【caffe I/O】数据变换器（图像的预处理部分） 代码注释

caffe.proto中TransformationParameter部分

// Message that stores parameters used to apply transformation
// to the data layer's data
message TransformationParameter {
  // For data pre-processing, we can do simple scaling and subtracting the
  // data mean, if provided. Note that the mean subtraction is always carried
  // out before scaling.
  //像素幅度缩放参数，默认不缩放
  optional float scale =  [default = ];
  // Specify if we want to randomly mirror data.
  //图像随机镜像开关。默认false，不进行随机镜像操作
  optional bool mirror =  [default = false];
  // Specify if we would like to randomly crop an image.
  //图像随机切块的大小。默认不切块。
  optional uint32 crop_size =  [default = ];
  // mean_file and mean_value cannot be specified at the same time
  //存储图像均值的文件
  optional string mean_file = ;
  // if specified can be repeated once (would subtract it from all the channels)
  // or can be repeated the same number of times as channels
  // (would subtract them from the corresponding channel)
  //均值数值。无需读取文件。
  //如果均值数目和通道数一致，则每个通道分别减去对应均值；
  //如果没有均值只有一个，则每个通道减去相同的均值。
  repeated float mean_value = ;
  // Force the decoded image to have 3 color channels.
  //强制为三通道三色图像输入 默认false
  optional bool force_color =  [default = false];
  // Force the decoded image to have 1 color channels.
  //强制为单通道灰度图像输入 默认false
  optional bool force_gray =  [default = false];
}

caffe.proto中Datum部分

//Datum用来从LMDB/LEVELDB 中读取数据，或将数据写入其中。和BlobProto有相似功能，只是
//BlobProto用于模型权值序列化反序列化，而Datum用于数据或特征图（feature map）提供序列化反序列化
message Datum {
  //数据维度信息：channels*height*width
  optional int32 channels = ;
  optional int32 height = ;
  optional int32 width = ;
  // the actual image data, in bytes
  //图像数据，以字节类型存储
  optional bytes data = ;
  //标签数据，统一用int32类型存储
  optional int32 label = ;
  // Optionally, the datum could also hold float data.
  repeated float float_data = ;
  // If true data contains an encoded image that need to be decoded
  optional bool encoded =  [default = false];//是否为编码数据，默认false
}

include/caffe/data_transformer.hpp

 #ifndef CAFFE_DATA_TRANSFORMER_HPP
 #define CAFFE_DATA_TRANSFORMER_HPP

 #include <vector>

 #include "caffe/blob.hpp"
 #include "caffe/common.hpp"
 #include "caffe/proto/caffe.pb.h"

 namespace caffe {

 /**
  * @brief Applies common transformations to the input data, such as
  * scaling, mirroring, substracting the image mean...
  */
 template <typename Dtype>
 class DataTransformer {
  public:
   explicit DataTransformer(const TransformationParameter& param, Phase phase);
   virtual ~DataTransformer() {}

   /**
    * @brief Initialize the Random number generations if needed by the
    *    transformation.
    */
   //初始化随机数种子函数
   void InitRand();

   /**
    * @brief Applies the transformation defined in the data layer's
    * transform_param block to the data.
    *
    * @param datum
    *    Datum containing the data to be transformed.
    * @param transformed_blob
    *    This is destination blob. It can be part of top blob's data if
    *    set_cpu_data() is used. See data_layer.cpp for an example.
    */
   //数据读取层中transform_param块所声明的变换应用到输入数据中。
   //以下为多个重载函数
   void Transform(const Datum& datum, Blob<Dtype>* transformed_blob);

