yolov3代码详解1-网络结构搭建

YOLOv3网络结构的搭建

import math
from collections import OrderedDict

import torch
import torch.nn as nn

1. 残差块的搭建

class BasicBlock(nn.Module):
    def __init__(self, inplanes, planes):
        super(BasicBlock, self).__init__()
        self.conv1 = nn.Conv2d(inplanes, planes[0], kernel_size=1,stride=1, padding=0, bias=False)
        self.bn1 = nn.BatchNorm2d(planes[0])
        self.relu1 = nn.LeakyReLU(0.1)
        
        self.conv2 = nn.Conv2d(planes[0], planes[1], kernel_size=3,stride=1, padding=1, bias=False)
        self.bn2 = nn.BatchNorm2d(planes[1])
        self.relu2 = nn.LeakyReLU(0.1)

    def forward(self, x):
        residual = x

        out = self.conv1(x)
        out = self.bn1(out)
        out = self.relu1(out)

        out = self.conv2(out)
        out = self.bn2(out)
        out = self.relu2(out)

        out += residual
        return out

2. Darknet53主网络结构的搭建

每一次的残差块堆叠过程都是先进行一步下采样，下采样的尺寸为3x3/2，再进行相应次数的堆叠，如下图所示

 一共要进行五次残差块处理，每次堆叠的次数分别为[1,2,8,8,4]

 

# 主网络结构的搭建
class DarkNet(nn.Module):
    def __init__(self, layers):
        super(DarkNet, self).__init__()
        self.inplanes = 32

        # [416,416,3] -> [416,416,32]
        self.conv1 = nn.Conv2d(3, self.inplanes, kernel_size=3, stride=1, padding=1, bias=False)
        self.bn1 = nn.BatchNorm2d(self.inplanes)
        self.relu1 = nn.LeakyReLU(0.1)

        # [416,416,32] -> [208,208,64]
        self.layer1 = self._make_layer([32, 64], layers[0])     # layers内为残差块的堆叠次数
        # 208,208,64 -> 104,104,128
        self.layer2 = self._make_layer([64, 128], layers[1])
        # 104,104,128 -> 52,52,256
        self.layer3 = self._make_layer([128, 256], layers[2])
        # 52,52,256 -> 26,26,512
        self.layer4 = self._make_layer([256, 512], layers[3])
        # 26,26,512 -> 13,13,1024
        self.layer5 = self._make_layer([512, 1024], layers[4])

        self.layers_out_filters = [64, 128, 256, 512, 1024]

        # 进行权值初始化
        for m in self.modules():
            if isinstance(m, nn.Conv2d):
                n = m.kernel_size[0] * m.kernel_size[1] * m.out_channels
                m.weight.data.normal_(0, math.sqrt(2. / n))
            elif isinstance(m, nn.BatchNorm2d):
                m.weight.data.fill_(1)
                m.bias.data.zero_()

    #---------------------------------------------------------------------#
    #   在每一个layer里面，首先利用一个步长为2的3x3卷积进行下采样
    #   然后进行残差结构的堆叠
    #---------------------------------------------------------------------#
    def _make_layer(self, planes, blocks):
        layers = []

        # 下采样，步长为2，卷积核大小为3
        layers.append(("ds_conv", nn.Conv2d(self.inplanes, planes[1], kernel_size=3,stride=2, padding=1, bias=False)))
        layers.append(("ds_bn", nn.BatchNorm2d(planes[1])))
        layers.append(("ds_relu", nn.LeakyReLU(0.1)))

        # 加入残差结构
        self.inplanes = planes[1]
        for i in range(0, blocks):
            layers.append(("residual_{}".format(i), BasicBlock(self.inplanes, planes)))
        return nn.Sequential(OrderedDict(layers))

    def forward(self, x):
        x = self.conv1(x)
        x = self.bn1(x)
        x = self.relu1(x)

        x = self.layer1(x)
        x = self.layer2(x)
        out3 = self.layer3(x)
        out4 = self.layer4(out3)
        out5 = self.layer5(out4)

        return out3, out4, out5

 

3. 加载预训练模型

def darknet53(pretrained, **kwargs):
    model = DarkNet([1, 2, 8, 8, 4])
    if pretrained:
        if isinstance(pretrained, str):
            '判断pretrained是否为字符串'
            model.load_state_dict(torch.load(pretrained))
        else:
            raise Exception("darknet request a pretrained path. got [{}]".format(pretrained))
    return model