说明

要训练 MNIST，实际上只需要 3 个脚本文件即可完成：

cd $CAFFE_ROOT
./data/mnist/get_mnist.sh
./examples/mnist/create_mnist.sh
./examples/mnist/train_lenet.sh

MNIST database,一个手写数字的图片数据库,每一张图片都是0到9中的单个数字。每一张都是抗锯齿(Anti-aliasing)的灰度图,图片大小2828像素,数字部分被归一化为2020大小,位于图片的中间位置,保持了原来形状的比例.

官方链接是没有提供jpg图片格式的，如果我们想对数据有个直观的了解，可以通过此链接下载 jpg 图片。

数据分布

MNIST数据库的来源是两个数据库的混合,一个来自Census Bureau employees(SD-3),一个来自high-school students(SD-1);有训练样本60000个,测试样本10000个.训练样本和测试样本中,employee和student写的都是各占一半.60000个训练样本一共大概250个人写的.训练样本和测试样本的来源人群没有交集.

MNIST数据库也保留了手写数字与身份的对应关系.

./data/mnist/get_mnist.sh

#!/usr/bin/env sh
# This scripts downloads the mnist data and unzips it.

DIR="$( cd "$(dirname "$0")" ; pwd -P )"
cd "$DIR"

echo "Downloading..."

for fname in train-images-idx3-ubyte train-labels-idx1-ubyte t10k-images-idx3-ubyte t10k-labels-idx1-ubyte
do
    if [ ! -e $fname ]; then
        wget --no-check-certificate http://yann.lecun.com/exdb/mnist/${fname}.gz
        gunzip ${fname}.gz
    fi
done

这个脚本仅仅是把官网上的 mnist 的文件下载下来，其中 4 个文件分别是（测试数据和训练数据）：

train-images-idx3-ubyte 
train-labels-idx1-ubyte 
t10k-images-idx3-ubyte 
t10k-labels-idx1-ubyte

./examples/mnist/create_mnist.sh

#!/usr/bin/env sh
# This script converts the mnist data into lmdb/leveldb format,
# depending on the value assigned to $BACKEND.
set -e

EXAMPLE=examples/mnist
DATA=data/mnist
BUILD=build/examples/mnist

BACKEND="lmdb"

echo "Creating ${BACKEND}..."

rm -rf $EXAMPLE/mnist_train_${BACKEND}
rm -rf $EXAMPLE/mnist_test_${BACKEND}

$BUILD/convert_mnist_data.bin $DATA/train-images-idx3-ubyte \
  $DATA/train-labels-idx1-ubyte $EXAMPLE/mnist_train_${BACKEND} --backend=${BACKEND}
$BUILD/convert_mnist_data.bin $DATA/t10k-images-idx3-ubyte \
  $DATA/t10k-labels-idx1-ubyte $EXAMPLE/mnist_test_${BACKEND} --backend=${BACKEND}

echo "Done."

步骤：

• 删除 examples/mnist 目录下的 mnist_train_lmdb 和 mnist_test_lmdb，其中BACKEND="lmdb"指定了数据格式，数据格式除了lmdb，还有leveldb。
• 将第一步下载好的文件，转换成lmdb格式，保存在examples/mnist 目录下。

最终会出现下面的两个文件：

./examples/mnist/train_lenet.sh

#!/usr/bin/env sh
set -e

./build/tools/caffe train --solver=examples/mnist/lenet_solver.prototxt $@

使用编译好的caffe，用lenet_solver.prototxt这个proto文件的规则来训练，如果使用CPU的话，需要将最后一行的solver_mode改为CPU。

# The train/test net protocol buffer definition
net: "examples/mnist/lenet_train_test.prototxt"
# test_iter specifies how many forward passes the test should carry out.
# In the case of MNIST, we have test batch size 100 and 100 test iterations,
# covering the full 10,000 testing images.
test_iter: 100
# Carry out testing every 500 training iterations.
test_interval: 500
# The base learning rate, momentum and the weight decay of the network.
base_lr: 0.01
momentum: 0.9
weight_decay: 0.0005
# The learning rate policy
lr_policy: "inv"
gamma: 0.0001
power: 0.75
# Display every 100 iterations
display: 100
# The maximum number of iterations
max_iter: 10000
# snapshot intermediate results
snapshot: 5000
snapshot_prefix: "examples/mnist/lenet"
# solver mode: CPU or GPU
solver_mode: CPU