   /**
    * @brief Applies the transformation defined in the data layer's
    * transform_param block to a vector of Datum.
    *
    * @param datum_vector
    *    A vector of Datum containing the data to be transformed.
    * @param transformed_blob
    *    This is destination blob. It can be part of top blob's data if
    *    set_cpu_data() is used. See memory_layer.cpp for an example.
    */
   void Transform(const vector<Datum> & datum_vector,
                 Blob<Dtype>* transformed_blob);
 //使用OPENCV
 #ifdef USE_OPENCV
   /**
    * @brief Applies the transformation defined in the data layer's
    * transform_param block to a vector of Mat.
    *
    * @param mat_vector
    *    A vector of Mat containing the data to be transformed.
    * @param transformed_blob
    *    This is destination blob. It can be part of top blob's data if
    *    set_cpu_data() is used. See memory_layer.cpp for an example.
    */
   void Transform(const vector<cv::Mat> & mat_vector,
                 Blob<Dtype>* transformed_blob);

   /**
    * @brief Applies the transformation defined in the data layer's
    * transform_param block to a cv::Mat
    *
    * @param cv_img
    *    cv::Mat containing the data to be transformed.
    * @param transformed_blob
    *    This is destination blob. It can be part of top blob's data if
    *    set_cpu_data() is used. See image_data_layer.cpp for an example.
    */
   void Transform(const cv::Mat& cv_img, Blob<Dtype>* transformed_blob);
 #endif  // USE_OPENCV

   /**
    * @brief Applies the same transformation defined in the data layer's
    * transform_param block to all the num images in a input_blob.
    *
    * @param input_blob
    *    A Blob containing the data to be transformed. It applies the same
    *    transformation to all the num images in the blob.
    * @param transformed_blob
    *    This is destination blob, it will contain as many images as the
    *    input blob. It can be part of top blob's data.
    */
   void Transform(Blob<Dtype>* input_blob, Blob<Dtype>* transformed_blob);

   /**
    * @brief Infers the shape of transformed_blob will have when
    *    the transformation is applied to the data.
    *
    * @param datum
    *    Datum containing the data to be transformed.
    */
    //获取执行变换后输出Blob的形状
   vector<int> InferBlobShape(const Datum& datum);
   /**
    * @brief Infers the shape of transformed_blob will have when
    *    the transformation is applied to the data.
    *    It uses the first element to infer the shape of the blob.
    *
    * @param datum_vector
    *    A vector of Datum containing the data to be transformed.
    */
   vector<int> InferBlobShape(const vector<Datum> & datum_vector);
   /**
    * @brief Infers the shape of transformed_blob will have when
    *    the transformation is applied to the data.
    *    It uses the first element to infer the shape of the blob.
    *
    * @param mat_vector
    *    A vector of Mat containing the data to be transformed.
    */
 #ifdef USE_OPENCV
   vector<int> InferBlobShape(const vector<cv::Mat> & mat_vector);
   /**
    * @brief Infers the shape of transformed_blob will have when
    *    the transformation is applied to the data.
    *
    * @param cv_img
    *    cv::Mat containing the data to be transformed.
    */
   vector<int> InferBlobShape(const cv::Mat& cv_img);
 #endif  // USE_OPENCV

  protected:
    /**
    * @brief Generates a random integer from Uniform({0, 1, ..., n-1}).
    *
    * @param n
    *    The upperbound (exclusive) value of the random number.
    * @return
    *    A uniformly random integer value from ({0, 1, ..., n-1}).
    */
   //产生取值0到n-1的随机整数，服从均匀分布
   virtual int Rand(int n);

   void Transform(const Datum& datum, Dtype* transformed_data);
   // Tranformation parameters
   //变换参数，该数据结构由ProtoBuffer工具自动生成
   TransformationParameter param_;

   //随机数生成器。声明在include/caffe/common.hpp中
   shared_ptr<Caffe::RNG> rng_;
   Phase phase_;//当前运行阶段。TRAIN或TEST。阶段不同，变换会有差异
   Blob<Dtype> data_mean_;//均值图像，从均值文件中读取
   vector<Dtype> mean_values_;//均值数值，从param_中提取
 };

