MNIST手写数字识别 —— 从二分类到十分类

将一个二分类模型改成多分类模型的方法

本案例中使用深度学习框架MindSpore,利用其友好的封装模块，模型结构定义、损失函数定义、梯度下降实现等过程，只需简单地函数调用，就能实现模型训练，极大地提高了模型开发的效率。

1.加载数据集

        加载完整的、十个类别的数据集 

import os
import numpy as np

import moxing as mox
import mindspore.dataset as ds

datasets_dir = '../datasets'
if not os.path.exists(datasets_dir):
    os.makedirs(datasets_dir)
    
if not os.path.exists(os.path.join(datasets_dir, 'MNIST_Data.zip')):
    mox.file.copy('obs://modelarts-labs-bj4-v2/course/hwc_edu/python_module_framework/datasets/mindspore_data/MNIST_Data.zip', 
                  os.path.join(datasets_dir, 'MNIST_Data.zip'))
    os.system('cd %s; unzip MNIST_Data.zip' % (datasets_dir))
    
# 读取完整训练样本和测试样本
mnist_ds_train = ds.MnistDataset(os.path.join(datasets_dir, "MNIST_Data/train"))
mnist_ds_test = ds.MnistDataset(os.path.join(datasets_dir, "MNIST_Data/test"))
train_len = mnist_ds_train.get_dataset_size()
test_len = mnist_ds_test.get_dataset_size()
print('训练集规模：', train_len, '，测试集规模：', test_len)

训练集规模：60000，测试集规模：10000 

        查看10个样本

from PIL import Image
items_train = mnist_ds_train.create_dict_iterator(output_numpy=True)

train_data = np.array([i for i in items_train])
images_train = np.array([i["image"] for i in train_data])
labels_train = np.array([i["label"] for i in train_data])

batch_size = 10  # 查看10个样本
batch_label = [lab for lab in labels_train[:10]]
print(batch_label)
batch_img = images_train[0].reshape(28, 28)
for i in range(1, batch_size):
    batch_img = np.hstack((batch_img, images_train[i].reshape(28, 28)))  # 将一批图片水平拼接起来，方便下一步进行显示
Image.fromarray(batch_img)

[0, 2, 2, 7, 8, 4, 9, 1, 8, 8]

 2.处理数据集

        数据集对于训练非常重要，好的数据集可以有效提高训练精度和效率，在使用数据集前，通常会对数据集进行一些处理。

进行数据增强操作

import mindspore.dataset.vision.c_transforms as CV
import mindspore.dataset.transforms.c_transforms as C
from mindspore.dataset.vision import Inter
from mindspore import dtype as mstype

num_parallel_workers = 1
resize_height, resize_width = 28, 28

# according to the parameters, generate the corresponding data enhancement method
resize_op = CV.Resize((resize_height, resize_width), interpolation=Inter.LINEAR)  # 对图像数据像素进行缩放
type_cast_op = C.TypeCast(mstype.int32)  # 将数据类型转化为int32。
hwc2chw_op = CV.HWC2CHW()  # 对图像数据张量进行变换，张量形式由高x宽x通道（HWC）变为通道x高x宽（CHW），方便进行数据训练。

# using map to apply operations to a dataset
mnist_ds_train = mnist_ds_train.map(operations=resize_op, input_columns="image", num_parallel_workers=num_parallel_workers)
mnist_ds_train = mnist_ds_train.map(operations=type_cast_op, input_columns="label", num_parallel_workers=num_parallel_workers)
mnist_ds_train = mnist_ds_train.map(operations=hwc2chw_op, input_columns="image", num_parallel_workers=num_parallel_workers)

buffer_size = 10000
mnist_ds_train = mnist_ds_train.shuffle(buffer_size=buffer_size)  # 打乱训练集的顺序

进行数据归一化

        对图像数据进行标准化、归一化操作，使得每个像素的数值大小在（0,1）范围中，可以提升训练效率。

rescale = 1.0 / 255.0
shift = 0.0

rescale_nml = 1 / 0.3081
shift_nml = -1 * 0.1307 / 0.3081

rescale_op = CV.Rescale(rescale, shift) 
mnist_ds_train = mnist_ds_train.map(operations=rescale_op, input_columns="image", num_parallel_workers=num_parallel_workers)

rescale_nml_op = CV.Rescale(rescale_nml, shift_nml)
mnist_ds_train = mnist_ds_train.map(operations=rescale_nml_op, input_columns="image", num_parallel_workers=num_parallel_workers)

mnist_ds_train = mnist_ds_train.batch(60000, drop_remainder=True)  # 对数据集进行分批,此处加载完整的训练集

