【PaddlePaddle系列】CIFAR-10图像分类

前言

本文与前文对手写数字识别分类基本类似的，同样图像作为输入，类别作为输出。这里不同的是，不仅仅是使用简单的卷积神经网络加上全连接层的模型。卷积神经网络大火以来，发展出来许多经典的卷积神经网络模型，包括VGG、ResNet、AlexNet等等。下面将针对CIFAR-10数据集，对图像进行分类。

1、CIFAR-10数据集、Reader创建

CIFAR-10数据集分为5个batch的训练集和1个batch的测试集，每个batch包含10,000张图片。每张图像尺寸为32*32的RGB图像，且包含有标签。一共有10个标签：airplane、automobile、bird、cat、deer、dog、frog、horse、ship、truck十个类别。

我在CIFAR-10网站中下载的是[CIFAR-10 python version](http://www.cs.toronto.edu/~kriz/cifar-10-python.tar.gz)。数据集完成后，解压得到上述六个文件。上述六个文件都是字典文件，使用cPickle模块即可读入。字典中‘data’需要重新定义维度为1000*32*32*3，维度分别代表[N H W C]，即10,000张32*32尺寸的三通道（RGB）图像，再经过转换成为paddlepaddle读取的[N C H W ]维度形式；而字典‘labels’为10000个标签。如此一来，可以建立读取CIFAR-10的reader（与官方例程不同），如下：

def reader_creator(ROOT,istrain=True,cycle=False):
    def load_CIFAR_batch(filename):
        """ load single batch of cifar """
        with open(filename,'rb') as f:
            datadict = Pickle.load(f)
            X = datadict['data']
            Y = datadict['labels']
            """ (N C H W) transpose to (N H W C) """
            X = X.reshape(10000,3,32,32).transpose(0,2,3,1).astype('float')
            Y = np.array(Y)
            return X,Y
    def reader():
        while True:
            if istrain:
                for b in range(1,6):
                    f   = os.path.join(ROOT,'data_batch_%d'%(b))
                    X,Y = load_CIFAR_batch(f)
                    length = X.shape[0]
                    for i in range(length):
                        yield X[i],Y[i]
                if not cycle:
                    break
            else:
                f = os.path.join(ROOT,'test_batch')
                X,Y = load_CIFAR_batch(f)
                length = X.shape[0]
                for i in range(length):
                    yield X[i],Y[i]
                if not cycle:
                    break
    return reader

2、VGG网络

VGG网络采用“减小卷积核大小，增加卷积核数量”的思想改造而成，这里直接采用paddlepaddle例程中的VGG网络了，值得提醒的是paddlepaddle中直接有函数img_conv_group提供卷积、池化、dropout一组操作，所以根据VGG的模型，前面卷积层可以划分为5组，然后再经过3层的全连接层得到结果。

PaddlePaddle例程中根据上图D网络，加入dorpout：

def vgg_bn_drop(input):
    def conv_block(ipt, num_filter, groups, dropouts):
        return fluid.nets.img_conv_group(
            input=ipt,
            #一组的卷积层的卷积核总数,组成list[num_filter num_filter ...]
            conv_num_filter=[num_filter] * groups, 
            conv_filter_size=3,
            conv_act='relu',
            conv_with_batchnorm=True,
            #每组卷积层各层的droput概率
            conv_batchnorm_drop_rate=dropouts, 
            pool_size=2,
            pool_stride=2,
            pool_type='max')

    conv1 = conv_block(input, 64, 2, [0.3, 0]) #[0.3 0]即为第一组两层的dorpout概率，下同
    conv2 = conv_block(conv1, 128, 2, [0.4, 0])
    conv3 = conv_block(conv2, 256, 3, [0.4, 0.4, 0])
    conv4 = conv_block(conv3, 512, 3, [0.4, 0.4, 0])
    conv5 = conv_block(conv4, 512, 3, [0.4, 0.4, 0])

    drop = fluid.layers.dropout(x=conv5, dropout_prob=0.5)
    fc1 = fluid.layers.fc(input=drop, size=512, act=None)

    bn = fluid.layers.batch_norm(input=fc1, act='relu')