 }  // namespace caffe

 #endif  // CAFFE_DATA_TRANSFORMER_HPP_

src/caffe/data_transformer.cpp

 #ifdef USE_OPENCV
 #include <opencv2/core/core.hpp>
 #endif  // USE_OPENCV

 #include <string>
 #include <vector>

 #include "caffe/data_transformer.hpp"
 #include "caffe/util/io.hpp"
 #include "caffe/util/math_functions.hpp"
 #include "caffe/util/rng.hpp"

 namespace caffe {

 template<typename Dtype>
 DataTransformer<Dtype>::DataTransformer(const TransformationParameter& param,
     Phase phase)
     : param_(param), phase_(phase) {//初始化
   // check if we want to use mean_file
   //查看是否使用均值文件
   if (param_.has_mean_file()) {
     //如果param_中指定了均值文件又指定了均值数值则报错，二选一
     CHECK_EQ(param_.mean_value_size(), ) <<
       "Cannot specify mean_file and mean_value at the same time";
     const string& mean_file = param.mean_file();//获取均值文件名
     if (Caffe::root_solver()) {
       LOG(INFO) << "Loading mean file from: " << mean_file;
     }
     BlobProto blob_proto;//从均值文件中读取数据到blob_proto对象中
     ReadProtoFromBinaryFileOrDie(mean_file.c_str(), &blob_proto);
     data_mean_.FromProto(blob_proto);//从blob_proto将均值反序列化到data_mean_内存中
   }
   // check if we want to use mean_value
   //查看是否使用均值数值
   if (param_.mean_value_size() > ) {
     CHECK(param_.has_mean_file() == false) <<
       "Cannot specify mean_file and mean_value at the same time";
     for (int c = ; c < param_.mean_value_size(); ++c) {
         //读取均值数值，不再读取均值文件
       mean_values_.push_back(param_.mean_value(c));
     }
   }
 }
 //若干重载变换函数
 //该函数以Datum作为输入，结构体在caffe.proto中可见。输出为数据指针
 template<typename Dtype>
 void DataTransformer<Dtype>::Transform(const Datum& datum,
                                        Dtype* transformed_data) {
   //获得datum数据字串、维度信息
   const string& data = datum.data();
   const int datum_channels = datum.channels();
   const int datum_height = datum.height();
   const int datum_width = datum.width();

   //从param_获得处理参数。如切块大小、幅度缩放、随机镜像、图像均值等
   const int crop_size = param_.crop_size();
   const Dtype scale = param_.scale();
   const bool do_mirror = param_.mirror() && Rand();
   const bool has_mean_file = param_.has_mean_file();
   const bool has_uint8 = data.size() > ;
   const bool has_mean_values = mean_values_.size() > ;

   CHECK_GT(datum_channels, );//保证输入通道数大于零
   CHECK_GE(datum_height, crop_size);//保证输入数据的宽和高大于切块尺寸
   CHECK_GE(datum_width, crop_size);

   //获得图像均值
   Dtype* mean = NULL;
   if (has_mean_file) {//若指定了图像均值文件
     //保证图像的均值文件的维度和输入图像数据的维度完全相同
     CHECK_EQ(datum_channels, data_mean_.channels());
     CHECK_EQ(datum_height, data_mean_.height());
     CHECK_EQ(datum_width, data_mean_.width());
     mean = data_mean_.mutable_cpu_data();//获得图像均值数据控制权
   }
   if (has_mean_values) {//未指定图像均值文件，直接给出均值数值
     //保证均值数据维数是1，或与输入图像数据的通道数相同
     CHECK(mean_values_.size() ==  || mean_values_.size() == datum_channels) <<
      "Specify either 1 mean_value or as many as channels: " << datum_channels;
     if (datum_channels >  && mean_values_.size() == ) {
       // Replicate the mean_value for simplicity
       //如果均值数据维度为1，且输入数据通道数大于1，则重复channels次
       for (int c = ; c < datum_channels; ++c) {
         mean_values_.push_back(mean_values_[]);
       }
     }
   }

   //输入图像的宽和高
   int height = datum_height;
   int width = datum_width;

