jpg压缩原理可以参考这篇文章，我很早以前转的一篇文章。

　　没有使用libjpeg的压缩代码可以看看这篇文章，也是我很早以前转的。

　　这次使用libjpeg库压缩和的解压正好对应起来，有好多函数名称我都是对称的起的，所以结合起来看效果更好。

　　和上一篇一样，只能处理24位和8位的图像。

　　代码如下：

#include 
     
      #include 
      
       extern "C"{#include "jpeglib.h"};#pragma comment(lib,"jpeg.lib")using namespace std;#pragma pack(2)        //两字节对齐，否则bmp_fileheader会占16Bytestruct bmp_fileheader{    unsigned short    bfType;        //若不对齐，这个会占4Byte    unsigned long    bfSize;    unsigned short    bfReverved1;    unsigned short    bfReverved2;    unsigned long    bfOffBits;};struct bmp_infoheader{    unsigned long    biSize;    unsigned long    biWidth;    unsigned long    biHeight;    unsigned short    biPlanes;    unsigned short    biBitCount;    unsigned long    biCompression;    unsigned long    biSizeImage;    unsigned long    biXPelsPerMeter;    unsigned long    biYpelsPerMeter;    unsigned long    biClrUsed;    unsigned long    biClrImportant;};FILE *input_file;FILE *output_file;struct bmp_fileheader bfh;struct bmp_infoheader bih;unsigned char *src_buffer;unsigned char *dst_buffer;void read_bmp_header(){        fread(&bfh,sizeof(struct bmp_fileheader),1,input_file);    fread(&bih,sizeof(struct bmp_infoheader),1,input_file);}void read_bmp_data(){    fseek(input_file,bfh.bfOffBits,SEEK_SET);    src_buffer=new unsigned char[bih.biWidth*bih.biHeight*bih.biBitCount/8];    fread(src_buffer,sizeof(unsigned char)*bih.biWidth*bih.biHeight*bih.biBitCount/8,1,input_file);    unsigned long width=bih.biWidth;    unsigned long height=bih.biHeight;    unsigned short depth=unsigned short(bih.biBitCount/8);    unsigned char *src_point;    unsigned char *dst_point;    dst_buffer=new unsigned char[width*height*depth];        src_point=src_buffer+width*depth*(height-1);    dst_point=dst_buffer+width*depth*(height-1);    for (unsigned long i=0;i
       
        alloc_sarray)            ((j_common_ptr)&cinfo,JPOOL_IMAGE,width*depth,1);    point=dst_buffer+width*depth*(height-1);    while (cinfo.next_scanline