这里面是详细的参数，并且附有详细的注释~~~w(ﾟДﾟ)w 其中最重要的应该是下面这句话：

# The train/test net protocol buffer definition
net: "examples/mnist/lenet_train_test.prototxt"

把lenet_train_test.prototxt全部贴出来，不做深入分析，对于 proto 语法，可以参考：Google Protocol Buffers 数据交换协议。

name: "LeNet"
layer {
  name: "mnist"
  type: "Data"
  top: "data"
  top: "label"
  include {
    phase: TRAIN
  }
  transform_param {
    scale: 0.00390625
  }
  data_param {
    source: "examples/mnist/mnist_train_lmdb"
    batch_size: 64
    backend: LMDB
  }
}
layer {
  name: "mnist"
  type: "Data"
  top: "data"
  top: "label"
  include {
    phase: TEST
  }
  transform_param {
    scale: 0.00390625
  }
  data_param {
    source: "examples/mnist/mnist_test_lmdb"
    batch_size: 100
    backend: LMDB
  }
}
layer {
  name: "conv1"
  type: "Convolution"
  bottom: "data"
  top: "conv1"
  param {
    lr_mult: 1
  }
  param {
    lr_mult: 2
  }
  convolution_param {
    num_output: 20
    kernel_size: 5
    stride: 1
    weight_filler {
      type: "xavier"
    }
    bias_filler {
      type: "constant"
    }
  }
}
layer {
  name: "pool1"
  type: "Pooling"
  bottom: "conv1"
  top: "pool1"
  pooling_param {
    pool: MAX
    kernel_size: 2
    stride: 2
  }
}
layer {
  name: "conv2"
  type: "Convolution"
  bottom: "pool1"
  top: "conv2"
  param {
    lr_mult: 1
  }
  param {
    lr_mult: 2
  }
  convolution_param {
    num_output: 50
    kernel_size: 5
    stride: 1
    weight_filler {
      type: "xavier"
    }
    bias_filler {
      type: "constant"
    }
  }
}
layer {
  name: "pool2"
  type: "Pooling"
  bottom: "conv2"
  top: "pool2"
  pooling_param {
    pool: MAX
    kernel_size: 2
    stride: 2
  }
}
layer {
  name: "ip1"
  type: "InnerProduct"
  bottom: "pool2"
  top: "ip1"
  param {
    lr_mult: 1
  }
  param {
    lr_mult: 2
  }
  inner_product_param {
    num_output: 500
    weight_filler {
      type: "xavier"
    }
    bias_filler {
      type: "constant"
    }
  }
}
layer {
  name: "relu1"
  type: "ReLU"
  bottom: "ip1"
  top: "ip1"
}
layer {
  name: "ip2"
  type: "InnerProduct"
  bottom: "ip1"
  top: "ip2"
  param {
    lr_mult: 1
  }
  param {
    lr_mult: 2
  }
  inner_product_param {
    num_output: 10
    weight_filler {
      type: "xavier"
    }
    bias_filler {
      type: "constant"
    }
  }
}
layer {
  name: "accuracy"
  type: "Accuracy"
  bottom: "ip2"
  bottom: "label"
  top: "accuracy"
  include {
    phase: TEST
  }
}
layer {
  name: "loss"
  type: "SoftmaxWithLoss"
  bottom: "ip2"
  bottom: "label"
  top: "loss"
}

训练结果

最终的训练结果，准确率竟然高达 98.98% w(ﾟДﾟ)w

测试自己的手写数字图片

在训练好模型之后，如何才能应用到实际当中呢？对于MNIST，就是要把符合格式的图片输入给神经网络，然后看预测是否符合标准。参考caffe笔记：测试自己的手写数字图片，用 caffe 测试自己的图片。在这里具体过程就不详细介绍，仅展示下最终结果（前面的链接写得非常全）：

./build/examples/cpp_classification/classification.bin \
> examples/mnist/classificat_net.prototxt \
> examples/mnist/lenet_iter_10000.caffemodel \
> examples/mnist/mean.binaryproto \
> examples/mnist/label.txt \
> examples/mnist/0.png

注意一下，classificat_net.prototxt是 examples/mnist/lenet_train_test.prototxt的副本，但是要删掉

layer {
  name: "mnist"
  type: "Data"
  top: "data"
  top: "label"
  include {
    phase: TRAIN
  }
  transform_param {
    scale: 0.00390625
  }
  data_param {
    source: "examples/mnist/mnist_train_lmdb"
    batch_size: 64
    backend: LMDB
  }
}
layer {
  name: "mnist"
  type: "Data"
  top: "data"
  top: "label"
  include {
    phase: TEST
  }
  transform_param {
    scale: 0.00390625
  }
  data_param {
    source: "examples/mnist/mnist_test_lmdb"
    batch_size: 100
    backend: LMDB
  }
}

以及

layer {
  name: "accuracy"
  type: "Accuracy"
  bottom: "ip2"
  bottom: "label"
  top: "accuracy"
  include {
    phase: TEST
  }
}
layer {
  name: "loss"
  type: "SoftmaxWithLoss"
  bottom: "ip2"
  bottom: "label"
  top: "loss"
}

并在最后增加一层：

layer {
  name: "prob"
  type: "Softmax"
  bottom: "ip2"
  top: "prob"
}

最终的预测结果如下，可以看到模型100%确定数字是0，预测正确：

---------- Prediction for examples/mnist/0.png ----------
1.0000 - "0"
0.0000 - "1"
0.0000 - "3"
0.0000 - "4"
0.0000 - "2"

总结

对于模型及原理，网上已经有很多好的文章，这里不作介绍。今天在 centos 上安装 caffe，实在太费劲了…… 我在编译的时候，把 leveldb 和 lmdb 全部都取消了，这就导致 MNIST 的数据格式不能识别！解决方法：更改配置，重新编译。

参考了那么多的 caffe 安装教程，这篇文章最好：caffe 安装（centos 7）