還是關于圖像格式上的東西。使用了libjpeg庫將jpeg圖像轉換到bmp格式。解壓原理還是相對復雜的，將來有機會說不定會詳細介紹。這里只是庫的使用而已。

　　首先需要下載libjpeg庫，網址在這里：http://www.ijg.org/

　　然后需要配置環境，我是在windows下用vs2010搞的，編譯庫可以參考這篇文章。編譯出jpeg.lib就可以了。當然實際編程還需要相應的頭文件，頭文件在下載的文件中。

　　如果不想編譯就在這下載吧：http://vdisk.weibo.com/s/jpiMs

　　下面是相應的例程，只能將24位彩色圖和8位深度圖的jpg轉換到bmp。

#include <iostream>
#include <stdio.h>
extern "C"{
#include "jpeglib.h"
};
#pragma comment(lib,"jpeg.lib")
using namespace std;

#pragma pack(2)        //兩字節對齊，否則bmp_fileheader會占16Byte
struct bmp_fileheader
{
    unsigned short    bfType;        //若不對齊，這個會占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));
    
    //寫入比較關鍵的幾個bmp頭參數
    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)        //灰度圖像要添加調色板
    {
        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);    //倒著寫數據，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)        //處理灰度圖
            {
                dst_width_buff[j]=point[j];
            }

            if (depth==3)        //處理彩色圖
            {
                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);    //一次寫一行
    }
}

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);    //一下為libjpeg函數，具體參看相關文檔
    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);    //讀取一行jpg圖像數據到buffer
        memcpy(point,*buffer,width*depth);    //將buffer中的數據逐行給src_buff
        point+=width*depth;            //一次改變一行
    }

    write_bmp_header(&cinfo);            //寫bmp文件頭
    write_bmp_data(&cinfo,src_buff);    //寫bmp像素數據

    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()有內存泄露，不想在源代碼上編輯了。

　　

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