jpg转bmp（使用libjpeg）

　　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;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    }}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;}

 注：write_bmp_data()有内存泄露，不想在源代码上编辑了。