3. 封装成函数

        到此，我们就完成了训练数据的准备工作，可以将以上操作封装成load_data_all函数和process_dataset函数，以便后面再次用到。

定义数据处理操作

定义一个函数process_dataset来进行数据增强和处理操作：

• 定义进行数据增强和处理所需要的一些参数。

• 根据参数，生成对应的数据增强操作。

• 使用map映射函数，将数据操作应用到数据集。

• 对生成的数据集进行处理。 

%%writefile ../datasets/MNIST_Data/process_dataset.py
def process_dataset(mnist_ds, batch_size=32, resize= 28, repeat_size=1,
                   num_parallel_workers=1):
    """
    process_dataset for train or test

    Args:
        mnist_ds (str): MnistData path
        batch_size (int): The number of data records in each group
        resize (int): Scale image data pixels
        repeat_size (int): The number of replicated data records
        num_parallel_workers (int): The number of parallel workers
    """

    import mindspore.dataset.vision.c_transforms as CV
    import mindspore.dataset.transforms.c_transforms as C
    from mindspore.dataset.vision import Inter
    from mindspore import dtype as mstype
    
    # define some parameters needed for data enhancement and rough justification
    resize_height, resize_width = resize, resize
    rescale = 1.0 / 255.0
    shift = 0.0
    rescale_nml = 1 / 0.3081
    shift_nml = -1 * 0.1307 / 0.3081

    # according to the parameters, generate the corresponding data enhancement method
    resize_op = CV.Resize((resize_height, resize_width), interpolation=Inter.LINEAR)
    rescale_nml_op = CV.Rescale(rescale_nml, shift_nml)
    rescale_op = CV.Rescale(rescale, shift)
    hwc2chw_op = CV.HWC2CHW()
    type_cast_op = C.TypeCast(mstype.int32)
    c_trans = [resize_op, rescale_op, rescale_nml_op, hwc2chw_op]

    # using map to apply operations to a dataset
    mnist_ds = mnist_ds.map(operations=type_cast_op, input_columns="label", num_parallel_workers=num_parallel_workers)
    mnist_ds = mnist_ds.map(operations=c_trans, input_columns="image", num_parallel_workers=num_parallel_workers)

    # process the generated dataset
    buffer_size = 10000
    mnist_ds = mnist_ds.shuffle(buffer_size=buffer_size)
    mnist_ds = mnist_ds.batch(batch_size, drop_remainder=True)
    mnist_ds = mnist_ds.repeat(repeat_size)

    return mnist_ds

定义数据加载函数 

%%writefile ../datasets/MNIST_Data/load_data_all.py
def load_data_all(datasets_dir):
    import os
    if not os.path.exists(datasets_dir):
        os.makedirs(datasets_dir)
    import moxing as mox
    if not os.path.exists(os.path.join(datasets_dir, 'MNIST_Data.zip')):
        mox.file.copy('obs://modelarts-labs-bj4-v2/course/hwc_edu/python_module_framework/datasets/mindspore_data/MNIST_Data.zip', 
                      os.path.join(datasets_dir, 'MNIST_Data.zip'))
        os.system('cd %s; unzip MNIST_Data.zip' % (datasets_dir))
    
    # 读取完整训练样本和测试样本
    import mindspore.dataset as ds
    datasets_dir = '../datasets'
    mnist_ds_train = ds.MnistDataset(os.path.join(datasets_dir, "MNIST_Data/train"))
    mnist_ds_test = ds.MnistDataset(os.path.join(datasets_dir, "MNIST_Data/test"))
    train_len = mnist_ds_train.get_dataset_size()
    test_len = mnist_ds_test.get_dataset_size()
    print('训练集规模：', train_len, '，测试集规模：', test_len)
    
    return mnist_ds_train, mnist_ds_test, train_len, test_len

4.加载处理后的测试集

import os, sys 
sys.path.insert(0, os.path.join(os.getcwd(), '../datasets/MNIST_Data'))
from process_dataset import process_dataset
mnist_ds_test = process_dataset(mnist_ds_test, batch_size= 10000)