   //开始图像切块
   int h_off = ;
   int w_off = ;
   if (crop_size) {//若不为0则进行切块，为0不切块
     height = crop_size;
     width = crop_size;
     // We only do random crop when we do training.
     // 在训练的时候随机crop图像块,这里需要自己实现Rand这个函数来确定是如何随机的
     if (phase_ == TRAIN) {
       h_off = Rand(datum_height - crop_size + );// 产生从0到datum_height - crop_size的随机数
       w_off = Rand(datum_width - crop_size + );//切块的width偏移量
     } else {//测试阶段只切取图像中心位置
       h_off = (datum_height - crop_size) / ;
       w_off = (datum_width - crop_size) / ;
     }
   }

   // 对数据进行变换，主要是将原来的像素值减去均值，然后乘以scale这么一个操作
   // 如果需要crop则最终转换的Blob的大小即为crop*crop
   // 如果不是，则最终的Blob大小即为datum_height*datum_width
   Dtype datum_element;//存放输入图像的像素值
   int top_index, data_index;//分别存放输出index和输入index
   for (int c = ; c < datum_channels; ++c) {
     for (int h = ; h < height; ++h) {
       for (int w = ; w < width; ++w) {
         data_index = (c * datum_height + h_off + h) * datum_width + w_off + w;
         if (do_mirror) {//若需要镜像操作，则输出index时设置width反向
           top_index = (c * height + h) * width + (width -  - w);
         } else {
           top_index = (c * height + h) * width + w;
         }
         if (has_uint8) {// Datum中如果是uint8存储图像数据则转换为float
           datum_element =
             static_cast<Dtype>(static_cast<uint8_t>(data[data_index]));
         } else {//否则为float
           datum_element = datum.float_data(data_index);
         }
         if (has_mean_file) {//若指定了均值文件
           transformed_data[top_index] =
             (datum_element - mean[data_index]) * scale;//去均值，幅度缩放
         } else {
           if (has_mean_values) {//若指定了均值数值
             transformed_data[top_index] =
               (datum_element - mean_values_[c]) * scale;//去均值，幅度缩放
           } else {
             transformed_data[top_index] = datum_element * scale;//不去均值，只幅度缩放
           }
         }
       }
     }
   }
 }

 //与上一个函数类似，只是输出变成blob
 template<typename Dtype>
 void DataTransformer<Dtype>::Transform(const Datum& datum,
                                        Blob<Dtype>* transformed_blob) {
   // If datum is encoded, decode and transform the cv::image.
   if (datum.encoded()) {//  检查datum是否经过编码的图像，如果是则解码
 #ifdef USE_OPENCV
     // 先检查是不是两个属性都设置， 如果是则说明参数设置有误
     CHECK(!(param_.force_color() && param_.force_gray()))
         << "cannot set both force_color and force_gray";
     cv::Mat cv_img;
     if (param_.force_color() || param_.force_gray()) {
     // If force_color then decode in color otherwise decode in gray.
     // 如果强制彩色或者强制灰度图像一个成立则使用DecodeDatumToCVMat解码
       cv_img = DecodeDatumToCVMat(datum, param_.force_color());
     } else {// 否则使用DecodeDatumToCVMatNative解码
       cv_img = DecodeDatumToCVMatNative(datum);
     }
     // Transform the cv::image into blob.将cv::image 变换为 blob
     return Transform(cv_img, transformed_blob);
 #else
     LOG(FATAL) << "Encoded datum requires OpenCV; compile with USE_OPENCV.";
 #endif  // USE_OPENCV
   } else {// 如果没有编码则检查force_color和force_gray是否设置，如果设置则不合法，因为该选项只适合于编码后的数据
     if (param_.force_color() || param_.force_gray()) {
       LOG(ERROR) << "force_color and force_gray only for encoded datum";
     }
   }

   const int crop_size = param_.crop_size();
   const int datum_channels = datum.channels();
   const int datum_height = datum.height();
   const int datum_width = datum.width();

   // Check dimensions.检查维度
   const int channels = transformed_blob->channels();
   const int height = transformed_blob->height();
   const int width = transformed_blob->width();
   const int num = transformed_blob->num();

