CNN中的BN（伪代码讲解）_cnn伪代码

https: // www.cnblogs.com / adong7639 / p / 9145911.
html
写的很好
'''
本文讲解的是在CNN中的batch normalization
'''
import torch
import torch.nn as nn
import copy
 
 
class Net(nn.Module):
    def __init__(self, dim, pretrained):
        super(Net, self).__init__()
        self.bn = nn.BatchNorm2d(dim, 0)
        if pretrained:
            self.pretrained()
 
    def forward(self, input):
        return self.bn(input)
 
    def pretrained(self):
        nn.init.constant_(self.bn.weight, 1)
        nn.init.constant_(self.bn.bias, 0)
 
 
def train():
    dim = 3
    model = Net(dim)
 
    print(sum(p.numel() for p in model.parameters() if p.requires_grad))
 
    for p in model.parameters():
        print(p, p.requires_grad)
    '''
    对于CNN特征图通道数为3的Batch normalization层而言，BN层的learnable parameter有6个，分别是gamma和beta
    在训练过程中gamma和beta才是需要被更新的
    6
    Parameter containing:
    tensor([0.2322, 0.9405, 0.9887], requires_grad=True) True
    Parameter containing:
    tensor([0., 0., 0.], requires_grad=True) True
    '''
 
    # model.eval()
    feature_map = torch.randn((2, 3, 2, 2))
 
    output1 = model(feature_map)
    state_dict = model.state_dict()
    for k, v in state_dict.items():
        print(k, v)
    '''
    bn.weight tensor([0.2860, 0.5986, 0.0594])
    bn.bias tensor([0., 0., 0.])
    bn.running_mean tensor([-0.2098,  0.1876, -0.3045])
    bn.running_var tensor([0.8099, 1.5140, 0.5880])
    bn.num_batches_tracked tensor(1)
    打印字典时，发现batch normalization层有5个参数
    其中bn.weight 对应论文中的gamma   bn.bias对应论文中的beta
    bn.running_mean则是对于当前batch size的数据所统计出来的平均值
    bn.running_var是对于当前batch size的数据所统计出来的方差
    '''
    print('bn.running_mean', state_dict['bn.running_mean'])
    print('bn.running_var', state_dict['bn.running_var'])
    #
    print(torch.mean(feature_map.permute(1, 0, 2, 3).contiguous().view(dim, -1), 1))
    print(torch.var(feature_map.permute(1, 0, 2, 3).contiguous().view(dim, -1), 1))
    '''
    bn.running_mean tensor([-0.2098,  0.1876, -0.3045])
    bn.running_var tensor([0.8099, 1.5140, 0.5880])
 
    tensor([-0.2098,  0.1876, -0.3045])
    tensor([0.8099, 1.5140, 0.5880])
    当然这是在设定BN层的momentum=1时，即当前时刻的统计量（running_mean和running_var）完全由统计平均值决定
    statistic_t_new=(1-momentum)*stastic_(t-1)+momentum)*stastic_(t)
    momentum决定当前时刻的bn.running_mean和bn.running_var数值
    （1）当momentum=1时，则数值完全由当前时刻计算出来的统计量决定
    （2）由于模型上一次的统计量（由于这里不进行模型的参数更新和迭代训练，故而模型的初始值
    bn.running_mean tensor([0., 0., 0.])
    bn.running_var tensor([1., 1., 1.])）   可能不是0 0 0   1  1  1，而是随机初始化
    故而如果将momentum设置为0，则模型会一直保持
    bn.running_mean tensor([0., 0., 0.])
    bn.running_var tensor([1., 1., 1.]) 
    （3）当设置默认参数momentum=0.1时
    bn.running_mean tensor([0.0233, 0.0166, 0.0469])
    bn.running_var tensor([0.9961, 1.0899, 0.9974])
    tensor([0.2329, 0.1663, 0.4691])    表示用tensor的方法计算出来的统计量
    tensor([0.9615, 1.8986, 0.9738])
    刚好bn.running_mean和bn.running_var是统计量的0.1倍
 
