeaturePyramidNetwork(
            in_channels_list=in_channels_list,
            out_channels=out_channels,
            extra_blocks=LastLevelMaxPool(),
        )
        super(BackboneWithFPN, self).__init__(OrderedDict(
            [("body", body), ("fpn", fpn)]))
        self.out_channels = out_channels


def resnet_fpn_backbone(backbone_name, pretrained):
    backbone = resnet.__dict__[backbone_name](
        pretrained=pretrained,
        norm_layer=misc_nn_ops.FrozenBatchNorm2d)
    # freeze layers
    for name, parameter in backbone.named_parameters():
        if 'layer2' not in name and 'layer3' not in name and 'layer4' not in name:
            parameter.requires_grad_(False)

    return_layers = {'layer1': 0, 'layer2': 1, 'layer3': 2, 'layer4': 3}

    in_channels_stage2 = 256
    in_channels_list = [
        in_channels_stage2,
        in_channels_stage2 * 2,
        in_channels_stage2 * 4,
        in_channels_stage2 * 8,
    ]
    out_channels = 256
    return BackboneWithFPN(backbone, return_layers, in_channels_list, out_channels)