mmdet/models/backbones/hourglass.py

import torch.nn as nn
from mmcv.cnn import ConvModule

from ..builder import BACKBONES
from ..utils import ResLayer
from .resnet import BasicBlock


class HourglassModule(nn.Module):
    """Hourglass Module for HourglassNet backbone.

    Generate module recursively and use BasicBlock as the base unit.

    Args:
        depth (int): Depth of current HourglassModule.
        stage_channels (list[int]): Feature channels of sub-modules in current
            and follow-up HourglassModule.
        stage_blocks (list[int]): Number of sub-modules stacked in current and
            follow-up HourglassModule.
        norm_cfg (dict): Dictionary to construct and config norm layer.
    """

    def __init__(self,
                 depth,
                 stage_channels,
                 stage_blocks,
                 norm_cfg=dict(type='BN', requires_grad=True)):
        super(HourglassModule, self).__init__()

        self.depth = depth

        cur_block = stage_blocks[0]
        next_block = stage_blocks[1]

        cur_channel = stage_channels[0]
        next_channel = stage_channels[1]

        self.up1 = ResLayer(
            BasicBlock, cur_channel, cur_channel, cur_block, norm_cfg=norm_cfg)

        self.low1 = ResLayer(
            BasicBlock,
            cur_channel,
            next_channel,
            cur_block,
            stride=2,
            norm_cfg=norm_cfg)

        if self.depth > 1:
            self.low2 = HourglassModule(depth - 1, stage_channels[1:],
                                        stage_blocks[1:])
        else:
            self.low2 = ResLayer(
                BasicBlock,
                next_channel,
                next_channel,
                next_block,
                norm_cfg=norm_cfg)

        self.low3 = ResLayer(
            BasicBlock,
            next_channel,
            cur_channel,
            cur_block,
            norm_cfg=norm_cfg,
            downsample_first=False)

        self.up2 = nn.Upsample(scale_factor=2)

    def forward(self, x):
        """Forward function."""
        up1 = self.up1(x)
        low1 = self.low1(x)
        low2 = self.low2(low1)
        low3 = self.low3(low2)
        up2 = self.up2(low3)
        return up1 + up2


@BACKBONES.register_module()
class HourglassNet(nn.Module):
    """HourglassNet backbone.

    Stacked Hourglass Networks for Human Pose Estimation.
    More details can be found in the `paper
    <https://arxiv.org/abs/1603.06937>`_ .

    Args:
        downsample_times (int): Downsample times in a HourglassModule.
        num_stacks (int): Number of HourglassModule modules stacked,
            1 for Hourglass-52, 2 for Hourglass-104.
        stage_channels (list[int]): Feature channel of each sub-module in a
            HourglassModule.
        stage_blocks (list[int]): Number of sub-modules stacked in a
            HourglassModule.
        feat_channel (int): Feature channel of conv after a HourglassModule.
        norm_cfg (dict): Dictionary to construct and config norm layer.

    Example:
        >>> from mmdet.models import HourglassNet
        >>> import torch
        >>> self = HourglassNet()
        >>> self.eval()
        >>> inputs = torch.rand(1, 3, 511, 511)
        >>> level_outputs = self.forward(inputs)
        >>> for level_output in level_outputs:
        ...     print(tuple(level_output.shape))
        (1, 256, 128, 128)
        (1, 256, 128, 128)
    """

    def __init__(self,
                 downsample_times=5,
                 num_stacks=2,
                 stage_channels=(256, 256, 384, 384, 384, 512),
                 stage_blocks=(2, 2, 2, 2, 2, 4),
                 feat_channel=256,
                 norm_cfg=dict(type='BN', requires_grad=True)):
        super(HourglassNet, self).__init__()

        self.num_stacks = num_stacks
        assert self.num_stacks >= 1
        assert len(stage_channels) == len(stage_blocks)
        assert len(stage_channels) > downsample_times

        cur_channel = stage_channels[0]

        self.stem = nn.Sequential(
            ConvModule(3, 128, 7, padding=3, stride=2, norm_cfg=norm_cfg),
            ResLayer(BasicBlock, 128, 256, 1, stride=2, norm_cfg=norm_cfg))

        self.hourglass_modules = nn.ModuleList([
            HourglassModule(downsample_times, stage_channels, stage_blocks)
            for _ in range(num_stacks)
        ])

        self.inters = ResLayer(
            BasicBlock,
            cur_channel,
            cur_channel,
            num_stacks - 1,
            norm_cfg=norm_cfg)

        self.conv1x1s = nn.ModuleList([
            ConvModule(
                cur_channel, cur_channel, 1, norm_cfg=norm_cfg, act_cfg=None)
            for _ in range(num_stacks - 1)
        ])

        self.out_convs = nn.ModuleList([
            ConvModule(
                cur_channel, feat_channel, 3, padding=1, norm_cfg=norm_cfg)
            for _ in range(num_stacks)
        ])

        self.remap_convs = nn.ModuleList([
            ConvModule(
                feat_channel, cur_channel, 1, norm_cfg=norm_cfg, act_cfg=None)
            for _ in range(num_stacks - 1)
        ])

        self.relu = nn.ReLU(inplace=True)

    def init_weights(self, pretrained=None):
        """Init module weights.

        We do nothing in this function because all modules we used
        (ConvModule, BasicBlock and etc.) have default initialization, and
        currently we don't provide pretrained model of HourglassNet.

        Detector's __init__() will call backbone's init_weights() with
        pretrained as input, so we keep this function.
        """
        # Training Centripetal Model needs to reset parameters for Conv2d
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                m.reset_parameters()

    def forward(self, x):
        """Forward function."""
        inter_feat = self.stem(x)
        out_feats = []

        for ind in range(self.num_stacks):
            single_hourglass = self.hourglass_modules[ind]
            out_conv = self.out_convs[ind]

            hourglass_feat = single_hourglass(inter_feat)
            out_feat = out_conv(hourglass_feat)
            out_feats.append(out_feat)

            if ind < self.num_stacks - 1:
                inter_feat = self.conv1x1s[ind](
                    inter_feat) + self.remap_convs[ind](
                        out_feat)
                inter_feat = self.inters[ind](self.relu(inter_feat))

        return out_feats