mmdetection/mmdet/models/necks/channel_mapper.py

# Copyright (c) OpenMMLab. All rights reserved.
from typing import List, Tuple

import torch.nn as nn
from mmcv.cnn import ConvModule
from mmengine.model import BaseModule
from torch import Tensor

from mmdet.registry import MODELS
from mmdet.utils import OptConfigType, OptMultiConfig


@MODELS.register_module()
class ChannelMapper(BaseModule):
    """Channel Mapper to reduce/increase channels of backbone features.

    This is used to reduce/increase channels of backbone features.

    Args:
        in_channels (List[int]): Number of input channels per scale.
        out_channels (int): Number of output channels (used at each scale).
        kernel_size (int, optional): kernel_size for reducing channels (used
            at each scale). Default: 3.
        conv_cfg (:obj:`ConfigDict` or dict, optional): Config dict for
            convolution layer. Default: None.
        norm_cfg (:obj:`ConfigDict` or dict, optional): Config dict for
            normalization layer. Default: None.
        act_cfg (:obj:`ConfigDict` or dict, optional): Config dict for
            activation layer in ConvModule. Default: dict(type='ReLU').
        num_outs (int, optional): Number of output feature maps. There would
            be extra_convs when num_outs larger than the length of in_channels.
        init_cfg (:obj:`ConfigDict` or dict or list[:obj:`ConfigDict` or dict],
            optional): Initialization config dict.
    Example:
        >>> import torch
        >>> in_channels = [2, 3, 5, 7]
        >>> scales = [340, 170, 84, 43]
        >>> inputs = [torch.rand(1, c, s, s)
        ...           for c, s in zip(in_channels, scales)]
        >>> self = ChannelMapper(in_channels, 11, 3).eval()
        >>> outputs = self.forward(inputs)
        >>> for i in range(len(outputs)):
        ...     print(f'outputs[{i}].shape = {outputs[i].shape}')
        outputs[0].shape = torch.Size([1, 11, 340, 340])
        outputs[1].shape = torch.Size([1, 11, 170, 170])
        outputs[2].shape = torch.Size([1, 11, 84, 84])
        outputs[3].shape = torch.Size([1, 11, 43, 43])
    """

    def __init__(
        self,
        in_channels: List[int],
        out_channels: int,
        kernel_size: int = 3,
        conv_cfg: OptConfigType = None,
        norm_cfg: OptConfigType = None,
        act_cfg: OptConfigType = dict(type='ReLU'),
        num_outs: int = None,
        init_cfg: OptMultiConfig = dict(
            type='Xavier', layer='Conv2d', distribution='uniform')
    ) -> None:
        super().__init__(init_cfg=init_cfg)
        assert isinstance(in_channels, list)
        self.extra_convs = None
        if num_outs is None:
            num_outs = len(in_channels)
        self.convs = nn.ModuleList()
        for in_channel in in_channels:
            self.convs.append(
                ConvModule(
                    in_channel,
                    out_channels,
                    kernel_size,
                    padding=(kernel_size - 1) // 2,
                    conv_cfg=conv_cfg,
                    norm_cfg=norm_cfg,
                    act_cfg=act_cfg))
        if num_outs > len(in_channels):
            self.extra_convs = nn.ModuleList()
            for i in range(len(in_channels), num_outs):
                if i == len(in_channels):
                    in_channel = in_channels[-1]
                else:
                    in_channel = out_channels
                self.extra_convs.append(
                    ConvModule(
                        in_channel,
                        out_channels,
                        3,
                        stride=2,
                        padding=1,
                        conv_cfg=conv_cfg,
                        norm_cfg=norm_cfg,
                        act_cfg=act_cfg))

    def forward(self, inputs: Tuple[Tensor]) -> Tuple[Tensor]:
        """Forward function."""
        assert len(inputs) == len(self.convs)
        outs = [self.convs[i](inputs[i]) for i in range(len(inputs))]
        if self.extra_convs:
            for i in range(len(self.extra_convs)):
                if i == 0:
                    outs.append(self.extra_convs[0](inputs[-1]))
                else:
                    outs.append(self.extra_convs[i](outs[-1]))
        return tuple(outs)