   CHECK_EQ(channels, datum_channels);
   CHECK_LE(height, datum_height);
   CHECK_LE(width, datum_width);
   CHECK_GE(num, );

   if (crop_size) {
     CHECK_EQ(crop_size, height);
     CHECK_EQ(crop_size, width);
   } else {
     CHECK_EQ(datum_height, height);
     CHECK_EQ(datum_width, width);
   }
   // 参数变换完毕，调用现有函数
   Dtype* transformed_data = transformed_blob->mutable_cpu_data();
   Transform(datum, transformed_data);
 }

 //对一组datum数据进行变换
 template<typename Dtype>
 void DataTransformer<Dtype>::Transform(const vector<Datum> & datum_vector,
                                        Blob<Dtype>* transformed_blob) {
   const int datum_num = datum_vector.size();
   const int num = transformed_blob->num();// 变换到的目标blob的形状
   const int channels = transformed_blob->channels();
   const int height = transformed_blob->height();
   const int width = transformed_blob->width();

   CHECK_GT(datum_num, ) << "There is no datum to add";
   CHECK_LE(datum_num, num) <<
     "The size of datum_vector must be no greater than transformed_blob->num()";
   Blob<Dtype> uni_blob(, channels, height, width);// 新建一个uni_blob,里面只有一个batch。临时Blob
   //依次对每一个datum进行变换，放入对应的Blob之中
   for (int item_id = ; item_id < datum_num; ++item_id) {
     int offset = transformed_blob->offset(item_id);
     uni_blob.set_cpu_data(transformed_blob->mutable_cpu_data() + offset);
     Transform(datum_vector[item_id], &uni_blob);
   }
 }

 #ifdef USE_OPENCV
 //对一组cv::Mat对象进行变换，放入Blob中
 template<typename Dtype>
 void DataTransformer<Dtype>::Transform(const vector<cv::Mat> & mat_vector,
                                        Blob<Dtype>* transformed_blob) {
   // 获取mat的参数
   const int mat_num = mat_vector.size();
   const int num = transformed_blob->num();
   const int channels = transformed_blob->channels();
   const int height = transformed_blob->height();
   const int width = transformed_blob->width();

   CHECK_GT(mat_num, ) << "There is no MAT to add";
   CHECK_EQ(mat_num, num) <<
     "The size of mat_vector must be equals to transformed_blob->num()";
   Blob<Dtype> uni_blob(, channels, height, width);
   for (int item_id = ; item_id < mat_num; ++item_id) {
     int offset = transformed_blob->offset(item_id);
     uni_blob.set_cpu_data(transformed_blob->mutable_cpu_data() + offset);
     Transform(mat_vector[item_id], &uni_blob);
   }
 }
 // 对一个cv::Mat对象进行变换，放入Blob中。
 // 如果是图像的话，需要减去均值乘以scale，判断是不是需要做镜像处理
 // 逻辑与前面类似
 template<typename Dtype>
 void DataTransformer<Dtype>::Transform(const cv::Mat& cv_img,
                                        Blob<Dtype>* transformed_blob) {
   const int crop_size = param_.crop_size();
   const int img_channels = cv_img.channels();
   const int img_height = cv_img.rows;
   const int img_width = cv_img.cols;

   // Check dimensions.检查维度
   const int channels = transformed_blob->channels();
   const int height = transformed_blob->height();
   const int width = transformed_blob->width();
   const int num = transformed_blob->num();

   CHECK_EQ(channels, img_channels);
   CHECK_LE(height, img_height);
   CHECK_LE(width, img_width);
   CHECK_GE(num, );

   CHECK(cv_img.depth() == CV_8U) << "Image data type must be unsigned byte";

   const Dtype scale = param_.scale();
   const bool do_mirror = param_.mirror() && Rand();
   const bool has_mean_file = param_.has_mean_file();
   const bool has_mean_values = mean_values_.size() > ;

   CHECK_GT(img_channels, );
   CHECK_GE(img_height, crop_size);
   CHECK_GE(img_width, crop_size);