5.定义网络结构和评价函数 

import mindspore
import mindspore.nn as nn
import mindspore.ops as ops
from mindspore.common.initializer import Normal

class Network(nn.Cell):
    def __init__(self, num_of_weights):
        super(Network, self).__init__()
        self.fc = nn.Dense(in_channels=num_of_weights, out_channels=10, weight_init=Normal(0.02))  # 定义一个全连接层
        self.nonlinearity = nn.Sigmoid()
        self.flatten = nn.Flatten()
    
    def construct(self, x):  # 加权求和单元和非线性函数单元通过定义计算过程来实现
        x = self.flatten(x)
        z = self.fc(x)
        pred_y = self.nonlinearity(z)
        return pred_y
    
def evaluate(pred_y, true_y): 
    pred_labels = ops.Argmax(output_type=mindspore.int32)(pred_y)
    correct_num = (pred_labels == true_y).asnumpy().sum().item()
    return correct_num

6.定义交叉熵损失函数和优化器

# 损失函数
net_loss = nn.SoftmaxCrossEntropyWithLogits(sparse=True, reduction='mean')

# 创建网络
network = Network(28*28)  
lr = 0.01
momentum = 0.9

# 优化器
net_opt = nn.Momentum(network.trainable_params(), lr, momentum)

7.实现训练函数 

def train(network, mnist_ds_train, max_epochs= 50):
    net = WithLossCell(network, net_loss)
    net = TrainOneStepCell(net, net_opt)
    network.set_train()
    for epoch in range(1, max_epochs + 1):
        train_correct_num = 0.0
        test_correct_num = 0.0
        for inputs_train in mnist_ds_train:
            output = net(*inputs_train)
            train_x = inputs_train[0]
            train_y = inputs_train[1]
            pred_y_train = network.construct(train_x)  # 前向传播
            train_correct_num += evaluate(pred_y_train, train_y)
        train_acc = float(train_correct_num) / train_len
        
        for inputs_test in mnist_ds_test:
            test_x = inputs_test[0]
            test_y = inputs_test[1]
            pred_y_test = network.construct(test_x)
            test_correct_num += evaluate(pred_y_test, test_y)
        test_acc = float(test_correct_num) / test_len
        if (epoch == 1) or (epoch % 10 == 0):
            print("epoch: {0}/{1}, train_losses: {2:.4f}, tain_acc: {3:.4f}, test_acc: {4:.4f}" \
                  .format(epoch, max_epochs, output.asnumpy(), train_acc, test_acc, cflush=True))

8. 配置运行信息

        在正式训练前，通过context.set_context来配置运行需要的信息，譬如运行模式、后端信息、硬件等信息。        

from mindspore import context
context.set_context(mode=context.GRAPH_MODE, device_target="CPU")  # device_target 可选 CPU/GPU, 当选择GPU时mindspore规格也需要切换到GPU

 9.开始训练

import time
from mindspore.nn import WithLossCell, TrainOneStepCell

max_epochs = 100
start_time = time.time()
print("*"*10 + "开始训练" + "*"*10)
train(network, mnist_ds_train, max_epochs= max_epochs)
print("*"*10 + "训练完成" + "*"*10)
cost_time = round(time.time() - start_time, 1)
print("训练总耗时： %.1f s" % cost_time)

**********开始训练**********

epoch: 1/100, train_losses: 2.2832, tain_acc: 0.1698, test_acc: 0.1626

epoch: 10/100, train_losses: 2.0465, tain_acc: 0.6343, test_acc: 0.6017

epoch: 20/100, train_losses: 1.8368, tain_acc: 0.7918, test_acc: 0.7812

epoch: 30/100, train_losses: 1.7602, tain_acc: 0.8138, test_acc: 0.8017

epoch: 40/100, train_losses: 1.7245, tain_acc: 0.8238, test_acc: 0.7972

epoch: 50/100, train_losses: 1.7051, tain_acc: 0.8337, test_acc: 0.8044

epoch: 60/100, train_losses: 1.6922, tain_acc: 0.8403, test_acc: 0.8047

epoch: 70/100, train_losses: 1.6827, tain_acc: 0.8454, test_acc: 0.8033

epoch: 80/100, train_losses: 1.6752, tain_acc: 0.8501, test_acc: 0.8051

epoch: 90/100, train_losses: 1.6689, tain_acc: 0.8536, test_acc: 0.8049

epoch: 100/100, train_losses: 1.6635, tain_acc: 0.8569, test_acc: 0.8037

**********训练完成**********

训练总耗时： 430.7 s

到目前为止，基于手写数字二分类的代码进行少量修改，就快速实现了手写数字识别的十分类。

修改的过程是非常简单的，但从上面的结果可以看到，该模型训练100个epoch，在手写数字识别十分类的任务上仅仅达到了80%的准确率，而在上一节二分类任务上，模型训练50个epoch达到了99%的准确率，说明在感知机这样简单的模型上，手写数字识别十分类要比二分类要难。