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CapsNet基本结构

参考CapsNet的论文，提出的基本结构如下所示：

capsnet_mnist.jpg

可以看出，CapsNet的基本结构如下所示：

普通卷积层Conv1：基本的卷积层，感受野较大，达到了9x9
预胶囊层PrimaryCaps：为胶囊层准备，运算为卷积运算，最终输出为[batch,caps_num,caps_length]的三维数据：
- batch为批大小
- caps_num为胶囊的数量
- caps_length为每个胶囊的长度（每个胶囊为一个向量，该向量包括caps_length个分量）
胶囊层DigitCaps：胶囊层，目的是代替最后一层全连接层，输出为10个胶囊

代码实现

胶囊相关组件

激活函数Squash

胶囊网络有特有的激活函数Squash函数：
$$
Squash(S) = \cfrac{||S||^2}{1+||S||2} \cdot \cfrac{S}{||S||}
$$
其中输入为S胶囊，该激活函数可以将胶囊的长度压缩，代码实现如下：

def squash(inputs, axis=-1):
    norm = torch.norm(inputs, p=2, dim=axis, keepdim=True)
    scale = norm**2 / (1 + norm**2) / (norm + 1e-8)
    return scale * inputs

其中：

norm = torch.norm(inputs, p=2, dim=axis, keepdim=True)计算输入胶囊的长度，p=2表示计算的是二范数，keepdim=True表示保持原有的空间形状。
scale = norm**2 / (1 + norm**2) / (norm + 1e-8)计算缩放因子，即$ \cfrac{||S||^2}{1+||S||2} \cdot \cfrac{1}{||S||}$
return scale * inputs完成计算

预胶囊层PrimaryCaps

class PrimaryCapsule(nn.Module):
    """
    Apply Conv2D with `out_channels` and then reshape to get capsules
    :param in_channels: input channels
    :param out_channels: output channels
    :param dim_caps: dimension of capsule
    :param kernel_size: kernel size
    :return: output tensor, size=[batch, num_caps, dim_caps]
    """
    def __init__(self, in_channels, out_channels, dim_caps, kernel_size, stride=1, padding=0):
        super(PrimaryCapsule, self).__init__()
        self.dim_caps = dim_caps
        self.conv2d = nn.Conv2d(in_channels, out_channels, kernel_size=kernel_size, stride=stride, padding=padding)

    def forward(self, x):
        outputs = self.conv2d(x)
        outputs = outputs.view(x.size(0), -1, self.dim_caps)
        return squash(outputs)

预胶囊层使用卷积层实现，其前向传播包括三个部分：

outputs = self.conv2d(x)：对输入进行卷积处理，这一步output的形状是[batch,out_channels,p_w,p_h]
outputs = outputs.view(x.size(0), -1, self.dim_caps)：将4D的卷积输出变为3D的胶囊输出形式，output的形状为[batch,caps_num,dim_caps]，其中caps_num为胶囊数量，可自动计算；dim_caps为胶囊长度，需要预先指定。
return squash(outputs)：激活函数，并返回激活后的胶囊

胶囊层DigitCaps

参数定义

def __init__(self, in_num_caps, in_dim_caps, out_num_caps, out_dim_caps, routings=3):
    super(DenseCapsule, self).__init__()
    self.in_num_caps = in_num_caps
    self.in_dim_caps = in_dim_caps
    self.out_num_caps = out_num_caps
    self.out_dim_caps = out_dim_caps
    self.routings = routings
    self.weight = nn.Parameter(0.01 * torch.randn(out_num_caps, in_num_caps, out_dim_caps, in_dim_caps))

参数定义如下：

in_num_caps：输入胶囊的数量
in_dim_caps：输入胶囊的长度（维数）
out_num_caps：输出胶囊的数量
out_dim_caps：输出胶囊的长度（维数）
routings：动态路由迭代的次数

另外，还定义了权值weight，尺寸为[out_num_caps, in_num_caps, out_dim_caps, in_dim_caps]，即每个输出和每个输出胶囊都有连接

前向传播

def forward(self, x):
    x_hat = torch.squeeze(torch.matmul(self.weight, x[:, None, :, :, None]), dim=-1)
    x_hat_detached = x_hat.detach()

    b = Variable(torch.zeros(x.size(0), self.out_num_caps, self.in_num_caps)).cuda()
    assert self.routings > 0, 'The \'routings\' should be > 0.'
    for i in range(self.routings):
        c = F.softmax(b, dim=1)
        if i == self.routings - 1:
            outputs = squash(torch.sum(c[:, :, :, None] * x_hat, dim=-2, keepdim=True))
        else:
            outputs = squash(torch.sum(c[:, :, :, None] * x_hat_detached, dim=-2, keepdim=True))
            b = b + torch.sum(outputs * x_hat_detached, dim=-1)
    return torch.squeeze(outputs, dim=-2)

前向传播分为两个部分：输入映射和动态路由。输入映射如下所示：

x_hat = torch.squeeze(torch.matmul(self.weight, x[:, None, :, :, None]), dim=-1)
- x[:, None, :, :, None]将数据维度从[batch, in_num_caps, in_dim_caps]扩展到[batch, 1,in_num_caps, in_dim_caps,1]
- torch.matmul()将weight和扩展后的输入相乘，weight的尺寸是[out_num_caps, in_num_caps, out_dim_caps, in_dim_caps]，相乘后结果尺寸为[batch, out_num_caps, in_num_caps,out_dim_caps, 1]
- torch.squeeze()去除多余的维度，去除后结果尺寸[batch,out_num_caps,in_num_caps,out_dim_caps]
x_hat_detached = x_hat.detach()截断梯度反向传播

