dpt_head.py

import math

import torch
import torch.nn as nn
from mmcv.cnn import ConvModule, Linear, build_activation_layer
from mmcv.runner import BaseModule

from mmseg.ops import resize
from ..builder import HEADS
from .decode_head import BaseDecodeHead


class ReassembleBlocks(BaseModule):
    """ViTPostProcessBlock, process cls_token in ViT backbone output and
    rearrange the feature vector to feature map.

    Args:
        in_channels (int): ViT feature channels. Default: 768.
        out_channels (List): output channels of each stage.
            Default: [96, 192, 384, 768].
        readout_type (str): Type of readout operation. Default: 'ignore'.
        patch_size (int): The patch size. Default: 16.
        init_cfg (dict, optional): Initialization config dict. Default: None.
    """

    def __init__(self,
                 in_channels=768,
                 out_channels=[96, 192, 384, 768],
                 readout_type='ignore',
                 patch_size=16,
                 init_cfg=None):
        super(ReassembleBlocks, self).__init__(init_cfg)

        assert readout_type in ['ignore', 'add', 'project']
        self.readout_type = readout_type
        self.patch_size = patch_size

        self.projects = nn.ModuleList([
            ConvModule(
                in_channels=in_channels,
                out_channels=out_channel,
                kernel_size=1,
                act_cfg=None,
            ) for out_channel in out_channels
        ])

        self.resize_layers = nn.ModuleList([
            nn.ConvTranspose2d(
                in_channels=out_channels[0],
                out_channels=out_channels[0],
                kernel_size=4,
                stride=4,
                padding=0),
            nn.ConvTranspose2d(
                in_channels=out_channels[1],
                out_channels=out_channels[1],
                kernel_size=2,
                stride=2,
                padding=0),
            nn.Identity(),
            nn.Conv2d(
                in_channels=out_channels[3],
                out_channels=out_channels[3],
                kernel_size=3,
                stride=2,
                padding=1)
        ])
        if self.readout_type == 'project':
            self.readout_projects = nn.ModuleList()
            for _ in range(len(self.projects)):
                self.readout_projects.append(
                    nn.Sequential(
                        Linear(2 * in_channels, in_channels),
                        build_activation_layer(dict(type='GELU'))))

    def forward(self, inputs):
        assert isinstance(inputs, list)
        out = []
        for i, x in enumerate(inputs):
            assert len(x) == 2
            x, cls_token = x[0], x[1]
            feature_shape = x.shape
            if self.readout_type == 'project':
                x = x.flatten(2).permute((0, 2, 1))
                readout = cls_token.unsqueeze(1).expand_as(x)
                x = self.readout_projects[i](torch.cat((x, readout), -1))
                x = x.permute(0, 2, 1).reshape(feature_shape)
            elif self.readout_type == 'add':
                x = x.flatten(2) + cls_token.unsqueeze(-1)
                x = x.reshape(feature_shape)
            else:
                pass
            x = self.projects[i](x)
            x = self.resize_layers[i](x)
            out.append(x)
        return out


class PreActResidualConvUnit(BaseModule):
    """ResidualConvUnit, pre-activate residual unit.

    Args:
        in_channels (int): number of channels in the input feature map.
        act_cfg (dict): dictionary to construct and config activation layer.
        norm_cfg (dict): dictionary to construct and config norm layer.
        stride (int): stride of the first block. Default: 1
        dilation (int): dilation rate for convs layers. Default: 1.
        init_cfg (dict, optional): Initialization config dict. Default: None.
    """

    def __init__(self,
                 in_channels,
                 act_cfg,
                 norm_cfg,
                 stride=1,
                 dilation=1,
                 init_cfg=None):
        super(PreActResidualConvUnit, self).__init__(init_cfg)

        self.conv1 = ConvModule(
            in_channels,
            in_channels,
            3,
            stride=stride,
            padding=dilation,
            dilation=dilation,
            norm_cfg=norm_cfg,
            act_cfg=act_cfg,
            bias=False,
            order=('act', 'conv', 'norm'))

        self.conv2 = ConvModule(
            in_channels,
            in_channels,
            3,
            padding=1,
            norm_cfg=norm_cfg,
            act_cfg=act_cfg,
            bias=False,
            order=('act', 'conv', 'norm'))

    def forward(self, inputs):
        inputs_ = inputs.clone()
        x = self.conv1(inputs)
        x = self.conv2(x)
        return x + inputs_


class FeatureFusionBlock(BaseModule):
    """FeatureFusionBlock, merge feature map from different stages.