   Dtype* mean = NULL;
   if (has_mean_file) {
     CHECK_EQ(img_channels, data_mean_.channels());
     CHECK_EQ(img_height, data_mean_.height());
     CHECK_EQ(img_width, data_mean_.width());
     mean = data_mean_.mutable_cpu_data();
   }
   if (has_mean_values) {
     CHECK(mean_values_.size() ==  || mean_values_.size() == img_channels) <<
      "Specify either 1 mean_value or as many as channels: " << img_channels;
     if (img_channels >  && mean_values_.size() == ) {
       // Replicate the mean_value for simplicity 复制均值便于操作
       for (int c = ; c < img_channels; ++c) {
         mean_values_.push_back(mean_values_[]);
       }
     }
   }

   int h_off = ;
   int w_off = ;
   cv::Mat cv_cropped_img = cv_img;
   if (crop_size) {
     CHECK_EQ(crop_size, height);
     CHECK_EQ(crop_size, width);
     // We only do random crop when we do training.只有训练阶段才随机切块
     if (phase_ == TRAIN) {
       h_off = Rand(img_height - crop_size + );
       w_off = Rand(img_width - crop_size + );
     } else {
       h_off = (img_height - crop_size) / ;
       w_off = (img_width - crop_size) / ;
     }
     cv::Rect roi(w_off, h_off, crop_size, crop_size);
     cv_cropped_img = cv_img(roi);
   } else {
     CHECK_EQ(img_height, height);
     CHECK_EQ(img_width, width);
   }

   CHECK(cv_cropped_img.data);

   Dtype* transformed_data = transformed_blob->mutable_cpu_data();
   int top_index;
   for (int h = ; h < height; ++h) {
     const uchar* ptr = cv_cropped_img.ptr<uchar>(h);
     int img_index = ;
     for (int w = ; w < width; ++w) {
       for (int c = ; c < img_channels; ++c) {
         if (do_mirror) {
           top_index = (c * height + h) * width + (width -  - w);
         } else {
           top_index = (c * height + h) * width + w;
         }
         // int top_index = (c * height + h) * width + w;
         Dtype pixel = static_cast<Dtype>(ptr[img_index++]);
         if (has_mean_file) {
           int mean_index = (c * img_height + h_off + h) * img_width + w_off + w;
           transformed_data[top_index] =
             (pixel - mean[mean_index]) * scale;
         } else {
           if (has_mean_values) {
             transformed_data[top_index] =
               (pixel - mean_values_[c]) * scale;
           } else {
             transformed_data[top_index] = pixel * scale;
           }
         }
       }
     }
   }
 }
 #endif  // USE_OPENCV

 //输入输出都是Blob
 template<typename Dtype>
 void DataTransformer<Dtype>::Transform(Blob<Dtype>* input_blob,
                                        Blob<Dtype>* transformed_blob) {
   const int crop_size = param_.crop_size();
   const int input_num = input_blob->num();
   const int input_channels = input_blob->channels();
   const int input_height = input_blob->height();
   const int input_width = input_blob->width();

   if (transformed_blob->count() == ) {
     // Initialize transformed_blob with the right shape.初始化变换后的Blob形状
     if (crop_size) {
       transformed_blob->Reshape(input_num, input_channels,
                                 crop_size, crop_size);
     } else {
       transformed_blob->Reshape(input_num, input_channels,
                                 input_height, input_width);
     }
   }

   const int num = transformed_blob->num();
   const int channels = transformed_blob->channels();
   const int height = transformed_blob->height();
   const int width = transformed_blob->width();
   const int size = transformed_blob->count();

