elas算法源码赏析（一）：PGM格式图片的读取和保存

// basic image I/O, based on Pedro Felzenszwalb's code

#ifndef IMAGE_H
#define IMAGE_H

#include <cstdlib>
#include <climits>
#include <cstring>
#include <fstream>

// use imRef to access image data.获取图片数据
#define imRef(im, x, y) (im->access[y][x])
  
// use imPtr to get pointer to image data.获取图片数据指针
#define imPtr(im, x, y) &(im->access[y][x])

#define BUF_SIZE 256

typedef unsigned char uchar;
typedef struct { uchar r, g, b; } rgb;

inline bool operator==(const rgb &a, const rgb &b) {
  return ((a.r == b.r) && (a.g == b.g) && (a.b == b.b));
}

// image class模板类 T可以用各种类型代替
template <class T> class image {
public:

  // create image
  image(const int width, const int height, const bool init = false);

  // delete image
  ~image();

  // init image
  void init(const T &val);

  // deep copy
  image<T> *copy() const;
  
  // get image width/height
  int width() const { return w; }
  int height() const { return h; }
  
  // image data
  T *data;
  
  // row pointers
  T **access;
  
private:
  int w, h;
};

template <class T> image<T>::image(const int width, const int height, const bool init) {
  w = width;
  h = height;
  data = new T[w * h];  // allocate space for image data为图片数据分配空间
  access = new T*[h];   // allocate space for row pointers为行指针分配空间
  
  // initialize row pointers
  for (int i = 0; i < h; i++)
    access[i] = data + (i * w);  //初始化行指针
  
  // init to zero
  if (init)
    memset(data, 0, w * h * sizeof(T));//初始化全0
}

template <class T> image<T>::~image() {
  delete [] data; 
  delete [] access;
}

template <class T> void image<T>::init(const T &val) {
  T *ptr = imPtr(this, 0, 0);//取（0，0）地址
  T *end = imPtr(this, w-1, h-1);
  while (ptr <= end)
    *ptr++ = val;
}


template <class T> image<T> *image<T>::copy() const {
  image<T> *im = new image<T>(w, h, false);//硬拷贝
  memcpy(im->data, data, w * h * sizeof(T));//memcpy 目的 源 大小
  return im;
}

class pnm_error {};

void pnm_read(std::ifstream &file, char *buf) {
  char doc[BUF_SIZE];
  char c;
  
  file >> c;
  while (c == '#') {
    file.getline(doc, BUF_SIZE);//至少读取BUF_SIZE个字符到doc里面
    file >> c;
  }
  file.putback(c);//把c重新放回输入流，可以被再次读取
  
  file.width(BUF_SIZE);//输入BUF_SIZE-1个字符
  file >> buf;
  file.ignore();//默认参数为cin.ignore(1,EOF),即把EOF前的1个字符清掉,没有遇到EOF就清掉一个字符然后结束
}

image<uchar> *loadPGM(const char *name) {
  char buf[BUF_SIZE];
  
  // read header
  std::ifstream file(name, std::ios::in | std::ios::binary);
  pnm_read(file, buf);
  if (strncmp(buf, "P5", 2)) {
  //str1与str2的前n个字符相同，则返回0；若s1大于s2，则返回大于0的值；若s1 小于s2，则返回小于0的值。
    std::cout << "ERROR: Could not read file " << name << std::endl;
    throw pnm_error();
  }

  pnm_read(file, buf);
  int width = atoi(buf);
  pnm_read(file, buf);
  int height = atoi(buf);

  pnm_read(file, buf);
  if (atoi(buf) > UCHAR_MAX) {//无符号char型所能表示的最大整数
    std::cout << "ERROR: Could not read file " << name << std::endl;
    throw pnm_error();
  }

  // read data
  image<uchar> *im = new image<uchar>(width, height);
  file.read((char *)imPtr(im, 0, 0), width * height * sizeof(uchar));

  return im;
}

void savePGM(image<uchar> *im, const char *name) {
  int width = im->width();
  int height = im->height();
  std::ofstream file(name, std::ios::out | std::ios::binary);

  file << "P5\n" << width << " " << height << "\n" << UCHAR_MAX << "\n";
  file.write((char *)imPtr(im, 0, 0), width * height * sizeof(uchar));
}

#endif

来说说头文件

基础好的同学自动忽视这部分内容。
cstdlib 相当于stdlib.h
可以提供一些函数与符号常量，具体如下：根据ISO标准，stdlib.h提供以下类型：
size_t, wchar_t, div_t, ldiv_t, lldiv_t
常量：
NULL, EXIT_FAILURE, EXIT_SUCCESS, RAND_MAX, MB_CUR_MAX
函数：
atof, atoi, atol, strtod, strtof, strtols, strtol, strtoll, strtoul, strtoull, rand, srand, calloc, free, malloc, realloc, abort, atexit, exit, getenv, system, bsearch, qsort, abs, div, labs, ldiv, llabs, tlldiv, mblen, mbtowc, wctomb, mbstowcs, wcstombs

climits定义了各种类型的最大值最小值

pgm格式

p5
width
height
UCHAR_MAX
所有图片数据

简约笔记

1.#ifdef #define #endif
2.atoi
3.template class classname{}
4.template(void)classname(type)::funcname
5.ofstream file(name, std::ios::out | std::ios::binary);