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
}
}

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。