爱华网写得很好：http://bbs.witech.com.cn/forum.php?mod=viewthread&tid=8131&extra=page=1&page=1
另外在libjpegb库中有现成的模型，在example.c中，有jpeg图片的压缩和解压代码。解压出来后是bmp格式的图片，填充好图片文件头，再把图像数据写入就oK，在下面的例子程序中体现。
相关理论：http://www.cnblogs.com/leaven/archive/2010/04/06/1705846.html
libjpeg库的交叉编译：http://blog.csdn.net/npy_lp/article/details/6991664/，详细可以参考下载的库中的README文件
转载：http://www.cnblogs.com/tiandsp/archive/2012/11/30/2796758.html
还是关于图像格式上的东西。使用了libjpeg库将jpeg图像转换到bmp格式。解压原理还是相对复杂的，将来有机会说不定会详细介绍。这里只是库的使用而已。
首先需要下载libjpeg库，网址在这里：http://www.ijg.org/
然后需要配置环境，我是在windows下用vs2010搞的，编译库可以参考这篇文章。编译出jpeg.lib就可以了。当然实际编程还需要相应的头文件，头文件在下载的文件中。
如果不想编译就在这下载吧：http://vdisk.weibo.com/s/jpiMs
下面是相应的例程，只能将24位彩色图和8位深度图的jpg转换到bmp。
#include
#include
extern "C"{
#include "jpeglib.h"
};
#pragma comment(lib,"jpeg.lib")
using namespace std;
#pragmapack(2)//两字节对齐，否则bmp_fileheader会占16Byte
struct bmp_fileheader
{
unsignedshortbfType;//若不对齐，这个会占4Byte
unsignedlongbfSize;
unsignedshortbfReverved1;
unsignedshortbfReverved2;
unsignedlongbfOffBits;
};
struct bmp_infoheader
{
unsignedlongbiSize;
unsignedlongbiWidth;
unsignedlongbiHeight;
unsignedshortbiPlanes;
unsignedshortbiBitCount;
unsignedlongbiCompression;
unsignedlongbiSizeImage;
unsignedlongbiXPelsPerMeter;
unsignedlongbiYpelsPerMeter;
unsignedlongbiClrUsed;
unsignedlongbiClrImportant;
};
FILE *input_file;
FILE *output_file;
void write_bmp_header(j_decompress_ptr cinfo)
{
structbmp_fileheader bfh;
structbmp_infoheader bih;
unsignedlong width;
unsignedlong height;
unsignedshort depth;
unsignedlong headersize;
unsignedlong filesize;
width=cinfo->output_width;
height=cinfo->output_height;
depth=cinfo->output_components;
if(depth==1)
{
headersize=14+40+256*4;
filesize=headersize+width*height;
}
if(depth==3)
{
headersize=14+40;
filesize=headersize+width*height*depth;
}
memset(&bfh,0,sizeof(struct bmp_fileheader));
memset(&bih,0,sizeof(struct bmp_infoheader));
//写入比较关键的几个bmp头参数
bfh.bfType=0x4D42;
bfh.bfSize=filesize;
bfh.bfOffBits=headersize;
bih.biSize=40;
bih.biWidth=width;
bih.biHeight=height;
bih.biPlanes=1;
bih.biBitCount=(unsigned short)depth*8;
bih.biSizeImage=width*height*depth;
fwrite(&bfh,sizeof(struct bmp_fileheader),1,output_file);
fwrite(&bih,sizeof(struct bmp_infoheader),1,output_file);
if(depth==1)//灰度图像要添加调色板
{
unsigned char *platte;
platte=new unsigned char[256*4];
unsigned char j=0;
for (int i=0;i<1024;i+=4)
{
platte[i]=j;
platte[i+1]=j;
platte[i+2]=j;
platte[i+3]=0;
j++;
}
fwrite(platte,sizeof(unsigned char)*1024,1,output_file);
delete[] platte;
}
}
void write_bmp_data(j_decompress_ptr cinfo,unsigned char*src_buff)
{
unsignedchar *dst_width_buff;
unsignedchar *point;
unsignedlong width;
unsignedlong height;
unsignedshort depth;
width=cinfo->output_width;
height=cinfo->output_height;
depth=cinfo->output_components;
dst_width_buff=new unsigned char[width*depth];
memset(dst_width_buff,0,sizeof(unsigned char)*width*depth);
point=src_buff+width*depth*(height-1);//倒着写数据，bmp格式是倒的，jpg是正的
for(unsigned long i=0;i
{
for (unsigned long j=0;j
{
if(depth==1)//处理灰度图
{
dst_width_buff[j]=point[j];
}
if(depth==3)//处理彩色图
{
dst_width_buff[j+2]=point[j+0];
dst_width_buff[j+1]=point[j+1];
dst_width_buff[j+0]=point[j+2];
}
}
point-=width*depth;
fwrite(dst_width_buff,sizeof(unsignedchar)*width*depth,1,output_file);//一次写一行
}
delete[]dst_width_buff;
}
void analyse_jpeg()
{
structjpeg_decompress_struct cinfo;
structjpeg_error_mgr jerr;
JSAMPARRAYbuffer;
unsignedchar *src_buff;
unsignedchar *point;
cinfo.err=jpeg_std_error(&jerr);//一下为libjpeg函数，具体参看相关文档
jpeg_create_decompress(&cinfo);
jpeg_stdio_src(&cinfo,input_file);
jpeg_read_header(&cinfo,TRUE);
jpeg_start_decompress(&cinfo);
unsignedlong width=cinfo.output_width;
unsignedlong height=cinfo.output_height;
unsignedshort depth=cinfo.output_components;
src_buff=newunsigned char[width*height*depth];
memset(src_buff,0,sizeof(unsigned char)*width*height*depth);
buffer=(*cinfo.mem->alloc_sarray)
((j_common_ptr)&cinfo,JPOOL_IMAGE,width*depth,1);
point=src_buff;
while(cinfo.output_scanline
{
jpeg_read_scanlines(&cinfo,buffer,1);//读取一行jpg图像数据到buffer
memcpy(point,*buffer,width*depth);//将buffer中的数据逐行给src_buff
point+=width*depth;//一次改变一行
}
write_bmp_header(&cinfo);//写bmp文件头
write_bmp_data(&cinfo,src_buff);//写bmp像素数据
jpeg_finish_decompress(&cinfo);
jpeg_destroy_decompress(&cinfo);
delete[]src_buff;
}
int main()
{
input_file=fopen("lena.jpg","rb");
output_file=fopen("lena.bmp","wb");
analyse_jpeg();
fclose(input_file);
fclose(output_file);
cout<<"good job."<<endl;
cin.get();
return0;
}