    Args:
        in_channels (int): Input channels.
        act_cfg (dict): The activation config for ResidualConvUnit.
        norm_cfg (dict): Config dict for normalization layer.
        expand (bool): Whether expand the channels in post process block.
            Default: False.
        align_corners (bool): align_corner setting for bilinear upsample.
            Default: True.
        init_cfg (dict, optional): Initialization config dict. Default: None.
    """

    def __init__(self,
                 in_channels,
                 act_cfg,
                 norm_cfg,
                 expand=False,
                 align_corners=True,
                 init_cfg=None):
        super(FeatureFusionBlock, self).__init__(init_cfg)

        self.in_channels = in_channels
        self.expand = expand
        self.align_corners = align_corners

        self.out_channels = in_channels
        if self.expand:
            self.out_channels = in_channels // 2

        self.project = ConvModule(
            self.in_channels,
            self.out_channels,
            kernel_size=1,
            act_cfg=None,
            bias=True)

        self.res_conv_unit1 = PreActResidualConvUnit(
            in_channels=self.in_channels, act_cfg=act_cfg, norm_cfg=norm_cfg)
        self.res_conv_unit2 = PreActResidualConvUnit(
            in_channels=self.in_channels, act_cfg=act_cfg, norm_cfg=norm_cfg)

    def forward(self, *inputs):
        x = inputs[0]
        if len(inputs) == 2:
            if x.shape != inputs[1].shape:
                res = resize(
                    inputs[1],
                    size=(x.shape[2], x.shape[3]),
                    mode='bilinear',
                    align_corners=False)
            else:
                res = inputs[1]
            x = x + self.res_conv_unit1(res)
        x = self.res_conv_unit2(x)
        x = resize(
            x,
            scale_factor=2,
            mode='bilinear',
            align_corners=self.align_corners)
        x = self.project(x)
        return x


@HEADS.register_module()
class DPTHead(BaseDecodeHead):
    """Vision Transformers for Dense Prediction.

    This head is implemented of `DPT <https://arxiv.org/abs/2103.13413>`_.

    Args:
        embed_dims (int): The embed dimension of the ViT backbone.
            Default: 768.
        post_process_channels (List): Out channels of post process conv
            layers. Default: [96, 192, 384, 768].
        readout_type (str): Type of readout operation. Default: 'ignore'.
        patch_size (int): The patch size. Default: 16.
        expand_channels (bool): Whether expand the channels in post process
            block. Default: False.
        act_cfg (dict): The activation config for residual conv unit.
            Defalut dict(type='ReLU').
        norm_cfg (dict): Config dict for normalization layer.
            Default: dict(type='BN').
    """

    def __init__(self,
                 embed_dims=768,
                 post_process_channels=[96, 192, 384, 768],
                 readout_type='ignore',
                 patch_size=16,
                 expand_channels=False,
                 act_cfg=dict(type='ReLU'),
                 norm_cfg=dict(type='BN'),
                 **kwargs):
        super(DPTHead, self).__init__(**kwargs)

        self.in_channels = self.in_channels
        self.expand_channels = expand_channels
        self.reassemble_blocks = ReassembleBlocks(embed_dims,
                                                  post_process_channels,
                                                  readout_type, patch_size)

        self.post_process_channels = [
            channel * math.pow(2, i) if expand_channels else channel
            for i, channel in enumerate(post_process_channels)
        ]
        self.convs = nn.ModuleList()
        for channel in self.post_process_channels:
            self.convs.append(
                ConvModule(
                    channel,
                    self.channels,
                    kernel_size=3,
                    padding=1,
                    act_cfg=None,
                    bias=False))
        self.fusion_blocks = nn.ModuleList()
        for _ in range(len(self.convs)):
            self.fusion_blocks.append(
                FeatureFusionBlock(self.channels, act_cfg, norm_cfg))
        self.fusion_blocks[0].res_conv_unit1 = None
        self.project = ConvModule(
            self.channels,
            self.channels,
            kernel_size=3,
            padding=1,
            norm_cfg=norm_cfg)
        self.num_fusion_blocks = len(self.fusion_blocks)
        self.num_reassemble_blocks = len(self.reassemble_blocks.resize_layers)
        self.num_post_process_channels = len(self.post_process_channels)
        assert self.num_fusion_blocks == self.num_reassemble_blocks
        assert self.num_reassemble_blocks == self.num_post_process_channels

    def forward(self, inputs):
        assert len(inputs) == self.num_reassemble_blocks
        x = self._transform_inputs(inputs)
        x = self.reassemble_blocks(x)
        x = [self.convs[i](feature) for i, feature in enumerate(x)]
        out = self.fusion_blocks[0](x[-1])
        for i in range(1, len(self.fusion_blocks)):
            out = self.fusion_blocks[i](out, x[-(i + 1)])
        out = self.project(out)
        out = self.cls_seg(out)
        return out