这一部分结束后，每个输入胶囊都产生了out_num_caps个输出胶囊，所以目前共有in_num_caps*out_num_caps个胶囊，第二部分是动态路由，动态路由的算法图如下所示：

dynamic_route.jpg

以下部分实现了该过程：

b = Variable(torch.zeros(x.size(0), self.out_num_caps, self.in_num_caps)).cuda()
    for i in range(self.routings):
        c = F.softmax(b, dim=1)
        if i == self.routings - 1:
            outputs = squash(torch.sum(c[:, :, :, None] * x_hat, dim=-2, keepdim=True))
        else:
            outputs = squash(torch.sum(c[:, :, :, None] * x_hat_detached, dim=-2, keepdim=True))
            b = b + torch.sum(outputs * x_hat_detached, dim=-1)

第一部分是softmax函数，使用c = F.softmax(b, dim=1)实现，该步骤不改变b的尺寸
第二部分是计算路由结果：outputs = squash(torch.sum(c[:, :, :, None] * x_hat, dim=-2, keepdim=True))
- c[:, :, :, None]扩展c的维度，以便按位置相乘时广播维度
- torch.sum(c[:, :, :, None] * x_hat, dim=-2, keepdim=True)计算出每个胶囊与对应权值的积，即算法中的$s_j$，同时在倒数第二维上求和，则该步输出的结果尺寸为[batch, out_num_caps, 1,out_dim_caps]
- 通过激活函数squash()
第三部分更新权重b = b + torch.sum(outputs * x_hat_detached, dim=-1)，两个按位相乘的变量尺寸分别为[batch, out_num_caps, in_num_caps, out_dim_caps]和[batch, out_num_caps, 1,out_dim_caps]，倒数第二维上有广播行为，因此最终结果为[batch, out_num_caps, in_num_caps]

其他组件

网络结构

class CapsuleNet(nn.Module):
    """
    A Capsule Network on MNIST.
    :param input_size: data size = [channels, width, height]
    :param classes: number of classes
    :param routings: number of routing iterations
    Shape:
        - Input: (batch, channels, width, height), optional (batch, classes) .
        - Output:((batch, classes), (batch, channels, width, height))
    """
    def __init__(self, input_size, classes, routings):
        super(CapsuleNet, self).__init__()
        self.input_size = input_size
        self.classes = classes
        self.routings = routings

        # Layer 1: Just a conventional Conv2D layer
        self.conv1 = nn.Conv2d(input_size[0], 256, kernel_size=9, stride=1, padding=0)

        # Layer 2: Conv2D layer with `squash` activation, then reshape to [None, num_caps, dim_caps]
        self.primarycaps = PrimaryCapsule(256, 256, 8, kernel_size=9, stride=2, padding=0)

        # Layer 3: Capsule layer. Routing algorithm works here.
        self.digitcaps = DenseCapsule(in_num_caps=32*6*6, in_dim_caps=8,
                                      out_num_caps=classes, out_dim_caps=16, routings=routings)

        # Decoder network.
        self.decoder = nn.Sequential(
            nn.Linear(16*classes, 512),
            nn.ReLU(inplace=True),
            nn.Linear(512, 1024),
            nn.ReLU(inplace=True),
            nn.Linear(1024, input_size[0] * input_size[1] * input_size[2]),
            nn.Sigmoid()
        )

        self.relu = nn.ReLU()

    def forward(self, x, y=None):
        x = self.relu(self.conv1(x))
        x = self.primarycaps(x)
        x = self.digitcaps(x)
        length = x.norm(dim=-1)
        if y is None:  # during testing, no label given. create one-hot coding using `length`
            index = length.max(dim=1)[1]
            y = Variable(torch.zeros(length.size()).scatter_(1, index.view(-1, 1).cpu().data, 1.).cuda())
        reconstruction = self.decoder((x * y[:, :, None]).view(x.size(0), -1))
        return length, reconstruction.view(-1, *self.input_size)

网络组件包括两个部分：胶囊网络和重建网络，重建网络为多层感知机，根据胶囊的结果重建了图像，这表示胶囊除了包括结果外，还可以包括一些空间信息。

注意胶囊网络的前向传播部分为：

x = self.relu(self.conv1(x))
x = self.primarycaps(x)
x = self.digitcaps(x)
length = x.norm(dim=-1)

最终的输出为每个胶囊的二范数，即向量的长度

代价函数

胶囊神经网络的胶囊部分的代价函数如下所示
$$
L_c = T_c max(0,m^+ - ||V_c||)^2 + \lambda (1 - T_c)max(0,||v_c|| - m^-) ^ 2
$$

以下代码实现了这个部分，其中L为胶囊的代价函数计算，这里$m^+=0.9,m-=0.1$，L_recon为重建的代价函数，为输入图像与复原图像的MSELoss函数。

def caps_loss(y_true, y_pred, x, x_recon, lam_recon):
    L = y_true * torch.clamp(0.9 - y_pred, min=0.) ** 2 + \
        0.5 * (1 - y_true) * torch.clamp(y_pred - 0.1, min=0.) ** 2
    L_margin = L.sum(dim=1).mean()
    L_recon = nn.MSELoss()(x_recon, x)
    return L_margin + lam_recon * L_recon

参考

CapsNet论文

CapsNet开源代码

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基于Pytorch的CapsNet源码详解