   CHECK_LE(input_num, num);
   CHECK_EQ(input_channels, channels);
   CHECK_GE(input_height, height);
   CHECK_GE(input_width, width);

   const Dtype scale = param_.scale();
   const bool do_mirror = param_.mirror() && Rand();
   const bool has_mean_file = param_.has_mean_file();
   const bool has_mean_values = mean_values_.size() > ;

   int h_off = ;
   int w_off = ;
   if (crop_size) {
     CHECK_EQ(crop_size, height);
     CHECK_EQ(crop_size, width);
     // We only do random crop when we do training.只有训练阶段随机切块
     if (phase_ == TRAIN) {
       h_off = Rand(input_height - crop_size + );
       w_off = Rand(input_width - crop_size + );
     } else {
       h_off = (input_height - crop_size) / ;
       w_off = (input_width - crop_size) / ;
     }
   } else {
     CHECK_EQ(input_height, height);
     CHECK_EQ(input_width, width);
   }

   Dtype* input_data = input_blob->mutable_cpu_data();
   if (has_mean_file) {
     CHECK_EQ(input_channels, data_mean_.channels());
     CHECK_EQ(input_height, data_mean_.height());
     CHECK_EQ(input_width, data_mean_.width());
     for (int n = ; n < input_num; ++n) {
       int offset = input_blob->offset(n);
       /*
        template <typename Dtype>
        void caffe_sub(const int N, const Dtype* a, const Dtype* b, Dtype* y);
        math_function中定义的caffe_sub目的是矩阵相减input_data（以offset开始的矩阵） = input_data（以offset开始的矩阵） - data_mean_
       */
       caffe_sub(data_mean_.count(), input_data + offset,
             data_mean_.cpu_data(), input_data + offset);
     }
   }

   if (has_mean_values) {
     CHECK(mean_values_.size() ==  || mean_values_.size() == input_channels) <<
      "Specify either 1 mean_value or as many as channels: " << input_channels;
     if (mean_values_.size() == ) {
       caffe_add_scalar(input_blob->count(), -(mean_values_[]), input_data);
     } else {
       for (int n = ; n < input_num; ++n) {
         for (int c = ; c < input_channels; ++c) {
           int offset = input_blob->offset(n, c);
           // 给input_data[offset]地址开始的每一个元素加上一个-mean_values_[c]
           caffe_add_scalar(input_height * input_width, -(mean_values_[c]),
             input_data + offset);
         }
       }
     }
   }
   // 如果什么均值都没有则直接复制
   Dtype* transformed_data = transformed_blob->mutable_cpu_data();

   for (int n = ; n < input_num; ++n) {
     int top_index_n = n * channels;
     int data_index_n = n * channels;
     for (int c = ; c < channels; ++c) {
       int top_index_c = (top_index_n + c) * height;
       int data_index_c = (data_index_n + c) * input_height + h_off;
       for (int h = ; h < height; ++h) {
         int top_index_h = (top_index_c + h) * width;
         int data_index_h = (data_index_c + h) * input_width + w_off;
         if (do_mirror) {
           int top_index_w = top_index_h + width - ;
           for (int w = ; w < width; ++w) {
             transformed_data[top_index_w-w] = input_data[data_index_h + w];
           }
         } else {
           for (int w = ; w < width; ++w) {
             transformed_data[top_index_h + w] = input_data[data_index_h + w];
           }
         }
       }
     }
   }
   if (scale != Dtype()) {
     DLOG(INFO) << "Scale: " << scale;
     caffe_scal(size, scale, transformed_data);
   }
 }

 //获得数据变换输出尺寸
 template<typename Dtype>
 vector<int> DataTransformer<Dtype>::InferBlobShape(const Datum& datum) {
   if (datum.encoded()) {
 #ifdef USE_OPENCV
 // 如果使用OpenCV则可以用先转换为CVMat，然后推断blob的形状
     CHECK(!(param_.force_color() && param_.force_gray()))
         << "cannot set both force_color and force_gray";
     cv::Mat cv_img;
     if (param_.force_color() || param_.force_gray()) {
     // If force_color then decode in color otherwise decode in gray.
       cv_img = DecodeDatumToCVMat(datum, param_.force_color());
     } else {
       cv_img = DecodeDatumToCVMatNative(datum);
     }
     // InferBlobShape using the cv::image.
     return InferBlobShape(cv_img);
 #else
     LOG(FATAL) << "Encoded datum requires OpenCV; compile with USE_OPENCV.";
 #endif  // USE_OPENCV
   }
   // 否则直接粗暴地从datum里面获取形状的数据
   const int crop_size = param_.crop_size();
   const int datum_channels = datum.channels();
   const int datum_height = datum.height();
   const int datum_width = datum.width();
   // Check dimensions.检查维度
   CHECK_GT(datum_channels, );
   CHECK_GE(datum_height, crop_size);
   CHECK_GE(datum_width, crop_size);
   // Build BlobShape. 创建BlobShape对象
   vector<int> shape();
   shape[] = ;
   shape[] = datum_channels;
   shape[] = (crop_size)? crop_size: datum_height;
   shape[] = (crop_size)? crop_size: datum_width;
   return shape;
 }