    再次回顾计算BN的方式：
    对于CNN的输入而言（即BN的输出时4-dimension），则
    在batch，H,W 维度上进行normalization，也被称为spatial batch normalization
    '''
 
 
if __name__ == '__main__':
    '''
    在BN层中，一般，bn.weight时随机初始化的，而bn.bias初始化为全0
    假设现在已知输入特征图的数值，和对应batch normalization的参数，求BN输出的结果
    momentum=0.1默认值    0.9*(t-1时刻的统计量)+0.1*(t时刻的统计量)
    '''
    dim = 3
    momentum = 0.1
    model = Net(dim, True)
    input = torch.randn((2, 3, 2, 2))
    output1 = model(input)
 
 
def bn_simple_train(input, model):
    '''
    :param input: 卷积神经网络特征图  shape [batch size,C,H,W]
    :return:
    '''
    mean = torch.mean(input.permute(1, 0, 2, 3).contiguous().view(dim, -1), 1)  # shape [dim]
    var = torch.var(input.permute(1, 0, 2, 3).contiguous().view(dim, -1), 1)  # shape [dim]
 
    init_mean = torch.zeros((dim))
    init_var = torch.ones((dim))
 
    run_mean = (1 - momentum) * init_mean + momentum * mean  # 滑动平均的方式计算新的均值，训练时计算，为测试数据做准备
    run_var = (1 - momentum) * init_var + momentum * var  # 滑动平均的方式计算新的方差，训练时计算，为测试数据做准备
 
    run_std = torch.sqrt(run_var + 1e-5)
 
    run_mean_exp = run_mean.view(1, input.shape[1], 1, 1).expand(input.shape)
    run_std_exp = run_std.view(1, input.shape[1], 1, 1).expand(input.shape)
    '''
    这里的tensor复制问题也让我想了很久
    
    tensor1=torch.tensor([1,2,3])
    需要得到一个2*3*2*2的tensor2，然后需要满足
    tensor2[:,0,:,:]=1
    tensor2[:,1,:,:]=2
    tensor2[:,2,:,:]=3
    这个，除了for循环，内部函数也可以实现
    
    先unsqueeze 到 ，(1， 2， 1， 1)  再 expand(2，3，2，2)
    expand, 只能再指定维度进行复制， 不能增加维度， 所以你要先unsqueeze到4个维度
    expand的时候会找channel相同的维度，这些维度不变，其他维度复制
    
    '''
    
    # run_mean_exp=torch.zeros((2,3,2,2))
    # for i in range(3):
    #     run_mean_exp[:,i,:,:]=run_mean[i]
 
    # run_std_exp = torch.zeros((2, 3, 2, 2))
    # for i in range(3):
    #     run_std_exp[:, i, :, :] = run_std[i]
 
    output2 = input - run_mean_exp
    output2 = output2 / run_std_exp
 
    init_weights = model.state_dict().items()['bn.weights']  # gamma
    init_bias = model.state_dict().items()['bn.bias']  # beta
    init_weights_exp = init_weights.view(1, input.shape[1], 1, 1).expand(input.shape)
    init_bias_exp = init_bias.view(1, input.shape[1], 1, 1).expand(input.shape)
    '''
    在训练过程中会一直更新（反向传播时）的可学习参数
    '''
    # init_weights_exp=torch.zeros((2, 3, 2, 2))
    # for i in range(3):
    #     init_weights_exp[:, i, :, :] = init_weights[i]
    #
    # init_bias_exp = torch.zeros((2, 3, 2, 2))
    # for i in range(3):
    #     init_bias_exp[:, i, :, :] = init_bias[i]
    output2 = output2 * init_weights_exp
    output2 = output2 + init_bias_exp
 
    return output2
 
 
def bn_for_test(input, model):
    '''
    测试过程中，BN层的running mean和running var都是固定值，不再时新的验证数据的统计量，在model.eval()模式下这两个参数会被固定住
    而gamma和beta也不发生改变
    :param input:
    :param model:
    :return:
    '''
    state_dict = model.state_dict()
    init_weights = state_dict.items()['bn.weight']
    init_bias = state_dict.items()['bn.bias']
    running_mean = state_dict.items()(['bn.running_mean']
    running_var = state_dict.tems()['bn.running_var']
 
    mean = running_mean.view(1, input.shape[1], 1, 1).expand(input.shape)
    var = running_var.view(1, input.shape[1], 1, 1).expand(input.shape)
    weights = init_weights.view(1, input.shape[1], 1, 1).expand(input.shape)
    bias = init_bias.view(1, input.shape[1], 1, 1).expand(input.shape)
 
    output = (input - mean) / torch.sqrt(var + 1e-5)
    output = output * weights + bias
    return output