目标检测算法改进系列之添加C2f-DCN模块

DCNv2简介

可变形卷积网络的卓越性能源于其适应物体几何变化的能力。通过对其自适应行为的检查，我们观察到，虽然对其神经特征的空间支持比常规的 ConvNet 更接近对象结构，但这种支持可能远远超出感兴趣区域，导致特征受到不相关图像内容的影响。为了解决这个问题，我们提出了可变形卷积网的重新表述，通过提高建模能力和更强的训练，提高了其关注相关图像区域的能力。通过更全面地集成网络内的可变形卷积，并引入扩大变形建模范围的调制机制，增强了建模能力。为了有效地利用这种丰富的建模能力，我们通过提出的特征模拟方案来指导网络训练，该方案帮助网络学习反映R-CNN特征的对象焦点和分类能力的特征。有了这些贡献，这个新版本的 Deformable ConvNets 与原始模型相比产生了显着的性能提升，并在对象检测和实例分割的 COCO 基准上产生了领先的结果。

原文地址：Deformable ConvNets v2: More Deformable, Better Results
效果图

代码实现

pytorch代码实现DCNv2模块

class DCNv2(nn.Module):
    def __init__(self, in_channels, out_channels, kernel_size, stride=1,
                 padding=1, dilation=1, groups=1, deformable_groups=1):
        super(DCNv2, self).__init__()

        self.in_channels = in_channels
        self.out_channels = out_channels
        self.kernel_size = (kernel_size, kernel_size)
        self.stride = (stride, stride)
        self.padding = (padding, padding)
        self.dilation = (dilation, dilation)
        self.groups = groups
        self.deformable_groups = deformable_groups

        self.weight = nn.Parameter(
            torch.empty(out_channels, in_channels, *self.kernel_size)
        )
        self.bias = nn.Parameter(torch.empty(out_channels))

        out_channels_offset_mask = (self.deformable_groups * 3 *
                                    self.kernel_size[0] * self.kernel_size[1])
        self.conv_offset_mask = nn.Conv2d(
            self.in_channels,
            out_channels_offset_mask,
            kernel_size=self.kernel_size,
            stride=self.stride,
            padding=self.padding,
            bias=True,
        )
        self.bn = nn.BatchNorm2d(out_channels)
        self.act = Conv.default_act
        self.reset_parameters()

    def forward(self, x):
        offset_mask = self.conv_offset_mask(x)
        o1, o2, mask = torch.chunk(offset_mask, 3, dim=1)
        offset = torch.cat((o1, o2), dim=1)
        mask = torch.sigmoid(mask)
        x = torch.ops.torchvision.deform_conv2d(
            x,
            self.weight,
            offset,
            mask,
            self.bias,
            self.stride[0], self.stride[1],
            self.padding[0], self.padding[1],
            self.dilation[0], self.dilation[1],
            self.groups,
            self.deformable_groups,
            True
        )
        x = self.bn(x)
        x = self.act(x)
        return x

    def reset_parameters(self):
        n = self.in_channels
        for k in self.kernel_size:
            n *= k
        std = 1. / math.sqrt(n)
        self.weight.data.uniform_(-std, std)
        self.bias.data.zero_()
        self.conv_offset_mask.weight.data.zero_()
        self.conv_offset_mask.bias.data.zero_()

构建Bottleneck_DCN模块

class Bottleneck_DCN(nn.Module):
    # Standard bottleneck with DCN
    def __init__(self, c1, c2, shortcut=True, g=1, k=(3, 3), e=0.5):  # ch_in, ch_out, shortcut, groups, kernels, expand
        super().__init__()
        c_ = int(c2 * e)  # hidden channels
        if k[0] == 3:
            self.cv1 = DCNv2(c1, c_, k[0], 1)
        else:
            self.cv1 = Conv(c1, c_, k[0], 1)
        if k[1] == 3:
            self.cv2 = DCNv2(c_, c2, k[1], 1, groups=g)
        else:
            self.cv2 = Conv(c_, c2, k[1], 1, g=g)
        self.add = shortcut and c1 == c2

    def forward(self, x):
        return x + self.cv2(self.cv1(x)) if self.add else self.cv2(self.cv1(x))

C2f_DCN模块定义

class C2f_DCN(nn.Module):
    # CSP Bottleneck with 2 convolutions
    def __init__(self, c1, c2, n=1, shortcut=False, g=1, e=0.5):  # ch_in, ch_out, number, shortcut, groups, expansion
        super().__init__()
        self.c = int(c2 * e)  # hidden channels
        self.cv1 = Conv(c1, 2 * self.c, 1, 1)
        self.cv2 = Conv((2 + n) * self.c, c2, 1)  # optional act=FReLU(c2)
        self.m = nn.ModuleList(Bottleneck_DCN(self.c, self.c, shortcut, g, k=(3, 3), e=1.0) for _ in range(n))

    def forward(self, x):
        y = list(self.cv1(x).split((self.c, self.c), 1))
        y.extend(m(y[-1]) for m in self.m)
        return self.cv2(torch.cat(y, 1))