gcnet.py

# Copyright (c) 2020 PaddlePaddle Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#    http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import os

import paddle
import paddle.nn.functional as F
from paddle import nn
from paddleseg.cvlibs import manager
from paddleseg.models.common import layer_libs
from paddleseg.utils import utils


@manager.MODELS.add_component
class GCNet(nn.Layer):
    """
    The GCNet implementation based on PaddlePaddle.

    The orginal artile refers to 
        Cao, Yue, et al. "GCnet: Non-local networks meet squeeze-excitation networks and beyond."
        (https://arxiv.org/pdf/1904.11492.pdf)

    Args:
        num_classes (int): the unique number of target classes.
        backbone (Paddle.nn.Layer): backbone network, currently support Resnet50/101.
        model_pretrained (str): the path of pretrained model. Defaullt to None.
        backbone_indices (tuple): two values in the tuple indicte the indices of output of backbone.
            the first index will be taken as a deep-supervision feature in auxiliary layer;
            the second one will be taken as input of GlobalContextBlock. Usually backbone 
            consists of four downsampling stage, and return an output of each stage, so we 
            set default (2, 3), which means taking feature map of the third stage (res4b22) 
            and the fourth stage (res5c) in backbone.
        backbone_channels (tuple): the same length with "backbone_indices". It indicates the channels of corresponding index.
        gc_channels (int): input channels to Global Context Block. Default to 512.
        ratio (float): it indictes the ratio of attention channels and gc_channels. Default to 1/4.
        enable_auxiliary_loss (bool): a bool values indictes whether adding auxiliary loss. Default to True.
    """

    def __init__(self,
                 num_classes,
                 backbone,
                 model_pretrained=None,
                 backbone_indices=(2, 3),
                 backbone_channels=(1024, 2048),
                 gc_channels=512,
                 ratio=1 / 4,
                 enable_auxiliary_loss=True,
                 pretrained_model=None):

        super(GCNet, self).__init__()

        self.backbone = backbone

        in_channels = backbone_channels[1]
        self.conv_bn_relu1 = layer_libs.ConvBnRelu(
            in_channels=in_channels,
            out_channels=gc_channels,
            kernel_size=3,
            padding=1)

        self.gc_block = GlobalContextBlock(in_channels=gc_channels, ratio=ratio)

        self.conv_bn_relu2 = layer_libs.ConvBnRelu(
            in_channels=gc_channels,
            out_channels=gc_channels,
            kernel_size=3,
            padding=1)

        self.conv_bn_relu3 = layer_libs.ConvBnRelu(
            in_channels=in_channels + gc_channels,
            out_channels=gc_channels,
            kernel_size=3,
            padding=1)

        self.conv = nn.Conv2d(
            in_channels=gc_channels, out_channels=num_classes, kernel_size=1)

        if enable_auxiliary_loss:
            self.auxlayer = layer_libs.AuxLayer(
                in_channels=backbone_channels[0],
                inter_channels=backbone_channels[0] // 4,
                out_channels=num_classes)

        self.backbone_indices = backbone_indices
        self.enable_auxiliary_loss = enable_auxiliary_loss

        self.init_weight(model_pretrained)

    def forward(self, input, label=None):

        logit_list = []
        _, feat_list = self.backbone(input)
        x = feat_list[self.backbone_indices[1]]

        output = self.conv_bn_relu1(x)
        output = self.gc_block(output)
        output = self.conv_bn_relu2(output)

        output = paddle.concat([x, output], axis=1)
        output = self.conv_bn_relu3(output)

        output = F.dropout(output, p=0.1)  # dropout_prob
        logit = self.conv(output)
        logit = F.resize_bilinear(logit, input.shape[2:])
        logit_list.append(logit)

        if self.enable_auxiliary_loss:
            low_level_feat = feat_list[self.backbone_indices[0]]
            auxiliary_logit = self.auxlayer(low_level_feat)
            auxiliary_logit = F.resize_bilinear(auxiliary_logit,
                                                input.shape[2:])
            logit_list.append(auxiliary_logit)

        return logit_list

    def init_weight(self, pretrained_model=None):
        """
        Initialize the parameters of model parts.
        Args:
            pretrained_model ([str], optional): the path of pretrained model. Defaults to None.
        """
        if pretrained_model is not None:
            if os.path.exists(pretrained_model):
                utils.load_pretrained_model(self, pretrained_model)
            else:
                raise Exception('Pretrained model is not found: {}'.format(
                    pretrained_model))


class GlobalContextBlock(nn.Layer):
    """
    Global Context Block implementation.

    Args:
        in_channels (int): input channels of Global Context Block
        ratio (float): the channels of attention map.
    """

    def __init__(self, in_channels, ratio):
        super(GlobalContextBlock, self).__init__()

        self.conv_mask = nn.Conv2d(
            in_channels=in_channels, out_channels=1, kernel_size=1)

        self.softmax = nn.Softmax(axis=2)
        
        inter_channels = int(in_channels * ratio)
        self.channel_add_conv = nn.Sequential(
            nn.Conv2d(
                in_channels=in_channels,
                out_channels=inter_channels,
                kernel_size=1),
            nn.LayerNorm(normalized_shape=[inter_channels, 1, 1]), nn.ReLU(),
            nn.Conv2d(
                in_channels=inter_channels,
                out_channels=in_channels,
                kernel_size=1))

    def global_context_block(self, x):
        batch, channel, height, width = x.shape

        # [N, C, H * W]
        input_x = paddle.reshape(x, shape=[batch, channel, height * width])
        # [N, 1, C, H * W]
        input_x = paddle.unsqueeze(input_x, axis=1)
        # [N, 1, H, W]
        context_mask = self.conv_mask(x)
        # [N, 1, H * W]
        context_mask = paddle.reshape(
            context_mask, shape=[batch, 1, height * width])
        context_mask = self.softmax(context_mask)
        # [N, 1, H * W, 1]
        context_mask = paddle.unsqueeze(context_mask, axis=-1)
        # [N, 1, C, 1]
        context = paddle.matmul(input_x, context_mask)
        # [N, C, 1, 1]
        context = paddle.reshape(context, shape=[batch, channel, 1, 1])

        return context

    def forward(self, x):
        context = self.global_context_block(x)
        channel_add_term = self.channel_add_conv(context)
        out = x + channel_add_term
        return out