    drop2 = fluid.layers.dropout(x=bn, dropout_prob=0.5)
    fc2 = fluid.layers.fc(input=drop2, size=512, act=None)

    predict = fluid.layers.fc(input=fc2, size=10, act='softmax')
    return predict

3、训练

训练程序与上一节例程一样，同样是选取交叉熵作为损失函数，不多累赘讲述。

def train_network():
    predict = inference_network()
    label = fluid.layers.data(name='label',shape=[1],dtype='int64')
    cost  = fluid.layers.cross_entropy(input=predict,label=label)
    avg_cost = fluid.layers.mean(cost)
    accuracy = fluid.layers.accuracy(input=predict,label=label)
    return [avg_cost,accuracy]

def optimizer_program():
    return fluid.optimizer.Adam(learning_rate=0.001)

def train(data_path,save_path):
    BATCH_SIZE = 128
    EPOCH_NUM  = 2
    train_reader = paddle.batch(
        paddle.reader.shuffle(reader_creator(data_path),buf_size=50000),
        batch_size = BATCH_SIZE)
    test_reader  = paddle.batch(
        reader_creator(data_path,False),
        batch_size=BATCH_SIZE)
    def event_handler(event):
        if isinstance(event, fluid.EndStepEvent):
            if event.step % 100 == 0:
                print("
Pass %d, Epoch %d, Cost %f, Acc %f" %
                      (event.step, event.epoch, event.metrics[0],
                       event.metrics[1]))
            else:
                sys.stdout.write('.')
                sys.stdout.flush()
        if isinstance(event, fluid.EndEpochEvent):
            avg_cost, accuracy = trainer.test(
                reader=test_reader, feed_order=['image', 'label'])
            print('
Test with Pass {0}, Loss {1:2.2}, Acc {2:2.2}'.format(
                event.epoch, avg_cost, accuracy))
            if save_path is not None:
                trainer.save_params(save_path)
    place = fluid.CUDAPlace(0) 
    trainer = fluid.Trainer(
        train_func=train_network, optimizer_func=optimizer_program, place=place)
    trainer.train(
        reader=train_reader,
        num_epochs=EPOCH_NUM,
        event_handler=event_handler,
        feed_order=['image', 'label'])

4、测试接口

测试接口也类似，需要特别注意的是图像维度要改为[N C H W]的顺序！

def infer(params_dir):
    place = fluid.CUDAPlace(0)
    inferencer = fluid.Inferencer(
        infer_func=inference_network, param_path=params_dir, place=place)
     # Prepare testing data. 
    from PIL import Image
    import numpy as np
    import os

    def load_image(file):
        im = Image.open(file)
        im = im.resize((32, 32), Image.ANTIALIAS)
        im = np.array(im).astype(np.float32)
        """transpose [H W C] to [C H W]"""
        im = im.transpose((2, 0, 1)) 
        im = im / 255.0

        # Add one dimension, [N C H W] N=1
        im = np.expand_dims(im, axis=0)
        return im
    cur_dir = os.path.dirname(os.path.realpath(__file__))
    img = load_image(cur_dir + '/dog.png')
    # inference
    results = inferencer.infer({'image': img})
    print(results)
    lab = np.argsort(results)  # probs and lab are the results of one batch data
    print("infer results: ", cifar_classes[lab[0][0][-1]])

5、运行结果

由于笔者没有GPU服务器，所以只迭代了50次，已经用了8个多小时，但是准确率只有15.6%，测试集方面准确率有17%，效果不理想，用于验证的结果也是错的！

Pass 300, Epoch 49, Cost 2.261115, Acc 0.156250
.........................................................................................
Test with Pass 49, Loss 2.2, Acc 0.17

Classify the cifar10 images...
[array([[0.05997971, 0.13485196, 0.096842 , 0.09973737, 0.11053724,
0.08180068, 0.13847008, 0.08627985, 0.06851784, 0.12298328]],
dtype=float32)]
infer results: frog

结语

网络比较深，且数据集比较大，训练时间比较长，普通笔记本上面的GT840M聊以胜无吧。

本文代码：02_cifar

参考：book/03.image_classification/