 template<typename Dtype>
 vector<int> DataTransformer<Dtype>::InferBlobShape(
     const vector<Datum> & datum_vector) {
   const int num = datum_vector.size();
   CHECK_GT(num, ) << "There is no datum to in the vector";
   // Use first datum in the vector to InferBlobShape.使用第一个推断
   vector<int> shape = InferBlobShape(datum_vector[]);
   // Adjust num to the size of the vector.
   shape[] = num;
   return shape;
 }

 #ifdef USE_OPENCV
 // 如果使用OpenCV
 // 使用CVMat中的信息来推断形状
 template<typename Dtype>
 vector<int> DataTransformer<Dtype>::InferBlobShape(const cv::Mat& cv_img) {
   const int crop_size = param_.crop_size();
   const int img_channels = cv_img.channels();
   const int img_height = cv_img.rows;
   const int img_width = cv_img.cols;
   // Check dimensions.
   CHECK_GT(img_channels, );
   CHECK_GE(img_height, crop_size);
   CHECK_GE(img_width, crop_size);
   // Build BlobShape.
   vector<int> shape();
   shape[] = ;
   shape[] = img_channels;
   shape[] = (crop_size)? crop_size: img_height;
   shape[] = (crop_size)? crop_size: img_width;
   return shape;
 }

 template<typename Dtype>
 vector<int> DataTransformer<Dtype>::InferBlobShape(
     const vector<cv::Mat> & mat_vector) {
   const int num = mat_vector.size();
   CHECK_GT(num, ) << "There is no cv_img to in the vector";
   // Use first cv_img in the vector to InferBlobShape.
   vector<int> shape = InferBlobShape(mat_vector[]);
   // Adjust num to the size of the vector.
   shape[] = num;
   return shape;
 }
 #endif  // USE_OPENCV

 // 初始化随机数种子
 template <typename Dtype>
 void DataTransformer<Dtype>::InitRand() {
   // 要么需要镜像要么训练阶段和需要crop同时满足的情况下才初始化随机数种子
   const bool needs_rand = param_.mirror() ||
       (phase_ == TRAIN && param_.crop_size());
   if (needs_rand) {
     const unsigned int rng_seed = caffe_rng_rand();// 获得随机数种子（通过熵池或者时间生成种子）
     rng_.reset(new Caffe::RNG(rng_seed));//初始化随机数种子并实例化随机数生成器
   } else {
     rng_.reset();//否则随机数生成器设置为空
   }
 }

 // 产生从0到n的随机数
 template <typename Dtype>
 int DataTransformer<Dtype>::Rand(int n) {
   CHECK(rng_);
   CHECK_GT(n, );
   caffe::rng_t* rng =
       static_cast<caffe::rng_t*>(rng_->generator());
   return ((*rng)() % n);
 }

 INSTANTIATE_CLASS(DataTransformer);
 /*
 初始化类的宏定义
 #define INSTANTIATE_CLASS(classname) \
   char gInstantiationGuard##classname; \
   template class classname<float>; \
   template class classname<double>
 */
 }  // namespace caffe

摘抄参考赵永科《21天实战caffe》

同时参考http://blog.csdn.net/langb2014/article/details/51050213