jpg转bmp（使用libjpeg）

写得很好：http://bbs.witech.com.cn/forum.php?mod=viewthread&tid=8131&extra=page%3D1&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 <iostream>

#include <stdio.h>

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16Byte

struct 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;

void write_bmp_header(j_decompress_ptr cinfo)

{

struct bmp_fileheader bfh;

struct bmp_infoheader bih;

unsigned long width;

unsigned long height;

unsigned short depth;

unsigned long headersize;

unsigned long 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)

{

unsigned char *dst_width_buff;

unsigned char *point;

unsigned long width;

unsigned long height;

unsigned short 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<height;i++)

{

for (unsigned long j=0;j<width*depth;j+=depth)

{

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(unsigned char)*width*depth,1,output_file); //一次写一行

}
delete[]dst_width_buff;

}

void analyse_jpeg()

{

struct jpeg_decompress_struct cinfo;

struct jpeg_error_mgr jerr;

JSAMPARRAY buffer;

unsigned char *src_buff;

unsigned char *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);

unsigned long width=cinfo.output_width;

unsigned long height=cinfo.output_height;

unsigned short depth=cinfo.output_components;

src_buff=new unsigned 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<height)

{

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();

return 0;

}